Voyager to Jupiter and Saturn

8/8/2019 Voyager to Jupiter and Saturn

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NASA SP-420

AND SATURN

Scientific and Technical Informatzon Ofice 1977NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington , D.C.


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For sale by the N ational Technical Information ServiceSpringfield, Virginia 22 15 1


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Foreword

THISublication briefly describes the National Aeronautics andSpace Administration's Voyager mission to explore the giantplanets of the out er solar system-Jupiter, Saturn, and possiblyUranus. Ou r Pioneer 10 and 1 1 missions to Jupit er have alreadygiven us a brief glimpse of the majesty of that giant planet and itssatellites. Based on that reconnaissance, the two Voyager space-craft will now explore in mo re d ept h the characteristics of theJovian system and make the first concerted reconnaissance ofSaturn, its satellites and mysterious rings. If all goes well, we mayget our first close look at Uranus almost eight years from now,extending man's presence n earer t o th e edge of ou r solar system.Jus t as Voyager is building upon results from Pioneers 10 and 1 1,these missions in turn will pave the w ay f or orbital and atmospheric-probe explorat ions in the 1980s.Voyager is an impo rtant increm ental and sequential step in man-kind's quest for knowledge about himself and his place in the uni-verse. By comp aring the o ute r planetary systems with each oth er,and with the terrestrial planets Earth (and its Moon), Mars, Venus,and Mercury, we will better understand how the solar system wasformed , how it evolves, how life originated, and how t he planetaryenvironments are affected by the S un.

NOEL W. HINNERSAssociate A dministratorfor Space ScienceJune 21,19 77


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ContentsPage

THE VOYAGER MISSION ....................................................... 1THE VOYAGER SPACECRAFT AND ITS INVESTIGA-.....................................................................................IONS 15

The Mission Module ............................................................ 15.............................................maging Science Investigation 22Infrared Radiation Investigation ......................................... 26...........................................hotop olarim etry Investigation 29Ultraviolet Sp ectroscopy Investigation ................................ 31Radio Science Investigation 3 4 .................................................Cosmic Ray Particles Investigation ...................................... 3 7Low-Energy Charged Particle Investigation ......................... 39Plasma Particles Investigation ........................................ 42Magnetic Fields Investigation ........................................ 4 5...................................................lasma Wave Investigation 47Planetary Radio Astronomy Investigation ........................... 50............................ppendix A-VOYAGER SCIENCE TEAMS 53

Appendix B-VOYAGER MANAGEMENT TEAM .................... 5 7


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VOYAGER TRAJECTORIES


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The Voyager MissionWHENhe first of two Voyager spacecraft approaches Saturnin November 1980 , it w ill already be an experienced interplanetaryexplorer. An extrem ely busy en counter w ith Jupiter and its satellitesystem will be behind it, and more than three years of cruiseobservations. A twin spacecraft will have reported its own observa-

tions of the Jupiter system four months after the first. Thescientists of the eleven Voyager investigating teams will have begunanalyzing the mass of data acquired during the Jupiter encounters,an d will be braced for the coming flood of observations fromSaturn. A preliminary peek at Saturn fro m the Pioneer 11 flyby inSeptember 1979 will have further whetted their anticipation and,perhaps, suggested last-minute changes in encounter plans. It will bean exciting time.The two Voyagers will have been launched from Cape Canaveralyears before, in August and September of 1977. Each will havetaken advantage of the moving Jupiter gravity field to get a free rideto Saturn. One Voyager will pass by Jupiter at a distance of13 0 000 km (nearly five times th e planet's radius) on March 5,1979, and go on to pass by Saturn at about three Saturn radii onNovember 13, 1 980. The other Voyager will travel at a moreleisurely pace. Its encounter with Jupiter on July 9, 1979, at adistance of ten Jupiter radii will boost it into a trajectory that willreach Saturn in August 1981. That's not all. The circumstances ofits encounter with Saturn are flexible enough to maintain theoption of a second gravity-assisted boost that would bring it toUranus early in 198 6. Theoretically, we might even repeat the trickat Uranus and send th e spacecraft on t o a 1 989 rendezvous withNeptune. Table 1 gives comparative data for Jupiter and S aturn andthe respective satellites that will be investigated by the Voyagers.

The emphasis of the Voyager missions is on comparative studiesof th e Ju piter and Sa turn planetary systems. Each spacecraft will useidentical sets of instruments to observe several satellites and theparent planet in each system: their b ody and surface characteristics,


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2 VOYAGERTable 1 -Comparative Data for Jovian and Saturnian Satellites

their atmospheres, and environmen ts. The journey from E arth toSaturn provides the opportunity to explore the interplanetarymedium at distances from 1 AU (the mean radius of the Earth'sorbit) to 10 AU. A fter leaving Saturn , the two spacecraft are likelyto continue sending data from distances as mu ch as 20 AU or mo re.Successive planetary encou nters by two identically instrumentedspacecraft make it possible to examine some of the bodies underdifferent illumination and observation angles, and to study changes

Planet

Earth ..........Jupiter ........

Saturn .... . .

Satellite

-Moon-ArnalthealoEuropaGanymedeCallistoLedaHimaliaLysitheaElaraAnankeCarmePasiphaeSinope-JanusMimasEnceladusTethysDioneRheaTitanHyperionIapetusPhoebe

Diameter,km

12 7563476142 80024036403050527050002 to 141706 to 32806 t o 2 88 to 408 to 466 to 36120 000?40 05501200115014505800160 to 920180060 to 320

DistancefromSun,1 0 6 k m149.6-778.4-------------

1424.6----------

Distancefromplanet,1 0 3 k m-

384.4-181.3421.6670.91 0 7 0

1 8801 1 1 1 01 1 4 7 011 71011 74020 70022 35023 30023 700-168.7185.8238.3294.9377.9527.61 222.61484.13 562.912 960

Period oforbit

1 yr27.32 days11.86 yr.49 day1.77 days3.55 days7.16 days16.69 days240days251days260 days260 days617 days692 days735 days758days29.46 yr.82 day.94 day1.37 days

1.89 days2.74 days4.52 days15.95 days21.28 days79.33 days550.45 days


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MISSION 3in their atmospheres. During most of their flights both spacecraftwill be located at different distances from the Sun. Comparing thearrival times of solar-wind disturbances will let us calculate thespeed at which these events travel through interplanetary space.Perhaps the most exciting objective of the missions is to learnmore about those mysterious rings that have made Saturn such anobject of wonder and curiosity for centuries.Launch and Near -Earth Phases

The Voyagers will start their journeys from Launch Complex 41at Cape Canaveral. The launch vehicle is the Titan IIIEICentaur (fig.1). Since it can't supply enough energy by itself to send the space-craft off to Jupiter, the extra increment of energy is supplied by asolid rocket which, although it functions as a fifth launch vehiclestage, actually constitutes the spacecraft's Propulsion Module.The Voyager spacecraft will leave Earth orbit at a higher velocitythan any previous spacecraft. It will pass the Moon's orbit in 10hours; it took Apollo astronauts (though moving many times fasterthan a rifle bullet) three days to get that far. The scientific instru-ments aboard the spacecraft will conduct observations of Earth andMoon for a few days, to check out and calibrate the instruments,and to provide a base line for the Jupiter and Saturn encounters.

Figure 1 -Titan IIIEICentaur launch vehicle with Voyager spacecraft.


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4 VOYAGEREarth-Jupiter Cruise Phase

About 10 days after launch, the spacecraft and the missionoperators and scientific investigators back on Earth will settle downto cruise routine. Most of the time, only the experiments thatmeasure the fields and particles of interplanetary space will beoperating regularly. Periodically, the pace of activity will be steppedup to conduct maneuvers recording measurements in all directionsof the sky. The spacecraft will spin slowly about its Earth-pointingaxis, meanwhile stepping its Science Scan Platform so that theremote-sensing instruments can survey the entire celestial sphere.These science maneuvers will occur each time the distance from theSun increases by 0.5 AU, which will be about every other month.Several trajectory correction maneuvers during cruise will cancellaunch injection errors and refine the aiming point for Jupiter.The mission plan takes no particular notice of the asteroid beltthat lies between 2.2 and 3.5 AU, beyond Mars' orbit. Until a fewyears ago there were speculations that a heavy concentration ofsmall meteoroids in this belt might pose a hazard to spacecraft.However, the results of the micrometeoroid experiments on thePioneer 10 and 11 missions showed no concentration within thebelt, and the Voyagers are expected to fly serenely through.Jupiter Encounter Phase

Eighty days before the closest approach to Jupiter, the remotesensing instruments will begin to observe the planet from about 80million kilometers out. At that distance, with Jupiter coveringabout 1/10 degree in the field of the narrow-angle televisioncamera, the images should be superior to the full-disk images madethrough telescopes on Earth.Over the next seven weeks or so, the narrow-angle camera willrecord hundreds of images of Jupiter through all of its color filters,and the infrared and ultraviolet spectrometers and the photopolar-imeter will begin their whole-planet observations. In addition, thelatter two instruments will scan the orbits of the large satellites forevidence of clouds of ions. (Such clouds are believed to be spreadout along the orbit of one satellite, 10.)A month before closest approach, with Jupiter about 30 millionkilometers away, the spacecraft will begin transmitting steadily atthe encounter data rate. The three Deep Space Network receivingstations with 64-meter antennas will provide full-time coverage. The


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MISSION 5various fields and particles experiments will go into their encountermodes, to observe the transition from the region dominated by thesolar wjnd to Jupiter's magnetosphere.Just before Jupiter fills the field of view of the narrow-anglecamera, the imaging system will take what amounts to a time-lapsecolor movie, with an image every 9 minutes through 10 revolutionsof the planet. Somewhat later, the camera will photograph the starfield from known Scan Platform orientations to permit opticalnavigation. The final pre-encounter trajectory correction maneuver,10 days before closest approach, will be based on the opticalnavigation data. After that, the narrow-angle camera will followselected clouds and storm features of the Jovian atmosphere, andthe spectrometers will scan across the atmosphere to determine thecomposition of the cloud bands.The busiest period of the entire mission will be the 48 hourscentered on closest approach. For the earlier-arriving Voyagerspacecraft, this will occur on March 5, 1979. Figure 2 illustrates thegeometry of this first encounter, as projected onto Jupiter'sequatorial plane. The concentric circles in the drawing are the orbitsof the tiny innermost satellite, Arnalthea, and of the four very largesatellites first seen by Galileo: 10, Europa, Ganymede, and Callisto.The spacecraft's track, which is marked off in Zhour increments,enters at the lower right. Although it will cross the orbit of Callistoabout 30 hours before periapsis, it will not photograph the satelliteat that time. In fact, the closest encounter with all of the satellitesexcept Amalthea will take place on the outbound leg.On the inbound leg, the imaging system will be largely occupiedwith selected regions of Jupiter. For example, figure 3 shows theplanned high-resolution coverage of the atmosphere in the vicinityof the Great Red Spot during one 3-hour period. As the spacecraftswings around the rapidly rotating planet, features that were imagedduring the approach at very low phase angles (i.e., very nearly fromthe Sun's direction) will be imaged again at increasing phase anglesuntil the Sun sets on them.Returning to figure 1, we see that at periapsis (closest approach)the spacecraft will be 4.9 Jupiter radii (one radius at the equator is7 1 600 km) from the planet's center. The giant planet's gravita-tional pull will whip the Voyager around behind it, imparting a largeincrease in velocity.The moment when a spacecraft disappears behind a planetarybody is a dramatic one for people in space flight operations. If the


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6 VOYAGER

Launch date: September 1, 1977Jupiter arrival date: March 5, 1979Satellite Flyby distance (km

Amalthea 440 000lo 25 000Europa 750 000Ganyrnede 130 000Callisto 130 000

Figure 2.-Voyager 1 encounter at 4.9 Jupiter radii. (View is normal to equatorof Jupiter; position shown for each satellite is point of closest approach byVoyager .)

occulting body has no atmosphere, th e cu toff of the steady streamof radio signals from the spacecraft is as sudden and startling as athunderclap. Conversely, an Earth occultation by a planet likeJupiter, with a deep, dense atmosphere, provides an excellent


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MISSION 7

Figure 3.-Imaging frames of Jupiter's Red Spot to be obtained at closestapproach.

opportunity to study the structure of the atmosphere and ion-osphere through t he refraction of radio waves.Before Voyager enters Earth occultation, it will encounter thesatellite 10 at the very close range of 25 000 km. 10, the innerm ostof the Four Galilean satellites, is fascinating scientifically, with anumber of remarkable features including 10's extraordinarily highreflectivity, and the toroidal cloud marking 10's orbit. The space-craft will pass between Ju pit er and 10, crossing the magnetic linesof force that seem, at times, to form an electrical pathway betweenthe two bodies.Before the spacecraft emerges from Earth occultation, it willent er Jupiter's shad ow. This Sun occultation provides an oppo rtuni-


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8 VOYAGERty f or the ultraviolet spe ctrome ter experiment to measure thecomposition of the upper layers of Jupiter's atmosphere. Mean-while spacecraft cameras will take long exposures of the planet'snight side in search of very bright meteors, lightning, and auroraldischarges.The Earth and Sun occultations do not pose any particularoperational problems for the mission. Scientific data taken duringEarth occultation will be recorded for later transmission. Since thespacecraft depends on radioisotope generators rather than solarenergy for pow er, it will no t lose pow er during th e solar eclipse.During the next 24 hours, the spacecraft will make its closestapproach to the other three Galilean satellites: Europa, Ganymede,and Callisto. A lthough Europa and Ganymede are to be seen atmuch shorter range by the second Voyage1 spacecraft, the dif-ference in phase angle between the two sets of encounters makesall the satellite imagery scientifically valuable.The first Voyager will recede from Jupiter at about a millionkilometers a day as it coasts toward its rendezvous with Saturn. Itspassage thro ugh th e magnetosphere's extende d tail mak es itdesirable to operate the fields and particles experiments atencounter rates for another 40 days. During that time there will beanother trajectory correction maneuver, based on both optical andradio navigation.Second Jupiter Enco unter

The second Voyager will fly by Jupiter o n July 9, 1979 . Th ebeginning of its encounter phase activities in mid-April will comejust as its predecessor is settling down into the cruise mode. As faras scientific investigations are concerned, the Jupiter system will beunder nearly continuous intensive observation for an eight-monthperiod.Figure 4 illustrates the geometry of the second Jupiter en-counter. This tim e the closest approach will be 10 Jupiter radii,which is just outside the orbit of Europa. Callisto, Ganymede,Europa, and Amalthea will all be encountered on the inbound leg.Tlie Ganym ede encou nter will be at the close range of abou t 5 0 00 0krn, providing image resolution about as fine as the very besttelescope photographs of the Moon from Earth. Only 10 will be toofar to be observed a t all.Since the path of the second Voyager will not be as sharplycurved by Jupiter 's gravity, the E arth and Sun occultations will take


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MISSION

Launch date: August 20 , 1 977Jupiter arrival date: July 9, 19 79Satellite Flyby distance ( k m )

Call sto 240 00 0Ganyrnede 50 00 0Europa 190 00 0Amalthea 550 00 0

Figure 4.-Voyager 2 encounter at 10 Jupiter radii.

place at m uch greater distances from th e planet. This will perm itthe grazing radio waves (and then the grazing ultraviolet rays) toexplore new regions of the Jovian atmosp here.As this Voyager recedes from Jupiter it, too, will see a crescentplanet, and will perform remote sensing observations of features inthe vicinity of the sunrise terminator.


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10 VOYAGERJupiter-S aturn Cruise Phase

During the period from the summer of 197 9 to the autumn of1980 , both spacecraft will be operating in the cruise mode. As theyproceed in tu rn from Jupiter 's 5.2 AU orbit out to Saturn's 9.5 AUorbit, they will observe gradual changes in the character andtemperature of the solar wind, and probably in the cosmic rayenvironment. The sky survey science maneuvers will continue atintervals of about 0.5 AU. The planetary radio astronomy exper-iment will increasingly be in a position to receive any radioemissions from Saturn.As the Earth proceeds along its orbit, there will be an annualsolar conjunction for each spacecraft, during which radio signalswill have to traverse the solar corona. Although the conjunctionswill temp orarily interfe re with com mu nication s, they will provideopportunities for the measurement of coronal electron densities.First Saturn E ncounter

The Voyager tha t flew by Jupiter four m onths ahead of its twinwill have widened its lead to nine mo nth s by th e time it approachesSaturn . Toward th e end of August 1980 , the en coun ter phase willbegin with the imaging of the planet and its rings from a distance of96 million kilometers. The fields and particles experiments will goto encounter mode in mid-October, in order to determine theexte nt of th e magnetosphere. By late October, the rings will be to olarge for the narrow-angle camera, and will be imaged in segments.Because the data rate that must be used at Saturn's distance isconsiderably less than that from Jupiter, most images will have tobe recorded before transmission. (It will tak e abo ut 1% hou rs forradio signals t o reach E arth.)Figure 5 shows the geometry of the Saturn encounter. The daybefore closest a ppro ach, the Voyager will fly past th e giant satelliteTitan at a range of only 4000 km. The known facts and specu-lations abou t Titan's dense, hydrocarbon-laden atmosp here make itan exceptionally impo rtant object t o study. The flyby of Titan willinclude an occultation from the Earth and the Sun, permitting goodmeasurements of the atmosphere's compo sition and density.On November 12, 1980, the spacecraft will fly past Saturn'ssouthern hemisphere at a distance of 3 .3 Saturn radii (about200 000 km ) from th e center of the planet. Shortly afterward itwill be occulted from the Earth by the rings, and then by the


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12 VOYAGERkrn, and finally Rhea at 60 000 krn. Then for another month theinstruments will look back at the crescent planet and its tilted rings.Second Saturn Encounter and the Uranus Option

The second Voyager will reach Saturn in late August of 1981.The details of the encounter depend on a decision that will have tobe made during the preceding winter or spring-whether to go for aUranus encounter or not. Uranus and its system of five satellites(and recently discovered ring system) offer an extremely temptingtarget. The opportunity to explore that whole new world will notcome again for another decade. In fact, it is only at 42-yearintervals that the planet offers the fascinating sight of a polar axispointed at the Sun.The main consideration in the decision is whether the firstVoyager encounter with Saturn has accomplished its objectives-most particularly those concerned with Titan and the rings. The con-dition of the second spacecraft is another important consideration.Will it be capable of performing the necessary maneuvers and activi-ties for an exploration of Uranus in January 1986, more than eightyears after leaving Earth?If the decision is made to use Saturn's gravity field as a means ofreaching Uranus, the Saturn encounter will take place August 27,1981, at a distance of 2.7 Saturn radii. The path is only about38 000 km beyond the visible edge of the outermost ring. Thegeometry of the encounter is shown in figure 6.The Voyager, entering at the upper right of the drawing, willcross high above Titan's orbit about a day before periapsis. Theflyby distance of about 350 000 km will not permit anotherthorough exploration of Titan. Five other satellites will be exam-ined at closer range: Rhea, Tethys, Enceladus, Mimas (at only34 000 km), and Dione. The planet will again occult the Earth, pro-viding an opportunity to supplement the previous encounter'soccultation observations. When the Voyager leaves the vicinity ofSaturn, it will be very nearly in the ecliptic plane.A decision not to go to Uranus would be implemented early inthe summer of 1981 by a trajectory correction maneuver. Theencounter would be essentially like the other Voyager's trip

through the Saturnian system. The geometrical conditions at Saturnand Titan would be just about the same. Rhea would be seen at agreater distance, but the other four satellites would be closer.


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MISSION 13

Launch date: August 20,1977Saturn arrival date: August 27,1981Satellite Flyby distance ( k m

Titan 353 000Rhea 253 000Tethys 159 000Enceladus 93 000Mimas 34 000Dione 196 000

Figure 6.-Voyager 2 encounter at 2.7 Saturn radii.

With either choice, the second Voyager will have completed itsSaturn observations by the end of September. In their first fouryears of exploration, the Voyagers will have performed fourdetailed investigations of two planets and their systems. One of the


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14 VOYAGERsafest predictions is that our conception of the outer solar systemwill be substantially altered.Out There

If the slower of the two Voyagers is indeed headed for Uranus (at19 AU from the Sun), it will reach that planet in 1986. At thattime, the Sun will be nearly over the north pole of Uranus, and theentire southern hemisphere will be in the middle of its years-longnight. The equatorial plane, which seems now to include a ringsystem as well as the satellites, will be broadside to the Sun. Thepath of the spacecraft will be nearly perpendicular to the equatorialplane, and should permit the exploration of several satellites inaddition to the planet itself.The solar system as a whole is moving through galactic space.After all their planetary encounters both Voyager spacecraft willcontinue to recede from the Sun in the general direction of thatmotion. At some time they will cross the heliopause-the boundarybetween the Sun's magnetic and plasma domain and the generalstellar wind. Since the location of the heliopause is unknown, theVoyagers may carry out that valedictory flourish of observationsbefore communication at last ceases.


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The Voyager Spacecraftand Its Investigations

The Mission ModuleWE ave been accustomed to thinking of planetary missions interms of a launch vehicle and a spacecraft. The launch vehicle sup-plies sufficient velocity to the spacecraft to enable it to coast over along time period to its encounter with the distant planet. Thelaunch vehicle usually consists of several stages, and the spacecraftis carried by the final stage within a nose fairing that protects it onits passage through the Earth's atmosphere. Once the spacecraft hasbeen injected into a coasting trajectory, it can make minor coursecorrections by burning propellant in its velocity-control rocketengine. It does not generally make large changes in its velocity enroute, although if it is to orbit a planet it must decrease its velocityby a substantial amount when i t arrives in the planet's vicinity.With this outer planets mission, however, the old distinction be-tween launch vehicle and spacecraft becomes blurred. The launch

vehicle, which consists of a Titan IIIE first stage and a Centaur D-ITupper stage, is not capable by itself of imparting enough energy toinject a Voyager spacecraft into a Jupiter trajectory. The final incre-ment of launch velocity is provided by a solid-propellant rocketthat constitutes the Propulsion Module of the spacecraft. The partof the spacecraft that makes the entire journey (i.e., the Voyager)is the Mission Module. This arrangement provides advantages inperformance and reliability through the use of the Mission Module'sguidance and control electronics and propellant to stabilize thecomposite spacecraft's a ttitude during the one-time operation ofthe Propulsion Module. The Propulsion Module is shown as part ofthe spacecraft in figure 7.The total mass of the composite spacecraft is 2016 kg. Shortlyafter the burnout of the solid rocket, the Propulsion Module isjettisoned, and the 792-kg Voyager is on its way to Jupiter. (Duringthe remainder of this description, "spacecraft" and "Voyager" will


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16 VOYAGER

Figure 7.-Voyager spacecraft consisting of Mission Module and Propu lsionModule shown in launch configuration with stowed appendages.be used to designate the Mission Module th at makes the journey tothe outer planets.)Although the basic design of the Voyager spacecraft is derivedfrom the earlier Mariner spacecraft and the Viking Orbiters that arenow circling Mars, the family resemblance is effectively hidden byVoyager's conspicuous appendages (fig. 8). Particularly evident isthe very large (3.7 m) parabolic anten na th at is located abo ut wherethe solar panels were attached to the predecessor spacecraft. This


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Narrow angle TVTV electronics

xtendable boom

interferometerand radiometer

compartmentssma wave anten Science instrumentcalibration panel

and shunt radiator

Figure 8.-Schematic diagram of deployed Voyager.


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18 VOYAGERsubstitution is a striking reminder of the vast distances separatingthe outer planets from both the Sun and Earth.Solar panels were discarded as the electrical power sourcebecause the solar illumination is so ineffective at the distanceswhere the spacecraft will operate. At Saturn's distance of 10 AU,the solar panel area required for a given power output would be 100 btimes that which is needed in Earth orbit. Therefore, radioisotopethermoelectric generators (RTGs) are used to supply electricalpower to the spacecraft. Three of these generators are mounted ona boom to help isolate the main subsystems from excess heat andradioactivity. The energy source for each RTG is heat from theradioactive decay of plutonium oxide. The generator consists of abank of thermoelectric elements that convert the temperature dif-ference between their ends into electrical power. The combinedthermal output will be about 7000 W by the time Voyager reachesSaturn, and it will be converted to 390 W of electrical power.The Voyager's radio transmitters use a high-gain antenna to con-centrate the radiated energy into a beam and to maintain a reason-able signal strength even at very great distances. The antenna is aparabolic reflector of very large aperture, when compared with theother spacecraft components. The antenna's beam pattern is furthersharpened by the use of X-band transmission in place of the S-bandfrequency employed by the Mariners. The antenna's beamwidthwith X-band transmission is a fourth of its S-band beamwidth. Thespacecraft transmits only in the S-band during the cruise phases ofthe mission, when a high data rate is not required.The data rate during cruise depends on the distance, decliningfrom 2560 bits (binary digits) per second (bps) near the Earth to80 bits beyond Saturn. When periodic science maneuvers are per-formed during the cruise, the rate is increased to 7200 bps. This isalso the rate allocated to engineering telemetry and science data(exclusive of the imaging science investigation) during the Jupiterand Saturn encounters. Imaging science data will use all the remain-ing available transmission capacity during the encounters. The totaldata rate at Jupiter can be as high as 115 200 bps, and at Saturn44 800 bps. Such high data rates at those distances are particularlyimpressive when we recall that only a dozen years ago Mariner 4transmitted the first pictures from the vicinity of Mars at theagonizingly slow rate of 8-1 3 bps.The axis of the parabolic reflector is also the roll axis of thespacecraft. It is kept pointing directly at the Earth throughout the


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SPACECRAFT 19mission, except during velocity-control and science maneuvers.Although the roll axis is not aimed at the Sun, as it was on otherMariners, the Sun is still the primary attitude reference. From thedistance of the outer planets, Earth and the Sun are never far apart.(At Saturn's distance, the maximum separation is 6O.) The space-craft Sun sensor views the Sun through a hole cut in the reflector.Another sensor, mounted approximately perpendicular to the axis,locates the star Canopus to provide a roll attitude reference. Duringmaneuvers, and when celestial reference signals are not available, aninertial reference unit maintains the orientation references.The spacecraft's main body is a polygonal 10-sided ring thathouses the electronic equipment. It provides some protection frommicrometeoroids and radiation, and it maintains a suitable thermalenvironment by means of louvers that adjust the area available forthermal radiation.Distributed around the periphery of the ring are the 16 thrustersthat maintain the spacecraft attitude and produce the velocitychanges required for trajectory corrections. On this spacecraft,unlike previous Mariners, one subsystem performs both functions.All the thrusters generate their thrust by the decomposition of themonopropellant hydrazine. Four of the thrusters point in the samedirection, and are operated simultaneously for velocity changes.The remaining thrusters produce torque couples about the roll,pitch, and yaw axes to control attitude. The hydrazine tankage andtubing that supply the spacecraft thrusters also supply the largerthrusters that stabilize the Propulsion Module during the bum ofits solid-propellant rocket. About one-twentieth of the hydrazinesupply is expended for that purpose. All the thrusters are under thecontrol of the same electronics.During the encounter phases of the mission, while the spacecraftmaintains an orientation that points the parabolic antenna directlytoward Earth, one set of sensors must be accurately aimed at a longsuccession of objects of scientific interest. In order to accomplishthis, the sensors for the imaging science, infrared radiation, polarim-etry, and ultraviolet spectroscopy investigations are mounted on aScience Scan Platform (fig. 9) that can be precisely rotated abouttwo axes. The sensors are boresighted to point in a common direc-tion.The boom that supports the Science Scan Platform provides aconvenient place to mount the particle sensors for the cosmic ray,low energy charged particle, and plasma particle investigations.


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20 VOYAGE93

Figure 9.-Science Scan Platform instruments.

These sensors all require fixed orientations with respect to thespacecraft, along with wide unobstructed fields of view.The third very conspicuous boom attached to the Voyager space-craft supports the sensors for the magnetic fields investigation. Thequality of the data from the low-field magnetometers depends ontheir remoteness from the spacecraft's own magnetic field. They aretherefore carried at the end of a 13-m-long deployable boom.This remarkable boom, together with its two low-field mag-netometers, is stowed like a jack-in-the-box inside a canister that isonly 23 cm in diameter and 66 cm long. It is a triangular trusswhose fiberglass longitudinal members are held in place by fiber-


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SPACECRAFT 2 1glass triangles that are spaced 14 cm apart. The truss is stiffenedwith tensioned, collapsible diagonal filaments.

The boom is stowed by twisting the entire structure so that thediagonal filaments interlace and the triangles are nearly in contactwith each other. This puts a considerable elastic force on theassembly. When the canister is opened by a pyrotechnical device, alanyard that is paid out at a controlled speed prevents the boomfrom popping out with destructive violence.The two remaining appendages are the long, thin, deployableantennas that are used jointly by the planetary radio astronomy andthe plasma wave investigations.To assure proper operation for the four-year flight to Saturn, andperhaps well beyond, all of the spacecraft's subsystems have beendesigned for high reliability. Much reliance is also placed on func-tional flexibility and extensive redundancy of key components.Three of the subsystems use reprogrammable digital computers tomaintain their flexibility of response to changing conditions andrequirements during the long mission. These are the command con-trol subsystem (CCS), the attitude and articulation control subsys-tem (AACS), and the flight data subsystem (FDS). Each has twocomputers for redundancy.The CCS is the heart of the on-board control system. It issuescommands to other spacecraft subsystems from its memory. It candecode commands from the ground t o update its memory, and canpass the commands along to the other subsystems. The CCS cansurvive any single internal fault, because each of its functional unitshas a duplicate elsewhere in the subsystem.The AACS controls the propulsion subsystem, maintains thespacecraft's att itude, and positions the Science Scan Platform. Ithas. two plated-wire memories and redundant processors for them.The FDS controls the scientific instruments and arranges all thescience and engineering data for telemetering to Earth. It has twodata processors and two memories that provide both flexibility andredundancy.In addition to its own memory units, the FDS controls the datastorage subsystem, which is a high-capacity digital tape recorder.The 8-track tape can record about 536 million bits (the equivalentof 100 image frames), and can play the data back at four differentspeeds.Redundancy is also greatly in evidence in the communicationscomponents. There are two receivers, two traveling-wave-tube


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22 VOYAGER(TWT) amplifiers for X-band transmission, and both a TWT and asolid-state amplifier for S-band transmission.Imaging Science Investigation

Visual images have been the most valuable source of remote-sensing information in deciphering the stratigraphy and structure ofthe solid planetary bodies that have been examined, and they pro-vide the means of relating the data acquired by other remotesensors to particular features of a planet's surface. Because of thediffering characteristics of the various planetary bodies, imagingscience plays a variety of roles in their exploration. The Galileansatellites of Jupiter, with their optically thin or nonexistent atmos-pheres, should be amenable to the same kinds of image analysis asthe inner planets. The surface of Saturn's largest satellite, Titan,may be obscured by its atmosphere in the manner of Venus. As forthe giant planets themselves, Jupiter and Saturn do not have solidsurfaces, and imaging science is principally concerned with the topsof their cloud layers.One set of specific objectives for the imaging science investiga-tion is concerned with the motions of the cloud systems of Jupiterand Saturn and what they may reveal about the global atmosphericcirculations. Sequences of images over an extended time period willobserve the motions in the boundary regions between the belts andzones, and the motions in the neighborhood of such large atmos-pheric features as the Great Red Spot.Other objectives are to characterize the colored materials in thecloud belts and zones and to determine the vertical structures ofthe clouds and the high-altitude scattering layers. For the latterpurpose, the Voyager will photograph the planets' limb and termi-nator regions during the close encounters.The mission will make comparative geological studies of thesatellites possible by photographing about ten of them at resolu-tions finer than 15 kilometers. Imaging science will determine thesize and shape of many of the smaller satellites, and establish co-ordinate systems for the larger ones.Several objectives apply to Saturn's rings. Their optical scatter-ing properties will be investigated by photometric imagery over awide range of phase angles, and at several wavelengths. The imagingscience experiment will investigate the radial distribution of thering particles by imaging large portions of the rings at moderate


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SPACECRAFT 23resolution from positions well above or below the plane of therings.The subsystem that will perform the imaging science observationsconsists of a pair of television cameras mounted on the ScienceScan Platform. Television systems employing slow-scan vidiconcameras have provided the visual imagery on planetary missionsfrom Mariner 4 onward. The image on the target plate of thesecameras is erased each time a picture has been scanned out, thusmaking the camera available for further photography at once, andover a period of years. Photographing a complete feature of interestat high resolution usually involves assembling a mosaic of a numberof pictures.

The pictures of Jupiter obtained on the Pioneer 10 and 11 mis-sions were from the imaging photopolarimeter experiment, whichwas neither a film camera nor a television camera. The imagingphotopolarimeter made ingenious use of the spin-stabilized Pioneerspacecraft to scan its light-sensitive field across the planet's face andbuild up a coarse two-dimensional image. The fully stabilized atti-tude of the Voyager spacecraft permits the use of television camexasto photograph very much fmer details at very much shorter timeintervals.Each camera of the imaging science subsystem (fig. 10) comprisesa lens, filter wheel, shutter, and slow-scan vidicon tube with asso-ciated electronics. The lens for the wide-angle camera has a focallength of 200 mm, and a relative aperture of fl3.5. The narrow-angle camera uses a catadioptric telescope (one that combinesreflecting and transmitting elements) with a focal length of 1500mm and a relative aperture of fl8.5. The field of view of the wide-angle camera is 56 by 56 mrad, so that at a range of 1000 km onepicture would cover a square 56 km on a side. The field of view ofthe narrow-angle camera is only 7.4 by 7.4 mrad. The optic axes ofthe two lenses are boresighted to point in precisely the same direc-tion, so that simultaneous imaging provides nested coverage.An eight-position filter wheel on each camera permits the selec-tion of a wide range of spectral transmittances and thereby allowsthe reconstruction of color images. The two filter wheels are inde-pendently controlled, and their positions can be changed frompicture to picture by commands stored in the memory of the flightdata subsystem.Slow-scan cameras, unlike those used for broadcast television,employ shutters to control the duration of an exposure. Exposure


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24 VOYAGER

Filter wheelShutter/filter

Mou nting structure

assembly Scan platf ormWide-angle camera

Corrector Vidicon

Narrow-angle camera

Figure 10.-Imaging science instru men ts.

times between 0.005 and 15 sec can be selected in the fixed mode,and much longer exposures in the long-exposure mode.

The sensing element of each camera is a vidicon tube. The imageformed by the lens is exposed by the shutter onto the vidicon'starget plate, which bears a layer of photoconductive material. Atwo-dimensional image in the form of electrostatic charges is re-corded on the plate. A scanning beam of electrons reads out theimage line by line, converting it into a sequence of electrical signals.In the normal readout mode it takes 48 sec to scan the 800 lines ofa single image. Slower scanning rates are also selectable. Each lineis composed of 800 picture elements (pixels). The electrical signalarising from each pixel is assigned one of 25 6 discrete intensity


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SPACECRAFT 25levels. Since 256 equals 2 , t takes eight binary digits (bits) to dis-tinguish that many levels, and each pixel is treated by the flightdata system as an 8-bit digital word.Images can either be transmitted to Earth in real time (as theyare read out by the scanning beam), or recorded in digital form ontape for later transmission. The normal readout mode is compatiblewith the 1 15 200 bit-per-second transmission rate at Jupiter's dis-tance. The tape recorder accepts imaging data at the same rate. Ithas storage capacity for about 100 frames. To transmit images fromSaturn in real time at 44 800 bps, the scanning time is increased to144 seconds per frame. These images include the full frame, at fullresolution. There are also editing modes available, in which not allthe pixels composing the frame are transmitted.

The encounters with the various planets and satellites during thismission will occur at widely varying distances. A knowledge of thenominal angular resolution of each camera makes it possible to pre-dict the sizes of detail that could be distinguished on the images.Since a pair of scan lines in the narrow-angle camera covers a fieldangle of 19 p a d , the camera could theoretically resolve about 19 mat a distance of 1000 km. Table 2 shows the predicted resolutionsfor the narrow-angle cameras at the currently planned closestTable 2.-Imaging Res olution o f Satellites

SatellitesJovian :10 ..........................................................Europa ..................................................Ganymede ........................................Callisto .................................................Arnalthea ..............................................Saturnian:Mimas ...................................................Tethys ..................................................Enceladus ........................................Rhea .....................................................

Dione ....................................................Iapetus ..................................................Hyperion ........................................Titan .....................................................

Resolution, km

0.5 to 24 to 61 t o 33 to 48 t o 102 to 34 to 66 to 82 t o 42 to 420 to 2510 t o 122 to 4

Coverage,%

50404 0353530303030301 51550


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26 VOYAGERapproach distances t o the various planetary bodies. Resolved detailin the wide-angle images will b e coarser by a factor of a bou t 7-112.Infrared Radiation Investigation

Infrared radiation comes to the Voyager spacecraft from theplanets and their satellites both by thermal emission and by thereflection of solar radiation. The temperatures of the emitting sur-faces o r atm ospheric layers a re so low that nearly all of the thermalradiation is in the far infrared, whereas solar radiation is moreintense a t near infrared a nd visible wavelengths.Atmospheric gases have various absorp tion bands throug hou t theinfrared, and the reflected solar radiation shows reduced intensi-ties at some of those wavelengths. The effectiveness of particularabsorption bands depends on the chemical composition and onpressure and temperature conditions. It is through these effectsthat the infrared radiation investigation will acquire inform ation onthe composition and thermal structure of planetary atmospheres(including that of Titan) to complement that provided by theultraviolet spectroscopy investigation.Infrared radiation from satellites with tenuous atmospherescontains information about surface composition, temperature, andthermal and optical properties, as well as atmospheric compositioninformation. The investigation will also obtain spectra fromSaturn's rings, permitting studies of their composition and radialstructure , and of the size and thermal properties of the ringparticles.The investigation includes a radiometer that measures the totalreflected radiation in the visible wavelengths and a portion of thenear infrared wavelengths. In combination with the reflection andthermal emission information derived from the infrared spectra,this permits studies of the energy balances of the planets and theirsatellites.A prime objective is the investigation of the heat balances of theouter planets. Jupiter is known to radiate about twice as much heatas it receives from th e S un. Recent telescope measurem ents indicatetha t U ranus also radiates mo re heat than it receives, while a t presentthe evidence on Saturn's overall heat balance is conflicting. Theexistence of an internal heat source such as Jupiter's bears on ques-tions concerning the origin and evolution of the planets as well asthe dynamics of deep atmospheres that are transporting heat out-ward.


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SPACECRAFT 2 7Studies of atmospheric dynamics require knowledge of local

energy balances. T he z ones and belts t ha t are so clearly visible inthe atmospheres of Jup iter and S aturn are doubtless associated withlatitudinal variations in heat flow as well as with the very rapidrotations of both planets. If Saturn's overall balance of internal andsolar heat turns out to be markedly different from that of Jupiter,local energy balance determinations may explain th e similarities inthe two planets' zo ne and belt structure s.The atmosphere of Uranus m ust reflect, in its regional heatbalance variations, the fact that the planet's polar axis is nearly inthe orbital plane. Titan's atmospheric behavior should be differentfrom those of the rapidly rotating planets if the presumedsynchronization of its rotation with its orbital period is correct.A second major objective is the study of the atmospheric com-positions of the planets and their satellites. The mixing ratios ofthe main gaseous constituents and the concentrations of the lesscommon gases can be derived from the infrared and ultravioletspectrometric investigations. The ratio of molecular hydrogen tohelium is of particular imp ortance to theories of the early forma-tion of t he solar system. Some theories assume that Jupiter, Saturn ,and Uranus are representative of the primordial solar nebula. If therelative abundances of the light elements are nearly the same forthe three p lanets, the case for th at assumption will be quite strong.The relative abundan ces of the isotopes of the light elements areimportant to cosmogonic theories for similar reasons. The ratio ofdeuter ium (H 2) to hydrogen, and tha t of C1 t o C1 are of greatestinterest. Because meth ane ( CH 4) has so man y absorption bands inthe infrared, it shou ld be possible t o infer the isotopic ratios ofboth elements from the infrared data.The investigation of clouds and hazes (aerosols) in theatmospheres of the planets and Titan is another objective. Theirchemical compo sitions can be d etermined from the positions ofabsorption features in the reflected solar infrared spectra. Cloudsand hazes can be differentiated in the emission portion of thespectrum. It is also possible to infer the heights of cloud tops andthe particle size and relative opacity of hazes. The study of Titan's

clouds is of particular biochemical interest because it is the onlysatellite in the solar system kno wn to have a cloudy atmosphere andit may have conditions favorable to the formation of complexorganic molecules.The surfaces of most of the satellites (10 is an exception) are at


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28 VOYAGERleast partially covered with ice. The ices of water, ammonia, andmethane are readily distinguished in spectra of the near infrared.Although minerals have more subtle spectral characteristics thanices, certain groups of silicate minerals can be distinguished. Somenon-silicate minerals of interest, such as the sulfates that arebelieved to be present on 10, are readily detectable. The spatialresolution of these measurements will permit surface compositionmapping of the Galilean satellites, and globally averaged determina-tions for some of the others.The objectives with respect to Saturn's rings include theinvestigation of the composition and size of the ring particles andtheir distribution within the rings. While the two bright rings areknown to contain water ice, the composition of the faint ringsremains to be determined. The instrument can determine particlesize over a wide range, from a radius of a few micrometers to a fewcentimeters.

There are currently two versions of the infrared instrument. Theimproved instrument is designed for the very distant Uranus en-counter. If i t is not completed and qualified in time for the mission,

Figure 1 1 -Infrared interfero meter spectrom eter.


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SPACECRAFT 29the earlier version, which was only designed for the Jupiter andSaturn encounters, will be carried.The new instrument (fig. 1 1) consists of a telescope for collectingthe infrared radiation, two interferometers for acquiring spectraldata in the near and far infrared, and a radiometer for measuringthe total reflection over a band of wavelengths.

The early instrument with a single Michelson interferometer hasa spectral wavelength range of 2.5 to 50 pm, a field of view of0.25O, and a system operating temperature of 200 K. The newinstrument has two Michelson interferometers to extend thespectral range, and it operates at a lower temperature to give greatersensitivity. It has a spectral wavelength range of 1.4 to 10 pm and15 to 200 pm, a field of view of 0.15", and a system operatingtemperature of 140 K. The two versions of the instrument areinterchangeable on the spacecraft.Pho topolarimetry Investigation

Non-luminous objects become visible by scattering incident light.Whereas the imaging science investigation has the function of resolv-ing the scattered light to produce images, the function of the photo-polarimetry investigation is to provide information about theproperties of light-scattering surfaces or atmospheric particles. Itdoes this by measuring the intensity of scattered light at selectedwavelengths and polarization angles.

The light that the Sun radiates over a broad band of wavelengthsis unpolarized. That is to say that the light waves vibrate equally inall of the infinite number of planes that are perpendicular to thedirection of propagation. When this light is scattered by particles inan atmosphere or on a surface, the scattered light is polarized tosome extent; i.e., the intensity of the light vibrating in some planesis higher than in others. With light that is completely plane-polarized (by means other than scattering), the intensity is zero inthe plane perpendicular to the plane of maximum intensity. Bymeasuring light intensities separately through three plane-polarizingfilters oriented 60" apart, a photopolarimeter can determine boththe degree and the plane of polarization.

The Voyager instrument makes its measurements in eight discretewavelength bands, ranging from 2350 A in t$e ultraviolet portion ofthe spectrum, through the visible, to 7500 A in the near infrared. Itmeasures at 5 positions of the polarization analyzer wheel, so that asingle observation comprises 40 measurements.


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30 VOYAGERTo know the complete light-scattering behavior of a given target,

it is generally necessary to repeat the observation under differentgeometrical conditions of illumination and viewing. The significantvariables involve one or more of three angles: phase angle, illumina-tion angle, and viewing angle. The phase angle is the angle made atthe target by the Sun's direction and the instrument's direction(projected to the plane containing both directions). The illumina-tion angle at a surface is the angle between the normal to thatsurface and the Sun's direction. The viewing angle is the anglebetween the surface normal and the instrumen t7sdirection.Earthbound photopolarimetry of the outer planets is limited to avery small range in these angles. The phase angle is never more than12' at Jupiter or 6 at Saturn. During Voyager encounters, theseplanets and their satellites will be viewed at phase angles up to160'.The main purpose of the photopolarimetry investigation is todetermine the physical and chemical properties of particulatematter in the atmospheres of the planets, Titan, and other satellitesthat may have thin atmospheres; and the surfaces of satellites thathave little or no atmospheres. The rings of Saturn, which may beaggregations of very small satellites, may also contain small amountsof particulate matter.A major set of objectives concerns the various kinds of clouds inthe atmospheres of Jupiter, Saturn, and Titan. One is to define theoverall structures of the clouds by determining the vertical distribu-tion of their particles. A related objective is to study the nature ofthe individual cloud particles in various regions: their size, shape,and probable composition.A second set of objectives is concerned with the surfaces of thosesatellites that do not have light-scattering atmospheres. The varia-tion of the polarization with phase angle provides information onthe physical characteristics of the material at the surface. It will bepossible to distinguish among bare rock surfaces, dust, frostdeposits, ice, and regoliths produced by repeated impacts.Learning the surface composition of as many satellites as possibleis an important Voyager objective. The photopolarimeter's capa-

bility for obtaining accurate spectral photometry can provide somedata on surface composition for all the satellites of Saturn and atleast seven of the Jovian satellites.The objectives with regard to the rings of Saturn are to provideinformation on the size distribution and shape of the ring particles,


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SPACECRAFT 3 1Aperture wheel

Data logic board

Figure 12.-Photopolarimeter configuration.

their composition, and their density and radial distribution. Thephotopolarimeter will acquire its data in two modes: scattered sun-light measurements, and observations through the shadowed por-tion of the rings of the extinction and scattering of the light frombright stars.Th e Voyager photopolarim eter, designed primarily for theencounter phase of the mission, is mounted on the spacecraft'sScience Scan Platform, with its line of sight boresighted to theimaging science system's narrow-angle camera. The instrument (fig.12) consists of a telescope, a photomultiplier tube, three motor-driven wheels that select the field of view and the wavelength bandand polarization plane of observation, and associated electronics.Ultraviolet Spectroscopy Investigation

The outer planets, because of their strong gravitationalattractions and low temperatures, retain even the lightest gases intheir atmospheres. Thus, the atmospheres are probably closeapproximations of the original composition of the primordial solarnebula at their respective heliocen tric distances.


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32 VOYAGERAtmospheric gases emit radiation at certain of the far ultraviolet

wavelengths as a result of either the resonance scattering of solarultraviolet radiation or excitation by bombardment with energeticparticles. This airglow can be analyzed by a sufficiently sensitivespectrometer. When sunlight passes through the atmosphere, reso-nance scattering causes a reduction in the transmitted energy atthose wavelengths. Most of the gases with wjlich this investigation isconcerned also have continuous absorption bands below somecharacteristic wavelength. A spectrometer aboard a spacecraft thatis entering or leaving a planet's shadow can analyze this atmosphericextinction spectrum by looking at the Sun. The Voyager ultravioletspectrometer will operate in both the airglow and solar occultationmodes during encounters with Jupiter, Saturn, Uranus, and some oftheir satellites.The ultraviolet wavelength band of the electromagnetic radiationspectrum extends from about 50 A to 4000 A. The Voyager ultra-violet spectrometer analyzes the portion of the ultraviolet bandbetween 500 and 1700A. This portion is the far ultraviolet.The primary objective of the ultraviolet spectroscopyinvestigation is to determine the concentration of the main con-stituents and the structure of the atmospheres of Jupiter, Saturn,Titan, and, possibly, Uranus. The atmospheres are believed toconsist mainly of atomic hydrogen, molecular hydrogen, helium,and methane.Another objective is to study the atmospheres of the Galileansatellites and to search for toroidal (ring-shaped) clouds of gas alongtheir orbits. 10's orbit has a t least two such clouds: a sodium cloudextending all around the orbit detected by Earth-based spectroscopy,and a cloud of hydrogen detected by Pioneer 10's ultravioletphotometer.The existence of far-ultraviolet spectrometers sensitive enough tomeet the foregoing objectives aboard two long-lived spacecraftprovides an unexcelled opportunity for some exploratoryastronomical observations. Long observation times in the airglowmode during the cruise phases of the missions will permit pursuit ofseveral additional objectives.One astronomical objective is to perform a comprehensive surveyof the stellar sources of extreme ultraviolet radiation that have beenidentified recently by rocket-borne spectrometers. There will beabout 8 hours of observation time for each source that was brieflyglimpsed from rocket flights.


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SPACECRAFT 33The ultraviolet spectrometer (fig. 13) is an instrument that mustfunction in two distinctly different modes. In the airglow mode, it

must be as sensitive as possible to the weak far-ultraviolet emissionsthat it observes. In the occultation mode, it must look directly atthe Sun and accurately measure rapidly changing atmosphericabsorption. The instrument is mounted on the spacecraft's ScienceScan Platform, with its airglow-observation field of view bore-sighted to that of the imaging science subsystem's narrow-anglecamera. A mirror offsets the occultation-observation field of viewby 20, so that the other instruments on the Science Scan Platformare not damaged by the Sun.Figure 14 is a simplified optical diagram of the ultraviolet spec-trometer. The field of view, which is fixed by the slits in a seriesof 13 opaque plates, is about 0. 1" (in the plane of the diagram) by0.9". The incident radiation is dispersed into a spectrum by thediffraction grating at the back of the spectrometer. Only reflective

Figure 13.-Ultraviolet spectrometer.


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3 VOYAGERColl imator:13 identicalape r tu re plates

Occul tat ion

lOcm- S u n s h a d e fieldFigure 14.-Schematic diagram of ultraviolet spectro met er.

optics can be used i n th e far ultraviolet, because no solid transmitswavelengths shorter than 1050 A. The spectrum produced by thediffraction grating is imaged on the d etect or array.Radio Science Investigation

Th e success of these ou ter planet missions requires new levels ofcommunications and tracking performance. Although the radioequipment is not especially dedicated to this investigation, somerequirements have been upgraded to improve capabilities for radioscience. Also, the sp acecraft traje ctories have been designed t o pro-vide radio occultations by the planets, the rings of Saturn, andTitan.As a planet is interposed between a spacecraft and Earth, theradio paths can traverse the planet's magnetosphere, ionosphere,and atmo sphere in tur n. Each region affects the radio signal charac-teristics in its own way, so that it is possible to study its structuresand disturbances from o ccultation d ata.Throughout the missions, the radio paths will be traversing theinterplanetary medium. With two spacecraft in the same part of thesky at different distances, the radio signals will permit a study ofthe flow patterns of solar wind disturbances. When the paths lienear the Sun, the effects of the solar corona and the relativisticsignal delay caused by the solar gravity field will be observe d.One set of objectives is to investigate the structure of the


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SPACECRAFT 35atmospheres of Jupiter, Saturn, and Titan. This will be done bycontinuous measurement of the received frequencies and intensitiesof the radio signals from the spacecraft's S-band and X-band trans-mitters.The atmospheric pressure of any planet's atmosphere increases asone descends toward the surface. That is the basis for the use ofbarometric altimeters in airplanes. The velocity of light (or radio)wave propagation, which is constant in a vacuum, decreases withincreasing atmospheric pressure. To put it another way, theatmosphere's index of refraction increases with pressure, Rays pass-ing through an atmosphere are refracted from a straight-line path bythe variation in pressure. When a spacecraft is entering occultationthe rays are increasingly refracted, and the lengthening optical pathproduces a Doppler shift in the received radio frequencies. Refrac-tive dispers?on also reduces the intensity of the received signals asthe angle of refraction increases. By cross-checking the frequencyand intensity measurements, it is possible to derive a profile of theatmosphere's refractivity. Then, with a knowledge of theatmosphere's composition and its temperature at some altitude(from data supplied by the mission's ultraviolet and infrared investi-gations), the refractivity profile can be converted to temperatureand pressure profiles.Radio signal intensity can also be diminished during the occulta-tion experiment by cloud layers in the atmosphere. Since cloudabsorption does not affect the Doppler frequency, it should bepossible to distinguish cloud layers, and to determine their altitudesand densities.Another objective is to study local and regional variations in theatmospheric profiles due to weather and turbulence. It should bepossible to sort out fluctuations in the frequency and intensitymeasurements that are due to turbulence by their characteristicrates and amplitudes at the S-band and X-band wavelengths.An important objective of the occultation investigations atJupiter, Saturn, and Titan is the measurement of their ionospheres.This can provide information bearing on the composition, photo-chemistry, and dynamics of the upper atmosphere. The refractivityof an ionospheric layer depends on its concentration of free elec-trons. Refractivity profiles of the ionosphere are derived from theradio data in a generally similar manner to those of the neutralatmosphere, except that the dual transmission at two wavelengthsbecomes a more powerful tool. That is because the ionospheric


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36 VOYAGERrefractivity is a function of the square of the wavelength, whereasthe refractivity of neutral atmospheres is essentially independent ofwavelength. Another difference is that ionospheric layers canrefract both upward and downward, producing ray crossings.

A significant source of help in separating the ionospheric occulta-tion data from the effects of the interplanetary medium and theEarth's ionosphere is the presence of a second transmitting space-craft in the same part of the sky as the spacecraft that is beingocculted.

The first Voyager spacecraft to encounter Saturn will be occultedby the ring system immediately after it emerges from occultationby the planet. The objective of this very important occultationinvestigation is to study the character of the rings. It should bepossible to infer the total amount of material in each major ringzone, its radial distribution, and something of the composition andsize distribution of the particles.

During encounters with planets and satellites, tracking reveals theeffects of their gravitational fields. A general objective is to measurethe masses of the bodies encountered and, where possible, to meas-ure the harmonic coefficients of their gravitational fields (that is,the fields' departures from a spherical shape). These results areneeded for modeling their interiors.

Since the two spacecraft will fly by Jupiter at much greaterdistances than Pioneers 10 and 11 did, little improvement in theknowledge of that planet's gravitational field is expected. However,the missions will provide improved information on the masses anddensities of the Galilean satellites, to help define their internalstructures.

The two Saturn encounters will be close enough to the planet topermit measurement of the harmonics of its gravitational field. Theeffect of Saturn's rings on the encounter trajectories should bedistinguishable from the planetary harmonics, and it appears possi-ble to derive good values for the total mass of the rings, and theindividual masses of the A and B rings. The very close approach toTitan will permit an accurate measurement of that satellite's massand density and some determination of its gravitational harmonics.

Another objective will involve studies of the solar corona and thesolar wind. Radio signals passing through the coro.na when thespacecraft is near solar conjunction will permit annual observationsof coronal electron density. Radio measurements will provide acontinuous monitoring of the solar wind electron density between


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38 VOYAGER

The low energy portion of the galactic cosmic ray spectrum isstrongly affected by disturbances that originate in the solar corona.This solar modulation is very far-reaching, but must diminishsomewhere well beyond Jupiter's heliocentric distance. One of theimportant objectives of the cosmic ray particle investigation is toobtain more representative abundances of these cosmic rays bymeasurements near and beyond the outer boundaries of themodulation region.Quiet-time observations during the most recent solar sunspotminimum have indicated that the elemental composition of cosmicrays changes substantially at energies below about 15 MeV/nucleon.Oxygen, nitrogen, and helium nuclei become much more abundant.It is an objective of the cosmic ray particle investigation to studythis anomalous component of the cosmic ray population and, ifpossible, to identify its sources. The capability for resolvingisotopes is important in meeting this objective. Since the sources arebelieved to be nearby in the galaxy, the measurement of substantialanisotropies in the cosmic ray flux may provide importantidentification data.The cosmic ray particles instrument includes three sets of particletelescopes: the high energy telescope system (HETS), the lowenergy telescope system (LETS), and the electron telescope (TET).In any telescope, the passage of a cosmic ray particle through adetector results in the liberation of an electrical charge. The charge,which is a measure of the energy lost by the particle in its passage,depends on the particle's charge, mass, and initial energy. Thischarge is converted to a voltage pulse that can be measured by apulse-height analyzer. The measurement of a particle event in atelescope involves combining the pulse-height measurements fromcertain selected detectors with information about the total numberof detectors that the particle has penetrated.The complete instrument package (fig. 15) is mounted on theboom that supports the Science Scan Platform, where the fixedfields of view of all the particle telescopes are unobstructed byspacecraft parts.The HETS consists of two identical double-ended telescopes: onefixed in the ecliptic plane, and the other perpendicular to it. TheHETS can measure the spectra of electrons and the nuclei of allelements from hydrogen to iron over a broad range of energies upto 500 MeV/nucleon. It can resolve individual isotopes through the


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SPACECRAFT 39

Figure 15.-Cosmic ray particles instrum ent.isotopes of oxygen. It can measure the energy of electrons withhigh precision between 3 and 10 MeV.The LETS is designed particularly to determine the three-dimensional flow patterns of cosmic rays and to extend theelemental measurements down to very low energies. It consists offour single-ended telescopes. The LETS can distinguish the nuclei ofelements from hydrogen to iron down to an energy level of about 1MeV/nucleon. Isotopes from hydrogen to sulfur can be resolved atenergies between 'about 1 and 75 MeVlnucleon.There is just one TET in the instrument package. Becauseelectrons are greatly outnumbered by nuclei (which constitutebackground noise) in the galactic cosmic ray flux, TET is designedto suppress the latter. TET is a single-ended telescope that measureselectron energies from about 5 to 1 10 MeV.Low-Energy Charged Particle Investigation

Three investigations study charged particles: the plasma particle,low energy charged particle, and cosmic ray particle investigations.The three complement each other in the energy ranges they can


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40 VOYAGERexamine, and in the types of data provided. Collectively, they com-plement the magnetic fields and plasma wave investigations inexamining the structure of the planetary magnetospheres and theinterplanetary magnetic fields.

The low energy charged particle (LECP) investigation deals withparticles of lower energy than does the cosmic ray particleinvestigation, although there is some overlap. There is a cover-age gap between the low-energy end of the LECP range atabout 10 000 electron volts and the plasma particle investigationrange of 10 to 5950 electron volts. Still, the LECP by itself canprovide a great deal of information about the flow velocities andtemperatures of hot plasmas when their densities are sufficientlyhigh.

The objectives of the LECP investigation fall into two broadgroups: those concerned with particles in the planetarymagnetospheres and near natural satellites; and those concernedwith particles in the interplanetary environment.

As a result of the Pioneer flybys of Jupiter, the general nature ofthe Jovian magnetosphere is known. The radiation belts have beenlocated, and their electron fluxes have been observed to beunexpectedly high. The objective here is to fill in details such as thecomposition, energy range, and angular distribution of the chargedparticle radiation, to answer such questions as the origin, transport,and loss of the particles, the sources of radio emission, and theeffects of the Galilean satellites.

Recent observation of radio emissions from Saturn indicates amagnetosphere exists. Objectives of the investigation during theSaturn encounter are to study the extent of this magnetosphereand to determine the compositions, spectral and angular distribu-tions, and fluxes of particles in various regions. The interaction ofthe magnetospheric charged particles with the satellites and withthe material of the rings will be investigated. The nature of theradio emission will also be examined.

The existence of a magnetosphere surrounding Uranus is alsolikely. Because the spin axis of Uranus will be pointing very nearlyat the Sun in 1986 (encounter date), the interaction of a rotatingmagnetosphere with the solar wind should differ considerably fromthose at Jupiter and Saturn.

The objectives of the LECP investigation during the intervalsbetween planetary encounters are concerned with sorting outcharged particles by source, composition, energy spectra, flux


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SPACECRAFT 41intensities, and favored directions. Galactic cosmic ray particlesspan a very wide range of energies. The LECP investigates the verylow end of this range, while the cosmic ray particle investigationachieves better discrimination at higher energies.The LECP instrument comprises two subsystems whose detectorassemblages are mounted in a common pointing apparatus (fig.16). The detector optimized for the magnetospheric environmentis the low energy magnetospheric particle analyzer (LEMPA). Theother, designed primarily for the interplanetary environment, is thelow energy particle telescope (LEPT). The overlap in energy andintensity measurement ranges is such that each subsystem performsmeasurements that are important to both environments. Both

Stationary dome

* U

C' @ .d

Figure 16.-Low energy charged particles instrument.


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42 VOYAGERoperate throughout the mission, except that the LEPT detectors areto be turned off near Jupiter's radiation belts.

An important part of all particle measurements is the detectionof anisotropies, or preferred directions, in particle fluxes. The lowenergy charged particle investigation does this by systematicallypointing the axes of the two sensors in successive 45" steps inazimuth. The stepping mechanism rotates in the plane of theecliptic when the spacecraft is in its usual flight attitude. The two-dimensional anisotropies that can be accurately measured by thisdirectional scanning are sufficient for studies of the interplanetarymedium. In order to measure three-dimensional anisotropies of thelow-energy electron and proton populations in the planetarymagnetospheres, an ingenious arrangement permits two detectors ofthe LEMPA to make stepwise scans in the direction perpendicularto the ecliptic plane. As the sensor assemblies are stepped throughthe sequence of azimuths, the unshielded viewing angles of the twosensors rotating at the base of the dome step through a sequence ofelevations.The LEPT is a double-ended array designed to measure thecharge and energy distributions of low and medium energy nuclei inenvironments where the intensity is relatively low.Plasma Particles Investiga tion

A plasma is a gas composed of charged particles. The gas as awhole is electrically neutral, or nearly so, but the positive andnegative particles can be studied separately. Since the plasma parti-cle investigation is one of three Voyager investigations dealing withcharged particles, it is instructive t o consider the differences.The plasma particles have lower kinetic energies than thosemeasured by the cosmic ray particle and low energy charged parti-cle instruments, and there are more of them. Whereas the otherinvestigations are instrumented to count individual particles, theplasma particles investigation is concerned with the collective prop-erties-the plasma's velocity, density, and pressure.The kinetic energy of a large number of particles can be con-sidered as a group velocity. The instrument measures in the energyrange from 10 to 6000 V/charge. This corresponds to velocities upto about 1000 km/sec for protons, and up to about 50 000 km/sec(one-sixth the speed of light) for electrons. By measuring the varia-tion of group velocity with direction, the instrument determinesthe plasma flow direction.


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SPACECRAFT 4 3The flow of a large number of charged particles is, of course, an

electric current. The particle density of a plasma can be measuredby the current on a collector plate. In principle, the instrumentmeasures the cu rrent impinging on a collector plate after the parti-cles with different energies have been sorted out by varying a con-trol grid's retarding po tential.The plasmas that will be investigated are the solar wind and thevarious regions o f planetary magnetospheres. T he solar wind, whoseaverage velocity is about 450 km/sec, is highly variable as tovelocity, pressure, and density a t the Earth's heliocentric distance,and remains so beyond Jupiter 's distance.Jupiter's magnetosphere is a plasma whose composition, flowstructure, and density vary greatly in different regions. It interactswith th e solar wind t ha t surro unds it, with the Jovian ionospherewhich it surrounds, with the Galilean satellites that revolve withinit, and w ith the magnetic fields that traverse i t.Saturn and Uranus probably have magnetospheres as well. Theproperties and configurations of those plasmas are prime subjectsfor investigation during enc ounters w ith those planets.The "flux tube" connecting the ionospheres of 10 and Jupite rmoves through the Jovian magnetosphere. The planned passage ofa Voyager spacecraft below 10 will provide the op por tunity f or theplasma particle instrum ent to investigate th e changes in the plasmaproperties in th e vicinity of th e flux tube .

Another objective is concerned with the structure and fluctua-tions of the solar wind to distances of 20 AU. Pioneer observationsshowed that solar-wind streams d o not sm ooth o ut their speeds atincreasing solar distances. Although the amplitude of the speedfluctuations does decrease, their steepness with respect to timeincreases. At 4 to 5 AU, nearly all the streams begin with abruptjumps in th e flow speed tha t are probably shock fron ts. The Pioneerinstrument could measure the variations in speed, but not theplasma den sity or temp erature variations.Th e plasma particles instrum ent (fig. 17) consists of two Fa radaycup plasma sensors. One d etec tor is aligned with t he main space-craft axis that points toward the Earth and approximately into thesolar wind. The other detector points laterally, in an orientationthat views the rotating flow of the magnetospheric plasma duringthe Jupiter encounter .A Faraday cup comprises a collector plate with several grids infront of it , and an aperture that defines the field of view. One of


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44 VOYAGER

Figure 17.-Photograph of plasma particles equipment.

the grids is a modulator grid; when a retarding potential is appliedto it, only those ions with a corresponding range of energy-to-charge ratios are allowed to reach the collector plate. The flux ofthe selected ions impinging on the collector constitutes a measura-ble electric current. This measurement can be made for a series ofenergy-per-charge intervals by systematically varying the retardingpotential.The lateral detector of the plasma particles instrument has twomodulator grids. To measure the flow of positive ions, positivepotentials are applied to the first grid, while the second is held atground. For electrons, the first grid is held at ground, and negativepotentials are applied to the second grid.The Earth-pointing detector is a composite Faraday cup. Itscollector plate is split into three segments. Each segment is set atan angle of 20' to the main axis (fig. 18), so that they form aflattened tetrahedron. There is a set of grids parallel to eachsegment, and then the outer surface, which is a tetrahedron with anaperture cut in each face. The composite detector is a direction


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SPACECRAFT 45I -Lateral\ etectorM a ~ n

Elec tron~cs ox

Figure 18.-Schematic diagram of plasma particles equipment.analyzer: when the same retarding potential is applied to the threemodulating grids, the relative currents to the three collector seg-ments provide information about the direction of the plasma flow.Magnetic Fields Investigation

Magnetic fields are everywhere in the solar system. Some planetshave their own magnetic fields, presumably of internal origin. Theseinclude Mercury, Earth, and Jupiter. Recent radio data from theIMP-6 satellite indicate that Saturn, too, has a magnetic field. Theinterplanetary medium is traversed by streams of charged particlesthat constitute the solar wind, and by the shifting patterns ofmagnetic fields that they bring with them. The interaction of the


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46 VOYAGERsolar wind with the planetary magnetic fields produces the variousfeatures of the planetary magnetospheres. Jupiter's inner satellites,whose orbits lie within its magnetosphere, must likewise interactwith it. Some of the satellites of Saturn and Uranus may do thesame. Finally, the solar wind must interact with the particles andmagnetic fields of the interstellar medium. The zone where thistakes place, the heliopause, is at an unknown distance from theSun.The magnetic fields investigation will gather data on all the fieldsencountered in the mission. Because of the extreme differences inthe strengths of the planetary and interplanetary magnetic fields,the instrument employs both a low-field and a high-fieldmagnetometer system.

An important objective of the investigation is to measure themagnetic fields of Jupiter, Saturn, and Uranus. Since Jupiter's fieldhas already been measured by magnetometers on the Pioneer 10and 11 missions, the new measurements at that planet can provide adata base for studying any long-term changes in the field. Pioneer11 is expected to make the first measurements of Saturn's magneticfield as well, in September 1979. The planet Uranus is also believedto have a strong magnetic field, although the IMP-6 observations ofradio emissions coming from that direction are not free ofambiguity.Although the structure of Jupiter's inner magnetosphere is fairlywell understood, the three-dimensional structure of the outer regionneeds additional observations. The structure of Saturn's mag-netosphere is still unknown, and the two mission traverses shoulddefine its general features. The presumed magnetosphere of Uranusshould have a particularly interesting structure because of theorientation of the planet's pole in the Sun's direction at the time ofencounter.In order to increase our understanding of the basic physicalmechanisms involved in the dynamics of the magnetospheres ofrapidly rotating planets and in their interactions with the solarwind, the magnetic fields instrument will acquire the data forcorrelative studies with the other particles and fields investigations.Investigating the interactions of satellites with the planetarymagnetospheres is another objective. Jupiter's magnetosphereincludes the orbits of Amalthea and the four Galilean satellites. lo,which controls the emission of radio bursts of decametricwavelengths from Jupiter's ionosphere, must exert this control


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SPACECRAFT 47through the magnetosphere. The plasma sheath (ionosphere)surrounding 10 evidently is electrically connected to the Jovianionosphere through a "flux tube" along the lines of force ofJupiter's magnetic field. The scheduled passage of one of thespacecraft through the flux tu be should permit the investigation ofthis interaction. 10 may possibly have an internal magnetic field aswell, and this will be considered in the analysis of the m agnetic fielddata. If Ganymede has an internal magnetic field, the planned closeencounter should permit detection of its interaction with themagnetosphere,Saturn's largest satellite, Tita n, revolves at a distance of abou t 20Saturn radii from the planet. Saturn's magnetosphere probablyextends that far, at least during undisturbed solar wind periods.Since Titan has a rather dense atmosphere, the interaction of thesatellite with its environment (whether it be the magnetosphere orthe sola r wind ) wil1,be part icula rly significant. Th e close app roac hdistance of 4 00 0 km should allow a thorough observation.The magnetic fields instrument consists of a high-fieldmagnetometer (HFM) system and a low-field magnetometer (LFM)system. Each system contains two identical triaxial fluxgatemagnetometers that measure the magnetic field intensity alongthree orthogonal axes simultaneously, producing a direct vectormeasurement. One LFM is located at the end of a 13-m boom,where the m agnetic field of the spacecraft is not expected to exceedabout 0.2 gamm a. (One gamm a equals 0.00 0 01 gauss, which is theunit of m agnetic field stre ngth . The average geomagnetic fieldstrength at the Earth's surface is about 0.5 gauss, or 50 00 0gamma.) The o ther LFM is located about 5.6 m in from the end ofthe boom . With simultaneous measurements using this arrangement,it is possible to separate the spacecraft field analytically from theambient field. The two high-field magnetometers are located abouta meter apart on the truss that supports the magnetom eter boom.Plasma Wave Investiga tion

Plasma waves are low-frequency oscillations that have theirorigins in instabilities within plasmas. As we have seen in a previoussection, the plasma particles investigation provides informationabout t he bulk properties and com position of th e gases of chargedparticles-the plasmas-that con stitute th e solar wind and th e plane-tary magnetospheres. When examined more closely, plasmas exhibit


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48 VOYAGERinstabilities of several kinds. There is turbulence in the flow of theparticles, and the electrically neutral plasma carries local concentra-tions of positively and negatively charged particles. When theseinstabilities become oscillatory, plasma waves are generated.Plasma waves can be categorized as either electrostatic oscilla-tions or as generalized electromagnetic waves of very low fre-quency. The Voyager plasma wave investigation measures only theelectric field component. Although the frequencies of interestextend from about 0.01 Hz to ab out 100 kHz, the plasma waveinstrument itself detects only the range of frequencies between 10Hz and 56 kHz. The Voyager magnetometer can measure themagnetic vectors of electromagn etic plasma waves below 10 Hz, andthe planetary radio astronomy instrument measures plasma waveswith frequencies over 56 kHz.The plasma ions and electrons both emit and absorb plasmawaves. These particle-wave interactions are known to affect themagneto spheric dynamics of the out er planets and th e propertiesof the distant interplanetary medium, but they have not beendirectly observed in these regions. In general, they can only beobserved by flying spacecraft through the interaction regions.Electrostatic plasma waves only propagate for short distances, andthe propagation of electromagnetic plasma waves is also greatlyrestricted when the frequency is lower than the gyrofrequency(cyclotron frequency) of th e

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Voyager to Jupiter and Saturn

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