NASA Facts Voyager

8/8/2019 NASA Facts Voyager

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NASAFactsAn Educational Publicationof theNational Aeronautics andSpaceAdm inistra t ionNF-87/10-77

to the Outer Planetsplanets Jupiter and Saturn-almost unimag-inably large and far away. Beyond Jupiter and Saturnare the even more remote planets Uranus, Neptune,and Pluto.

Although space exploration by the National Aero-nautics and Space Administration has become a famil-iar achievement in the last decade, the planets thathave been measured and photographed by NASAsMariner and Viking spacecraft have been the nearbyplanets Mars, Venus, and Mercury (though nearby inthis sense may mean millions of kilometers fromEarth).

Exploring the giant outer planets, however, posesproblems with a whole new set of dimensions. For thisreason, a different type of spacecraft, named Voyager,has been developed to perform this formidable task.NASAs mission to Jupiter and Saturn consists of

two unmanned Voyager spacecraft scheduled to belaunched in August and September of 1977 and toarrive at Jupiter in 1979. As they fly by Jupiter, theirinstruments look closely at the planet and its manysatellites. The spacecraft instruments find out howthese bodies affect the solar wind-the blizzard ofprotons and electrons streaming outward from the Sun.They also investigate how charged particles, such aselectrons and protons, and the magnetic field of Jupiteract on each other.

The trajectories of the spacecraft past Jupiter havebeen chosen so that the gravity and orbital motion ofthe giant planet act as a slingshot to send bothspacecraft to the ringed planet Saturn. Arrival at Saturnis scheduled for November 1980 and August 1981.The first spacecraft to reach Saturn is expected torepeat the types of experiments and measurements itmade as it flew by Jupiter and also to investigate theimportant satellite Titan and the spectacular ring sys-

tem of Saturn. If these measurements have beensuccessful, the second spacecraft to arrive might be

sent to the planet Uranus, the next planet beyonSaturn. Recent discoveries have revealed that Uranuis also a ringed planet, like Saturn, which had prevously been thought to be the only planet in our SolaSystem to have a system of rings. Thus, Voyager hathe potential of exploring three of the giant planets othe outer Solar System and providing a close look otheir larger satellites.

Each spacecrafts scientific instruments seek newinformation about the atmospheres and the environments of the planets, the surface features and atmospheres of the satellites, the nature of Saturns rings, themagnetic fields and the flow of charged particles in theplanetary systems, and the effects of the planets ansatellites and rings on these particles and fields.The Voyager Project

The Voyager project is managed by the NASA JePropulsion Laboratory, Pasadena, California. The Laboratory is responsible for building the two spacecrafand for conducting tracking, communications, and mission operations. NASAs Lewis Research CenterCleveland, Ohio, has responsibility for the launcvehicles.

There are eleven science teams with a total of 8scientists concentrating on different aspects of thescientific investigation. The leaders of the teams areshown in Table 1.The Spacecraft

Voyager is the most far-reaching space mission tobe flown by NASA, since it includes a possible flight tUranus, which is 2.87 billion kilometers from the Sun-19 times the distance from the Sun to the Earth.

The spacecraft (Figure l),because they have ttravel so far from the Sun, differ somewhat from theearlier Mariners from which they were developedPanels carrying solar cells to convert sunlight to electrcity, needed for the instruments and the electronics oearlier Mariner spacecraft, are missing from the new
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Figure 1 . Artist's concept of the Voyager spacecraft showing its large precision antenna and instrument booms and antennas

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Table 1. Science Experiments and Team LeadersExper iment Science Team Leader

Imaging Science(2 TV cameras)Infrared Interferometry

Spectrometer andRadiometer

Ultravio let Spectrometer

PhotopolarimeterPlasma (dual plasma

detectors)

Low-Energy ChargedParticles

Cosmic RaysMagnetometerPlanetary RadioAstronomyPlasma WavesRadio Science

Bradford Smith, University ofArizona, Tucson, Arizona

Rudolf Hanel, Goddard SpaceFlight Center, Maryland

A. Lyle Bradfoot, Kitt PeakNational Observatory,Arizona

Charles Lillie, University ofColorado, Colorado

Herbert Bridge, MassachusettsInstitute of Technology,Massachusetts

S. M. Krimigis, Johns HopkinsApplied Physics Laboratory,MarylandR. E. Vogt, California Instituteof Technology, CaliforniaNorman Ness, Goddard Space

Flight Center, MarylandJames Warwick, University ofColorado, ColoradoFrederick L. Scarf, TRW Sys-

rems Group, CaliforniaVon R . Eshleman, StanfordUniversity, California

spacecraft. At Jupiter, sunlight is only 1/25th as brightas it is at the Earth. At Uranus, it is a mere 1/350th asbright as Earths sunlight. So, instead of solar cells,Voyager spacecraft rely upon radioisotope thermo-electric generators, which provide electricity throughthe conversion of heat from the radioactive decay ofplutonium.

The generators are carried on a spar-like boomextended from the spacecraft to prevent their radiationfrom affecting the science instruments.They develop atotal of about 400 watts at the time the spacecraftreaches Saturn. Radio communication with Earth con-sumes 100 watts of this available power. Scienceinstruments consume 108 watts, and the remainder isavailable for the other needs of the spacecraft.

The antenna of the new spacecraft differs from thatof the earlier Mariner spacecraft. To return informationwith a given amount of transmitter power over theimmense distances to the outer planets, a much largerantenna has to be carried than for exploring planetssuch as Mars, Venus, and Mercury. Voyagers dish-shaped antenna is 3.7 m (12 ft) in diameter. It isconstructed very accurately to beam the high-frequency radio waves without scattering them toomuch.

Coupled with an improved spacecraft transmitterand the large receiving antennas of the Deep Space

Network on the ground, Voyager can send informatto Earth at a rate of 115,000 bits per second froJupiter, and 44,000 bits per second from Saturn. (contrast, no matter how fast you talk, you can onsend information over your telephone at a rate of 1bits per second.) The large, high-gain antenna Voyager is pointed toward Earth by use of electroeyes on the Spacecraft. There is also a low-gantenna, mounted in front of the high-gain antenna,that there can be some radio contact even if for somreason the large antenna cannot be pointed directlyEarth.

The spacecraft can transmit to Earth at two rafrequencies. During cruise between the planets tlower frequency-known as S-band-is used to sedata to Earth at a relatively low rate. This is adequafor cruise science and releases the big 64-m ( 2 1diameter antennas of the Deep Space Network other tasks. Information can be received on the smler, 26-m (854) diameter ground antennas. For ecounters with the planets, when very large amountsdata must be transmitted quickly, a higher frequen(X-band) is used.

The X-band transmitters power output is 21 or watts. The S-band transmitters power output is 28, 2or 10 watts. Both transmitters are duplicated, in castransmitter should fail during the long mission.

The new spacecraft is, like the earlier Mariners, baround a group of compartments that house the eletronics. On top is the big antenna. In the center of tcompartments is a large spherical tank containrocket propellant. Unlike earlier spacecraft, the neVoyager does not use a single main rocket enginecorrect its trajectory through space; instead, it useshydrazine rocket propellant in 16 small thrusters maneuvers and attitude control. The spacecraft can positioned by reference to the Sun and the star Canpus, or by its own internal system of gyroscopes (calan inertial reference unit) using the thrusters. For somcorrections to its path through space, Voyager mufire these small rocket thrusters for as long as ohour.

To add the final velocity required at launch to escaEarth and attain a trajectory to take it to Jupiter, easpacecraft has a solid rocket system weighing abo1210 kg (2668 Ib) capable of a thrust of 71,2newtons (16,000 Ib). The rocket system is droppfrom the spacecraft after it has been fired.

The mission module, which is the planetary spacraft, weighs about 810 kg (1786 Ib), of which abo105 kg (231 Ib) consists of science instruments.

Much of the electronics of the spacecraft is ducated. This duplication will allow the spacecraftswitch-in alternative electronics in case of damage high-energy, charged-particle radiation when closethe giant planets or in case of equipment failure durthe long flight time required to explore the outer SoSystem.


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COSMICR A Y7 1PLASMA

HIGH-GAINULTRAVIOLETSPECTROMETE

HIGH-FIELDMAGNETOMETERINFRARED SPECTROMETEAND RADIOMETER

PHOTOPOLARIMETERMAGNETOMETER LOW-ENERGYCHAR GED PARTICLE

HYDRAZINE AC/TCM THRUSTERS (16)OPTICAL CALIBRATION

RADIOISOTOPETHERMOELECTGENERATOR (3)

CANOPUS STAR TRACKER(NOT VISIBLE IN THIS VIEW (2)

PLANETARY RADIO ASTRONOMYANDPLASMA WAVE ANTENNA (2 )

Figure 2. Drawingof the Voyager spacecraft to identify the major components and the science instruments.

The spacecraft has been designed so that thescientific measurements will not be affected greatly bythe magnetic field of the spacecraft itself.

Voyager ScienceOriginally the mission was concerned primarily withthe two giant planets Jupiter and Saturn. However,increasing interest in the satellites of these planets ledto their being included in the missions objectives.There are, indeed, five planet-sized satellites that willbe inspected closely during the flight. They are alllarger than Earths Moon, and one of them, Titan, hasan atmosphere whose density is comparable to that ofthe Earths atmosphere.The science instruments are quite varied. Camerasphotograph the planets and satellites to a detail neverpossible before. Other instruments investigate the un-usual environments of these large planetary systems,which are like miniature solar systems. Many of thescience instruments are identified in Figure 2.

The cameras and spectrometers are mounted on movable platform at the end of a science boom (seFigure 2) so that the instruments can look around thlarge high-gain antenna.A narrow-angle camera system of 1500 mm (59 infocal length acts like a telephoto lens to show smaareas of the planets and satellites in very great detail.completely new wide-angle camera system was designed for the mission. It has a 200-mm (7.87-in.) focalength telescope which covers a greater area but iless detail. Both camera systems have eight filters

some of which can be used to produce color pictures othe planets and their satellites.

One spectrometer looks at the planets in ultraviolelight. There are also an infrared spectrometer, a radometer, and a photopolarimeter. On the same boombut not on the movable platform, there are sciencinstruments for measuring planetary and interplanetarparticles of various energies.4
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Another boom, extending 13 m (42.65 ft) from thespacecraft, carries devices to measure the magneticfields of the planets.Two long antennas project from the spacecraft todetect radio waves emitted from the planets andplasma waves in the extremely rarefied gases in thespace between the planets.

The ExperimentsThe infrared spectrometer measures temperaturesat various depths in the atmospheres and gives infor-mation about the gases in the atmospheres of theplanets and their satellites.The ultraviolet spectrometer also provides informa-tion about the gases in the atmospheres of the planetsand satellites. It is particularly useful in searching forhydrogen and helium.The photopolarimeter provides information aboutaerosols in planetary atmospheres and about thecharacteristics of the satellite surfaces.To provide information on the types of charged

particles and their direction of flow-both in interplane-tary space and in the vicinity of the planets andsatellites-several types of detectors are used. Forexample, high-energy particles expected in interplane-tary space are detected by a cosmic ray telescope.

These measurements of charged particles and mag-netic fields can help our understanding of the basicphysics that permits electrons to be accelerated to highvelocities by Jupiter. Information about the compositionof the radiation belts of Jupiter and of Saturn may allowscientists to deduce how these belts are structured.Also, from the magnetic moments of the planets, theinternal structure of the planets may be inferred.

The satellites of Jupiter, and probably those ofSaturn also, offer obstacles to charged particles thatrotate with the planet, as the inner planets of the SolarSystem offer obstacles to the solar wind. The way inwhich the satellites affect the charged particles sur-rounding the planets is investigated during the mission.Close approaches of the spacecraft to lo , Ganymede,and Callisto allow the instruments to search for wakesin the ocean of charged particles like those behindships at sea, and for interactions of the particles withJupiters magnetosphere-that space around the plan-et where its magnetic field predominates. These sci-ence measurements can throw light on how quickly thesatellites sweep up charged particles from the mag-netosphere of Jupiter and how quickly the wakes of thesatellites are smoothed by charged particles movinginward toward Jupiter from outer regions of the planetsmagnetosphere.

The 10-m (32.8-ft) long whip antennas of the plane-tary radio astronomy experiment are used to detectradio waves from the planets. Radio science investiga-

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tions also make use of the spacecrafts radio tranmissions to Earth to observe the effects as the spaccraft passes behind the planets, the satellites, and trings of Saturn. These observations provide informtion about the size of the planets and the satellitetheir atmospheres, the composition of the rings, athe sizes of particles making up the rings. Also, tradio signals are used to measure the gravity and thmass of each planet and satellite and to determine veaccurately their position in space and their orbimotions.

At Saturn the spacecraft should confirm the preence of a magnetic field and determine whether thplanet has a magnetosphere. Some recent expements with an Earth satellite suggested that Satudoes possess a magnetic field, but it is difficult to bsure from Earth-based observations. Voyager can loofor evidence of the solar wind hitting a magnetosphe(known as a bow shock) and the way in which any sumagnetosphere is affected by the rings and the satelites of Saturn. The experiments can also find outSaturns magnetic field is tilted relative to the spin axof the planet, as are Jupiters and Earths fields relatito their axes of rotation.The presence of Saturns rings is expected to hamajor effects upon trapped charged particles if thplanet does have radiation belts, especially if thmagnetic field is tilted to the spin axis of the planet.

The huge satellite of Saturn, Titan, is a fascinatinworld. Revolving around Saturn at a mean distance1,222,600 km (760,000 mi), Titan is larger than Mercry, and it has a substantial atmosphere. Gases lofrom Titans atmosphere into space might be expectto form a doughnut (torus) around Saturn, which coube detected by the spacecrafts instruments. If thtorus is outside Saturns magnetosphere and in thsolar wind, it may give rise to a detectable bow shocThis would provide a unique observation of the inteaction of the solar wind with a gas cloud.

Even if Uranus and Neptune have substantial manetic fields, it is unlikely that these can be detectfrom Earth. But Voyager should easily detect them, even if the magnetic fields are extremely weak they aexpected to give rise to very extensive magnespheres at these planets.The other major group of experiments beyond parcles and fields is concerned with the use of thinstruments on the camera platform. The atmosphereof all four planets and many of their satellites a

investigated by these instruments which, together wiradio science, can considerably increase our undestanding of the gaseous outer planets.Planetary atmospheres are important because thetell the story of the way in which the planets might haformed and how they evolved. Knowledge of thpresent states and compositions of planetary atmspheres is vital to an understanding of how the Sol


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System originated and became what it is today. Anytheory about the formation and evolution of the SolarSystem must account for the different atmospheres ofthe planets.

Scientists need to know such things as the tempera-ture, pressure, density, and gaseous and particulatecompositions of the atmosphere. It has been found thatheat from inside Jupiter plays a major role in thecirculation of that planets atmosphere. The same maybe true of Saturn and the more distant planets. Voy-agers experiments are expected to tell us what isreally taking place in the atmospheres of the outerplanets. In addition, the mission looks at the atmo-spheres of satellites, which may provide importantclues about how these bodies evolved.

Observations made for several weeks while ap-proaching each of the planets provide motion picturesof the swiftly rotating atmospheres and their clouds.This is especially important for Saturn, Uranus, andNeptune, since even coarse details of cloud patternsare impossible to see from Earth with the best oftelescopes.

Water ice has been detected in the rings of Saturn byradar observations from Earth, but it is believed thatother substances are also present; perhaps silicatesand ammonia ices. These can all be checked byVoyagers instruments during the Saturn encounters.Also, by looking at how sunlight is scattered by therings, scientists hope to determine the sizes of parti-cles in the rings. Radio waves passing from thespacecraft to Earth through the rings can help todetermine the size of particles. Whether or not therings are surrounded by an invisible atmosphere of gascan be checked. If one of the spacecraft is sent toUranus, it will be able to obtain information about therecently discovered rings of that planet.

Far-encounter pictures of 12 satellites using the tele-photo camera system are expectedto reveal details ontheir surfaces only 5 to 15 km (3 to 9 mi) across. Thespacecraft approaches close enough to three of thesatellites to reveal details as small as 1000 m (3280 ft).The cameras look for geological details of thesurfaces: craters, plains, scarps, mountains, and polarcaps. The wide-angle pictures may reveal global distri-bution of geological areas and perhaps show why thereare variations of color and albedo on the satellites.Sizes and shapes can be measured to between 0.1and 1 percent.

Mission ProfileThe trajectories used for the Voyager mission takeadvantage of the outer-planet alignment in the year1977, which is most favorable for launching a space-craft via Jupiter to Saturn with a relatively short flighttime between the two giant planets. Each spacecraft isto be launched by a Titadcentaur rocket from Ken-nedy Space Center, Florida, during a 30-day periodbeginning August 20, 1977. The first spacecraft to be

launched needs more energy and takes longer treach Jupiter. It arrives there on July 9, 1979. Thsecond spacecraft, which is launched later in thlaunch period, follows a more opportune path anarrives at Jupiter before the first spacecraft, namely oMarch 5, 1979.The first spacecraft to arrive at Jupiter is calle

Voyager 1, and it is targeted to fly by Jupiter in such way that it can proceed to Saturn and make a closencounter with Titan. The trajectory for Voyager 1 called JST (Jupiter, Saturn, Titan). If this first spaccraft is successful in its Titan encounter, the seconspacecraft to arrive at Jupiter, Voyager 2, is targetedfly by Saturn in a way that lets it continue on to thplanet Uranus and perhaps even to Neptune. Thtrajectory for Voyager 2 is called JSX (Jupiter, Saturwith option).

A spacecraft cannot be targeted to fly close to Titain its encounter with Saturn and also fly to Uranubecause the orbits of Titan arid Uranus are in differeplanes.Arrival at Saturn is scheduled for November 12, 198

for Voyager 1, and August 27,1981, for Voyager 2. If thflight continues to Uranus, Voyager 2 arrives there oJanuary 30, 1986. It may also be possible to reacNeptune about 1990. Thus Voyager 2 may be thnations longest space mission: 12 years flying througinterplanetary space to cover a distance of 30 astrnomical units (4.5 billion km or 2.8 billion mi).On the approach to Jupiter, the cameras start photgraphing the planet about 80 days in advance of thclosest approach, Le., in December 1978. The spaccraft also look for hydrogen clouds surrounding thplanet and in the orbits of the satellites. The pictureshow more detail than any obtained by Earth-basphotographs, and at 10 days before closest approacthey are better than the best obtained by the Pionespacecraft in 1973 and 1974. About 8 days befoclosest approach the entire planet is surveyed by thwide-angle camera, while the narrow-angle (telephocamera concentrates on detailed pictures of specifeatures of the turbulent atmosphere of Jupiter.At Jupiter, Voyager 1 flies by within 4.9 Jupiter ra(about 350,000 km or 217,500 mi) from the center of thplanet at 12:49 GMT on March 5, 1979 (see Figure 3The spacecraft passes within 415,000 km (258,000 mof Amalthea, Jupiters small innermost satellite, anwithin 22,000 km (13,670 mi) of lo, the innermost of thbig Galilean satellites.* The spacecraft flies almo

along los orbit below the satellite for about 5 houthereby providing good views of the south polar rgions. Shortly afterward the spacecraft is occulted bthe bulk of Jupiterfrom the Earth and from the Sun. Aftemerging from behind the planet, Voyager 1 thepasses within 733,000 km (455,500 mi) of Europ

The four largest satellites of Jupiter (lo, Europa, Ganymede, anCallisto) are called Galilean satellites in honor of Galileo, whdiscovered them in the early 1600s.

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makes a close pass by Ganymede at 115,000 km(71,500 mi), and by Callisto at about the same distance.Afterwards, the spacecraft continues to observeJupiter for about another month until just before thenext spacecraft to arrive, Voyager 2, starts its observa-tions of the planet.The encounter of Voyager 2 is somewhat different, inthat the spacecraft must fly by further away fromJupiter to preserve the option to fly to Uranus. Voyager

2 starts photographing Jupiter in April 1979, about 80days ahead of the closest approach. This is at 10Jupiter radii (714,000 km or 443,700 mi) at 11:00 p.m.July 9, 1979 (see Figure 4). Voyager 2 passes Callistoat a distance of 220,000 km (136,700 mi), then makesGanymede, followed by a passage within 200,000 km(124,300 mi) of Europa. Through the use of the twospacecraft, Ganymede and Callisto are seen beforeand after closest approach so that both hemispheres ofthe satellites are observed and photographed. Amal-thea is passed at a distance of 550,000 km (342,000mi), but since this satellite is so close to Jupiter as to beextremely difficult to observe from Earth, the Voyagerpictures are important even though the approaches arenot very close. On its way out from its closest approachto Jupiter, Voyager 2 passes through the occultationzones for both Earth and Sun.

At Saturn, Voyager 1 first makes a close approach of7000 km (4350 mi) to Titan. Sixteen hours later itmakes its closest approach to Saturn (see Figure 5). At1:00 a.m. GMT on November 13, 1980, Voyager 1 is

a very close approach of 55,000 km (34,200 mi) to

L A U N C H D A T E 9 1 7 7J U P IT E R A R R I V A L D A T E 3 5 79 V I E W N O R M A L TOA M A L T H E A 415.000 hili l 2 5 8 . 0 0 0 i i 7 i lE U R O P A 733 000 k in 1455 500 1110 S H O W N A T S P A C E C R A F T ' SG A N Y M E D E 1 1 5 0 0 0 k in 17 1 5 0 O i i i i l C L O S E S T A P P R O A C H TOC A L L I S T O 115000 k in 17 1 5001nO J U P I T E R

J U P I T E R S E O U A T O RIO 22000k in 113,670 11111 S A T E L L I T E P O S IT I O NS

Figure 3. Diagram looking down on the north pole of Jupitershowing the path of Voyager 1 (JST trajectory) past the planet.

Figure 4. The path of Voyager 2 (JSX trajectory) durinencounter with Jupiter.

3.3 Saturn radii (197,300 km or 122,600 mi) fromcenter of the planet and has a spectacular view ofsouth polar regions and the open ring system. Thenspacecraft passes within 96,000 km (60,000 mMimas, 230,000 km (143,000 mi) of Encela140,000 km (87,000 mi) of Dione, and 60,000(37,300 mi) of Rhea. This is the first time thsatellites are seen as worlds rather than fuzzy poinlight. The spacecraft passes behind the ringsviewed from Earth and through their shadow, and passes behind the planet and through its shadow Figure 6) . Voyager 2, if the Uranus option is chofirst encounters Titan, but at a distance of 353,000(219,000 mi) below the spacecraft. An approacwithin 254,000 km (158,000 mi) of Rhea is followeone of 159,000 km (98,800 mi) of Tethys. As spacecraft swings round the planet (see Figure passes within 94,000 km (58,400 mi) of Encela33,000 km (20,500 mi) of Mimas, and 196,000(121,800 mi) of Dione. Voyager 2 then passes Earth and Sun occultations by the planet and proceout from Saturn, but it continues to look back observe the planet until the end of September, 198is then on its way to Uranus for a rendezvous 4112 yelater (see Figure 8 ) .

Both Saturn flybys are outside the ring systemthey provide good closeup views of the rings. Voy2 does not see the rings as well as does VoyagWhen Voyager 2 passes within 2.7 planetary rad1:OO p.m. GMT on August 27, 1981, it is viewingdark side of the rings.


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V I E W N O R M A L T OL A UN C H U A l t 0 1 / IS A I U R N A R R I V A L I ~ A ~ E1 1 17811 S A T U R N ' S E Q U A T O R

Figure 5. At Saturn, Voyager 1 makes a close approach toTitan, then flies by Saturn at a distance of only3.3 Saturn radiifrom the center of the planet.

Figure 6. As seen from the Earth, the spacecraft passesbehind Saturn and its rings at the times following closestapproach shown in the drawing. Alongside is the view of therings obtained 20 minutes before closest approach.

As Voyager 2 approaches Uranus, equipment withinthe spacecraft is prepared for the new encounter,which is quite different from the encounters with Jupiterand Saturn. This capability to change operation ofequipment within the spacecraft many millions of milesfrom Earth is a powerful new technique for exploringseveral planets by one mission. This technique usescomputers on board the spacecraft, the operatinginstructions for which can be changed by commandssent to it from Earth. Spacecraft operations can then be

L A U N C H U A T E E 20 7 7S A r U R N A R R IV A L D A T t 8 77 81 V I E < . ' I O H M A i 10i A T U R j S OL,AT(JHS A T E L I I T E P O S IT I O NSSHOb%r l AT SPACECRAFT SC L O S E S T A P P R O A C t I T OS A T U R l l

Figure 7. If the Uranus option is chosen, Voyager 2 will passby Saturn along the JS X trajectory shown in this drawing.

changed to fit the special needs of the different plane-tary encounters. This capability also permits controllersat the Mission Control Center to work around equip-ment failures that might occur because of radiationclose to Jupiter and Saturn or because of the longperiod of operation of the spacecraft in space.Approaching Uranus, Voyager 2 is prepared for thisstrange planetary system in which the planet's axisof rotation is in the plane of its orbit around the Sun.The satellites orbit Uranus in its equatorial plane.The orbits of the satellites face the oncoming Voyagerso that the Uranian system looks like a target with theplanet at the bull's-eye. The flyby could take placewhen all the satellites are on one side of the planet, butno more than one can be close to the spacecraft if thespacecraft is to approach close to the planet. Detailsare worked out during the long voyage to Uranus. Al lthe satellites are, however, photographed clearlyenough to measure their sizes and to see surfacedetail-near impossibilities from Earth. The same con-

dition applies to Neptune and its satellite system.Thus, by 1990, the planets and satellites of the outerSolar System may be known in detail comparable toour current knowledge about the innermost planets-Mercury, Venus, and Mars (before Viking)-and thiswealth of information will have been gathered by twospacecraft taking advantage of a unique configurationof the planets in their orbits, a configuration that will notbe repeated for many human generations.

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Figure 8.with Uranus 452 years later in January 1986.Following the encounter with Saturn as shown here. Voyager 2 will speed through the outer solar system to a rendezvous

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STUDENT INVOLVEMENTProject One: Satellite Encounter

Make an enlarged sketch from Figure 3 showingJupiter and its shadow, the path of the spacecraft from40 hours before to 8 hours after closest approach (thetick marks on the spacecrafts path shown in Figure 3are 2 hours apart), and the orbit of Ganymede. On theorbit of Ganymede draw the dot which represents theposition of the satellite relative to the spacecraft at thetime of its closest approach to the spacecraft. Placeanother dot on the path of the spacecraftto indicate itsposition at this time. This should coincide approxi-mately with the eighth tick mark before closestapproach.

Working backward and forward along the orbit ofGanymede, place tick marks to show the position of thesatellite every two hours for the 24 hours before andafter the closest approach to the spacecraft. Gany-mede is assumed to be in a circular orbit at 1,071,000km from the center of Jupiter. Its period in this orbit is7.1546 days. Calculate the number of degrees ittravels along its orbit in 2 hours, and use a protractor tomake the tick marks.

Draw lines connecting the position of the spacecraftwith the position of the satellite at each of the 2-hourlyconfigurations. Measure the distances along theselines and convert to distances in kilometers from yourknowledge of the radius of Ganymedes orbit. Make atable of distances for the 24 hours before and afterclosest encounter with the satellite. Plot these as agraph. What is the distance at 6 hours 40 minutesbefore and after encounter? If the time of closestapproach for Voyager 1 is 12:49 GMT on March 51979, what is this in terms of your local time? Calculatein your local time the time of closest approach ofVoyager 1 to Ganymede.

The diameter of Ganymede is approximately 5280km. From the data you have in the table of distancesand times, make a series of 24 drawings of the disc ofGanymede showing its relative size as seen from thespacecraft each 2 hours before and after closestapproach. Scale your drawings so that the one for theclosest approach is 5.28 cm in diameter. What is therelative size of the disc of Ganymede 24 hours beforeand 24 hours after closest approach? Calculate theangle subtended by the satellite at the spacecraft foreach of these 2-hourly positions. If the high-resolutioncamera has a resolution of 4 seconds of arc (1 milli-degree), calculate the size in meters of the smallestobject that can be recognized on the satellite 24 hoursbefore closest approach, at closest approach, and 24hours after closest approach. Assume that the smallestobject recognizable is the same as the resolution of thecamera system.

Again making use of the illustration of Figure 3,which shows the direction to the Sun, calculate thephase of Ganymede as seen from the spacecraft at

each of the 2-hourly positions. Draw the position of tterminator (the boundary between day and night) the discs of Ganymede you have already drawn, as tterminator would appear from the spacecraft. Darkthe night side of the satellite.Assume that on the first of your sketches, i.e., tdisc at 24 hours before closest approach, there is

large impact basin in the exact center of the satelliteseen from the Spacecraft. This basin is 300 kmdiameter. Draw i t on this first sketch of the satellite seen from Voyager 1. Assuming that Ganymede (i.e., turns one hemisphere always towards Jupiter the Moon does to Earth), show in your series sketches of the disc of Ganymede the apparent movment of the big impact basin on the disc as viewed frothe spacecraft. Does the basin disappear from viearound the limb of the satellite? If so, when?

tates synchronously with its revolution about Jupi

When would you expect to see most detail on tfloor of the big basin? Remember that as the banears the limb the details in its floor are foreshortenby the angle of view. At the time you can see the mdetail, what is the size in meters of the smallest objyou can see at the 4 seconds of arc resolution of thigh-resolution camera?

Project Two: Relative Sizes ofMagnetospheres

The Earths surface magnetic field of approximat0.3 gauss produces a magnetopause (boundary btween the magnetosphere and the solar wind) at approximate distance from Earths center of 10 Earadii. Jupiters magnetic field of 12 gauss producesmagnetopause at 53 radii from Jupiter, the distanvarying with solar wind activity. Make a large drawito scale to show the relative sizes of Earth and magnetopause and Jupiter and its magnetopause, shown in the sketchalongside. Scale thedisc of Jupiter (diame-ter of 142,900 km) as1.43 cm diameter.Alongside Jupiter, drawthe Earth and its mag-netopause. If Saturnhas a surface magneticfield of 1 gauss, its magnetopause may be recordedVoyager at a distance of 39 Saturn radii from the cenof the planet. Draw Saturn to scale, together with magnetopause, alongside Jupiter and Earth. (Tdiameter of Saturn is 119,600 km).

M A G N E T O P A U

PLANETcThen draw the Sun to the same scale. Its diamete1,426,000 km. Are the magnetospheres of Jupiter aSaturn comparable in size with the Sun itself? Hmuch bigger 01 smaller are they?

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GLOSSARYAlbedo: The ratio of the light reflected by a planet or asatellite to the sunlight falling upon it.Boom: A slender structure or a single pole extending froma spacecraft to locate equipment or science instru-ments away from the body of the spacecraft.Charged Particles: Ionized atoms (i.e., atoms that have lostone or more of their orbital electrons) and electronsmoving freely in space.Closest Approach: The position on the path of a spacecraftthat is closest to the planet the spacecraft is flying

by.Cosmic Ray Telescope: A device that detects high-energycharged particles passing through a system of de-tectors arranged so that the direction of the par-ticles can be determined and their energymeasured.Cruise: That part of an interplanetary flight where the space-craft is traveling between the Earth and the planetwhich it is to fly by or orbit.Data Bits: Informationcan be sent from one place to anotherin the form of a code, like the Morse code. Digitaldata is in a code of two bits, zero and one,equivalent to a lamp being either off or on. Therate at which these zeros and ones can be sentfrom a transmitter to a receiver is called the databit rate.Deep Space Network: A system of large antennas arrangedat stations around the world so that as the Earthturns on its axis, constant communication can bemaintained with a distant spacecraft. The antennasin Australia, Spain, and California pass informa-tion back and forth between spacecraft and thecontrol center at the Jet Propulsion Laboratory,Pasadena, California. Each of the three DeepSpace Network stations has a 64-m antenna andsmaller antennas.Encounter: That period in a space mission when a space-craft is actively gathering scientific informationabout the planet it is flying by or orbiting.Focal Length: The distance from a lens or mirror at whichthe image af a very distant object is focused; longfocal lengths produce larger images than do shorterfocal lengths, but the images are of reduced inten-sity. Long-focal-lengthtelescopes are often referredto as telephoto, narrow-angle, or high-resolutionsystems; short focal lengths are used in wide-angleor low-resolution systems.

Ionosphere: Upper region of a planetary atmosphere, inwhich atoms and molecules become electricallycharged by incoming radiation from the Sun.Launch Window: A period of time during which, because ofthe relative positions of the planets and the Earthand the rotation of Earth on its axis, a space-craft can be launched to reach a given planet witha certain class of launch vehicle. For Voyager, thelaunch window is about one hour each day during alaunch period from August 20 to September 20,1977. The beginning of the window varies betweenabout 9:00 a.m. and 10:30 a.m., EDT.

Magnetic Moment: A measure of the magnetizing force produced by a magnetized body such as a planet.Magnetosphere: The region surrounding a planet in whicthe magnetic field of the planet predominates ovethe magnetic field carried by the particles of thsolar wind. The transition between the solar winand the magnetosphere is a boundary known as thbow shock. The magnetosphere traps particlefrom the solar wind which are contained withiradiation belts.Occultation: The passage of a spacecraft behind a planet oa satellite so that the spacecraft is hidden fromthe observer.Photopolarimeter: An instrument that measures the lighintensity of a planetor satellite by using a polarizindevice.Radiation Belt: A region of trapped electrically chargeparticles, mainly protons (nucleiof hydrogen atomsand electrons, in the magnetosphere of a planet.Radiometer: A device to measure heat (infrared) radiatiofrom a planet or a satellite.S-band and X-band: Two bands or sections of the radifrequency spectrum allocated internationally fospace communications. S-band is from 2290 t2300 MHz and X-band is from 8400 to 8500 MHzSilicate: Rock containing large amounts of silicon, such aquartz, pyroxene, and feldspar.Solar Wind: The flow of electrons and protons streaminoutward from the Sun throughout the Solar SystemSpectrometer: A device that measures the different frequencies of radiation from a body and their relativintensities.Spin Axis: The axis of rotation of a planet passing througthe north and south poles of the planet.Whip Antenna: A long, wire-like, flexible antenna supporteat one end only.Work Around: A way of overcoming a failure within a spacecraft so that the mission of the spacecraft can stbe accomplished.

Suggested ReadingPIONEER ODYSSEY, R. 0. Fimmel, W. SwindeTHE ATMOSPHERE OF TITAN, D. H. Hunten (edTHE RINGS OF SATURN, F. D. Palluconi and G. HPettengill (eds.), NASA SP-343, 1974.JUPITER, THE GIANT PLANET, T. Gehrels (ed.University of Arizona Press, Tucson, 197THE OUTER PLANETS, D. M. Hunten, ScientifAmerican, v. 233, September 1975, pTHE SMALLER BODIES OF THE SOLAR SYSTEMW. K. Hartmann, Scientif ic American, v. 23September 1975, pp. 143- 159.

E. Burgess, NASA SP-396, 1977.NASA SP-340, 1974.

130-141.

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