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Passage to a Ringed World

T h e C a s s i n i – H u y g e n s M i s s i o n t o S a t u r n a n d T i t a n

On the Cover: A col-lage of images showsthe destination of theCassini–Huygens mis-sion — the Saturn sys-tem. The insets, fromleft, are Enceladus,Saturn and Titan. Thecollage represents themission’s five chiefareas of scientificinvestigation: icy sat-ellites, Saturn, Titan,rings and the mag-netosphere. (Mag-netospheric andplasma processesproduce the “spokes”that run out acrossthe rings as diffuse,dark markings.)

Passage to a Ringed World

T h e C a s s i n i – H u y g e n s M i s s i o n t o S a t u r n a n d T i t a n

Linda J. Spilker, Editor

Jet Propulsion Laboratory

California Institute of Technology

NASA SP-533

National Aeronautics and Space Administration

Washington, D.C.

October 1997

F O R E W O R D

This book is for you. You will find that it lives up to the promise of its cover

and title. The text is authoritative, but at the same time easy to understand. It is

written for any layperson who is interested in space exploration. In this book,

we will give you a good look at what is involved in sending a large spacecraft

to the outer solar system. Join us and share in the excitement of this extended

voyage of discovery!

One of the first things you will learn is that Cassini–Huygens is an inter-

national mission. Seventeen countries are involved. You will also learn that

Huygens is an atmospheric probe that the Cassini spacecraft will deliver to

Titan, the largest moon of Saturn. Titan has a gaseous atmosphere that is

thicker and more obscuring than the atmosphere here on Earth. We will look

through Titan’s atmosphere with Cassini’s radar and discover surface details

for the first time. The mission will find out if there are really liquid hydrocar-

bons on Titan’s surface in the form of lakes or seas.

The Cassini spacecraft itself will spend four years in orbit about Saturn.

We will examine the rings and visit many of the satellites. We will sample spe-

cial locations in the magnetosphere that are believed to harbor interesting —

some might say strange — plasma processes (that is, electromagnetic interac-

tions involving electrons, protons and ions).

You can see in the table of contents the “menu” we have prepared for

you. One chapter explains the mission, another the spacecraft. Other chapters

tell you about Saturn, Titan, the rings and the various other parts of the Saturn

system. These chapters reflect the facts and theories as we know them today.

The chapters were prepared by expert researchers in each of the areas cov-

ered. Professional writers then edited the text for clarity and to make sure that

you would not be overwhelmed with jargon and technospeak. Working hand-

in-hand with them were graphic designers and illustrators who created the

page layouts and the many informative diagrams. Finally, to assure that the fi-

nal product was accurate, the book was reviewed by scientists and engineers

who work on the Cassini–Huygens mission.

Come with us now as we set out on this voyage of exploration. We have

over three billion kilometers to go! It will take more than six years to reach Sat-

urn. We will get there on July 1, 2004. By the end of the Cassini–Huygens mis-

sion, four years later, we will have completed the most complicated scientific

experiment ever performed.

We wish you good reading!

Richard J. Spehalski Dennis L. MatsonCassini Program Manager Cassini Project Scientist

T

A C K N O W L E D G M E N T S

his book would not have been possible without the outstanding efforts of many

talented people, from the authors who wrote the chapters, to the editors and

reviewers, to the graphic designers, artists and finally the printer. I would like

to thank all of them for their excellent work. Many long hours went into develop-

ing the exciting Cassini–Huygens book you hold in your hand. Cassini–Huygens

plans to reap a bountiful harvest of new information about the Saturn system,

so stay tuned!

Many individuals worked diligently on writing the chapters in this book.

These dedicated authors include Vince Anicich, Scott Bolton, Jim Bradley, Bonnie

Buratti, Marcia Burton, Roger Diehl, Stephen Edberg, Candy Hansen, Charley

Kohlhase, Paulett Liewer, Dennis Matson, Ellis Miner, Nicole Rappaport, Laci Roth,

Linda Spilker and Brad Wallis. I also thank Stephen Edberg, Charley Kohlhase,

Ellis Miner and Randii Wessen for their support in creating the Appendices.

The creative design of this book is the ingenious work of the Jet Propulsion

Laboratory’s Design Services team. The team worked tirelessly to complete this

work before the Cassini–Huygens launch. My thanks to Audrey Steffan-Riethle

for design and art direction. Special thanks go to Sanjoy Moorthy who spent

many long hours writing and editing and organizing the material. My thanks also

to David Hinkle for illustration and production, David Seal for computer artwork,

Adriane Jach and Elsa King-Ko for design and production, Marilyn Morgan for

editing and Robert Chandler for printing coordination.

Many people provided reviews of the chapters. Special thanks for detailed

reviews and editorial support go to Stephen Edberg, Charley Kohlhase, Jean-Pierre

Lebreton, Ellis Miner, Nicole Rappaport, Laci Roth and Randii Wessen. My thanks

to Marjorie Anicich, Michel Blanc, Jeff Cuzzi, Luke Dones, Michele Dougherty,

Charles Elachi, Larry Esposito, Bill Fawcett, Ray Goldstein, Don Gurnett, Len Jaffe,

Bill Kurth, Janet Luhman, Jonathan Lunine, Carl Murray, Andy Nagy, Toby Owen,

Carolyn Porco, Dave Young and Philippe Zarka for serving as reviewers.

Linda J. SpilkerCassini Deputy Project Scientist

Chapter 1 A Mission of Discovery 1

Chapter 2 The Ringed World 17

Chapter 3 The Mysterious Titan 27

Chapter 4 Those Magnificent Rings 41

Chapter 5 The Icy Satellites 53

Chapter 6 A Sphere of Influence 67

Chapter 7 Planning a Celestial Tour 81

Chapter 8 Vehicle of Discovery 89

Chapter 9 Tools of Discovery 101

Chapter 10 An Ensemble Effort 121

Afterword 131

Appendices

A Glossary of Terms 135

B Acronyms & Abbreviations 143

C General Index 146

D Solar System Characteristics 148

E Program & Science Management 151

F Bibliography 152

Contents

A M I S S I O N O F D I S C O V E R Y 1

S

C H A P T E R 1

aturn, the second most massive planet in the solar sys-tem, offers us a treasure of opportunities for explorationand discovery. Its remarkable system of rings is a sub-ject intensively studied, described and cataloged. Itsplanet-sized satellite, Titan, has a dense, veiled atmo-

sphere. Some 17 additional icy satellites are knownto exist — each a separate world to explore in itself.Saturn’s magnetosphere is extensive and maintainsdynamic interfaces with both the solar wind andwith Titan’s atmosphere.

From Observation to ExplorationSaturn was known even to the an-cients. Noting the apparitions of theplanets and recording other celestialevents was a custom of virtually everyearly civilization. These activitiesknew no oceanic bounds, suggestinga curiosity universal to humankind.Real progress in understanding theplanets came with the invention and

use of instruments to accuratelymeasure celestial positions, enablingastronomers to catalog the planets’positions.

Developments in mathematics and thediscovery of the theory of gravitation-al attraction were also necessarysteps. With the invention of the tele-scope and its first application in ob-

serving the heavens by Galileo Ga-lilei, the pace of progress quickened.

Today, the Cassini–Huygens missionto Saturn and Titan is designed tocarry out in-depth exploration of theSaturn system. A payload of scienceinstruments will make in situ measure-ments or observe their targets underfavorable geometric and temporalcircumstance. For example, an instru-ment might make observations overa range of various angles of illumina-tion and emission, or study eventssuch as occultations or eclipses.

Cassini–Huygens’ interplanetary jour-ney starts in October 1997, with thelaunch from Cape Canaveral in Flo-rida. Upon arrival at Saturn, Cassini–Huygens will go into orbit about theplanet. The spacecraft consists of twoparts — the Cassini Orbiter and asmaller spacecraft, the HuygensProbe, which is targeted for Titan,Saturn’s largest moon.

Huygens will arrive at Titan in No-vember 2004. After using its heatshield for deceleration in Titan’s up-per atmosphere, Huygens will deploya parachute system. Six instrumentswill make scientific measurementsand observations during the long

Three separateimages, taken byVoyager 2 throughultraviolet, violetand green filters,respectively, werecombined to cre-ate this false-colorimage of Saturn.

A Mission of Discovery

2 P A S S A G E T O A R I N G E D W O R L D

G E T T I N G T O K N O W S A T U R N

A T I M E L I N E O F D I S C O V E R Y

~ 800 BC Assyrian and Babylonian observations

~ 300 AD Mythological view of Saturn the god

1610 Galileo notes the “triple planet” Saturn

with his telescope

1655–59 Huygens discovers Saturn’s largest satellite, Titan

1671–84 Cassini discovers a division in the ring;

he also discovers the satellites Iapetus,

Rhea, Dione and Tethys

1789 Herschel discovers satellites Mimas and

Enceladus — and notes thinness of rings

1848 Bond and Lassel discover the satellite Hyperion

1850 Bond, Bond and Daws discover inner ring

1857 Maxwell proves that rings are not solid

1895 Keeler measures ring velocities

1898 Pickering discovers satellite Phoebe

1932 Wildt discovers methane and ammonia on Saturn

1943–44 Kuiper discovers methane and ammonia on Titan

1979 Pioneer 11 flies past Saturn

1980 Voyager 1 encounters Saturn

1981 Voyager 2 encounters Saturn

1989 Hubble Space Telescope’s

Wide Field and Planetary Camera images Saturn

1995 Wide Field and Planetary Camera 2 images

ring plane crossing

1997 Cassini–Huygens launches

2004 Cassini–Huygens enters Saturn orbit;

Huygens explores Titan


descent to the surface. The HuygensProbe data will be transmitted to theOrbiter and then to Earth.

The Orbiter then commences a tourof the Saturn system. With its comple-ment of 12 instruments, Cassini is ca-pable of making a wide range of insitu and remote-sensing observations.The Orbiter will make repeated closeflybys of Titan to make measurementsand obtain observations.

The flybys of Titan will also providegravity-assisted orbit changes, en-abling the Orbiter to visit other satel-lites and parts of the magnetosphereand observe occultations of the ringsand atmospheres of Saturn and Titan.Over the span of the four-year-longorbital mission, Cassini is expected torecord temporal changes in many ofthe properties that it will observe.

Vision for a MissionThe Cassini–Huygens mission hon-ors two astronomers who pioneeredmodern observations of Saturn. TheOrbiter is named for Jean-DominiqueCassini, who discovered the satellitesIapetus, Rhea, Dione and Tethys, aswell as ring features such as the Cas-sini division, in the period 1671–1684. The Titan Probe is named forChristiaan Huygens, who discoveredSaturn’s largest satellite in 1655.

The mission is a joint undertaking bythe National Aeronautics and SpaceAdministration (NASA) and the Euro-pean Space Agency (ESA). The Huy-gens Probe is supplied by ESA andthe main spacecraft — the Orbiter —is provided by NASA. The Italian

space agency (Agenzia SpazialeItaliana, or ASI), through a bilateralagreement with NASA, is providinghardware systems for the Orbiterspacecraft and instruments. Otherinstruments on the Orbiter and theProbe are provided by scientificgroups and/or their industrial part-ners, supported by NASA or by thenational funding agencies of memberstates of ESA. The launch vehicle andlaunch operations are provided byNASA. NASA will also provide themission operations and telecommuni-cations via the Deep Space Network(DSN). Huygens operations are car-ried out by ESA from its operationscenter in Darmstadt, Germany.

Late in 1990, NASA and ESA simul-taneously selected the payloads forthe Orbiter and for the HuygensProbe, respectively. Both agenciesalso selected interdisciplinary investi-gations. The NASA Orbiter selectioncomprises seven principal investiga-tor instruments, five facility instru-ments and seven interdisciplinaryinvestigations. The ESA Huygensselection comprises six principal in-vestigator instruments and three inter-disciplinary scientist investigations.

This complex, cooperative undertak-ing did not come into being over-night. Rather, it was the end productof a process of joint discussions andcareful planning. The result — theCassini–Huygens mission — is an en-terprise that, from the initial vision tothe completion of the nominal mis-

sion, will span nearly 30 years!The formal beginning was in 1982,when a Joint Working Group wasformed by the Space Science Com-mittee of the European Science Foun-dation and the Space Science Boardof the National Academy of Sci-ences in the United States.

The charter of the group was tostudy possible modes of cooperationbetween the United States and Eu-rope in the field of planetary sci-ence. The partners were cautiousand did not enter lightly into the de-cision to carry out the Cassini–Huy-gens mission. Their precept was thatthe mission would be beneficial forthe scientific, technological and in-dustrial sectors of their countries.

The Quest for UnderstandingIn carrying out this voyage, we arefollowing a basic, evolutionally nur-tured instinct to explore our environ-ment. Whether exploration resultsin the discovery of resources or therecognition of hazards, or merelyprovides a sense of place or accom-plishment, it has always provedbeneficial to be familiar with ourenvironment.

It is not surprising, therefore, thatsuch exploration is a hallmark ofgrowing, thriving societies. Parallelscan be drawn between historicalvoyages of exploration and the eraof solar system exploration. Avail-able technology, skilled labor, possi-ble benefits, cost, risk and tripduration continue to be some of themajor considerations in deciding —to go or not to go? All these factorswere weighed for Cassini–Huygens— and we decided to go.


In traveling to Saturn with Cassini–Huygens, we will also be satisfying acultural desire to obtain new knowl-edge. This drive is very strong be-cause modern society places a veryhigh value on knowledge. For exam-ple, the whole field of education is fo-cused on transferring knowledge tonew generations. Often, a substantialportion of a person’s existence isspent in school. The resources ex-pended in creating new knowledgethrough inventions, research andscholarship are considered to be in-vestments, with the beneficial returnto come later. Knowledge is advanta-geous to us.

With Cassini–Huygens, we do nothave to wait for our arrival at Saturn,because the return of new knowledgehas already occurred. The challenge

of this mission has resulted in newtechnological developments and in-ventions. Some of these have alreadybeen spun off to new applications,and new benefits are being realizednow. But as with any investment, themain return is expected later, whenCassini–Huygens carries out its mis-sion at Saturn.

Cassini–Huygens is the next logicalstep in the exploration of the outersolar system. The Jupiter system hasbeen explored by Galileo. Now it isSaturn’s turn. With Cassini–Huygens,we will explore in depth a new partof the solar system. Not only will welearn about the Saturn system, butas a result we will also learn moreabout Earth as a part of the solarsystem — rather than as an isolatedplanet.

Physics and chemistry are the sameeverywhere. Thus, knowledge gainedabout Saturn’s magnetosphere or Ti-tan’s atmosphere will have applica-tion here on Earth. Interactions withthe solar wind and impacts of cometsand asteroids are just two of the pro-cesses that planets have in common.Information gleaned at Saturn aboutthese shared histories and processeswill also lead us to new informationabout Earth and its history.

The Cassini–Huygens mission also al-lows the realization of other goals.Bringing people of different countriestogether to work toward a commongoal promotes understanding andcommon values. The internationalcharacter of Cassini–Huygens permitstalented engineers and scientists to

V E H I C L E O F D I S C O V E R Y

4-meter High-Gain Antenna

11-meterMagnetometerBoom

Radio andPlasma WaveSubsystemAntenna (1 of 3)

Remote-Sensing Pallet

445-newtonEngine (1 of 2)

Low-GainAntenna(1 of 2)

Radar Bay

Fields andParticles Pallet

Huygens TitanProbe

RadioisotopeThermoelectricGenerator(1 of 3)

are attached andfueled, the space-craft weighs over5600 kilograms.

Its body standsalmost seven me-ters tall and is overfour meters wide.Here the spacecraftis shown without itsthermal blankets.

After a deep spacevoyage, Cassini willspend years studyingthe vast Saturn system.

A big mission re-quires a big space-craft — and Cassinifits the bill. WhenOrbiter and Probe


address the challenges of the mission.More people will share in the discov-eries, the costs and the benefits.

About three quarters of a million peo-ple from more than 80 countries haveinvolved themselves in the Cassini–Huygens mission by requesting thattheir signatures be placed aboard thespacecraft for the trip to the Saturnsystem. What will the mission bring tothese people? The answers are var-ied: satisfaction of curiosity, a senseof participation, aesthetic inspiration,perhaps even understanding of ourplace in this vast universe.

The SpacecraftAt the time of launch, the mass of thefully fueled Cassini spacecraft will beabout 5630 kilograms. Cassini con-sists of several sections. Starting atthe bottom of the “stack” and movingupward, these are the lower equip-ment module, the propellant tanks to-gether with the engines, the upperequipment module, the 12-bay elec-tronics compartment and the high-gain antenna. These are all stackedvertically on top of each other. At-tached to the side of the stack is anapproximately three-meter-diameter,disk-shaped spacecraft — the Huy-gens Titan Probe. Cassini–Huygensaccommodates some 27 scientific in-vestigations, supported by 18 special-ly designed instruments: 12 on theOrbiter and six on the Probe.

Most of the Orbiter’s scientific instru-ments are installed on one of twobody-fixed platforms — the remote-sensing pallet or the fields and parti-cles pallet — named after the typeof instruments they support. The big,

11-meter-long boom supports sensorsfor the magnetometer experiment.Three thin 10-meter-long electricalantennas point in orthogonal direc-tions; these are sensors for the Radioand Plasma Wave Science experi-ment. At the top of the stack is thelarge, four-meter-diameter high-gainantenna. Centered and at the verytop of this antenna is a relativelysmall low-gain antenna. A secondlow-gain antenna is located nearthe bottom of the spacecraft.

Two-way communication with Cassiniwill be through NASA’s Deep SpaceNetwork (DSN) via an X-band radiolink, which uses either the four-meter-diameter high-gain antenna or one ofthe two low-gain antennas. The high-gain antenna is also used for radioand radar experiments and for re-ceiving signals from Huygens.

The electrical power for the space-craft is supplied by three radioiso-tope thermoelectric generators.Cassini is a three-axis-stabilizedspacecraft. The attitude of the space-craft is changed by using either reac-tion wheels or the set of 0.5-newtonthrusters. Attitude changes will bedone frequently because the instru-ments are body-fixed and the wholespacecraft must be turned in order topoint them. Consequently, most of theobservations will be made without areal-time communications link toEarth. The data will be stored ontwo solid-state recorders, each witha capacity of about two gigabits.

Scientific data will be obtained pri-marily by using one or the other oftwo modes of operation. Thesemodes have been named after thefunctions they will carry out and arecalled the “remote-sensing mode”and the “fields and particles anddownlink mode.”

During remote-sensing operations, therecorders are filled with images andspectroscopic and other data that areobtained as the spacecraft points tovarious targets. During the fields andparticles and downlink mode, thehigh-gain antenna is pointed at Earthand the stored data are transmittedto the DSN. Also, while in this mode,the spacecraft is slowly rolled aboutthe axis of the high-gain antenna.This allows sensors on the fields andparticles pallet to scan the sky anddetermine directional components forthe various quantities they measure.

Mission OverviewThe Cassini–Huygens mission is de-signed to explore the Saturn systemand all its elements — the planet Sat-urn and its atmosphere, its rings, itsmagnetosphere, Titan and many ofthe icy satellites. The mission will payspecial attention to Saturn’s largestmoon, Titan, the target for Huygens.

The Cassini Orbiter will make re-peated close flybys of Titan, both forgathering data about Titan and forgravity-assisted orbit changes. Thesemaneuvers will permit the achieve-ment of a wide range of desirablecharacteristics on the individual or-bits that make up the tour. In turn,this ability to change orbits will en-able close flybys of icy satellites, re-


Instrument Investigations

Cassini Plasma Spectrometer In situ study of plasma within and near Saturn’s magnetic field

Cosmic Dust Analyzer In situ study of ice and dust grains in the Saturn system

Dual Technique Magnetometer Study of Saturn’s magnetic field and interactions with the solar wind

Ion and Neutral Mass Spectrometer In situ study of compositions of neutral and charged particleswithin the magnetosphere

Magnetospheric Imaging Instrument Global magnetospheric imaging and in situ measurements of Saturn’smagnetosphere and solar wind interactions

Radio and Plasma Wave Science Measurement of electric and magnetic fields, and electron densityand temperature in the interplanetary medium and withinSaturn’s magnetosphere

Cassini Radar Radar imaging, altimetry, and passive radiometry of Titan’s surface

Composite Infrared Spectrometer Infrared studies of temperature and composition of surfaces,atmospheres and rings within the Saturn system

Imaging Science Subsystem Multispectral imaging of Saturn, Titan, rings and icy satellites to observetheir properties

Radio Science Instrument Study of atmospheric and ring structure, gravity fields andgravitational waves

Ultraviolet Imaging Spectrograph Ultraviolet spectra and low-resolution imaging of atmospheres and ringsfor structure, chemistry and composition

Visible and Infrared Mapping Spectrometer Visible and infrared spectral mapping to study composition andstructure of surfaces, atmospheres and rings

Aerosol Collector and Pyrolyser In situ study of clouds and aerosols in Titan’s atmosphere

Descent Imager and Spectral Radiometer Measurement of temperatures of Titan’s atmospheric aerosols andsurface imagery

Doppler Wind Experiment Study of winds by their effect on the Probe during descent

Gas Chromatograph and Mass Spectrometer In situ measurement of chemical composition of gases and aerosolsin Titan’s atmosphere

Huygens Atmospheric Structure Instrument In situ study of Titan’s atmospheric physical and electrical properties

Surface Science Package Measurement of the physical properties of Titan’s surface

C A S S I N I – H U Y G E N S S C I E N C E I N V E S T I G A T I O N S

Cassini Saturn

Orbiter Fields and

Particles Instruments

Cassini Saturn

Orbiter Remote-Sensing

Instruments

Huygens Titan

Probe Instruments


Participant Nations*

* First flag in each row represents nation of Principal Investigator or Team Leader.


connaissance of the magnetosphereover a variety of locations and the ob-servation of the rings and Saturnat various illumination and occultationgeometries and phase angles.

The spacecraft will be injected into a6.7-year Venus–Venus–Earth–JupiterGravity Assist (VVEJGA) trajectory toSaturn. Included are gravity assistsfrom Venus (April 1998 and June1999), Earth (August 1999) and Jupi-ter (December 2000). Arrival at Sat-urn is planned for July 2004.

During most of the early portion of thecruise, communication with the space-craft will be via one of the two low-gain antennas. Six months after theEarth flyby, the spacecraft will turn topoint its high-gain antenna at Earthand communications from then on willuse the high-gain antenna.

Following the Jupiter flyby, the space-craft will attempt to detect gravitation-

al waves using Ka-band and X-bandradio equipment. Instrument calibra-tions will also be done during cruisebetween Jupiter and Saturn. Scienceobservations will begin two yearsaway from Saturn (about one anda half years after the Jupiter flyby).

The most critical phase of the missionfollowing launch is the Saturn orbitinsertion (SOI) phase. Not only willit be a crucial maneuver, but it willalso be a period of unique scientificactivity, because at that time thespacecraft will be the closest it willever be to the planet.

The SOI phase of the trajectory willalso provide a unique opportunity forobserving the rings. The spacecraft’sfirst orbit will be the longest in the or-bital tour. A periapsis-raise maneuverin September 2004 will establish thegeometry for the Huygens Probe en-try at the spacecraft’s first Titan flybyin November.

Huygens’ Encounter with Titan. In No-vember 2004, the Huygens Probewill be released from the Cassini Or-biter, 21–22 days before the first Ti-tan flyby. Two days after the Probe’srelease, the Orbiter will perform a de-flection maneuver; this will keep theOrbiter from following Huygens intoTitan’s atmosphere. It will also estab-lish the required radio-communicationgeometry between the Probe and theOrbiter, which is needed during theProbe descent phase, and will alsoset the initial conditions for the satel-lite tour — which starts right after thecompletion of the Probe mission.

The Huygens Probe has the task ofentering Titan’s atmosphere, makingin situ measurements of the satellite’sproperties during descent by para-chute to the surface. The Probe con-sists of a descent module enclosed bya thermal-protection shell. The frontshield of this shell is 2.7 meters in di-

Cassini will takenearly seven yearsto make the longtrip from Earth toSaturn. Gravityassists from Venus(twice), Earth andJupiter will giveCassini slingshot-

T A K I N G T H E L O N G W A Y R O U N D

type boosts that willhurl the spacecrafton toward its July 1,2004, encounter with

Saturn. It may takea while, but Cassiniwill get there!

Venus SwingbyApril 21, 1998

Venus SwingbyJune 22, 1999

Earth SwingbyAugust 17, 1999

Deep SpaceManeuverJanuary 20, 1999

Venus OrbitEarth Orbit

Jupiter SwingbyDecember 30,2000

Saturn ArrivalJuly 1, 2004

Launch from EarthOctober 6, 1997


P R O B I N G A M O O N O F M Y S T E R Y

Huygens’ encounterwith Titan is plannedfor November 2004,about three weeksbefore the Orbitermakes its first flybyof Saturn’s largestsatellite. The Probe,which consists of adescent module en-veloped by a conicalthermal-protection

ameter and is a very bluntly shapedconical capsule with a high drag co-efficient. The shield is covered witha special thermal-ablation material toprotect the Probe from the enormousflux of heat generated during atmo-spheric entry. On the aft side is aprotective cover that is primarilydesigned to reflect away the heat ra-diated from the hot wake of the Probeas it decelerates in Titan’s upper at-mosphere. Atmospheric entry is atricky affair — entry at too shallow anangle can cause the Probe to skip outof the atmosphere and be lost. If theentry is too steep, that will cause theProbe measurements to begin at alower altitude than is desired.

After the Probe has separated fromthe Orbiter, the electrical power forits whole mission is provided by fivelithium–sulfur dioxide batteries. TheProbe carries two S-band transmittersand two antennas, both of which willtransmit to the Orbiter during the

Probe’s descent. One stream of telem-etry is delayed by about six secondswith respect to the other to avoiddata loss if there are brief transmis-sion outages.

Once the Probe has decelerated toabout Mach 1.5, the aft cover ispulled off by a pilot parachute. An8.3-meter-diameter main parachute isthen deployed to ensure a slow andstable descent. The main parachuteslows the Probe and allows the decel-erator and heat shield to fall away.

To limit the duration of the descent toa maximum of two and a half hours,the main parachute is jettisoned atentry +900 seconds and replacedby a smaller, three-meter-diameterdrogue chute for the remainder of thedescent. The batteries and other re-sources are sized for a maximum mis-sion duration of 153 minutes. This

corresponds to a maximum descenttime of two and a half hours, with atleast three minutes — but possibly upto half an hour or more — on the sur-face, if the descent takes less timethan expected.

The instrument operations are com-manded by a timer in the top part ofthe descent and on the basis of mea-sured altitude in the bottom part ofthe descent. The altitude is measuredby a small radar altimeter during thelast 10–20 kilometers.

Throughout the descent, the HuygensAtmospheric Structure Instrument(HASI) will measure more than half adozen physical properties of the at-mosphere. The HASI will also processsignals from the Probe’s radar altime-ter to gain information about surfaceproperties. The Gas Chromatographand Mass Spectrometer (GCMS) willdetermine the chemical composition

shell almost threemeters in diameter,carries six scienceinstruments designedto make in situ mea-

Titan Probe Entryand Orbiter FlybyNovember 27, 2004

Probe ReleaseNovember 6, 2004

Saturn OrbitInsertionJuly 1, 2004

surements of Titan’satmospheric and sur-face properties.

1 0 P A S S A G E T O A R I N G E D W O R L D

of the atmosphere as a function of al-titude. The Aerosol Collector and Py-rolyser (ACP) will capture aerosolparticles, heat them and send the ef-fused gas to the GCMS for analysis.

The optical radiation propagationin the atmosphere will be measuredin all directions by the Descent Imag-er and Spectral Radiometer (DISR).The DISR will also image the cloudformations and the surface. As thesurface looms closer, the DISR willswitch on a bright lamp and measurethe spectral reflectance of the surface.

Throughout its descent, the Dopplershift of Huygens’ telemetric signal willbe measured by the Doppler WindExperiment (DWE) equipment on theOrbiter to determine the atmosphericwinds, gusts and turbulence.

In the proximity of the surface, theSurface Science Package (SSP) willactivate a number of its devices tomake measurements near and on thesurface. If touchdown occurs in a liq-uid, such as in a lake or a sea, theSSP will measure the liquid’s physicalproperties.

The Orbital Tour. After the end of theProbe mission, the Orbiter will startits nearly four-year tour, consistingof more than 70 Saturn-centeredorbits, connected by Titan-gravity-assist flybys or propulsive maneuvers.The size of these orbits, their orienta-tion to the Sun–Saturn line and theirinclination to Saturn’s equator aredictated by the various scientificrequirements, which include Titanground-track coverage; flybys of icysatellites, Saturn, Titan, or ring occul-tations; orbit inclinations; and ring-plane crossings.

D E S C E N T I N T O T H E M U R K Y D E P T H S

As Huygens’ descentinto Titan slows, it re-leases a small para-chute, which thendeploys the main para-chute. The deceleratorshield jettisons and theProbe drifts to about40 kilometers abovethe surface. Then, asmaller, drogue chutecarries Huygens therest of the way to thesurface.

Main Chute Deploys

Instruments’ InletPort Opens

DeceleratorJettisons

Drogue Chute DeploysAlti

tude

, kilo

met

ers

Time, Hours After Entry0 2.5

1000

500

300

192

170

0

A M I S S I O N O F D I S C O V E R Y 1 1

T O U R T 1 8 - 3

T A R G E T E D S A T E L L I T E F L Y B Y S *

Cassini Flybys, Saturn Voyager ClosestPlanned Satellite Approach, kilometers

1 Iapetus 909,000

4 Enceladus 87,000

1 Dione 162,000

1 Rhea 74,000

* Actual distances will be based on the type of observations planned.

Titan is the only Saturn satellite thatis large enough to enable significantgravity-assisted orbit changes. Thesmaller icy satellites can help some-times with their small perturbations,which can be useful in trimming atrajectory. Designing the Cassiniorbital tour is a complicated andchallenging task that will not becompleted for at least several years.

One of the tours under consideration(called T18-3) can be used to illus-trate the complexity involved in thistype of navigational planning. T18-3was the eighteenth tour designed forCassini: This example is the thirditeration of that tour. Tour T18-3comprises 43 Titan flybys and seven

of less than 100,000 kilometers. Ata range of 100,000 kilometers, thepixel resolution of Cassini’s narrow-angle camera is about 0.6 kilometerfor several of the icy satellites, pro-viding better resolution than thatachieved by Voyager.

Science ObjectivesAll the activities on Cassini–Huygensare pointed toward the achievementof the mission’s science objectives.The origin of the objectives can betraced all the way back to the meet-ings of the Joint Working Group in1982. They were further developedduring the Joint NASA–ESA assess-ment study in 1984-85. As a resultof this study, the science objectives

“targeted” flybys of the icy satellitesIapetus, Enceladus, Dione and Rhea.“Targeted” means that the flyby dis-tance can be chosen to best accom-modate the planned observations —generally in the 1000-kilometerrange. The closest approaches tothe satellites made by the Voyagerspacecraft in the early 1980s areused for reference.

In addition to the targeted flybys,there are other, unplanned — seren-dipitous — flybys that occur as a re-sult of the tour path’s geometry. Ifthese flybys are close enough to thesatellites, they will provide valuableopportunities for scientific observa-tions. In our sample tour, T18-3, thereare 28 of these flybys, with distances

T O U R T 1 8 - 3

S E R E N D I P I T O U S S A T E L L I T E F L Y B Y S *

Saturn 5000–25,000 25,000–50,000 50,000–100,000 Voyager FlybySatellite kilometers kilometers kilometers Range, kilometers

Mimas – 1 4 88,000

Enceladus 3 – 3 87,000

Tethys 1 3 6 93,000

Dione – 2 2 161,00

Rhea – 1 1 74,000

Phoebe – 1 – 2,076,000

* For flybys with closest approach on the sunlit side.


appeared for the first time in theirpresent form in the group’s final re-port, issued by ESA in 1985. Theobjectives then became formally es-tablished with their inclusion in theNASA and ESA announcements ofopportunity (ESA, 1989; NASA,1989, 1991). The science objectivesfor Cassini–Huygens are organizedby mission phase and target.

Scientific Objectives During Cruise.Given the trajectory for the long voy-age to Saturn, Cassini–Huygens hasthe opportunity to carry out a numberof experiments during the cruisephase. After launch, there will be in-strument checkouts and maintenanceactivities. Searches for gravity waves

will be carried out during the threesuccessive oppositions of the space-craft, beginning in December 2001.

These searches are radio experi-ments that involve using the DSN fortwo-way, Ka–band tracking of thespacecraft. During two solar conjunc-tions of the spacecraft, a series of ra-dio-propagation measurementsobtained from two-way X-band andKa-band DSN tracking will provide atest of general relativity, as well asdata on the solar corona.

In early 1992, the Cassini projectteam at the Jet Propulsion Laboratoryredesigned the mission and the Or-biter to meet NASA budgetary con-

straints expected for the followingyears. As a result, most of the scienceobjectives for targets of opportunity— asteroid flyby, Jupiter flyby andcruise — were deleted. This was partof the effort to control developmentcosts and the cost of operation duringthe first few years in flight. In the cur-rent baseline plan, the scientific dataacquisition will start two years beforearrival at Saturn; that is, well after theJupiter flyby.

Scientific Objectives at Saturn. The listof scientific objectives for Cassini–Huygens is extensive. There are spe-cific objectives for each of the typesof bodies in the system — the planetitself, the rings, Titan, icy satellitesand the magnetosphere. Not only is

E N C O U N T E R S W I T H A C E L E S T I A L G I A N T

Iapetus Orbit

Titan Orbit

Initial Orbit

The actual makeupof Cassini’s four-year “tour” of theSaturn system is yetto be finalized, butmany possible sce-narios are alreadyunder examination.

This sample tour,named T18-3, con-tains over 70 orbits ofSaturn, over 40 flybysof Titan and a numberof close flybys of sev-eral other satellites.


C A S S I N I – H U Y G E N S M I S S I O N

S C I E N C E O B J E C T I V E S

S A T U R N

Determine temperature field, cloud properties and composition of the atmosphere.

Measure global wind field, including wave and eddy components; observe synoptic cloudfeatures and processes.

Infer internal structure and rotation of the deep atmosphere.

Study diurnal variations and magnetic control of ionosphere.

Provide observational constraints (gas composition, isotope ratios, heat flux) on scenariosfor the formation and evolution of Saturn.

Investigate sources and morphology of Saturn lightning (Saturn electrostatic discharges,lightning whistlers).

T I T A N

Determine abundances of atmospheric constituents (including any noble gases); establish isotope ratiosfor abundant elements; constrain scenarios of formation and evolution of Titan and its atmosphere.

Observe vertical and horizontal distributions of trace gases; search for more complex organicmolecules; investigate energy sources for atmospheric chemistry; model the photochemistry of thestratosphere; study formation and composition of aerosols.

Measure winds and global temperatures; investigate cloud physics and general circulation andseasonal effects in Titan’s atmosphere; search for lightning discharges.

Determine physical state, topography and composition of surface; infer internal structure.

Investigate upper atmosphere, its ionization and its role as a source of neutral and ionized materialfor the magnetosphere of Saturn.

M A G N E T O S P H E R E

Determine the configuration of the nearly axially symmetrical magnetic field and its relation to themodulation of Saturn kilometric radiation.

Determine current systems, composition, sources and sinks of the magnetosphere’s charged particles.

Investigate wave–particle interactions and dynamics of the dayside magnetosphere and magnetotailof Saturn, and their interactions with solar wind, satellites and rings.

Study effect of Titan’s interaction with solar wind and magnetospheric plasma.

Investigate interactions of Titan’s atmosphere and exosphere with surrounding plasma.

R I N G S

Study configuration of rings and dynamic processes (gravitational, viscous, erosional andelectromagnetic) responsible for ring structure.

Map composition and size distribution of ring material.

Investigate interrelation of rings and satellites, including embedded satellites.

Determine dust and meteoroid distribution in ring vicinity.

Study interactions between rings and Saturn’s magnetosphere, ionosphere and atmosphere.

I C Y S A T E L L I T E S

Determine general characteristics and geological histories of satellites.

Define mechanisms of crustal and surface modifications, both external and internal.

Investigate compositions and distributions of surface materials, particularly dark, organicrich materials and low-melting-point condensed volatiles.

Constrain models of satellites’ bulk compositions and internal structures.

Investigate interactions with magnetosphere and ring system and possible gas injectionsinto the magnetosphere.


C A S S I N I — H U Y G E N SC H R O N O L O G Y O F A P L A N E T A R Y M I S S I O N

1 9 8 2

Space Science Committee of the

European Science Foundation and

the Space Science Board of the

National Academy of Sciences form

working group to study possible

U.S.–European planetary science

cooperation. European scientists

propose a Saturn orbiter–Titan

probe mission to the European

Space Agency (ESA), suggesting

a collaboration with NASA.

1 9 8 3

U.S. Solar System Exploration

Committee recommends NASA

include a Titan probe and a radar

mapper in its core program and

also consider a Saturn orbiter.

1 9 8 4 – 8 5

Joint ESA–NASA assessment study

of a Saturn orbiter–Titan probe

mission.

1 9 8 7

ESA Science Program Committee

approves Cassini for Phase A study,

with conditional start in 1987.

1 9 8 7 – 8 8

NASA carries out further definition

and work on Mariner Mark 2

spacecraft and the missions de-

signed to use it: Cassini and Comet

Rendezvous/Asteroid Flyby (CRAF).

Titan probe Phase A study carried

out by joint ESA/NASA committee,

supported by European industrial

consortium led by Marconi Space

Systems.

1 9 8 8

Selection by ESA of Cassini mis-

sion Probe to Titan as next science

mission; Probe named Huygens.

1 9 8 9

Funding for CRAF and Cassini

approved by U.S. Congress. NASA

and ESA release announcements

of opportunity to propose scien-

tific investigations for the Saturn

orbiter and Titan probe.

1 9 9 2

Funding cap imposed on CRAF/

Cassini: CRAF canceled; Cassini

restructured. Launch rescheduled

from 1996 to 1997.

1 9 9 5

U.S. House Appropriations Sub-

committee targets Cassini for

cancellation, but the action is

reversed.

1 9 9 6

Spacecraft and instruments inte-

gration and testing.

A P R I L 1 9 9 7

Cassini spacecraft shipped to Cape

Canaveral, Florida.

S U M M E R 1 9 9 7

Final integration and testing.

O C T O B E R 1 9 9 7

Launch from Cape Canaveral,

Florida.

A P R I L 1 9 9 8

First Venus gravity-assist flyby.

J U N E 1 9 9 9

Second Venus gravity-assist flyby.

A U G U S T 1 9 9 9

Earth gravity-assist flyby.

D E C E M B E R 2 0 0 0

Jupiter gravity-assist flyby.

D E C E M B E R 2 0 0 1

First gravitational-wave experiment.

J U N E 1 2 , 2 0 0 4

Phoebe flyby; closest approach is

52,000 kilometers.

J U L Y 1 , 2 0 0 4

Spacecraft arrives at Saturn and

goes into orbit around the planet.

N O V E M B E R 6 , 2 0 0 4

Release of Huygens Probe on a tra-

jectory to enter Titan’s atmosphere.

N O V E M B E R 2 7 , 2 0 0 4

Huygens returns data as it descends

through Titan’s atmosphere and

reaches the surface; Orbiter begins

tour of the Saturn system.

J U L Y 2 0 0 8

Nominal end of mission.


Cassini–Huygens designed to deter-mine the present state of these bodiesand the processes operating on or inthem, but it is also equipped to dis-cover the interactions that occuramong and between them.

These interactions within the Saturnsystem are important. An analogy canbe drawn to a clock, which has manyparts. However, a description of eachpart and the cataloging of its proper-ties, alone, completely misses the “es-sence” of a clock. Rather, the essenceis in the interactions among the parts.So it is for much of what the Cassini–Huygens mission will be studying inthe Saturn system.

It is the ability to do “system science”that sets apart the superbly instrument-ed spacecraft. The very complex inter-actions that are in play in systemssuch as those found at Jupiter and Sat-urn can only be addressed by suchinstrument platforms. This is becausethe phenomena to be studied are of-ten sensitive to a large number of pa-rameters — a measurement mighthave to take into account simulta-neous dependencies on location,time, directions to the Sun and planet,the orbital configurations of certainsatellites, magnetic longitude and lati-tude and solar wind conditions. Todeal with such complexity, the righttypes of instruments must be on thespacecraft to make all the necessaryand relevant measurements — and all

the measurements must be made es-sentially at the same time. Identicalconditions very seldom, if ever, recur.Thus, it would be totally impossiblefor a succession, or even a fleet, of“simple” spacecraft to obtain thesame result. Furthermore, requiringthe instruments to operate simulta-neously has a major impact onspacecraft resources such as electri-cal power. This demand — and theneed for a broadly based, diversecollection of instruments — is the rea-son that the Cassini–Huygens space-craft is so large.

Both the Huygens Probe and the Cas-sini Orbiter will study Titan. Whilethe formal set of scientific objectivesis the same for both, the Cassini–Huy-gens mission is designed so that asynergistic effect will be realizedwhen the two sets of measurementsare combined. In other words, the to-tal scientific value of the two sets ofdata together will be maximized be-cause of certain specific objectivesfor the Probe and the Orbiter. Eachtime the Orbiter flies by Titan, it willmake atmospheric and surface re-mote-sensing observations that in-clude re-observations along the flightpath of the Probe. The Probe’s mea-surements will be a reference set ofdata for calibrating the Orbiter’s ob-servations. In this way, the Probe andOrbiter data together can be used tostudy the spatial and seasonal varia-tions of the composition and dynam-ics of the atmosphere.

Launch: Ending and BeginningLaunch is both an ending and a be-ginning. Nowhere is there a more

profound test of the work that hasbeen done, nor a more dramaticstatement to mark a shift in programpriorities. Nowhere is there morehope — or more tension. A successfullaunch is everything.

These will be our thoughts as we sur-vey the scene at Cape Canaveral AirStation in Florida. It is October 6,1997. The launch vehicle is the TitanIVB with two stout Solid Rocket MotorUpgrades (SRMUs) attached. Anadditional Centaur rocket — the up-permost stage — sits on top of thepropulsion stack. This system putsCassini–Huygens into Earth orbitand then, at the right time, sendsit on its interplanetary trajectory.

The “core” Titan vehicle has twostages. The SRMUs are anchoredto the first, or lower, stage. These“strap-on” rockets burn solid fuel; theTitan uses liquid fuel. The Centaur is aversatile, high-energy, cryogenic-liq-uid-fueled upper stage with two multi-ple-start engines. The Titan IVB/SRMU–Centaur system is capable ofplacing a 5760-kilogram payloadinto a geostationary orbit. On top ofall this propulsive might sits Cassini–Huygens, protected for its trip throughthe lower atmosphere by a 20-meter-long payload fairing.

Lift-off is from Cape Canaveral AirStation, launch complex 40. It is earlyin the morning. The launch sequencebegins with the ignition of the two sol-id-rocket motors, which lift the wholestack off the pad. About 10 seconds


after lift-off, the stack continues to ac-celerate and then it starts to tilt androtate. The rotation continues until therequired azimuth is reached.

At plus two minutes, the first stage ofthe Titan is ignited, at an altitude ofapproximately 58,520 meters. A fewseconds later, the two solid-rocketmotors, now spent, are jettisoned.One and a half minutes pass, and at109,730 meters, the payload fairingis released.

About five and a half minutes into theflight, the Titan reaches an altitude of167,330 meters. Here the first stageof the Titan separates and the secondstage fires. At launch plus nine min-utes, the second stage has burnt outand drops away. Now it is the Cen-taur’s turn to fire. It boosts the remain-ing rocket–spacecraft stack into a“parking” orbit around Earth andturns off its engines.

Some 16 minutes later, the Centaurignites for a second time. This burnlasts between seven and eight min-

utes, and when it is over, the Centaurseparates from the spacecraft. TheCassini–Huygens spacecraft is nowon an interplanetary trajectory, head-ing first for Venus, then Venus again,around to Earth, to Jupiter — and atlast, Saturn!

T H E R I N G E D W O R L D 1 7

S

C H A P T E R 2

Historical ObservationsIn 1609, Galileo Galilei built his owntelescope and put it to use in makingobservations of the night sky. In hisinitial observations of Saturn, Galileothought he was seeing three planetsbecause of the poor quality of thelenses of his telescope. By 1614, hisnotes indicated that he observed therings for the first time — though hedid not identify them as such.

The optical aberrations in early tele-scopic lenses prevented clear view-ing. Because of the way Saturn’s rings

aturn was the most distant planet that could be seen by an-cient astronomers. Like Mercury, Venus, Mars and Jupiter,it appeared to be a bright, star-like wanderer that movedabout the fixed stars in the night sky. For this reason, these

celestial bodies were called “planetes,” the Greek word for“wanderers.” Little was learned about Saturn, besides itsapparent celestial motions, until the telescope was inventedin Holland in the early 1600s.

appeared when viewed through earlytelescopes, observers spent the earlyyears trying to understand what theyinterpreted as the planet’s odd shapeand behavior. Some saw the ringsas cup handles. By 1659, ChristiaanHuygens had developed the conceptof a planetary ring system, whichhelped the astronomers of the dayunderstand what they were seeing.

Because of its dense cloud cover,all we can see of Saturn is the atmo-sphere — that is, if we do not look atthe rings! Saturn has been a mystery

to us since Galileo’s first telescopicobservations in the early 17th centu-ry. Up to the time of the visits madeby Voyagers 1 and 2, Saturn ap-peared as a fuzzy yellow ball withsome visible banding and some poledarkening. We now know that the“surface” of Saturn is gaseous andthat the patterns present are due toclouds in its gaseous envelope.

From Galileo’s time through the next300 years, telescopes improved andmore moons were discovered to be

In this diagramfrom his book,Systema Saturnium,Christiaan Huygensexplained the incli-nation of Saturn’srings according tothe planet’s orbitalposition with re-spect to Earth.

The Ringed World


orbiting Saturn. But not until the firstspacecraft were able to fly by theplanet did we really advance ourknowledge of Saturn’s atmosphere,complicated ring structure, magneticfield, magnetosphere and satellites.

So far, there have been three flybymissions to Saturn. Detailed informa-tion about Saturn’s structure was ob-tained by these spacecraft. The first,Pioneer 11, flew by in August 1979.Pioneer’s photopolarimeter made im-portant measurements of Saturn’s at-mosphere, and the data could beassembled into low-resolution images.The atmosphere’s appearance chang-es so rapidly, however, that the timerequired to take a picture was toolong to capture the details. Only sub-sequent flyby missions by Voyager 1in October 1980 and Voyager 2 inAugust 1981 were able to show usthe complex structure of the atmo-sphere and its rapidly changing fea-tures. In addition to taking images,the Voyagers conducted many differ-ent experiments — the data collectedgreatly added to our knowledge ofSaturn’s interior structure, clouds andupper atmosphere.

The composition of planets in thesolar system is largely controlled bytheir temperatures, as determinedby their distances from the Sun. Re-fractory compounds (those with highmelting points) were the first to con-dense, at temperatures around1500 kelvins, followed by silicatesat 1400 kelvins. These substancesformed the rocky cores of the planets.

While hydrogen is the predominantelement in the universe and in our so-lar system, other gases are present,including water, carbon dioxide andmethane, all of which condense be-low 500 kelvins. As ices, these pre-dominate in the cooler, outer solarsystem. This gaseous material col-lected as envelopes around the plan-ets and moons. (Where conditionsare right, large quantities of water inits liquid state can form, as exhibitedby Earth’s oceans, Mars’ flood plainsand potentially beneath the surfaceof Europa, a moon of Jupiter.)

The large outer planets contain muchof the primordial cloud’s gases thatwere not trapped by the Sun. Hydro-gen is the most abundant material inthe Sun and in all the large gaseousplanets — Jupiter, Saturn, Uranusand Neptune. Each of these giantplanets — known as the “gas giants”— has many moons. These moonsform satellite systems, suggesting thatminiature solar systems formedaround the gas giants by processessimilar to those that formed the solarsystem itself.

Characteristics of SaturnCool and Slow. Saturn is the sixthplanet from the Sun. It is nine-and-a-half times farther from the Sun thanEarth. The diameter of the Sunviewed from Saturn is about one-tenth the size of the Sun we see fromhere on Earth. Sunlight spreads as ittravels through space; an area onEarth receives 90 times more sunlightthan an equivalent area on Saturn.Because of this fact, the same light-driven photoprocessing in Saturn’satmosphere takes 90 times longer

than it would on Earth. You would nothave to worry about getting a sun-burn on Saturn!

Remembering the astronomer–mathe-matician Johannes Kepler’s laws ofplanetary motion, the farther awayfrom the Sun, the slower a planet trav-els in its orbit, and the longer it takesto complete its orbit about the Sun.Saturn travels at an average velocityof only 9.64 kilometers per second,whereas Earth travels at an averagevelocity of 29.79 kilometers per sec-ond. Saturn’s yearly orbit about theSun — a “Saturn year” — is equalto 29.46 Earth years. If you livedon Saturn, you would have only onebirthday every 29-plus (Earth) years.

Since the orbit of Saturn is not circu-lar but is elliptical in shape, its dis-tance from the Sun changes duringits orbital revolution around the Sun.The elliptical orbit causes a smallchange in the amount of sunlight thatreaches the surface of the planet overthe Saturn year, and may affect theplanet’s upper atmospheric composi-tion over that period.

Slightly Squashed. Saturn’s period ofrotation around its axis depends onhow it is measured. The cloud topsshow a rotation period of 10 hoursand 15 minutes at the equator, but23 minutes longer at higher latitudes.A radio signal associated with Sat-urn’s magnetic field shows a periodof 10 hours and 39.4 minutes.

The high rotation rate creates astrong centrifugal force, causing anequatorial bulge and a flattening of


This Voyager 1 im-age shows Saturn’s“squashed” appear-ance — the planet’sequatorial bulge andflattened poles.


This cutaway diagramshows the basic struc-ture of Saturn’s interior.

the planet’s poles. As a result, Sat-urn’s equator is 60,330 kilometersfrom the center, while the poles areonly 54,000 kilometers from the cen-ter. Almost 10 Earths can be lined upalong Saturn’s equatorial diameter.

The Mysterious InteriorLow Density. To understand Saturn’sinterior and evolution, we must applyour basic knowledge of physics to ob-servations of Saturn’s volume, mass,gravity, temperature, magnetic fieldand cloud movement. The clues pro-vided by this body of knowledge arenot always easy to decipher, but aswe gain more information on Saturn,we are more and more able to relatethe pieces of the puzzle.

Saturn has the lowest density of allthe planets, because of its vast, dis-tended, hydrogen-rich outer layer. Inthe early 1900s, we thought that thegiant planets might consist entirely ofgas. Actually, the giant planets con-tain cores of heavy elements like ironas well as other components of refrac-tory and silicate compounds, andthus have several times the mass anddensity of Earth.

Saturn’s density is not uniform fromits center to the surface — the densityin the core is many times that of thesurface. An understanding of this dis-tribution is obtained through observa-tions of planetary probes sent fromEarth. Observations of the probe tra-jectories can be used to determinethe density distribution throughoutSaturn’s interior.

The Rocky Core. Using data from theVoyager flybys, planetary scientistshave put together a picture of Sat-urn’s interior. We believe that Saturnhas a molten rocky core of about thesame volume as Earth, but with threeor more times the mass of Earth’score. This increased density is due togravitational compression resultingfrom the pressure of the liquid andatmospheric layers above the core.

The rocky core is believed to be cov-ered with a thick layer of metallic liq-uid hydrogen, and beyond that, alayer of molecular liquid hydrogen.The great overall mass of Saturn pro-

duces a very strong gravitationalfield, and at levels just above the corethe hydrogen is compressed to a statethat is liquid metallic and conductselectricity. (On Earth, liquid hydrogenis usually made by cooling the hydro-gen gas to very cold temperatures.On Saturn, liquid hydrogen is veryhot, with temperatures of many thou-sands of kelvins, and is formed underseveral million times the atmosphericpressure found on Earth.) It is be-lieved that this conductive liquid me-tallic hydrogen layer, spinning withthe rest of the planet, is the source ofSaturn’s magnetic field — turbulenceor convective motion in this layer maybe creating the field.

A remarkable characteristic of Sat-urn’s magnetic field is that its axis ofrotation is the same as that of theplanet. This is different from that ofthe five other known magnetic fields,of Mercury, Earth, Jupiter, Uranus andNeptune. Present theory suggests thatwhen the axes of rotation and mag-netic field are aligned, the magneticfield cannot be maintained.

Rock

Metallic HydrogenMolecular Hydrogen


An Extended AtmosphereFrom Liquid to Gas. Above the layerof liquid molecular hydrogen, there isa vast atmosphere that is only sur-passed by the atmospheres surround-ing Jupiter and our Sun. On Earth,there is a definite separation betweenthe land, the oceans and the atmo-sphere, but Saturn has only layers ofhydrogen that transform graduallyfrom a liquid state deep inside to agaseous state in the atmosphere, with-out a well-defined boundary. This isan unusual condition that results fromthe very high pressures and tempera-tures found on Saturn.

Because the pressure of the atmo-sphere is so great, at the point wherethe separation would be expected tooccur, the atmosphere is compressedso much that it actually has a densityequal to that of the liquid. This amaz-ing condition is referred to as “super-critical.” This can happen to anyliquid and gas compressed to a pointabove critical pressure. Saturn thuslacks a distinct surface, so scientistsmake measurements from the cloudtops. The reference point is a pressureof one bar (or one Earth atmosphere,760 millimeters of mercury).

The major component of Saturn’s at-mosphere is hydrogen gas. If theplanet were composed solely of hy-drogen, there would not be much ofinterest to study. However, the compo-sition of Saturn’s atmosphere includessix percent helium gas by volume and0.0001 percent of other trace ele-ments. Using spectroscopic analysis,scientists know that these atmosphericelements can interact to form ammo-

nia, phosphine, methane, ethane,acetylene, methylacetylene and pro-pane. Even a small amount is enoughto freeze or liquefy and make cloudsof ice or rain possessing a variety ofcolors and forms.

With the first pictures of Saturn takenby the Voyager spacecraft in 1980,we could see that the clouds and thewinds were almost as complex asthose found on Jupiter just the yearbefore. There has been an effort tolabel the belts and zones seen inSaturn’s cloud patterns. The bandingresults from temperature-driven con-vective flows in the atmosphere, verymuch the same process that occursin Earth’s atmosphere, but on agrander scale and with a differentheat source.

Saturn has different rotation rates inits atmosphere at different latitudes.Differences of 500 meters per sec-ond were seen between the equatorand nearer the poles, with higherspeeds at the equator. This is fivetimes greater than the wind velocitiesfound on Jupiter.

The Exosphere. Like all planets andmoons with atmospheres, Saturn’soutermost layer of atmosphere isextremely thin. The exosphere is thetransition from lower layers to thevery tenuous gases and ions withinthe magnetosphere, which extendsout to the moon Titan and beyond.Since its density is so low, the exo-sphere is easily heated by absorptionof sunlight. At the outer edge of the

Two satellites orbitfar above the rag-ing storm clouds ofSaturn in this false-color image takenby Voyager 1.


exosphere, the temperature of Sat-urn’s atmosphere is between 400and 800 kelvins, cooling closer toits base.

The gases in the exosphere are heavi-ly bombarded with light and thus dis-sociate to the atomic level, that is, allthe chemicals in the exosphere areseparated into atoms or atomic ions.This material has no way to dissipatethe energy absorbed from sunlight ex-cept through a rare collision with an-other atom. Absorbed solar photonsheat the exosphere up and the colli-sions cool it down. How fast the exo-sphere heats or cools depends uponhow far away it is from the bulk ofthe atmosphere.

The Ionosphere. Lower in the atmo-sphere, below the exosphere, is theionosphere. It is characterized by alarge abundance of electrons andions. There are equal numbers of neg-atively charged electrons and posi-tively charged ions, because theelectrons and the ions are formed atthe same time. When a high-energyphoton of light is absorbed by a neu-tral or ion, a negative electron is en-ergized to escape, leaving an ionbehind with an equal amount of posi-tive charge. The maximum number ofcharged particles in the ionosphereoccurs at a distance of 63,000 kilo-meters from the planet center, or3000 kilometers above the one-barpressure level.

Between the exosphere and the mid-dle of the ionosphere, the predomi-nant ions are H+, H2

+, and H3+. In the

lower ionosphere, where there is anincrease in pressure, more complicat-ed hydrocarbon ions predominate.The number of ions present is only avery small percentage of the mole-cules in the atmosphere, but as inEarth’s atmosphere, the ionospherehas an important influence. The iono-sphere is like a pair of sunglasses forthe planet, filtering out the more ener-getic photons from sunlight. These en-ergetic photons are the cause of theionization.

Cloud Decks. Approaching the planetand from a distance, the Voyagerspacecraft could only see a slightlysquashed, fuzzy yellow sphere. Asthe spacecraft drew nearer to Saturn,light and dark bands parallel to theequator appeared. Closer yet, cy-clonic storms could be seen all overthe planet. Following the paths of thestorms, scientists could measure thevelocities of the winds. Scientists ob-served the tops of clouds covering

the “surface” of Saturn, which indi-cated very high wind speeds at theequator and giant storm patterns inbands around the planet. There wereholes in the clouds, and more layersbelow.

A minimum temperature is reached inSaturn’s atmosphere at about 250 ki-lometers above the one-bar level. Atthis altitude, the temperature is about82 kelvins. At such low temperatures,the trace gases in the atmosphereturn into liquids and solids, andclouds form. The highest clouds areassociated with ammonia ice at theone-bar level, ammonium sulfide at80 kilometers below this point, andwater ice at 260 kilometers belowthe ammonia clouds. The gases arestirred up from the lower altitudesand condense into ice grains be-cause the temperature is low and thepressure has dropped to about onebar. At this pressure, the condensa-tion process starts and the ammonia,ammonium sulfate and water take theform of ice crystals.

Voyager 2 measure-ments showed thenearly identical at-mospheric structureat two different posi-tions on Saturn.

0.0001

0.001

0.01

0.1

1

10

Pres

sure

, bar

s

60 100 140 180

Temperature, kelvins

250

km

36.5°N

31°S


Haze layers are also observed abovethe temperature minimum, at pres-sures of 0.1 and 0.01 bar. Boththe origin and composition of thesehazes are unknown.

Winds and WeatherSaturn is completely covered withclouds. The cloud tops show the ef-fects of the temperature, winds andweather occurring many kilometersbelow. Hot gases rise. As they rise,they cool and form clouds. As thesegas clouds cool, they begin to sink;this convective motion is the source ofthe billowy clouds we see in Saturn’scloud layer. The cyclonic storms weobserve in the planet’s cloud tops aremuch like the smaller versions we seein our daily satellite weather reportson Earth.

The horizontal banding in Saturn’sclouds is a result of the differentwind speeds at different latitudes.The fastest winds are at the equator;the banding results from the windshear between different zones of lati-tude. One possible model suggeststhat the atmosphere is layered incylinders that rotate at different ratesand whose axes parallel the planet’saxis. If this is true, there should beother notable consequences thatwill be revealed in future observa-tions of Saturn.

There are variations in temperatureson Saturn, as well, which are thedriving forces for the winds and thuscloud motion. The lower atmosphereis hotter than the upper atmosphere,driving the vertical motion of gases,and the equator is warmer than the

poles because it receives more directsunlight. Temperature variations com-bined with the planet’s rapid rotationrate drive the horizontal motion ofwinds in the atmosphere.

A Comparison to JupiterSaturn is similar to Jupiter in size,shape, rotational characteristics andmoons, but Saturn is less than one-third the mass of Jupiter and is almosttwice as far from the Sun. Saturn radi-ates more heat than it receives fromthe Sun. This is true of Jupiter as well,but Jupiter’s size and cooling ratesuggest that it is still warm from theprimordial heat generated from con-densation during its formation. Thesmaller Saturn, however, has hadtime to cool, so some mechanism —such as helium migration to the core

A S T A R I S B O R N

Studies of star for-mation indicatethat our solar sys-tem formed out ofa collection of gas-es and dust, drawntogether by gravita-tional attractionand condensedover millions ofyears into manystars. The giant gascloud condensedinto rotating poolsof higher densityin a process calledgravitational col-lapse, because ascondensation pro-

ceeds, it acceler-ates. These rotatingpools of materialcondense more rap-idly until their tem-peratures anddensities are greatenough to formstars. Surroundingeach new star, theleftover materialflattens into a diskrotating approxi-mately in the planeof the star’s equa-tor. This materialcan eventually formplanets — appar-

ently what occurredto form our ownsolar system. Thisimage, from theHubble Space Tele-scope’s Wide Fieldand PlanetaryCamera 2, showsa part of the OrionNebula, which isknown as a “nurs-ery for youngstars.” The insetimages at far rightare possible exam-ples of young stars.


0

— must be found to explain its con-tinuing radiation of heat.

From Voyager measurements, welearned that Saturn’s ratio of heliumto molecular hydrogen is 0.06, com-pared to Jupiter’s value of 0.13(which is closer to the solar abun-dance and that of the primordial so-lar nebula). The helium depletion inSaturn’s upper atmosphere is be-lieved to be due to helium rainingdown to the lower altitudes; this sup-ports the concept of helium migrationas the heat source in Saturn. Mea-surements of the planet’s energy, ra-diation and helium abundance willhelp explain the residual warmth weobserve.

Saturn’s “surface” features are domi-nated by atmospheric clouds. Theyare not as distinct as Jupiter’s clouds,primarily because of a haze layercovering the planet that is a result ofthe weaker insolation from the Sun.This reduced solar radiation leads togreater wind velocities on Saturn.Both Saturn’s and Jupiter’s weatherare driven by heat from below.

Cassini’s Global StudiesThe Cassini Orbiter’s remote-sensinginstruments will be the primary datagatherers for global studies of Sat-urn’s atmospheric temperatures,clouds, and composition. Each instru-

ment will provide unique types ofdata to help solve the puzzles of Sat-urn’s atmosphere.

The Composite Infrared Spectrometer(CIRS), operating in the thermal infra-red at very high spectral resolution, isspecifically designed to determinetemperatures and will spend long pe-riods scanning Saturn’s atmosphere.Over the course of a Saturn day, thewhole planet will rotate through thisinstrument’s field of view, providingdata that will produce a thermal map.Such measurements will help us eval-uate the amount of solar heating ascompared with the heating generatedfrom the planet’s interior. In addition,CIRS can independently measure thetemperatures of the different gasescomposing Saturn’s atmosphere. Thiswill give information about how thetemperature of Saturn’s atmospherechanges at different depths.

Radio science experiments use thespacecraft’s radio and ground-basedantennas as the science instrument.The Cassini Orbiter will send micro-wave radio signals through the atmo-sphere of Saturn to Earth; Saturn’sionosphere and atmosphere willchange the signal as it passesthrough. The radio science “probe”will be used to measure the electrondensity, temperature, pressure andwinds in the ionosphere. Temperaturedata from radio science experimentsdepend on the composition of the at-mosphere in a unique manner com-pared with other instruments; thus,the most sensitive determination ofhelium abundance will be obtainedby combining radio science datawith measurements from the CIRS.

The fastest winds onSaturn are at the equa-tor, here shown sur-passing 400 metersper second.

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Similar measurements made by theUltraviolet Imaging Spectrograph(UVIS) and the Visible and InfraredMapping Spectrometer (VIMS) as theSun disappears behind Saturn (asseen from the Orbiter) are also veryinformative. These experiments willobserve the atmosphere’s variation incomposition and opacity with depthby measuring the intensities of vari-ous colors of sunlight as the Sun setsinto haze and clouds. Each moleculein Saturn’s atmosphere can be identi-fied by its unique absorption andemission of the sunlight.

Cassini–Huygens’ three spectrometerscomplement each other in measure-

ments of atmospheric composition.The UVIS and the VIMS measurechemical composition at wavelengthsspanning the ultraviolet and near in-frared — useful in determining manyexpected atmospheric components.The CIRS and the UVIS can measureatomic composition as well; theformer can delineate the atomic com-position of molecules, while the lattercan measure atoms directly. The im-portant abundances of deuterium andother interesting isotopes will be in-vestigated to shed light on the originof our solar system.

The UVIS will follow up on HubbleSpace Telescope observations withstudies of the energetics of Saturn’s

aurora. The VIMS and potentially theCIRS can contribute also, and the im-aging system will monitor the auro-ra’s changing morphology.

The Imaging Science Subsystem (ISS)will play a major role in understand-ing the dynamics of Saturn’s cloudsand weather systems. With windspeeds of 1800 kilometers per hourat the equator, the cloud bands seenbeneath the haze layer are subject toconsiderable turbulence. Indeed, thebands themselves are defined by thewinds, separated by zones of highwind shear. In addition, storms canbe discerned among the bands, and

I N T H E E Y E O F T H E B E H O L D E R

Taken from ChristiaanHuygens’ Systema Sat-urnium, these earlydrawings of Saturnrepresent the views of:I. Galileo, 1610;II. Scheiner, 1614;III. Riccioli, 1614 or1643; IV–VII. Hevel;VIII, IX. Riccioli,1648, 1650; X. Di-vini, 1646–1648;XI. Fontana, 1636;XII. Biancani, 1616;

XIII. Fontana, 1644–1645. Some of thedrawings, such as IX,had a very ring-likeappearance yearsbefore Huygens’ the-ory was accepted.


their dynamics are of considerableinterest. Images of the same regionmade with different filters can be in-terpreted to indicate the altitudes ofvarious phenomena.

“Saturn-annual” white spots, whichappear at approximately 30-yearintervals, are not expected duringthe portion of the cycle when Cassi-ni is studying the planet, but other

white spots may fortuitously appearand will be studied by all the opti-cal remote-sensing instruments.Cassini may be able to confirmthe theory that the white spots arecaused by the upwelling and conden-sation of ammonia-ice crystals in theatmosphere.

By carefully tracking the Cassini Or-biter’s motion around Saturn, data onthe deep layers — all the way to the

core — of Saturn can be acquired.In these studies, radio signals fromthe spacecraft will be monitored forchanges, especially Doppler shiftsof frequency that are different fromthose predicted from a simple basemodel. Using the new measurementsfrom the Cassini Orbiter, more de-tailed models of the internal structureof Saturn will be constructed.

T H E M Y S T E R I O U S T I T A N 2 7

T

C H A P T E R 3

he exploration of Titan is at the very heart of the Cassini–Huygens mission. As one of the primary scientific interestsof the joint NASA–ESA mission, Titan is the sole focusof the Huygens Probe and one of the main targets of the

Cassini Orbiter. Through the combined findings from Earth-based telescopes and the Voyager spacecraft, Titan hasbeen revealed to be a complicated world, more similar toa terrestrial planet than a typical outer-planet moon.

A Smoggy SatelliteWith a thick, nitrogen-rich atmosphere,possible oceans and a tar-like soil,Titan is thought to harbor organiccompounds that may be important inthe chain of chemistry that led to lifeon Earth.

Titan was discovered by ChristiaanHuygens about 45 years after Galileodiscovered the four large moons ofJupiter. In the early 1900s, the astron-omer Comas Sola reported markingsthat he interpreted as atmosphericclouds. Titan’s atmosphere was con-firmed in 1944, when Gerard Kuiperpassed the sunlight reflecting off Titanthrough a spectrometer and discov-ered the presence of methane.

Later observations by the Voyagerspacecraft showed that nitrogenwas the major constituent of the atmo-sphere, as on Earth, and establishedthe presence of gaseous methane inconcentrations of several percent.The methane participates in sunshine-driven chemistry, which has produceda photochemical smog.

Due to Titan’s thick natural smog,Voyager could not see the surface,and instead the images of Titan’s diskshowed a featureless orange face.

Spectroscopic observations by Voyag-er’s infrared spectrometer revealedtraces of ethane, propane, acetyleneand other organic molecules in addi-tion to methane. These organic com-pounds, known as hydrocarbons, areproduced by the interaction of solarultraviolet light and electrons from Sat-urn’s fast-rotating magnetosphere strik-ing Titan’s atmosphere.

Hydrocarbons produced in the atmo-sphere eventually condense out andrain down on the surface; so Titan mayhave lakes of ethane and methane,perhaps enclosed in the round bowlsof impact craters. Alternatively, liquidethane and methane may exist insubsurface reservoirs. Titan’s hiddensurface may have exotic features:mountains sculpted by hydrocarbon

This spectacularview of the edgeof Titan was takenby Voyager 1from a distance of22,000 kilometers.Tenuous high-alti-tude haze layers(blue) are visibleabove the opaquered clouds. Thehighest of thesehaze layers isabout 500 kilo-meters above themain cloud deck,and 700 kilome-ters above Titan’ssurface.

The Mysterious Titan


rain, rivers, lakes and “waterfalls.”Water and ammonia magma fromTitan’s interior may occasionallyerupt, spreading across the surfaceand creating extraordinary land-scapes.

The Cassini–Huygens study of Titanwill provide a huge step forward inour understanding of this haze-cov-ered world, and is expected to yieldfundamental information on the pro-cesses that led to the origin of lifeon Earth.

By combining the results from theCassini–Huygens mission with Earth-based astronomical observations, lab-oratory experiments and computermodeling, scientists hope to answerbasic questions regarding the originand evolution of Titan’s atmosphere,the nature of the surface and the struc-ture of its interior. Earth’s atmospherehas been significantly altered by theemergence of life. By studying Titan’satmosphere, scientists hope to learnwhat Earth’s atmosphere was likebefore biological activity began.

The investigation of Titan is dividedinto inquiries from traditional plane-tary science disciplines: What is itsmagnetic environment, what is the at-mosphere like, what geological pro-cesses are active on the surface andwhat is the state of its interior? In thischapter, we describe how our knowl-edge of Titan has developed over theyears, focusing on the primary areasof Titan science that the Cassini–Huy-gens mission will address. The finalsection discusses how Huygens’ andCassini’s instruments will be employed

to address fundamental questions,such as: What is the nature of Titan’ssurface, how have the atmosphereand surface evolved through timeand how far has prebiotic chemistryproceeded on Titan?

Saturn’s MagnetosphereSaturn’s magnetic field rotates withthe planet, carrying with it a vastpopulation of charged particlescalled a plasma. (Further discussionof Saturn’s magnetosphere can befound in Chapter 6.) The interactionof Saturn’s magnetosphere with Titancan be explained by the deflection ofSaturn’s magnetic field and the ion-ization and collision of Titan’s atmo-sphere with the charged particlestrapped in Saturn’s magnetosphere.During the Voyager 1 flyby of Titan,

clear indications of changes were ob-served in the magnetosphere due tothe presence of Titan. The signaturesincluded both plasma and plasma-wave effects, along with a draping ofSaturn’s magnetic field around Titan.

Titan in Saturn’s Magnetosphere. Titanorbits Saturn at a distance of about20.3 RS (RS = one Saturn radius).The conditions of the plasma environ-ment in which Titan is submerged canvary substantially, because Titan issometimes located in the magneto-sphere of Saturn and sometimes outin the solar wind. Additionally, theangle between the incident flow andthe solar irradiation varies duringTitan’s orbit. During the Voyager 1encounter, Titan was found to bewithin Saturn’s magnetosphere and

C A S S I N I S C I E N C E O B J E C T I V E S

A T T I T A N

Determine the abundance of atmospheric constituents (including any

noble gases); establish isotope ratios for abundant elements; and constrain

scenarios of the formation and evolution of Titan and its atmosphere.

Observe the vertical and horizontal distributions of trace gases; search

for more complex organic molecules; investigate energy sources for

atmospheric chemistry; model the photochemistry of the stratosphere;

and study the formation and composition of aerosols.

Measure the winds and global temperatures; investigate cloud physics,

general circulation and seasonal effects in Titan’s atmosphere; and search

for lightning discharges.

Determine the physical state, topography and composition of the surface;

infer the internal structure of the satellite.

Investigate the upper atmosphere, its ionization and its role as a source

of neutral and ionized material for the magnetosphere of Saturn.


the interaction was described as atransonic flow of plasma (above thelocal speed of sound).

Scientists expect the characteristics ofthe incident flow to vary substantiallydepending on the location of Titanwith respect to Saturn’s magneto-sphere. When Titan is exposed to thesolar wind, the interaction may besimilar to that of other bodies in thesolar system such as Mars, Venus orcomets (these bodies have substantialinteraction with the solar wind, and,like Titan, have atmospheres but nostrong internal magnetic fields).

Titan’s Wake. The magnetosphericplasma flowing into and aroundTitan produces a wake. As with theGalilean satellites at Jupiter, the fast,corotating plasma of Saturn’s mag-netosphere smashes into Titan frombehind, producing a wake that isdragged out in front of the satellite.The process is similar to one in whicha wake is created behind a motorboat speeding through the water.

In this analogy, Titan is the motorboat and Saturn’s magnetosphere isthe water. Instead of the boat goingthrough the water, the water is rush-ing past the boat. Titan is movingalso, in the same direction as Saturn’smagnetosphere, but slower, so themagnetospheric plasma actually push-es the wake out in front of the satel-lite. Scientists expect the wake to bea mixture of plasma from Titan andSaturn’s magnetosphere. The wakemay be a source of plasma for Sat-urn’s magnetosphere, producing atorus of nitrogen and other elementsabundant in Titan’s atmosphere.

Titan’s wake and thepossible bow shockinduced by the inter-action of Titan withcorotating plasma inSaturn’s vastmagnetosphere.

MagnetosphericPlasma

Titan

Wake

Titan’s ExtendedAtmosphere andIonosphere

Exploring Titan is likeinvestigating a full-fledged planet. Witha radius of 2575 kilo-meters, Titan is Sat-urn’s largest moon —larger than the plan-ets Mercury and Plu-to. Titan is the secondlargest moon in thesolar system, sur-passed only by Jupi-ter’s Ganymede. ThisVoyager image of Ti-tan shows the asym-metry in brightnessbetween the moon’ssouthern and northernhemispheres. Titan’snatural smoggy hazeblocked Voyager’sview of the surface.

Bow Shock


Titan’s Torus. The interaction of Titanwith Saturn’s magnetosphere providesa mechanism for both the magneto-spheric plasma to enter Titan’s atmo-sphere and for the atmosphericparticles to escape Titan. Voyagerresults suggested that the interactionproduces a torus of neutral particlesencircling Saturn, making Titana potentially important source ofplasma to Saturn’s magnetosphere.The characteristics of this torus are yetto be explored and will be addressedby the Cassini Orbiter. The interactionof ice particles and dust from Saturn’srings will play a special role as thedust moves out toward Titan’s torusand becomes charged by collisions.When the dust is charged, it behavespartially like a neutral particle orbit-ing Titan, according to Kepler’s laws(gravity driven), and partially like acharged particle moving with Saturn’s

magnetosphere. The interaction ofdust with Saturn’s magnetosphere willprovide scientists with a detailed lookat how dust and plasma interact.

Atmosphere–Magnetosphere Interac-tion. Saturn’s magnetospheric plas-ma, which is trapped in the planet’sstrong magnetic field, corotates withSaturn, resulting in the plasma flow-ing into Titan’s back side. The flowpicks up ions created by the ioniza-tion of neutrals from Titan’s exosphereand is slowed down while the mag-netic field wraps around the satellite.The characteristics of the incidentflow are important because the in-coming plasma is a substantial sourceof atmospheric ionization that triggersthe creation of organic molecules inTitan’s atmosphere. Thus, the aerono-my (study of the physics of atmo-spheres) of the upper atmosphereand ionosphere is dependent on the

plasma flow and the solar radiationas a source of energy. Scientists ex-pect that heavy hydrocarbons are thedominant ions in Titan’s ionosphere.

Lightning on Titan? The extensiveatmosphere of Titan may host Earth-like electrical storms and lightning.Although no evidence of lightningon Titan has been observed, theCassini–Huygens mission providesthe opportunity to determine whethersuch lightning exists. In addition tothe visual search for lightning, thestudy of plasma waves in the vicinityof Titan may offer another method.Lightning discharges a broad bandof electromagnetic emission, part ofwhich can propagate along magneticfield lines as whistler-mode emission.The emissions are known as “whis-tlers” because, as detected by radioand plasma-wave instruments, they

The flow of chargedparticles in Saturn’smagnetosphere pastTitan is slowed by theions created fromcollisions with Titan’sextended atmosphere.Saturn’s magnetic fieldbecomes drapedaround Titan. Titan

Ionosphere

Titan’s Extended Atmosphere

Saturn’s Magnetic Field

Incident Flow


have a signature of a tone decreasingwith time (because the high frequen-cies arrive before the low frequen-cies). Lightning whistlers have beendetected in both the Earth and Jupitermagnetospheres and, besides beingdetectable from large distances, theyoffer an opportunity to estimate thefrequency of lightning flashes.

Titan’s Magnetic Field. The questionof whether or not Titan has an inter-nal magnetic field remains open,although Voyager results did not sug-gest the presence of one. Recent Gali-leo results from Jupiter indicate thepossibility of a magnetic field associ-ated with the moon Ganymede. ForTitan there are two possibilities — amagnetic field could be induced fromthe interaction of Titan’s substantialatmosphere with the flow of Saturn’smagnetosphere (such as at Venus,with the solar wind), or a magneticfield could be generated internallyfrom dynamo action in a metallicmolten core (such as at Earth).

In addition to being important tounderstanding the Titan interactionwith Saturn’s magnetosphere, a Titanmagnetic field, if generated internal-ly, would place strong constraints onits interior structure.

Titan’s AtmosphereBackground. The discovery of meth-ane absorption bands in Titan’sspectrum in 1944 was the first confir-mation that Titan has an atmosphere.Theoretical analysis followed —would Titan be enshrouded within awarm, atmospheric greenhouse orpossess a thin, cold atmosphere witha warm layer at high altitudes? Thereddish appearance of Titan’s atmo-sphere led scientists to suggest thatatmospheric chemistry driven by ultra-violet sunlight and/or the interactionwith Saturn’s magnetospheric plasmaproduced organic molecules. Theterm “organic” refers here to carbon-based compounds, not necessarilyof biological origin.

Later, laboratory experiments and the-oretical research were aimed at re-producing the appearance of Titan’sspectrum and learning if Titan’s atmo-sphere could indeed be considereda prebiological analog to Earth’satmosphere. Researchers filled labo-ratory flasks with various mixturesof gases, including methane, andexposed the flasks to ultraviolet ra-diation. The experiments produceddark, orange–brownish polymersdubbed “tholins,” from the Greekword meaning “muddy.”

Voyager Results. In 1980 and 1981,the Voyager spacecraft flybys re-turned a wealth of data about Titan.Voyager 1 skimmed by at a distanceof just 4000 kilometers. Voyagerimages revealed an opaque atmo-sphere with thin, high hazes. Therewas a significant difference in bright-ness between the northern and south-ern hemispheres and polar hoods,attributed to seasonal variations.

S I X G I A N T S A T E L L I T E S

Satellite Titan Moon Io Europa Ganymede Callisto(Planet) (Saturn) (Earth) (Jupiter) (Jupiter) (Jupiter) (Jupiter)

Distance from 1,221,850 384,400 421,600 670,900 1,070,000 1,883,000Parent, kilometers

Rotation Period, 15.945 27.322 1.769 3.551 7.155 16.689days

Radius, 2575 1738 1815 1569 2631 2400kilometers

Average Density, 1.88 3.34 3.57 2.97 1.94 1.86grams percubic centimeter


Voyager’s solar and Earth occulta-tion data, acquired as the spacecraftpassed through Titan’s shadow, re-vealed that the dominant atmosphericconstituent is nitrogen. Methane, theatmospheric gas detected from Earth,represents several percent of thecomposition of the atmosphere. Titan’ssurface pressure, one and a half bars,is 50 percent greater than Earth’s —in spite of Titan’s smaller size. The sur-face temperature was found to be94 kelvins, indicating that there is lit-tle greenhouse warming. The tempera-ture profile in Titan’s atmosphere hasa shape similar to that of Earth: warm-er at the surface, cooling with increas-ing altitude up to the tropopause at42 kilometers (70 kelvins), then in-creasing again in the stratosphere.

The opacity of Titan’s atmosphereturned out to be caused by photo-chemical smog: Voyager’s infraredspectrometer detected many minorconstituents generated primarily byphotochemistry of methane, whichproduces hydrocarbons such asethane, acetylene, and propane.Methane also interacts with nitrogenatoms created by the break up ofnitrogen to form “nitriles” such ashydrogen cyanide. Titan may welldeserve the title “smoggiest worldin the solar system.”

A person standing on Titan’s surfacein the daytime would experience alevel of daylight equivalent to per-haps 1/1000 the daylight at Earth’ssurface, given Titan’s greater dis-

tance from the Sun and the hazeand gases blocking the Sun. Thelight will still be 350 times brighterthan moonlight on an Earth night witha full Moon.

In the years following the Voyagermission, a stellar occultation ob-served from Earth in 1989 providedmore data at a Titan season differentfrom the season that Voyager ob-served. Extensive theoretical andlaboratory experiments have alsoincreased our understanding of Ti-tan’s complex atmosphere. For in-stance, this work has helped us tounderstand complex atmosphericphotochemistry and the compositionof the haze particles.

The change in Titan’stemperature withaltitude is like Earth’s:decreasing with in-creasing altitude upto 50 kilometers, thenincreasing againabove that. This illus-tration shows atmo-spheric structure, thelocation of the surface-obscuring ethane hazeand the possible loca-tion of clouds.

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To summarize, prior to Cassini, theprimary constituents of Titan’s atmo-sphere have been detected, the pro-cess by which photolysis of methanehas produced a smoggy haze is fairlywell understood and the pressure–temperature profile as a function ofaltitude in the atmosphere has beendetermined. We do not know thesource of the atmosphere, if there isactive “weather” (clouds, rain, light-ning) nor how the atmosphere circu-lates. These are the intriguing sciencequestions the Cassini Orbiter and theHuygens Probe will soon investigate.

Atmospheric Origin and Chemistry.What is the source of molecular nitro-gen, the primary constituent of Titan’scurrent atmosphere? Is it primordial(accumulated as Titan formed) orwas it originally accreted as ammo-

nia, which subsequently broke downto form nitrogen and hydrogen? Ordid the nitrogen come from comets?These important questions can beinvestigated by looking for argon inTitan’s atmosphere.

Both argon and nitrogen condenseat similar temperatures. If nitrogenfrom the solar nebula — out of whichour solar system formed — was thesource of nitrogen on Titan, the ratioof argon to nitrogen in the solar neb-ula should be preserved on Titan.Such a finding would mean that wehave truly found a sample of the“original” planetary atmospheres.Argon is difficult to detect, however,because it is a noble gas — it wasnot detectable by Voyager instrumen-tation. The upper limit that has been

set observationally is one percent rel-ative to nitrogen; the solar nebula ra-tio is close to six percent.

Methane is the source of the manyother hydrocarbons detected in Ti-tan’s atmosphere. It breaks down insunlight into fragments such as CH2

and H2. The CH2 fragments recom-bine to produce hydrocarbons.Ethane is the most abundant by-prod-uct of the photochemical destructionof methane. The leftover hydrogenescapes from Titan’s atmosphere. Thisis an irreversible process, and thecurrent quantity of methane in Titan’satmosphere — if not replaced — willbe exhausted in 10 million years.

The hydrocarbons spend time as theaerosol haze in Titan’s atmosphereobscuring the surface. Polymerizationcan occur at this stage, especially forhydrogen cyanide and acetylene,forming additional aerosols that even-tually drift to the surface. Theoretical-ly, the aerosols should accumulate onthe surface, and, over the life of thesolar system, produce a global oceanof ethane, acetylene, propane and soon, with an average depth of up toone kilometer. A large amount of liq-uid methane mixed with ethane couldtheoretically provide an ongoingsource of methane in the atmosphere,analogous to the way the oceans onEarth supply water to the atmosphere.Radar and near-infrared data ob-tained from Earth-based observationsshow, however, that there is no glo-bal liquid ocean, although therecould be lakes and seas, or possiblysubsurface reservoirs. The ultimatefate of Titan’s hydrocarbons, whichare expected to exist as liquids orsolids on its surface, is a mystery.

C O N S T I T U E N T S O F T H E

T I T A N A T M O S P H E R E

Chemical Common AtmosphericConstituent Name Concentration

N2 Nitrogen 90–97 percent

Hydrocarbons

CH4 Methane 2–10 percent

C2H2 Acetylene 2.2 parts per million

C2H4 Ethylene 0.1 parts per million

C2H6 Ethane 13 parts per million

C3H8 Propane 0.7 parts per million

Nitriles

HCN Hydrogen cyanide 160 parts per billion

HC3N Cyanoacetylene 1.5 parts per billion


Titan has been described as havingan environment similar to that of Earthbefore biological activity forever al-tered the composition of Earth’s atmo-sphere. It is important to emphasizethat a major difference on Titan is theabsence of liquid water — absolutelycrucial for the origin of life as weknow it. Liquid water may occur forshort periods after a large comet ormeteoroid impact, leading perhaps tosome interesting prebiotic chemistry.However, the surface temperatures atTitan are almost certainly cold enoughto preclude any biological activity atTitan today.

Winds and Weather. Titan’s methanemay play a role analogous to that ofwater on Earth. Might we expect Titanto have methane clouds and rain? Atthis point, the data suggest otherwise.Voyager infrared data are “best fit”with an atmosphere that is supersatu-rated with methane. This means thatthe methane wants to form rain andsnow but lacks the dust particles onwhich the methane could condense.

Titan appears to have winds. The tem-perature difference from the equatorto 60 degrees latitude may be asmuch as 15 kelvins, which suggeststhat Titan might have jet streams simi-lar to those in Earth’s stratosphere.Wind speeds in Titan’s stratospheremay reach 100 meters per second.The occultation of the star 28 Sgr ob-served from Earth in 1989 confirmedthis theoretical analysis by detectingthe shape of Titan’s atmosphericbulge, which is influenced by high-altitude winds. In the troposphere, thetemperature as a function of latitude

varies by only a few degrees, and theatmosphere should be much calmer.

Titan’s SurfaceThe surface of Titan was not visiblethrough the limited spectral rangeof the Voyager cameras. Our knowl-edge of the surface of Titan comesfrom much more recent Earth-basedimages, acquired at longer wave-lengths with the Wide Field and Plan-etary Camera aboard the HubbleSpace Telescope. Brightness varia-tions are evident, including a large,continent-size region on Titan’s sur-face with a distinctly higher albedo(reflectance) at both visible and near-infrared wavelengths.

Preliminary studies suggest that asimple plateau or elevation differenceon Titan’s surface cannot explain theimage features, and that the bright-ness differences must be partly dueto a different composition and/orroughness of material. Like othermoons in the outer solar system, Titanis expected to have a predominantlywater-ice crust. Water at the tempera-

A composite ofimages of Titan’ssurface acquiredin 1994 by theHubble SpaceTelescope showsbrightness varia-tions, including alarge region thatappears bright atvisible and near-infrared wave-lengths. Thisregion is also agood reflector ofradar.

tures in the outer solar system is assolid and strong as rock. Observa-tions show weak spectral featuresindicative of ice on Titan’s surface,but some dark substance is alsopresent. This suggests that somethingon the surface is masking the waterice.

Surface Geology. At the resolutionprovided by the Hubble Space Tele-scope, we can establish Titan’s rota-tion rate, and also agree that thismoon has a continent-size albedofeature. The surfaces of solid bodiesin the solar system have been alteredprimarily by three processes: impactcratering, volcanism and tectonics.Erosion may also be important onbodies with atmospheres. By studyingthe surface of a body, scientists candetermine how it has evolved —when the surface solidified, the subse-quent geological processes and howthe surface and atmosphere interact.

Planetary geologists use crater statis-tics to determine the relative age ofa surface. Since the population of im-


By contrast, in thecase of Jupiter’smoons, Callisto’s cra-tered surface frozeearly in the historyof the solar system,and has experiencedonly limited evolutionsince that time. Con-tinuous volcanismgives Io (left) theyoungest surfacein the solar system.

The processes ofplate tectonics anderosion have erasedthe signature of allbut the most recent ofEarth’s impact cra-ters, such as the Me-teor Crater inArizona.


pactors was greatest early in the his-tory of the solar system, the more cra-ters there are on a surface, the olderthe surface must be. Looking at abody like Jupiter’s moon Callisto, cov-ered with impact craters, it is clearthat the surface is old and that the ef-fect of geological processes has beenminimal.

On Earth, tectonic forces and erosionhave obliterated all old craters anderosion has modified the few recentones. On a body like Io, anothermoon of Jupiter, active volcanism hascovered all signs of craters, and theyoung landscape is dominated byvolcanic features. At which end ofthis spectrum will we find Titan?

Earth-based radar data suggest a sur-face similar to that of Callisto. Titan’ssize alone suggests that it may havea surface similar to Jupiter’s moonGanymede — somewhat modifiedby ice tectonics, but substantially cra-tered and old. If Titan’s tectonic activi-ty is no more extensive than that ofGanymede, circular crater basinsmay provide storage for lakes of liq-uid hydrocarbons. Impactors createa layer of broken, porous surfacematerials, termed “regolith,” whichmay extend to depths from one tothree kilometers. The regolith couldprovide subsurface storage for liquidhydrocarbons as well.

In contrast to Ganymede, Titanmay have incorporated as much as15 percent ammonia as it formed inthe colder, Saturn region of the solarnebula. As Titan’s water-ice surfacefroze, ammonia–water liquid wouldhave been forced below the surface.This liquid will be buoyant relative

Jupiter’s moonGanymede is theonly moon in thesolar system largerthan Titan. The twomoons may havesimilar geology —with regions modifiedby ice tectonics, buta surface that basi-cally solidified threeto four billion yearsago, now scarredby craters. This viewof Ganymede showsa Galileo image su-perimposed on amuch less detailedVoyager image.

to the surface water-ice crust;thus, ammonia–water magma mayhave forced its way along cracks tothe surface, forming exotic surfacefeatures.

What amount of weathering of thesurface might Cassini–Huygens see?On Earth, water accomplishes weath-ering because as it expands in itsfreeze–thaw cycle, rocks are brokenup. In its liquid phase, water acts asthe medium for many chemical reac-tions. On Titan, condensed hydrocar-bons may or may not participate ina weathering process, but may also

be expected to form a solid and/orliquid veneer over the icy surface.

The high-contrast features seen in theHubble Space Telescope images arenot consistent with a surface uniform-ly covered with liquid, suggestingsome transport of the hydrocarbonsinto lakes or subsurface reservoirs.

Interior StructureTitan’s average density is 1.88 gramsper cubic centimeter, suggesting amixture of roughly 50 percent rockysilicate material and 50 percent wa-ter ice. Given the temperature andpressure of the solar nebula at Sat-urn’s distance from the Sun when Ti-


This model of Titan’sinterior envisions anunderground oceanof liquid ammoniaand water, 250 kilo-meters beneath thesolid surface.

tan was accreting, it is possible thatmethane and ammonia would havebeen mixed with the water ice. Theformation of Titan by accretion wasundoubtedly at temperatures warmenough for Titan to “differentiate,”that is, for the rocky material to sepa-rate to form a dense core, with a wa-ter–ammonia–methane ice mantle.

The mixture of ammonia with watercould ensure that Titan’s interior is stillpartially unsolidified, as the ammoniawill effectively act like antifreeze. Ra-dioactive decay in the rocky materialin the silicate-rich core would heat thecore and mantle for approximatelythree billion years, further strengthen-ing the case that a liquid layer in themantle could exist to the present.Titan’s solid water-ice crust may belaced with methane clathrate — meth-ane trapped in the structure of waterice. This could provide a possiblelong-term source for the methane inTitan’s atmosphere — if the methanewere freed by ongoing volcanism.An alternative model for providingthe methane is a porous crust wherethe methane–water clathrate ratio isapproximately 0.1.

Cassini–Huygens ExperimentsOn the first orbit after Cassini has ex-ecuted its Saturn orbit insertion ma-neuver, the Huygens Probe will betargeted for Titan. Probe data are ob-tained during the descent through Ti-tan’s atmosphere and relayed to theOrbiter; then the Orbiter mission con-tinues with over 40 more Titan flybys.

Magnetospheric Investigations. As theOrbiter encounters Titan, the science

instruments will work together to in-vestigate the fields and particles sci-ence associated with Titan and helpdetermine if the satellite has an in-trinsic magnetic field. The CassiniPlasma Spectrometer will map out theplasma flow. The Magnetospheric Im-aging Instrument will measure high-energy particles and image thedistribution of neutral particles sur-rounding Titan during the approachand departure of the Orbiter.

Through the measurement of plasmawaves, the Radio and Plasma WaveScience instrument will investigate theinteraction between the plasmas and

high-energy particles, and will alsosearch for radio emission associatedwith Titan. The Cosmic Dust Analyzerwill map out the dust distribution. TheIon and Neutral Mass Spectrometerand the Cassini Plasma Spectrometerwill measure the composition of Ti-tan’s atmosphere and exosphere di-rectly during the flyby, to determinecomposition and study the interactionwith Saturn’s magnetosphere.

Atmospheric Investigations. The mainpurpose of the Huygens Probe is toreturn in situ measurements as it de-scends through Titan’s atmosphereand lands on the surface. The Probe’s

0

Mostly Solid SurfaceSubsurfaceLiquid Methane

Methane Clathrate Hydrateand Water Ice

Ammonia–Water Ocean

Ammonia–Water Ice

Rock Core

250

450

700

Dep

th, k

ilom

eter

s


suite of instruments has been opti-mized for the return of informationon critical atmospheric parameters.The atmospheric structure instrumentwill measure temperature, density andpressure over a range of altitudes,and also measure the electrical prop-erties of the atmosphere and detectlightning.

The Aerosol Collector and Pyrolyserwill vaporize aerosols, then direct theresulting gas to the Gas Chromato-graph and Mass Spectrometer, whichwill provide a detailed inventory ofthe major and minor atmospheric spe-cies, including organic molecules andnoble gases — in particular argon.The instrument for the Doppler WindExperiment will track the course of thedescending Probe, giving wind direc-tion and speed at various altitudesin Titan’s atmosphere. The Descent

Imager and Spectral Radiometer willtake pictures and record spectra ofthe clouds and surface. Measure-ments of the Sun’s aureole collectedby this instrument will help scientistsdeduce the physical properties of theaerosols (haze and cloud particles)in Titan’s atmosphere. The instrumentwill also acquire spectra of the atmo-sphere at visible and near-infraredwavelengths and measure the solarenergy deposited at each altitude lev-el of the atmosphere.

The data acquired by the Probe willbe compared with the continuing cov-erage provided by the Orbiter’s sub-sequent flybys. The Orbiter’s threespectrometers (the Ultraviolet ImagingSpectrograph, the Visible and Infra-red Mapping Spectrometer and theComposite Infrared Spectrometer)will detect chemical species at a vari-ety of levels in the atmosphere. By

mapping the distribution of hydrocar-bons as a function of time, the infor-mation from the spectrometers willaddress the sources, sinks and effi-ciency of the photochemical process-es making up the hydrocarbon cycle.The Ultraviolet Imaging Spectrographcan also detect argon and establishTitan’s deuterium–hydrogen ratio.

The Orbiter will study the circulationof Titan’s atmosphere using the Visi-ble and Infrared Mapping Spectro-meter and the Imaging ScienceSubsystem’s cameras to track clouds.For the study of Titan’s weather, scien-tists will combine data from the Orbit-er’s Composite Infrared Spectrometerand the Radio Science Instrumentexperiments to develop temperatureprofiles and thermal maps.

Titan’s subsurfacestructure may be keyto understanding theproperties of its sur-face. Photochemicalmodels of Titan’satmosphere predictthe existence of aglobal ocean of liq-uid ethane and meth-ane, acetylene andother hydrocarbonprecipitates — while

Cosmic Impacts

Liquid Percolation

Methane Reservoirs

Methane GasExchange

ExplosiveMethaneRelease

Ammonia–WaterVolcanic Activity

Ammonia–WaterMagma

“Regolith” Porous Surface LayerBroken Up by Impacts

Earth-based radardata and near-infra-red images suggesta solid surface. IfTitan’s crust is suffi-ciently porous, stor-age space could existfor large quantities ofliquid hydrocarbons,which would notshow up in the radardata. The porous re-golith produced byimpacts could beover one kilometerthick, providing suf-ficient volume tohide the expectedhydrocarbons.


Surface Science. The highest resolu-tion, clearest images of Titan’s surfacewill come from the Descent Imagerand Spectral Radiometer on the Huy-gens Probe. This camera will takeover 500 images under the hazelayer — where the atmosphere maybe very clear — during the Probe’sjourney to the surface.

If the Probe survives a landing on liq-uid, aerosol drifts or ice, chemicalsamples may be analyzed using thegas chemical analyzer. The camerawill image the surface, while informa-tion on the atmosphere will be re-layed by the Huygens AtmosphericStructure Instrument and DopplerWind Experiment instrument. TheProbe’s surface science instrumenta-tion will measure deceleration uponimpact, test for liquid surface (ifwaves are present, the instrument candetect the bobbing motion) or fluffysurface and conduct investigations ofliquid density, optical refractivity andthermal and electrical properties.

The Cassini Orbiter instruments havethe capability to penetrate the hazyatmosphere for data gathering. TheOrbiter’s radar can map the surfacethrough the clouds with high-resolu-tion swaths in a manner similar to theMagellan mission’s mapping of Ve-nus’ cloud-enshrouded surface. Theradar will provide data on the extentof liquid versus dry land and map sur-face features at high resolution (hun-dreds of meters) in narrow swaths.

The Orbiter’s camera and visible andinfrared radiometer can provide glo-bal coverage by imaging throughlong-wavelength “windows,” whereatmospheric aerosols do not block

This artist’s renderingillustrates the CassiniOrbiter’s radar as itmaps swaths of Ti-tan’s surface.

Artist’s conception ofthe Huygens Probedescending throughTitan’s atmosphere.

sunlight. The combined set of datafrom these instruments will allowscientists to determine the dominantgeological processes — cratering,volcanism and erosion. Titan’s spinvector and reference features for mak-ing maps will be established, anddata from the radar altimeter will pro-vide estimates of the relative eleva-

tions. If large bodies of liquid exist,winds may be detected through mea-surements of the surface roughness.

Interior Structure. Titan’s interior struc-ture can be determined indirectly. Asit passes close to Titan, the CassiniOrbiter’s path will be altered by the


satellite’s gravitational field. The result-ing change in the Orbiter’s velocitywill be detected on Earth as a Dop-pler shift in the spacecraft’s radio sig-nal. Understanding the basic structureof Titan’s gravitational field allows sci-entists to determine the satellite’s mo-ment of inertia, thereby establishingwhether Titan has a differentiatedcore and layered mantle.

Titan’s eccentric orbit carries it tovarying distances from Saturn. As Ti-tan gets nearer and farther from Sat-urn, the shape of the satellite changesdue to tidal forces. This may cause adetectable change in Titan’s gravita-tional field that can be measured byflying close to Titan a number of timeswhen the satellite is at different loca-tions in its orbit. Such deformation isa measure of the rigidity of Titan, as itflexes because of tidal forces, whichcan then be used to infer whether ornot it has a liquid layer in the mantle.

A World of Its OwnThe interaction between Titan’s atmo-sphere and magnetosphere is verycomplex and many questions still re-main. Most of the progress to date

has been made using Voyager 1 re-sults, combined with computer model-ing. Questions concerning preciseinteraction, such as the importance ofionization, neutral particle escape,shape of the Titan wake and how theinteraction varies over the Titan orbitcould all soon be answered by theCassini–Huygens mission.

With over 40 flybys planned, theCassini Orbiter should be able toanswer the question of whether ornot Titan has a significant internalmagnetic field.

Understanding Titan’s methane–hy-drocarbon cycle is a major goal forCassini–Huygens. Investigating thecirculation of the atmosphere andthe transport of these hydrocarbons isimportant to understanding Titan’s cli-mate and weather in many time do-mains: over days, over seasons andover the age of the solar system. Thesource of atmospheric methane is anintriguing puzzle — methane in Ti-

tan’s atmosphere will be depletedby irreversible photolysis in less than10 million years, so there needs to bea replenishing source of methane ator near the surface.

Titan presents an environment thatappears to be unique in the solar sys-tem, with a thick, hazy atmosphere;a possible ocean of hydrocarbons;and a surface coated with precipi-tates from the atmosphere — similarto the scenario that scientists believeled to the origin of life on Earth. Inthe three centuries since the discoveryof Titan, we have come to see it as aworld strangely similar to our own —yet located about one and a half bil-lion kilometers from the Sun.

When the Cassini Orbiter and Huy-gens Probe arrive at Titan, they willprovide us with our first close-upview of Saturn’s largest satellite.With the Probe’s slow descentthrough Titan’s atmosphere — withimages of the surface and chemicalcomposition data — and the Or-biter’s radar maps of the moon’ssurface, we will once again beshown another world.

T H O S E M A G N I F I C E N T R I N G S 4 1

G

C H A P T E R 4

Historical BackgroundFor reasons that are lost to us, Gali-leo initially paid little attention to Sat-urn beyond his declaration that it wasa triple planet. To him and other earlyviewers, the nature of the rings wasmasked by the poor quality of thetelescopes of the time. After his initialobservations, Galileo seemed to loseinterest in Saturn and did not viewthe planet again for two years.

When Galileo again chanced to lookat Saturn, in 1612, the planet’s ap-pearance had totally changed. Sat-urn no longer seemed “triple,” butappeared as a single “ball,” similarto the appearance of Jupiter in hissmall instrument. The puzzle of Sat-urn, which would grow for centuries,had begun to take shape.

In 1616, when Galileo once againreturned to his observations of Sat-urn, the planet had changed again!The features he had initially seen asseparate bodies now had an entirelydifferent appearance; he termed them“ansae” or “handles.”

Ring NatureOnce the changing appearance ofSaturn had been explained by a ringaround the planet, the next question

alileo first pointed his tiny telescope at Saturn in July of1610. Since then, the rings of the giant planet have be-come one of the great, enduring mysteries of our solarsystem. Even today, those magnificent rings are the most

requested object for viewing at any public telescope ses-sion. While the beauty of the rings is universally known,the mystery of their existence has challenged the explana-tions of astronomers for four centuries.

This enhancedVoyager 2 imageshows some hintsof possible varia-tions in the chemi-cal composition ofSaturn’s rings.

Those Magnificent Rings


H U Y G E N S ’H A N D L E

O N

T H ER I N G S

Following Galileo’s characterization of Saturn’s

“handles,” various observations were made by

astronomers until 1655, when Christiaan Huygens

turned a telescope of far better quality than Gali-

leo’s toward the planet and discovered its large

moon, Titan.

Within a year, Huygens had arrived at a theory to

explain some of the mystery regarding the continu-

ally changing appearance of Saturn — he correctly

asserted that Saturn was surrounded by a disk or

ring whose appearance varied because of the incli-

nation of Saturn’s equatorial plane and the planet’s

orbital motion around the Sun. Shown here is the

model of Saturn and its

rings used by

the Acca-

demia del Ci-

mento to test

Huygens’

hypothesis.

Huygens had

some details

about the rings

wrong, but his overall

theory was a concise

explanation of the ring

phenomenon and

was soon widely

accepted. By

1671, when

Huygens’

theory cor-

rectly pre-

dicted the rings’

“disappearance,”

his model was ac-

cepted as fact — he

indeed had grasped the “handle”

on the rings.

to occupy astronomers concernedthe nature of the ring, or rings. Werethe rings solid, or were they a swarmof tiny satellites in orbit, as suggestedin 1660 by Christiaan Huygens’close friend, Jean Chapelain? The ac-counts of various observers causedadjustments to Huygens’ theory, andthe observations of Jean-DominiqueCassini added information to the sci-entific debate.

Cassini noted that the brighter anddimmer portions of the rings wereseparated by a dark band, which heinterpreted to be a “gap.” This meantthat there were actually two rings,but the thought of two solid spinningdisks around Saturn was a bit muchfor astronomers to believe. The “soliddisk” theory began to lose its appeal.Instead, by the beginning of the18th century, the notion that Saturn’srings were actually a swarm of tiny,orbiting satellites gained wide favorin the astronomical community.

During the 19th century, as telescopequality improved, astronomers madenumerous studies of Saturn’s rings.The quality of observations also gen-erally improved — the matter of therings’ nature was being addressed.In 1857, James Clerk Maxwell putforth a now-famous theory about thenature of the rings, attributing themto swarms of particles and providinga good explanation for the existenceof such a system. In 1866, DanielKirkwood noted that the Cassini divi-sion (or gap) lay at an orbital reso-nance point1 with the orbit of Mimas;he later noted that the Encke divisionalso lay at a resonance point.

In 1872, James Keeler noted a nar-row gap in the outer A-ring, termedby later observers as the Keelergap2. In 1889, E. E. Barnard mademeasurements of an occultation ofIapetus by the translucent C-ring,finding it to be a semitransparentring of nonuniform optical density.In 1895, Keeler proved using spec-troscopic evidence that the ringswere composed of swarms of parti-cles moving in Keplerian orbits, thusvalidating Maxwell’s theory. In1908, Barnard made a series ofobservations at the time of Earth’spassage through Saturn’s ring plane,concluding that the Cassini divisionwas not devoid of particles.

Over the years, interesting featuresnoted by Earth-based astronomerswere not readily subject to verifica-tion. In fact, many observationsabout Saturn remained in disputeuntil the first robotic probes — Pio-neer 11, Voyager 1 and Voyager 2— flew close to the planet. ViewingSaturn from Earth involves peeringthrough a turbulent atmosphere.Visual acuity varies from person toperson: what one observer glimpsesfor a few seconds now and thenmight never be seen by another ob-server at a less favorable observingsite or with less acute eyesight.

Even the advent of photography inthe late 19th century did not improvediscovery verification, since photo-graphic studies were more subjectto the woes of looking throughEarth’s turbulent atmosphere. Onlythe passes by Pioneer and Voyager


Voyager 1 capturedthis striking image ofSaturn and its rings,with the rings cuttinga dark shadowacross the planet.

This false-color imageof Saturn’s outer A-ring was constructedfrom green, violetand ultravioletframes. Voyager 2captured the imageon August 23, 1981.


crystallized centuries of Saturn ob-servations, almost instantly sortingobservational fact from fiction —just as the Mariner flybys shatteredthe myths of Martian canals andcivilizations.

Ring StructureLarge-Scale Features. Viewing Saturnand its rings through a modest tele-scope easily allows us to see that thering system is divided into severalvisually different radial zones. Theouter zone (visible to an Earth ob-server) is known as the A-ring, whilethe brighter, inner zone is known asthe B-ring. Between these two zoneswe can sometimes see the gap

known as the Cassini division; on anexceptional viewing night, we maysee an inner, semitransparent ring,the C or Crepe ring.

This view of the rings, while modest,nevertheless introduces us to the sys-tem’s unexplained features. Whyare there several morphologicallydistinct rings? Beyond the A, B, Cand E-rings, which can be seen orimaged from Earth-bound telescopes,Saturn has several other distinctrings — the D, F and G-rings —detected in the images from Pioneerand Voyager.

The mysteries of Saturn’s rings lieat an even more fundamental level:

S A T U R N ’ S F L Y I N G S N O W B A L L S

Saturn’s rings haveintrigued scientistson Earth for nearlyfour centuries. Seenup close, the ringsmay be even moreastonishing thanthey appear at adistance. Althoughthe rings stretchover 40,000 kilo-meters in width,they may be as littleas 100 meters in

thickness. The ringparticles — froma few micrometers(one micrometer =one millionth of ameter) to a fewmeters in size —have been describedas ice-covered rocksor icy snowballs. Sci-entists are continuingin their attempts to

analyze ring compo-sition, hoping that theCassini–Huygens mis-sion will bring themclearer clues to theicy puzzle.

Why are the rings there at all? Howdid the rings form? How stable is thering system? How does the systemmaintain itself? So far, we have onlybits and pieces of answers and muchspeculation about these questions.

Smaller-Scale Features. As Pioneerand Voyager moved closer to Saturn,the spacecraft captured images thatmade clear the features glimpsedover the centuries, as well as manypreviously unseen features. Pioneerfound a new ring feature (the F-ring)outside the A-ring, which became anobject of major interest when Voyag-er later arrived. The fields and parti-cles sensors on Pioneer 11 showedthe possible presence of satellites inthe vicinity of the new F-ring. This ringproved to be particularly interesting— it is a narrow ring, and the satel-lites that (as we now know) maintainthe ring in position were the firstproof of a theory put forth in 1979 toexplain the narrow rings of Uranus.

Distance, Width,Ring kilometers* kilometers

D 66,970 7,500

C 74,500 17,500

B 92,000 25,400

A 122,170 14,610

F 140,180 50

G 170,180 8,000

E 180,000 300,000

* Distance from Saturn to closest edgeof ring.

T H E R I N G S O F

S A T U R N


Prometheus andPandora, two tinysatellites, shepherdSaturn’s F-ring, whichis multistranded andkinked in places.

This Voyager 1 im-age clearly shows thekinky, braided struc-ture of the F-ring.


Voyager’s high-resolution azimuthalstudies of the new F-ring showed thatthe ring was even more unusual thanexpected: It comprises a number ofstrands that appear to be intertwinedand braided in some places, andkinked in other locations. While thegravitational effects of nearby moonsmay be responsible for some F-ringattributes, they do not account for thecomplex structure that Voyager discov-ered. Continuing studies of Voyagerimages over the years have unraveledmore of the complex properties of thisunusual ring.

The possibility of numerous, small sat-ellites occurring within Saturn’s ringsystem was a puzzle the Voyager mis-sion had hoped to solve. Voyager’sbest-resolution studies of the ring sys-tem were aimed at revealing any bod-

provide Cassini with many good op-portunities for conducting azimuthalimaging studies.

Occultation ExperimentsCassini will be able to perform anumber of the experiments that Voy-ager used to detect other gravita-tional effects on Saturn’s ring mate-rial. One experiment, termed an“occultation experiment,” involveswatching as light from a star (a stel-lar occultation) or the radio beamfrom the spacecraft (a radio occulta-tion) passes through the ring materialto see how the beam is affected dur-ing the occultation. As the beampasses through the ring material, itmay be attenuated or even extin-guished. These experiments providedan extremely high-resolution study ofa single ring path with resolutions upto about 100 meters.

ies larger than about 10 kilometersin diameter; nevertheless, only fourmoonlets — Atlas, Pan, Prometheusand Pandora — were found in theimages. Only one, Pan, was locatedin the main ring system.

The Voyager high-resolution studiesdid, however, detect signs of smallmoonlets not actually resolved in theimages. When a small, dense bodypasses near a section of low-densityring material, its gravitational pulldistorts the ring and creates whatare known as “edge waves.”3 Whilethe high-resolution, azimuthal cov-erage of the ring system is a time-consuming endeavor, such studiesare the only means of detectingthese waves and thereby findingsatellites too small to be seen in im-ages. Soon, four years in orbit will

S A T U R N ’ S C R O W N J E W E L S

Saturn’s rings are eas-ily the crown jewels ofthe entire solar sys-tem. And the varietyof rings is staggering;the count in high-reso-lution images suggestsanywhere from 500to 1,000 separaterings! Named in or-der of discovery, therings’ labels do notindicate their relativepositions. From theplanet outward, theyare known as D, C, B,A, F, G and E.


In the stellar occultation experiment,the Voyager 2 spacecraft had a sin-gle opportunity to observe as lightfrom a star was occulted by the ringsystem. As the light passed throughthe rings, it was attenuated and extin-guished by the particles comprisingthe system. The velocity of the space-craft, the brightness of the star, thesensitivity of the instrumentation andthe sampling rate of the instrumentsprovide the limitations for such stellaroccultation experiments.

The primary instruments used to con-duct the stellar occultation study wereVoyager’s photopolarimeter and ultra-violet spectrometer, with the photopo-larimeter providing the higherresolution data set. Resolving featuresas small as about 100 meters, thestellar occultation experiments detect-ed many small-scale ring structuresand found that the F-ring was farmore complex than images had sug-gested. The data set, furthermore,showed that the B-ring was quiteopaque in many regions and con-firmed that the Cassini “division” wasnot at all empty. It also provided adirect measurement of the maximumthickness of the ring system in severallocations, finding it to be much lessthan 100 meters thick.

In the radio occultation experiment,Voyager 1 had a single opportunityto watch as the spacecraft’s radiobeacon was occulted by the ringsystem. As this radio signal passedthrough the ring material, its signalwas also attenuated. The antennas ofthe Deep Space Network receivedthe attenuated signal, allowing scien-

tists to study the complex data beingreturned. While the radio-sciencedata set was complicated to processand interpret, it yielded informationon particle size distribution in thering system not available from stellaroccultation studies. From the radio-science data set, we know some-thing of the size distribution of thelarger particles in the ring system.We know that much of the ring massis in particles from a few centimetersto a few meters in diameter; boul-ders more than 10 meters in diame-ter do not comprise much of themass of Saturn’s ring system.

Both occultation experiments de-scribed here detected features inSaturn’s rings caused by the gravita-tional effects of the planet’s satellites.Locations where gravitational reso-nance effects had partially clearedmaterial were identified by theseexperiments and are also visible atlower resolution in imaging data.While the existence of resonanceshad been recognized since 1866 —when it was pointed out that theCassini division lay near a gravita-tional resonance point — the extentto which the structures occur waslargely unexpected.

At these resonances, gravitational-wave effects were detected in thephotopolarimeter, ultraviolet spec-trometer and radio-science occulta-tion data sets — known as densityand bending waves — that are simi-lar in behavior to the arms of spiralgalaxies. While some of these struc-

tures were later detected in the imag-ing data, the high-resolution datasets of the photopolarimeter, ultravio-let spectrometer and radio-scienceexperiments provided the details thatallowed Voyager science teams toderive information about the mass,density and viscosity4 of the ringmaterial in their vicinity. Much ofthe fine structure of the A-ring is well-explained by such gravitational reso-nances with satellite orbits. However,the resonance theory does not pro-vide a universal solution and muchof the structure of the B-ring is leftunexplained.

The occultation results, along with im-aging studies, showed that a greatmany of the narrow ringlet featuresat Saturn are slightly eccentric — notcircular in shape — and that theseeccentric ringlets lie embedded inthe mass of nearly circular rings thatconstitute the majority of Saturn’sring system. These same studies alsoshowed that many small ringlets areasymmetrical in structure and thatthere are very few truly empty“gaps” in the ring system.

Results from the Voyager experimentsprovided a wealth of information toexplain some physical properties ofSaturn’s rings, but at the same time,they also created many mysteriesthat were unexpected and not under-standable through Voyager data.For instance, Voyager data did notdetect the number of moonlets weexpected. Are these moonlets non-existent or just not yet discovered?Cassini will have far more time to


perform ring searches and will pro-vide higher resolution data sets, bothrequired to answer this question.

Ring SpokesAs Voyager approached closer to Sat-urn, images began to show dark, ra-dial structures on the rings. Whilethese features had been sighted byEarth-based observers over the de-cades, the Voyager images showedthe “spokes” in great detail, allowingthem to be studied as they formedand rotated about the planet. Spokesseemed to appear rapidly — as asection of ring rotated out of the dark-ness near the dawn terminator —and then dissipate gradually, rotating

around toward the dusk ring termina-tor. A spoke’s formation time seemedto be very short; in some imagingstudies one was seen to grow over6000 kilometers in distance in justfive minutes.

As it passed from the day side to thenight side of the planet, Voyager pro-vided unique views of Saturn’s ringsand was able to observe the ringsand spokes in forward-scatteredlight where they appeared as farbrighter features than in backscat-tered light. This light-scattering prop-erty showed that the spokes werelargely composed of very small, mi-crometer-sized particles.

The spokes in Saturn’s rings format the distance from Saturn where therotational speed of the ring particlesmatches that of the planet’s magneticfield lines. Further studies of this pos-sible interaction revealed a link be-tween the spoke formation and aregion on the planet associated withintense long-wavelength radio emis-sions (now referred to as the Saturnkilometric radiation, or SKR, zone).All these studies pointed to a sizableinteraction between the ring systemand Saturn’s electromagnetic fields.But what was it? Once again, Voyag-er had provided a tantalizing tidbit ofinformation, but had left a riddle.

Ring Particle PropertiesThe Voyager trajectories allowedthe rings to be viewed from a muchwider range of angles than couldever be achieved by Earth-basedobservers. This allowed for extensiveobservations of the light scattering,or photometric, properties of therings. The proximity of the spacecraftto Saturn also provided for higherresolution photometric and spectralstudies than could ever have beenachieved by Earth-based observers.These studies yielded a great dealof information about the nature of thering particles and the composition ofthe rings.

Observations in forward-scatteredlight made while the spacecraft wason the dark side of the planet showedthat some areas of the rings weremuch brighter in forward-scatteredlight than others. Small particles inthe micrometer-size range are muchmore efficient in producing forward

The numerous“spoke” features inthe B-ring are evidentin this image ob-tained by Voyager 2.


scattering than are larger particles,so this tells us that these areas of therings have an abundance of small,micrometer-size particles. The D, E, Fand G rings all show such forward-scattering properties as do the spokesof the B-ring. These data, combinedwith ring occultation data from thephotopolarimeter, ultraviolet spectrom-eter and radio-science experiments,give us some information about thevariation of particle-size distributionacross Saturn’s ring system. Cassiniwill have the opportunity to conductextensive studies of these diffuse ringsand their particle distributions.

Ring CompositionGround-based infrared spectral stud-ies of the A and B rings show thatthey are composed largely of verynearly pure water ice. The spectralcharacteristics of the rings are alsovery similar to those of several of Sat-urn’s inner satellites. The extendedwavelength range over which Cassiniwill be able to make spectral mea-surements will yield information onboth the nature of the chemical con-taminants in the rings’ water ice aswell as on the relationship betweenring and satellite composition.

Studies of the color distribution (asa sign of compositional variation)in the main rings show that the ringsystem is not completely uniform in itsmakeup and that some sorting of ma-terials within A and B rings exists.Why such a nonuniform compositionexists is unknown; Cassini data willprovide some clues toward solvingthis problem. The other, diffuse rings

of Saturn are much more difficultto study in this manner, but we doknow that the E-ring is somewhat blu-ish in color — and thus different inmakeup from the main rings. Specu-lation exists that the moon Enceladusis the source of E-ring material, butthis is unconfirmed.

Since ring particles larger thanabout one millimeter represent aconsiderable hazard to the Cassinispacecraft, the mission plan will in-clude efforts to avoid dense particleareas of Saturn’s ring plane. Thespacecraft will be oriented so as toprovide maximum protection for itself

and its sensitive instrumentation pack-ages. Even with such protective steps,passage through the ring plane willallow the Cassini particle measure-ment experiments (the Cosmic DustAnalyzer, the Ion and Neutral MassSpectrometer and the Cassini PlasmaSpectrometer) to perform importantstudies of the particles making up theless dense regions of the Cassini ringplane. Such measurements could pro-vide insight into the composition andenvironment of the ring system andSaturn’s icy satellites.

This false-color imageof Saturn’s B and Crings reveals fine de-tails and subtle colordifferences.


Formation and EvolutionA century after their discovery, theorigin of Saturn’s rings was a pointof speculation. Then, for many years,the matter was actually thought to bewell understood. Today’s view is thatthe origin of those magnificent ringsinvolves a complex set of issues.The size, mass and composition ofthe rings make their formation andevolution rather difficult to explain.

Among the early notions about therings’ origin was a theory by EdouardRoche. He suggested that the ringswere fragments left over from a moonthat had at one time orbited Saturn.

This moon had broken up, Roche ex-plained, because of the tremendousstresses placed upon it by Saturn’shuge gravitational field.

The Roche theory does not hold upwell, however, under scrutiny. Forinstance, did the rings form out of theinitial solar system nebula, or afterone or more satellites were torn apartby Saturn’s gravity? Moreover, if therings were the result of the numerouscomets captured and destroyed bySaturn’s gravity, why are Saturn’srings so different in nature from therings of the other giant planets?Over their lifetime, the rings musthave been bombarded continually

by comets and meteors — theyshould have accumulated a greatamount of carbonaceous and silicatedebris — yet their composition seemsalmost entirely to be water ice.

Another issue concerns the stability ofthe ring system. The effects of torqueand gravitational drag, along withthe loss of momentum via collisionalprocesses should have produced asystem only one-tenth to one-hun-dredth the age of the solar system it-self. If this hypothesis is correct, thenwe cannot now be observing a ringsystem around Saturn that formedwhen the solar system coalesced.

Voyager 1 lookedback on November16, 1980, four daysafter flying by theplanet, to captureSaturn and its spec-tacular rings fromthis perspective.


In fact, Saturn’s rings — as well asthe rings of all the other large planets— may have formed and dissipatedmany times since the beginning of thesolar system. If anything, we may in-fer that ring systems are in a constant,steady state of renewal and regenera-tion. We are quite likely a long wayfrom understanding all the processesand dynamics of ring formation, re-newal and evolution.

In conducting an array of ring stud-ies, the Cassini mission will ultimatelyfocus on four critical questions: Howdid the rings form? How old are therings? How are the rings maintained?What are the dynamics and relation-ships of the rings to Saturn, its satel-lites and its electromagnetic fields?

In attempting to answer these ques-tions, whereas the Voyager space-craft had only a few days close toSaturn’s rings — time only enoughto suggest some tantalizing clues —Cassini will have several months toobserve the rings in detail and returnenough data to substantially increaseour knowledge base.

Voyager Legacy, Cassini MissionInformation about Saturn’s rings “dis-covered” by the Voyager mission —which was then studied and docu-mented — was not entirely new toEarth-bound astronomers. Ring gaps,narrow rings, spokes, diffuse rings,sharp ring edges, satellite resonances— these were all features seen or in-ferred before. Nevertheless, scientistswere stunned by the sheer abun-dance of these features on every

scale once Voyager began returningactual, high-resolution images of thering system.

As more and more data arrived onEarth, one thing was increasinglyclear: Rather than solving the mysteryof Saturn’s rings, Voyager was in-stead showing us how little we knewabout them, even after four centuriesof study!

After two decades of studying theVoyager data, and then studyingmore recent data from the Hubble

Space Telescope, scientists have de-veloped a new set of questions forthe Cassini mission to investigatewhen the spacecraft reaches Saturnin 2004. The answers Cassini mayfind to these questions will help toexplain some of the processes in therings’ origin. Scientists are aware,however, that the data Cassini returnswill quite probably present us withanother round of surprises — and yetmore questions.

Early observers were puzzled by thechanges that Saturn’s rings showed

Scientists have longsought to de-terminewhat therings ofSaturn aremade of, howthey got thereand what keepsthem in orbitaround the giantplanet. Some of thequestions began tobe answered whenVoyager 1 arrived atSaturn on November12, 1980, and Voy-ager 2 followed suiton August 25, 1981.The Voyagers’ im-ages revealed therings as the mostexquisite sight in thesolar system. Still, thediscoveries presentedmany more new puz-zles than solutions.

Now, as Cassini–Huygens preparesto tackle the many

issues raised by theVoyager encounters,the Voyagers them-selves are headedtoward the outerboundary of thesolar system.The twin spacecraftcontinue their grandtour of the solar sys-tem in search of theheliopause, the re-gion where the Sun’sinfluence wanes andthe beginning of inter-stellar space can be

sensed. The Voyagershave enough electri-cal power and thrusterfuel to operate until atleast 2015. By then,

V O Y A G E R — T H E G R A N D E S T T O U R

Voyager 1 will be19.8 billion kilome-ters away from theSun, and Voyager 2will be 16.8 billionkilometers away.


from year to year. Now, after centu-ries of Earth-bound observations fol-lowed by spacecraft explorations,scientists do not question if Cassiniwill find changes in Saturn’s ringsystem. Instead, they wonder whatchanges the rings have undergonesince Voyager last looked in 1981.

Among other things, Cassini willprobe the many questions raisedby Voyager, and Pioneer before that,about Saturn’s rings. In this endeavor,the new spacecraft has several ex-pected advantages over its prede-cessors. Cassini’s science instrumen-tation covers a broader range of theelectromagnetic spectrum and is ofgreater sensitivity and resolution.

In addition, while Voyager was amission of discovery into previouslyuncharted frontiers, Cassini is afocused return visit, designed toaddress specific issues. Of course,

FOOTNOTES

[1] If a satellite and a ring particle have arotational period that are integer multiples ofone another, their periods are said to be inresonance.

If a ring particle orbits in exactly one-half theamount of time it takes a given satellite tocomplete its orbit, we have a 2:1 resonance.The ring particles in such an orbit obtain re-peated gravitational “tugs,” which createmore collisions, which eventually thin the re-gion of particles. The Cassini gap is the bestknown and best defined such zone, with theJanus 7:6 point at its exterior limit and theMimas 2:1 point at its interior limit. In re-gions where the resonance effect is less pro-nounced, we find density waves instead ofgaps in the rings.

[2] Keeler probably discovered what isknown as the Encke gap, but the astronomi-cal community has been somewhat remiss incorrecting this error.

[3] Further searching in the Encke gap,where such edge waves were seen, led tothe discovery of one small satellite inside thedivision. Called Pan, this satellite may wellbe responsible for clearing and maintainingthe gap.

[4] The particles constituting the rings arenot completely isolated from each other;they interact with one another in orbitthrough collisions and their mutual gravita-tional attraction. These interactions makethe rings behave somewhat like a fluid,rather than a swarm of isolated, orbitingparticles. The degree to which the particlesinteract is characterized as a measure ofthe rings’ “viscosity.”

Cassini is also expected to make itsown discoveries about Saturn.

The ring system of Saturn — whichcontains the widest array of attributesof all the ring systems known to us —is the ideal laboratory for studyingthe ring phenomenon. The Voyagermission showed us how little we real-ly know about Saturn’s rings, evenafter four centuries of observations.While Cassini provides us with awealth of information to help solvesome of the rings’ mystery, this newmission will also uncover many newclues for future investigation.

For now, scientists anxiously awaitthe volumes of data — about thosemagnificent rings — that Cassini isexpected to return to Earth between2004 and 2008.

T H E I C Y S A T E L L I T E S 5 3

The satellites ofSaturn comprisea diverse set ofobjects. This com-posite image shows,from top, Mimas,Rhea, Iapetus,Tethys, Enceladusand Dione.

T

C H A P T E R 5

he moons, or satellites, of Saturn represent a diverse set ofobjects. They range from the planet-like Titan, which has adense atmosphere, to tiny, irregular objects only tens of ki-lometers in diameter. These bodies are all believed to haveas major components some type of frozen volatile, prima-

rily water ice, but also other components such as meth-ane, ammonia and carbon dioxide existing either aloneor in combination with other volatiles. Saturn has at least18 satellites; there may exist other small, undiscoveredsatellites in the planet’s system.

Overall CharacteristicsIn 1655, Christiaan Huygens discov-ered Titan, the giant satellite of Sat-urn. Later in the 17th century, Jean-Dominique Cassini discovered thefour next largest satellites of Saturn.It was not until more than 100 yearslater that two smaller moons of Saturnwere discovered. As telescopes ac-quired more resolving power in the19th century, the family of Saturn’ssatellites grew.

Most of the smallest satellites were dis-covered during the Voyager space-craft flybys. The 18th satellite, Pan,was found nearly 10 years after theflybys during close analysis of Voyag-er images; it is embedded in the A-ring of Saturn. Saturn’s ring planecrossings — when the obscuring lightfrom Saturn’s bright rings dims, as therings move to an edge-on orientation— represent the ideal configurationfor discovering new satellites. Images

obtained by the Hubble Space Tele-scope during the ring plane crossingsin 1995 did not reveal any unambig-uous discoveries of new satellites.

Physical and DynamicPropertiesMost planetary satellites present thesame hemisphere toward their prima-ries, a configuration that is the resultof tidal evolution. When two celestialbodies orbit each other, the gravita-tional force exerted on the near sideis greater than that exerted on the farside. The result is an elongation ofeach body that forms tidal bulges,which can consist of either solid,liquid or gaseous (atmospheric) mate-rial. The primary tugs on the satellite’stidal bulge to lock its longest axis onto the primary satellite line. The satel-lite, which is said to be in a state ofsynchronous rotation, keeps the sameface toward the primary. Since thisdespun state occurs rapidly (usuallywithin a few million years), most natu-ral satellites are in synchronous rota-tion. Of Saturn’s icy satellites, two,Hyperion and Phoebe, are known toexhibit asynchronous rotation.

The Icy Satellites


The Hubble SpaceTelescope obtainedthese images of Sat-urn and its rings andsatellites. The top im-age, obtained in Au-gust 1995, showsTitan (far left, castinga shadow) and (farright, from the left),Mimas, Tethys, Janusand Enceladus. In themiddle image, ob-tained in November1995, Tethys (upperleft) and Dione ap-pear. In the bottomimage, obtained inDecember 1994, astorm is raging in thecenter of the planet.[Images courtesy of:top and middle — E.Karkoschka, Univer-sity of Arizona Lunarand Planetary Labo-ratory; bottom — R.Beebe, New MexicoState University]


Before the advent of spacecraft explo-ration, planetary scientists expectedthe satellites of the outer planets to begeologically dead. They assumed thatheat sources were not sufficient tohave melted the satellites’ mantlesenough to provide a source of liquidor even semiliquid ice or ice silicateslurries. The Voyager and Galileospacecraft have radically altered thisview by revealing a wide range ofgeological processes on the moonsof the outer planets. Enceladus maybe currently active. Several of Sat-urn’s medium-sized satellites arelarge enough to have undergone in-ternal melting with subsequent differ-entiation and resurfacing.

Recent work on the importance oftidal interactions and subsequent

heating has provided the theoreticalfoundation for explaining the exist-ence of widespread activity in theouter solar system. Another factor isthe presence of non-ice components,such as ammonia hydrate or methaneclathrate, which lower the meltingpoints of near-surface materials. Par-tial melts of water ice and variouscontaminants — each with their own

melting point and viscosity — providematerial for a wide range of geologi-cal activity.

Because the surfaces of so manyouter planet satellites exhibit evi-dence of geological activity, plane-tary scientists have begun to think interms of unified geological processesoccurring on the planets — includingEarth — and their satellites. For ex-ample, partial melts of water ice withvarious contaminants could provideflows of liquid or partially molten slur-ries that in many ways mimic the ter-restrial or lunar lava flows formedby the partial melting of silicate rockmixtures. The ridged and grooved ter-rains on satellites such as Enceladusand Tethys may have resulted fromtectonic activities occurring through-

Hyperion is an irregu-lar, pockmarked sat-ellite in an unusual,nonsynchronousrotational state.

T H E K N O W N M O O N S O F S A T U R N

Diameter, Distance, Orbital Year Discovered:Moon kilometers kilometers Period, days Discoverer

Pan 20 133,580 0.56 1990: Showalter

Atlas 30 137,670 0.60 1980: Terrile

Prometheus 100 139,350 0.61 1980: Collins

Pandora 90 141,700 0.63 1980: Collins

Epimetheus 120 151,450 0.69 1966: Walker

Janus 190 151,450 0.69 1966: Dolfus

Mimas 392 185,520 0.94 1789: Herschel

Enceladus 500 238,020 1.37 1789: Herschel

Tethys 1060 294,660 1.89 1684: Cassini

Telesto 30 294,660 1.89 1980: Smith

Calypso 26 294,660 1.89 1980: Smith

Dione 1120 377,400 2.74 1684: Cassini

Helene 32 377,400 2.74 1980: Laques and Lecacheux

Rhea 1530 527,040 4.52 1672: Cassini

Titan 5150 1,221,830 15.94 1655: Huygens

Hyperion 290 1,481,100 21.28 1848: Bond

Iapetus 1460 3,561,300 79.33 1671: Cassini

Phoebe 220 12,952,000 550.40 1898: Pickering


out the solar system. Finally, the ex-plosive volcanic eruptions possiblyoccurring on Enceladus may be simi-lar to those occurring on Earth,which result from the escape of vola-tiles released as the pressure de-creases in upward-moving liquids.

Formation and Bulk CompositionThe solar system — the Sun, the plan-ets and their families of moons —condensed from a cloud of gas anddust about 4.6 billion years ago. Thisage is derived primarily from radio-metric dating of meteorites, whichare believed to consist of primordial,unaltered matter. Because there ex-isted a temperature gradient in theprotosolar cloud, or nebula, volatilematerials (those with low condensa-tion temperatures) are the major com-ponents of bodies in the outer solarsystem. These materials include water

ice, ice silicate mixtures, methane,ammonia and the hydrated formsof the latter two materials. Two typesof dark contaminants exist on thesurfaces of the satellites: C-type orcarbon-rich material, and D-type ma-terial, which is believed to be rich inhydrocarbons.

The planets and their satellite systemsformed from the accretion of succes-sively larger blocks of material, orplanetesimals. One important con-cept of planetary satellite formationis that a satellite cannot accrete with-in Roche’s limit, the distance at whichthe tidal forces of the primary be-come greater than the internal cohe-sive forces of the satellite. Except forTitan, Saturn’s satellites are too smallto possess gravity sufficiently strongto retain an appreciable atmosphereagainst thermal escape.

EvolutionSoon after the satellites accreted,they began to heat up from the re-lease of gravitational potential ener-gy. An additional heat source wasprovided by the release of mechani-cal energy during the heavy bom-bardment of their surfaces byremaining debris. Mimas and Tethyshave impact craters (named Herscheland Odysseus, respectively) causedby bodies that were nearly largeenough to break them apart; proba-bly such catastrophes did occur. Thedecay of radioactive elements foundin silicate materials provided anothermajor source of heat. The heat pro-duced in the larger satellites mayhave been sufficient to cause meltingand chemical fractionation; the densematerial, such as silicates and iron,went to the center of the satellite toform a core, while ice and other vola-tiles remained in the crust. A fourthsource of heat is provided by the re-lease of frictional energy as heat dur-ing tidal and resonant interactionsamong the satellites and Saturn.

Several of Saturn’s satellites under-went periods of melting and activegeology within a billion years of theirformation and then became quies-cent. Enceladus may be currentlygeologically active. For nearly a bil-lion years after their formation, thesatellites all underwent intense bom-bardment and cratering. The bom-bardment tapered off to a slower rateand presently continues. By countingthe number of craters on a satellite’ssurface and making certain assump-tions about the flux of impacting ma-

planet-like Titan iseasily the largest ofSaturn’s moons. Ithas a dense atmo-sphere and possibly

S M A L L O N E S , B I G O N E S …

The moons, or sat-ellites, of Saturncomprise a diverseset of objects. Ofthe 18 currentlyknown satellites,nine are illustratedbelow to show theirrelative sizes. The

liquid oceans on itssurface. Hyperion,by contrast, is irreg-ularly shaped andseems to be cov-ered with ice and

dark, rocky mate-rial. The Cassini–Huygens mission,through its detailedobservations, willprovide us withmuch more informa-tion about Saturn’smany moons.

Phoebe

Rhea

Titan

IapetusHyperion

Dione

Tethys

Enceladus

Mimas


The bright surfaceof Saturn’s moonEnceladus showsevidence of exten-sive resurfacingover time.

The Herschel cra-ter covers approxi-mately one third ofthe diameter of thesurface on themoon Mimas.


terial, geologists are able to estimatewhen a specific portion of a satellite’ssurface was formed. Continual bom-bardment of satellites causes the pul-verization of both rocky and icysurfaces to form a covering of finematerial known as a regolith.

Meteoritic bombardment of icy bodiesalters the optical characteristics of thesurface by excavating and exposingfresh material. Impacts can also causevolatilization and the subsequent es-cape of volatiles to create a lag de-posit enriched in opaque, darkmaterials. Both the Galilean satellitesof Jupiter and the medium-sized satel-lites of Saturn tend to be brighter onthe hemispheres leading in the direc-tion of orbital motion (the so-called“leading” side, as opposed to the“trailing” side); this effect is thought tobe due to preferential micrometeoritic“gardening” on the leading side.

Many geologists expected the cratersformed on the outer planets’ satellitesto have disappeared from viscousrelaxation. Voyager images revealcraters that in many cases have mor-phological similarities to those foundin the inner solar system, includingcentral pits, large ejecta blankets andwell-formed outer walls. Scientists nowbelieve that silicate mineral contami-nants or other impurities in the iceprovide the extra strength required tosustain impact structures.

Individual SatellitesMedium-Sized Icy Satellites. These sixsatellites of Saturn are smaller than Ti-tan and Jupiter’s giant Galilean satel-

The six medium-sizedicy satellites of Sat-urn. Clockwise fromupper left: Rhea,Tethys, Mimas,Enceladus, Dioneand Iapetus.

lites, but they are still sizable — andas such they represent a unique classof icy satellite. Earth-based telescopicmeasurements showed the spectralsignature of ice for Tethys, Rhea andIapetus; Mimas and Enceladus areclose to Saturn and difficult to ob-

serve because of scattered light fromthe planet. The satellites’ low densi-ties and high albedos imply that theirbulk composition is largely water ice,possibly combined with ammoniaor other volatiles. They have smalleramounts of rocky silicates than the

Artist’s view of Saturnas seen from the plan-et’s heavily crateredmoon, Mimas. In themiddle of the image isa central pit of a largeimpact crater.


Galilean satellites. Most of what ispresently known of the Saturn systemwas obtained from the Voyager flybysin 1980 and 1981.

Saturn’s innermost, medium-sized sat-ellite, Mimas, is covered with craters,including one named Herschel thatcovers a third of the moon’s diameter.There is a suggestion of surficialgrooves that may be features causedby the impact. The craters on Mimastend to be high-rimmed, bowl-shapedpits; apparently surface gravity isnot sufficient to have caused viscousrelaxation. The application of crater-counting techniques to Mimas sug-gests that it has undergone severalepisodes of resurfacing.

The next satellite outward from Saturnis Enceladus, an object that wasknown from telescopic measurementsto reflect nearly 100 percent of thevisible radiation incident on it (forcomparison, Earth’s Moon reflectsonly about 11 percent). The onlylikely composition consistent with thisobservation is almost pure water iceor some other highly reflective vola-tile. Voyager 2 images show an ob-ject that had been subjected, in therecent geological past, to extensiveresurfacing; grooved formations simi-lar to those on the Galilean satelliteGanymede are prominent.

The lack of impact craters on this ter-rain is consistent with an age of lessthan a billion years. Some form of icevolcanism may be currently occurringon Enceladus. A possible heatingmechanism is tidal interactions, per-

haps with Dione. About half of thesurface observed by Voyager is ex-tensively cratered, consistent with anage of four billion years.

A final element to the puzzle ofEnceladus is the possibility that it isresponsible for the formation of the E-ring of Saturn, a tenuous collection oficy particles that extends from insidethe orbit of Enceladus to past the or-bit of Dione. The maximum thicknessposition of the ring coincides with theorbital position of Enceladus. If someform of volcanism is presently activeon the surface, it could provide asource of particles for the ring. An al-ternative source mechanism is the es-cape of particles from the surfacedue to meteoritic impacts.

Tethys is covered with impact craters,including Odysseus, the largestknown impact structure in the solarsystem. The craters tend to be flatterthan those on Mimas or the Moon,probably because of viscous relax-ation and flow over the eons underTethys’ stronger gravitational field.Evidence for episodes of resurfacingis seen in regions that have fewercraters and higher albedos. In addi-tion, there is a huge trench formation,the Ithaca Chasma, which may be adegraded form of the grooves foundon Enceladus.

Dione, about the same size as Tethys,but more dense, exhibits a wide di-versity of surface morphology. Nextto Enceladus, it has the most exten-

sive evidence for internal activity. Itsrelatively high density may provideadded radiogenic heat from siliceousmaterial to spur this activity. Most ofthe surface is heavily cratered, butgradations in crater density indicatethat several periods of resurfacing oc-curred during the first billion years ofits existence. The leading side of thesatellite is about 25 percent brighterthan the other, due possibly to moreintensive micrometeoritic bombard-ment on this hemisphere. Wispystreaks, which are about 50 percentbrighter than the surrounding areas,are believed to be the result of inter-nal activity and subsequent emplace-ment of erupting material. Dionemodulates the radio emission fromSaturn, but the mechanism responsi-ble for this phenomenon is unknown.

Rhea appears to be superficially verysimilar to Dione. Bright wispy streakscover one hemisphere. However,there is no evidence for any resurfac-ing events early in its history. Theredoes seem to be a dichotomy be-tween crater sizes — some regionslack large craters while other regionshave a preponderance of such im-pacts. The larger craters may be dueto a population of larger debris moreprevalent during an earlier episodeof collisions. The craters on Rheashow no signs of viscous relaxation.

When Cassini discovered Iapetus in1672, he noticed almost immediatelythat at one point in its orbit aroundSaturn it was very bright, but on theopposite side of the orbit, the moonnearly disappeared. He correctly de-


The cratered surfaceof Tethys, includingthe groove-like IthacaChasma and thecrater Telemachusat the upper right.

The heavily crateredface of Dione isshown in thisVoyager 1 image.Bright wispy streaksare visible on thelimb.


shepherding satellites, the co-orbitalsand the Lagrangians. All threegroups of satellites are irregularlyshaped and probably consist primari-ly of ice.

The three shepherds, Atlas, Pandoraand Prometheus, are believed to playa key role in defining the edges ofSaturn’s A and F rings. The orbit ofSaturn’s second innermost satellite,Atlas, lies several hundred kilometersfrom the outer edge of the A-ring. Theother two shepherds, which orbit oneither side of the F-ring, constrain thewidth of this narrow ring and maycause its kinky appearance.

The co-orbital satellites, Janus andEpimetheus, which were discoveredin 1966 and 1978, respectively, ex-ist in an unusual dynamic situation.

duced that one hemisphere is com-posed of highly reflective material,while the other side is much darker.Voyager images show that the brightside, which reflects nearly 50 percentof the incident radiation, is fairly typi-cal of a heavily cratered icy satellite.The other side, which is centered onthe direction of motion, is coated witha redder material that has a reflectivi-ty of about three to four percent.

Scientists do not agree on whetherthe dark material originated from anexogenic source or was endogeni-cally created. One scenario for theexogenic deposit of material entailsdark particles being ejected fromPhoebe and drifting inward to coatIapetus. The major problem with thismodel is that the dark material on Ia-petus is redder than Phoebe, althoughthe material could have undergonechemical changes after its expulsionfrom Phoebe that made it redder.One observation lending credence toan internal origin is the concentrationof material on crater floors, which im-plies an infilling mechanism. In onemodel, methane erupts from the interi-or and is subsequently darkened byultraviolet radiation.

Other characteristics of Iapetus areodd. It is the only large Saturn satel-lite in a highly inclined orbit. It is lessdense than objects of similar albedo;this fact implies a higher fraction ofice or possibly methane or ammoniain its interior.

Small Satellites. The Saturn system hasa number of unique small satellites.Three types of objects have beenfound only in the Saturn system: the

They move in almost identical orbitsat about two and a half Saturn radii.Every four years, the inner satellite(which orbits slightly faster than theouter one) overtakes its companion.Instead of colliding, the satellites ex-change orbits. The four-year cyclethen begins again. Perhaps these twosatellites were once part of a largerbody that disintegrated after a majorcollision.

The three other small satellites of Sat-urn — Calypso, Helene and Telesto— orbit in the Lagrangian points oflarger satellites, one associated withDione and two with Tethys. Lagrang-ian points are locations within anobject’s orbit in which a less massivebody can move in an identical, stableorbit. The points lie about 60 degreesin front of and behind the largerbody. Although no other known satel-

An array of brightstreaks is visible inthis view of Rhea,Saturn’s secondlargest satellite.


Planetary satellites, including Saturn’s, are generally named after

figures in classical Greek or Roman mythology associated with the

namesakes of their primaries. Hyperion, Iapetus, Phoebe, Rhea and

Tethys, for example, were siblings of Kronos, the Greek counterpart

of the Roman god Saturn.

The satellites also carry scientific designations, which comprise the

first letter of the primary followed by a sequential Arabic numeral,

assigned in order of discovery: Mimas is S1, Titan is S2 and so on.

When satellites are first discovered, but not yet confirmed or offi-

cially named, they are known by the year in which they were

discovered, the initial of

the primary and a

number assigned con-

secutively for all solar

system discoveries.

So, Pan was first

called 1981S13

(although Pan

was discovered in

1990, the Voyager

image in which the

moon was found

was obtained

in 1981).

After planetary

scientists were

able to map

geological for-

mations of the

satellites from

spacecraft

images, they

named many

of the features after characters or locations from world mythologies.

The official names for all satellites and surface features are assigned

by the International Astronomical Union.

H O W T ON A M E

A M O O N

lites in the solar system are Lagrang-ians, the Trojan asteroids orbit in twoof the Lagrangian points of Jupiter.

Telescope observations showed thatthe surface of Hyperion, which liesbetween the orbits of Iapetus andTitan, is covered with ice. BecauseHyperion has a relatively low albedo,however, this ice must be mixed witha significant amount of darker, rockymaterial. The color of Hyperion issimilar to that of the dark side of Ia-petus and D-type asteroids: All threebodies may be rich in primitive mate-rial rich in organics. It is darker thanthe medium-sized, inner satellites,presumably because resurfacingevents have never covered it withfresh ice.

Although Hyperion is only slightlysmaller than Mimas, it has a highlyirregular shape, which along withthe satellite’s battered appearance,suggests that it has been subjectedto intense bombardment and frag-mentation. There is also good evi-dence that Hyperion is in a chaoticrotation, perhaps a collision withinthe last few million years knocked itout of a tidally locked orbit.

Saturn’s outermost satellite, Phoebe,a dark object with a surface composi-tion probably similar to that of C-typeasteroids, moves in a highly inclined,retrograde orbit, suggesting it is acaptured object. Voyager imagesshow definite variegations consistingof dark and bright (presumably icy)


patches on the surface. Although it issmaller than Hyperion, Phoebe hasa nearly spherical shape. With arotation period of about nine hours,Phoebe is the only Saturn satelliteknown to exhibit a simple, asynchro-nous rotation.

Pan, the 18th known satellite of Sat-urn, was discovered in 1990 in Voy-ager 2 images that were obtained in1981. This small object is embeddedwithin the A-ring and helps to clearthe Encke division of particles.

The Cassini–Huygens MissionThe two Voyager spacecraft providedthe first detailed reconnaissance ofthe Saturn system. They provided thefirst evidence that the satellites hadbeen geologically active after theirformation and that one icy satellite,Enceladus, may be still active. Thereare significant gaps in our knowledgeabout Saturn’s moons that can onlybe addressed by a more detailedobservational plan and sophisticatedinstruments. The Cassini mission is de-signed to undertake this endeavor.

The Cassini payload represents acarefully chosen suite of instrumentsthat will address the major scientificquestions surrounding Saturn’smoons, as follows:

First, what is the composition of thesurfaces of these satellites? Althoughground-based spectroscopic measure-ments showed water ice to be preva-lent on their surfaces, the existenceof additional volatiles, hydrates, clath-rates and impurities is a critical factorin understanding the satellites’ evolu-tion. Because many impurities lower

the freezing point of ice, their exist-ence could provide an explanationfor the satellites’ activity. Hydratedammonia, for example, could be thedriver for activity on Enceladus andDione, but its spectroscopic signatureis so subtly different from water ice,that only close reconnaissance byCassini’s Visible and Infrared Map-ping Spectrometer (VIMS) may beable to detect its presence.

In most cases, the satellites are toodim for detailed Earth-based spec-troscopic studies. Much of the keyspectroscopic evidence exists in theultraviolet and 3–20-micrometer re-gion, which is opaque to the terrestri-al atmosphere. The nature of the darkmaterial on the dark side of Iapetus,Hyperion, Phoebe and some

of the small satellites is mysterious.Is it unprocessed and primordial? Isthis material rich in organics and, assuch, is it related to the origin of life?Is it similar to the dark material foundon comets, some asteroids and othersatellites in the outer solar system?

Next, what is the detailed morpholo-gy of the satellites, and what is therelationship between geological struc-tures and compositional units? Witha synergistic payload offering an Im-aging Science Subsystem (ISS) thatis capable of tens of meters resolutionon the satellites and high-resolutionspectrometers in the 0.055–1000 mi-crometer region — the CompositeInfrared Spectrometer (CIRS), theUltraviolet Imaging Spectrograph(UVIS) and the VIMS — and comple-mentary radio-metric radar Ku-bandmeasurements, it will be possible tocorrelate geological units with spe-cific compositions.

This Voyager 2 imageof Iapetus shows bothbright and dark ter-rains on the moon.


For example, what is the detailed dis-tribution of dark material on Iapetus’craters, and what does that say aboutthe origin of the material? Are thebright wispy streaks of Dione andRhea rich in ammonia and thus plau-sibly formed from a hydrated ammo-nia slurry exuded onto the surface?What is the distribution of surficial im-purities, and can they offer an expla-nation for the varying degrees ofviscous relaxation on the satellites?Are the inner satellites coated withmaterial from the E-ring?

Third, are there compositional similar-ities between the rings, dust andsome of the satellites, and if so, doesthat imply they are interrelated? Arethe rings comminuted satellites? Doany of the dust or icy particles, whichwill be measured in detail by Cassi-ni’s Cosmic Dust Analyzer (CDA),appear to come from satellites? IsEnceladus the source of the E-ring?

Also, do any of the satellites havethin, tenuous atmospheres of molecu-

lar oxygen, OH or other material,similar to those recently found onJupiter’s moons Europa and Gany-mede? Cassini’s UVIS or possibly theISS should be able to detect suchatmospheres, which may in turnprovide material to Saturn’s mag-netosphere. If there is significant ero-sion of material from the satellites’surfaces (or in the case of Enceladus,expulsion), the Ion and Neutral MassSpectrometer (INMS), the CDA andthe Radio and Plasma Wave Scienceinstrument (RPWS) will study theircomposition, size and mass.

Fifth, are there any additional satel-lites? Are there any embedded in thering system? How do these satellitesinteract dynamically with the rings?High-resolution imaging by the ISSwill be able to detect kilometer-sizedbodies and resolve the issue of 10 orso unconfirmed observations of addi-tional satellites.

Further, what is the relationship be-tween Saturn’s magnetosphere andthe satellites? Are the satellites a sig-nificant source of magnetosphericparticles? Do any of the satelliteshave magnetic fields? Why doesDione modulate Saturn’s radio emis-sion? Is there a flux tube of some sortbetween the satellite and the plane-tary magnetic field? The Dual Tech-nique Magnetometer (MAG), theMagnetospheric Imaging Instrument(MIMI), the Cassini Plasma Spectrom-eter (CAPS), the Ion and NeutralMass Spectrometer (INMS) and theRPWS will be the key instruments foranswering these questions.

Seventh, is Enceladus still active?High-resolution imaging by the ISSwill detect any recent features, aswell as geysers or plumes. Furthercompositional analysis of the plumesand deposits will help to determinethe physical mechanism of the activityand its similarity to other activity onJupiter’s moon Io, Neptune’s moonTriton and Earth.

The small satellites ofSaturn. Clockwisefrom left: Atlas,Pandora, Janus,Calypso, Helene,Telesto, Epimetheusand Prometheus.


This artist’s renditionshows Saturn as itmight appear fromthe surface of its sec-ond largest satellite,Rhea.

Artist’s conceptionof Ithaca Chasma, ahuge trench on themoon Tethys.


Then, what are the internal structuresof the satellites? Did they fully differ-entiate? Close flybys of the satellitesentailing precise measurements bythe RSS of gravitational perturbationswill be able to determine which satel-lites have cores. Accurate measure-ments of the density of each satellite,coupled with spectroscopically deter-

mined compositional information,will yield determinations of their bulkdensities.

And finally, what was the origin ofPhoebe? Is it a captured Kuiper Beltobject or a more pedestrian asteroid?Is its surface material related to thedark material on either Iapetus orHyperion?

With four years to investigate Saturnand its icy satellites, the Cassini–Huygens mission hopes to shed somelight on the innumerable mysteriesthat remain about this solar systemgiant. Any answers that Cassini un-covers, of course, are equally likelyto lead to even more questions.

A S P H E R E O F I N F L U E N C E 6 7

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C H A P T E R 6

aturn, its moons and its awesome rings sit inside anenormous cavity in the solar wind created by the planet’sstrong magnetic field. This “sphere of influence” of Sat-urn’s magnetic field — called a magnetosphere — re-sembles a similar magnetic bubble surrounding Earth.

The region is not at all spherical; rather, the supersonicsolar wind, flowing at 300–1000 kilometers per secondagainst Saturn’s magnetic field, compresses the magneto-sphere on the side facing the Sun and draws it out into along magnetotail in the direction away from the Sun.

Inside the MagnetosphereInside Saturn’s vast magnetosphericbubble is a mixture of particles, in-cluding electrons, various speciesof ions and neutral atoms and mole-cules, several populations of very en-ergetic charged particles (like those inEarth’s Van Allen Belts) and chargeddust grains. The charged particlesand dust grains all interact with boththe steady and the fluctuating electric

and magnetic fields present through-out the magnetosphere.

These ionized gases contain chargedparticles (electrons and ions) such asoccur in the solar wind and planetarymagnetospheres and are called plas-mas. The steady fields can causeorganized motions of the chargedparticles, creating large currents inthe plasma.

Plasma behavior is more complexthan that of neutral gases because,unlike neutral particles, the chargedparticles interact with each otherelectromagnetically as well as withany electric and magnetic fieldspresent. The plasma’s fluctuatingfields (including wave fields) can“scatter” the charged particles ina manner similar to collisions in aneutral gas and cause a mixing ofall the magnetospheric components.

An artist’s rendi-tion of Saturn’simmense magneto-sphere. ΩS is theplanet’s rotationaxis, closelyaligned with themagnetic axis. [Im-age courtesy ofLos Alamos Na-tional Laboratory]

A Sphere of Influence

Solar Wind

Fast Neutrals

Pickup Ions

Solar Wind

Bow Shock

Ring Current

AuroraCusp Polar Wind

Titan Tail

Aurora

Titan NeutralTorus

Magnetopause

Magnetosheath

Plasma Sheet

Planetary Ions

SKR

H+, He++

H+, He++

H+, He++H+, H2

+, H3+

H+, H2+, H3

+

ΩS


Main Ring

F-Ring

G-Ring E-Ring

Rhea

H2O

TitanDioneTethys

Enceladus

Mimas

NeutralHydrogen Cloud

Most of what we know about Saturn’smagnetosphere comes from the briefvisits by Pioneer 11 and Voyagers 1and 2, but remote observations by theHubble Space Telescope and otherspacecraft have also provided us withintriguing information.

Magnetospheric Particle SourcesSaturn has a variety of sources for theparticles in its magnetosphere. Parti-cles can escape from any moon, ringor dust particle surface, or they canbe “sputtered” off by energetic parti-cles or even micrometeoroid impacts.

The primary particle sources arethought to be the moons Dione andTethys. But, the solar wind, iono-

sphere, rings, Saturn’s atmosphere,Titan’s atmosphere and the other icymoons are sources as well. RecentHubble Space Telescope results showlarge numbers of neutral hydrogenatoms (the neutral hydrogen cloud inthe illustration above) throughout themagnetosphere that probably comefrom a number of these sources. It haseven been proposed that water ions

and molecules may form a dense“ionosphere” above Saturn’s rings.

Recent Hubble Space Telescope re-sults show large numbers of neutralhydrogen atoms throughout the mag-netosphere that probably come froma number of the sources mentioned.Determining the relative importanceof the varied sources in differentparts of Saturn’s space environmentis a prime objective for the Magneto-spheric and Plasma Science (MAPS)instruments aboard the Cassinispacecraft.

Neutral particles can escape fromany moon, ring or dust particle sur-

cles can be created by processeswithin the magnetosphere or theycan leak in from the solar wind.These and many other magneto-spheric phenomena were seen bythe three earlier spacecraft.

The mysterious “spokes” in the ringsof Saturn, clearly seen in Voyagerimages, are probably caused by elec-trodynamic interactions between thetiny charged dust particles in therings and the magnetosphere. Auro-ras, which exist on Saturn as well asEarth, are produced when trappedcharged particles precipitating fromthe magnetosphere collide with atmo-spheric gases.

face — or they can be “sputtered”off by energetic particles or evenmicrometeoroid impacts. Whenthese particles become ionized, theycan excite electromagnetic waveswith a frequency that can be usedto determine their type. The icy ringsabsorb the energetic particles inwardof the moon Mimas. Energetic parti-

Despite many exciting discoveries,many more questions about the phys-ical processes in Saturn’s magneto-sphere remain unanswered. Thischapter examines the current stateof knowledge about Saturn’s mag-netosphere and discusses the obser-vations we expect to make withCassini’s instruments and the knowl-edge we expect to gain from forth-coming explorations.

Sources of par-ticles in Saturn’smagnetosphere.


The MAPS InstrumentsCoordinated observations are re-quired from all the Magnetosphericand Plasma Science (MAPS) instru-ments aboard Cassini to fully under-stand Saturn’s various dynamicmagnetospheric processes. TheCassini Plasma Spectrometer willmeasure in situ Saturn’s plasma popu-lations including measurements ofelectron and ion species (H+, H2

+,He++ , N+ , OH+, H2O

+, N2) and de-termine plasma flows and currentsthroughout the magnetosphere.

The Cosmic Dust Analyzer will makemeasurements of dust particles withmasses of 10–19–10–9 kilograms, de-termining mass, composition, electriccharge, velocity and direction of in-coming dust particles. Perhaps thisinstrument’s most important capabilitywill be measuring the chemical com-position of incoming dust particles,making it possible to relate individualparticles to specific satellite sources.

The Ion and Neutral Mass Spec-trometer will measure neutral speciesand low-energy ions throughout themagnetosphere and especially atTitan. The Dual Technique Magne-tometer will measure the strengthand direction of the magnetic fieldthroughout the magnetosphere. Thefirst ever global images of Saturn’shot plasma regions will be obtainedby the Magnetospheric Imaging In-strument, which will also measurein situ energetic ions and electrons.

The Radio and Plasma Wave Scienceinstrument will detect the radio andplasma wave emissions from Saturn’s

magnetosphere, which will tell usabout plasma sources and interac-tions in the magnetosphere. The Ra-dio Science Instrument will measurethe ionosphere of Saturn and searchfor ionospheres around Titan, the oth-er moons and the rings. The Ultravio-let Imaging Spectrograph will mapthe populations of atomic hydrogenand weak emissions from neutralsand ions including auroral emissions.

The Magnetic EnigmaSaturn’s magnetic field presents anenigma. Planetary fields such asthose of Earth and Saturn can beapproximated by a dipole, a simplemagnetic field structure with northand south poles, similar to that pro-duced by a bar magnet. Magneticfield measurements from the threeprevious flybys revealed a dipole-like field at Saturn with no (less thanone degree) measurable tilt betweenSaturn’s rotation and magnetic dipoleaxes. This near-perfect alignment ofthe two axes is unique among theplanets. The Earth and Jupiter havedipole tilts of 11.4 and 9.6 degrees,respectively. The polarity of Saturn’s

magnetic dipole, like Jupiter’s, is op-posite to that of Earth.

There is a general consensus that theinternal magnetic fields of the giantplanets arise from dynamo actionsomewhere inside the planets’ gas-eous atmospheres. Of course, we donot really know what is inside Saturnor where the field is generated, al-though we have a number of theo-ries. The inside of Saturn is probablyquite exotic because of the greatpressures caused by its large size.There may be a rocky (Earth-like) cen-ter with a molten core, but wrappedaround this core we would expect tofind layers of other uncommon materi-als (like liquid helium). The Saturn wesee with telescopes and cameras isreally only the cloud tops.

Although the measured field is sym-metrical about the rotation axis, anumber of observed phenomena canonly be explained by an asymmetryin the magnetic field. Two examplesare the occurrence of major emis-sions of Saturn kilometric radiation

Saturn’s aurora, im-aged in the far ultra-violet by the WideField and PlanetaryCamera 2 aboardthe Hubble SpaceTelescope. The au-rora (the bright re-gion near the pole) iscaused by energeticcharged particles ex-citing atoms in theupper atmosphere.


(SKR), the principal radio emissionfrom Saturn, at the presumed periodof the planetary rotation and a simi-lar variation in the formation of thespokes in the B-ring. The SKR obser-vations can be explained by a mag-netic anomaly in the otherwisesymmetric field of less than five per-cent of the field at Saturn’s surface(0.2 gauss), small enough to be im-perceptible at the closest approachdistances of the previous flybys of theVoyagers and Pioneer.

With magnetic field measurementsmade close to the planet over a widerange of latitudes and longitudes, theDual Technique Magnetometer onCassini will measure the details ofthe magnetic field and tell us moreabout Saturn’s interior. The magne-tometer will measure the strengthand direction of the magnetic fieldthroughout the magnetosphere, closeto the planet where the field is dipo-lar and further from the planet wherethe field is non-dipolar due to distor-tion by current systems. The magne-tometer will measure the field withsufficient accuracy to determine if itis indeed symmetrical. If so, the basictenets of dynamo theory may need tobe reexamined.

Solar Wind InteractionA planetary magnetosphere formswhen the magnetized solar wind (thesupersonic, ionized gas that flows ra-dially outward from the Sun) impingesupon a planet with a sufficiently largemagnetic field. Like Earth and the oth-er giant planets, Saturn has a strongmagnetic field and an extensive mag-netosphere. Although the morphologyand dynamics of planetary magneto-spheres vary according to the strengthand orientation of their internal fields,

magnetospheres share many com-mon features.

Because the solar wind flow is almostalways supersonic, a “bow shock”forms Sunward of the magneto-sphere. The bow shock heats, de-flects and slows the solar wind.Pioneer 11 made the first in situ mea-surements of Saturn’s bow shock in1979 when discontinuous jumps insolar wind parameters (magneticfield strength, density, temperature)were observed. Because of the vari-ation in characteristics of the solarwind with distance from the Sun,by the time the orbit of Saturn isreached, the average Mach number,which determines the strength of thebow shock, is quite large. The bowshock of Saturn is a high Mach num-ber shock similar to that of Jupiterand differs from the low Mach num-ber shocks of the terrestrial planets.Saturn’s bow shock provides aunique opportunity to study the struc-ture of strong astrophysical shocks.

The magnetopause marks the bound-ary of the magnetosphere, separat-ing the solar wind plasma andmagnetospheric plasma. Betweenthe bow shock and the magneto-pause is a layer of deflected andheated solar wind material formingthe magnetosheath. The boundariesmove in and out in response tochanging solar wind conditions. Theaverage distance to the nose of themagnetopause at Saturn is roughly20 RS (RS = one Saturn radius or

60,330 kilometers). These bound-aries, shown in the image on the firstpage of this chapter, are of interest inunderstanding how energy from thesolar wind is transferred to the planetto fuel magnetospheric processes.

Extensive observations of Earth’smagnetosphere have demonstratedthat solar wind energy is coupled intothe magnetosphere primarily througha process called magnetic reconnec-tion, in which field lines break andreconnect to change the magnetic to-pology. Similar processes must in-deed occur at Saturn. Given theproper relative orientation of inter-planetary and planetary magneticfields on the sunward side of themagnetosphere, the field lines recon-nect and a purely planetary magneticfield line (with both ends attached tothe planet) becomes a field line withone end attached to the planet andthe other end open to interplanetaryspace.

It is on these open field lines that format high Saturn latitudes that energeticparticles of solar, interplanetary orcosmic origin can enter the magneto-sphere. These regions of the mag-netosphere over the northern andsouthern poles are referred to as thepolar caps. The open field lines arethen pulled back by the drag of thediverted solar wind flow to make themagnetotail. Because charged parti-cles and magnetic field lines are “fro-zen” together, this drives a tailwardflow within the magnetosphere.

In the magnetotail, reconnectionagain occurs. Here, the magnetic


field reverses direction across thetail’s plasma sheet (a thin sheet ofplasma located approximately in theplanet’s equatorial plane, where cur-rents flow and particles are accelerat-ed. The process of reconnection andopening of field lines on the sunwardside of the magnetosphere is thus bal-anced by reconnection that closesfield lines in the magnetotail. Thenewly closed field lines contract backtoward the planet, pulling the plasmaalong and driving a circulation pat-tern, as shown in the figure below.

The process of reconnection on theSunward side of the magnetosphereis thus closely coupled to processesthat occur in the magnetotail. Theseprocesses are known to be strongly

affected by the changing conditionsin the solar wind. At Earth, reconnec-tion processes can give rise to large,erratic changes in the global configu-ration of the magnetosphere referredto as geomagnetic storms. Cassini’sMAPS instruments will investigate tosee if similar magnetospheric stormsoccur at Saturn.

Voyager 1 made the first direct mea-surement of Saturn’s magnetotail,finding it to resemble its terrestrialand Jupiter counterparts. The mag-netotail was detected to be roughly40 RS in diameter at a distance 25 RS

downstream; it may extend hundredsof Saturn radii in the downstream so-lar wind. Understanding the process-es that occur in the magnetotail isfundamental to understanding overall

magnetospheric dynamics; coordinat-ed measurements by the MAPS instru-ments during the deep tail orbitsplanned for the Cassini tour will con-tribute to that understanding. In turn,by understanding overall magneto-spheric dynamics, scientists will gaininsight into how Saturn’s magneto-sphere harnesses energy from the so-lar wind.

Current Magnetospheric SystemsVarious large-scale current systemsexist in Saturn’s magnetosphere dueto the collective motions of chargedparticles. Cross-tail currents flow fromdusk to dawn in the plasma sheet lo-cated near the center of the magneto-tail. An equatorial ring currentdistorts the magnetic field from its di-

S O L A R W I N D C I R C U L A T I O N

Solar Wind Reconnection

Magnetopause

Reconnection

Tailward Flow

Large-scale circula-tion driven by thesolar wind as it oc-curs at Earth. Ananalogous processoccurs at Saturn. Theorientations of mag-netic field lines andplasma flows areshown. When the in-terplanetary magneticfield is oriented south-ward, as shown, fieldlines reconnect at thenose of the magne-topause and thenagain in the magne-totail, driving theflows described inthe chapter text.


polar configuration, particularly in theouter magnetosphere where it stretch-es the magnetic field lines in the equa-torial plane. This ring current, causedby electrons and ions drifting aroundthe planet in opposite directions, isprobably primarily due to the ener-getic particles discussed later in thischapter. The effect of this ring currentis moderate when compared with Ju-piter, however. Another major contri-bution to Saturn’s total magnetic fieldcomes from currents flowing in themagnetopause, which result from in-teraction with the solar wind.

Cassini’s Dual Technique Magnetom-eter, measuring the magnetic field,and the Cassini Plasma Spectrometer,measuring the currents, will help mapthe current systems. These measure-ments, together with those taken bythe other Cassini plasma instruments,will allow scientists to make a globalmodel of Saturn’s magnetic fieldthroughout the magnetosphere.

Major Magnetospheric FlowsThere are two primary sources of en-ergy driving magnetospheric process-es: the planet’s rotation and the solarwind. Correspondingly, there are twotypes of large-scale plasma flow with-in the magnetosphere — corotationand convection. The nature of thelarge-scale circulation of particles inthe magnetosphere depends on whichsource is dominant. At Earth, the ener-gy is derived primarily from the solarwind; at Jupiter it is derived from theplanet’s rapid rotation rate. Saturn’smagnetosphere is especially interest-ing because it is somewhere in be-

tween: both energy sources shouldplay an important role.

Saturn’s ionosphere is a thin layer ofpartially ionized gas at the top of thesunlit atmosphere. Collisions betweenparticles in the atmosphere and theionosphere create a frictional dragthat causes the ionosphere to rotatetogether with Saturn and its atmo-sphere. The ionosphere, which ex-tends from 1500 kilometers abovethe surface (defined as the visiblecloud layer) to about 5000 kilome-ters, has a maximum density of about10,000 electrons per cubic centime-ter at about 2000–3000 kilometers.

The rotation of Saturn’s magneticfield with the planet creates a largeelectric field that extends into themagnetosphere. The electromagneticforces due to the combination of thiselectric field and Saturn’s magneticfield cause the charged magneto-spheric plasma particles to “corotate”(rotate together with Saturn and itsinternal magnetic field) as far out asRhea’s orbit (about nine RS).

Convection, the other large-scaleflow, is caused by solar wind pullingthe magnetic field lines toward thetail. This leads to a plasma flowfrom day side to night side on openfield lines and to a return flow fromnight side to day side on closed fieldlines (particularly near the equatorialplane).

On the dawn side, the corotationand convective flows will be in the

same direction, but on the dusk side,they are opposing flows. The interac-tion of these flows may be responsi-ble for some of the large variabilityobserved in the outer magnetosphere.While at present we can only specu-late about the consequences of theseplasma flow patterns, we may expectsome answers from investigations byCassini’s plasma instruments (espe-cially the Cassini Plasma Spectrome-ter and the Magnetospheric ImagingInstrument).

Magnetospheric Plasma RegionsSaturn’s magnetosphere can bebroadly divided in two parts: a fairlyquiet inner magnetosphere extendingto about 12 RS (beyond all moonsexcept Titan), and an extremely vari-able hot outer magnetosphere. Inboth regions, the plasma particlesare concentrated in a disk near theequatorial plane, where most plasmaparticle sources are located.

In their brief passages throughSaturn’s inner and outer magneto-spheres, Pioneer and both Voyag-ers passed through several differentplasma regions. The spacecraft ob-served a systematic increase in elec-tron temperature with distance fromSaturn, ranging from one electronvolt (equivalent to a temperature of11,600 kelvins) at four RS in the in-ner magnetosphere and increasingto over 500 electron volts in the outermagnetosphere.

The thickness of the plasma disk in-creases with distance from Saturn.Inside about four RS a dense (about100 per cubic centimeter) popula-


tion of low-energy ions and electronsis concentrated in a thin (less than0.5 RS) equatorial sheet. The low tem-perature is probably due to interac-tions with ring material; it has evenbeen proposed that water ions andmolecules may form a dense “iono-sphere” above Saturn’s rings.

In the inner magnetosphere, there isan oxygen-rich Dione–Tethys torus ex-

S A T U R N ’ S M A G N E T I C F I E L D

Saturn’s magneticfield. Field lines areshown for a dipolefield model (solidline) and a modelcontaining a dipoleplus a ring current(dashed line). Thestretching out of thefield lines due to thering current (shadedregion) is moderate.

tending from four to about eight RS,beyond Rhea’s orbit. The icy surfacesof Dione and Tethys and other moonsand rings in the magnetosphere arecontinually bombarded by both parti-cles and solar radiation. Water mole-cules released by the bombardmentform a disk-shaped cloud of watermolecules and fragments of these

molecules. The charged particle den-sity in this region is a few particlesper cubic centimeter and is com-posed of about 20 percent light ions(primarily hydrogen ions) and about80 percent heavy ions with massesbetween 14 and 18 (species suchas O+ and OH+).

In between Saturn’s inner torus andouter magnetosphere is an extended

8

6

4

2

2

4

6

8

Dis

tanc

e, S

atur

n ra

dii

10

10

2 10 12 14 16 18

Distance, Saturn radii

Voyager 2

Voyager 1 Pioneer 11

Dione

Enceladus

Mimas Tethys Rhea

4 6 8

Ring Current


Solar Wind

Magnetopause

Saturn

Corotation

Convection

M A J O R M A G N E T O S P H E R I C F L O W S

equatorial plasma sheet of chargedparticles with densities between 0.1and 2 particles per cubic centimeter.The inner edge of the sheet has “hot”(temperatures in the thousands of elec-tron volts) ions and coincides with avast cloud of neutral hydrogen, ex-tending to 25 RS, which probably es-caped from the moon Titan, and othersources as well. Possibly, the hot ionsare newly born ions from the neutralcloud that were heated by Saturn’srotational energy.

The Voyager spacecraft saw consider-able variability in both the chargedparticle density and temperature inthe outer magnetosphere on veryshort time scales. This has been inter-preted as “blobs” of hot plasma inter-spersed with outward moving coldplasma and may be pieces of theplasma sheet that have broken off.

The variability may also be due todense “plumes” of hydrogen or nitro-gen escaping from Titan that wraparound Saturn. Alternately, the varia-tions may be caused by fluctuationsin the solar wind, since both the outermagnetosphere and the magnetotailare thought to be the primary regionswhere solar wind energy enters themagnetosphere.

The dynamics, composition andsources of the outer magnetosphericplasma particles are not well under-stood. Investigation of this region isone important Cassini objective, sothe MAPS instruments will make coor-dinated observations in this region.Until Cassini determines the compo-sition here, the extent of the role ofTitan in Saturn’s outer magnetospherewill remain unknown.

In the inner region of the magneto-sphere, most of the particles “coro-tate” with the planet. The corotationspeed of charged particles differsfrom the speed of normal orbital mo-tion (determined by gravity). Beyondapproximately eight RS, the chargedparticles lag behind the corotationspeed by 10–30 percent. Here, thegravitational orbit velocity is muchslower than the corotation speed; thelag is probably due to new ions bornfrom the neutral hydrogen cloud orTitan’s atmosphere that have not yetbeen brought up to Saturn’s rotationrate (the corotation speed).

In the outer magnetosphere, theplasma rotation rate is about 30 per-cent lower than the corotation speed.When neutral particles from themoons or rings are ionized, theybegin to move relative to the other

Major flows in Sat-urn’s magnetosphere.The solar wind flowsin from the left; themagnetotail is to theright. Convection, atailward plasma flow,is caused by the solarwind dragging mag-netic field lines pastthe planet. Corota-tion, magnetospheric

rotation at the rate ofSaturn, is caused bythe corotation of Sat-urn’s ionosphere.


neutral particles because of the differ-ence in the orbital speed and thecorotation speed. Since these newions add to the mass of the corotatingplasma population, they can slow itdown, as suggested by the observa-tions. Outward motion of plasmafrom the inner magnetosphere mayalso contribute to slowing it down.Cassini’s MAPS instruments will inves-tigate the relative importance of thesetwo effects on the corotation rate inthe outer magnetosphere.

Energetic Particle PopulationsSaturn’s magnetosphere, like that ofother planets, contains populations

of highly energetic particles similarto those in Earth’s Van Allen radiationbelts (kilo electron volt to mega elec-tron volt energies). These particlesare trapped by Saturn’s strong mag-netic field.

In a uniform magnetic field, chargedparticles move in helical orbits alongmagnetic field lines. In Saturn’s dipo-lar magnetic field, the field strengthalong a field line increases towardthe planet. At some point determinedby the particle speed and the mag-netic field strength, the particle is “re-flected” or “mirrored” and it reversesdirection along the same field line.

R E G I O N S O F S A T U R N ’ S M A G N E T O S P H E R E

Regions of Saturn’smagnetosphere. Thevarious plasma re-gions — inner torus,extended plasma

sheet, variable out-er magnetosphere,etc. — are shownin relationship tothe location of the

moons and themagnetosheath.The temperature isindicated by thecolor scale, goingfrom cold (blue)

to hot (pink). Notethe asymmetry: Theleft side shows thenoon magnetosphere(that portion closestto the Sun). [Based onSittler, et al., 1983]

A “trapped” charged particle movesin such an orbit in Saturn’s field,bouncing back and forth along a sin-gle magnetic field line. The radiationbelts are made up of energetic parti-cles moving in such orbits. Collisionswith neutral particles or interactionswith the fluctuating electric and mag-netic fields in the plasma can changea charged particle’s orbit.

Voyager 2 data showed Saturn’smagnetosphere to be populatedlargely by low-energy (tens of elec-tron volts) electrons in the outer re-gions with more energetic electrons

Solar Wind Magnetosheath

Titan

DetachedPlasmaBlobs

ExtendedPlasmaSheet E-Ring

InnerPlasmaTorus

Mimas

Enceladus

Tethys

Dione RheaHotOuterMagnetosphere

Noon Dawn

1000

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1

ElectronTemperature(electron volts)


dominating further inward. Substantialfluxes of high-energy protons were ob-served inside the orbits of Enceladusand Mimas, forming the hard core ofthe radiation belts. Pioneer 11 investi-gators concluded that these protonsprobably originated from the interac-tion of cosmic rays with Saturn’s rings.

The origin of these and other ener-getic particles is unclear and will beinvestigated by Cassini’s MAPS instru-ments. In particular, the Magneto-spheric Imaging Instrument will makein situ measurements of energetic ionsand electrons. Some energetic ionssuch as helium and carbon mayoriginate in the solar wind, but othersmay come from lower energy parti-cles that are energized in Saturn’smagnetosphere.

The energetic particles drifting inthe dipole-like magnetic field create

Saturn’s magnetosphere. Cassini’sMagnetospheric Imaging Instrumentwill use these energetic neutral atomsas if they were photons of light tomake global images and study theoverall configuration and dynamicsof Saturn’s magnetosphere. The instru-ment will obtain the first global im-ages of Saturn’s hot plasma regionswith observations of features such asSaturn’s ring current and Titan’s hy-drogen torus. Cassini will be the firstspacecraft to carry an instrument toimage the magnetosphere using ener-getic neutral atoms.

Polar Region InteractionsAurora. Most energetic particlesbounce back and forth along fieldlines in trapped particle orbits. If,however, the mirror point is belowthe top of the atmosphere, the parti-cle can deposit its energy in the up-

the ring currents discussed above.Charged particles moving in trappedparticle orbits along dipole field linesalso drift in circles around the planet.Electrons and ions drift in oppositedirections and this causes the ringcurrent discussed previously in thischapter.

Measurements of energetic parti-cles indicate that the satellites ofSaturn play an important role inshaping their spatial distributions.In the inner region of the magneto-sphere, charged particles undergosignificant losses as they diffuse in-ward and are swept up by collisionswith the satellites.

Some of the energetic ions undergocollisions with the surrounding neutralgases that result in the exchange ofan electron, producing a populationof fast or energetic neutral atoms in

C H A R G E D P A R T I C L E O R B I T S

Charged particleorbits in a magneticfield. Left: in a uni-form field, chargedparticles are tied tofield lines and movealong them in heli-cal orbits. Right: ina dipole-like field,trapped chargedparticles move inhelical orbits alongfield lines, but atsome point “mirror”or “reflect,” leadingto a bounce motionalong the field line.Charged particles insuch trapped orbits

also drift in circlesaround the planet dueto the inhomogenousmagnetic field. Ions

drift in one directionand electrons in theother, leading to aring current that modi-fies the planetarymagnetic field.

MagneticField Line

UniformMagnetic Field

Drift of Ions Drift of Electrons

“Mirror” Point

Trajectory ofTrapped Particles

MagneticField Line

Dipole-LikeMagnetic Field


per atmosphere. Energetic particlesreaching the atmosphere create theauroral emission by exciting gases inthe upper atmosphere (molecular andatomic hydrogen lines in the case ofSaturn; oxygen and nitrogen inEarth’s atmosphere).

Saturn’s aurora was first detected bythe Voyager ultraviolet spectrometer.While is it not clear which magneto-spheric particles (electrons, protonsor heavy ions) create the aurora, itis clear that planets with higher fluxesof energetic particles have strongerauroral emissions. Cassini’s MAPSinstruments will make coordinatedstudies of Saturn’s aurora, with theUltraviolet Imaging Spectrometer pro-viding images.

Saturn Kilometric Radiation. For about20 years prior to the Voyager visits toSaturn, radio astronomers had beensearching for Saturn’s radio emis-

sions. We now know that Saturn isa much weaker radio source thanJupiter. Confirmation of radio emis-sions from Saturn came only whenVoyager 1 approached within threeastronomical units of the planet.

Saturn emits most strongly at kilo-metric wavelengths. Like the radioemission of other planets, Saturn kilo-metric radiation (SKR) comes from theauroral regions of both hemispheresand the radio beams are fixed in Sat-urn’s local time. However, the emit-ting regions are on the night side forEarth and on the dayside for Saturn.The emission appears to come fromlocalized sources near the poles —one in the north and one in the south— that “light up” only when theyreach a certain range of local timesnear Saturn’s noon.

The periodicity of these emissions isabout 10 hours, 39 minutes, assumedto be the rotation rate of Saturn’s con-ducting core. This is somewhat longerthan the atmospheric rotation rate of10 hours, 10 minutes observed at thecloud tops near the equator. The peri-odicity in SKR emission is unexpectedfor a planet with such a symmetricalmagnetic field. Possibly, a magneticanomaly exists that allows energeticelectrons to penetrate further downinto the polar region (at some point)and here the SKR radiation is gener-ated at the electron’s natural fre-quency of oscillation.

Based on the local time of emission,the source energy for the SKR ap-pears to be the supersonic solar windand, in fact, changes in the solarwind strongly control the SKR power.For example, a solar wind pressureincrease by a factor of about 100results in an increase by a factor of

Energetic neutral im-aging. Simulation ofan energetic neutralatom (ENA) image ofthe type that will beobtained by Cassini’sMagnetospheric Im-aging Instrument.The Saturn magneto-sphere appears closeto the center of theimage and Titan ison the left.


about 10 in the SKR power. For aperiod of about two to three daysfollowing the Voyager 2 encounter,no SKR emission was detected. It isthought that since Saturn was im-mersed in Jupiter’s long magnetotailat this time, the planet’s magneto-sphere was shielded from the solarwind. One of the main objectives ofthe Radio and Plasma Wave Scienceinstrument aboard Cassini is to makemeasurements of the SKR, study itsvariation with variations in the solarwind and map the source region.

Other MagnetosphericEmissionsWe are all familiar with waves in avacuum (electromagnetic waves) andwaves in a gas and fluid (electromag-netic waves, sound waves, gravitywaves). A magnetized plasma sup-ports all these waves and more.Because of the electromagnetic inter-actions between charged plasmaparticles and the magnetic field, newtypes of waves can propagate thathave no counterpart in a neutral gasor fluid. Waves in the magnetospherecan be produced via various pro-cesses, for example by ionizationof atmospheric neutral atoms in the

magnetospheric plasma or by cur-rents flowing between differentplasma populations.

These waves, as well as other typesof waves (Alfvén waves, magnetoson-ic waves and ion and electron cyclo-tron waves, to name a few) canpropagate in a plasma and be de-tected by sensors such as the mag-netic and electrical antennas ofCassini’s Radio and Plasma WaveScience instrument. These waves aretrapped within the magnetosphereand thus can only be sampled insideit. Saturn produces a variety of radio

T H E F O U R T H S T A T E O F M A T T E R

Plasma is the fourthstate of matter. Aplasma is an ion-ized gas containingnegatively chargedelectrons and posi-tively charged ionsof a single or manyspecies; it may alsocontain neutral parti-cles of various spe-cies. Examples arethe Sun, the super-sonic solar wind,Earth’s ionosphereand the interstellarmaterial. Plasmasbehave differentlyfrom neutral gases;the charged parti-cles interact witheach other electro-

magnetically and withany electric and mag-netic fields present.The charged particlesalso create and mod-ify the electric andmagnetic fields. Ina highly conductingplasma, the magneticfield lines move with(are “frozen to”) theplasmas. The X-rayimage here showsa million-degreeplasma, the solarcorona, which is thesource of the super-sonic solar windplasma that pervadesthe solar system. [Im-age from the Yohkohsatellite]


and plasma wave emissions fromnarrow (single frequency) and burstyto broadband (several frequencies)and continuous. The primary goalof the RPWS instrument is to studythese wave emissions. As mentioned,neutral atoms from various sourcessupply Saturn with magnetosphericplasma. As they do, they leave a“signature” in the plasma wavesthat can be used to determine theirspecies.

Emissions in magnetospheres arewaves of the plasma driven to largeamplitudes by magnetospheric pro-cesses that tap some reserve of free

energy. There are many modes, inter-actions and energy reserves; theemissions are studied to help discov-er the interactions and energy sourc-es driving them.

Free Energy Sources. We have seenhow energetic particles from the solarwind are one source of energy. Bothnonuniform and nonthermal plasmadistributions represent additionalsources of free energy. Generally,waves that grow at the expense of anonthermal or nonuniform feature in-teract back on the plasma distributionto try to eliminate the nonuniform or

nonthermal feature. For example, thePioneer 11 magnetometer saw low-frequency waves associated with Di-one; these have been interpreted asion cyclotron waves, apparently reso-nant with oxygen ions. These waveswere probably generated by newlyborn oxygen ions, created fromDione’s ice as a sputtering product,interacting with the corotating mag-netosphere plasma and tapping theenergy in the plasma rotation.

The waves generated by these newions then act to thermalize their high-ly nonthermal distribution. A modula-tion of the radio emission was also

The “spokes” in Sat-urn’s rings were firstseen from Earth, butVoyager observationsallowed the first studyof how these surprisingfeatures evolve. Thespokes are cloud-likedistributions of mi-crometer-sized parti-cles that occasionallyappear in the regionfrom approximately1.75 RS to 1.9 RS. Voy-ager saw spokes formradially over thou-sands of kilometers inless than five minutes.Subsequent Keplerianmotion (motion due togravity) changes thesespokes into “wedges.”The images shownhere form a time se-

quence from upper leftto lower right. Mostlikely, these nonradialfeatures result frominteractions of tinycharged ring dust par-ticles with the electro-magnetic fields and/orcharged particles inthe magnetosphere.Moreover, the spokesoccur preferentially atthe same longitudeand the same periodic-ity as the Saturn kilo-metric radiation, albeitat different local times,suggesting a relation-ship to the magneticanomaly. Cassini’sMAPS and imaginginstruments will makecoordinated studies ofthe formation and evo-lution of the spokes.

T H O S E S U R P R I S I N G S P O K E S


associated with the orbital phase ofDione, raising the possibility that Di-one is venting gases. Plasma wavescan also scatter particles into orbits,taking them down into the upper at-mosphere, where they drive auroralprocesses. Although the RPWS instru-ment is the primary detector of plas-ma waves, the causes and effects ofthe plasma waves are seen in mea-surements by Cassini’s other MAPSinstruments and coordinated observa-tions of wave phenomena will be im-portant in understanding the sourcesand sinks of magnetospheric plasmaand dynamic processes in general.

Atmospheric Lightning. Lightning inSaturn’s atmosphere is thought tocause the unusual emissions desig-

nated Saturn electrostatic discharges(SED). These are short, broadbandbursts of emission apparently comingfrom very localized regions (presum-ably atmospheric storms). It was de-termined that the source acts like asearchlight and is not fixed relativeto the Sun, as is the case for SKRemissions.

A 10 hours, 10 minutes periodicitywas seen in the emissions by Voy-ager 1, quite different from the10 hours, 39 minutes periodicity ofthe SKR emissions. From Voyager im-

C A S S I N I ’ S M A P S I N S T R U M E N T S

Instrument Objective

Cassini Plasma Spectrometer Measures composition, density, velocity and temperature of ions and electrons

Cosmic Dust Analyzer Measures flux, velocity, charge, mass and composition of dust and ice

particles from 10–16–10–6 grams

Dual Technique Magnetometer Measures the direction and strength of the magnetic field

Ion and Neutral Mass Spectrometer Measures neutral species and low-energy ions

Magnetospheric Imaging Instrument Images Saturn’s magnetosphere using energetic neutral atoms, and mea-

sures the composition, charge state and energy distribution of energetic

ions and electrons

Radio and Plasma Wave Science Measures wave emissions as well as electron density and temperature

Radio Science Instrument Measures the density of Saturn’s ionosphere

Ultraviolet Imaging Spectrograph Measures ultraviolet emissions to determine sources of plasma in Saturn’s

magnetosphere

aging results, the rotational periodof the equatorial cloud tops had alsobeen measured at 10 hours, 10 min-utes, consistent with the interpretationof the source as lightning. Cassiniwill further investigate the nature ofthese bursts, which give potential in-sight into Saturn’s atmospheric pro-cesses, the planet’s “weather.” TheCassini RPWS instrument will makemeasurements of SKR emissions,electromagnetic emissions fromlightning and SED as well. Measure-ments by the Cassini MAPS instru-ments will enhance an understandingof Saturn’s complex and fascinatingmagnetosphere.

P L A N N I N G A C E L E S T I A L T O U R 8 1

S

C H A P T E R 7

pace mission design aims to maximize science returnwithin the constraints and limitations imposed by the lawsof nature and the safe, reliable operation of the space-craft. Major mission design variables that may influencescience return from the Cassini–Huygens mission are de-sign and selection of the interplanetary trajectory, design

and selection of spacecraft orbits and the use of thespacecraft and instruments in making observations andreturning data to Earth. This chapter discusses the roleof planetary swingbys in achieving the Cassini–Huy-gens trajectory to Saturn, along with the spacecraft’sactivities during this time.

The Interplanetary MissionThe Cassini–Huygens mission uses tra-jectories requiring planetary swingbysto achieve the necessary energy andorbit shaping to reach Saturn. Theprimary trajectory for Cassini is aVenus–Venus–Earth–Jupiter GravityAssist (VVEJGA) transfer to Saturn.

As the name implies, the VVEJGAtrajectory makes use of four gravity-

assist planetary swingbys betweenlaunch from Earth and arrival atSaturn. The use of planetary gravityassists reduces launch energy require-ments compared to other Earth–Sat-urn transfer modes, and allows thespacecraft to be launched by theTitan IVB/Centaur. Direct Earth–Sat-urn transfers with this launch vehicleare not possible for Cassini–Huygens.

T H E E A S Y W A Y T O F L Y

The Venus–Venus–Earth–Jupiter GravityAssist (VVEJGA) tra-jectory that Cassini–Huygens will takerequires a deepspace maneuver be-tween the two Venusswingbys. This ma-neuver reduces thespacecraft orbit peri-helion (the closestpoint with respect tothe Sun) and places iton the proper courseto encounter Venusfor a second time inJune 1999. After theEarth swingby in Au-gust 1999, the Cas-

sini–Huygens space-craft will be on itsway to the outerplanets, flying byJupiter in late Decem-ber 2000. The fortu-itous geometry ofVVEJGA provides theunique opportunityof a double gravity-assist swingby, Ve-nus 2 to Earth, within

56 days, reducing thetotal flight time to Sat-urn to under sevenyears. The scientificinformation obtainedduring the interplane-tary cruise phase is

limited primarily togravitational wavesearches during threesuccessive Sun oppo-sitions, beginning inDecember 2001.

The nominal launch period of the pri-mary mission opens on October 6,1997, and closes on November 4,1997, providing a 30-day launch pe-riod. A contingency launch period isextended beyond the nominal launchperiod to November 15, 1997, to in-crease the chances of mission success— although possibly degrading tosome extent the scientific accomplish-ments of the nominal mission. The

Venus SwingbyApril 21, 1998

Venus SwingbyJune 22, 1999

Earth SwingbyAugust 17, 1999

Deep SpaceManeuverJanuary 20, 1999

Venus OrbitEarth Orbit

Jupiter SwingbyDecember 30,2000

Saturn ArrivalJuly 1, 2004

Planning a Celestial Tour

Launch from EarthOctober 6, 1997


Milestone Date

Launch 10/06/1997

Venus 1 Swingby 04/21/1998

High-Gain Antenna Opportunity 12/16/1998 – 01/10/1999

Deep Space Maneuver 01/20/1999

Venus 2 Swingby 06/22/1999

Earth Swingby 08/17/1999

Asteroid Belt Crossing 12/12/1999 – 04/10/2000

High-Gain Antenna Earth-Pointing 02/01/2000

Jupiter Swingby 12/30/2000

Gravity Wave Experiment 11/26/2001 – 01/05/2002

Conjunction Experiment 06/06/2002 – 07/06/2002

Cruise Science On 07/02/2002

Gravity Wave Experiment 12/06/2002 – 01/15/2003

Phoebe Flyby 06/11/2004

Saturn Orbit Insertion 07/01/2004

Periapsis Raise 09/25/2004

Probe Separation 11/06/2004

Orbiter Deflection 11/08/2004

Probe Titan Entry 11/27/2004

Titan 1 Flyby 11/27/2004



Enceladus 1 Flyby 03/09/2005



Enceladus 2 Flyby 07/14/2005


Titan Flybys Continue 2005 – 2008

Mission Ends 07/01/2008

opening and closing of the nominallaunch period are chosen such thatthe launch vehicle’s capabilities arenot exceeded and the mission perfor-mance and operational requirementsare met.

After launch, the trajectory is con-trolled through a series of trajectorycorrection maneuvers designed to cor-rect errors in the planetary swingbys.Each planetary swingby has the effectof a large maneuver on the trajectoryof the spacecraft. For the prime trajec-tory, the four swingbys can supply theequivalent of over 20 kilometers persecond of Sun-relative speed gains —an amount not achievable using con-ventional spacecraft propulsion.

Typically, interplanetary swingbys arecontrolled using two approach ma-neuvers and one departure maneuver,with additional maneuvers added atcritical points. About 20 maneuverswill be needed to deliver the space-craft from launch to Saturn. The con-trollers’ knowledge of the actuallocation of the spacecraft is obtainedusing measurements made from theDeep Space Network. This system ofmaneuvers is capable of deliveringthe spacecraft to various planetaryswingbys with an accuracy varyingfrom about five kilometers at Earth to150 kilometers at Jupiter. The final ap-proach to Saturn is predicted to havean accuracy of about 30 kilometers.

Huygens Probe MissionBased on the primary launch opportu-nity, Cassini–Huygens will arrive atSaturn on July 1, 2004. On arrival,the Orbiter will make a close flybyand execute a Saturn orbit insertion

M A J O R M I S S I O N M I L E S T O N E S

C A S S I N I – H U Y G E N S


(SOI) propulsive maneuver to initiatea highly elliptical, 148-day orbitaround the planet. This orbit willset up the geometry for the first en-counter with Titan for the HuygensProbe mission, currently planned forNovember 27, 2004.

On November 6, 2004, approxi-mately 22 days before the first Titanflyby, the Huygens Titan Probe willbe released from the Orbiter. TheOrbiter will turn to orient the Probeto its entry attitude, spin it up to about7.29 revolutions per minute and re-lease it with a separation velocity ofabout 0.33 meter per second. Atleast two navigational maneuvers willbe performed before separation toensure accurate targeting for atmo-spheric entry. Two days after separa-tion, the Orbiter will perform anOrbiter deflection maneuver (ODM)to ensure that the Orbiter will not fol-low the Probe into Titan’s atmosphere,and to establish the proper geometryfor the Probe data relay link.

The Orbiter is targeted to similar aim-point conditions at the second Titanflyby to permit a contingency Probemission opportunity if anything pre-vents the Probe from being deliveredon the first flyby.

Following completion of the predicteddescent, the Orbiter will continue tolisten to the Probe for 30 minutes, inthe event the Probe transmissions con-tinue after landing. The longest pre-dicted descent time is 150 minutes.

When Probe data collection is com-pleted, that data will be write-protect-ed on each of the Orbiter’s solid-staterecorders. The spacecraft will thenturn to view Titan with optical remote-sensing instruments, until about onehour after closest approach.

Soon after closest approach, the Or-biter will turn the high-gain antennatoward Earth and begin transmittingthe recorded Probe data. The com-plete, four-fold redundant set of Probedata will be transmitted to Earthtwice, and its receipt verified, beforethe write protection on that portionof the recorder is lifted by groundcommand — marking Probe missioncompletion.

Saturn Orbiter TourThe tour phase of the mission will be-gin at Probe mission completion andends four years after the SOI. Thereference tour described here, calledTour T18-3, consists of 74 orbits ofSaturn with various orientations,orbital periods ranging from sevento 155 days and Saturn-centered pe-riapsis radii ranging from about 2.6to 15.8 RS (Saturn radii).

Orbital inclinations with respect toSaturn’s equator range from zero to75 degrees, providing opportunitiesfor ring imaging, magnetosphericcoverage and radio (Earth), solarand stellar occultations of Saturn,Titan and the ring system.

Shown here is theCassini–Huygensflight path for the ap-proach to Saturn, theSaturn orbit insertionmaneuver, the initialSaturn orbit and theapproach to the firstTitan encounter. Apropulsive maneuver— called the periap-sis raise maneuver— during the initialorbit, near apoapsis(the farthest point

from Saturn), raisesperiapsis (the nearestpoint to Saturn in theorbit) to correctly tar-get the Orbiter for thefirst Titan flyby. If anyproblem arises withthe Orbiter, Probe orground system thatprevents execution of

the Probe mission atthe first Titan flyby,mission controllerscan decide to havethe spacecraft fly byTitan without releas-ing the Probe, delay-ing the Probe’smission till the second

Titan flyby on thesecond orbit of Sat-urn. The secondTitan flyby is cur-rently planned forJanuary 14, 2005.

Titan Probe Entryand Orbiter FlybyNovember 27, 2004

Probe ReleaseNovember 6, 2004

Saturn Orbit InsertionJuly 1, 2004

J U S T D R O P P I N G B Y


CleanupManeuver

Apoapsis

TargetingTitan

Sun

A total of 43 Titan flybys occur duringthe reference tour. Of these, 41 haveflyby altitudes less than 2500 kilome-ters and two have flyby altitudesgreater than 8000 kilometers. Titanflybys are used to control the space-craft’s orbit about Saturn as well asfor Titan science acquisition. Our ref-erence tour also contains seven closeflybys of icy satellites and 27 addi-tional distant flybys of icy satelliteswithin 100,000 kilometers.

Close Titan flybys could make largechanges in the Orbiter’s trajectory.For example, a single close flyby ofTitan can change the Orbiter’s Saturn-

The Cassini–Huygenstour is navigated witha combination of Dop-pler data and imagesagainst a star back-ground. Each swing-

spacecraft will per-form about 135 ma-neuvers. Shown hereis a three-maneuversequence to controlthe trajectory from oneTitan encounter to thenext. The “cleanup”maneuver corrects forerrors in the previousflyby. The sequenceuses between eightand 11 meters persecond, per encoun-ter. The plan is to al-low some variation inthe actual Titan swing-by conditions that willreduce the amount ofpropellant required toaccomplish the tour.The use of both radio-metric and opticaldata results in a Titandelivery accuracy of10 kilometers or less.

770-meters-per-sec-ond spacecraft ma-neuver. (The icysatellites are toosmall to significantlymodify the trajectory.)In a tour with 45 tar-geted encounters, the

relative velocity by hundreds ofmeters per second. For comparison,the total such change possible fromthe Orbiter’s main engine and thrust-ers is about 500 meters per secondfor the entire tour. The limited amountof propellant available for tour oper-ations is used only to provide smalltrajectory adjustments necessary tonavigate the Orbiter, or to turn it inorder to obtain science observationsor to communicate with Earth.

Titan is the only satellite of Saturnmassive enough to use for orbit con-trol during a tour. The masses of theother satellites are so small that even

close flybys (within several hundredkilometers) can change the Orbiter’strajectory only slightly. Consequently,Cassini tours consist mostly of Titanflybys. This places restrictions on howthe tour must be designed. Each Titanflyby must place the Orbiter on a tra-jectory that leads back to Titan. TheOrbiter cannot be targeted to a flybyof a satellite other than Titan unlessthe flyby lies almost along a returnpath to Titan. Otherwise, since thegravitational influence of the othersatellites is so small, the Orbiter willnot be able to return to Titan — and

S T E E R I N G B Y T H E S T A R S

by of Titan modifies thespacecraft’s trajectory— on average, eachTitan encounter will pro-vide the equivalent of a


the tour cannot continue. Of course,the large number of Titan flybys willproduce extensive coverage of Titan.

Tour Terminology“Rev” numbers ranging from 1 to 74are assigned to each tour orbit, as-suming each orbit begins at apoap-sis. The partial orbit from SOI to thefirst apoapsis is Rev 0. The rev num-ber is incremented at each succeed-ing apoapsis. Two or more revsabout Saturn may occur between suc-cessive Titan flybys; therefore, thereis no correspondence between revnumber and flyby number. Titan fly-bys are numbered consecutively, asare the targeted flybys of icy satel-lites. For example, the first flyby ofEnceladus is Enceladus 1; the first ofRhea is Rhea 1.

“Orbit orientation” is the angle mea-sured clockwise at Saturn from theSaturn–Sun line to the apoapsis. Thisis an important consideration for ob-servations of Saturn’s magnetosphereand atmosphere. The time availablefor observations of Saturn’s lit sidedecreases as the orbit rotates towardthe anti-Sun direction. Arrival condi-tions at Saturn fix the initial orienta-tion at about 90 degrees. Due to themotion of Saturn around the Sun, theorbit orientation increases with time,at a rate of orientation of about onedegree per month, which over thefour-year tour results in a total rota-tion of about 48 degrees clockwise(as seen from above Saturn’s northpole). Period-changing targeted fly-bys that rotate the line of apsides (or-bital points nearest or farthest fromthe center of Saturn) may be used to

add to or subtract from this drift in or-bit orientation.

The “petal” plot on the facing pageshows how targeted flybys combinewith orbit drift to rotate the orbit fromthe initial orientation clockwise mostof the way around Saturn to near theSun line. In the coordinate systemused in this diagram, the direction tothe Sun is fixed. Encounters of satel-lites occur either inbound (before Sat-urn-centered periapsis) or outbound(after Saturn-centered periapsis).

A “targeted flyby” is one where theOrbiter’s trajectory has been de-signed to pass through a specifiedaimpoint (latitude, longitude, altitude)at closest approach. At Titan, theaimpoint is selected to produce a de-sired change in the trajectory usingthe satellite’s gravitational influence.At targeted flybys of icy satellites, theaimpoint is generally selected to opti-mize the opportunities for scientificobservations, since the gravitationalinfluence of those satellites is small.However, in some cases the satel-lite’s gravitational influence is greatenough to cause unacceptably largevelocity-change penalties for a rangeof aimpoints, which makes it neces-sary to constrain the range of allow-able aimpoints to avoid the penalty.

If the closest approach point duringa flyby is far from the satellite, orif the satellite is small, the gravita-tional effect of the flyby can be smallenough that the aimpoint at the flybyneed not be tightly controlled. Suchflybys are called “nontargeted.” Fly-bys of Titan at distances greater than25,000 kilometers — as well as fly-

bys of satellites other than Titan atdistances of greater than a few thou-sand kilometers — are considerednontargeted flybys.

Flybys of satellites other than Titan atdistances up to a few thousand kilo-meters must be treated as targetedflybys to achieve science objectives,even though the satellite’s gravita-tional influence is small. Opportuni-ties to achieve nontargeted flybys ofsmaller satellites will occur frequentlyduring the tour and are important forglobal imaging.

If the transfer angle between two fly-bys is 360 degrees (that is, the twoflybys occur with the same satellite atthe same place), the orbit connectingthe two flybys is called a “resonantorbit.” The period of a Titan-resonantorbit is an integer multiple of Titan’sorbital period. The plane of the trans-fer orbit between any two flybys isformed by the position vectors of theflybys from Saturn.

If the transfer angle is either 360 de-grees (that is, the two flybys occurwith the same satellite at the sameplace) or 180 degrees, an infinitenumber of orbital planes connects theflybys. In this case, the plane of thetransfer orbit can be inclined signifi-cantly to the planet’s equator. Anyinclination can be chosen for thetransfer orbit, as long as sufficientbending is available from the flybyto get to that inclination. If a space-craft’s orbital plane is significantlyinclined to the equator, the transferangle between any two flybys form-


ing this orbital plane must be nearly180 or 360 degrees.

If the angle between the positionvectors is other than 180 or 360 de-grees — as is usually the case — theorbital plane formed by the positionvectors of the two flybys is unique andlies close to the satellites’ orbitalplanes (except for Iapetus), which areclose to Saturn’s equator. In this case,the orbit is “nonresonant.” Nonreso-nant orbits have orbital periods thatare not integer multiples of Titan’speriod. Nonresonant Titan–Titantransfer orbits connect inbound Titanflybys to outbound Titan flybys, or viceversa.

General Tour StrategyThis section contains specific descrip-tions of the segments in the referencetour T18-3.

Titan 1–Titan 2. The first three Titanflybys reduce orbital period and incli-nation. The Orbiter’s inclination is re-duced to near zero with respect toSaturn’s equator only after the thirdflyby; so, these three flybys mustall take place at the same place inTitan’s orbit. The period-reducing fly-bys were designed to be inbound,rather than outbound, to accomplishthe additional goal of rotating theline of apsides counterclockwise. Thismoves the apoapsis toward the Sunline in order to provide time for ob-servations of Saturn’s atmosphere,and to allow Saturn occultationson subsequent orbits to occur at dis-tances closer to Saturn.

Titan 3. After the inclination has beenreduced to near Saturn’s equator, atargeted inbound flyby of Enceladusis achieved on the fourth orbit on the

way to an outbound flyby of Titanon orbit five on March 31, 2005.Changing from an inbound to an out-bound Titan flyby here orients the lineof nodes nearly normal to the Earthline. This minimizes the inclinationrequired to achieve an occultationof Saturn, preparing for the series ofnear-equatorial Saturn and ring occul-tations that follows.

A Titan flyby occurring normal tothe Earth line can be inbound or out-bound (like any Titan flyby). For Titanflybys occurring nearly over the dawnterminator as in the reference tour,the spacecraft is closer to Saturn dur-ing the occultation if the Titan flybyis outbound than if it is inbound. Thescience return from the occultationis much greater if the spacecraft isclose to Saturn than if it is far away.

In particular, the antenna “footprint”projected on the rings is smaller whenthe occultation occurs closer to Sat-urn, improving the spatial resolutionof the “scattered” radio signal obser-vations. This is an important influenceon the design of the tour. Loweringinclination to the equator, switchingfrom inbound to outbound Titan flybysand rotating the orbit counterclock-wise near the start of the tour all helpkeep the spacecraft close to Saturnduring the subsequent series of equa-torial occultations.

Titan 4–Titan 7. Here, the minimuminclination required to achieve equa-torial occultations is about 22 de-grees. The two outbound flybys onMarch 31 and April 16, 2005, in-

M A I N C H A R A C T E R I S T I C S O F T O U R S E G M E N T S

Titan Flyby Comments

T1–T2 Reduce period and inclination, target for Probe mission.

T3 Rotate counterclockwise and transfer from inbound to outbound.

T4–T7 Raise inclination for eight equatorial Saturn–ring occultations and loweragain to equator.

T8–T15 Rotate clockwise toward anti-Sun direction.

T16 Increase inclination and rotate for magnetotail passage.

T17–T31 180-degree transfer sequence (including several revolutions for ring obser-vations).

T32–T34 Target to close icy satellite flybys of Enceladus, Rhea, Dione and Iapetus.

T35–T43 Increase inclination to 71 degrees (maximum value in tour).


crease inclination to this value. Thesecond of these also changes the pe-riod to 18.2 days. At this period,seven orbiter revolutions and eightTitan revolutions are completed be-fore the next Titan flyby, producingseven near-equatorial occultations ofEarth by Saturn (one on each orbit).On all eight of these revolutions, theOrbiter crosses Saturn’s equator nearEnceladus’ orbit; on the fourth revolu-tion, the second targeted flyby ofEnceladus occurs. Enceladus’ gravityis too weak to displace inclinationsignificantly from the value requiredto achieve occultations. The Titan fly-

bys on August 22 and September 7,2005, reduce inclination once againto near Saturn’s equator.

Titan 8–Titan 15. After inclination isreduced and the spacecraft’s orbitalplane again lies near Saturn’s equa-tor, a series of alternating outbound/period-reducing and inbound/period-increasing flybys — lasting about10 months — is used to rotate theorbit clockwise toward the magneto-tail. The first flyby in this series occurson September 26, 2005, and the lastoccurs on June 1, 2006.

Titan 16. After rotating the orbit toplace apoapsis near the anti-Sun line,

inclination is raised to about 10 de-grees with one flyby on July 6, 2006,to achieve passage through the cur-rent sheet in the magnetotail region.At distances this far from Saturn, thecurrent sheet is assumed to be sweptaway from Saturn’s equatorial planeby the solar wind. After this flyby,apoapsis distance is about 49 RS,exceeding the 40 RS magnetosphericand plasma science requirement as-sociated with magnetotail passage.Besides providing a magnetotail pas-sage, this inclination-raising flyby isthe first of a sequence of flybys thatmakes up the 180-degree transfersequence described next.

This view from aboveSaturn’s north poleshows all possibleorbits in a rotatingcoordinate system, inwhich the Sun direc-tion is fixed, for a pos-sible Saturn systemtour referred to bymission designers as“Tour T18-3.” Thistype of diagram is of-ten referred to as a“petal” plot due to itsresemblance to thepetals of a flower.The broad range of

orbit orientationsmade possible by thismission design allowsa detailed survey ofSaturn’s magneto-sphere and atmo-sphere. Whateverthe final tour lookslike, Cassini’s jauntaround the Saturnsystem begins July 1,2004 and continuestill July 1, 2008.

Sun

O N A F O U R - Y E A R T O U R


Titan 17–Titan 31. This series of flybyscompletes a 180-degree transfer se-quence. The first several flybys of thissequence — all inbound — are usedto raise inclination as quickly as possi-ble using the minimum altitude of950 kilometers at each flyby. The Ti-tan flyby of August 22, 2006, reduc-es the period to 16 days, as well asraising inclination. The period is thenkept constant at 16 days as inclina-tion is raised, except during aninterval of 48 days between flybyson October 25 and December 12,2006. The flyby on October 25,2006, reduces the period to 12 days(a resonance of three Titan revs for ev-ery four spacecraft revs) in order toprovide extra spacecraft revs betweenTitan flybys for observing the rings,for which the geometry is particularlyfavorable at this point in the tour. Theflyby on December 12, 2006, in-creases the period back to 16 days.

As inclination is raised, the periapsisradius increases and apoapsis radiusdecreases until the orbit is nearly cir-cularized at an inclination of about60 degrees. The Orbiter’s trajectorythen crosses Titan’s orbit at not one,but two points (the ascending and de-scending nodes), making possible a180-degree transfer from an inboundTitan flyby to an outbound Titan flyby.After this 180-degree transfer is ac-complished, the next seven Titan fly-

bys, all of which are outbound, re-duce inclination as quickly as possi-ble to near Saturn’s equator. This180-degree transfer flyby sequence(raising inclination, accomplishing the180-degree transfer, then loweringinclination again) rotates the line ofapsides about 120 degrees so thatapoapsis lies between the Sun lineand Saturn’s dusk terminator.

Titan 32–Titan 34. The flybys immedi-ately following the completion of the180-degree transfer sequence andthe return of the spacecraft’s orbitalplane to near Saturn’s equator areused to target flybys of Enceladus,Rhea, Dione and Iapetus. The Encela-dus and Rhea flybys occur on succes-sive orbits (46 and 47) between theTitan flybys on May 28 and July 18,2007. The Titan flyby on Septem-ber 1, 2007, raises inclination to sev-en degrees to target to Iapetus.

Titan 35–Titan 43. Following the Iape-tus flyby on September 18, 2007, theOrbiter is targeted to an outbound Ti-tan flyby on October 3, 2007, whichplaces the line of nodes close to theSun line. Starting with this flyby, therest of the tour is devoted to a “maxi-mum-inclination sequence” of flybysdesigned to raise inclination as highas possible for ring observationsand in situ fields and particles mea-surements (in this case, to about75 degrees). In this reference tour,the orbits during this maximum-incli-nation flyby sequence are oriented

nearly toward the Sun, opposite themagnetotail, to ensure several occul-tations of Earth by Saturn and therings at close range.

During this flyby sequence, first, orbit“cranking” and then, orbit “pump-ing” (after a moderate inclinationhas been achieved) are used to in-crease inclination, eventually reduc-ing the orbit period to just over sevendays (nine Orbiter revolutions, fourTitan revolutions). The closest ap-proach altitudes during this sequenceare kept at the minimum allowed val-ue to maximize gravitational assist ateach flyby.

End of MissionThe reference tour ends on July 1,2008, four years after insertion intoorbit about Saturn, 33.5 days andfour and a half spacecraft revs afterthe last Titan flyby (which occurs onMay 28, 2008). The spacecraft’s or-bital period is 7.1 days (a resonanceof four Titan revs to nine spacecraftrevs), its inclination is 71.1 degreesand its periapsis radius is four RS.

The aimpoint at the last flyby is cho-sen to target the orbiter to a Titan fly-by on July 31, 2008 (64 days afterthe last flyby in the tour), providingthe opportunity to proceed with moreflybys during an extended mission,if resources allow. Nothing in thedesign of the tour precludes an ex-tended mission.

V E H I C L E O F D I S C O V E R Y 8 9

T

C H A P T E R 8

he Cassini Orbiter, with its scientific instruments, is anamazing “tool” for making remote observations. How-ever, much work has to be done to get the spacecraft to itsdestination, and that is the story of mission design and ex-ecution, covered in the previous chapter, and of missionoperations, covered in Chapter 10. In this chapter, we

will examine the structure and design of the Cassini space-craft, including onboard computers, radio receivers andtransmitters, data storage facilities, power supplies andother items to support data collection by the science pay-load. The instruments themselves, on the Orbiter and withinthe Huygens Probe, are covered in Chapter 9.

Orbiter DesignThe functions of the Cassini Orbiterare to carry the Huygens Probeand the onboard science instrumentsto the Saturn system, serve as theplatform from which the Probe islaunched and science observationsare made and store information andrelay it back to Earth.

The design of the Orbiter was drivenby a number of requirements and

parts: the high-gain antenna (pro-vided by the Italian space agency),the upper equipment module, thepropulsion module and the lowerequipment module. Attached to thisstack are the remote-sensing palletand the fields and particles pallet,both with their science instruments,and the Huygens Probe system(provided by the European SpaceAgency). The overall height of theassembled spacecraft is 6.8 meters,making Cassini–Huygens the largestplanetary spacecraft ever launched.

The Huygens Probe contains instru-ments for six investigations. TheOrbiter carries instrumentation for12 investigations. Four science in-strument packages are mounted onthe remote-sensing pallet; three moreare mounted on the fields and parti-cles pallet. Two are associated withthe high-gain antenna (HGA). Themagnetometer is mounted on its own11-meter boom. The remaining in-struments are mounted directly onthe upper equipment module.

challenges that make Cassini–Huy-gens different from most other mis-sions to the planets. Among theseare the large distance from Saturn tothe Sun and Earth, the length of themission, the complexity and volumeof the science observations and thespacecraft’s path to Saturn. That pathincludes four “gravity assists” fromplanets along the way to Saturn.

General Configuration. The main bodyof the Orbiter is a stack of four main

This illustrationshows the main de-sign features of theCassini spacecraft,including the Huy-gens Titan Probe.

445-newtonEngine (1 of 2)

RadioisotopeThermoelectricGenerator (1 of 3)

HuygensTitan Probe

Fields andParticles Pallet

Radar Bay

Low-Gain Antenna (1 of 2)

Remote-Sensing Pallet

Radio and Plasma WaveSubsystem Antenna (1 of 3)

11-meter MagnetometerBoom

4-meter High-Gain Antenna

Vehicle of Discovery


Power Source. Because of Saturn’s dis-tance from the Sun, solar panels ofany reasonable size cannot providesufficient power for the spacecraft. Togenerate enough power, solar arrayswould have to be the size of a coupleof tennis courts and would be far tooheavy to launch.

The Cassini Orbiter will get its powerfrom three radioisotope thermoelectricgenerators, or RTGs, which use heatfrom the natural decay of plutoniumto generate direct current electricity.These RTGs are of the same designas those already on the Galileo andUlysses spacecraft and have the abil-ity to operate many years in space.At the end of the 11-year Cassini–Huygens mission, they will still becapable of producing 630 wattsof power! The RTGs are mounted onthe lower equipment module and areamong the last pieces of equipment

to be attached to the spacecraft priorto launch.

Fault Protection. The distance ofSaturn from Earth is especially im-portant, because it affects communi-cation with the spacecraft. WhenCassini is at Saturn, it will be be-tween 8.2 and 10.2 astronomicalunits (AU) from Earth (one AU is thedistance from Earth to the Sun, or150 million kilometers). Because ofthis, it will take 68–84 minutes forsignals to travel from Earth to thespacecraft, or vice versa.

In practical terms, this means thatmission operations engineers on theground cannot give the spacecraft“real-time” instructions, either for day-to-day operations or in case of unex-pected events on the spacecraft. Bythe time ground personnel becomeaware of a problem and respond,nearly three hours will have passed.

Onboard fault protection is thereforeessential to the success of the space-craft. The Cassini–Huygens space-craft system fault protection isdesigned to ensure that the space-craft can take care of itself in theevent of onboard problems longenough to permit ground personnelto study the problem and take appro-priate action. In practical terms, thismeans that if a fault is detected thatmight pose a substantial risk to anypart of the spacecraft, onboard com-puters automatically initiate appropri-ate “safing” actions. These mayinclude terminating preprogrammedactivities and establishing a safe,commandable and relatively inactivespacecraft state for up to severalweeks without ground intervention.

Command and Data Subsystem. Theprimary responsibility for command,control (including the fault protection

Radioisotope ther-moelectric gen-erators (RTGs)have no movingparts and arehighly reliablepower sources.

Heat SourceSupport

CoolingTubes

Gas ManagementAssembly

Aluminum OuterShell Assembly

General PurposeHeat Source

Active CoolingSystem Manifold

Pressure ReliefDevice

RTG MountingFlange

MultifoilInsulation

Silicon GermaniumUnicouple

Midspan HeatSource Support


discussed earlier) and data handlingis performed by the actively redun-dant command and data subsystem(CDS). This computer executes se-quences of stored commands, eitheras a part of a normal preplannedflight activity or as a part of fault-protection routines. The CDS also pro-cesses and issues real-time commandsfrom Earth, controls and selects datamodes and collects and formats sci-ence and engineering data for trans-mission to Earth.

The CDS electronics are located inBay 8 of the 12-bay upper equipmentmodule. Commands and data fromthe CDS to each instrument and datafrom the instruments are handled bybus interface units (BIUs), located inthe electronics boxes of each instru-ment. The BIUs are also used by theCDS to control data flow and allow-able power states for each instrument.The CDS can accommodate data col-lection from the instruments and engi-neering subsystems at a combinedrate in excess of 430,000 bits persecond while still carrying on its com-mand and control functions!

Solid-State Recorders. The Cassini–Huygens spacecraft has been de-signed with a minimum of movableparts. In keeping with that designphilosophy, data storage is accom-plished by means of solid-state re-corders rather than tape recorders.The two redundant solid-state record-ers each had a storage capacity oftwo gigabits when they were built.Storage capacity is guaranteed to beat least 1.8 gigabits 15 years after

launch. The solid-state recorders arelocated in Bay 9 of the upper equip-ment module.

The solid-state recorders have theability to record and read out datasimultaneously, record the samedata simultaneously in two differentlocations on the same recorder andrecord simultaneously on both record-ers. The recorders will be used tobuffer essentially all of the collecteddata, permitting data transmissionto Earth to occur at the highest avail-able rates, rather than being re-

stricted to instantaneous data collec-tion rates. They can also be parti-tioned by command from the CDS.The recorders will be used to storebackup versions of memory loads foralmost all computers on the space-craft and keep a running record ofrecent engineering activities to assistin the analysis of possible problems.

Attitude and Articulation Control. Theattitude and articulation control sub-system (AACS) is primarily responsi-ble for maintaining the orientation of

The Cassini–Huygensspacecraft, in launchconfiguration, in theJet Propulsion Labora-tory’s High Bay.


Cassini–Huygens in space. Specifical-ly, the AACS is required to do the fol-lowing tasks:

• Acquire a “fix” on the Sun followingseparation of the spacecraft fromthe launch vehicle.

• Point the antenna (either the high-gain or one of the two low-gainantennas) toward Earth whenrequired.

• Point the high-gain antenna towardthe Huygens Probe during its three-hour data collection period as it de-scends through the atmosphere andlands on Titan’s surface.

• Point the high-gain antenna atappropriate radar or radio sciencetargets.

• Point the instruments on the remote-sensing pallet toward targets thatare themselves in motion relative tothe spacecraft.

• Stabilize the spacecraft for Proberelease and gravity wave measure-ments.

• Turn the spacecraft at a constantrate around the axis of the high-gain antenna for fields and particlesmeasurements during transmissionof data to Earth or receipt of com-mands from Earth.

• Point one of the two redundantmain propulsion engines in thedesired direction during mainengine burns.

• Perform trajectory correction maneu-vers of smaller magnitude using theonboard thrusters.

• Provide sufficient data in the trans-mitted engineering data to supportscience data interpretation and mis-sion operations.

The two redundant computers for theAACS are located in Bays 1 and 10of the upper equipment module, butparts of the AACS are spread acrossthe spacecraft. Redundant Sun sen-sors are mounted to the high-gainantenna. Redundant stellar referenceunits are mounted on the remote sens-ing platform. Three mutually perpen-dicular reaction wheels are mountedon the lower equipment module: Afourth reaction wheel, mounted onthe upper equipment module, is abackup that can be rotated to beparallel to any one of the three otherreaction wheels. Redundant inertialreference units, consisting of fourhemispherical resonator gyroscopeseach, are mounted to the upperequipment module. The main engineactuators and electronics are mount-ed near the bottom of the propulsionmodule. An accelerometer, used tomeasure changes in the spacecraft’svelocity, shares Bay 12 of the upperequipment module with the imagingscience electronics.

A sophisticated pointing systemknown as inertial vector propagationis programmed into the AACS com-puters. It keeps track of spacecraftorientation, the direction and dis-tance of the Sun, Earth, Saturn andother possible remote-sensing targetsin the Saturn system and the space-craft-relative pointing directions of allthe science instruments — and there-by points any specified instrument at

its selected target. The AACS uses thestellar reference unit to determine theorientation of the spacecraft by com-paring stars seen in its 15-degreefield of view to a list of more than3000 stars stored in memory.

Propulsion Module Subsystem. Thelargest and most massive subsystemon the spacecraft is the propulsionmodule subsystem (PMS). It consists ofa bipropellant element for trajectoryand orbit changes and a hydrazineelement for attitude control, small ma-neuvers and reaction wheel desatura-tion (“unloading”).

The bipropellant components aremonomethyl hydrazine (fuel) and ni-trogen tetroxide (oxidizer), and areidentical and stacked in tandem in-side the cylindrical core structure,with the fuel closer to the spacecrafthigh-gain antenna. The fuel tank isloaded with 1130 kilograms ofmonomethyl hydrazine; the oxidizertank contains 1870 kilograms of ni-trogen tetroxide. Their combinedmass at launch, 3000 kilograms, con-stitutes more than half the total mass

A diagram of theCassini spacecraftshowing elements ofthe propulsion mod-ule subsystem. (Acro-nyms are defined inAppendix B.)

SSH

AFCSSE

BRU

VDE &EGE

RWA

EGA

IRI & RWIACC


of the spacecraft! Below the oxidizertank are the redundant main engines,which can be articulated in each oftwo axes by the AACS. A heat shieldprotects the gimbal assembly and thespacecraft from engine heat.

The hydrazine tank is spherical andis mounted external to the PMS cylin-drical structure. Hydrazine is usedto power the small thrusters as com-manded by the AACS. The 16 thrust-ers are located in clusters of foureach on four struts extending out-ward from the bottom of the PMS.At launch, the tank holds 132 kilo-grams of hydrazine.

A larger cylindrical tank withrounded ends, also exterior to thePMS cylindrical structure but on theopposite side from the hydrazinetank, holds nine kilograms of helium.The helium tank supplies pressurantto expel propellants from the two bi-propellant tanks and from the hydra-zine tank.

Telecommunications. The long-distancecommunications functions on thespacecraft are performed by the ra-dio frequency subsystem (RFS) andthe antenna subsystem. For telecom-munications from the spacecraft toEarth, the RFS produces an X-bandcarrier at a frequency of 8.4 giga-hertz, modulates it with data receivedfrom the CDS, amplifies the carrierand delivers the signal stream to theantenna subsystem for transmission.In the opposite direction, the RFS ac-cepts X-band ground commands anddata signals from the antenna sub-system at a frequency of 7.2 giga-

hertz, demodulates them and deliversthe telemetry to the CDS for storageand/or execution. The RFS is con-tained in Bays 5 and 6 of the upperequipment module.

The antenna subsystem includes thefour-meter-diameter high-gain antenna(HGA) and two low-gain antennas(LGA1 and LGA2). LGA1 is attachedto the secondary reflector of the HGAand has a hemispherical field of viewcentered on the HGA field of view.LGA2 is mounted on the lower equip-ment module and has a hemispheri-cal field of view centered on adirection approximately perpendicu-

lar to the HGA field of view. The twolow-gain antennas are primarily usedfor communications during the firsttwo and a half years after launch,when the HGA needs to be pointedat the Sun (to provide shade for mostof the spacecraft subsystems) andcannot therefore be pointed at Earth.After the first two and a half years,the high-gain antenna is used almostexclusively for communications.

The spacecraft receives commandsand data from Earth at a rate of1000 bits per second during HGAoperations. It transmits data to Earthat various rates, between 14,220

The propulsion mod-ule subsystem wastested in the Jet Pro-pulsion Laboratory’sSpacecraft AssemblyFacility.


and 165,900 bits per second. Lowerrates of receipt and transmission ap-ply for low-gain antenna operations.In general, during operations fromSaturn, data will be recorded on thesolid-state recorders for 15 hourseach day, while the HGA is notpointed at Earth. Then, for nine hourseach day (generally during Gold-stone, California, tracking station cov-erage) the data from the solid-staterecorders will be played back whiledata collection from the fields andparticles investigations continues. Inthis fashion, approximately one giga-bit of data can be returned each“low-activity” day via a 34-meterDeep Space Network antenna. Simi-larly, using a 70-meter antenna on“high-activity” days, approximatelyfour gigabits of data can be returnedin nine hours.

In addition to redundant X-band re-ceivers and transmitters, the HGAalso houses feeds for a Ka-band re-ceiver, a Ka-band transmitter and anS-band transmitter, all for radio sci-ence. Feeds for Ku-band transmittersand receivers for radar science are

also on the HGA, as is a feed foran S-band receiver for receipt of theHuygens Probe data.

Power and Pyrotechnics Subsystem.The radioisotope thermoelectric gen-erators (discussed earlier) are a partof the power and pyrotechnics sub-system (PPS). The RTGs generate thepower used by the spacecraft; thepower conditioning equipment con-ditions and distributes that power tothe rest of the spacecraft; the pyroswitching unit provides redundantpower conditioning and energy stor-age and, upon command, redundantpower switching for firing electro-explosive or pyro devices.

The power conditioning equipmentconverts the RTG output to provide aregulated 30-volt direct current pow-er bus. It also provides the capabilityto turn power on and off to the vari-ous spacecraft power users in re-sponse to commands from the CDS.If any power user experiences anovercurrent condition, the powerconditioning equipment detects thatcondition; if the level of overcurrent

exceeds a predetermined level, thepower to that user is switched off.

The pyro switching unit controls thefiring of pyro devices on 32 com-mandable circuits, most of whichopen or shut valves to control thepressures and flows within the propul-sion module plumbing. The pyrodevices are also used to:

• Separate the spacecraft from thelaunch vehicle after launch.

• Pull the pin that unlatches thespare reaction wheel articulationmechanism.

• Pull the pin that permits the Radioand Plasma Wave Science instru-ment’s Langmuir probe to deploy.

• Jettison science instrument covers.

• Separate the Huygens Probe fromthe Cassini Orbiter.

• Jettison the articulated cover thatprotects the main engine nozzlesfrom damage by high-velocity parti-cles in space (in the event the coversticks in a closed position).

Temperature Control Subsystem. TheCassini spacecraft has many electri-cal and mechanical units that are sen-sitive to changes in temperature. Thefunction of the temperature controlsubsystem (TCS) is to keep these unitswithin their specified temperature lim-its while they are on the spacecraft,during both prelaunch and post-launch activities. The TCS monitorstemperatures by means of electricaltemperature sensors on all criticalparts of the spacecraft. The TCS thencontrols the temperature of the vari-ous units by using one or more of thefollowing techniques:

• Turning electrical heaters on or offto raise or lower temperatures.

The Cassini high-gainantenna was providedby the Italian spaceagency.

SubsystemTestbed

Cassini High-Gain Antenna

Radar Antenna


• Opening or shutting thermal louvers(which resemble Venetian blinds) tocool or heat electronics in one ofthe bays of the upper equipmentmodule.

• Using small radioisotope heaterunits to raise the temperatures ofselected portions of the spacecraftthat would otherwise cool to unac-ceptably low temperatures.

• Installing on the spacecraft thermalblankets (gold or black in color) toprevent heat from escaping tospace.

• Installing thermal shields above theRTGs to keep them from radiatingexcessive heat to sensitive scienceinstruments.

• Taking advantage of the orientationof the spacecraft to keep most ofthe spacecraft in the shadow of thehigh-gain antenna or the HuygensProbe.

From launch until Cassini–Huygensis well beyond the distance of Earth,the spacecraft is oriented to point thehigh-gain antenna at the Sun, therebyshading most of the rest of the space-craft. Deviations from this orientationare permitted for a maximum of halfan hour for each occurrence untilthe spacecraft reaches a distanceof 2.7 AU (405 million kilometers)from the Sun. After that time, the heatinput from the Sun will have dimin-ished sufficiently to permit almost anyspacecraft orientation.

Although they are not a part of theTCS responsibility, several instrumentshave radiator plates to cool their de-tectors. Even at the distance of Saturn

from the Sun, spacecraft orientationsthat point those radiator plates in thesame half of the sky as the Sun canseverely degrade the data collectedby some of the science instruments. Inat least one case, it is even necessaryto simultaneously avoid having Sat-urn illuminate the radiator plate(s).

New Technology. Whereas previousinterplanetary spacecraft used on-board tape recorders, Cassini will pi-oneer a new solid-state data recorderwith no moving parts. The recorderwill be used in more than 20 otherNASA and non-NASA missions.

Similarly, the main onboard com-puter, which directs Orbiter opera-tions, uses a novel design incorporat-ing new families of electronic chips.Among them are very high speed in-tegrated circuit (VHSIC) chips. Thecomputer also contains powerful newapplication-specific integrated circuit(ASIC) parts, each replacing a hun-dred or more traditional chips.

Also on the Orbiter, the power systemwill benefit from an innovative solid-state power switch that will eliminatethe rapid fluctuations or “transients”that can occur with conventionalpower switches. This will signifi-cantly extend component lifetime.

Past interplanetary spacecraft haveused massive mechanical gyroscopesto provide inertial reference duringperiods when Sun and star referenceswere unavailable. Cassini employshemispherical resonator gyroscopesthat have no moving parts. These de-vices utilize tiny “wine glasses” aboutthe size of a dime that outperformtheir more massive counterparts bothin accuracy and in expected lifetime.

Huygens Probe DesignThe Huygens Probe, supplied by theEuropean Space Agency (ESA), willscrutinize the clouds, atmosphereand surface of Saturn’s moon Titan.It is designed to enter and brake in

Final European SpaceAgency inspection ofthe Huygens Probeprior to shipment tothe United States forintegration with theCassini Orbiter.


Heat Shield

Front Shield

Back Cover

Parachute Compartment

Descent Module withScientific Instruments

1

2

3

4

5

Titan’s atmosphere and parachute afully instrumented robotic laboratorydown to the surface. The HuygensProbe system consists of the Probe it-

The Probe will remain dormantthroughout the 6.7-year interplane-tary cruise, except for health checksevery six months. These checkoutswill follow preprogrammed descentscenario sequences as closely as pos-sible, and the results will be relayedto Earth for examination by systemand payload experts.

Prior to the Probe’s separation fromthe Orbiter, a final health check willbe performed. The “coast” timer willbe loaded with the precise time nec-essary to turn on the Probe systems(15 minutes before the encounter withTitan’s atmosphere), after which theProbe will separate from the Orbiterand coast to Titan for 22 days withno systems active except for its wake-up timer.

The Probe system is made up of anumber of engineering subsystems,some distributed between the Probeand the Probe support equipment onthe Orbiter. The Huygens payloadconsists of a complement of six sci-ence instrument packages.

Probe Support Equipment. The Probesupport equipment includes the elec-tronics necessary to track the Probe,recover the data gathered during itsdescent and process and deliver thedata to the Orbiter, from which thedata will be transmitted or “down-linked” to the ground.

The Probe engineering subsystemsare the entry subsystem, the innerstructure subsystem, the thermal con-trol subsystem, the electrical powersubsystem, the command and data

self, which will descend to Titan, andthe Probe support equipment, whichwill remain attached to the orbitingspacecraft.

1

23

4 5

The Huygens Probecarries six scienceinvestigations tostudy the clouds,atmosphere and sur-face of Titan. Threeseparate parachutesare carried in theProbe’s parachutecompartment. Theheat shield, frontshield and backcover are jettisonedbefore data collec-tion begins.


management subsystem and theProbe data relay subsystem.

Entry Subsystem. The entry subsys-tem functions only during the releaseof the Probe from the Orbiter and itssubsequent entry into the Titan atmo-sphere. It consists of three main ele-ments: the spin–eject device thatpropels the Probe away from theOrbiter; a front shield covered withspecial thermal protection materialthat protects the Probe from the heatgenerated during atmospheric entry;and an aft cover, also covered withthermal protection material, to reflectaway heat from the wake of theProbe during entry.

The Probe will be targeted for a high-latitude landing site on the “day” sideof Titan and released from the Or-biter on November 6, 2004. Thespin–eject device will impart a rela-tive velocity of about 0.3 meter persecond and a spin rate (for stabiliza-tion) of about seven revolutions perminute. The Probe’s encounter withTitan is planned for November 27,when it will enter the atmosphere ata velocity of 6.1 kilometers per sec-ond. The entry phase will last aboutthree minutes, during which theProbe’s velocity will decrease toabout 400 meters per second.

Three parachutes will be used duringthe Probe’s descent. When the on-board accelerometers detect a speedof Mach 1.5 near the end of the de-celeration phase, the two-meter-diam-eter pilot chute will deploy, pullingoff the aft cover. This will be followedimmediately by deployment of the8.3-meter-diameter main parachute.

About 30 seconds after deploymentof the main chute, the Probe’s velocitywill have dropped from Mach 1.5 toMach 0.6, and the front heat shieldwill be released.

At this point, scientific measurementscan begin. About 15 minutes later,the main chute will be released anda smaller, three-meter drogue chutewill permit the Probe to reach Titan’ssurface, about two and a half hoursafter data taking starts. The Probewill hit the surface at a velocity ofabout seven meters per second andis expected to survive the impact. TheProbe will continue to transmit datafor an additional 30–60 minutes.

Inner Structure Subsystem. The innerstructure of the Probe consists of twoaluminum honeycomb platforms andan aluminum shell. It is linked to thefront heat shield and the aft cover byfiberglass struts and pyrotechnicallyoperated release mechanisms. Thecentral equipment platform carries,

on both its upper and lower surfaces,the boxes containing the electricalsubsystems and the science experi-ments. The upper platform carriesthe parachutes (when stowed) andthe antennas for communication withthe Orbiter.

Thermal Control Subsystem. At dif-ferent times during the mission, theProbe will be subjected to extremethermal environments requiring a vari-ety of passive controls to maintain therequired temperature conditions. Forinstance, during the two Venus flybys,the solar heat input will be high. TheProbe will get some protection fromthe shadow of the high-gain antenna,and when the antenna is off Sun-pointfor maneuvers or communication, theProbe will be protected by multilayerinsulation that will burn off during thelater atmospheric entry.

The Probe will be at its coldest justafter it separates from the Orbiter.To ensure that none of the equipment

AQ60 tiles, similarto those used onthe Space Shuttle,will help to dissi-pate heat during theHuygens Probe’s de-scent through Titan’satmosphere.


S P A C E C R A F T S U B S Y S T E M S

M A S S A N D P O W E R

Mass, Power,Subsystem or Component kilograms watts Comments on Power Usage

Engineering Subsystems

Structure Subsystem 272.6 0.0

Radio Frequency Subsystem 45.7 80.1 During downlinking of data

Power and Pyrotechnics Subsystem 216.0 39.1 Shortly after launch

Command and Data Subsystem 29.1 52.6 With both strings operating

Attitude and Articulation Control Subsystem 150.5 115.3 During spindown of spacecraft

(Engine Gimbal Actuator) 31.0 During main engine burns

Cabling Subsystem 135.1 15.1 Maximum calculated power loss

Propulsion Module Subsystem (dry)* 495.9 97.7 During main engine burns

Temperature Control Subsystem 76.6 117.8 During bipropellant warmup

6.0 Temperature fluctuation allowance

2.0 Radiation and aging allowance

20.0 Operating margin allowance

Mechanical Devices Subsystem 87.7 0.0

Packaging Subsystem 73.2 0.0

Solid-State Recorders 31.5 16.4 Both recorders reading and writing

Antenna Subsystem 113.9 0.0

Orbiter Radioisotope Heater Subsystem 3.8 0.0

Science Instrument Purge Subsystem 3.1 0.0

System Assembly Hardware Subsystem 21.7 0.0

Total Engineering Mass* 1756.6

Science Instruments

Radio Frequency Instrument Subsystem 14.4 82.3 Both S-band and Ka-band operating

Dual Technique Magnetometer 8.8 12.4 Both scalar and vector operations

Science Calibration Subsystem 2.2 44.0 In magnetometer calibration mode

Imaging Science (narrow-angle camera) 30.6 28.6 Active and operating

Imaging Science (wide-angle camera) 25.9 30.7 Active and operating

Visible and Infrared Mapping Spectrometer 37.1 24.6 In imaging mode

Radio and Plasma Wave Science 37.7 17.5 During wideband operations

Ion and Neutral Mass Spectrometer 10.3 26.6 Neutral Mass Spectrometer operating

Magnetospheric Imaging Instrument 29.0 23.4 High-power operations

Cosmic Dust Analyzer 16.8 19.3 Operating with articulation

Cassini Radar 43.3 108.4 Operating in imaging mode

Cassini Plasma Spectrometer 23.8 19.2 Operating with articulation

Ultraviolet Imaging Spectrograph 15.5 14.6 On in sleep state

Composite Infrared Spectrometer 43.0 43.3 Active and operating

Total Science Mass 338.2

Huygens Probe (including Probe support) 350.0 249.8 During Probe checkouts

Launch Vehicle Adaptor Mass 136.0 0.0

TOTALS* 2580.7 680.5 Power available at middle of tour

* Note that the masses given above do not include approximately 3141 kilograms of propellant and pressurant.


falls below its storage temperaturelimits, the Probe will carry a numberof radioisotope heater units that eachgenerate one watt.

At described above under the entrysubsystem, the front heat shield willprotect the Probe during initial atmo-spheric entry. The front shield iscovered with Space Shuttle–like tilesmade of a material known as AQ60,developed in France. This materialis essentially a low-density “mat” ofsilica fibers. The tile thickness of thefront shield is calculated to ensurethat the temperature of the structurewill not exceed 150 degrees Celsius,below the melting temperature oflead. The rear side of the Probe willreach much lower temperatures, soa sprayed-on layer of “Prosial” silicafoam material will be used on therear shield. The overall mass of thethermal control subsystem will bemore than 100 kilograms, or almostone-third of the entire Probe mass.

Electrical Power Subsystem. DuringProbe checkout activities, the Probewill obtain power from the Orbiter viathe umbilical cable. After separation,the Orbiter will continue to supplypower to the Probe support equip-ment, but power for the Probe itselfwill be provided by five lithium–sul-phur dioxide batteries.

Much of the battery power will beused to power the timer for the22 days of “coasting” to Titan. Thehigher current needed for Probe mis-

sion operations is only required forthe science data collection period,three to three and a half hours. Theelectrical power subsystem is de-signed to survive the loss of one ofits five batteries and still support acomplete Probe mission.

Command and Data Management.The command and data managementsubsystem provides monitoring andcontrol of all Probe subsystem andscience instrument activities. Specifi-cally, this subsystem performs the fol-lowing functions:

• Time the 22-day coast phase toTitan and switch the Probe onjust prior to atmospheric entry.

• Control the activation of deploy-ment mechanisms during the de-scent to Titan’s surface.

• Distribute telecommands to theengineering subsystems and sci-ence instruments.

• Distribute to the science instrumentsa descent data broadcast providinga timeline of conditions on whichthe instruments can base the sched-uling of mode changes and otheroperations.

• Collect science and housekeepingdata and forward the data to theOrbiter via the umbilical cable(before Probe separation) or viathe Probe data relay subsystem(during descent).

Probe Data Relay Subsystem. TheProbe data relay subsystem providesthe one-way Probe-to-Orbiter commu-nications link and includes equipmenton both Probe and Orbiter. The ele-ments that are part of the Probe sup-port equipment on the Orbiter are theProbe system avionics and the radiofrequency electronics, including anultrastable oscillator and a low-noiseamplifier.

The Cassini–Huygensmission: an interna-tional collaboration.

Switzerland

Denmark

Belgium

United States France Germany

Italy

Great Britain

Netherlands

Austria

Finland

NorwaySwedenHungaryIreland

Czech Republic

Spain

International Participation inCassini Saturn Orbiter and

Huygens Titan Probe

1 0 0 P A S S A G E T O A R I N G E D W O R L D

The Probe carries two redundantS-band transmitters, each with its ownantenna. The telemetry in one link isdelayed by about four seconds withrespect to the other link, to preventdata loss if there are brief transmis-sion outages. Reacquisition of theProbe’s signal by the Orbiter wouldnormally occur within this interval.

Concluding RemarksThe design of the Cassini–Huygensspacecraft is the end result of exten-sive trade-off studies that consideredcost, mass, reliability, durability, suit-ability and availability of hardware.

To forestall the possibility of mechani-cal failures, moving parts were elimi-nated from the spacecraft whereverfeasible. Early designs that includedarticulating instrument scan platformsor turntables were discarded in favorof body-fixed instruments whosepointing will require rotation of theentire spacecraft.

Tape recorders were replaced withsolid-state recorders. Mechanical gy-roscopes were replaced with hemi-spherical resonator gyroscopes. Anarticulated Probe relay antenna wasdiscarded in favor of using the high-gain antenna to capture the Probe’ssignal.

Spacecraft engineers, both thosewho designed and built the hard-ware and those who will operatethe spacecraft, relied heavily onextensive experience to provide aspacecraft design that is more sophis-ticated, reliable and capable thanany other spacecraft ever built forplanetary exploration. Because ofthat care in design, the Cassini–Huy-gens spacecraft will be far easier tooperate and will return more sciencedata about its targets than any priorplanetary mission!

T O O L S O F D I S C O V E R Y 1 0 1

T

C H A P T E R 9

he 18 scientific instruments carried by the Cassini–Huy-gens spacecraft have been designed to perform the mostdetailed studies ever of the Saturn system. Twelve instru-ments are mounted on the Cassini Orbiter and six are car-ried inside the Huygens Probe. Each instrument will make

its own unique measurements, providing comprehensive,synergistic information on Saturn, its rings, its satellitesand its magnetosphere. Cassini–Huygens’ scientific investi-gations will build on the wealth of data provided by Pio-neer 11 and Voyager 1 and 2.

The Cassini–Huy-gens spacecraftstands almostseven meterstall and overfour meters wide.In launch-readyconfiguration,the spacecraftweighs over5600 kilograms.

Orbiter Instruments OverviewThe Cassini–Huygens mission is a co-operative, international endeavor. Inaddition to the United States’ involve-ment, a number of other countrieshave provided instrument hardwareand software as well as investigatorsto the instrument teams. The UnitedStates’ international partners in oneor more elements of the Cassini–Huygens mission are Austria, Belgium,the Czech Republic, Denmark, the Eu-ropean Space Agency, Finland,France, Germany, Hungary, Ireland,Italy, The Netherlands, Norway,Spain, Sweden, Switzerland and theUnited Kingdom. There are 12 scientif-ic instruments aboard the Cassini Or-biter, which will spend four yearsstudying the Saturn system in detail.

All 12 Orbiter instruments are body-fixed to the spacecraft; some havearticulating platforms that allow themto scan a portion of the sky withouthaving to move the spacecraft. Eachinstrument has one or more micropro-cessors for internal control and datahandling. The Orbiter experiments canbe divided into three basic categories— optical remote sensing, microwaveremote sensing and fields, particles

purposes. General science objectivesfor the ISS include studying the atmo-spheres of Saturn and Titan, the ringsof Saturn and their interactions withthe planet’s satellites and the surfacecharacteristics of the satellites, includ-ing Titan. The following tasks are in-volved:

• Map the three-dimensional structureand motions in the Saturn and Titanatmospheres.

and waves — and comprise a total of27 sensors. The total mass of the sci-ence payload is 365 kilograms.

Orbiter Instrument Descriptions

Optical Remote Sensing

Imaging Science Subsystem. The opti-cal remote-sensing Imaging ScienceSubsystem (ISS) will photograph awide variety of targets — Saturn, therings, Titan, the icy satellites and starfields — from a broad range of ob-serving distances for various scientific

Tools of Discovery


• Study the composition, distributionand physical properties of cloudsand aerosols.

• Investigate scattering, absorptionand solar heating in the Saturn andTitan atmospheres.

• Search for evidence of lightning,aurorae, airglow and planetary os-cillations.

• Study gravitational interactionsamong the rings and the satellites.

• Determine the rate and nature ofenergy and momentum transferwithin the rings.

• Determine ring thickness and thesize, composition and physical na-ture of ring particles.

• Map the surfaces of the satellites,including Titan, to study their geo-logical histories.

• Determine the nature and composi-tion of the icy satellites’ surface ma-terials.

• Determine the rotation states of theicy satellites.

The ISS comprises a narrow-anglecamera and a wide-angle camera.The narrow-angle camera provideshigh-resolution images of the target ofinterest, while the wide-angle cameraprovides extended spatial coverageat lower resolution. The cameras canalso obtain optical navigation frames,which are used to keep the space-craft on the correct trajectory.

The Cassini–Huygens imagers differprimarily in the design of the optics.The wide-angle camera has refractiveoptics with a focal length of 200 milli-meters, a focal length–diameter ratio(ƒ number) of 3.5 and a 3.5-degree-square field of view (FOV). Refractiverather than reflective optics were cho-sen primarily to meet mass and costconstraints; these optics were avail-able in the form of spares from theVoyager mission.

The narrow-angle camera has Rit-chey–Chretien reflective optics witha 2000-millimeter focal length, anƒ number of 10.5 and a 0.35-de-gree FOV.

Filters are mounted in two rotatablewheels per camera; they have a two-wheel filter-changing mechanismwhose design is derived from thatof the Hubble Space Telescope’sWide Field and Planetary Camera 2.The wide-angle camera has 18 filterscovering the range 380–1100 na-nometers; the narrow-angle camerahas 24 filters covering the range200–1100 nanometers.

Shutters of the same type used onboth Voyager and Galileo missionscontrol exposure times: The shortestplanned is five milliseconds and thelongest is 20 minutes. The sensingelement of each camera is a 1024 ×1024–element, solid-state array, orcharge-coupled device (CCD). TheCCD is phosphor-coated for ultravio-let response and radiator-cooled to180 kelvins to reduce dark current(residual current in the CCD beyondthat released by incident light).

Visible and Infrared Mapping Spec-trometer. The Visible and InfraredMapping Spectrometer (VIMS) will

O P T I C A L R E M O T E S E N S I N G

Four optical remote-sensing instrumentsare mounted on afixed, remote-sensingpallet, with their opti-cal axes coaligned.The entire spacecraftmust be rotated topoint these instru-ments at the target ofinterest. The instru-ments include an Im-

aging Science Sub-system (ISS) com-prising narrow-and wide-anglecameras, a Visibleand Infrared Map-ping Spectrometer(VIMS), a Compos-ite Infrared Spec-trometer (CIRS) andan Ultraviolet Imag-

ing Spectrograph(UVIS). A fifthremote-sensing in-strument, the imag-ing portion of theMagnetosphericImaging Instru-ment (MIMI), ismounted on theupper equipmentmodule, not far

from the remote-sensing platform,and is also bore-sighted with theoptical remote-sensing instru-ments. This ex-periment has thecapability to im-age the chargedparticle popula-tion of Saturn’smagnetosphere.

Stellar ReferenceUnit 1

Stellar ReferenceUnit 2

VIMS Infrared

VIMS Visible

Ultraviolet ImagingSpectrograph

ISS Wide-AngleCamera

Composite Infrared Spec-trometer (Optics Module)

ISS Narrow-Angle Camera


map the surface spatial distributionof the mineral and chemical featuresof a number of targets, includingthe rings and satellite surfaces andthe atmospheres of Saturn and Titan.The VIMS science objectives are asfollows:

• Map the temporal behavior ofwinds, eddies and other featureson Saturn and Titan.

• Study the composition and distribu-tion of atmospheric and cloud spe-cies on Saturn and Titan.

• Determine the composition and dis-tribution of surface materials on theicy satellites.

• Determine the temperatures, inter-nal structure and rotation of Sat-urn’s deep atmosphere.

• Study the structure and compositionof Saturn’s rings.

• Search for lightning on Saturn andTitan and active volcanism on Titan.

• Observe Titan’s surface.

The VIMS comprises a pair of imag-ing–grating spectrometers that aredesigned to measure reflected andemitted radiation from atmospheres,rings and surfaces to determine theircompositions, temperatures and struc-tures. The VIMS is an optical instru-ment that splits the light receivedfrom objects into its component wave-lengths. The instrument uses a diffrac-tion grating for this purpose.

The VIMS obtains information over352 contiguous wavelengths from0.35 to 5.1 micrometers. The instru-ment measures the intensities of indi-vidual wavelengths. The data areused to infer the composition andother properties of the object thatemitted the light (such as a distant

The Imaging ScienceSubsystem (ISS) nar-row-angle camera.

The Imaging ScienceSubsystem (ISS) wide-angle camera.

The Visible andInfrared MappingSpectrometer (VIMS).


star), that absorbed specific wave-lengths of the light as it passedthrough (such as a planetary atmo-sphere), or that reflected the light(such as a satellite surface). The VIMSprovides images in which every pixelcontains high-resolution spectra of thecorresponding spot on the ground.

The VIMS has separate infrared andvisible sensor channels. The infraredchannel covers the wavelength range0.85–5.1 micrometers. Its optics in-clude an ƒ/3.5 Ritchey–Chretientelescope that has an aperture of230 millimeters, and a secondarymirror that scans on two axes to pro-duce images varying in size from0.03 degree to several degrees.

Radiation gathered by the telescopepasses through a diffraction grating,which disperses it in wavelength. Acamera refocuses it on to a linear de-tector array. The detector array is ra-diator-cooled to as low as 56 kelvins;the spectrometer’s operating tempera-ture is 125 kelvins.

The visible channel produces multi-spectral images spanning the spec-tral range 0.35–1.05 micrometers. Itutilizes a Shafer telescope and a grat-ing spectrometer. The silicon CCD ar-ray is radiator-cooled to 190 kelvins.

Composite Infrared Spectrometer.The Composite Infrared Spectrometer(CIRS) will measure infrared emis-sions from atmospheres, rings andsurfaces. The CIRS will retrieve verti-cal profiles of temperature and gascomposition for the atmospheres ofTitan and Saturn, from deep in theirtropospheres (lower atmospheres), tohigh in their stratospheres (upper at-mospheres). The CIRS instrument will

also gather information on the ther-mal properties and compositions ofSaturn’s rings and icy satellites.

The CIRS science objectives are asfollows:

• Map the global temperature struc-ture in the Saturn and Titan atmo-spheres.

• Map the global gas composition inthe Saturn and Titan atmospheres.

• Map global information on hazesand clouds in the Saturn and Titanatmospheres.

• Collect information on energeticprocesses in the Saturn and Titanatmospheres.

• Search for new molecularspecies in the Saturn and Titanatmospheres.

• Map the global surface tempera-tures at Titan’s surface.

10–8

CIRS

0.000110–7 10–6 10–5 0.001 0.01Wavelength, meters

VIMS IRVIMS VISISS WACISS NAC

UVIS UVUVIS EUV

0.10.001Particle Energy, electron volts

10 1000

MIMI CHEMS

MIMI LEMMS p

MIMI LEMMS e

CAPS IMS

CAPS IBS

CAPS ELS

INMS

The bar charts atright show the oper-ating wavelength cov-erage for the opticalremote-sensing inves-tigations (above) andenergy range for thefields, particles andwaves investigations(below).

105 107


• Map the composition and thermalcharacteristics of Saturn’s rings andicy satellites.

The CIRS is a coordinated set of threeinterferometers designed to measureinfrared emissions from atmospheres,rings and surfaces over wavelengthsfrom 7 to 1000 micrometers. TheCIRS uses a beamsplitter to divide in-coming infrared light into two paths.The beamsplitter reflects half theenergy toward a moving mirror andtransmits half to a fixed mirror. Thelight is recombined at the detector.As the mirror moves, different wave-lengths of light alternately canceland reinforce each other at a ratethat depends on their wavelengths.This information can be used to con-struct an infrared spectrum.

The CIRS data will be collected bytwo of the instrument’s three interfer-ometers, which are designed to makeprecise measurements of wavelengthwithin a specific range of the electro-magnetic spectrum. The CIRS far in-frared interferometer covers thespectral range 17–1000 microme-ters; its FOV is circular and 0.25 de-gree in diameter.

The CIRS mid infrared interferometeris a conventional Michelson instru-ment that covers the spectral rangeof 7–17 micrometers. The third CIRSinterferometer is used for internal ref-erence. Light enters the instrumentthrough a 51-centimeter-diametertelescope and is sent to the interfer-ometers. The mid infrared detectorsconsist of two 1 × 10 linear arrays.Each square detector in the arraysis 0.015 degree on a side. The midinfrared arrays are cooled to 70 kel-

vins by a passive radiator. The re-mainder of the instrument, includingthe infrared detectors, is cooled to170 kelvins.

Ultraviolet Imaging Spectrograph. TheUltraviolet Imaging Spectrograph(UVIS) is a set of detectors designedto measure ultraviolet light reflectedby or emitted from atmospheres, ringsand surfaces to determine their com-positions, distributions, aerosol con-tent and temperatures. The UVIS willmeasure the fluctuations of sunlightand starlight as the Sun and starsmove behind the rings and atmo-spheres of Saturn and Titan, and willdetermine the atmospheric concen-trations of hydrogen and deuterium.

The UVIS science objectives involvethe following:

• Map the vertical and horizontalcompositions of the Saturn and Ti-tan upper atmospheres.

• Determine the atmospheric chemis-try occurring in the Saturn and Ti-tan atmospheres.

• Map the distributions and proper-ties of aerosols in the Saturn andTitan atmospheres.

• Infer the nature and characteristicsof circulation in the Saturn and Ti-tan atmospheres.

• Map the distributions of neu-trals and ions in Saturn’smagnetosphere.

The Composite Infra-red Spectrometer(CIRS).

The Ultraviolet Imag-ing Spectrograph(UVIS).


• Study the radial structure of Sat-urn’s rings by means of stellaroccultations.

• Study surface ices and any tenu-ous atmospheres associated withthe icy satellites.

The UVIS instrument is a two-channelimaging spectrograph (far and ex-treme ultraviolet) that includes a hy-drogen–deuterium absorption celland a high-speed photometer. An im-aging spectrograph is an instrumentthat records spectral intensity informa-tion in one or more wavelengths ofenergy and then outputs digital datathat can be displayed in a visualform, such as a false-color image.

Each of the two spectrographicchannels utilizes a reflecting tele-scope, a grating spectrometer andan imaging, pulse-counting detector.The telescopes have a focal length of100 millimeters and an FOV of 3.67× 0.34 degrees. The far ultravioletchannel has a wavelength range of

115–190 nanometers; the range ofthe extreme ultraviolet channel is 55–115 nanometers. Spectral resolutionranges from 0.21 to 0.24 nanome-ters. For solar occultation observa-tions, the extreme ultraviolet channelincludes a mechanism that allows sun-light to enter when the Sun is 20 de-grees off the telescope axis.

The absorption cell channel is aphotometer that measures hydrogenand deuterium concentrations. Thehigh-speed photometer measuresundispersed light from its own para-bolic mirror with a photomultipliertube detector. The wavelength rangefor this photometer channel is 115–185 nanometers. The FOV is 0.34× 0.34 degrees. The time resolution is2 milliseconds.

Microwave Remote Sensing

Cassini Radar. The Cassini Radar (RA-DAR) will investigate the surface ofSaturn’s largest moon, Titan. Titan’ssurface is covered by a thick, cloudy

atmosphere that is hidden to normaloptical view, but can be penetratedby radar.

The RADAR uses the five-beam Ku-band (13.78 gigahertz) antenna feedon the high-gain antenna (HGA) tosend RADAR transmissions toward tar-gets. These signals, after reflectionfrom the target, will be captured bythe HGA and detected by the RA-DAR. The RADAR will also operatein a passive mode wherein the instru-ment will measure the blackbodyradiation emitted by Titan. Scienceobjectives of the RADAR include thefollowing:

• Determine if large bodies of liquidexist on Titan, and if so, determinetheir distribution.

• Investigate the geological featuresand topography of Titan’s solidsurface.

• Acquire data on other targets(rings and icy satellites) as condi-tions permit.

The RADAR will take four types of ob-servations: imaging, altimetry, back-scatter and radiometry. In imagingsynthetic aperture radar (SAR) mode,the instrument will record echoes ofmicrowave energy off the surface ofTitan from different incidence angles.The recorded echoes will allow con-struction of visual images of the targetsurface with a surface resolution of0.35–1.7 kilometers.

Radar altimetry similarly involvesbouncing microwave pulses off thesubsatellite surface of the target bodyand measuring the time it takes theecho to return to the spacecraft. Inthis case, however, the goal will notbe to create visual images but ratherto obtain numerical data on the pre-

M I C R O W A V E R E M O T E S E N S I N G

Two microwave re-mote-sensing experi-ments, the CassiniRadar (RADAR) andthe Radio ScienceInstrument (RSS),share the space-craft’s high-gainantenna (HGA).This antenna re-ceives spacecraftcommands fromEarth and sendsscience andspacecraft data

the radio frequencyelectronics subsys-tem (RFES), sits ina penthouse-like at-tachment betweenthe upper equip-

ment moduleand the

HGA.

The RSS compo-nents are housedin the spacecraftbays; the instru-ment also includesradio receiver ele-ments located atNASA’s DeepSpace Networkstations on Earth.

back to Earth; it isalso used for com-munications withthe Huygens Probe.Some RADAR com-ponents are housedin equipment bays inthe upper equipmentmodule, below theHGA; one piece,


cise distance between the surfacefeatures of Titan and the spacecraft.

When these data are combined withthe spacecraft ephemeris, a Titan to-pography map (altitude of surfacefeatures) can be created. The altime-ter resolution is 24–27 kilometers hor-izontal, 90–150 meters vertical.

In scatterometry mode, the RADARwill bounce pulses off Titan’s surfaceand then measure the intensity of theenergy returning. This returning en-ergy, or backscatter, is always lessthan the original pulse since surfacefeatures inevitably reflect the pulsein more than one direction. Fromthe variations in these backscattermeasurements, scientists can inferthe composition and roughness ofthe surface.

Finally, in radiometry mode, the RA-DAR will operate as a passive instru-ment, simply recording the energyemanating from the surface of Titan.This information will tell scientistsabout Titan’s surface properties. Inthe interferometry and scatterometrymodes, the low-resolution (0.35-de-gree) data will be acquired fromlarge amounts of Titan’s surface byscanning the HGA.

The RADAR can operate at altitudesbelow approximately 100,000 kilo-meters. From 100,000 to 25,000 ki-lometers, the RADAR will operate inradiometry mode only to gather dataabout surface temperature and emis-sivity that is related to composition.

Between 22,500 and 9000 kilome-ters, the RADAR will switch betweenscatterometry and radiometry to ob-tain low-resolution global maps of Ti-tan’s surface roughness, backscatterintensity and thermal emissions. At al-

titudes between 9000 and 4000 kilo-meters, the instrument will switch be-tween altimetry and radiometry,collecting surface altitude and ther-mal emission measurements along thesuborbital track. Below 4000 kilome-ters, the RADAR will switch betweenimaging and radiometry.

At the RADAR’s lowest plannedaltitude, 950 kilometers, the maxi-mum image resolution will be 540 ×350 meters. In imaging mode, theRADAR will use all five beams in afan shape to broaden the swath toapproximately six degrees.

Radio Science Instrument. The RadioScience Instrument (RSS) will use thespacecraft radio and ground anten-nas — such as those of NASA’s DeepSpace Network (DSN) — to studythe compositions, pressures and tem-peratures of the atmospheres andionospheres of Saturn and Titan; theradial structure of Saturn’s rings andthe particle size distribution within therings; and planet–satellite masses,gravity fields and ephemerides withinthe Saturn system. During the long in-terplanetary cruise to Saturn, the RSSwill also be used — near solar oppo-sitions to search for gravitationalwaves coming from beyond our solarsystem, and near solar conjunctionsto test general relativity and study thesolar corona. The instrument’s scienceobjectives include the following:

• Search for and characterize gravi-tational waves coming from beyondthe solar system.

• Study the solar corona and gen-eral relativity when Cassini passesbehind the Sun.

• Improve estimates of the massesand ephemerides of Saturn andits satellites.

• Study the radial structure of andparticle size distribution in Saturn’srings.

• Determine temperature and compo-sition profiles within the Saturn andTitan atmospheres.

• Determine temperatures and elec-tron densities in the Saturn and Ti-tan ionospheres.

The RSS instrument is unique in thatit consists of elements on the groundas well as on the spacecraft (seethe sidebar on the next page). Theground elements are DSN 70-meterand 34-meter high-efficiency andbeam waveguide stations.

On the spacecraft, the elements of theRSS are contained in the radio fre-quency subsystem and in the radio

The Cassini Radar(top) and the RadioScience Instrument(RSS).


frequency instrument subsystem. Abody-fixed HGA with its boresightalong the spacecraft’s z-axis will bepointed by moving the spacecraft.The HGA will be configured to oper-ate at S-band, X-band and Ka-bandto support radio science.

Occultation experiments measure therefractions, Doppler shifts (frequencyshifts) and other modifications toradio signals that occur when thespacecraft is “occulted” by (passesbehind) planets, moons, atmospheresor physical features such as planetaryrings. From these measurements, sci-entists can derive information aboutthe structures and compositions of theocculting bodies, atmospheres andthe rings.

Cruise experiments and gravity fieldmeasurements use Doppler and rang-ing two-way measurements. The sig-nal is generated from a reference onthe ground, transformed to the trans-mitting frequencies, amplified andradiated through a Deep Space Net-work antenna. The spacecraft tele-communications subsystem receivesthe carrier signal, transforms it todownlink frequencies coherent withthe uplink, amplifies it and returnsit to Earth. The signal is detectedthrough a ground antenna, amplifiedand saved for later analysis.

Fields, Particles and Waves

Cassini Plasma Spectrometer. TheCassini Plasma Spectrometer (CAPS)will measure the composition, density,

flow velocity and temperature of ionsand electrons in Saturn’s magneto-sphere. The CAPS science objectivesare as follows:

• Measure the composition of ionizedmolecules originating from Saturn’sionosphere and Titan.

• Investigate the sources and sinks ofionospheric plasma — ion inflow/outflow and particle precipitation.

• Study the effect of magnetosphericand ionospheric interaction on ion-ospheric flows.

• Investigate auroral phenomena andSaturn kilometric radiation (SKR)generation.

• Determine the configuration of Sat-urn’s magnetic field.

• Investigate plasma domains and in-ternal boundaries.

• Investigate the interaction of Sat-urn’s magnetosphere with the solarwind and solar-wind driven dynam-ics within the magnetosphere.

• Study the microphysics of the bowshock and magnetosheath.

• Investigate rotationally driven dy-namics, plasma input from the satel-lites and rings and radial transportand angular momentum of the mag-netospheric plasma.

• Investigate magnetotail dynamicsand substorm activity.

• Study reconnection signatures inthe magnetopause and tail.

• Characterize the plasma input tothe magnetosphere from the rings.

• Characterize the role of ring–mag-netosphere interaction in ring parti-cle dynamics and erosion.

• Study dust–plasma interactions andevaluate the role of the magneto-sphere in species transport betweenSaturn’s atmosphere and rings.

R A D I O S C I E N C E I N S T R U M E N TI N O P E R A T I O N

Radio Signal(S-, X- and Ka-Bands)

Processing and Analysis

Occultation ex-periments useone-way mea-surements. Theradio signal isgenerated by anultrastable oscilla-tor aboard thespacecraft. Thespacecraft radiofrequency sub-systems conditionand amplify thesignal for transmis-sion to Earth. After

passing throughthe “feature” ofinterest, the signalis detected via aparabolic DSNantenna, ampli-fied andsaved forlater analysis.


• Study the interaction of the mag-netosphere with Titan’s upper atmo-sphere and ionosphere.

• Evaluate particle precipitation as asource of Titan’s ionosphere.

• Characterize plasma input to themagnetosphere from icy satellites.

• Study the effects of satellite inter-action on magnetospheric particledynamics inside and around thesatellite flux tube.

The CAPS will measure the flux (thenumber of particles arriving at theCAPS per square centimeter persecond per unit of energy and solidangle) of ions in Saturn’s magneto-sphere. The CAPS consists of threesensors: an electron spectrometer,an ion beam spectrometer and anion-mass spectrometer. A motor-drivenactuator rotates the sensor packageto provide 208-degree scanning inazimuth about the –y-symmetry axisof the Cassini Orbiter.

The electron spectrometer makes mea-surements of the energy of incomingelectrons; its energy range is 0.7–30,000 electron volts. The sensor’sFOV is 5 × 160 degrees. The ion-beam spectrometer determines theenergy-to-charge ratio of an ion;its energy range is 1 electron volt to50 kilo–electron volts; its FOV is 1.5× 160 degrees. The ion-mass spec-trometer provides data on both theenergy-to-charge and mass-to-chargeratios. Its mass range is 1–60 atomicmass units (amu); its energy range is1 electron volt to 50 kilo–electronvolts; its FOV is 12 × 160 degrees.

Ion and Neutral Mass Spectrometer.The Ion and Neutral Mass Spectrom-eter (INMS) will determine the compo-

sition and structure of positive ionand neutral species in the upper at-mosphere of Titan and magneto-sphere of Saturn, and will measurethe positive ion and neutral environ-ments of Saturn’s icy satellites andrings. The science objectives are asfollows:

• Measure the in situ compositionand density variations, with al-titude, of low-energy positiveions and neutrals in Titan’s up-per atmosphere.

• Study Titan’s atmosphericchemistry.

• Investigate the interaction of Titan’supper atmosphere with the mag-netosphere and the solar wind.

• Measure the in situ composition oflow-energy positive ions and neu-

trals in the environments of the icysatellites, rings and inner magneto-sphere of Saturn, wherever densi-ties are above the measurementthreshold and ion energies arebelow about 100 electron volts.

The INMS will determine the chemi-cal, elemental and isotopic compo-sition of the gaseous and volatilecomponents of the neutral particlesand the low-energy ions in Titan’satmosphere and ionosphere, Sat-urn’s magnetosphere and the ringenvironment. It will also determinegas velocity.

The principal components of theINMS include an open source (forboth ions and neutral particles), aclosed source (for neutral particles

Six instruments de-signed to study mag-netic fields, particlesand waves aremounted in a varietyof locations on thespacecraft. The

fields and particlespallet holds theCassini PlasmaSpectrometer(CAPS), an Ion andNeutral Mass Spec-trometer (INMS)

and two additionalcomponents of theMagnetosphericImaging Instrument(MIMI). The Cos-mic Dust Analyzer(CDA) and the Ra-

dio and PlasmaWave Science(RPWS) antennasand the magneticsearch coils, alsopart of the RPWS,are attached tovarious locationson the upperequipment mod-ule. The RPWSalso includes aLangmuir probe.The Dual Tech-nique Magnetome-ter sensors arelocated on anextendible boom.

F I E L D S , P A R T I C L E S A N D W A V E S

MIMI Low-EnergyMagnetosphericMeasurementSystem

Ion and NeutralMass Spectrometer

Cassini PlasmaSpectrometer

MIMI Charge-EnergyMass Spectrometer

12-BayElectronicsBus


only) and a quadrupole mass anal-yzer and detector.

The open source analyzes ions asthey enter, but the neutral gases mustfirst be ionized within the instrumentby the impact of electrons from anelectron gun. The closed source mea-sures neutrals along the direction ofincoming atomic and molecular spe-cies. In both ion sources, the neutralsare ionized by electron impact.

Ions emerging from the ion sourcesare directed into the quadrupolemass analyzer; as the ions exit theanalyzer, they go into an ion detec-tor and are counted.

The FOV of the open source for neu-tral species is 16 degrees; the closedsource has a hemispherical FOV. Themass range of the INMS is 1–8 amuand 12–99 amu.

Cosmic Dust Analyzer. The CosmicDust Analyzer (CDA) will provide di-rect observations of small ice or dustparticles in the Saturn system in orderto investigate their physical, chemicaland dynamic properties and studytheir interactions with the rings, icysatellites and magnetosphere of Sat-urn. The CDA science objectives areas follows:

• Extend studies of interplanetarydust (sizes and orbits) to the orbitof Saturn.

• Define dust and meteoroid distri-bution (sizes, orbits, composition)near the rings.

• Map the size distribution of ringmaterial in and near the knownrings.

• Analyze the chemical compositionsof ring particles.

The Cassini PlasmaSpectrometer (CAPS).

The Ion and NeutralMass Spectrometer(INMS).

The Cosmic DustAnalyzer (CDA).


• Study the erosional and electromag-netic processes responsiblefor E-ring structure.

• Search for ring particles beyondthe known E-ring.

• Study the effect of Titan on theSaturn dust complex.

• Study the chemical composition oficy satellites from studies of ejectaparticles.

• Determine the role of icy satellitesas a source of ring particles.

• Determine the role of dust as acharged-particle source or sink inthe magnetosphere.

The CDA measures the flux, velocity,charge, mass and composition ofdust and ice particles in the massrange from 10–16 to 10–6 gram. It hastwo types of sensors, high-rate detec-tors and a dust analyzer. The twohigh-rate detectors, intended primarilyfor measurements in Saturn’s rings,count impacts up to 10,000 per sec-ond, giving the integral flux of dustparticles above the mass thresholdof each detector. The threshold varieswith impact velocity, but since ringparticle orbits are nearly circular,their velocity at a given point doesnot vary much.

The dust analyzer determines theelectric charge carried by dust parti-cles, the flight direction and impactspeed and the mass and chemicalcomposition, at rates up to one par-ticle per second, and for speeds of1–100 kilometers per second. Somedust particles strike a separate chem-ical analyzer in the dust analyzerand also produce impact ionization.A grid accelerates positive ions;ions reaching the grid signal thestart time for the time-of-flight massspectrometer.

Other dust particles pass throughinto the spectrometer and eventuallyreach the ion collector and electronmultiplier. Their time of flight is an in-verse function of the ion mass. Thedistribution of ion masses (atomicweights) gives the chemical compo-sition of the dust particle. The massresolution of the ion spectrum is ap-proximately 50.

An articulation mechanism allows theentire CDA instrument to be rotatedor repositioned, relative to the Cas-sini Orbiter body.

Dual Technique Magnetometer. TheDual Technique Magnetometer(MAG) will determine planetarymagnetic fields and study dynamicinteractions in the planetary environ-ment. The MAG science objectivesinclude the following:

• Determine the internal magneticfield of Saturn.

• Develop a three-dimensional modelof Saturn’s magnetosphere.

• Determine the magnetic state ofTitan and its atmosphere.

• Derive an empirical model of Ti-tan’s electromagnetic environment.

• Investigate the interactions of Titanwith the magnetosphere, magneto-sheath and solar wind.

• Survey ring and dust interactionswith the electromagnetic environ-ment.

• Study the interactions of the icy sat-ellites with Saturn’s magnetosphere.

• Investigate the structure of the mag-netotail and the dynamic processestherein.

The MAG consists of direct-sensinginstruments that detect and measure

the strength of magnetic fields in thevicinity of the spacecraft. The MAGcomprises both a flux gate magne-tometer and a vector/scalar heliummagnetometer. The flux gate magne-tometer is used to make vector fieldmeasurements. The vector/scalar he-lium magnetometer is used to makevector (magnitude and direction) andscalar (magnitude only) measure-ments of magnetic fields.

Since magnetometers are sensitive toelectric currents and ferrous compo-nents, they are generally placed onan extended boom, as far from thespacecraft as possible. On Cassini,the flux gate magnetometer is located

The Dual TechniqueMagnetometer(MAG) comprises theflux gate magnetome-ter (above) and thevector/scalar heliummagnetometer (left).


Composite Infrared Spec-trometer – CIRS

Imaging Science Sub-system – ISS

Ultraviolet Imaging Spec-trograph – UVIS

Visible and Infrared Map-ping Spectrometer – VIMS

Cassini Radar –RADAR

Radio Science Instrument– RSS

Magnetospheric ImagingInstrument – MIMI

Cassini Plasma Spectrome-ter – CAPS

Cosmic Dust Analyzer –CDA

Dual Technique Magne-tometer – MAG

Ion and Neutral MassSpectrometer – INMS

Radio and Plasma WaveScience – RPWS

C A S S I N I O R B I T E R I N S T R U M E N T S

Measurements

High-resolution spectra,7–1000 µm

Photometric images throughfilters, 0.2–1.1 µm

Spectral images, 0.055–0.190 µm;occultation photometry, 2 ms; Hand D spectroscopy, 0.0002-µmresolution

Spectral images, 0.35–1.05 µm(0.073-µm resolution); 0.85–5.1 µm (0.166-µm resolution);occultation photometry

Ku-band RADAR images (13.8GHz); radiometry resolution lessthan 5 K

Ka-, S- and X-bands; frequency,phase, timing and amplitude

Image energetic neutrals and ionsat less than 10 keV to 8 MeV pernucleon; composition, 10–265keV/e; charge state; directionalflux; mass spec: 20 keV to 130MeV ions; 15 keV to greater than11 MeV electrons, directional flux

Particle energy/charge, 0.7–30,000 eV/e; 1–50,000 eV/e

Directional flux and mass of dustparticles in the range 10–16–10–6 g

B DC to 4 Hz up to 256 nT; scalarfield DC to 20 Hz up to 44,000 nT

Fluxes of +ions and neutrals inmass range 1–66 amu

E, 10 Hz–2MHz; B, 1 Hz–20 kHz;plasma density

Techniques

Spectroscopy using 3 interferometricspectrometers

Imaging with CCD detectors; 1 wide-angle camera (61.2 mr FOV); 1narrow-angle camera (6.1 mr FOV)

Imaging spectroscopy, 2 spectrometers;hydrogen–deuterium absorption cell

Imaging spectroscopy, 2 spectrometers

Synthetic aperture radar; radiometrywith a microwave receiver

X- and Ka-band transmissions toCassini Orbiter; K

a-, S- and X-band

transmissions to Earth

Particle detection and imaging; ion-neutral camera (time-of-flight, totalenergy detector); charge energy massspectrometer; solid-state detectorswith magnetic focusing telescope andaperture-controlled ~45° FOV

Particle detection and spectroscopy;electron spectrometer; ion-mass spec-trometer; ion-beam spectrometer

Impact-induced currents

Magnetic field measurement; fluxgate magnetometer; vector–scalarmagnetometer

Mass spectrometry

Radio frequency receivers; 3 electricdipole antennas; 3 magnetic searchcoils; Langmuir probe current

Partner Nations

US, France, Germany,Italy, UK

US, France, Germany,UK

US, France, Germany

US, France, Germany,Italy

US, France, Italy, UK

US, Italy

US, France, Germany

US, Finland, France,Hungary, Norway, UK

Germany, Czech Republic,France, The Netherlands,Norway, UK, US

UK, Germany, US

US, Germany

US, Austria, France,The Netherlands,Sweden, UK

Optical Remote Sensing

Radio Remote Sensing

Particle Remote Sensing & In Situ Measurement

In Situ Measurement


midway out on the magnetometerboom, and the vector/scalar heliummagnetometer is located at the end ofthe boom. The boom itself, composedof thin, nonmetallic rods, will be fold-ed during launch and deployed longafter the spacecraft separates fromthe launch vehicle and shortly beforethe Earth flyby. The magnetometerelectronics are in a spacecraft bay.

The use of two separate magnetome-ters at different locations aids in dis-tinguishing the ambient magneticfield from that produced by thespacecraft. The helium magnetom-eter has full-scale flux ranges of 32–256 nanoteslas in vector mode and256–16,000 nanoteslas in scalarmode. The flux gate magnetometer’sflux range is 40–44,000 nanoteslas.

Magnetospheric Imaging Instrument.The Magnetospheric Imaging Instru-ment (MIMI) is designed to measurethe composition, charge state and en-ergy distribution of energetic ions andelectrons; detect fast neutral species;and conduct remote imaging of Sat-urn’s magnetosphere. This informationwill be used to study the overall con-figuration and dynamics of the mag-netosphere and its interactions withthe solar wind, Saturn’s atmosphere,Titan, rings and the icy satellites. Thescience objectives are as follows:

• Determine the global configurationand dynamics of hot plasma in themagnetosphere of Saturn.

• Monitor and model magneto-spheric, substorm-like activity andcorrelate this activity with Saturnkilometric radiation observations.

• Study magnetosphere/ionospherecoupling through remote sensing ofaurora and measurements of ener-getic ions and electrons.

• Investigate plasma energizationand circulation processes in themagnetotail of Saturn.

• Determine through imaging andcomposition studies the magneto-sphere–satellite interactions at Sat-urn, and understand the formationof clouds of neutral hydrogen, nitro-gen and water products.

• Measure electron losses due to in-teractions with whistler waves.

• Study the global structure andtemporal variability of Titan’satmosphere.

• Monitor the loss rate and compo-sition of particles lost from Titan’satmosphere due to ionizationand pickup.

• Study Titan’s interaction with themagnetosphere of Saturn and thesolar wind.

• Determine the importance of Ti-tan’s exosphere as a source for theatomic hydrogen torus in Saturn’souter magnetosphere.

• Investigate the absorption of ener-getic ions and electrons by Saturn’srings and icy satellites.

• Analyze Dione’s exosphere.

The MIMI will provide images of theplasma surrounding Saturn and deter-mine ion charge and composition.The MIMI has three sensors that per-form various measurements: the low-energy magnetospheric measurementsystem (LEMMS), the charge-energy-mass spectrometer (CHEMS) and theion and neutral camera (INCA).

The LEMMS will measure low- andhigh-energy proton, ion and electronangular distributions (the number ofparticles coming from each direction).The LEMMS sensor is mounted on a

scan platform that is capable of turn-ing 180 degrees. The sensor providesdirectional and energy information onelectrons from 15 kilo–electron voltsto 10 mega–electron volts, protons at15–130 mega–electron volts and oth-er ions from 20 kilo–electron volts to10.5 mega–electron volts per nucle-on. The LEMMS head is double-end-ed, with oppositely directed FOVs of15 and 45 degrees.

The CHEMS uses an electrostatic ana-lyzer, a time-of-flight mass spectrome-ter and microchannel plate detectorsto measure the charge and composi-tion of ions at 10–265 kilo–electronvolts per electron. Its mass/charge

The MagnetosphericImaging Instrument(MIMI) low-energymagnetosphericmeasurement system,LEMMS (above),and the MIMI charge-energy-mass spectrom-eter, CHEMS (left).


range is 1–60 amu/e (elements hy-drogen–iron); its molecular ion massrange is 2–120 amu.

The third MIMI sensor, the INCA,makes two different types of measure-ments. It will obtain three-dimensionaldistributions, velocities and the roughcomposition of magnetospheric andinterplanetary ions with energies from10 kilo–electron volts to about eightmega–electron volts per nucleon forregions with low energetic ion fluxes.The instrument will also obtain remoteimages of the global distribution ofthe energetic neutral emission ofhot plasmas in the Saturn magneto-sphere, measuring the compositionand velocities of those energetic neu-trals. The FOV is 120 × 90 degrees.

The MIMI sensors share common elec-tronics and provide complementarymeasurements of energetic plasmadistribution, composition and energyspectrum, and the interaction of thatplasma with the extended atmosphereand satellites of Saturn.

Radio and Plasma Wave Science.The Radio and Plasma Wave Science(RPWS) instrument will measure theelectrical and magnetic fields in theplasma of the interplanetary mediumand Saturn’s magnetosphere, as well

as electron density and temperature.Science objectives of the RPWS instru-ment include the following:

• Study the configuration of Saturn’smagnetic field and its relationshipto Saturn kilometric radiation (SKR).

• Monitor and map the sources ofSKR.

• Study daily variations in Saturn’sionosphere and search for outflow-ing plasma in the magnetic cuspregion.

• Study radio signals from lightningin Saturn’s atmosphere.

• Investigate Saturn electric dis-charges (SED).

• Determine the current systems inSaturn’s magnetosphere and studythe composition, sources and sinksof magnetospheric plasma.

• Investigate the dynamics of themagnetosphere with the solar wind,satellites and rings.

• Study the rings as a source of mag-netospheric plasma.

• Look for plasma waves associatedwith ring spoke phenomena.

• Determine the dust and meteroiddistributions throughout the Saturnsystem and interplanetary space.

• Study waves and turbulence gener-ated by the interaction of chargeddust grains with the magnetosphericplasma.

• Investigate the interactions of theicy satellites and the ring systems.

• Measure electron density and tem-perature in the vicinity of Titan.

• Study the ionization of Titan’s up-per atmosphere and ionosphereand the interactions of the atmo-sphere and exosphere with thesurrounding plasma.

• Investigate the production, trans-port, and loss of plasma fromTitan’s upper atmosphere andionosphere.

• Search for radio signals fromlightning in Titan’s atmosphere,a possible source for atmosphericchemistry.

• Study the interaction of Titan withthe solar wind and magnetosphericplasma.

• Study Titan’s vast hydrogen torusas a source of magnetosphericplasma.

• Study Titan’s inducedmagnetosphere.

The MIMI ion andneutral camera,INCA (right), andthe Radio and Plas-ma Wave Science(RPWS) instrument(below).


The RPWS instrument will be usedto investigate electric and magneticwaves in space plasma at Saturn. Thesolar wind is a plasma; plasma maybe “contained” within magnetic fields(that is, the magnetospheres) of bod-ies such as Saturn and Titan. TheRPWS instrument will measure theelectric and magnetic fields in the in-terplanetary medium and planetarymagnetospheres and will directlymeasure the electron density and tem-perature of the plasma in the vicinityof the spacecraft.

The major components of the RPWSinstrument are an electric field sensor,a magnetic search coil assembly andthe Langmuir probe. The electric fieldsensor is made up of three deploy-able antenna elements mounted onthe upper equipment module of theCassini Orbiter. Each element is acollapsible beryllium copper tube thatis rolled up during launch and subse-quently unrolled to its 10-meter lengthby a motor drive.

The magnetic search coil assemblyincludes three orthogonal coilsabout 25 millimeters in diameterand 260 millimeters long. The mag-netic search coils are mounted on asmall platform attached to a supportfor the HGA. The Langmuir probe,which measures electron densityand temperature, is a metallic sphere50 millimeters in diameter. The probeis attached to the same platform by aone-meter deployable boom.

Signals from the sensors go to a num-ber of receivers, which provide lowand high time and frequency resolu-tion measurements. The instrumentranges are one hertz to 16 mega-hertz for electric fields; one hertz

to 12.6 kilohertz for magnetic fields;electron densities of 5–10,000 elec-trons per cubic centimeter; and elec-tron temperatures equivalent to 0.1–4 electron volts.

Probe Instruments OverviewThe Huygens Probe, provided by theEuropean Space Agency (ESA), willfly to the Saturn system aboard theCassini–Huygens spacecraft. Carry-ing six science instruments, the Probewill study the atmosphere and surfaceof Saturn’s largest satellite, Titan, de-scending through Titan’s atmosphereand landing — either on solid land orin a liquid lake or ocean.

Many countries have participatedin the development of the HuygensProbe’s six instruments. The instru-ments are the Huygens AtmosphericStructure Instrument (HASI), the Aero-sol Collector and Pyrolyser (ACP), theGas Chromatograph and Mass Spec-

trometer (GCMS), the Descent Imagerand Spectral Radiometer (DISR), theDoppler Wind Experiment (DWE)and the Surface Science Package(SSP). These instruments comprise39 sensors; the total mass of the sci-ence payload is 48 kilograms.

Once the Probe has separated fromthe Orbiter, it is wholly autonomous.The instruments are turned on in apreprogrammed sequence after theProbe’s heat shield is released. Probeinstrument operation is controlled bytimers, acceleration sensors, altime-ters and a Sun sensor. All instrumentsuse time-based operation from thebeginning of the descent down to20 kilometers. Three of the instru-ments, the DISR, the HASI and theSSP, use measured altitude below20 kilometers. Up to three hours ofProbe data will be relayed to the Or-biter for later transmission to Earth.

Probe Instrument DescriptionsHuygens Atmospheric Structure Instru-ment. The Huygens AtmosphericStructure Instrument (HASI) investi-gates the physical and electricalproperties of Titan’s atmosphere, in-cluding temperature, pressure and at-mospheric density as a function ofaltitude and wind gusts — and in theevent of a landing on a liquid surface— wave motion. Comprising a vari-ety of sensors, the HASI will alsomeasure the ion and electron conduc-tivities of the atmosphere and searchfor electromagnetic wave activity.

Atmospheric pressure is measured bythe deflection of a diaphragm. A tubeinlet is mounted on a stub extendingoutside the Probe. Thermometersmounted on the stub measure atmo-spheric temperature.

The Cassini–Huygensspacecraft, with ther-mal blankets on, in itsflight-ready state.


Huygens AtmosphericStructure Instrument – HASI

Gas Chromatograph and MassSpectrometer – GCMS

Aerosol Collector andPyrolyser – ACP

Descent Imager and SpectralRadiometer – DISR

Doppler Wind Experiment –DWE

Surface Science Package –SSP

H U Y G E N S P R O B E I N S T R U M E N T S

Measurements

Temp: 50–300 K; Pres: 0–2000mbar; Grav: 1 µg–20 mg; AC E-field: 0–10 kHz, 80 dB at 2 µVm–1

Hz–0.5; DC E-field: 50 dB at 40 mV/m; electrical conductivity: 10–15 Ω/m to ∞; relative permittivity: 1–∞;acoustic: 0–5 kHz, 90 dB at 5 mPa

Mass range: 2–146 amu; Dynamicrange: >108; Sensitivity: 10–12 mix-ing ratio; Mass resolution: 10–6 at60 amu

2 samples: 150–45 km,30–15 km altitude

Upward and downward spectra:480–960 nm, 0.87–1.7 µm; reso-lution 2.4–6.3 nm; downward andside-looking images: 0.66–1 µm;solar aureole photometry: 550 nm,939 nm; surface spectralreflectance

(Allan Variance)1/2: 10–11 (in 1 s),5 × 10–12 (in 10 s), 10–12 (in 100 s),corresponding to wind velocitiesof 2 m/s to 200 m/s, Probe spin

Gravity: 0–100 g; Tilt: ±60°; Temp:65–100 K; thermal conductivity:0–400 mW m–1K–1; speed of sound:150–2000 m/s; liquid density:400–700 kg m–3; refractive index:1.25–1.45

Partner Nations

Italy, Austria,Finland, Germany,France, The Neth-erlands, Norway,Spain, US, UK

US, Austria,France

France, Austria,Belgium, US

US, Germany,France

Germany, France,Italy, US

UK, Italy, TheNetherlands, US

Techniques

Direct measurements using“laboratory” methods

Chromatography and mass spec-trometry; 3 parallel chromato-graphic columns; quadrupole massfilter; 5 electron impact sources

3-step pyrolysis; 20°C, 250°C,650°C

Spectrometry, imaging, photometryand surface illumination by lamp

Doppler shift of Huygens Probetelemetry signal, signal attenuation

Impact acceleration; acousticsounding, liquid relative permittivi-ty, density and index of refraction

To determine the density of the atmo-sphere, an accelerometer measuresacceleration along the spin axis.Three additional accelerometers mea-sure acceleration along all three axesof the Probe over a range of ±20 g.A microphone is also part of theHASI; it senses acoustic noise fromsources such as thunder, rain orwind gusts.

A permittivity and wave analyzer,consisting of an array of six elec-

trodes, is mounted on two deploy-able booms. Measurements of themagnitude and phase of the receivedsignal give the permittivity and elec-tronic conductivity of the atmosphereand the surface. Electromagnetic sig-nals such as those of lightning canalso be detected. The instrument alsoprocesses the signal from the Probe’sradar altimeter to obtain informationon surface topography, roughnessand electrical properties.

Aerosol Collector and Pyrolyser. TheAerosol Collector and Pyrolyser(ACP) captures aerosol particles fromTitan’s atmosphere using a deploy-able sampling device extended inthe air flow below the nose of theProbe. The samples are heated inovens to vaporize the volatiles anddecompose the complex organic ma-terials. The products are then passedto the GCMS for analysis.

The ACP will obtain samples at twoaltitude ranges. The first sample, at


the surface. Following a safe landing,the GCMS can determine Titan’s sur-face composition.

The GCMS uses an inlet port to col-lect samples of the atmosphere andhas an outlet port at a low pressurepoint. The instrument contains threechromatographic columns. Onecolumn has an absorber chosento separate carbon monoxide, nitro-gen and other gases. Another columnhas an absorber that will separatenitriles and other highly polar com-pounds. The third is to separate hy-drocarbons up to C8. The mass rangeis 2–146 amu.

The Mass Spectrometer serves as thedetector for the Gas Chromatograph,for unseparated atmospheric samplesand for samples provided by theACP. Portions of the GCMS are iden-tical in design to the Orbiter’s INMS.

Descent Imager and Spectral Radiome-ter. The Descent Imager and SpectralRadiometer (DISR) uses several instru-ment fields of view and 13 sensors,operating at wavelengths of 350–1700 nanometers, to obtain a varietyof imaging and spectral observations.The thermal balance of the atmo-sphere and surface can inferred bymeasuring the upward and down-ward flux of radiation.

Solar aureole sensors will measurethe light intensity around the Sun re-sulting from scattering by aerosols,permitting calculations of the size andnumber density of suspended parti-cles. Infrared and visible imagers willobserve the surface during the latterstages of the descent. Using theProbe’s rotation, the imagers will

build a mosaic of pictures of the Titanlandscape. A side-looking visible im-ager will view the horizon and takepictures of the clouds, if any exist. Forspectral measurements of the surface,a lamp will be turned on shortly be-fore landing to provide enough lightfor measuring surface composition.

The DISR will obtain data to help de-termine the concentrations of atmo-spheric gases such as methane andargon. DISR images will also deter-mine if the local surface is solid or liq-uid. If the surface is solid, DISR willreveal topographic details. If the sur-face is liquid, and waves exist, DISRwill photograph them.

DISR sensors include three framingimagers, looking downward and hori-zontally; a spectrometer dispersinglight from two sets of optics lookingdownward and upward; and foursolar aureole radiometers. The spec-tral range of the imagers is 660–1000 nanometers; the spectrometer’srange is 480–960 nanometers; theaureole radiometers operate at 475–525 and 910–960 nanometers, withtwo different polarizations.

Separate downward- and upward-looking optics are linked by fiberopticbundles to an infrared grating spec-trometer. The infrared detectors havea spectral range of 870–1700 na-nometers. There are also two violetphotometers, looking downwardand upward with a bandwidth of350–470 micrometers.

To provide reference and timing forthe other measurements, the DISRuses a Sun sensor to measure the so-lar azimuth and zenith angle relativeto the rotating Probe.

The interior of theHuygens Probe,showing its scienceinstruments.

altitudes down to 40 kilometersabove the surface, will be obtainedprimarily by direct impact of the at-mosphere on a cold filter target. Thesecond sample will be obtained atabout 20 kilometers by pumping theatmosphere through the filter.

After each collection, the filter will betransferred to an oven and heated tothree successively higher tempera-tures, up to about 650 degrees Cel-sius, to vaporize and pyrolyze thecollected material. The product ateach temperature will be swept up bynitrogen carrier gas and transferredto the GCMS for analysis.

Gas Chromatograph and Mass Spec-trometer. The Gas Chromatographand Mass Spectrometer (GCMS) pro-vides a quantitative analysis of thecomposition of Titan’s atmosphere.Atmospheric samples are transferredinto the instrument by dynamic pres-sure as the Probe descends throughthe atmosphere. The Mass Spectrome-ter constructs a spectrum of the molec-ular masses of the gas driven into theinstrument. Just prior to landing, theinlet port of the GCMS is heated tovaporize material on contact with


Doppler Wind Experiment. The Dop-pler Wind Experiment (DWE) usestwo ultrastable oscillators (USOs),one on the Probe and one on theOrbiter, to give Huygens’ relay linka stable carrier frequency. Orbitermeasurements of the shift in Probefrequency (Doppler shift) will provideinformation on the Probe’s motionfrom which a height profile of thezonal wind (the component of windalong the line of sight) and its turbu-lence can be derived.

The output frequency of each USO isset by a rubidium oscillator. The sig-nal (which is like a very high preci-sion clock) received from the Probeis compared with that of the OrbiterUSO, and the difference in frequencyis recorded and stored for transmis-sion to Earth. This information is usedto determine the Doppler velocity be-

tween the Probe and the Orbiter.Modulation of this signal will provideadditional data on the Probe spinrate, spin phase and parachuteswing. Winds will be measured toa precision of one meter per second.

Surface Science Package. The SurfaceScience Package (SSP) contains anumber of sensors to determine thephysical properties and compositionof Titan’s surface. Some of the SSPsensors will also perform atmosphericmeasurements during descent.

During descent, measurements ofthe speed of sound will give infor-mation on atmospheric compositionand temperature. An accelerometerrecords the deceleration profile at im-pact, indicating the hardness of thesurface. Tilt sensors (liquid-filled tubeswith electrodes) will measure anypendulum motion of the Probe during

descent, indicate the Probe orienta-tion after landing and measure anywave motion.

If the surface is liquid, an openingat the bottom of the Probe body, witha vent extending upward along theProbe axis, will admit liquid, whichwill fill the space between a pair ofelectrodes. The capacitance betweenthe electrodes gives the dielectric con-stant of the liquid; the resistance givesthe electrical conductivity. A float withelectrical position sensors determinesthe liquid’s density.

A sensor to measure the refractive in-dex of the liquid has light-emitting di-ode (LED) light sources, a prism witha curved surface and a linear photo-diode detector array. The position ofthe light/dark transition on the detec-tor array indicates the refractive in-

Like the CassiniOrbiter, Huygensrepresents an inter-national collabora-tion. The partnersinvolved in theProbe effort areAustria, Belgium,the EuropeanSpace Agency, Fin-land, France, Ger-many, Italy, TheNetherlands, Nor-way, Spain, theUnited Kingdomand the UnitedStates. The Huy-gens science instru-

P R O B I N G T I T A N ’ S D E P T H S

ments comprise atotal of 39 sensors.The total mass ofthe Probe sciencepayload is 48 kilo-grams. Once theProbe separatesfrom the Orbiter, itis wholly autono-mous. The instru-ments will turn on ina preprogrammedsequence after theProbe cover is re-leased. Probe oper-ation is controlledby timers, accelera-tion sensors, altime-ters and a Sunsensor. As muchas three hours of

Probe data will be re-layed to the Orbiterand later transmittedto Earth.

Gas Chromatographand Mass Spectrometer

Aerosol Collectorand Pyrolyser

Surface Science package“Top Hat”

Batteries

Huygens AtmosphericStructure Instrument(HASI) DeployableBooms

HASI Stud

Radar AltimeterAntenna


dex. A group of platinum resistancewires, two of which can be heated,will measure temperature and thermalconductivity of the surface and loweratmosphere and the heat capacityof the surface material. The acousticsounder, which will then work asa sonar, will conduct an acousticsounding of liquid depth — if theProbe lands in liquid.

Instrument OperationsCassini Orbiter. The power availableto the Cassini Orbiter is not sufficientto operate all the instruments and en-gineering subsystems simultaneously.Operations are therefore divided intoa number of operational modes. Forexample, during much of the orbitaltour of Saturn, 16 hours in remote-sensing mode will often alternate witheight hours in fields and particles andwaves and downlink mode.

Other science modes will be usedduring satellite flybys, occultations,cruise to Saturn and so on. Instru-ments that are not gathering datawill generally not turn off duringthe orbital tour, but will be in a low-power “sleep” state. This strategyis designed to reduce on–off thermalcycling, keep high voltages on (toavoid having to turn up voltageslowly each time it is required) andto preserve the onboard computermemories (to avoid having to reloadthem each time).

The operational modes differ incharacteristics other than power.In remote sensing, for example, theOrbiter is oriented to point remote-sensing instruments toward their ob-jects of interest. This means that the

HGA generally cannot be pointedtoward Earth, so telemetry is storedin the solid-state recorders for latertransmission.

In fields, particles, waves and down-link mode, the HGA is pointed to-ward Earth — permitting transmissionof stored and real-time telemetry —and the Orbiter is rolled about theantenna axis to provide scanningabout another axis in addition to thearticulation axes of some instruments.

The bit rate available on the com-mand and data subsystem databusis not high enough to permit all instru-ments to output telemetry simulta-neously at their maximum rates. TheOrbiter is switched among a numberof different telemetry modes in which

the available bit rate is allocated dif-ferently among the instruments.

Some of the instruments will adjusttheir operating state or parametersdepending on the activities of thespacecraft or other instruments andthe kind of environment the Orbiter isencountering. When these other con-ditions are predictable from the com-mand sequence, commands for theinstruments will be set accordingly.

When conditions are not predictable,or if it is simpler to handle the adjust-ment on board, the command anddata subsystem relays information tothe instruments that need it. Specifi-cally, information on spacecraft atti-tude and its rate of change, warningsof thruster firings, measurements ofthe magnetic field vector and noticesof operation of the sounder and Lang-muir probe in the RPWS instrument

The Huygens Probeis released on a tra-jectory to enter Ti-tan’s atmosphere.


are broadcast for use by the CAPS,CDA and MIMI.

Huygens Probe. There is no radiotransmission link to the HuygensProbe after it separates from theOrbiter; the Probe is wholly autono-mous. The instruments are turned onin a preprogrammed sequence afterthe Probe cover is released. TheProbe goes through five successivepower configurations in which theavailable power is allocated differ-ently among the various instruments.Operation is controlled by timers,acceleration sensors, altimeters anda Sun sensor. The Probe’s commandand data management subsystembroadcasts altitude and spin-rate datato the instruments. Data collection in-volves three successive steps in whichthe available data rate is allocateddifferently among the instruments.

SummaryThe Cassini–Huygens mission willcarry 18 scientific instruments to theSaturn system. After the spacecraft isinserted into Saturn orbit, it will sepa-rate into a Saturn Orbiter and an at-mospheric Probe, called Huygens,

ice particles. Cassini’s maximumdownlink rate from Saturn to Earthis 166 kilobits per second.

The Probe is spin-stabilized for thecoast to Titan. A heat shield deceler-ates it and protects it from heat dur-ing entry into Titan’s atmosphere.Parachutes then slow its descent tothe surface and provide a stable plat-form for taking images and makingother measurements. A set of smallvanes on the bottom skirt of the Probeforces it to rotate, providing 360-de-gree viewing of the landscape.

The Probe carries six instruments, in-cluding sensors to determine atmo-spheric physical properties andchemical composition. Radiometricand optical sensors will provide dataon temperatures and thermal balanceand obtain images of Titan’s atmo-sphere and surface. Doppler mea-surements over the radio link fromProbe to Orbiter will provide windprofiles. Surface sensors are carriedto measure impact acceleration, ther-mal properties of the surface materialand, if the surface is liquid, its densi-ty, refractive index, electrical proper-ties and acoustic velocity. The Probereturns its data via an S-band link tothe Orbiter.

which will descend to the surface ofTitan. The Orbiter will orbit the planetfor four years, with close flybys ofEnceladus, Dione, Rhea, Hyperionand Iapetus, and multiple close flybysof Titan.

The Orbiter is three-axis stabilized.Its 12 science instruments are body-mounted; the spacecraft must beturned to point them toward objectsof interest. Optical instruments pro-vide imagery and spectrometry atwavelengths from 55 nanometers toone millimeter. A radar instrumentsupplies synthetic aperture imaging,altimetry and microwave radiometry.S-band, X-band and Ka-band linkmeasurements between the Orbiterand Earth will provide informationabout intervening material and grav-ity fields. Fields and particles instru-ments will measure magnetic andelectric fields, plasma properties andthe flux and properties of dust and

A N E N S E M B L E E F F O R T 1 2 1

M

C H A P T E R 1 0

ission operations are the ensemble of actions to plan andexecute the launch and subsequent activities of a space-craft, including data return. And since that data flow is di-vided into uplink and downlink, operations processes alsofall into these two categories. Uplink encompasses plan-

General Mission Operations

Mission operations involve complexinteractions among people, comput-ers, electronics, and even heavymachinery (at NASA’s Deep SpaceNetwork antenna sites). Planning formission “ops” begins with the concep-tion of the mission itself, and involvesassumptions about the frequency ofcommunication with the spacecraftand data transfer rates and volumes.These, in turn, drive the design of thespacecraft hardware and softwareand of the ground system that willbe manipulating the spacecraft duringits interplanetary cruise and missionoperations phases.

Sequence Planning

After basic design decisions havebeen made, their effects on day-to-day and long-term operations areconsidered, and a system is devel-oped to enable spacecraft engineer-ing, health and safety requirementsand science data acquisition require-ments to be met. The period of mis-sion operations, starting shortly afterlaunch, is divided into a series of in-tervals. These intervals may be con-strained by engineering requirements(e.g., limitations on available space-craft memory), mission events (e.g., aplanetary flyby), flight rules (e.g., thespacecraft must have communications

with the ground every 240 hours) orother constraints.

A “sequence” is prepared for eachplanning interval. The sequence isa series of computer commands thattells the spacecraft and its engineer-ing and science subsystems whatto do and when. These commandscover everything from the mundane,such as turning a heater on, to thesublime, such as aiming the cameraand taking a sequence of approachimages to make into a movie.

Uplink and Downlink

Most of the human interaction occursat the beginning of sequence plan-ning, when the push-pull of spacecraftengineering requirements and sci-ence measurement requirements anddesires must be resolved. (Note thatthe push-pull of competing science re-quests must have been resolved first.)After what may be long hours of ne-gotiations, the interests of all the con-cerned parties are met (more or less)and the actual work of sequencepreparation can begin. This involvesfirst generating human-readable com-mands (for proofreading purposes)and then converting them into thecomputer code words that the space-

ning mission activities; developing and radiating instru-ment and spacecraft commands, and execution of thosecommands. Downlink encompasses data collection; trans-mission to the ground, and data processing and analysisfor system performance evaluation and science studies.

“Aces,” as missioncontrollers areknown, provideround-the-clockmonitoring of deepspace explorationmissions such asCassini–Huygens.

An Ensemble Effort


craft can understand. The computercodes are then packaged into radio-transmissible “tones,” instructions aregiven to the antenna on where topoint, and the sequence is radiatedto the spacecraft. The transmission isreceived and decoded by the space-craft, which sends an acknowledg-ment and executes the commands.This portion of mission operations isreferred to as uplink.

Downlink is what follows. In thecourse of executing the commands,both engineering subsystems and sci-ence instruments generate data thatare often stored on the spacecraft fora while before they are coded andpackaged for transmission to theground as telemetry. On Earth, thedata are decoded and forwarded tothe engineering and science teamsconcerned with those particularactivities on the spacecraft. The dataare analyzed and engineering reportsand science publications are gener-ated for dissemination to their respec-tive communities.

Signals Through Space

None of this happens instantaneouslynor easily. Electromagnetic radiation— whether it is visible light withwavelengths in the range of 400to 700 nanometers (400–700 bil-lionths of a meter) or radio waveswith wavelengths from 1 millimeter to30 kilometers or longer (frequenciesof 300 gigahertz to 10 kilohertz orlower) — take time to travel throughspace. The velocity of electromagneticradiation in a vacuum is 299,792 ki-lometers per second. One-way light-times in excess of a few seconds (the

equivalent distance to the Moon isabout 1.3 seconds) make “joystick-ing” the spacecraft impractical.While provision is made for realtimecommands, especially planned onesand those needed in response toemergencies, preplanned sequencesare almost always used for com-manding the spacecraft.

The amount of electromagnetic radia-tion (emitted by a point source) pass-ing through a unit area decreases asthe square of the distance. Thus, aspacecraft twice as far as Earth isfrom the Sun receives only one-fourthof the light and heat that it would re-ceive at Earth’s distance.

What is true of light and heat is trueof radio transmissions as well, and itaffects how telecommunications areaccomplished. The radio transmittersand receivers on the ground andon the spacecraft are equipped withparaboloidal antennas that directtransmissions to the receiver and con-centrate received radio energy, maxi-mizing signal to the preamplifier–receiver–amplifier chain.

Large antennas are a necessity.While a commercial radio sta-tion may generate as much as50,000 watts in its signal to yourcar radio a few tens or hundreds ofkilometers away, a spacecraft typical-ly has only a 20-watt transmitter thatmust reliably send signals over hun-dreds of millions or billions of kilome-ters across the solar system.

Spacecraft communications are typi-cally accomplished using radio fre-quencies in S-band (2–4 gigahertz)or X-band (7–12 gigahertz). Someradio science measurements madewith a spacecraft — for example,tests of Einstein’s general theory ofrelativity — are made in Ka-band(32–34 gigahertz). (Radio astronomi-cal observations from the ground andfrom spacecraft are made across awide region of the spectrum.)

A Worldwide Effort

Cassini operations involve numerouspeople at many sites around theworld. Careful coordination of the ef-forts of the science, engineering,planning, navigation and sequencingteams will permit the Orbiter andProbe to return a cache of scientificdata on Saturn and its environs thatwill increase our understandingmanyfold and allow scientists manyyears of additional study after theend of the mission.

Cassini–Huygens Operations

The purpose of Cassini–Huygens mis-sion operations is to launch an instru-mented spacecraft to orbit Saturn anddeliver a Probe to Titan in order toaccomplish primary scientific objec-tives. The program is a joint undertak-ing among NASA, the EuropeanSpace Agency (ESA) and the Italianspace agency, Agenzia Spaziale Ital-iana (ASI).

Mission Description

Cassini will be launched aboard aTitan IVB/Centaur launch vehicle withSolid Rocket Motor Upgrades and in-jected into a 6.7-year trajectory to


Development

Sequence Input Development

Administration

P L A N E T A R Y M I S S I O N O P E R A T I O N ST H E J P L W A Y

Sequence Input Development

Function Primary Activities

Mission Planning Generate a mission plan and mission phase plans. Generate mission and flight

rules and constraints.

Integrated Sequence Generate detailed timelines of activities. Generate integrated files of valid com-

mands. Generate sequence and realtime command loads for radiation to the

spacecraft.

Mission Control Configure and control the ground system. Monitor in real time the spacecraft’s

and payload’s activities, health and safety.

Navigation Predict and reconstruct the spacecraft’s trajectory. Design maneuvers to correct

the trajectory to achieve mission objectives.

Spacecraft Analysis and Plan, design and integrate engineering activities. Maintain the health and safety

of the spacecraft through non-realtime analyses and anomaly identification and

resolution.

Payload Analysis and Generate a science operations plan. Design and integrate instrument observa-

tions. Maintain the health and safety of the payload through non-realtime analy-

ses and anomaly identification and resolution.

Science Processing Generate and archive processed data in support of payload analysis and for the

purpose of scientific research.

Database Management Provide throughout the mission realtime accessible engineering and science te-

lemetry and ancillary data for non-realtime spacecraft and instruments

analyses.

Data Acquisition Acquire the downlink signal. Generate Doppler and ranging tracking data. Ex-

tract and digitize the original information from the modulated subcarrier of the

radio signal. Eliminate bit errors.

Digital Processing Perform telemetry decommutation.* Display and convert telemetry channels data.

Data Transport Ensure that communications are in place and functioning so that data can be

routed to the project database and scientists.

Computer and Communication Implement and administer computer and communication systems.

Training Train personnel on the processes and on the use of the ground data system.

System Engineering Design a mission operations system that matches the requirements, takes con-

straints into account and has acceptable performance.

*Decommutation is the process by which a single telemetry signal is separated into its component signals.


by a Principal Investigator (the INMSis operated like a Principal Investiga-tor instrument). The Huygens TitanProbe is operated from the HuygensProbe Operations Centre in Darms-tadt, Germany.

Capabilities and Constraints

The Cassini–Huygens missionscientific objectives involve complexmission, spacecraft and payload de-mands. In addition, mission opera-tions must fit within certain inherentconstraints. However, operational ca-pabilities built into the spacecraft canhelp to keep operations simple.

Operational Capabilities

Robustness. Robustness is provided byredundant engineering subsystem as-semblies, healthy margins of Orbiterconsumables and fault-protection soft-ware that provides protection for thespacecraft and the mission in theevent of a fault.

Automation. Automation is providedby the onboard inertial vector propa-gator, which generates turn profiles,bringing the spacecraft from its lastcommanded attitude and rate to apossibly time-varying target attitudeand rate. The target attitude is ob-tained by applying an offset to abase attitude, which is defined byprimary and secondary pointingconstraints as follows:

• Primary constraint — Align aspacecraft fixed vector (primarybody vector) with a specified iner-tial vector (primary inertial vector).

• Secondary constraint — Aligna spacecraft fixed vector (second-ary body vector) with a specifiedinertial vector (secondary inertialvector).

Flexibility. Flexibility is provided bythe capability of storing telemetrydata on a solid-state recorder forlater playback and by trigger-com-mand capabilities. Most instrumentsoperate from commands stored intheir memories; a trigger command isissued from the central computer toinitiate a preestablished series of in-strument-internal commands.

Inherent Constraints

Cost. The major operational con-straint of the Cassini–Huygens mis-sion is that mission operations anddata analysis activities must be con-ducted within fixed budgets (withinboth the U.S. and the Europeanspace communities).

Thermal. At Sun range closer than2.7 astronomical units (AU), thermalconstraints make it necessary to usethe Cassini’s high-gain antenna toshade the spacecraft from the Sun —except for infrequent turns away fromthe Sun (off-Sun turns), such as thoseused for trajectory correction maneu-vers. The allowable duration of theoff-Sun turns roughly scales with thesquare of the Sun range. For exam-ple, at 0.61 AU, the spacecraft couldwithstand a transient off-Sun durationof 30 minutes.

Telecommunications. While the high-gain antenna is used to shade thespacecraft, telecommunicationsmostly will be restricted to one of

Saturn, gathering energy from two fly-bys of Venus, one flyby of Earth andone of Jupiter. Upon arrival at Saturn,the spacecraft will be placed into or-bit around Saturn and a four-year tourof the Saturn system will begin.

The Huygens Probe will be deliveredon the first (nominal) or second (back-up) flyby of Titan. Multiple close Titanflybys will be used during the tour forgravity assists and studies of the satel-lite. The tour also includes icy satelliteflybys for satellite studies and orbits ata variety of inclinations and orienta-tions with respect to the Sun–Saturnline for ring, atmospheric, magneto-spheric and plasma science studies.

Spacecraft and Payload

The Cassini spacecraft is a three-axisstabilized spacecraft. The HuygensProbe and the Orbiter instruments areaffixed to the body of the spacecraft.There are 18 science instrument sub-systems, which are divided into fourgroups: optical remote sensing; micro-wave remote sensing; fields, particlesand waves; and Probe instruments.

Orbiter instruments that serve multipleinvestigations are called facility instru-ments. Facility instruments are provid-ed by the Jet Propulsion Laboratory(JPL), the NASA Goddard SpaceFlight Center or by JPL and ASI. Thefacility instruments, except for the Ionand Neutral Mass Spectrometer(INMS), are operated by a JPL teamcalled the distributed operations inter-face element.

Instruments that serve individual inves-tigations are provided and operated


two low-gain antennas1 and will pro-ceed at very low bit rates (generallyless than 40 bits per second).

Power. The electrical power fromCassini’s radioisotope thermoelectricgenerators (676 watts at the begin-ning of the tour and 641 watts at theend of the tour) is not sufficient to op-erate all instruments and engineeringsubsystems simultaneously.

Data Rates. The bit rate available onthe command and data subsystemdata bus — which is limited to a totalof 430 kilobits per second (kbps) —is not high enough to permit all instru-ments to output telemetry simulta-neously at their maximum rates.Instruments and engineering sub-systems also must share the number ofbits stored on solid-state recordersand transmitted to the ground. Duringthe tour, expected data rates are onthe order of 14–166 kbps.

Pointing. The major pointing constraintarises from the fact that the instru-ments are fixed to the body of thespacecraft. Other pointing constraintsprevent exposing sensitive spacecraftand payload components to undesir-able thermal input. The pointingaccuracy is two milliradians whenthe spacecraft is not rotating andpointing toward a fixed inertial direc-tion. For target-relative pointing, theaccuracy is limited by navigationaluncertainties.

Navigation. During the tour, thespacecraft’s orbit as a whole is con-trolled by Saturn’s gravity. For satelliteflybys, preflyby tracking is needed tosupport the maneuver that places

the spacecraft in the final flyby trajec-tory. Postencounter maneuvers arerequired only for Titan as a result ofits gravitational effect on the trajec-tory. A compromise must be reachedto balance the desire to acquire sci-ence data during Titan’s flyby withthe desire of performing the post-encounter maneuver soon after clos-est approach in order to minimizethe amount of propellant used forthat maneuver.

Propellant. This is the principal con-sumable of the mission. At launch,the spacecraft will carry 2919 kilo-grams of bipropellant for the mainengine and 132 kilograms of hydra-zine for the thrusters. Once the bi-propellant is gone, no significantmaneuver can be planned (unlesssignificant leftover hydrazine isused). Once the hydrazine is gone,the spacecraft will begin to loseattitude control. For this reason,propellant budgets are subject toparticular scrutiny.

The Probe. After separation from theOrbiter, the Probe will be poweredby five batteries. After 22 days ofcoasting, there will be enough powerto operate the Probe for three hoursfrom entry, including two and a halfhours of descent and 30 minutes onthe surface. The data rate over theProbe–Orbiter link will be 16 kbps.During cruise, Probe checkouts mustbe performed to verify the capabilityof executing the Probe mission. Thecheckouts simulate as closely aspossible the sequence of activitiesto be performed when the Probeapproaches Titan.

B R I N G I N G I M A G E S F R O M

S P A C E T O E A R T H

NASA brings us unforgettable images of space. In ad-

dition to their enormous scientific value, these images

dazzle and capture our imagination. Bringing images

from space to Earth — in effect, doing long-distance pho-

tography — is a complicated process that depends on our

ability to communicate with spacecraft millions of kilo-

meters from Earth. This communication is the responsi-

bility of NASA’s Deep Space Network (DSN), a global

system of powerful antennas.

Taking the picture is the job of the spacecraft’s imaging

system: digital camera, computer and radio. Reflected

light from the target — for example, Saturn’s rings or the

large satellite Titan — passes through the lens and one

or more color filters before reaching a charge-coupled

device (CCD). The surface of the CCD is made up of thou-

sands of light-sensitive picture elements, or “pixels.”

Each pixel assigns a number value for the light it

senses; the values are different for each color filter. The

spacecraft’s computer converts the data into digital code

— bits — which are transmitted to Earth. The bit stream

is received by huge antenna receivers at one of the three

DSN sites: Goldstone, California; Canberra, Australia;

and Madrid, Spain. The data are then sent to JPL in Pas-

adena, California, where the bits are reformatted, cali-

brated and processed to ensure a true representation

of the target, then recorded on high-quality black-and-

white or color film for archiving and distribution.

Of course, the DSN must handle not only image data

but all the other kinds of data Cassini transmits to Earth

as well. For flexibility, Cassini will use a variety of an-

tenna configurations — 34-meter, 70-meter or an array

of 34- and 70-meter dishes.


Operational Choices

The following operational choiceswere made to reduce costs and sim-plify mission operations.

Limited Science

To reduce costs, there is no plan toacquire science data during innerand outer cruise phases. The onlyexception is a gravitational waveexperiment which, following theJupiter flyby, will attempt to detectgravitational waves emitted by super-massive dynamical objects such asquasars, active galactic nuclei or bi-nary black holes.

Science data will not be collectedduring the planetary flybys2 exceptfor calibrations of the Cassini PlasmaSpectrometer, the Dual TechniqueMagnetometer and the Magneto-spheric Imaging Instrument at Earth,and calibration of the Radio and Plas-ma Wave Science instrument at Jupi-ter. Other science activities during theinner and outer cruise phases will belimited to deployments, maintenance,characterizations and checkout. Sci-

ence observations will begin twoyears before Saturn orbit insertion.

Limited Operational Selections

To simplify operations, the number ofallowable operational selections inseveral areas of mission operationshas been reduced to a limited set ofconfigurations.

Telemetry Modes. A set of telemetrymodes has been defined to providerates for recording data on the solid-state recorder and for downlink inthe context of changing telecommuni-cation capabilities. Each telemetrymode represents a unique configura-tion of data sources, rates and desti-nations for telemetry data gatheredand distributed by the command datasubsystem. There are five types oftelemetry data: engineering, sciencehousekeeping, scientific, playbackfrom the solid-state recorder, andProbe. The telemetry modes aregrouped into nine functional catego-ries. The three primary modes for op-erations during the orbital tour arerealtime engineering, science and en-

gineering record and realtime engi-neering and science playback.

Operational Modes. The concept ofoperational modes was invented toreduce operational complexity arisingfrom the facts that there is insufficientpower to operate all the instrumentssimultaneously, and that all the instru-ments cannot be optimally pointedin a simultaneous fashion becausethey are fixed to the body of thespacecraft.

The operational modes concept pre-scribes that instruments will operate ina series of standard, well-character-ized configurations. Each operationalmode is characterized by the state ofeach instrument (for example, “on,”“off,” “sleep,” etc.); minimum andmaximum power and peak datarate allocations for each instrument,and states of certain engineering sub-systems: the radio frequency sub-system, the solid-state recorder, theattitude and articulation control sub-system and the propulsion modulesubsystem.

W H A T G O E S U P … M U S T C O M E D O W N

This diagramshows the uplinkand downlinkdata flow process.(Housekeepingdata provide infor-mation about sci-ence instruments.)

InstrumentsScience

ObservationPlans,InstrumentCommands

Science,Housekeeping,Tracking andAncillary Data

GroundSystem

Sequences,RealtimeCommands

Science,Housekeeping,Engineering andTracking Data

Spacecraft

CommandLoads

Telemetry

UPLINK

DOWNLINK


Cassini–Huygens willlaunch on October 6,1997, from CapeCanaveral in Florida,aboard a Titan IVB/Centaur launch vehicle.

During much of Cassini’s orbital tour,15 hours in an optical remote sensingmode will alternate with nine hours ina fields, particles, waves and down-link mode. Other modes will be usedfor radar and radio science observa-tions. Instruments that are not collect-ing data are generally left in alow-power sleep state, so as to re-duce on–off thermal cycling, keephigh voltages on and avoid reloadinginstrument memories each time theyneed to operate.

Basic Mission. This concept prescribesthat 98 percent of the tour will beconducted with a small number ofreusable multi-instrument sequencemodules and templates. The remain-ing two percent (seven days a year)may consist of unique sequences.These sequence constructs are de-fined as follows:

• A module is a reusable sciencesequence of commands integratingsystem-level and trigger commands,pointing functions and telemetrymode selections.3

• A template is a sequential seriesof fixed-duration modules, fixedsequences, gaps of fixed durationsor other templates whose relativetiming is set.

• A fixed sequence is of fixed dura-tion and is designed and validatedonce for multiple uses, does not useoperational modes and has a fixedlist of variable inputs.

• A unique sequence has a specificpurpose, is used once and doesnot use modules.

Data Return. Deep Space Network(DSN) coverage of Cassini during

the tour will consist of one pass perday, with occasional radio sciencepasses. To reduce operational com-plexity, the coverage is divided intohigh-activity and low-activity days,with four gigabits or one gigabit ofdata returned per day, respectively.A quarter of each orbit, or sevendays, whichever is less, will be con-sidered high-activity.

To simplify operations while maintain-ing some flexibility for data gather-ing, three configurations for datareturn have been selected. They differessentially by the DSN antennas used(34-meter antennas, 70-meter anten-nas or 34-meter and 70-meter anten-nas arrayed) and the durations ofthe passes.

Tour Maneuvers. Propulsive maneu-vers during the orbital tour will gen-erally occur three days before andtwo days after each Titan flyby, nearapoapsis (the farthest point in thespacecraft’s orbit).

Workforce Management

Distributed Operations. Distributed op-erations places observing decisions,including generation of instrument-internal subsequences, in the handsof the science teams. The implementa-tion of distributed operations for theCassini mission is achieved throughcomputers, computer-resident soft-ware and communication lines pro-vided by JPL to the remote sites, aswell as science participation in theuplink (mission planning, sequencedevelopment) and downlink (PrincipalInvestigator instrument health monitor-ing) processes.

Virtual Teams. Cassini uses virtualteams for mission planning and

sequence development. These teamsbring together people for the devel-opment of a given product. Theirmembership varies, depending on theparticular subphase or sequence tobe developed. Each virtual teammember maintains membership inhis/her mission team of origin andcontinues to work for that team whileproviding expertise to and generatingproducts for the virtual team.

Uplink Processes

Cassini mission operations uses fiveuplink processes: science planning,mission planning, sequence develop-ment, realtime command develop-ment and radiation.

Science Planning

The purpose of the science planningprocess is to plan conflict-free scienceactivities. During the inner and outercruise phases, the science planningprocess is particularly simplified be-


cause the acquisition of science datais limited to deployments, mainte-nance, calibrations, checkout and thegravitational wave experiment.

In preparation for the science cruisephase and the tour, Cassini scientistswill prepare a science operationsplan. The science office and the fourdiscipline working groups — atmo-spheres, magnetospheric and plasmascience, rings and satellite surfaces —will play a major role in coordinatingthis effort.

Mission PlanningThe mission planning process is theresponsibility of the mission planningvirtual team. This process focuses onparticular subphases of the missionand consists of two subprocesses: mis-sion plan and mission phase plans up-dates, and phase update packagegeneration.

Mission Plan and Mission Phase PlansUpdates. The mission plan is theprincipal reference for a high-leveldescription of the Cassini–Huygensmission. It documents spacecraft de-sign and trajectory, mission phases,high-level activities and operationalstrategies for collection of scientificand engineering data. The planserves as a common starting pointand a guide for the implementationof mission operations.

Mission phase plans describe specificmission phases. Examples include the“Cassini Spacecraft System Mainte-nance, Calibration and DeploymentHandbook,” which identifies thoseactivities listed in the document’s titleas well as other “one-time only”4 or

science activities of the inner andouter cruise phases, and the CassiniInner Cruise Activity Plan, whichdescribes all spacecraft activitiesfrom two days after launch until theend of the inner cruise phase. Theseplans are updated for a given mis-sion subphase to provide levels ofdetail adequate for implementation.Changes in spacecraft performanceor in ground capabilities may makeit necessary for the mission planningvirtual team to perform trade studiesand mission analyses.

Phase Update Package Generation.This activity consists of translating theupdated sections of the plans into aset of inputs, called a Phase UpdatePackage, for the sequence develop-ment team(s). A major portion of thistask consists of developing timelinesfor each sequence of the mission sub-phase. Activities contained in thosetimelines include spacecraft events,geometric events, data rates, solid-state recorder strategies and DSNallocations, with a resolution timebetween one hour and one day.

Sequence Development

The sequence development process isthe responsibility of the sequence vir-tual team. This process focuses onparticular sequences and consistsof three subprocesses: activity plan-ning, subsequence generation andsequence integration and validation.

Activity Planning. The sequence time-line is refined to one-minute resolu-tion. Inclusion of the latest DSNallocations, revisions arising from theverification of resource allocationsand other revisions such as timingchanges are included in the timeline.The timeline is verified for compli-

ance with activity level mission andflight rules and constraints.

Subsequence Generation. Science, en-gineering, system-level and ground-event subsequences are implementedto the command level. The subse-quences are generated by the appro-priate teams. For instance, the teamknown as the flight system operationselement generates the subsequencesfor activities, the distributed opera-tions interface element generates thesubsequences for the facility instru-ments, the Orbiter Principal Investiga-tor teams generate the subsequencesfor their instruments and the HuygensProbe Operations Centre commandsthe Probe (prior to its separation fromthe spacecraft).

Sequence Integration and Validation.The set of subsequences is mergedinto an integrated sequence. The se-quence virtual team leader verifiesthat the syntax is correct and thatmission and flight rules and con-straints are not violated. There maybe a need to validate the sequencethrough simulation, which can bedone in the Cassini high-speed simu-lator or in the Cassini integration testlaboratory.

The high-speed simulator is a high-fidelity simulator of the commanddata subsystem and attitude and artic-ulation control subsystem’s computers.The integration test laboratory con-tains hardware identical to hardwareflown on the spacecraft, in particularthe command and data subsystem,solid-state recorders, attitude and


articulation control subsystem’scomputers, sensors and actuators.

Realtime Commands

A realtime command is one that isgenerated and transmitted subse-quent to the nominal sequence devel-opment process. It can be executedimmediately after receipt by thespacecraft or its execution may be de-layed. There are several types of real-time commands. Some are plannedand pregenerated, and are used forrepetitive transmissions. They arenot part of the sequence, but the se-quence contains a window to sendthe realtime command if needed.Some realtime commands areplanned but not generated, and aregenerally used for updating parame-ters. The need to change the parame-ter value is known, but the value towhich it should be changed is not. Re-altime commands that are unplanned,with varying degrees of time critical-ity, are emergency commands.

System-level realtime commands arehandled by the sequence virtualteam. There is a plan to have instru-ment-internal realtime commands han-dled automatically after the requesterhas generated, verified and deliveredthe realtime command file to theproject’s central database.

Radiation

After translation into appropriate for-mat by a JPL team called the realtimeoperations element, sequence andcommand files are sent or “radiated”to the spacecraft by a DSN antenna.

Downlink Processes

Cassini–Huygens mission operationsuse six downlink processes: realtimedata processing, data storage and

distribution, realtime monitoring, non-realtime data analysis, science dataanalysis and science dataarchiving.

Realtime Data Processing

A deep space station antenna ac-quires the signal transmitted by thespacecraft, passes it through a low-noise amplifier, which boosts the en-ergy level of the signal, and distrib-utes it to the signal processing center.

In the signal processing center, thesignal goes through a receiver, whereit is converted into an electrical sig-nal and the subcarrier is separatedfrom the carrier. (The carrier, which isthe main component of the radio sig-nal, is modulated with information-carrying variations; the subcarrier isa modulation applied to the carrier.)The receiver distributes the subcarrierand the tracking data to separate as-semblies. In the telemetry string, the

telemetry is separated from the sub-carrier and digitized, the digitizedencoded data are corrected to elimi-nate bit errors (loss of information bya change in value of a bit by somechance effect) and they are blockedinto frame-size records.

The full records are then routed viathe ground communication facilityinterface to the telemetry input sub-system, where science, instrumenthousekeeping and engineering dataare separated. The realtime opera-tions element operates the telemetryinput subsystem.

Data Storage and Distribution

The science packets and instrumenthousekeeping and engineeringchannelized data are stored on thetelemetry delivery system. Selecteddata are broadcast for realtime moni-toring and displayed by the datamonitor and display program. Fornon-realtime analysis, data are ob-tained through queries of the telem-etry delivery system.

The flyby of Earth inCassini–Huygens’primary trajectoryoccurs on August 16,1999, 57 days afterthe second Venusflyby. This artist’srendition shows thespacecraft over SouthAmerica.


FOOTNOTESRealtime Monitoring

The mission controllers, engineeringteams and science teams monitortelemetry and look for anomalies inreal time. They process the channel-ized data, looking for evidence ofanomalous behavior, displaying thedata in various ways and performingspecial processing.

Non-Realtime Data Analysis

The flight system operations elementretrieves engineering data used todetermine the health, safety and per-formance of the spacecraft, and pro-cesses the tracking data to determineand predict the spacecraft’s trajectory.The team generates ancillary informa-tion that is delivered to the projectcentral database.

The distributed operations interfaceelement for the facility instruments, thePrincipal Investigator teams for theirinstruments and the operations teamfor the Probe process the data, con-verting telemetry and raw trackingdata into products usable for verifica-tion that planned activities have beenperformed as expected and of instru-ment health and safety evaluation andscience data analysis.

Science Data Analysis

The Cassini science team membersand the interdisciplinary scientists(who use scientific information fromtwo or more instruments provided byscience teams) are responsible forthis process. Science data analysisis the process by which scientificstudies using the data products areconducted to evaluate the informa-tion content of the measurements,characterize physical phenomena,identify causes and effects, appraiseexisting theories and develop newones — and produce highly pro-cessed data and publications to in-crease and disseminate scientificknowledge.

Science Data Archiving

This is the process by which dataare recorded and stored in approvednational data archives for future refer-ences and scientific studies.

[1] At approximately 14 months afterlaunch, the Earth–spacecraft–Sun angle willbe small enough to allow use of the high-gain antenna for telecommunications. That25-day period will be used for a checkout ofall the Orbiter instruments. This will be thefirst real opportunity after launch for the sci-ence teams to verify that their instruments areworking properly.

[2] Science at Jupiter is currently notplanned, but is not precluded.

[3] The inertial vector propagation and trig-ger command capabilities are key to en-abling the implementation of modules.

[4] An example of a one-time only activity isthe launch plus 14 months Orbiter instrumentcheckout.

A F T E R W O R D 1 3 1

I t has been said that in making scientific and technological advances, each

generation of scientists and engineers stands on the shoulders of the previous

generation. This is true of all types of exploration, and it is true with the

Cassini–Huygens mission.

On the Shoulders of Giants

The story begins in late 1609, when Galileo Galilei built a telescope and had

the inspiration to aim it at the sky. Galileo’s discoveries about the Moon, Venus,

Jupiter and the Milky Way were a quantum leap forward in our understanding

of the universe, but he was puzzled by Saturn. With his telescope’s poor optics,

Galileo could only see that Saturn looked like a “triple” planet. To add to the

mystery, the planet lost its “companions” a few years later!

Fast forward more than 40 years: Christiaan Huygens turned his tele-

scope toward Saturn and discovered a real companion to the planet — like the

four discovered around Jupiter by Galileo — a satellite now called Titan. Huy-

gens also recognized for the first time the great ring surrounding Saturn. He re-

alized that the plane of the ring is tilted with respect to the plane of the planet’s

orbit, so that about every 14 years, Earth crosses through the plane and the

ring can disappear temporarily — explaining Galileo’s puzzling observation.

Shortly thereafter, Jean-Dominique Cassini discovered four additional satellites

and spied a “gap” in the ring, making it two rings separated by what is now

called the Cassini division.

Over the next century, more satellites were discovered. And two centuries

later, James Clerk Maxwell, well-known for his explanation of electromagne-

tism, showed that the rings were not solid disks but were instead composed of

myriad particles, each in its own orbit. Astrophysical techniques, in develop-

ment since the 1860s, enabled in 1932 the discovery of methane and ammo-

nia in Saturn’s atmosphere — and thus a great advancement in understanding

the nature of Saturn and the other “gas giants” — Jupiter, Uranus and Neptune.

This was followed by Gerard Kuiper’s surprising discovery in 1943–44 of a

dense atmosphere, unknown among other planetary satellites, on Titan.

The birth of space exploration in 1957, and the National Aeronautics and

Space Administration in 1958, led to dreams of human and robotic exploration

of the planets. The wanderlust of the Renaissance was reborn as both scientists

A F T E R W O R D


and the public became interested in visiting distant worlds. Ground-based plane-

tary astronomy experienced a rebirth, as new techniques and the need for more

information about the planets to be explored pushed the science along.

The evolution of planetary exploration began with the first steps to the

Moon, was followed by the exploration of Venus and Mars and continued in

plans for exploring the outer planets (that is, Jupiter and beyond). Fanciful

“Grand Tour” missions to the outer planets were proposed. Pioneers 10 and 11

explored Jupiter and its environment in 1973 and 1974, respectively. The suc-

cess of Pioneer 10 permitted the retargetting, after Jupiter, of Pioneer 11 to

arrive at Saturn in 1979, just as Voyager 1 was arriving at Jupiter.

The Voyagers, 1 and 2, carried out a thorough reconnaissance of Jupiter

and then Saturn. Following the success at Saturn, Voyager 2 was able to com-

plete the Grand Tour, missing only tiny Pluto in its travels.

The contributions of these missions, complemented by the continuing

developments of science and technology, have brought us to the Cassini–Huy-

gens mission. We can only guess, and not well, at what the Cassini–Huygens

spacecraft will find. Surely, many questions raised by previous observations will

be answered. But the answers, as always in science, will only generate more

questions. And, it is safe to predict there will be some unimaginable surprises.

Technology — the Bridge from the Ethereal to the Material

As technology has improved through history, our understanding of natural phe-

nomena has also improved. And, this technology has affected our daily lives.

Improved telescopes provided more knowledge about Saturn and Titan, gener-

ating more questions — which drove the development of technology in optics.

This has not only helped us do remote-sensing science better, it has allowed

eyeglass wearers to see better, via improved lens materials and coatings.

The development of auxiliary instruments changed astronomy from a de-

scriptive science into a “measuring” science, uncovering new mysteries for us to

puzzle over, like Titan’s atmosphere. Improvements in detectors, from eyeballs

through photography to electronic imagers, have made possible the permanent,

unbiased recording of images. Digitized images can be transmitted, without loss

of detail, across the solar system — and soon, directly into your home, with the

image quality on your television matching that in a movie theater.

Advancements in detectors and image-analysis software have been ap-

plied in diverse areas: helping patients with limited night vision “see” in the

dark; enabling doctors to analyze magnetic resonance, positron emission and

X-ray images; and protecting airport personnel from exposure to radiation from

security devices.

A F T E R W O R D 1 3 3

Developments tied to the Cassini–Huygens mission have had an effect

on everyday life. The solid-state recorder, using silicon chips to store data, has

found application on other spacecraft and in whole industries, from aerospace

to entertainment. Powerful computer chips and a radio transponder from the

mission are being used in other spacecraft, providing better performance and

lowering overall cost. Solid-state power switches developed for the mission can

be used in electrical and electronic products for both industry and consumers.

A new resource-trading exchange permits subsystems on the Cassini–

Huygens spacecraft to help balance conflicting needs. It delegates any decisions

to those who understand the problem best, so better decisions result. Already,

this system has been used by California’s South Coast Air Quality Management

District to help regulate air pollution; the state of Illinois is adapting it to man-

age volatile organic wastes.

The Cassini Management Information System tracks a myriad of receiv-

ables and deliverables by establishing “contracts” among those involved. Critical

steps and objects are easier to identify — and problems can be identified earlier,

so they can be addressed more easily and with less expense.

The Cassini–Huygens mission involves hundreds of scientists, engineers

and technicians from 17 countries. Learning to cooperate and work together

toward a common goal is good practice, for technicians as well as govern-

ments. International cooperation distributes the expense among the partner

nations and gives them all the opportunity to participate in the adventure and

the science. More people with different backgrounds than ever before will

share in the direct and derived benefits from this mission.

Space exploration inspires us to look beyond our everyday existence

and to the greater universe. We come into this world as explorers, engineers

and scientists, learning initially by taste and feel and going on, first crawling,

then walking, then running — as the preceding generations try to keep up! —

to see and experience the world around us. Exploration is instinctive in us; the

Cassini–Huygens mission is another way for us to extend our senses physically

to new places and over a broader range of phenomena.

Beautiful Saturn, with its rings, its moons and its many other wonders,

beckons to us to explore and uncover its secrets, to partake in the thrill of dis-

covery. Cassini–Huygens is the vehicle.

Apoapsis. The farthest point in an orbit

from the body being orbited.

Arrayed antennas. Ground antennas

made to work together in order to enhance

the telemetry capability.

Astronomical unit (AU). The mean

distance from the Sun to Earth —

149,597,870.694 kilometers.

Aureole. A luminous area surrounding

the Sun or other bright light when seen

through a thin atmosphere.

B

Bit error. A loss of information that

occurs as a result of the change in value

(1 to 0, or 0 to 1) of a single bit by some

chance effect.

Black hole. An object whose gravita-

tional field is so strong that even light can-

not escape from it.

Bow shock. A standing shock wave

that forms upstream of a planet. The bow

shock forms because the planet’s magnetic

field creates an obstacle to the incoming

solar wind flow. The bow shock heats the

solar wind and slows it to subsonic speeds

so that it can flow around the planet’s

magnetosphere.

C

Carbonaceous (C-type) material.

Carbon silicate primordial material rich

in simple organic compounds. C-type ma-

terial is spectrally flat and exists on the

surfaces of several outer planet satellites

and C-type asteroids.

Carrier. The main component of a

radio signal generated by a transmitter.

Central Data Base. A database ad-

ministered by the Cassini Program that

stores relevant project-specific data such

as sequence files and ancillary data.

Clathrate. A chemical compound con-

sisting of a lattice molecule and inclusions

of a smaller molecule within the crystal

lattice; ice or other substance that traps

molecules in its crystal structure without

chemical bonding.

A P P E N D I X A

Very Large Array, Socorro, New Mexico

A

Accrete. Gravitationally accumulate mass;

the process by which planets form.

Aeronomy. The science of the physics of

upper atmospheres.

Aerosols. Large molecules or particulates

that remain suspended in air over long peri-

ods of time.

Albedo. The fraction of incident radiation

reflected by a planet or satellite. If the value

is integrated over all wavelengths, it is called

the bolometric albedo. If the value is integrat-

ed over all directions, it is called the Bond al-

bedo. The geometric albedo is the ratio of the

brightness at a phase angle of zero degrees

(full illumination) compared with a diffuse,

perfectly reflecting disk of the same size.

Apparition. The duration of visibility of a

planet during any given year.

Glossary of Terms

G L O S S A R Y O F T E R M S 1 3 5


D

DSN pass. An interval of time during

which a Deep Space Network (DSN)

station is used to communicate with a

spacecraft.

D-type material. Primordial, low-

albedo, reddish material believed to be rich

in organic compounds. It exists on D-type

asteroids and on the surfaces of some of

Saturn’s satellites.

Data block. A segment of data with

fixed sizes, organized with a specific struc-

ture with a header containing a variety of

codes, such as spacecraft identification or

source of data.

Data packet. A group of data that has

been structured into standardized, discrete

units for transmission by the spacecraft.

Deceleration. Diminishing speed.

Deuterium. Heavy hydrogen, so named

because the nucleus of a deuterium atom

contains a neutron in addition to the proton

carried by ordinary hydrogen atoms.

Differentiation. Melting and chemical

fractionation of a planet or satellite into

a core and mantle; the gravitational sepa-

ration of different kinds of material in dif-

ferent layers in the interior of a planet,

generally with denser layers deeper, as

a result of heating.

Doppler data. Data that determine the

speed at which a spacecraft is approaching

or departing Earth. The Doppler data are

derived from the change of frequency of a

downlink signal.

Downlink. On the path from the space-

craft to the ground.

Dynamo theory. The theory that at-

tributes planetary magnetic fields to the

flow of electric currents in the interior of

the planet.

E

Eclipse. The passage of a satellite or

spacecraft through the shadow of a larger

planet or satellite; the Sun is not visible

from the smaller body during an eclipse.

Emission. The sending out or giving off

of light, infrared radiation, radio waves or

matter by a body.

Endogenic. Caused by some process

originating within a planet or satellite.

A P P E N D I X A

Command. A data transaction which,

when issued to a recipient, causes an ac-

tivity to take place within that recipient’s

hardware, software or both. A command is

defined by a name and input parameters.

Commands in a sequence are time-tagged.

Comminuted satellite. A satellite

(moon) reduced to tiny fragments.

Conjunction. Occurs when the direction

from Earth to the spacecraft is the same as

that to the Sun.

Convection. Transport of energy by

means of motion of the material (generally

refers to charged particles moving through

a magnetic field, gases moving through

surrounding atmosphere, or vertical mo-

tions of liquids or partially melted solids

within a satellite).

Corotating. Moving with the planet’s

rotation (generally refers to the charged

particles in Saturn’s magnetosphere that

are being carried along by Saturn’s mag-

netic field, which rotates with the planet).

Cosmic rays. Very energetic atomic

nuclei, the most energetic of which come

from outside of our solar system. Cosmic

rays can enter planetary magnetospheres

and interact with particles there.

Jean-Dominique Cassini, astronomer

1 3 7G L O S S A R Y O F T E R M S 1 3 7

Engineering subsystems. The space-

craft subsystems that maintain day-to-day

functions of the spacecraft and support the

science instruments.

Exogenic. Caused by exterior processes

acting on a planet or satellite.

F

Facility instrument. One of the five

Cassini Orbiter science instruments (INMS,

ISS, RADAR, RSS and VIMS) provided by

the Cassini Program for use by a selected

team of scientists (as opposed to being

provided by the selected team).

G

Galilean satellites. The four largest

satellites of Jupiter, discovered by and

named for Galileo Galilei.

Gravitational field. The region of

space surrounding a planet or satellite

within which gravitational forces from that

planet or satellite can be detected; gener-

ally also characterized at each point by a

strength and direction.

Gravitational wave. Distortion of space

and time by a massive object as predicted

by Einstein’s general theory of relativity.

Gravity. The force of attraction between

masses in the universe.

Gravity assist. Modification of a space-

craft trajectory by passage near a planet;

modification of a spacecraft orbit around

a planet by passage close to a satellite of

that planet.

H

Housekeeping data. Data that provide

information about the status of a science

instrument.

Hydrate. A chemical compound with

water ice bound to the crystal lattice or ad-

sorbed to the crystal surface.

I

Illumination. The lighting up, or the

geometry associated with the lighting up,

of a surface or atmosphere.

In situ. The Latin meaning is “in (its

original) place.” Spacecraft measurements

that sample the local environment make in

situ measurements in contrast to cameras,

which sense the environment “remotely.”

Interdisciplinary. Utilizing information

from two or more scientific instruments to

deduce the nature of some phenomenon.

Ion. An atom or molecule electrically

charged by the loss or addition of one or

more electrons. In planetary atmospheres

or magnetospheres, most ions are posi-

tively charged, implying that one or more

electrons are lost.

Ionosphere. The electrically conducting

plasma region above the atmosphere of a

planet (or Titan) in which many of the at-

oms are ionized, where charged particles

(ions and electrons) are abundant.

Isotope. One of two or more forms of

an element that differ in atomic mass due

to differing numbers of neutrons in the nu-

clei of its atoms.

Galileo Galilei, scientist

A P P E N D I X A


L

Lag deposit. A residual surface de-

posit remaining after other components

of the original surface material have been

removed.

Lagrange points. Equilibrium points

in the orbit of a planet or satellite around

its primary. Smaller bodies may reside

near these points, which are about 60 de-

grees ahead of or behind the planet or sat-

ellite in its orbit.

M

Mach number. The ratio of the speed

of the solar wind to the speed of compres-

sional waves. The Mach number of a bow

shock is an indication of the bow shock’s

strength.

Magma. Subsurface molten material

that may cause volcanic activity on a

moon or planet if it breaks through the

surface.

Magnetic anomaly. A perturbation

within an otherwise symmetric dipolar

magnetic field. At Saturn, a magnetic

anomaly is thought to allow charged parti-

cles to precipitate along field lines deep

into the magnetosphere and account for

Saturn kilometric radiation and auroras.

J

Jovian planet. A planet with general

characteristics similar to those of Jupiter,

also referred to as a “gas giant” planet.

The Jovian planets in our solar system are

Jupiter, Saturn, Uranus and Neptune.

K

Kelvin. A unit of a temperature scale

with its zero level at absolute zero temper-

ature (–273.16 degrees Celsius).

Kilometer. Preferred distance unit in

planetary studies, equal to 1000 meters;

about 62 percent of a mile.

Kuiper Belt. A belt of small bodies of

ice and rock left over from the formation

of the solar system. These bodies reside in

near-circular orbits beyond Neptune.

Magnetic field. The region of space

surrounding a magnetized planet or satel-

lite in which a moving charge or magnetic

pole experiences a force; generally also

characterized at each point by a strength

and direction.

Magnetic reconnection. A process

whereby magnetic field lines are “cut”

and “reconnected” to different field lines,

allowing the magnetic topology to change.

Magnetopause. The boundary of the

magnetosphere that separates the incoming

solar wind from the magnetosphere.

Magnetosheath. Layer of deflected and

shock-heated solar wind plasma between

the bow shock and the magnetopause.

Magnetosphere. The region around a

planet where the planet’s magnetic field

and associated charged particles (plasma)

dominate over the relatively weak inter-

planetary field carried by the solar wind.

Magnetotail. The region of the mag-

netosphere that stretches away from the

Sun (up to several hundred planetary radii)

due to the drag of the solar wind flow past

the planet’s magnetosphere.

A P P E N D I X A

Saturn’s immense magnetosphere

1 3 9

Meteorite. A piece of celestial debris

that hits the surface of planets or satellites;

it may range in size from invisible dust to

asteroid-sized chunks.

Morphology. The external structure of

a satellite surface in relation to form or

topographic features that may lead to

interpretation of the geological history of

that surface.

N

Nitrile. A compound containing

nitrogen.

Noble gas. A nonreactive gas; specifi-

cally, helium, neon, argon, krypton, xenon

or radon.

Nonresonant orbit. An orbit wherein

the spacecraft orbital period is not an inte-

ger multiple of the satellite’s orbital period.

Used for orbits that have consecutive fly-

bys with the same satellite at different or-

bit locations.

P

Packet. The data transmitted by the

spacecraft are structured into standardized,

discrete data packets to ease their han-

dling and to reduce data noise.

Periapsis. The point in an orbit closest

to the body being orbited.

Phase angle. The angle at the target

between the observer (or spacecraft) and

the Sun.

Photochemistry. Chemical reactions

caused or promoted by the action of light

(usually ultraviolet light) which excites or

dissociates some compounds and leads to

the formation of new compounds.

Plasma. A completely ionized gas, the

so-called fourth state of matter (besides

solid, liquid and gas), in which the temper-

ature is too high for atoms as such to exist

and which consists of free electrons and

free atomic nuclei.

Plasma sheet. Plasma of “hot” parti-

cles surrounding the neutral sheet in the

magnetotail, where the magnetic field re-

verses from an orientation toward to an

orientation away from the planet.

Nontargeted flyby. A relatively

close (less than 100,000 kilometers)

flyby that is not tightly controlled but

occurs by serendipity during the

Saturn tour.

Nucleation. The process whereby rain-

drops condense on a preexisting solid

grain of material that is suspended in the

atmosphere.

O

Occultation. Passage of one object be-

hind another object as viewed by the ob-

server or spacecraft.

Opposition. Occurs when the direction

from Earth to a spacecraft is the opposite

of that to the Sun.

Orbit cranking. Changing the orbit of a

spacecraft from a gravity-assist flyby with-

out changing its orbital period.

Orbit pumping. Changing the orbital

period of a spacecraft from a gravity-assist

flyby.

Organic molecule. A molecule con-

taining carbon; organic molecules are not

necessarily associated with life or living

organisms.

Orthogonal. Descriptive of two or three

directions in space that are at right angles

(perpendicular) to each other.

A P P E N D I X A



Plasma wave. A wave characterized

by displacement motions of ions within a

plasma.

Polar caps. The polar regions in the

northern and southern hemispheres in

which the magnetic field lines are open

to the solar wind. The aurorae form at the

boundaries of the polar caps.

Polymers. Structures in which the

same molecular configuration repeats

again and again.

Primary body. The celestial body

(usually a planet) around which a satellite

orbits.

Prograde motion. The orbital motion

of a planet or satellite that orbits its pri-

mary in the same direction as the rotation

of the primary.

Q

Quasar. Also called quasi-stellar object.

An object with a dominant starlike compo-

nent with spectral lines showing a large

redshift.

Remote site. An institution outside the

Jet Propulsion Laboratory from which a

Principal Investigator or a Team Leader

conducts his or her investigation.

Resonance. A pattern of recurring orien-

tations in the orbital positions or rotational

states of planets or satellites, leading to re-

peated gravitational perturbations.

Resonance point. A radial distance

within the Saturn ring system where the or-

bital period of the ring particles is an integer

multiple of a relatively nearby satellite’s or-

bital period.

Resonant orbit. An orbit in which the

spacecraft orbital period is an integer multi-

ple of the satellite’s orbital period. Used for

orbits that have consecutive flybys with the

same satellite at the same orbit location.

A P P E N D I X A

R

Radiometric dating. A method of esti-

mating the age of an object. It entails mea-

suring the fraction of a radioactive material

that has decayed and inferring the time

elapsed based on laboratory measure-

ments of the decay rate of the radioactive

substance.

Ranging data. Data that determine the

distance between a tracking antenna on

Earth and a spacecraft, produced by im-

pressing a ranging code on the radio sig-

nal between Earth and the spacecraft.

Reconnaissance. Preliminary survey

of a planetary system revealing the gen-

eral characteristics of that system.

Refractivity. A measure of the amount

of bending experienced by a light beam

traveling from one transparent material

into another; related to the relative speeds

of light in the two materials.

Regolith. The broken or pulverized sur-

face layer or rocky debris fragmented by

meteorite impacts.

Remote sensing. Observations using

instrumentation that collects data on an

object from a location remote from that

object.

Saturn orbit insertion for Cassini

1 4 1

Science payload. The assemblies used

for science data collection. For Cassini, this

includes the science instruments on the Or-

biter and the Huygens Probe.

Sequence. A computer file of valid

commands used to operate the spacecraft

for a predetermined period of time.

Slurry. A mixture of liquid and solid

material, often of the same chemical

composition.

Solar wind. The highly ionized plasma

streaming radially outward from the Sun

at supersonic speeds. It consists largely of

protons and electrons in nearly equal num-

bers with a small amount of ionized helium

and ions of heavier elements. Embedded in

it is the weak interplanetary magnetic field

that originates at the Sun.

Spectroscopic analysis. Determina-

tion of the composition and other charac-

teristics of a surface or gas based on the

relative brightnesses at a number of differ-

ent colors.

A P P E N D I X A

Retrograde motion. The orbital motion

of a planet or satellite that orbits its pri-

mary in a direction opposite that of the ro-

tation of the primary.

Roche limit. The distance (equal to

about 2.44 times the radius of the pri-

mary) within which the tidal forces exerted

by the primary on an icy satellite exceed

the internal gravitational forces holding the

satellite together.

S

Satellite. An object orbiting a planet;

natural satellites are often referred to as

“moons.”

Saturn electrostatic discharge. Radio

bursts from Saturn with a periodicity of

about 10 hours, 10 minutes; thought to be

due to lightning activity in Saturn’s equato-

rial atmosphere.

Saturn kilometric radiation. The

major type of radio emission from Saturn,

characterized by wavelengths in the kilo-

meter range and a periodicity of about

10 hours, 39.4 minutes. The emission

comes from regions in the auroral zones

when these regions are near local noon.

Saturn local time. Solar time as mea-

sured for a local region on Saturn; the Sun

is on the local meridian at local noon.

Sputtering. Erosion of a satellite’s

surface as a result of bombardment

by charged particles corotating with the

magnetosphere.

Stratosphere. The layer of the atmo-

sphere between the troposphere and the

ionosphere; a stable layer in which heat

is predominantly radiated (as opposed to

conducted) away.

Subcarrier. Modulation, applied to

a carrier, which is itself modulated with

information-carrying variations.

Subsequence. A portion of a sequence

that pertains to a single investigation or

engineering subsystem.

Synchronous rotation. A dynamic

state caused by tidal interactions in which

the satellite always presents the same face

toward the primary.

Synergism. The combination of results

from different science studies that pro-

duces a greater amount of information

than the sum of the individual results.

T

Target. The object that is observed at a

given time by one or more spacecraft sci-

ence instruments.

The Huygens Probe descends to Titan



Transonic flow. Flowing or moving

near the local speed of sound.

Tropopause. The boundary between the

troposphere and the stratosphere.

Troposphere. The atmospheric layer

closest to the surface of Saturn or Titan in

which convection is the dominant process

for transporting heat; the layer in which

“weather” takes place.

Turbulence. A state of commotion or

stormy agitation within an atmosphere.

U

Uplink. On the path from Earth to the

spacecraft.

V

Viscosity. The quality or property of a

fluid or aggregate of particles that causes

it to resist free flow.

Viscous relaxation. The slumping and

subsequent disappearance of a feature due

to gravitational forces acting on a material.

W

Wake. The track left by a body moving

through a fluid.

Targeted flyby. A flyby that passes

through a specified (usually very close)

aimpoint at the time of closest approach.

Tectonics. A branch of geology dealing

with surface structure, especially folding

and faulting.

Telecommunications. The process

of commanding and receiving information

from a remotely located spacecraft via ra-

dio signals.

Telemetry. A stream of data bits radi-

ated through space on electromagnetic

waves or transmitted by wire or electronic

pulses that represents the output of some

sensor or scientific instrument.

Tenuous. Very low density; difficult

to detect (descriptive of E-ring particle

distribution).

A P P E N D I X A

Tholin. Greek word meaning “muddy.”

Describes the orange–brown products of

laboratory experiments aimed at simulat-

ing the chemistry in Titan’s atmosphere.

Torus. A collection of charged or neu-

tral particles, shaped like a torus or

doughnut, and generally associated with

the orbital track of a satellite (like Titan).

Tracking data. Data needed to track a

spacecraft. These data are extracted from

the properties of the radio signal received

from the spacecraft. Examples of tracking

data are Doppler and ranging data.

Trajectory. The path of a body (i.e., a

spacecraft) in space.

Deep space burn during cruise to Saturn

1 4 3

A

A-ring Outermost of Saturn’s main

rings

AACS Attitude and articulation control

subsystem

ACC Accelerometer

ACP Aerosol Collector and Pyrolyser

(Huygens science instrument)

AFC AACS Flight Computer

AQ60 Material used for Huygens

Probe heat tiles

ASI Agenzia Spaziale Italiana, the

Italian space agency

ASIC Application-specific integrated

circuit

AU Astronomical unit, the mean dis-

tance from Earth to Sun =

149,597,870.694 kilometers

B

B-ring Brightest and densest of Saturn’s

rings

BIU Bus interface unit

A P P E N D I X B

C

C-ring Faint ring inward of Saturn’s

B-ring

C-type Asteroid material, rich in carbon

C2H2 Acetylene

C2H

4Ethylene

C2H6 Ethane

C3H8 Propane

CH4 Methane

CAPS Cassini Plasma Spectrometer

(Orbiter science instrument)

CDA Cosmic Dust Analyzer (Orbiter

science instrument)

CDS Command and data subsystem

CIRS Composite Infrared Spectrometer


CRAF Comet Rendezvous/Asteroid

Flyby spacecraft or mission

D

D-ring Innermost Saturn ring

D-type Asteroid material, rich in

hydrocarbons

DISR Descent Imager and Spectral

Radiometer (Huygens science

instrument)

DSN Deep Space Network (tracks op-

erating spacecraft)

DWE Doppler Wind Experiment (Huy-

gens science instrument)

E

E-ring Outermost Saturn ring

EGA Engine gimbal actuator

EGE Engine gimbal electronics

ESA European Space Agency

F

F-ring Narrow ring outward of A-ring

G

G-ring Diffuse ring at inner edge of

E-ring

GCMS Gas Chromatograph and Mass

Spectrometer (Huygens science

instrument)

GHz Gigahertz (billions of cycles per

second)

H

H+ Hydrogen atom with one elec-

tron removed, i.e., a proton

H2+ Hydrogen molecule with one

electron removed

Acronyms & Abbreviations

A C R O N Y M S & A B B R E V I A T I O N S 1 4 3


M

MAG Dual Technique Magnetometer


MAPS Magnetospheric and Plasma

Science

MIMI Magnetospheric Imaging

Instrument

MP Magnetopause (outer edge of

Saturn’s magnetic field)

N

N+ Nitrogen atom with one electron

removed

N2

Nitrogen molecule

NASA National Aeronautics and Space

Administration

O

O+ Oxygen atom with one electron

removed

ODM Orbiter deflection maneuver

OH Hydroxyl radical

OH+ Hydroxyl radical with one elec-

tron removed

ORS Optical remote sensing

H2O+ Water molecule with one elec-

tron removed

HASI Huygens Atmospheric Structure

Instrument (Huygens science in-

strument)

HC3N Cyanoacetylene

HCN Hydrogen cyanide

He++ Helium atom with two electrons

removed, i.e., an alpha particle

HGA High-gain antenna

I

INMS Ion and Neutral Mass Spectrom-

eter (Orbiter science instrument)

IRI Inertial reference interface

IRU Inertial reference unit

ISS Imaging Science Subsystem


J

JPL Jet Propulsion Laboratory

L

LGA1 Low-gain antenna #1

LGA2 Low-gain antenna #2

A P P E N D I X B

P

PMS Propulsion module subsystem

PPS Power and pyrotechnics

subsystem

R

RADAR Cassini Radar (Orbiter science

instrument)

RFES Radio frequency electronics

subsystem

RFS Radio frequency subsystem

RPWS Radio and Plasma Wave Science


RS

Unit of distance in radii of Saturn

(one RS = 60,330 kilometers)

RSS Radio Science Instrument (Orbiter

science instrument)

RTG Radioisotope thermoelectric

generator

RWA Reaction wheel assembly

RWI Reaction wheel interface

1 4 5

S

S Bow shock sunward of Saturn’s

magnetic field

SED Saturn electrostatic discharge

SKR Saturn kilometric radiation

SOI Saturn orbit insertion

SRMU Solid Rocket Motor Upgrade

(part of Titan IV launch vehicle)

SRU Stellar reference unit

SSE Sun sensor electronics

SSH Sun sensor head

SSP Surface Science Package (Huy-

gens science instrument)

A P P E N D I X B

T

TCS Temperature control subsystem

U

UK United Kingdom

US United States

UVIS Ultraviolet Imaging Spectrograph


V

VDE Valve drive electronics

VHSIC Very high-speed integrated

circuit

VIMS Visible and Infrared Mapping

Spectrometer (Orbiter science

instrument)

VVEJGA Venus–Venus–Earth–Jupiter

Gravity Assist

A C R O N Y M S & A B B R E V I A T I O N S 1 4 5


A

Aerosol Collector and Pyrolyser 10, 38,

115, 116

Atlas 61

Attitude and articulation control subsystem

91

Aurora 76

B

Basic mission 81

Bond 2

Bow shock 70

C

Calypso 61

Cape Canaveral 15

Cassini, J.-D. 2, 3, 42, 53

Cassini division 3, 42

Cassini Orbiter 5, 83, 89, 101, 119

Cassini Plasma Spectrometer 37, 49, 64,

69, 72, 108

Command and data subsystem 90, 119

Composite Infrared Spectrometer 24, 38,

63, 104

Cosmic Dust Analyzer 37, 49, 64, 69,

110

D

Data rates 93, 125

Deep Space Network 5, 107, 127

Descent Imager and Spectral Radiometer

38, 115, 117

Dione 59

Doppler Wind Experiment 38, 115, 118

Downlink 122, 129

Dual Technique Magnetometer 64, 69, 70,

111

E

Enceladus 59, 64

Epimetheus 61

European Space Agency 3

Exosphere 21

F

Fields and particles pallet 109

G

Galileo, G. 17, 41

Gas Chromatograph and Mass Spectro-

meter 9, 38, 115, 116

H

Helene 61

Huygens, C. 17, 42, 53

Huygens Atmospheric Structure Instrument

9, 115

Huygens Probe 8, 38, 82, 95, 115, 125

Hyperion 53, 62

I

Iapetus 59, 61

Imaging Science Subsystem 25, 63, 101

Ion and Neutral Mass Spectrometer 37, 49,

64, 69, 109

Ionosphere 22

J

Janus 61

Jet Propulsion Laboratory 12

K

Kirkwood, D. 42

Kuiper, G. 27

L

Launch vehicle 15

Lightning 30, 80

A P P E N D I X C

General Index

1 4 7

A P P E N D I X C

M

Magnetosphere 28, 30, 67

Magnetospheric Imaging Instrument

37, 64, 69, 76, 113

Maxwell, J. C. 42

Mimas 59, 62

Mission 5, 63, 81

Mission operations 121

N

Navigation 125

O

Operational modes 126

Orbital tour 11, 83

P

Pan 63

Pandora 61

Phoebe 62, 66

Pioneer 11, 12, 18, 44

Plasmas 67

Pointing 125

Power and Pyrotechnics Subsystem 94

Probe

Command and data management sub-

system 99

Data relay subsystem 99

Electrical power subsystem 99

Entry subsystem 97

Inner structure subsystem 97

Support equipment 96

Thermal control subsystem 97

Prometheus 61

Propellant 92, 125

Propulsion module subsystem 92

R

Radar 39, 106

Radio and Plasma Wave Science 37, 64,

69, 78, 114

Radio science 24, 69, 107

Radioisotope thermoelectric generator 90

Realtime commands 129

Remote-sensing platform 102

Rhea 58, 59

Ring spokes 48, 68

Rings 41

Roche theory 50

S

Saturn 18

Saturn kilometric radiation 48, 70, 77

Sequence development 128

Solid-state recorders 91

Surface Science Package 115, 118

T

Telecommunications 93, 124

Telesto 61

Temperature Control Subsystem 94

Tethys 58, 59

Titan 27

U

Ultraviolet Imaging Spectrograph 25, 38,

63, 69, 105

Uplink process 121, 127

V

Visual and Infrared Mapping Spectrometer

25, 38, 63, 102

Voyager 18, 28, 44, 47, 48, 51, 74

VVEJGA 81

W

Whistlers 30

G E N E R A L I N D E X 1 4 7


A P P E N D I X D

Solar System CharacteristicsS T A T I S T I C S O F T H E P L A N E T S

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Mean Distance from Sun

Astronomical Units 0.387 0.723 1.000 1.524 5.203 9.555 19.218 30.110 39.545

Millions of kilometers 57.9 108.2 149.6 227.9 778.3 1429.4 2871.0 4504.3 5913.5

Orbital Period, days 87.97 224.7 365.26 686.98 4332.7 10759.5 30685 60190 90800

Diameter, kilometers

Equatorial 4878 12104 12756 6794 142984 120660 51118 49528 2300

Polar 4879 12104 12713 6760 133829 107629 49586 48044 2300?

Mass [Earth = 1] 0.055 0.815 1.000 0.107 317.92 95.184 14.54 17.15 0.0022?

Density, t/cubic m 5.42 5.25 5.52 3.94 1.33 0.69 1.27 1.64 2.03

Gravity [Earth = 1] 0.38 0.90 1.00 0.38 2.53 1.07 0.91 1.14 0.07

Rotation Period, days 58.646 243.01 23.9345 24.6229 9.9249 10.6562 17.24 16.11 6.3872

Orbit Inclination, degrees 7.0 3.4 0.0 1.8 1.3 2.5 0.8 1.8 17.1

Footnotes

1 Astronomical Unit [AU] = 149,597,870.694 kilometers

Data from the Observer’s Handbook 1997, courtesy of The Royal Astronomical Society of Canada and the Cassini Physical Constants document.

M A G N E T O S P H E R E S O F T H E P L A N E T S

Mercury Earth Jupiter Saturn Uranus Neptune

Field Strength at Equator, gauss•RP3 0.0033 0.3076 4.28 0.218 0.228 0.133

Dipole Tilt to Rotational Axis, degrees 169 11.4 9.6 <1 58.6 46.8

Typical Distance to Sunward “Nose” ~1 10 65 20 18 25

of Magnetosphere, planet radii

1 4 9

A P P E N D I X D

T H E S A T E L L I T E S — A C O M P A R I S O N

Diameter, Distance from Planet,Planet No., Designation Name kilometers kilometers – planet radii Discovery

Saturn XVIII, 1981S13 Pan 20 133583 – 2.214 1990

XV, 1980S28 Atlas 37.0×34.4×26.4 137640 – 2.281 1980

XVI, 1980S27 Prometheus 148×100×68 139350 – 3.310 1980

XVII, 1980S26 Pandora 110×88×62 141700 – 2.349 1980

XI, 1980S3 Epimetheus 138×110×110 151422 – 2.510 1966

X, 1980S1 Janus 194×190×154 151472 – 2.511 1966

I Mimas 418.2×392.4×382.8 185520 – 3.075 1789

II Enceladus 512.6×494.6×489.2 238020 – 3.945 1789

III Tethys 1071.2×1056.4×1051.6 294660 – 4.884 1684

XIII Telesto 30×25×15 294660 – 4.884 1980

XIV Calypso 30×16×16 294660 – 4.884 1980

IV Dione 1120 377400 – 6.256 1684

XII, 1980S6 Helene 35 378400 – 6.266 1980

V Rhea 1528 527040 – 8.736 1672

VI Titan 5150 1221850 – 20.253 1655

VII Hyperion 360×280×225 1481100 – 24.550 1848

VIII Iapetus 1436 3561300 – 59.030 1671

IX Phoebe 230×220×210 12952000 – 214.7 1898

Earth – Moon 3476 384500 – 60.285 –

Mars I Phobos 21 9400 – 2.767 1877

II Deimos 12 23500 – 6.918 1877

Jupiter I Io 3630 422000 – 5.903 1610

II Europa 3140 671000 – 9.386 1610

III Ganymede 5260 1070000 – 14.967 1610

IV Callisto 4800 1885000 – 26.367 1610

Neptune I Triton 2700 5510000 – 222.491 1846

Data from the Observer’s Handbook 1997, courtesy of The Royal Astronomical Society of Canada and the Cassini Physical Constants document.

S O L A R S Y S T E M C H A R A C T E R I S T I C S


A P P E N D I X D

R I N G S Y S T E M S O F T H E O U T E R P L A N E T S

Planet Equatorial Radius Ring Inner Edge Ring Outer Edgekilometers – radii Ring Designation kilometers – radii kilometers – radii

Jupiter 71490 – 1.00 Halo 71490 – 1.00 122130 – 1.71

Main 122130 – 1.71 129130 – 1.81

Gossamer 129130 – 1.81 210000 – 2.94

Saturn 60330 – 1.00 D 66970 – 1.11 74510 – 1.24

C 74510 – 1.24 92000 – 1.53

B 92000 – 1.53 117580 – 1.95

A 122170 – 2.03 136780 – 2.27

F 140180 – 2.32 (width 50 kilometers)

G 170180 – 2.82 (width variable)

E 181000 – 3 483000 – 8

Uranus 25560 – 1.00 1986U2R 37000 – 1.45 39500 – 1.55

6 41837 – 1.64 (width 2 kilometers)



Alpha 44718 – 1.75 (width 10 kilometers)

Beta 45661 – 1.79 (width 10 kilometers)

Eta 47176 – 1.85 (width 1 kilometers)

Gamma 47626 – 1.86 (width 3 kilometers)

Delta 48303 – 1.89 (width 6 kilometers)

Lambda 50024 – 1.96 (width 2 kilometers)

Epsilon 51149 – 2.00 (width 20–90 kilometers)

Neptune 24765 – 1.00 Galle 41900 – 1.69 (width 15 kilometers)

Leverrier 53200 – 2.15 (width 30 kilometers)

Lassell 53200 – 2.15 (narrow)

Arago 53200 – 2.15 59000 – 2.38

Unnamed 61950 – 2.50 (indistinct)

Adams 62900 – 2.54 (width 50 kilometers; has

3 or more denser arcs)

1 5 1

A P P E N D I X E

P R O G R A M M A N A G E M E N T

S C I E N C E L E A D E R S H I P

Program & Science Management

Name Title

D. Gautier Interdisciplinary Scientist

T. Gombosi Interdisciplinary Scientist

J. Lunine Interdisciplinary Scientist

T. Owen Interdisciplinary Scientist

F. Raulin Interdisciplinary Scientist

L. Soderblom Interdisciplinary Scientist

D. Strobel Interdisciplinary Scientist

C. Porco ISS Team Leader

D. Southwood MAG Principal Investigator

S. Krimigis MIMI Principal Investigator

C. Elachi RADAR Team Leader

D. Gurnett RPWS Principal Investigator

A. Kliore RSS Team Leader

J. Zarnecki SSP Principal Investigator

L. Esposito UVIS Team Leader

R. Brown VIMS Team Leader

P R O G R A M & S C I E N C E M A N A G E M E N T

Name Title

R. Brace Product Assurance Manager

D. Kindt Huygens Probe Integration Manager

W. Fawcett Science Instruments Manager

P. Doms Mission and Science Operations Manager

J. Gunn Mission and Science Operations Development Manager

A. Tavormina Mission and Science Operations Operations Manager

J. Leising Program Engineering Manager

R. Wilcox Launch Approval Engineering Manager

Name Title

E. Huckins Program Director

R. Spehalski Program Manager

R. Draper Deputy Program Manager

H. Hassan Huygens Project Manager

C. Kohlhase Science and Mission Design Manager

T. Gavin Spacecraft System Manager

C. Jones Spacecraft Development Manager

G. Parker Spacecraft Integration and Test Manager

Name Title

H. Brinton Program Scientist

D. Matson Project Scientist

J.-P. Lebreton Huygens Project Scientist

L. Spilker Deputy Project Scientist

E. Miner Science Manager

G. Israel ACP Principal Investigator

D. Young CAPS Principal Investigator

E. Grün CDA Principal Investigator

V. Kunde CIRS Principal Investigator

M. Tomasko DISR Principal Investigator

M. Bird DWE Principal Investigator

H. Niemann GCMS Principal Investigator

M. Fulchignoni HASI Principal Investigator

H. Waite INMS Team Leader

M. Blanc Interdisciplinary Scientist

J. Cuzzi Interdisciplinary Scientist


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