the near earth asteroid rendezvous

8/7/2019 The Near Earth Asteroid Rendezvous

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Sun

(433) ErosOrbit

EarthOrbit

RTH SWINGBY1/23/98

LAUNCH

2/17/96Mathild

Flyby

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A Guide to the Mission,the Spacecraft, and the People

The

Near

EarthAsteroidRendezvous

al at

s

00

Eros flyby12/23/98

(253) MathildeOrbit


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Public Affairs Contacts

NASA HeadquartersWashington, D.C.

Dona ld Savage(202) 358-1727dona ld .savage @hq.nasa .gov

The Johns Hopkins UniversityApplied Physics Laboratory

Laurel, Maryland

Helen Worth(240) [email protected]

Jet Propulsion Laboratory

Pasadena, California

Diane E. Ainsworth(818) [email protected]

Kennedy Space Center

Cape Ca naveral, Florida

Ge orge Diller(407) [email protected]

The Boeing Compa ny

Huntington Beach, California

Keith Takaha shi(714) [email protected]

The Johns Hopkins University Ap p lied Physics Labora to ry

December 1999


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The

Near

EarthAsteroidRendezvous

A Guide to the Mission, the Spacec ra ft,

and the Peop le

The NEAR m ission is man aged by The John s Hopk ins University Applied Physics

Laboratory for the National Aeronau tics an d Space Adm inistration .

NEAR mission Web site: http:/ / near.jhuapl.edu


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The only c onc eivab le wa y in which a co ntinuing p ac e of p ioneering

plane tary missions c an be ma inta ined is by ma king the spa c ec raft sma ll, light,and elega nt while a t the sam e time sac rific ing little in the wa y of sc ientific

p rod uc tivity. . . But espec ially for the inner sola r system , extraordina ry op portu-

nities see m to be befo re us. NEAR is the first.

Ca rl Sag a n

Cornell University

a nd The Planeta ry Soc iety

(Comments read at the Low-Cost Planetary Mission Conference, held at The Johns

Hopkins University App lied Physics Laborato ry, Apr il 1996.)


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CONTENTS

Introduction .......................................................................................................................iv

The Mission ......................................................................................................................... 1Journey to Eros ............................................................................................................... 1

Orbital Phase .................................................................................................................. 2

Mission Operations ......................................................................................................... 3

Science Data Center ....................................................................................................... 4

Mission Costs ................................................................................................................. 4

Mission Operations Flow ................................................................................................ 5

Mission Timeline ............................................................................................................... 6

NEAR Science Objectives .................................................................................................. 7

The Asteroids ...................................................................................................................... 8

Asteroids and Meteors .................................................................................................... 8

Near-Earth Asteroids ....................................................................................................... 9

The Spacecraft .................................................................................................................. 12

Spacecraft Descript ion ................................................................................................. 12

Onboard Subsystems .................................................................................................... 13

Instruments .................................................................................................................. 15

The People ........................................................................................................................ 19

NASA NEAR Mission Management ............. .............. .............. ............. .............. ........... 19

JHU/APL NEAR Project Management .............. .............. .............. ............. .............. ...... 19

JPL NEAR Project Management .................................................................................... 19

NEAR Science Team Leaders ........................................................................................ 19

The Discovery Program .................................................................................................. 20

Discovery Goals ............................................................................................................ 20

Discovery Missions ....................................................................................................... 20NEAR Organization Chart ........................................................................ Inside Back Cover

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Introduction

The encounter of the Near Earth Asteroid Rendezvous (NEAR) spacecraft with asteroid 433 Eroson Feb. 14, 2000, begins a journey to a better understanding of asteroids, the Earths formation, andthe seeds of our solar system. Asteroids, comets, and meteorites have stirred h uman imaginations forhundreds of years, inspiring great speculation as well as scient ific observations. NEARs yearlongstudy of Eros comes at a time of unprecedented p ublic interest in asteroids and their possible collision

with the Earth and at a time of sufficient technical capabilities to unravel many mysteries that sur-round these near-Earth ob jects.

As the first mission in NASAs Discovery Program, NEAR is setting the stage for future asteroidexploration and will undoubtedly form a base of knowledge that will be the framework for futuremissions. This document describes the NEAR mission, which is being managed by The Johns HopkinsUniversity Applied Physics Laboratory (JHU/APL) from its Laurel, Md., campus.

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The Mission

Journey to Eros

Launch

On Feb. 17, 1996, the NEAR spacecraft thefirst asteroid orbiter of the Space Age wassuccessfully launched from Cape Canaveral AirStation in Florida aboard a Delta-2 rocket. Solar

panels that deployed minutes after launch providepower for the mission. In fact, NEAR is the firstspacecraft to operate beyond the orbit of Marssolely on solar power.

Mathilde Flyby

On June 27, 1997, the NEAR spacecraft flew

within 753 miles (1,212 kilometers) of asteroid253 Mathilde. The Mathilde flyby was the closest

spacecraft encounter with an asteroid and is thefirst close encounter with a C-type asteroid (seedescription of asteroid types on page 8). TheMultispectral Imager, a visible-light and infraredcamera, provided a wealth of data on theasteroids surface. These data, together with

Mathildes mass as determined from radio track-ing data, also gave important information aboutMathildes comp osition and density. (See p age 11

for a discussion of the science results.)Deep Spa c e Ma neuver

On July 3, 1997, the first firing of the space-crafts large bipropellant engine occurred to slowthe spacecraft by 602 mph (269 meters per sec-ond) relative to the sun. This maneuver was neces-

sary to reduce the perihelion distance of NEARstrajectory the closest point to the sun from0.99 to 0.95 astronomical unit (AU). (One AU isequal to the mean distance between the Earth andthe sun.) NEARs maneuver changed the perihe-

lion distance from 92 million miles (148 million ki-lometers) to 88 million miles (142 million kilome-ters), directing the spacecraft back to Earth for amission-critical gravity assist.

Earth Swingb y

On Jan. 23, 1998, the NEAR spacecraft flewby Earth for a gravity assist that put it onto the

correct trajectory for its rendezvous with theasteroid Eros. Flying as close as 335 miles (540kilometers) above southwestern Iran, the space-craft produced a series of images of Asia, Africa,and Antarctica. By designing the NEAR mission toinclude an Earth swingby, the less expensive

Delta-2 launch vehicle could be used, rather thanthe more powerful, but more expensive, Atlas-class vehicle.

Burn Abort and Eros Flyby

On Dec. 20, 1998, NEAR tried to fire its bipro-pellant engine for the first and largest of four

rendezvous maneuvers needed to first match thespeed of the spacecraft with that of Eros, and tothen ease the spacecraft into orbit around theasteroid. The spacecraft computer aborted theburn when preset acceleration limits were ex-ceeded and the spacecraft tumbled, causing loss

of communication for a day.While NEAR was found to be healthy, when

data were sent to ground controllers they foundthat about 64 lbs (29 kg) of fuel had been

expended by the onboard systems during theattempt to stop the tumbling and to point the

solar panels toward the sun.So instead of approaching Eros slowly, the

spacecraft hurtled past the asteroid on Dec. 23 at2,158 mph (965 meters per second) at a distanceof 2,378 miles (3,827 kilometers). Scientists andengineers quickly prepared commands to observe

Eros during this unexpected pass. They wereable to obtain dozens of low-resolution (1,500feet or 470 meters per p ixel) images during a full5-hour rotation, which gave them important

information about the asteroids size, shape, spin,and gravity (see p ages 1011). This information is

valuable for planning NEARs delayed orbitaloperations, reducing risk during the approach,and enhancing the future science re turn.

On Jan. 3, 1999, two major burns using thebipropel lant engine were combined andperformed correctly.

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Asteroid App roa ch a nd Rendezvous

After NEAR sped past Eros, it became neces-sary to fire the bipropellant engine as soon aspossible to match NEARs speed with the speed ofEros. Both the distance from the asteroid and the

time needed to return to Eros were increasing rap-idly. But the maneuver details had to be studiedcarefully to prevent another misfire. On Jan. 3,1999, NEAR performed its second large DeepSpace Maneuver, which lowered its speed relativeto the sun by 2,084 mph (932 meters per second)

and to Eros by 2,017 mph (902 meters per second).During the rest of 1999 and early 2000, NEAR

will slowly loop back toward Eros, arriving at theasteroid with enough fuel to complete the origi-nally planned orbital phase observations in spiteof the large fuel loss in December 1998. Rela-

tively small maneuvers, which will be p erformedwith NEARs simple, often-used, mon opropellant(hydrazine) system, will bring NEAR to a slowarrival at Eros on Feb. 14, 2000. The largest ofthese maneuvers, a speed change of 48 mph or21 meters per second, w as performed on Aug. 12,

1999. For the remainder of the mission, all ofNEARs maneuvers will be performed using thehighly reliable hydrazine system.

The NEAR mission was saved by a combina-tion of a generous fuel supply and a robust contin-gency plan developed before the aborted rendez-

vous maneuver. The delayed arrival at Eros givesscientists and engineers more time to develop andtest computer software for NEAR, and to finalizeand improve procedures for the spacecrafts ren-dezvous and orbital maneuvers.

The final app roach sequence in February 2000

will start with a maneuver on Feb. 2 that will cutthe spacecrafts speed relative to Eros from 44mph (19 meters per second) to 21 mph (9 metersper second). An additional small maneuver isscheduled for Feb. 8 to correct any errors in theFeb. 2 maneuver. During the final approach, Feb.

8-13, a search for satellites and debris around Erosshould detect anything bigger than about 17 feet(5 meters). The observations during the 1998flyby, which would have detected objects tentimes that size, had found none.

At 11:48 p.m. EST on Feb. 13, NEAR will be

124 miles (200 kilometers) from the center ofEros, passing directly between the asteroid andthe sun. Special infrared observations will then

be p ossible using the Near-Infrared Spectrometer(see page 16). The time was selected so that thelongest dimension of Eros would appear broad-

side, allowing observation of a maximum amountof Eros surface. Eleven hours later, NEAR will beat a viewpoint where one-half of the side of Eros

facing NEAR appears sunlit, the best locationfrom which most of NEARs instruments canobserve the asteroid. At that time, the hydrazine

engines will be fired to remove most of the re-maining speed relative to Eros, and insert NEARinto orb it around the asteroid. NEAR will then be160 million miles (258 million kilometers) fromEarth, 138 million miles (222 million kilometers)from the sun, and 209 miles (336 kilometers)

from Eros center.

Orbital Phase

With the orbit insertion burn on Feb. 14, 2000,the NEAR spacecraft will begin orbiting Eros. ByApril 10, 2000, mission planners will have gradu-ally changed the egg-shaped orbit to a circularorbit 62 miles (100 kilometers) from Eros, withplans to gradually lower the spacecrafts orbit to

22 miles (35 kilometers) by December 2000.Uncertainties in the mass, density, shape , and

rotation pole data obtained during the Dec. 23,1998, flyby make it impossible to finalize a de-tailed tour of Eros earlier than a few weeks after

orbit insertion. Adjustments to the spacecraftorbit orientation will keep the asteroid within thefields of view for the science instruments, enablecommunications antenna coverage of the Earth,and provide illumination of the solar panels bythe sun to power the spacecraft. As the space-craft maneuvers closer to the asteroid, estimates

of mass, moments of inertia, gravity harmonics,spin state, and landmark locations informationnecessary for safely navigating the closer orbits will be determined with increasing precision.

NEAR will remain in orbit around Eros for

more than 12 months. This long time allows theNEAR instruments to de termine the physical andgeological properties of Eros and to measureits elemental and mineralogical composition.Many of these measurements require lengthyobservations at close range that could not havebeen made during a flyby of the asteroid.

As NEAR orbits Eros the spacecrafts inclina-tion and radius will vary to satisfy mission science

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objectives. From May through August 2000, it willtravel in a circular orbit at a radius of 31 miles (50kilometers) from the center of Eros. It will then be

boosted to a higher orbit to view Eros from thedirection of the sun. In the second half of Decem-ber 2000, NEAR will descend to a 22-mile (36-

kilometer) orbit and will operate at that level, orlower, for the remainder of the mission. Althoughevery instrument will be operating during the low-

altitude phase, the highest-priority science w ill bethe measurement of elemental composition.

When NEAR first enters its orbit around Eros,the north p ole of the asteroid points 30 degreesaway from the sun, keeping much of its southernhemisphere on the night side for the entire

rotation period. The Multispectral Imager, Near-Infrared Spectrometer, and X-Ray Spectrometercan observe only the sunlit portions of Eros; theGamma-Ray Spectrometer, Magnetometer, andLaser Rangefinder are independent of sunlight.To make the full set of measurements over the

entire surface and particularly to image all ofEros at highest resolution NEAR must wait forseasonal changes as Eros moves in its orbitaround the sun. About five months after the ren-dezvous begins, all of Eros will become sunlitover the course of one rotation.

The irregular shape of Eros requires that NEARremain in retrograde orbit relative to the asteroid

spin once the spacecraft reaches very low altitude.When in a retrograde orbit, the spacecraft andEros spin in opposite directions. As comparedwith a direct orbit, a retrograde orbit tends to be

more stable because the spacecraft is not affectedas much by the unevenness of Eros gravity field. Ifboth NEAR and Eros were rotating in the same di-rection, the spacecraft could be ejected from itsorbit around Eros, or it could be pulled in and hitthe asteroids surface. The orientation of NEARs

orbit relative to the rotation pole of Eros will changeslowly during the orbital phase due to the chang-

ing relative p ositions of Eros, Earth, and the sun.When data are to be downlinked, the space-

craft will turn, if necessary, to point the h igh-gainantenna at Earth. The instruments face 90 de-

grees from the direction of the antenna, so theycan point at Eros as the spacecraft rolls in its or-bit. All or any combination of the instruments canoperate simultaneously, taking measurementsand storing data on solid-state recorders.

Mission Operations

NEARs mission operations are conductedfrom the Mission Operations Center at The JohnsHopkins University Applied Physics Laboratory(JHU/APL) campus in Laurel, Md. The Mission

Operations Center is the first non-NASA spacecenter to direct a NASA planetary mission. A teamof flight controllers and mission analysts isresponsible for the day-to-day operations of thespacecraft. The operations team works closelywith the science teams, JHU/APL Mission Design,and the Navigation Team at NASAs Jet Propul-

sion Laboratory (JPL) in Pasadena, Calif.Together, Mission Operations personnel and

the science teams plan spacecraft and instrumentactivities. The science teams prep are requests foroperating the five science instruments and trans-

mit them to Mission Operations two weeksbefore their intended execution. During NEARsrendezvous with Eros, activities will include com-mands to point instruments and image selectedareas of the asteroids surface or activate theNEAR Laser Rangefinder to measure the distancebetween Eros surface and the spacecraft.

Working with Mission Design and the JPLNavigation Team, Mission Operations executesthe orbit maneuvers by designing command se-quences for the spacecrafts propulsion system.Mission Design determines what the maneuversshould be and w hen th ey should be made.

The schedule calls for two rendezvous burns(one optional cleanup maneuver) using thepropulsion systems thrusters to slow th e space-craft during its approach to Eros. Orbit insertionis completed during the final rendezvous maneu-ver on Feb. 14, 2000. During the yearlong orbital

phase, orbit correction maneuvers adjustingNEARs orbit around Eros are expected as thespacecrafts instruments reveal more informationabout the gravity of Eros and its rotation.

All activities are integrated by MissionOperations into weekly command loads and are

thoroughly tested through software simulationand verification. Once approved, NEAR flightcontrollers uplink as much as a weeks activitiesto the spacecraft through NASAs Deep SpaceNetwork (DSN). Transmission time to NEARsonboard computers typically takes 15 minutes.

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Once uploaded to NEARs flight computers,commands automatically execute at predeter-mined times. The science and engineering data are

recorded to onboard solid-state recorders. Once aday, NEAR will turn from its normal asteroid-point-ing orientation to an Earth-pointing orientation to

play back the recorded data. The NEAR data willtravel back to Earth along the same path as thecommands uplinked through the DSN.

All data will then pass back through theMission Operations Center, where computersextract the science data from the incomingdata stream and forward it to the Science DataCenter. Flight controllers monitor NEAR engi-neering telemetry in real time to verify spacecraft

operations.

Science Data Center

The NEAR spacecraft transmits all of its data

to a global network of antenna tracking stations.The data are then forwarded to JHU/APLs Sci-

ence Data Center (SDC) for p rocessing, distribu-

tion to the science teams, and archiving. The SDCwas established for the NEAR mission as the cen-

tral site for data processing activities. It performsthe common data processing tasks cleaning

and merging the data coming down from the

spacecraft by sorting, removing duplicates, anddeleting errors.

The SDC also creates separate instrumentfiles from the incoming data, which it then dis-

tributes to the respective science teams over theInternet. This way, critical mission data are deliv-ered to the scientists desktops without delay.

Serving as the mission s library, the SDCmaintains an archive of telemetry, instrument,and command his tories , along with the

spacecrafts navigation and p ointing information.The data are used to p roduce th e images that areposted on the NEAR Web site, including the

image-of-the-day.The SDCs entire archive is available on-line

over the Internet on the NEAR Web site: http:/ /

near.jhuapl.edu. Thanks to the World WideWeb, NEAR mission information is accessible tothe scientific community and the general publicsoon after the data arrive at the SDC.

The SDC also sends NEAR data to NASAsPlanetary Data System (PDS), where the data are

archived under the small bodies section of thePDS Web site. The PDS makes digital data on

NASA missions available to the worldwide sci-

ence commu nity. The PDS Web site is http:/ /pds.jpl.nasa.gov.

Mission Costs

The total mission cost is projected to be

$224.1 million. The cost for spacecraft develop-ment came to $124.9 million,* and launch sup-port and tracking amounted to $44.6 million. Thecost for mission operations and data analysis is$54.6 million.

*These are official NASA figures. However, due to anunderrun during the development phase, approxi-mately $8 million was carried forward to th e missionoperations phase.

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Mission TimelineNote: The NEAR mission is the first to orbit a small body, and much is unknown. Because themission is exploring new frontiers, NEAR operations must remain fluid to respond to evolving

scientific findings. Therefore, dates, altitudes, and event sequences listed may be adjusted as themission unfolds. Check the NEAR Web site, http://near.jhuapl.edu, or the NEAR Hotline,

240-228-5413, for the most up-to-date information.

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Feb. 17, 1996NEAR successfully launches from Cape Canaveral ona Delta-2 rocket.

Feb. 18, 1997NEAR establishes record for the greatest distance fromthe sun for a solar-powered spacecraft (203 millionmiles/327 million kilometers).

June 27, 1997In a flyby of asteroid Mathilde, NEAR comes within 753miles (1,212 kilometers) of the asteroid.

Jan. 23, 1998An Earth swingby puts NEAR on its final approach path

for an encounter with asteroid 433 Eros. At its closestpoint to Earth, the spacecraft passes about 335 miles(540 kilometers) above Ahvaz in southwestern Iran.

April 1, 1998NEAR sets the record as the most distant manmadeobject detected by optical means when an amateurastronomer in New South Wales, Australia, spots thespacecraft at a distance of 20.91 million miles(33.65 million kilometers) from Earth. The previousrecord was the 1992 sighting of the Galileo spacecraftat a distance of 5 million miles (8.06 million kilometers)from Earth.

Dec. 20, 1998NEARs initial Eros rendezvous maneuver aborts

moments after thruster firing starts. Contact withMission Operations is regained after 27 hours of silence,revealing a healthy spacecraft that lost 64 pounds(29 kilograms) of propellant during its attempt torecover communications.

Dec. 23, 1998NEAR comes within 2,378 miles (3,827 kilometers) ofEros at 2,158 miles per hour (965 meters per second).

Jan. 3, 1999Large bipropellant thruster burn executed to close thegap between NEARs orbital speed and that of Eros.

Jan. 20, 1999Hydrazine thruster burn completed to fine-tune thespacecrafts trajectory and speed.

Aug. 12, 1999Last major trajectory correction completed with 2-minute burn of the hydrazine engine, slowing thespacecraft to 188 mph relative to Eros.

Feb. 3 and Feb. 8, 2000A two-part rendezvous maneuver refines NEARsspeed and trajectory for final approach to Eros.

Feb. 13, 2000Zero-phase measurements occur using the Near-Infrared Spectrometer as the spacecraft flies betweenEros northern hemisphere and the sun.

(You are looking at zero-phase when the sun is directlyoverhead and casting no shadows.)

Feb. 14, 2000NEAR enters an orbit 202 miles (325 kilometers) fromthe center of Eros.

Feb. 14 to April 30, 2000High-Orbit Phase. NEAR orbits Eros at distances de-creasing from 311 to 31 miles (500 to 50 kilometers)

from the center of the asteroid.March 3, 2000NEAR spacecraft descends to 125-mile (200-kilometer)orbit.

April 10, 2000NEAR reaches orbit of 62 miles (100 kilometers).

April 30, 2000NEAR arrives at a polar orbit of 31 miles (50 kilometers),where the spacecraft spends 100 days.

April 30 Aug. 27, 2000Low-Orbit Phase. NEAR travels in nearly circular orbitsat about 31 miles (50 kilometers) from Eros.

The X-Ray/Gamma-Ray Spectrometer measures ele-

ment abundances, which will help to determine therelationship between meteorites and asteroids.

July 6, 2000For the first time since NEAR arrived at Eros, all of Eros,excluding deep polar craters, is illuminated by the sun.Sunlight shines directly over Eros equator as the sub-solar point moves south.

Aug. 27 Dec. 20, 2000High-Orbit Phase. NEAR travels in orbits of 31 to311 miles (50 to 500 kilometers) from Eros. During thisperiod, the retrograde orbit shifts from nearly polar tonearly equatorial, where NEAR travels opposite thedirection of Eros spin.

Oct. 15, 2000

Zero-phase measurements occur using the Near-Infrared Spectrometer as the spacecraft flies betweenEros southern hemisphere and the sun.

Dec. 20, 2000Low-altitude operations begin as the spacecraftpasses within 21 miles (35 kilometers) or closer duringeach orbit.

Feb. 14, 2001Mission ends.


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7

NEAR Science Objectives

Except for the moon, near-Earth asteroids(NEAs) are Earths nearest and most accessibleplanetary neighbors. These bodies have played a

significant role in shaping the Earth; impacts oflarge NEAs have affected the evolution of theEarths atmosphere and biosphere. Along withcomets and meteorites, asteroids preserverecords of processes and conditions that existed inthe early solar system.

By February 2001, the NEAR mission will pro-vide the first comp rehensive picture of the physi-cal geology, composition, and geophysics of anasteroid. The overall science goals of the NEARmission can be summarized as follows: To characterize the physical and geological

properties of a near-Earth asteroid and to inferits elemental and mineralogical comp osition

To clarify relationships among asteroids, com-ets, and meteorites

To further the understanding of processes andconditions during the formation and early

evolution of the planets.

High-resolut ion imagery will offer insight intothe regolith the rocky debris layer that forms

on airless solar system bodies and the historyof impacts as recorded in the crater pop ulation.Spectroscopic analysis will provide maps of min-

eralogy at 1,000-foot (300-meter) resolution. TheRadio Science and Magnetometer experimentswill yield information on the strength and charac-

ter of the magnetic field and on global densityand density distribution.

The primary measurement objectives atEros are : To determine the gross physical properties of

the asteroid, including size, shape, configura-

tion, volume, mass, density, and spin state To measure surface composition, elemental

abundances, and mineralogy

To investigate surface morphology (structure)through comprehensive imaging under a vari-ety of lighting conditions.

Other measurement objectives are: To determine regolith properties and texture

by imaging to sub-meter scales. These obser-vations will be made during special closepasses to within 1 mile (1.6 kilometers) or lessof the surface near the end of the mission.


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in October 1991, when the Galileo spacecraftflew by the asteroid 951 Gaspra at a distance of1,000 miles (1,600 kilometers). In August 1993,Galileo passed within 1,500 miles (2,400 kilome-

ters) of another asteroid, 243 Ida. Later analysis ofthe Ida images revealed a small moon, Dactyl,about 1 mile (1.6 kilometers) in diameter. OnJune 27, 1997, the NEAR spacecraft flew within753 miles (1,212 kilometers) of the C-type aster-oid Mathilde. (Science results are discussed on

page 11.) Gaspra, Ida, and Mathilde are all mainbelt asteroids.

On July 28, 1999, NASAs Deep Space 1 flewwithin an estimated 16 miles (26 kilometers) ofnear-Earth asteroid 9969 Braille, which was dis-covered in 1992. The asteroid was found to be

elongated1.3 miles (2.2 kilometers) by 0.6 mile(1 kilometer)with very high reflectivity. Thespacecrafts infrared sensor confirmed thatBraille is similar to asteroid 4 Vestaone of thelargest asteroids in the main asteroid belt with adiameter of about 310 miles (500 kilometers).

Vesta was discovered in 1807 and is one of themain belts most reflective asteroids. Scientistsare now trying to determine whether Braille is achip that has broken off Vesta or perhaps a sib-ling coming from a larger asteroid.

Asteroids are classified into different types

according to their albedo and spectra seen in re-flected sunlight. Albedo refers to an objects mea-sure of reflectivity. A white, perfectly reflecting

To measure interactions with the solar windand search for possible intrinsic magnetism

To search for evidence of current activity as

indicated by dust or gas in the vicinity of theasteroid

To investigate the internal mass distribution

through measurements of the asteroids gravityfield and the time-variation of its spin state.To accomplish these objectives, NEAR

carries the following science p ayload: Multispectral Imager to map the morphol-

ogy and color at 10-foot (3-meter) resolution

The Asteroids

Asteroids and MeteorsAsteroids are small bodies without atmo-

spheres that orbit the sun but are too small to beclassified as planets. Dubbed minor planets,tens of thousands of asteroids are known to con-gregate in the main asteroid belt: a vast, doughnut-shaped ring located between the orbits of Mars

and Jupiter from approximately 2 to 4 AU (186 to370 million miles/299 to 598 million kilometers).

Asteroids are thought to be primordial mate-rial that w as prevented by Jupiters strong gravity

from accreting into a planet-sized body when thesolar system was born 4.6 billion years ago. The

estimated total mass of all asteroids would make abody about 930 miles (1,500 kilometers) in diam-eter less than half the size of the moon.

Known asteroids range in size from the largest Ceres, the first-discovered asteroid (discov-ered in 1801), measuring about 580 miles (930

kilometers) in diameter down to tens ofmeters. Sixteen asteroids have diameters of 150miles (240 kilometers) or more. Most main beltasteroids follow slightly elliptical, stable orbits,

revolving in the same direction as the Earth andtaking from three to six years to complete a full

circuit of the sun.Our understanding of asteroids comes from

three main sources: Earth-based remote sensing,laboratory analysis of meteorites, and data fromthe Galileo, NEAR, and Deep Space 1 flybys. Theyearlong encounter with Eros is an exciting pros-

pect for scientists whose appetites were w hetted

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Near-Infrared Spectrometer to map the min-eralogy at 1,000-foot (300-meter) resolution

X-Ray/Gamma-Ray Spectrometer to mea-sure the abundance of key elements

NEAR Laser Rangefinder to measure thetopography to 15-foot (5-meter) vertical

resolution

Magnetometer to search for a magnetic field

Radio Science to determine the mass and in-ternal structure of Eros using the spacecraftstelecommunications system.


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M-type (m etallic): This group includes manyof the remaining known asteroids and inhab-its the main belts middle region. With an

albedo of 0.10 to 0.18, these asteroids are rela-tively bright. Their comp osition is app arentlydominated by metallic iron.

Numerous other types of asteroids have beenidentified. The proportions of asteroids in theknown population do not simply reflect the ac-

tual populations because, for example, sometypes are easier to see than others.

The relationship betw een asteroids and mete-oroids remains a puzzle. Smaller than asteroids,meteoroids are interplanetary bodies. Meteoroidsthat enter the Earth s atmosphere are called

meteors, and the fragments that hit the groundare meteorites.

The most common meteorites, known as ordi-nary chondrites, are composed of small grains ofrock and appear relatively unchanged since thesolar system formed. Stony-iron meteorites, on the

other hand, appear to be remnants of larger bodiesthat were once melted so that the heavier metalsand lighter rocks separated into different layers.

A long-standing scientific debate exists overwhether the most common asteroids in the innerasteroid belt the S-types are the source of or-

dinary chondrites. Spectral evidenceso far suggests that the S-type aster-

oids may be geochemically pro-cessed bodies akin to the stony-irons. If S-types are unrelated to ordi-nary chondrites, then another par-

ent source must be found. If the twoare related, however, then scientistsneed an explanation for why theircolor properties are not similar.

Near-Earth Asteroids

Asteroids with orbits that bringthem within 1.3 AU (121 million

miles/195 million kilometers) ofthe sun are known as near-Earth as-teroids (NEAs). It is believed thatmost NEAs are fragments jarred

from the main belt by a combina-tion of asteroid collisions and thegravitational influence of Jupiter.Some NEAs may be the nuclei ofdead, short-period comets. The

The MainAsteroid Belt

Jupiter

99-1030B-3(Orbits drawn approximately to scale)

Sun

Mars

Earth

9

surface has an albedo of 1.0; a black, perfectly ab-sorbing surface has an albedo of 0.0.

The spectra of asteroids provide information

on their compositions and bear similarities tothose of known meteorite types. It is inferredthat asteroids display a wide variety of composi-

tions: some are rocky (for example, basaltic);some are metallic; some have hydrated minerals;and some are probably rich in organics.

The principal types of asteroids include: C-type (carbonaceous), including asteroid

253 Mathilde: This category includes morethan 75 percent o f the known asteroids. Theyare very dark, with an albedo of 0.03 to 0.09.Their composition is thought to be similar to

that of the sun, but dep leted in hydrogen, he-lium, and other volatiles substances thatvaporize easily. Carbon comp ounds similar tocoal are thought to p redominate. C-type aster-oids inhabit the main be lts outer regions.

S-type (silicaceous), including asteroid Eros,

Gaspra, and Ida: These asteroids accountfor about 17 percent of the know n populationand dominate th e inner asteroid belt. They arerelatively bright, with an albedo of 0.10 to0.22. Their comp osition is metallic iron mixedwith iron and magnesium silicates.


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NEA population appears to be representative ofmost or all asteroid types found in th e main belt.

Traditionally, NEAs have been classified intothree categories according to their orbits.Amorsare asteroids that cross Mars orbit but do not

quite reach the orbit of Earth. Eros the targetof the NEAR mission is a typical Amor.Apollosare asteroids that cross Earths orbit with a periodgreater than one year. Atens are asteroids thatcross Earth s orbit with a period less than one year.

NEAs are a dynamically young population,

meaning that their orbits evolve on 100-million-year timescales because of collisions and gravita-tional interactions with the sun and the planets. An

asteroids orbit can also change suddenly if a colli-sion occurs.

Approximately 800 NEAs have been found to

date, probably only a small percentage of theirtotal population. The largest presently known is1036 Ganymed, with an approximate diameter of25.5 miles (41 kilometers). Estimates suggest thatat least 700 NEAs may be large enough 0.6 mile (1 kilometer) or more in diameter to

threaten civilization if they were tostrike the Earth.

Many bodies have struck Earth

and its moon in the past. Onewidely accepted theory blamesthe impact 65 million years ago of

an asteroid or comet at least 6miles (10 kilometers) in diameterfor mass extinctions among many

life forms, including the dino-saurs. Other theories suggest thatthe chemical building blocks oflife and much of Earths water ar-rived on asteroids or comets thatbombarded the p lanet in its youth.

On June 30, 1908, a smallasteroid 330 feet (100 meters) indiameter exploded over the re-mote region of Tunguska inSiberia, devastating more than halfa million acres of forest. One of

the most recent close calls oc-curred on March 23, 1989, whenan asteroid 0.25 mile (0.4 kilome-ter) wide came within 400,000miles (640,000 kilometers) ofEarth. Surprised scientists esti-

mated that Earth and the asteroid weighing 50million tons and traveling at 46,000 mp h (74,000

kilometers per hour) had passed the samepoint in space just six hours apart.

433 Eros

The target of the NEAR mission is 433 Eros, thefirst near-Earth asteroid to be discovered and thesecond largest. Eros also is one of the most elon-gated asteroids, a potato-shaped body. It is one of

only three known NEAs with diameters of morethan 6 miles (10 kilometers).

Eros was discovered on Aug. 13, 1898, byGustav Witt, director of the Urania Observatory

in Berlin, and independently observed on thesame date by Auguste H. P. Charlois in Nice,

France. As a member of the NEA group known asthe Amors, Eros has an orbit that crosses Marspath but does not intersect the path of Earth. Theasteroid follows a slightly elliptical trajectory, cir-cling the sun in 1.76 years at an inclination of10.8 degrees to the ecliptic. Perihelion dis-

tance the closest point of the orbit to the

10Comparative Asteroid Sizes


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sun is 1.13 AU (105 million miles/169 millionkilometers); aphelion the farthest distancefrom the sun is 1.78 AU (165 million miles/

266 million kilometers). Eros average distancefrom the sun is 1.46 AU (135 million miles/218million kilometers).

The closest approach of Eros to Earth in the20th century was on Jan. 23, 1975, at approxi-mately 0.15 AU (14 million miles/22 million kilo-

meters). Previous close approaches occurred in1938 at 0.215 AU (20 million miles/32 millionkilometers) and in 1931 at 0.17 AU (16 millionmiles/26 million kilometers). Because of itsrepeated close encounters with Earth, Eros hasbeen an important object h istorically for refining

the mass of the Earth-moon system and the valueof the astronomical unit.

More than a century of ground-basedstudy including a worldwide observation cam-paign during the 1975 close approach hasmade Eros the best observed of the NEAs. As-

tronomers assign the asteroid a rotation period of5.27 hours. Albedo is 0.16. Thermal studies indi-cate the presence of a regolith, and radar sug-gests a rough surface. Eros is known to be com-positionally varied: one side appears to have ahigher pyroxene content and a facet-like surface,

while the opposite side displays higher olivinecontent and a convex-shaped surface.

Eros has no atmosphere and no evidence ofwater. During the day, the temperature averages100 degrees C (212 degrees F). At night, the tem-perature plunges to minus 150 degrees C (minus

238 degrees F). Gravity on Eros is very weak butsufficient to hold a spacecraft in orbit. A 100-pound (45-kilogram) object on Earth would weighabout an ounce on Eros, and a rock thrown fromthe asteroids surface at 22 mph (10 meters persecond) would escape into space.

Eros is known to be an S-type asteroid withhigh concentrations of silicate minerals and

metal. However, few details about its structure orcomposition are observable from the ground.The NEAR flyby on Dec. 23, 1998, produced evi-dence of variations in surface color and reflected

light (or albedo) that suggest the asteroid has adiverse surface makeup. Closer observations dur-ing the comprehensive yearlong orbital study ofEros will be needed to determine its prec ise com-position. Images taken during orbit are expected

to have more than 200 times better resolutionthan those obtained during the flyby and will betaken from as close as nine miles (15 kilometers)

from the asteroids surface.The science team has determined that Eros is

slightly smaller than originally estimated from

ground-based observations, with a size of 21 by 8by 8 miles (33 by 13 by 13 kilometers), versus anestimate of 25.3 by 9 by 8 miles (40.5 by 14.5 by 14

km). The asteroid rotates once every 5.27 hoursand has no discernible moons.

The asteroids density is approximately 1.55ounces per cubic inch (2.7 grams per cubic centi-meter), close to the average density of Earth scrust. This makes Eros about twice as dense as

asteroid 253 Mathilde, a C-type, carbon-rich aster-oid that NEAR flew past in June 1997, and aboutthe same density as S-type asteroid 243 Ida, whichNASAs Galileo spacecraft flew past in 1993. Erosand Ida are the only S-type asteroids for which amass and density have been determined.

Flyby imaging of the asteroids surface re-vealed a prominent elongated ridge that extendsalong its length for as much as 12 miles (20 kilo-meters). This ridge-like feature, combined withthe measurements of high density, suggests thatEros is a homogeneous body rather th an a collec-

tion of rubble such as Mathilde appears to be. Itmight even be a remnant of a larger body that was

shattered by an impact.The surface of Eros is pocked with craters.The two largest craters are four miles and 5.3miles (8.5 and 6.5 kilometers) in diameter, less

than half the size of asteroid Mathildes largestcraters. The existence of fewer, smaller craterscould be an indication that Eros has a relativelyyoung surface w hen compared to Ida.

253 Mathilde

Asteroid 253 Mathilde was discovered onNov. 12, 1885, by Johann Palisa in Vienna, Aus-

tria. The name was suggested by V. A. Lebeuf, astaff member of the Paris Observatory, who firstcomputed an orbit for the new asteroid. Thename is thought to honor the w ife of astronomer

Moritz Loewy, then the vice-director of the ParisObservatory.

Although Mathildes existence has beenknown for more than a century, not until 1995did observations with ground-based telescopes

11


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first identify the asteroid as a C-type. The 1995observations also revealed an orbital period of4.30 years. Perihelion is 1.94 AU (180 million

miles/290 million kilometers). Mathildes inclina-tion is 6.7 degrees and its albedo is 0.036.

On June 27, 1997, the NEAR spacecraft flew

within 753 miles (1,212 kilometers) of asteroid253 Mathilde. Mathilde was revealed as a verydark, heavily cratered object measuring 41 by 30

by 28 miles (66 by 48 by 46 kilometers). TheMultispectral Imager, one of the six instrumentson the spacecraft, found at least five craterslarger than 12 miles (20 kilometers) in diameter,just on the sunlit side of the asteroid.

Mathilde showed no color or albedo variations

over the 60 percent of its surface that was visibleto the NEAR spacecraft. The asteroid reflects 3 to 5percent of the suns light, making it tw ice as darkas a chunk of charcoal. Such a dark surface is be-

lieved to consist of carbon-rich material unalteredby planet-building processes, which melt and mixup the solar systems original materials.

The dark surface and color are suggestive of aparticular type of meteorite found on the Earthssurfacethe so-called CM carbonaceous

chondrites. However, the volume derived fromthe images and the mass of the asteroid deter-mined from the spacecraft tracking data yielded a

bulk density for Mathilde of 1.3 grams per cubiccentimeter, only about half that of CM chondrites.This suggests that asteroid Mathilde may have avery porous interior structure.

Mathilde rotates extraordinarily slowly. Itsrotation period is 17.4 days, the third-longest

known for an asteroid. In contrast, the Earth rotateson its axis in one day. The asteroids collision his-tory could be a factor, but more research needs tobe done. No moons have been discovered yet.

12

The Spacecraft

Spa cecraft Description

NEAR is the first solar-powered spacecraft to

fly beyond the orbit of Mars a technicalinnovation in spacecraft design. It has a designlifetime of four years and the capability to oper-ate at distances of 2.2 AU (203 million miles/

327 million kilometers) from the sun.Simplicity and low cost w ere the main drivers

in developing the spacecraft. Simplicity was

achieved by requiring that three major compo-nents instruments, solar panels, and high-gain

antenna be fixed and body-mounted. Al-though this requirement somewhat increases thecomplexity of spacecraft operations, it was an

important factor in overall cost.The NEAR system is designed to be highly fault-

tolerant. Fully redundant subsystems include the

Three-axis stabilized

Total weight: 1,775 pounds(805 kilograms)

Propellants: 717 pounds(325 kilograms)

Experiments:124 pounds(56 kilograms)

Dual-mode propulsion system Bipropellant (N2H4/N2O4) Monopropellant (N2H4)V capability: 1450 m/sec

Solar array power @ 1 AU: 1800 watts

Data rate @ Eros rendezvous 34-meter DSN antenna: 4.4 kbps 70-meter DSN antenna: 17.7 kbps

Two solid-state recorders: 1.6 gigabit capacity

Gallium arsenidesolar panels

Instruments

1.5-m antenna

99-1030B-5


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complete telecommunication system (except thehigh-gain and medium-gain antennas), as well asthe solid-state recorders, command and telemetry

processors, data buses, attitude interface unit andflight computers for guidance and control, andpower subsystem electronics. Additional fault-

tolerance is provided by use of redundant compo-nents: NEAR has two inertial measurement units(one operational, one backup), five sun sensors,

and 11 small thrusters.

Onboard Subsystems

The spacecraft has six onboard subsystems:mechanical, propulsion, power, guidance andcontrol, telecommunications, and command anddata handling.

Mechanica l SubsystemThe spacecraft structure is an eight-sided box

made of 18.3 feet square (1.7 meters square) alu-minum honeycomb panels connected to forwardand aft aluminum honeycomb decks. The NEARspacecraft launch mass, including propellant, is1,775 pounds (805 kilograms).

NEAR is designed with two independent struc-

tures: the spacecraft structure and the propulsionsystem structure, which are coupled at the aft deck.This design expedited spacecraft development byallowing the propulsion subsystem to be indepen-

dently designed and tested.Mounted on the outside of the forward deck

are the X-band high-gain antenna, the four solarpanels, and the X-ray solar monitor system. Mostelectronics are moun ted on the inside of the for-ward and aft decks, and all but one of the scienceinstruments are fixed in position on the outsideof the aft deck. The magnetometer is mounted on

the high-gain antenna feed. A star camera pointsout to the side of the spacecraft away from theinstruments so that a star-filled view is available

during asteroid operations. The interior of thespacecraft contains the propulsion module.

Prop ulsion Sub system

The NEAR propulsion subsystem, which wassupplied by Gencorp Aerojet of Sacramento, Ca-lif., contains the fuel and oxidizer tanks, 11 smallmonopropellant thrusters, a large bipropellantthruster, and a helium pressurization system. The

location of the tanks was selected to maintain thespacecrafts center of mass along the thrust vectorof the large thruster throughout the mission as the

biprop ellant is dep leted. The total change-in-veloc-ity capability is approximately 3,240 mph (1,450meters per second).

The monopropellant system is composed offour 5-pound (21-new ton) large, fine velocity con-trol thrusters and seven 1-pound (3.5-newton)

small, fine velocity control thrusters, all fueled bypure hydrazine. The specific impulses of themonopropellant thrusters range from 206 to 234seconds. They are arranged in six thruster mod-ules mounted to the forward and aft decks and arelocated so that the loss of any one thruster does

not affect performance. The 5-pound thrusters,which point in the same direction as the mainthruster, are used for thrust vector control duringthe bipropellant burns. The 1-pound thrusters areused for momentum dumping and orbit mainte-nance around the asteroid. A minimum change-in-

velocity increment of 0.02 mph (10 millimetersper second) is achievable in all directions.

The bipropellant thruster, or large velocityadjustment thruster, burns a mixture of hydra-zine and nitrogen te troxide (NTO) to p roduce amaximum 100 pounds (450 newtons) of thrust,

with a specific impulse of 313 seconds. The largethruster is used for the major velocity changes of

the NEAR mission.The propulsion system carries 461 pounds(209 kilograms) of hydrazine and 240 pounds(109 kilograms) of NTO oxidizer in three fuel and

two oxidizer tanks. The 14.5-gallon (55.1-liter)oxidizer tanks are located along the launchvehicle spin axis equidistant from the spacecraftcenter of mass. The 24-gallon (91-liter) fuel tanksare arranged 120 degrees apart in the mainthruster plane.

Power Subsystem

The power system comprises four 6- by 4-foot (1.8- by 1.2-meter) gallium arsenide solarpanels, a super nickel cadmium (NiCad) battery,and power system electronics. The solar array,

which was produced by Spectrolab Inc., Sylmar,Calif., provides 400 watts of power at NEARsmaximum solar distance of 2.2 AU (203 millionmiles/327 million kilometers) and 1,800 watts at1 AU (93 million miles/150 million kilometers).

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The power provided by the solar array is afunction of the spacecraft-to-sun distance and theincident solar angle, which must remain 30 de-

grees or less during the rendezvous at Eros. Thesolar power system is divided into 20 strings, sofailure of any one string would lead to only a

5-percent reduction in available power.The battery, which was produced by Hughes

Aircraft Co., Torrance, Calif., is a 9-ampere-hour,

22-cell super NiCad battery with cells fabricatedby Eagle-Picher Industries, Joplin, Mo. Batterycapacity provided p ower to the spacecraft beforethe solar arrays were deployed to make solarpower available. Thereafter, the battery wasrecharged, and it remains on-line to provide bus

voltage regulation. The battery serves as a backupsource of power in the event of momentary loadincreases or brief solar pow er deficits.

Guida nce and Control Subsystem

The guidance and control subsystem iscomposed of a suite of sensors for attitude deter-

mination, actuators for attitude corrections, andprocessors to provide continuous, closed-loopattitude control.

The sensor suite comprises five digital solarattitude detectors, a star tracker, and an inertialmeasurement unit. The inertial measurement

unit contains hemispherical resonator gyro-scopes for rate determination and accelerom-eters for measuring change in velocity.

The actuator complement contains four re-action wheels plus the 11 small monoprop ellantthrusters and the large biprop ellant thruster. All

normal attitude control is achieved using thereaction w heels alone. Any three of the reactionwheels provide complete 3-axis control, so asingle reaction wheel failure results in no loss infunctionality. The thrusters are used to dumpexcess angular momentum from the reaction

wh eels, accomp lish rapid slew maneuvers whenneeded, and p erform prop ulsive maneuvers.

Attitude control is to 0.1 degree; line-of-sightpointing stability is within 50 microradians over1 second; and post-processing attitude knowl-edge is to 50 microradians.

Telec om munication Subsystem

The telecommunication subsystem is anX-band system capable of simultaneously trans-

mitting telemetry data, receiving spacecraft com-mands, and providing Doppler and ranging track-

ing. In addition to the 5-foot (1.5-meter) high-gainantenna, there are two low-gain antennas and amedium-gain antenna w ith a fan-shaped radiationpattern. The w orldwide stations of NASAs Deep

Space Network (DSN) provide contact with thespacecraft after launch.

Eight discrete downlink data rates aresupported. In operation with the DSN 111-foot(34-meter) h igh-efficiency and beamguide anten-nas, the rates are 9.9 bits per second (bps) (emer-

gency mode), 39.4 bps, 1.1 kilobits per second(kbps), 2.9 kbps, 4.4 kbps, and 8.8 kbps. During

critical operations, the DSN 230-foot (70-meter)antennas can provide downlink rates of 17.6 and26.5 kbps. The downlink hardware, which wasdeveloped by JHU/APL, uses a solid-state power

amplifier with an outp ut level of 5 watts. The nor-mal uplink data rate is 125 bps. Emergency modeuplink is 7.8 bps.

Comm and and Data Hand lingSub system

The command and data handling subsystem

consists of four major segments: two redundantcommand and telemetry processors, two redun-dant solid-state recorders, a pow er switch ing unitto control spacecraft relays, and an interface totwo redundant 1553 standard data buses for com-

municating with other processor-controlled sub-systems. The functions provided are commandmanagement , te lemetry management , andautonomous op erations.

The solid-state recorders, which were pro-vided by SEAKR Engineering, Englewood, Colo.,

are constructed from 16-megabit IBM Luna-C

dynamic random access memories. One recorderhas 0.67 gigabit of storage; the other has 1.1-gigabitcapacity because it contains an additional memoryboard. This extra board is designated as the flightspare to replace either of the other memory

boards in a ground test failure.

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Instruments

The NEAR instrument payload consists of aMultispectral Im ager fi t ted with a charge

coupled device (CCD) imaging detector capableof photographing details on Eros surface as small

as 10 feet (3 meters) in diameter, aNear-InfraredSpectrom eter, an X-Ray/ Gam m a-Ray Spectrom -eter, a Laser Rangefinder, a Magnetometer, anda radio science experim ent. Several of the instru-

ments are derived from designs developed byJHU/APL for Department of Defense spacecraft,an example of dual-use technology transferred tothe civilian sector.

Despite the lower cost and rapid developmentschedule of the NEAR spacecraft, the instrument

designs incorporate many technical innovations:

X-Ray Sensor

Solar Panel(backside)

Magnetometer

MultispectralImager

InfraredSpectrometer

NEAR LaserRangefinder

Gamma-Ray Sensor

99-1030B-7

15

Multispec tral Imager (MSI)

MSI is a high-resolution, visible-light andinfrared camera that will determine the overall

size, shape, and spin characteristics of Eros andwill map the morphology and mineralogy ofsurface features. The imager also will be used foroptical navigation at Eros and to search for satel-lites. Images taken during approach, flyby, andorbit of Eros can detect surface features as small as

10 feet (3 meters).

First space flight of a laser incorporating an in-flight calibration system (Laser Rangefinder)

First space flight using a near-infrared systemwith a radiometric calibration target and an

indium-gallium-arsenide focal plane array thatdoes not require cooling with liquid nitrogen(Near-Infrared Spectrometer).

First space flight of a silicon solid-state

detector viewing the sun and measuring thesolar input X-ray spectrum at high resolution(X-Ray Spectrometer)

First space flight of a bismuth germanate anti-coincidence shielded gamma-ray detector(Gamma-Ray Spectrometer)

Adapted by JHU/APL from a military remotesensing system, MSI is a 537- by 244-pixel CCDcamera with five-element, radiation-hard refrac-tive optics. The instrument covers the spectralrange from 0.4 to 1.1 microns. It has an eight-posi-tion filter wheel with filters chosen to optimizesensitivity to minerals expected to occur on Eros.

MSI has a field-of-view of 2.26 degrees by 2.95 de-grees and a pixel resolution that corresponds to 31


22/27

by 53 feet (9.6 by 16.2 meters) from 62 miles (100kilometers). The instrument has a maximum fram-ing rate of one p er second with images digitized to

12 bits. It has a dedicated digital processing unit

with an image buffer, autoexposure capability,and onboard image compression.

MSI Science Team Leader

Joseph Veverka, Cornell University

MSI Instrument Scientist

Scott L. Murchie, JHU/APL

MSI Lead Engineer

S. Edward Hawkins III, JHU/APL

NEAR Payload Manager

Robert E. Gold, JHU/APL

MSI Development

JHU/APL

Nea r-Infrared Spec trom eter (NIS)

NIS data will provide the main evidence forthe distribution and abundance of surface miner-als like olivine and pyroxine. Together with themeasurements of elemental composition fromthe X-Ray/Gamma-Ray Spectrometer (XGRS) and

color imagery from MSI, NIS will provide a link

between asteroids and meteorites and clarify theprocesses by which asteroids formed and evolved.NIS will measure the spectrum of sunlight re-flected from Eros in the near-infrared range from0.8 to 2.7 microns in 64 channels.

NIS also adapted from a military remotesensing instrument is a grating spectrometer

that disperses light from the slit field-of-viewacross a pair of passively cooled, one-dimensionalarray detectors. One detector is a germanium ar-

ray covering the lower wavelengths from 0.8 to1.5 microns; the other is an indium-gallium-ars-enide array covering 1.3 to 2.7 microns. The NIS

slit field-of-view is 0.38 degree by 0.76 degree inthe n arrow position and 0.76 degree by 0.76 de-gree in the wide position. At 62 miles (100 kilo-

meters) from the asteroid, these p ositions corre-spond to 0.4 to 0.8 mile (0.65 to 1.3 kilometers)and 0.8 by 0.8 mile (1.3 by 1.3 kilometers). A scanmirror slews the field-of-view over a 140-degreerange. Mirror scanning combined with spacecraftmotion will be used to build up hyperspectral

images. NIS also carries a diffuse gold calibrationtarget that can reflect sunlight into the spectrom-eter and p rovide in-flight spectral calibration.

NIS Science Team Leader

Joseph Veverka, Cornell University

NIS Instrument Scientist

Noam R. Izenberg, JHU/APL

NIS Lead Engineer

Jeffrey W. Warren, JHU/APL



NIS Development

JHU/APL, Sensor Systems Group Inc.,

Sensors Unlimited

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17

X-Ray/Ga mm a-Ray Spec trom ete r(XGRS)

XGRS will measure and map abundances ofseveral dozen key elements at and near the sur-

face of Eros. X-rays from the sun striking the as-teroid can p roduce significant count rates of fluo-rescence X-rays from surface elements such asmagnesium, aluminum, and silicon. The elemen tssulfur, calcium, titanium, and iron are alsopresent in asteroids, but count rates are lowerand data take longer to accumulate. Similarly,

cosmic ray protons (and energetic particles asso-ciated with solar flares) can interact with theasteroid surface to produce gamma rays charac-teristic of the nuclear energy levels of a given

element. Gamma rays also can be spontaneouslyemitted by naturally occurring radioactive ele-

ments such as potassium, uranium, and thorium.The XGRS consists of two state-of-the-art

sensors: an X-ray spectrometer and a gamma-rayspectrometer.

X-Ray Spectrometer (XRS). XRS is an X-rayresonance fluorescence spectrometer that detects

the characteristic line emissions excited by solarX-rays from major elements in the asteroid surface.XRS covers the energy range from 1 to 10kiloelectron volts using three gas-proportional

counters. The balanced, differential filter tech-nique is used to separate the closely spaced mag-

nesium, aluminum, and silicon lines below2 kiloelectron volts. The gas-prop ortionalcounters directly resolve higher energy lineemissions from calcium and iron. A mechanical

collimator gives XRS a 5-degree field-of-view tomap the chemical composition at spatial resolu-tions as low as 1.2 miles (2 kilometers). XRS in-

cludes a separate solar monitor system to continu-ously measure the incident spectrum of solar X-rays. In-flight calibration capability also is pro-

vided.Gamm a-Ray Spectrometer (GRS). Abundances

of several important elements, such as potas-

sium, silicon, and iron, will be measured in fourquadrants of the asteroid. GRS detects character-istic gamma rays in the 0.3- to 10-megaelectronvolt range emitted from specific elements in theasteroid surface. GRS uses a body-mounted, pas-sively cooled sodium iodide detector enveloped

by an active bismuth germanate anti-coincidenceshield to provide a 45-degree field of view.

XGRS Science Team LeaderJacob I. Trombka,

NASA/Goddard Space Flight Center

XGRS Instrument Scientist

Ralph L. McNutt, Jr., JHU/APL

XGRS Lead Engineer

John O. Goldsten, JHU/APL



XGRS Development

JHU/APL, NASA/Goddard Space

Flight Center, Metorex, EMR Photoelectric

NEAR Laser Rangefinder (NLR)

NLR will determine the distance from thespacecraft to the asteroid by precisely measuringthe delay time between th e firing of a laser pu lseand its return reflection from the surface. It sends

a small portion of each emitted laser pulsethrough an opt ical fiber of known length and intothe receiver, providing continuous in-flightcalibration of the timing circuit.

The ranging data will be used to construct aglobal shape model and a global topographic

map of Eros with horizontal resolution of about1,000 feet (300 meters). NLR also will measuredetailed topographic profiles of surface featureson Eros with a best spatial resolution of about12 feet (4 meters). The profiles will be used asconstraints on models of the origin and evolution

of surface features.


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18

MAG is a 3-axis fluxgate sen sor mounted on atripod bracket above the high-gain antenna, alocation chosen for minimum exposure to space-

craft-generated magnetic fields. Magnetometerelectronics are located on the top deck. Theinstrument can determine the strength of the

field to w ithin 2 nano teslas.MAG Science Team Leader

Mario H. Acua, NASA/

Goddard Space Flight Center

MAG Team Member

Christopher T. Russell,

University of California, Los Angeles

MAG Instrumen t Scientists

Lawrence J. Zanetti and

Brian J. Anderson, JHU/APL

MAG Lead Engineer

David A. Lohr, JHU/APL



MAG Develop ment

NASA/Goddard Space Flight Center,

JHU/APL

Radio Sc ience Experiment

The radio science experiment will use the

NEAR radio tracking system to determine the mass

and mass distribution of the asteroid. Measure-ments will be made of the two-way Doppler shiftin radio frequency between the spacecraft andEarth to an accuracy better than 0.025 inch persecond (0.1 millimeter per second). These mea-

surements will determine line-of-sight velocityvariations induced in the spacecrafts motion bythe changing gravitational effects produced by theneighboring asteroid. Combined with data fromother NEAR instruments, this information will al-low accurate modeling of Eros density and mass

distribution.

Radio Science Team Leader

Donald K. Yeomans, NASA/Jet Propulsion

Laboratory



Radio Science Development

Motorola

NLR uses a neodymium-doped, yttrium-aluminum-garnet, solid-state laser and a compactreflecting telescope.

NLR Science Team Leader

Maria T. Zuber, MIT and NASA/

Goddard Space Flight CenterNLR Instrument Scient ist

Andrew F. Cheng, JHU/APL

NLR Lead Engineer

Timothy D. Cole, JHU/APL



Magnetom eter (MAG)

MAG will measure the strength of Eros mag-

netic field. Data from the Galileo spacecraftflybys of the asteroids Gaspra and Ida suggest thatboth of these bodies are magnetic, but the resultsare inconclusive. Discovery of an intrinsic mag-netic field at Eros would be the first definitive de-

tection of magnetism at an asteroid and wouldhave important implications about its thermaland geologic history.


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The People

NASA NEAR Mission Management

Associate Administrator,

Office of Space Science .................................................................. Edward Weiler

Director, Mission and Payload Development

Division, Office of Space Science................................................... Kenneth Ledbetter

Program Executive ............................................................................. Anthony Carro

Program Scientist ............................................................................... Thomas Morgan

Discovery Program Manager .............................................................. David Jarrett

JHU/APL NEAR Project Management

Space Department Head .................................................................... Stamatios Krimigis

Project Manager ................................................................................. Thomas CoughlinProject Scientist ................................................................................. Andrew Cheng

Mission Director................................................................................. Robert Farquhar

Mission Design Team Leader ............................................................. David Dunham

Mission Operations Manager.............................................................. Mark Holdridge

Spacecraft Team Leader ..................................................................... Andrew Santo

Payload Manager ................................................................................ Robert Gold

Science Data Center Manager ............................................................ Douglas Holland

JPL NEAR Project Management

Navigation Team Leader .................................................................... Bobby Williams

Deep Space Network Team Leader ................................................... Al Berman

NEAR Science Team Leaders

Multispectral Imager/Near-Infrared Spectrometer............................. Joseph Veverka,

Cornell University

X-Ray/Gamma-Ray Spectrometer ....................................................... Jacob Trombka,

NASA/Goddard Space Flight

Center

Magnetometer .................................................................................... Mario Acua,

NASA/Goddard Space FlightCenter

NEAR Laser Rangefinder .................................................................... Maria Zuber, MIT and NASA/

Goddard Space Flight Center

Radio Science ..................................................................................... Donald Yeomans, NASA/Jet

Propulsion Laboratory

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The Discovery Program

Discovery Goals

The Discovery Program NASAs inno vative

approach to faster, better, cheaper planetarymissions marked its inaugural launch with th e

NEAR mission. Formally initiated in NASAs fiscal 1994

budget within the Solar System Exploration Division,

the Discovery Program grew out of NASA discussions

with the science community to design a planetary ex-

ploration program that balances science return and

mission cost in an era of dec lining space budgets. The

Discovery Program represents a significant departure

from previous NASA planetary programs in terms of

total mission cost, development time, management

approach, and scope of science objectives.

The Discovery Program goals and criteria include:

Lower Cost: The cost of design and developmentthrough launch is limited to $190 million (fiscal

1999 dollars). Total mission cost is limited to $299

million and includes preliminary analysis, defini-

tion, launch services, and mission operations.

NASA-provided launch vehicles for Discovery

missions must be medium (Delta-2) class or smaller. Rapid Developm ent Tim e: To meet the Discovery

Program goal of launches every 12 to 18 months,

constraints on mission development and definition

times are tight. Design and development is limited

to 36 months or less from start through launch

plus 30 days. Stream lined Man agem ent Approach: Teaming is

encouraged among industry, educational/non-

profit institutions, and government partners.

NASA field centers are welcome as team members,

as are non-U.S. individuals and organizations. Com-

petitively selected teams have mission responsibil-

ity and auth ority, with a large degree of freedom in

accomplishing objectives. NASA oversight and re-

porting requirements focus on the essentials for

mission success and agreed-upon science re turn.

New Technology/Technology Transfer: The Dis-

covery selection process recognizes the inclusion

of new technology to achieve performance en-hancements and total mission cost reductions. The

teaming of industry, universities, and government

is meant to foster technology transfer occurring in

parallel with tech nology development.

Public Awareness and Education: Activities are

encouraged to enhance the level of public under-

standing and awareness of solar system

exploration. Such activities may include informa-

tion programs to inform the public through the

media or other means and educational activities

coordinated with schools and science centers.

Discove ry Missions

Since NEARs launch, three Discovery missions

have been successfully launched. Mars Pathfinder

sent back thousands of images and measurements af-

ter landing on the red planet on July 4, 1997. Dr. Mat-

thew Golombek of NASA/JPL was project scientist.

Lunar Prospector, launched in January 1998, sent

back data that enabled scientists to create the first

maps of the gravity, magnetic prop erties, and elemen-

tal composition of the moons entire surface. Led by

Dr. Alan Binder of the Lunar Research Institute, the

mission also detected a strong possibility of water ice

at both lunar poles. The mission ended on July 31,1999, with a controlled crash into a crater near the

south pole of the moon , in an attempt to con firm theo-

ries about abundant w ater ice buried in the lunar soil.

The Stardustmission, launched on Feb. 7, 1999,

will return the first samples of a comet. The spacecraft

will collect comet particles, volatiles and dust, along

with samples of interstellar dust, which will be

dropped back to Earth in a reen try capsule. Dr. Donald

E. Brow nlee o f the University of Washington is serving

as principal investigator.

The Genesis mission, due to launch in January

2001, will gather samples of the charged particles in

the solar wind and return them to Earth using a

sample-gathering techn ique similar to th at used by the

Stardust mission.Dr. Donald Burnett of the California

Institute of Technology is the lead scientist.

The Com et Nucleus Tou r (CONTOUR), led by Dr.

Joseph Veverka of Cornell University, will fly by three

near-Earth comets. Set to launch in June 2002, CON-

TOUR will provide images and spectral maps of comet

nuclei and analysis of comet dust.

Th e Mercu ry Surface, Space En viron m ent,

Geochem istry an d Ran ging (MESSENGER) mission,

scheduled for launch in spring 2004, will study

Mercurys shape, interior, and magnetic field, and sendback the first global images of the planet. Led by Dr.

Sean Solomon of the Carnegie Institution, MESSENGER

will be built and managed by JHU/APL.

The Deep Im pactmission will launch in January

2004 and send a projectile into comet P/Tempel 1 on

July 4, 2005, to create an explosion as a way to study

the interior of a comet. Led by Dr. Michael AHearn of

the University of Maryland, the mission will be man-

aged by NASA/JPL and built by Ball Aerospace.

20


27/27

NASA Science Team

Co-chairT. Morgan/A. F. Cheng

Multispectral Imager/Near-Infrared SpectrometerJ. Veverka

X-Ray/Gamma-Ray SpectrometerJ. I. Trombka

MagnetometerM. H. Acuna

NEAR Laser RangefinderM. T. Zuber

Radio ScienceD. K. Yeomans

JHU/APLInstrument Scientists

Multispectral ImagerS. L. Murchie

Near-Infrared SpectrometerN. R. Izenberg

X-Ray/Gamma-Ray SpectrometerR. L. McNutt, Jr.

MagnetometerL. J. Zanetti/B. J. Anderson

NEAR Laser RangefinderA. F. Cheng

Mission DirectorR. W. Farquhar

Project ScientistA. F. Cheng

DSN

A. L. Berman

Navigation

B. G. Williams

MissionDesign

D. W. Dunham

Mission Ops

M. E. Holdridge

Spacecraft

A. G. Santo

PayloadManager

R. E. Gold

Science DataCenter

D. B. Holland

NASA HQ Program Office

~

NEAROrganization ChartFlight PhaseFebruary 1996 to Mission End

Project Manager T. B. Coughlin

Program Executive A.Carro

Program Scientist T. Morgan

the near earth asteroid rendezvous

Documents