nasa facts interplanetary explorer satellites

8/7/2019 NASA Facts Interplanetary Explorer Satellites

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Vol. II, No. 1 Page 1NASA FACTSNASA FACTS N u m b e r Volume I of NASA FACTSconsis t s of al l issues publ i shed pr ior to Ju ly 1964and running from A-62 to8 - 2 - 6 4 . Volume II beginswith NASA FACTS, Interplanetary Explorer Satellites ,Vol . II , No .1 . An Educational Services Publication of theNational Aeronautics and Space Administration

INTERPLANETARY EXPLORER .SATELLITES N64-2871!{ ) . i t ~ / ~ J

Interplanetary Explorer satellite (Explorer XVIII) in space (artist's conception). InterplanetaryExplorers formerly were called Interplanetary Monitoring Platforms (IMP).

Before man can travel to other celestial bodies,he must solve mysteries about space which havedefied centuries of observation. A significantstep toward this goal is being made through aseries of satellites called Interplanetary Explorers,the first of which, Explorer XVIII, was launchedNovember 26, 1963.

Interplanetary Explorers are aimed primarilyat acquiring additional knowledge about radia-

t ion and magnetic fields in space between theearth and moon during a majo r part of the solarcycle. The solar cycle refers to a period of approximately 11 years during which the frequencyof solar eruptions reaches a maximum, a minimum, and then again, a maximum.

As their name implies, Interplanetary Explorersare designed principally to gather information onconditions in interplanetary space, which begins


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Explorer XVIII is prepared fo r test of its ability to withstand vibration .

Delta rocket vehicle launches Explorer XVIII. This wasthe twentieth straight time that the Delta vehicle hadperformed as planned .

at th e outer border of earth's magnetic field.The satellites contain arrays of instruments fo racquiring and transmitting detailed data aboutthe solar ' wind and cosmic rays (both defined be-low), an.d magnetic fields.

Such information is essential to design of protective shielding and communications systems fo rmanned spacecraft journeying to the moon andbeyond. The data also are expected to contribute to development of techniques for forecasting solar f lares-sudden outbursts of matterfrom the sun's surface-that shower space withlethal radiation. Lunar journeys could then betimed to avoid these dangerous periods.


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NASA FACTS Vol. II, No . 1

Engineers install nose cone around Explorer XVIIIprior to launch.

THE SOLAR WIND AND MA(;NETIC FIELDSTraveling above the earth's magnetic field, In-

terplanetary Explorers measure the solar wind,which is made up of hot electrified gases thatrush constantly from the turbulent surface of thesun. Its strength depends upon the level ofsolar activity. However, the solar wind is lesssubstantial than winds on earth, consisting of arelatively scant 10 to 20 atomic particles (chieflyprotons of hydrogen atoms) per cubic inch ascompared with earth winds. Nevertheless, thewind appears to be a dominant feature of interplanetary space.

Results from NASA's Mariner " spacecraft,coupled with other observations and assumptions, indicate that the wind pulls with it partsof the sun's magnetic field and distributes thesehroughout the solar system where they become

interplanetary magnetic fields.

Page 3Moreover, other evidence, particularly that

provided by artificial satellites, ind icates that thesolar wind compresses earth's magnetic field toan approximately 40,OOO-mile altitude on thesunny side and stretches or blows it out to asyet undefined limits on the night side . As a re-sult of these findings and carefu l analyses, scien-tists presume that the earth's magnetic field isshaped like a tear drop with the portion of themagnetic field on earth's night side trail ing ourplanet like the tail of a comet.

Interplanetary Explorers are expected to shednew l ight on the limits of earth's magnetic fieldand on the interplay between earth's and inter-planetary magnetic fields and the solar wind.

COSMIC RAYSCosmic rays, the most penetrating kind of

harmful radiation known, pose a major dangerto man in interplanetary space. They consist ofprotons (nuclei of hydrogen atoms), alpha particles (nuclei of helium atoms), nuclei of atomsheavier than hydrogen or helium, and electrons.

Energies of cosmic rays may be millions oftimes greater than the energies of particles in thesolar wind. Solar wind particles have energiesin the hundreds and thousands of electron volts.(The electron volt is a scientific measurement unitfor comparing energies of atomic particles.The electron volt is a tiny unit of energy. As anexa mple, it would take the equivalent of 550sext i l l ion-55 followed by 22 zeros-electronvolts to keep a 25-watt light bulb burning for anhour.)

Characteristic energies of cosmic rays emittedby the sun, usually in conjunction with a solarf lare, are about one hundred mill ion electron

NASA's Delta launch vehicle at ta ined It s twentiethconsecutive success with th e launch of Explorer XVIII.Delta has also been used in launching communicationssatelli tes including Echo, Relay, Syncom , and Telstar;th e TlROS series of meteorological satellites; and scien-tific satellites including th e Orbiting Solar Observatory,Explorers XII, XIV, XV, and XVII, and Ariel, the world'sfirst international satellite (built by the United Kingdomand the United States).


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LOWER ENERGYINTERSTELLARCOSMIC RAYS

;, ~ NASA FACTS Vol. II, No . 1

\LOCATION OF. SOLAR FLARE __

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LOWER ENERGYINTERSTELLARCOSMIC RAYS

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Artist's conception of cloud of cosm ic rays an d magnetic field emitted fram the sun following a so lar flare andhow it envelops earth creating th e Forbush effect (see text)_

volts. Energies of cosmic rays from beyond thesolar system, principally from our Milky Waygalaxy but sometimes from other parts of theUniverse, are in the millions, billions, and trillionsof electron volts.

SOLAR FLARESThe most violent of solar eruptions is the solar

flare which fires a vast cloud of cosmic rays intospace. Scientists theorize that the cloud dragsa part of the sun's magnetic field with it .

If the cloud reaches earth's vicinity, its accompanying magnetic field screens earth from someof the cosmic rays that originate outside of thesolar system. This is believed to account fo r thephenomenon in which a sudden decrease of thesecosmic rays striking earth was observed to folIowa solar flare. The event is called the For-

bush decrease fo r Scott E. Forbush, CarnegieInstitution, Washington, D.C., who first noted itmore than tw o decades ago .

At the same time, however, the solar cosmicrays follow the lines of force of the solar magnetic field and penetrate earth 's magnetic fieldto a depth where they collide with the air molecules and atoms of the atmosphere . Amongthe effects on earth that are linked to this phenomenon are radio black-outs , or interference,and the auroras, or Northern and Southernl ights, that from time to time illum inate the nightskies over certain areas of the globe.

During such an event, astronau ts travelingabove th e protective shields of earth's atmosphere and magnetic field could be endangeredby lethal radiation. Accurate long-range forecasts of solar flares are vital to preparations fo rProject Apollo which calls for manned exploration of the moon in this decade.


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NASA FACTS Vol. II, No. 1

Call-outs designate major equipment of Explorer XVIII,the first Interplanetary Explorer.

DESCRIPTION OF EXPLORER XVIIIExplorer XVIII, launched from Cape Kennedy

(formerly Canaveral), Florida, on November 26 ,1963, is th e first Interplanetary Explorer. Itsoctagon-shaped structure, about 2% feet acrossand a foot in breadth, has eight compartmentsor modules into which different kinds of particledetectors such as Geiger counters and ion chambers are installed, or plugged. Magnetometers,devices for measuring magnetic fields, are attached to the ends of booms extending from thebasic structure. The satellite weighs 138pounds .

Holding the magnetometers away from thespacecraft 's body keeps them from picking upthe weak magnetic forces generated by materialsand electric currents in the platform. The magnetometer booms are telescoped at launch andextended in orbit.

A rubidium vapor magnetometer, designed togather comprehensive data about magnetic

Th e launch of Explarer XVIII kept Intact the pe rfec trecord of scientific satellite launches since th e beginning of 1962: eight successes in eight attempts. Otherscientific satellites launched in 1962 and 1963 ar eAlouette , Canada ' s first satel l i te ; th e Orbitin g SalarObserva ta ry; Ariel, th e United Kingdom-Uni t edStates satellite; and Explorers XIV, XV, XVI, and XVII.All were launched by Delta with the except ions afAlouette, launched by Thor-Agena a, and ExplorerXVI, launched by Scaut.

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Artist's drawing (not in scale) of initial orb it of Explorer XVIII.

fields, is housed in a bulbous container at theend of an approximately 6-foot-long boom (seeillustration of Explorer XVIII) . Two Auxgatemagnetometers, less comprehensive in the information they supply than the rubidium vapormagnetometer but more capable of detectingfaint magnetic fields, ar e at th e tips of two 7-foot-long booms.

COMMUNICATIONS AND POWERFour whip-like antennas, projecting at 45-

degree angles from the satellite, are part of thecommunications system by which ground stationstrack and acquire information from ExplorerXVIII. The satellite's four-watt transmitter operates on radio frequencies of 136.110 megacycles for telemetry (long distance transmissionof information about the satellite and aboutspace through which it is traveling) and 136.9megacycles fo r tracking.

The spacecraft is powered by 11,520 solarcells, photoelectric devices that convert sunlightto electricity. The solar cells, which generate atotal of 73 watts, are divided among four solarpaddles, each of which is 2 ],i feet long and 1Y2feet wide. In addition, 13 rechargeable nickelcadmium storage batteries furnish electricitywhen the satellite is in the earth ' s shadow.(They also provide electricity from launch untilthe solar paddles are extended in orbit.)


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Antennas at the lima , Peru, station of NASA 's STADANrepresent one of several kinds that track an d gatherdata from unmanned satellites .

ORBITAL INFORMATIONA Delta launch vehicle rocketed Explorer XVIII

initially into a cigar-shaped orbit reaching as faras 122,792 miles from earth and dipping asnear as 119.6 miles. The satellite takes about95 hours to complete each orbit. Orbital in-clination is 33.3 to the equator. This meansthat the satellite's orbit intersects the earth'sequatorial plane at a 33.3 angle. (The equatorial plane bisects the earth at the equator.)Because Explorer XVIII races so far into space,its orbit is noticeably affected by the interplay of

--NASA FACTS Vol. II, No .

the sun's, moon's, and the earth's gravitationapulls. One result is that the spacecraft's pergee, or closest approach to earth, will riseAnother is that the inclination or tilt of its orbhom the equator will increase. Such pronouncechanges expected over a relatively short timpresent scientists with an unusual opportunity tstudy the motions of an object in space that arsignificantly influenced by several gravitationafields.

FIRST USE OF NEW TRACKING SYSTEMThe Explorer XVIII experiment marks the firs

use of NASA's recently completed range anrange rate system which complements other facilities of NASA's Satellite Tracking and DatAcquisition Network (STADAN). STADAN isworld-wide complex of ground stations thatracks and acquires data from unmanned satellites. The stations locate the spacecraft, tracit, control th e satellite ' s data gathering equipment, and receive information acquired anstored by the satellite while passing betweestations.

One of the methods of tracking employed bySTADAN is the original Minitrack system established during the International Geophysical Yea(IGY) in 1957 and 1958. In this system, thchanging angle of the satellite's radio beam i

OTHER SCIENTIFIC SPACECRAnThe interplanetary Explorer series is on e of several spacecraft progra ms designed to augment information about th e space

environment an d celestial bodies. Other Explorer satellite projects s tudy such phenomena as air density , magnetic fields , temperature, meteoroids, and solar and cosmic radiation. Among additional unmanned spacecraft programs are :EARTH SATELLITES LUNAR AND INTERPLANETARY SPACECRAFTAloue"e-Canadian satellite orbited by a U.S. launch vehicle.Th e satellite wa s Instrumented to provide new data on electrondensity of th e Ionosphere.Arie l-United Kingdom-U.S. satellites designed fo r ionosphericand radiation studies.Orbiting Astronomical Observatory - to m a k e telescopic an dothe r studies of th e skies at an al t i tude where atmosphericinterference is negligible.Orbiting Geophysical Observatory - to make correlated studieswithin earth 's atmosphere, th e magnetosphere, or cislunar spaceon energetic particles , galactic an d solar radiation, ionizationprocesses, meteoroids, and other space phenomena.Orbiti"9 Solar Observatory -to s tudy the features and behavior of th e su n fo r prolonged periods, porticularly it s emission of ultraviolet light, g a m m a radiation, and X-rays.

Lunar Orbi ter- to be launched into an orbit around th e moon .It will be equipped to take pictures af the moon's surface an dprovide other scientific data .Mariner- to fl y In th e vicinities of and send information aboutMars and Venus . Mariner II, launched December 14 , 1962,provided informat ion abQut Venus, including th e possibilitytho t th e planet ' s surface may be as hot as 800 Fahrenheit .Pioneer- to investigate the interplanetary environment as faras SO million miles from earth.Ranger - t o send close-range pictures of th e lunar surface be fore crashing to destruction on th e moon .Su rveyo r - to contribute to the technology fo r a soft landingon th e moon and send information about the lunar environmen t before and after landing. Such Information m ay includepictures of th e moonscape .


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Magnetic f ields, solar wind (energetic particles), the shock wave, and region of turbulence as indicated byExplorer XVIII (see text) . The magnetosphere is the region of space dominated by the geomagnetic field.The magnetopause is the outer boundary of earth's magnetic f ield .

in relation to tw o or more tracking ansystems, each of which has been precisely

From the a n ~ u l a r measurements anddistances between the antennas, other

details, including altitude and speed of theare calculated.

The new range and range rate system prothe spacecraft's altitude and velocity moreaccurately. In the range an d range

system, th e ground station beams radio sig-to the satellite which in turn transmits to the

The round-trip time of the signals furan immediate m e a ~ u r e m e n t of range, or

The range rate, or velocity, of the ve-is obtained by measuring the Doppler shift

f the signals received by the ground station.The phenomenon known as the Doppler shifts first expounded in 1842 by Christian Johann

Doppler of Prague (now the capital of Czechoslovakia). Our most famil iar experience with theDoppler shift is the apparent change in pitch, orfrequency, of a train whistle as the train approaches and passes us. Actually, the pitch ofthe train whistle has not changed.

What occurs is that as the train approaches,the sound waves sent by its whistle in effect pileup in our direction, resulting in a shorter pitch,frequency, or wave length. As the train pullsaway, the reverse takes place.

In a similar manner, a satellite's radio transmitter keeps sending signals at the sa me frequency. However, the signals received by theground station change frequency as the satellitemoves toward an d away from the station. Therate of change in frequency indic


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Shock wave (curved dark band) is created by small early model of Mercury spacecraft in supersonic wind tunne l.Special photographic processes make visible both the shock wave an d turbulent ai r in craft's wake.

PRELIMINARY RESULTSPreliminary analyses of data from Explorer

XV'" have provided significant new informationabout a shock wave that envelops the earth andabout a band of radiation that lies above theVan Allen Radiation Region.

A shock wave is formed in a medium suchas ai r or water when a body moves through themedium, or the medium moves past the body, atvery high speed. The material of the mediumis compressed in a wave which streams outaround the body.

The shock wave beyond the Van Allen regionis created by the impact of the speeding solarwind against earth's magnetic field (the geomagnetic field). The result is the same as when highspeed air is blown at a blunt object in a supersonic wind tunnel. The shock wave is producedsome distance ahead of th e object and flowsback on each side.

Explorer XVIII detected the shock wave about53,000 miles from earth on the earth's sunlitside. On this side, the geomagnetic field extends to an altitude of about 40,000 miles.

Between the shock wave and earth 's magneticfield, Explorer XVIII reported a region of turbulent magnetic forces and fluctuating quantities ofenergetic particles. Such particles include elec-tron 's and protons, constituents of atoms, andconstitute much of the radiation in space.

Both this region and the geomagnetic fieldfan ou t around the earth and trail off in a direction away from the sun. They may reach asfar as the orbit of the moon. As a result, themoon may be bombarded periodically by radiation as it passes through the region. The radiation is no t believed to pose a serious hazard fo rspace travelers .

SHOCK WAVE: a compression wa ve fo rmed whenever _th e speed of a body re la tiv e to a medium exceeds tha t a twhich th e medium (such as air or water) ca n transmit sound, an d character ized by a disturbed region of small but limi ted thickness within which very abrupt changes occur in th e pressure, density, and velocity of th e medium (e .g. passage of a shock wa ve from an explosion . . or from th e leading wing edge of a supersonic airplane) through acompress ible fluid such as air.

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