Pioneer G Press Kit

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NATIONAL AERONAUTICS ANDSPACE ADMINISTRATIONtj IW s' Washington, D. C. 20546202.7 55.8370FOR RELEASE:April 1, 1973

PROJECT: PIONEER G

contentsGENERAL RELEASE -------------------------------------- 1-8MISSION PROFILE --------------------------------------- 9-17Instrument TurnO--On -------------------------------- 10-11Midcourse Correction ------------------------------- 11

Asteroid Traverse -------------------------------- 12Factors in Planet Encounter ----------------------- 12-13Targeting ---------------------------------------- 13-14Flyby Operations -------------------------------- 14-15Beyond Jupiter ---------------------------------- 16-i7The Message Plaque ------------------------------

THE ASTEROlDS ---------------------------------------- 18-19COMETS ----------------------------------------------- 20JUPITER ---------------------------------------------- 20-27What We Know----------------------------------------- 20-21Clouds, Currents, and Visual Appearance ----- 21-22Magnetic Fields and Radiation Belts ---------- 22-23Jovian Radio Signals ------------------------ 23Temperature --------------------------------- 24Jpiter Unknowns ------------------------------- 24

Life ----------------------------------------- 24-25Planet Structure ---------------------------- 25A Hot Planet -------------------------------- 25-26Magnetic Field ------------------------------ 6Jupiter's Moons ---------------------------------- 26-27THE HELIOSPHERE ---------------------------------------- 28-31Solar Wind, Magnetic Field, and Solar Cosmic Rays 29-30Planetary Interactions and the Interstellar Gas-- 30Galactic Cosmic Rays ---------------------------- 30-31

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PIONEER 10 EXPERIMENT RESULTS ------------------------- 32-37Asteroid Belt and Interplanetary Dust -----------3-34The Solar Wind ---------------------------------- 34-35Magnetic Field ----------------------------------- 35-36Galactic Cosmic Rays ---------------------------- 36High-Energy Solar Particles --------------------- 36Interstellar Gas -------------------------------- 37

THE SPACECRAFT -------------------------------------- 38-48Pioneer G Description ---------------------------- 39-40Orientation and Navigation ---------------------- 40-41Propulsion and Attitude Control ----------------- 41-43Nuclear-Electric Power -------------------------- 43-44Communications ---------------------------------- 44-45Command System ---------------------------------- 45Data Handling ------------------------------------ 45-46Timing ------------------------------------------- 46-47Temperature Control ------------------------------- 48Magnetic Clea.,liness ----------------------------- 48Reliability --------------------------------------- 48

THE EXPERIMENTS -------------------------------------- 49-61Magnetic Fields ---------------------------------- 51-52

Helium Vector Magnetometer -------------------- 51Fluxgate Magnetometer ------------------------ 51-52

Interplanetary Solar Wind and Heliosphere -------- 52Plasma Analyzer ------------------------------- 52

Cosmic Rays, Jupiter's Radiation Belts and RadioEmissions -------------------------------------- 52-53Charged Particle Composition Instrument ------ 52-53Cosmic Ray Telescope ------------------------- 53

Jupiter's Charged Particles ---------------------- 54-55Geiger Tube Telescopes ----------------------- 54Jovian Trapped Radiation Detector ------------ 54-55

Asteroids, Meteoroids, Interplanetary Dust ------- 55Asteroid-Meteoroid Detector ------------------ 55Meteoroid Detector --------------------------- 56

Celestial Mechanics ------------------------------ 56Celestial Mechanics -------------------------- 56

Interplanetary Hydrogen, Helium, and Dust;Jupiter's Atmosphere, Temperatures, Auroras,Moons ------------------------------------------ 57-59

Ultraviolet Photometer ----------------------- 57-58Imaging Photopolarimeter --------------------- 58-59

Jupiter's Atmosphere, Ionosphere, Temperature ---- 60-61Infrared Radiometer -------------------------- 60Occultation Experiment ----------------------- 60-61

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EXPERIMENTS AND INVESTIGATORS ------------------------ 62-66Magnetic Fields Experiment ----------------------- 62Jovian Magnetic Fields Experiment ---------------- 62Plasma Analyzer Experiment ----------------------- 62Charged Particle Composition Experiment ---------- 63Cosmic Ray Energy Spectra Experiment. -------------- 63Jovian Charged Particles Experiment -------------- 64Jovian Trapped Radiation Experiment -------------- 64Asteroid-Meteoroid Astronomy Experime.-nt ---------- 64Meteoroid Detection Experiment ------------------- 64Celestial Mechanics Experiment ------------------- 64Ultraviolet Photometry Experiment ---------------- 65Imaging Photopolarimetry Experiment -------------- 65Jovian Infrared Thermal Structure Experiment ----- 65S-Band Occultation Experiment -------------------- 66

LAUNCH VEHICLE ----------------------------------------- 67Launch Vehicle Characteristics --------------------- 68-69

FLIGHT SEQUENCE --------------------------------------- 70-72Atlas Phase ---------------------------------------- 70Centaur Phase ------------------------------------- 70Third Stage Phase -------------------------------- 70Retromaneuver -------------------------------------- 71Atlas/Centaur/TE-M-364-4 Flight Sequence (AC-27)--- 72

LAUNCH WINDOWS ---------------------------------------- 73LAUNCH OPERATIONS ------------------------------------- 74-75MISSION OPERATIONS ------------------------------------ 76-77TRACKING AND DATA RETRIEVAL --------------------------- 78-80PIONEER PROJECT MANAGEMENT TEAM ----------------------- 81-82PIONEER G CONTRACTORS --------------------------------- 83-84

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\NATIONALAERONAUTICS ANDSPACE ADMINISTRATIONWashington, D. C. 20546AC 202/755-8370

FOR WELEASE: SundayNick Panagak:)s April 1, 1973(Phone: 202/755-3680)Peter Waller (Ames)(Phone: 415/965-5091)RELEASE NO: 73-41

PIONEER G READIED FOR LAUNCH TO JUPITER

Man will send his second spacecraft to Jupiter with the

launch of Pioneer G by the National Aeronautics and Space

Administration from Kennedy Space Center, Fla., betweenApril 5 and 26, 1973.

Pioneer G's flight to the largest and most active of

Lhe planets will take less than two years for most launch dates.

The unmanned spacecraft, to be named Pioneer 11 after

successful launch, will have a number of mission options --both at Jupiter and far beyond. These options will dependon results returned by Pioneer G's predecessor, Pioneer 10,

now three-quarters of the way to Jupiter. Pioneer 10 will

make the first reconnaissance of the giant planet during the

period between Dec. 1 and 6, 1973

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If Pioneer 10 is unsuccessful, Pioneer G will repeatits mission. Otherwise, Pioleer G may rli a differentcourse over the planet's surface, making measurements andtaking pictures that will complement those of Pioneer 10.The spacecraft may also investigate another of Jupiter'splanet-sized moons. After flyby, it may follow Pioneer 10and become the second man-made object to escape the solarsystem, or mission directors may choose a solar orbit nearJupiter's orbit. One possible trajectory could take it toSaturn in 1980.

One of Pioneer G's objectives, Jupiter, is unique amongplanets. It is enormous, colorful, and the most dynamicplanet in the solar system. It appears to have its owninternal energy source and is so maE3ive that it is almosta small star. Its dense, cloudy, and turbulent atmospheremay have the necessary ingredients to produce life.. Jupiter'svolume is more than 1,000 times that of Earth, and it hasmore than twicu the mass of all the other planets combined.Striped in glowing yellow-orange and blue-gray, it floats inspace like a bright colored rubber ball. It has a huge red"spot" in its southern hemisphere and spins more than twiceas fast as Earth. Of its 12 moons, two, Ganymede and Callisto,are the size of the planet Mercury, and two others, Io andEuropa, are as large as Earth's Moon.

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On its way to Jupiter, Pioneer G will make the secondspacecraft passage through the Asteroid Belt betweenAugust 1973 and February 1974.

It will carry a plaque identical to one on Pioneer 10for possible identification of its origin by another intelli-gent species, if any exist, should Pioneer G follow Pioneer 10out of the solar system into the Galary.

Spacecraft controllers can accomplish the various optionsopen to Pioneer G by varying the spacecraft's swing-by tra-jectory around Jupiter -- using the planet's gravity and orbitalmotion to change spacecraft speed and direction.

The trajectory will be chosen on the basis of scientificquestions raised by Pioneer 10's flight past the planet.Some proposed flyby trajectories call for an approach as closeas one-quarter of Pioneer 10's closest distance. This wouldbe only 35,000 kilometers (27,000 miles) above Jupiter'sstriped cloud tops.

Nearness of approach to Jupiter may depend on Pioneer 10findings about the planet's radiation belts. The belts ar-believed by many scientists to be as much as a million timesstronger than the Earth's, with intensity increasing rapidlyas one approaches the planet. If this is correct, the beltscould disable a spacecraft approaching too closely. Otherunknown hazards near Jupiter also might affect approachdistance and flyby trajectory.

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Pioneer 10 so far has found that the Asteroid Beltbetween tne orbits C 4ars and Jupiter apparently offersno serious hazard to spacecraft. During its seven-month,330-million-kiloe.i.ter (20S-million-mile) journey throughthe belt, Pioneer 10 received no damaging hits by high-velocity asteroid particles.

Pioneer 10 also has found some surprising features ofthe solar atmosphere (the heliosphere), found various elementsand isotopes among solar particles, and has made some findingsabout the "interstellar wind" outside the heliosphere.

Most conditions of the Pioneer G mission are at theextreme edge of space flight technology. The Atlas-Centaur-TE-M-364-4 launch vehicle will drive Pioneer G away from theEarth initially at 51,800 kilometers per hour (32,000 milesper hour) -- equaling the speed of Pioneer 10, which flew fasterthan any previous man-made object. Pioneer G will pass theMoon in about 11 hours.

At Jupiter, Pioneer G will be so far from Earth that

radio messages at the speed of light will requirea round-

trip time of 90 minutes. This will demand precisely plannedcommand operations. Pioneer G will be controlled by frequentinstructions from the Earth. The 64-meter (210-foot) "big dish"antennas of NASA's Deep Space Network (DSN) will have to listenoveA. a distance of 800 million kilometers (a half billion miles)as Pioneer C approaches Jupiter.

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The Pioneer G spacecraft is identical to Pioneer 10except that a second magnetometer has been added to measurehigh magnetic fields close to Jupiter. Pioneer's scientificexperiments should provide new knowledge about Jupiter, theAsteroid Belt, the outer solar system, and our galaxy. Theywill return images of Jupiter and will observe Jupiter'sdusk side, a view never seen from the Earth.

The 14 experiments will make 20 types of measurementsof Jupiter's atmosphere, radiation be , heat balance,magnetic field, internal structure, moons, and other phenomena.They will characterize the heliosphere (the solar atmosphere),the interstellar gas, cosmic rays, asteroids, and meteoroidsbetween Earth and beyond Jupiter.

Pioneers 10 and G are a new design for the outer solarsystem but are based on the subsystems of their predecessors,Pioneers 6 through 9. .ll four of these are still operatingin interplanetary space, with Pioneer 6 in its eight year.

The 260-kilogram (570-pound) Pioneer G is spin-stabilized, giving its instruments a full-circle soan fivetimes a minute. It uses nuclear sources for electric powerbecause solar radiation is too weak as Jupiter for anefficient solar-powered system.

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To maintain communications, Pioneer G's2.75-mete3r (9-foot)

dish antenna must point at Earth throughout the mission,changing its view by changes in spacecraft attitude, as thehome planet moves about in its orbit around the Sun.

Potential benefits of the Pioneer Jupiter missionsinclude increased knowledge of collisionless plasmas of thesolar wind. The findings may also lead to better under-standing of Earth's radiation belts, ionosphere and possiblyweather cycles, and to insights into Earth's atmosphericcirculation through study of Jupiter's rapidly rotating atrmo-sphere.

NASA's Office of Space Science has assigned project manage-ment for the two Pioneer Jupiter spacecraft to NASA's AresResearch Center, Mountain View, Calif., near San Francisco.The spacecraft were built by TRW Systems Inc., Redondo Beach,Calif. The scientific instruments have been supplied byNASA Centers, universities and private industry.

Tracking is conducted by NASA's Deep Space Network,operated by the Jet Propulsion Laboratory, Pasadena, Calif.

NASA's Lewis Research Center, Cleveland, manages the launchvehicle, which is built by General Dynamics, San Diego,Calif. Kennedy Space Center's Unmanned Launch OperationsDirectorate is responsible for Le launch.

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Cost of two Pioneer Jupiter spacecraft, scientificinstruments, and data processing and analysis is about $100million. This does not include costs of launch vehiclesand data acquisition.

The 30- to 45-minute evening launch window opensprogressively earlier each day -- approximately 9:00 p.m. EST

on April 5, and 6:00 p.m. by April 26.

Depending on launch date, the trip to Jupiter willtake from 609 to 825 days, with arrival dates betweenDec. 5, 1974 and July 30, 1975.

Studies of Pioneer G's target, Jupiter, have producedmany mysteries. The planet broadcasts predictably modulatedradio signals of enormous power. It also apparently radiatesabout three times as much thermal energy as it receives fromthe Sun.

Jupiter's atmosphere contains ammonia, methane, hydro-gen, and probably water, the same ingredients believed tohave produced life on Earth about four billion years ago.Many scientists believe that large regions below the frigidcloud layer may be at room temperature. These conditionscould allow the planet to produce living organisms despitethe fact that it receives only 1/27th of the solar energyreceived by Earth.

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Jupiter is composed mostly of hydrogen and helium, themain constituents of the universe. The planet may have nosolid surface. Due to its high gravity, it may go from athick gaseous atmosphere down to oceirs of liquid hydrogen,to a slushy layer, and then to a solid hydrogen core. Ideasof how deep beneath its striped cloud layers any solid hydro-gen "icebergs" or "continents" m`ght lie vary by thousandsof kilometers.

Astronomers have long seen violent circulation of theplanet's large-scale cloud currents and other features. Apoint on Jupiter's equator rotates at 45,600 km/hr (28,200 m.p.h.)during its ten-hour day, compared with 1,600 km/hr (1,000 m.p.h.)for a similar point on Earth's equator.

An odd feature of the planet is the Greet Red Spot, the"Eye of Jupiter." This huge bright red oval is 48,000 kilo-meters (30,000 miles) long and 13,000 kilometers (8,000 miles)wide, large enough to swallow up several Earths, The RedSpot may be an enormous standing column of gas or even araft of hydrogen ice floating on a bubble of warm hydrogenin the cooler hydrogen atmosphere and bobbing up and downat 30-year intervals, disappearing and reappearing. The spot'sred color may be due to the presence of organic compounds, thebuilding blocks of life, manufactured in Jupiter's atmosphere.

(END OF GENEMAL RELEASE, BACKGROUND INFORMATION FOLLOWS)


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J9-MISSION PROFILE

Pioneer G will be launched toward Jupiter on a direct-ascent trajectory from Cape Kennedy in a direction 18 degreessouth of straight east, passing over South Africa shortlyafter launch vehicle burnout.The trip will follow a curving path about a billionkilometers long (620 million miles) between the orbits ofEarth and Jupiter. The path will cover about 160 degreesgoing around the Sun between launch point and Jupiter.Because of the changing positions of the Earth and Jupiter,the shortest trip times are for launches during the earlydays of the 22-day launch period. These early dates would

put the Earth-Jupiter line far from the Sun during flyby,resulting in less interference by the Sun with data trans-mission from the spacecraft and with simultaneous telescopeobservations of Jupiter from Earth.The high-energy launch marks the second use of a thirestage, the TE-M-364-4, with the Atlas-Centaur launch vehicle.After liftoff, burnout of the 1,919,300-newton (431,500-pound) -thrust, stage-and-a-half Atlas booster will occur inabout four minutes. Stage separation and ignition of the131,200-newton (29,500-pound) -thrust Centaur second stagewill then take place and the hydrogen-fueled Centaur enginewill burn for about 7.5 minutes. The 10.7-meter-long (35-foot)aerodynamic shroud covering the third stage and the spacecraftwill be jettisoned after leaving the atmosphere, about 12seconds after Centaur engine ignition.At about 13 minutes after liftoff, small solid-f 1rockets will spin up the 66,270-newton (14,900-pound thrustthird stage and the attached spacecraft to 60 rpm for stabilityduring stage firing. The third stage will then ignite andburn for about 44 seconds.About two minutes after third stage burnout (16 minutesafter liftoff), Pioneer G will separate from the third stageand be on Jupiter trajectory.During powered flight, launch vehicle and spacecraftwill be monitored from the Mission Control Center at CapeKennedy via Eastern Test Range tracking stations and by atracking ship in mid-Atlantic.Twenty minutes after launch, mission control will shiftfrom the Ames Pesea-ch Center's Mission Director at CapeKennedy to the Ames Flight Director at the Pioneer MissionArea, at the Jet Propulsion Laboratory (JPL), Pasadena, Calif.

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Twenty-two minutes after launch, NASA's Pccension Islandtracking station will acquire the spacecraft.About 27 minutes after launch, the Johannesburg, SouthAftica, station of the Deep Space Network (DSN) will lockon the spacecraft and will be able to send commands. Bythen Pioneer G will have emerged from the Earth's shadowand begin to get timing data from its Sun-sensor.About 32 minutes after launch, Pioneer G's on-boardsequencer will initiate thruster firing for despin down toabout 21 rpm from the 60-rpm spin imparted before third stageignition. one sequencer then will start deployment of thefour nuclear power sources, the radioisotope thermoelectric

generators (RTG's), using spacecraft spin to slide them outon their trusses 3 meters (1 0 feet) from the center of thespacecraft. The sequencer will next initiate deploymentof the magnetometer to 6.6 meters (21.5 feet) from thespacecraft center by unfolding its lightweight boom. Whenboom and truss deployments are complete, spacecraft spinrate will be down to five rpm.Instrument Turn-On

At launch plus 40 minutes, the Johannesburg station willturn on the Geiger tube telescope and charged jarticle in-tru-ments to calibrate them against measurements of the knownradiation in EarLh's Van Ailen Belts. At 50 minutes,

thehelium vector magnetometer will be turned on.

Because of the possibility of high voltage arcing, someinstruments will be allowed to outgas before turn-on. Fliahtdirectors will turn then, on between six hours and two weeksafter launch.About t-wo hours after liftoff, controllers will commandthe spacecraft to reposition itself so that its high-gaindish antenna points to within 20 degrees of Earth. This willgreatly increase signal strength. Before this, the space-craft will have been communicating via its rear-facing, low-gain antenna. Thi3 spacecraft turning maneuver will takeabout three hours to complete. The high-gain antenna willnot be pointed directly at Earth, in order to avoid a spare-craft-Sun angle at which the intense solar radi;aion of spacestrikes the payload directly. Later in the mission, whenthe antenna must be poin'ed precisely at Earth for goodcommunications, the Sun-angle will have changed greatly so thatthree meter dish antenna will shade most of the payload.At eight hours after laanch, Pioneer G will "rise" atthe DSN's Goldstone. Calif., station. Goldstone will trimspin rate to the normal 4.8 rpm.

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In the first few days after launch, JPL specialistsat Pasadena will perform intensive computer calculationsto establish the precise trajectory.

Also during the first days, the imaging photopolari-meter experiment will make maps of the brightness of thesky, in order to correlate Pioneer's measurements of zodiacallight with simultaneous measurements fro)m Earth by DudleyObservatory, Albany, N.Y., and from Mount Haleakala, Hawaii.Midcourse Correction

Four days of accurate tracking will have determined theprecise launch velocity and direction. The automatic Conscansystem (whereby the spacecraft uses the pattern of theincoming radio signal to refine its Earth-pointing) will betested and then used to establish precise Earth-point forreference. Ames controllers will then cc-mmand change ofspacecraft attitude to point thrusters in the proper directionfor the first midcourse velocity change to eliminate errorsin aininq at Jupiter. Thrusters will then be fired and thespacecraft returned to it- early-mission pointing directionof 20 degrees away from Earth.

At about a week after launch, the spacecraft and itsoperations team will have settled into the interplanetaryoperations and scientific data gathering phase of the mission.Analysts will regularly assess performance of systems andinstruments.

The solar wind and magnetic field instruments will bemapping tile interplanetary medium. The particle experimentswill map distribution of solar and galactic cosmic ray parti-cles, and the ultraviolet photometer will measure neutralhydrogen. The meteoroid and dust detectors will gather dataon sizes and distribution of interplanetary matter. Zodiacallight measurements will be made periodically by the photo-polarimeter instrument and the meteoroid telescopes to helpdetermine amounts of interplanetary material.Spacecraft attitude change maneuvers to obtain desiredEarth-point will be commanded routinely every three to seven

days, and later in the mission every week or two.Near the end of the first month, a second midcoursevelocity change will be made if needed.When spacecraft operation has become routine, Amespersonnel controlling the spacecraft will move from thePioneer Mission Area at Pasadena to the Pioneer MissionOperations Center at the Ames Research Center, Mountain View,Calif. Ames personnel are at JPL in Pasadena during the firstweeks after launch because of the greater computer and displaycapability there.

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Asteroid TraverseAbout four months after launch, Pioneer G will enterthe Asteroid Belt and begin observations of light scatteredby asteroid material with its four onboard telescopes, andthe counting of particle penetrations with the gas-cellmeteoroid detector.Experimenters will compare numbers of asteroid particles'with the relatively few particles measured by Pioneer 10.Controllers will ready emergency procedures for possiblesubsystem malfunctions due to impact of high velocity- around48,000 km/hr (30,000 m.p.h.) -dust particles.Early in the mission, command, tracking, and data returnwill be primarily by the DSN's 26-meter (85-foot) antennaslocated at 120-degree intervals aroune the Earth at Goldstone,Calif.; Madrid, Spain, or Johannesburg, South Africa; andCanberra, Australia. At greater ranges from Earth, theDSN's 64-meter (210-foot) dish antennas at Goldstone,Madrid, and Canberra will track the spacecraft. The highlysensitive 64-meter dishes also will be used for criticalmaneuvers, such as midcourse velocity corrections.At Jupiter distance, the 26-meter dish antennas willbe able to receive 64 data bits per second (bps); the 64-meterdishes, 1,024 bps.About 315 days from launch, the spacecraft will passalmost directly behind the Sun, causing communication diffi-culties for about a week because of the Sun's radio noise.

Factors in Planet EncounterAbout a year after launch (April 1974), scientists will haveanalyzed findings from Pioneer 10's swing around Jupiter fourmonths earlier (December 1973). Project officials will thendecide on Pioneer G's exact path past the planet and makethe required course changes.The question of Jupiter's radiation belts will becritical. The belts may be a million times stronger thanEarth's belts. Number and energy fluy of particles in thebelts are believed to increase 100 times for each Jupiterradius closer to the planet. High energy protons (hydrogennuclei) and electrons could penetrate deeply into the space-craft, damaging vital solid state electronic circuitry. Thespacecraft could be crippled or destroyed.

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For most future missions to Jupiter,the design of the

spacecraft will be affected by the belzs because spacecraftmust approach the planet very closely through the most intensepart. For a more distant approach, the spacec.iaft would haveto carry exorbitant amounts of fuel for retrofire. Plannersfor these missions must know what particle radiation hazardto protect against. It could prove impossible to come closerto Jupiter than Pioneer 10's 140,000 km (87,000 miles).Extremely high-velocity dust particles near Jupiter,pulled in by the planet's high gravity, may pose anotherdanger. Though such particles are presumed to be few, theirincoming velocities of up to 220,000 km/hr (137,000 m.p.h.).could be opposed to the 125,600 km/h. (78,000 m.p.h.) speed

of the spacecraft at periapsis, possibly producing verypenetrating impacts.Because of Jupiter's gravity, any flyby course will bea turn around the planet. On a flyby trajectory likePioneer 101s, Pioneer G would approach from the sunlit side,then swing almost completely around its dark Lide travelingin a counterclockwise direction (looking down at the northpole). Planet rotation aside, it would fly about two thirdsof the way around Jupiter at various altitudes in a week'stime. It would pass over part of the southern hemisphere,cross the equator, and exit over the northern hemisphere.With a different flyby point from Pioneer 10's, flightpaths could be entirely different and even include flight aroundJupiter in the reverse direction (clockwise).

TargetingAs mentioned previously, prime consideration in selectionof the target region is to get the best scientific data onJupiter and its environment, especially the radiation belts.Hopefully, Pioneer 10 results will permit narrowing the rangeof possible targets from points more distant than two planetradii, 140,000 km (87,000 miles), down to about one-halfradius above the loud tops, 35,400 km (22.000 miles). Ifa close approach is desirable scientifically, it could be

possible for Pioneer G to fly to Saturn in 1980. For sucha Saturn flyby, launch from Earth would have to have beenaccurate enough to leave thruster gas for necessary latercourse corrections.Another trageting consideration will be close passage

to one of Jupiter's moons.

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A basic flyby factor will be the simultaneous trackingfrom two Earth stations during the critical five hours beforeperiapsis (closest approach).If possible, experimente-s would also like to have oneof the Jovian moons pass between the spacecraft and Earth,using effects on the spacecraft radio signal to detect anatmosphere on that moon. They also would like to view theGreat Red Spot and the "hot shadow" of a moon on Jupiter'ssurface to measure temperatures.

Flyby OperationsDuring flyby, spacecraft command and analysis activitywill increase greatly. Because communication time from Earthto Jupiter will be around 45 minutes, commands must beprecisely timed in advance for performance at a particularpoint over the planet. Five commands can be stored in advanceon the spacecraft.Ten days before periapsis (closest approach),controllerswill command a final Earth point maneuver to establish aprecise spacecraft attitude for the flyby. Biases may beincluded for effects of the Jovian magnetic field measuredby Pioneer 10, which could cause drift in spacecraft attitude.After that, no further thruster firings will be made because

these might distort several of the scientific measurementsmade near Jupiter.As Pioneer G is drawn in by the giant planet's gravity,its velocity relative to Jupiter will soar. For a Pioneer 10-type trajectory, velocity would increase in 20 days from anapproach speed of 33,000 km/hr (20,000 m.p.h.) to 125,600 km/hr(78,000 m.p.h.) at periapsis. For an approach to Jupiter'scloud tops as close as one-half a Jupiter radius, periapsisspeed would be as high as 173,000 km/hr (107,000 m.p.h.).Tracking stations will be prepared to handle effectsof this four-to-five-fold increase in speed on the two-wayDoppler measurements used to track the spacecraft. Doppler

shift will grow very large, and this will be complicatedby loss of communication when the spacecraft is behind theplanet.Experimenters will calibrate their instruments beforeflyby, and will change sensitivity ranges of the high energyparticle, solar wind and magnetic field instruments for thefar more intense phenomena at the planet.

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These experiments will look for the first Jupiter effects-the bow shock wave in the solar wind, Jupiter's magneticfield, and Jovian effects on streams of high energy solarparticles.For the flyby itself, on a Pioneer 10-type trajectory,Pioneer G would be close to Jupiter for about four days (100hours). At 50 hours before periapsis, 5 million km (3 millionmiles) away, Pioneer instruments would see the planet inalmost full sunlight. At 40 minutes before periapsis, Jupiterwould be half dark, and at periapsis itself about 60 percentof Jupiter would be dark. After periapsis, i-he spacecraftwould continue to see less than a half-phase Jupiter.For various other flyby trajectories, planet views andtimes will vary, but total length of time near Jupiter wouldbe about the same, and the same types of observations willbe made.The imaging photopolarimeter will have begun makingpolarization and intensity measurements of Jupiter's reflectedlight several weeks out from the p~anet. Starting at about1.3 million km (about 800,000 miles), from periapsis, theinstrument will begin to alternate these measurements withthe imaging. The polarization ana intensity measurementswill provide information on the physical properties of Jupiter'sclouds and atmosphere.The ultraviolet photometer will examine Jupiter's upperatmosphere for hydrogen/helium ratio, atmosphere mixing rate,temperature, and evidence of an auroral oval near the poleson the planet's day side. On most trajectories, it will haveseveral viewing periods during planet approach.The infrared radiometer will look for Jovian radiationfrom an internal heat source, hot spots in the outer atmosphere,and cold spots at the poles indicating icecaps of frozenmethane. It will measure the atmosphere's hydrogen/heliumratio. Its observation period will be close to periapsis. Itwill have good views of the boundary between Jupiter's lightand dark hemispheres (the terminator).As the spacecraft passes behind the planet and re-emerges,ground stations will record diffusion effects on its radiosignals to calculate the density and composition of Jupiter'satmosphere.After periapsis, solar wind, particle,and magnetometerexperiments will look for planetary effects on the interplanetarymedium behind the planet. Going away from the planet, thepolarimeter will again measure Jupiter's atmosphere.

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/ WPIONEER G PICTURE COVERAGE ONPlyE 10-TYPE FLYBY -

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Beyond JupiterAfter planet encounter, Pioneer will return to its inter-planetary data-gathering mode of operation.As in the case of Pioneer 10, the basic Pioneer G missionis planned to last through encounter and about three monthsthereafter. (The costs of about $100 million for the Pioneer 10and G flight- cover only this basic mission.) After that, itis not possible to predict exactly how long it will continueto function. Beside possible radiation damage, the life ofparts and exact rate of power system degradation are not certain.Another variable is supply of thruster gas for Earth-pointand course-change maneuvers.On a Pioneer 10-type flyby, Jupiter's gravity and orbitalvelocity would speed up Pioneer G to solar system escape speed,and bend the spacecraft trajectory inward toward Jupiter'sorbital path. Cruise velocity relative to the Sun (not theplanet) of 38,500 km/hr (24,000 m.p.h.) would n.ave increasedto 79,200 km/hr (49,320 m.p.h.).The most interesting experimental questions for Pioneers 10and G beyond Jupiter will be: What is the variation in the fluxof galactic cosmic rays; how do the solar wind and magneticfield change with increasing solar distance; and what is thedistribution of interstellar neutral hydrogen and helium? What

do these tell about the interstellar space ' yond the boundaryof the heliosphere (the Sun's atmosphere, The plasma, magneticfield, high energy particle, and ultraviolet photometer experi-ments will share these searches.During the post-encounter period, for most flyby tra-jectories, strength of incoming signals will decline steadily,eventually becoming almost infinitesimal. Communicationsdistance will grow steadily longer. Time required to commandthe spacecraft and get a response will lengthen to hours.Only the most intensive and sophisticated efforts bythe Deep Space Network will allow communications with the

spacecraft at all.On a Pioneer 10-type, solar-system-escape trajectory,Pioneer G would cross the otbit of Saturn at about 1.5 billionkilometers (930 million miles) from the Sun, about four yearsafter launch. It would reach Uranus' orbit at about 2.9billion kilometers (1.8 billion miles) from the Sun 7.5 yearsafter launch - but communication would be only marginallypossible at Uranus' great distance.

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Pioneer G would then continue into interstellar space.Six years after Jupiter encounter, at around 3.2 billion kilo-meters (2 billion miles), the spacecraft flight path wouldhave curved 87 degrees further around the Sun than theJupiter flyby poi:.t. Its velocity relative to the Sun atthis point would be 53,300 km/hr (33,100 m.p.h.) From thenon, its flight path would curve still another 25 degrees aroundthe Sun. At this point, far out in interstellar space itwould proceed away from the Sun in essentially a straight line,and its velocity would have dropped to a permanent 41,000 km/hr(25,500 m.p.h.) away from the Sun.The Message Plaque

Like Pioneer 10, Pioneer G will carry a pictorial messageintended for other intelligent species, if any exist, whomight find the spacecraft thousands of years from now insome other star system.The plaque tells when the Pioneers were launched, fromwhere, and by whom. Location of the Sun is shown by theintersection point of signals from 14 pulsars (cosmic radiosources). Binary symbols show the frequencies of the pulsarstoday, and these could be used even a million years later tocalculate the time of launch. Th electron reversal in thehydrogen atom is shown to provide a measurement standard(its 8-inch radio wavelength) for both pulsar frequenciesand the size of figures on the plaque. These are a man and

woman shown standing in front of the Pioneer spacecraft.The Sun and nine planets also are shown, as is the spacecraft'strajectory, leaving the third planet, Earth, passing Mars andswinging by the fifth planet, Jupiter.The plaque was designed by Dr. Carl Sagan, Director ofthe Laboratory for Planetary Studies, Cornell University; hiswife, Linda Salsman Sagan, a painter and film maker; andDr. Frank Drake, Director of the National Astronomy andIonosphere Center, Cornell. A detailed description of itappears in the February ?S, 1972, issue of the journal Science.

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THE ASTEROIDS

The asteroids travel around the Sun in elliptical orbitslike small planets. The Asteroid Belt forms a doughnut shapedregion, lying between the orbits of Mars and Jupiter, 300million to 545 million km (186 million to 338 million miles)from the Sun.The Belt is roughly 245 million kilometers (152 millionmiles) wide and extends about 40 million kilometers (25 millionmiles) above and below the plane of the Earth's orbit.Scientists believe the asteroids either condensed indi-vidually from the primordial gas cloud which formed the Sun

and planets, or somewhat less likely, that theyare debris

from the break-up of a very small planet. Clearly, theycontain important information on the origin of the solar system.Passage of Pioneer 10 through the Asteroid Be-- appears

to have shown that high-velocity, smaller asteroid particlespose only a minor hazard to spacecraft. This is an importantdiscovery because the belt is too thick to fly over or under,so all outer planet missions must fly through it.Some estimates suggest that there is enough material inthe Belt to makp a planet with a volume about 1/1000th thatof the Earth. \Astronomers have identified and calculated orbits for1,776 asteroids. There may be 50,000 in the size range fromthe largest, Ceres,,which has a diameter of 770 km (480 miles),to bodies 1 mile in diameter.In addition, the Belt is presumed to contain hundreds ofthousands of asteroid fragments, and uncountable billions ofdust particles.In the center of the Belt, asteroids and particles orbitthe Sun at about 61,200 km (38,000 miles) per hour. Theseparticles would impact the spacecraft (which has its ownvelocity in somewhat the sate direction) at about 49,000 km/hr

(about 30,000 m.p.h.). In short,asteroidal material appears

to be thinly spread, but penetrating.A few asteroids stray far beyond the Belt. Hermes cancome within about 354,000 km (220,000 miles) of the Earth,or closer than the Moon. Icarus comes within 9 million milesof the Sun.

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-19-Many meteorites which survive atmosphere entry and landon Earth are believed to be asteroidal material. Thesemeteorites are mostly stony, but some are iron. Some containlarge amounts of carbon, including organic chemicals likeamino acids.What effects could the Asteroid Belt have on Pioneer G?None of the known larae asteroids will come close to the space-craft. The threat of baseball-, green pea-, or even BB-sizedasteroids is negligible. The most serious hazard comes fromparticles of 1/10 to 1/1000th of a gram mass. Smaller parti-cles are too tiny to do damage. (There are 28 grams to oneounce).Meteoroids with a mass of 1/100th gram, which travel atabout 54,000 km/hr (33,500 m.p.h.) relative to the spacecraftnear Earth, can penetrate a single sheet of aluminum one centi-meter thick. At Jupiter's orbit these particles travel onlyabout 25,200 km/hr (15,660 m.p.h.).Results from Pioneer 10 indicate that Pioneer G appearsto have an excellent chance of passing through the belt with-out a damaging hit by a particle in the dangerous size rangeof 1/10th to 1/1000th gram.

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-20-COMETS g

Throughout the Pioneer G mission, cometary particlespresent some hazard.In the vicinity of Earth, about half the cometary parti-cles travel in streams which follow the orbital paths ofexisting or dissipated comets. When these streams (theLeonids, the Perseids, etc.) intersect Earth's orbit, meteorshowers result as the particles burn up in the atmosphere.Cometary particle streams orbit the Sun in long ovals. Mostof these orbits lie within 30 degrees of the ecliptic.The other half of the cometary particles near the Earthappear sporadically.Cometary particles near the Earth have average speedsrelative to the spacecraft of 72,000 km/hr (45,000 m.p.h.),

and a 1/100th gram cometary particle can penetrate a onecentimeter-thick aluminum sheet. Their speed at Jupiter'sorbit will be down to 32,200 km/hr (20,000 m.p.h.).

JUPITERWhat We Know

Though many mysteries concerning this big, brilliantplanet remain to be solved, hundreds of years of astronomicalobservations and analysis have provided a stock of information.Galileo made the first telescopic observations and discoveredJupiter's four larger moons in 1610.Seen from Earth, Jupiter is the second brightest planet,and fourth brightest object, in the sky. It is 773 millionkilometers (480 million miles) from the Sun, and circles itonce in just under 12 years. The planet has 12 moons; thefour outer ones orbit the planet in a direction opposite tothat of other known moons. Two of the moons, Ganymede andCallisto, are about the size of the planet Mercury. Twoothers, Io and Europa, are similar in size to the Earth'smoon.Jupiter spins once every 10 hours, the shortest day ofany of the nine planets. Because of Jupiter's size, thismeans that a point at the equator on its visible surface (cloudtnos) races along at 35,400 km/hr (22,000 m.p.h.), comparedto a speed of 1,600 km/hr (1,aoo m.p.h.) for a similar pointon Earth.

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JUPITER'S VISIBLE SURFACENORTH POLAR REGIONN NORTH NORTH NORTH TEMPERATE BELTNORTH NORTH TEMPERATE ZONENORTH NORTH TEMPERATE BELTNORTH TEMPERATE ZONE

NORTH TEMPERATE B6fNORTH TROPICAL ZONENORTH EQUATORIAL BELTEQUATORIAL ZONEEQUATORIAL BANDEQUATORIAL ZONENORTH COMPONENT OF SOUTHi..... xEQUATORIA' BELT oiSOUTH EQUATORIAL BELT

SOUTH COMPONENT OF SOUTHEQUATORIAL BELT-----wSOUTH TROPICAL ZONEGREAT RED SPOTSOUTH TEMPERATE BELTSOUTH TEMPERATE ZONESOUTH SOUTH TEMPERATE BELT

S SOUTH SOUTH TEMPERATE ZONESOUTH POLAR REGION

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This tremendous rotational speed (and fluid characterof the planet) makes Jupiter bulge at its equator. Jupiter'spolar diameter of about 124,000 kilometers (77,000 miles) is19,000 kilometers (11,800 miles) smaller than its equatorialdiameter of about 143,000 kilometers (88,900 miles).

Jupiter's visible surface (cloud top area) is 62 billionsquare kilometers (about 24 billion square miles). Theplanet's gravity at cloud top is 2.36 times that of Earth.The mass of the planet is 318 times the mass of Earth.

Its volume is 1,000 times Earth's. Because of the resultinglow density (one-fourth of the Earth's or 1.3 times thedensity of water), most scientists are sure that

the planet.is made up of a mixture of elements simila; to that in theSun or the primordial gas cloud which farmed the Sun andplanets. This means there are very large proportions (atleast three quarters) of the light gases hydrogen and helium.Scientists have identified hydrogen, deuterium (the Heavyisotope of hydrogen), methane (carbon and hydrogen), andammonia (nitrogen and hydrogen) by spectroscopic studies ofJupiter's clouds.

Clouds, Currents, and Visual Appearance. Seen through atelescope, the lighted heimiisphere ofJuiter is almost cer-tainly a view of the tops of gigantic regions of toweringmulti-colored clouds.Overall, due to its rotation, the planet is striped orbanded, parallel with its equator, with large dusky, grayregions at both poles. Between the two polar regions arefive, bright salmon-colored stripes, known as zones, and fourdarker, slate-gray stripes, known as belts -- the SouthEquatorial Belt, for example. The planet as a whole changeshue periodically, possibly as a result of the Sun's 11-yearactivity cycles.The Great Red Spot in the southern hemisphere is frequent-ly bright red, and since 1665 has disappeared completelyseveral times. It seems to brighten and darken at 30-yeaL

intervals.Scientists agree that the cold cloud tops in the zonesare probably largely ammonia vapor and crystals, and the graypolar regions many be condensed methane. The bright cloudzones have a complete range of colors from yellow and delicategold to red and brorze. C'ouds in the belts range from grayto blue-gray.

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-22-In addition to the belts and zones, many smaller features --streaks, wisps, arches, loops, patches, lumps, and spots --can be seen. Most are hundreds of thousands of kilometers insize.Circulation of these cloud features has been identifiedin a number of observations. The Great Equatorial Current(The Equatorial Zone), 20 degrees wide, sweeps around the planet410 km/hr (225 m.p.h.) faster than the cloud regions on eitherside of it, and is like similar atmospheric jet streams onEarth. The South Tropical circulating current is a well-knownfeature, as is a cloud current which sweeps completely aroundthe Great Red Spot.When Jupiter passed in front of stars in 1953 and again

in 1971, astronomers were able to calculate roughly the molecularweight of its upper atmosphere by the way it refracted thestars' light. They found a molecular weight of around 3.3which means a large proportion of hydrogen (molecular weight 2)because all other elements are far heavier. (Helium is 4, car-bon 12, and nitrogen 14.)Under Jovian gravity, atmospheric pressure at the cloudtops is calculated to be up to ten times one atmosphere on Earth.The transparent atmosphere above the clouds can be observedspectroscopically and in polarized light. It is believed to beat least 60 km (35 miles) thick.Scientists have suggested from cloud-top observations bytelescope that the general circulation pattern of Jupiter'satmosphere is like that of Earth, with circulation zonescorresponding to Earth's equatorial, tropical, sub-tropical,temperate, subpolar, and polar regions. However Jupiter'spolar regions (from an atmospheric circulation standpoint)appear to begin at about 26 degree latitude from the equator,instead of at 60 degrees as on Earth.

Magnetiz Fields and Radiation Belts. Among the nine planets,Chly Jupiter and Earth are Known to have magnetic fields. Evi-dence for Jupiter's magnetic field and radiation belts comesfrom its radio emissions. The only phenomenon known that couldproduce the planet's decimetric (very high frequency) radiowaves is trapped electrons gyrating around the lines of such amagnetic field. When such electrons approach the speed of light,they emit radio waves.

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Radio emissions indicate that Jupiter's magnetic fieldis toroidal (doughnut-shaped) with north and south poles likethe Earth's. It appears around 20 times as strong as Earth'sfield and presumably contains high energy protons (hydrogennuclei) and electrons trapped from the solar wind. The field'scenter appears to be near the planet's axis of rotation andsouth of the equatorial plane. Jupiter's powerful magnetic fieldcan hold mcre particles trapped from the solar wind than canEarth's field, and with increased particle energies. As aresult, particle concentrations and energies could be up toa million times higher than for Earth's radiation belts, aflux of a billion particles or more per square centimeter persecond.Because Jupiter has such high gravity and is so cold atthe top of its atmosphere, the transition region between denseatmosphere and vacuum is very narrow. As a result, the radiationbelts may come much closer to the planet than for Earth.

Jovian Radio Signals. Earth receives more radio noise fromJupiter than from any other extraterrestrial source except theSun. Jupiter broadcasts three kinds of radio noise: (1) ther-mal -- from the temperature-induced motions of the moleculesin its atmosphere (typical wavelength, 3 centimeters); (2) deci-metric (centimeter range) -- from the gyrations of electronsaround the lines of force of the planet's magnetic field(typical wavelength 3-70 centimeters); and (3) decametric(up to lOs of meters) -- believed to be from huge dischargesof electricity (like lighting flashes) in Jupiter's ionosphere(wavelength 70 centimeters to 60 meters).

The powerful decametric radio waves originate at knownlongitudes of Io with respect to Jupiter and have been shownto be modulated by passages of Jupiter's close moon, Io, whoseorbit is 2.5 planet diameters, 350,000 kilometers (about 217,000miles), above the tops of Jupiter's cloud layer. Some scientistsbelieve that the conductivity of Io must be sufficient to linkup magnetic lines of force to the planet's ionosphere, allowinghuge discharges of built-up electrical potential.The power of these decametric radio bursts is equal tothe power of several hydrogen bombs. Their average peak valueis 10,000 times greater than the power of Jupiter's decimetricsignals.

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Temperature. The average temperature at the tops of Jupiter'sclouds appears to be about -145 degrees C. (-229 degrees F.),based on many observations of Jupiter's infrared radiation.But recent stellar occultation studies indicate that much ofthe diffuse outer atmosphere is close to room temperature, andthat the top layer is at about 20 degrees C. (68 degrees F.).Only about 1/27th as much heat from the Sun arrives at Jupiteras arrives at Earth. Recent infrared measurements madeifromhigh altitude aircraft suggest that the giant planet radiatesabout 2.5 to three times more energy than it absorbs from theSun. The question is, what is the source of this energy? Theshadows of Jupiter's moons on the planet appear to measure hot-ter than surrounding sunlit regions.

Jupiter UnknownsScientists are reasonably certain of:most of the preceding

phenomena and observations, though it will be important to checkthem at close range with Pioneers 10 and G. They have few ex-planations for these observed phenomena, and they know verylittle about other aspects of the planet. What is hidden underthe heavy Jovian clouds? How intense are the radiation belts?Life. Perhaps the most intriguing unknown is the possible pre-sence of life in Jupiter's atmosphere.

Estimates of the depth of the Jovian atmosphere beneaththe cloud layer vary from 100 to 6,000 kilometers (60 to 3,600miles). The compositions and interactions of the lases makingup the atmosphere are unknown. If the atmosphere is deep, itmust also be dense. By one estimate, with an atmospheric depthof 4,200 kilometers (2,600 miles), pressure at the Jovian "sur-face" would be 200,000 times Earth's atmospheric pressure dueto the total weight of gas in the high Jovian gravity. Onesource cites eight different proposed models of Jupiter'satmosphere.However, scientists do appear to agree on the presence

of liquid water droplets in the atmosphere. Since the planetis believed to have a mixture of elements similar to thatfound in the Sun, it is almost sure to have abundant oxygen.And most of this oxygen has probably combined with the abundantJovian hydrogen as water.If large regions of Jupiter's atmosphere come close toroom temperature, both liquid water and water ice should bepresent.Jupiter's atmosphere contains ammonia, methane, and hy-drogen. The constituents, along with water, are the ingredi-ents of the primordial "soup" believed to have produced the

first life on Earth by chemical evolution. On this evidence,contain the building blocks of life.


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Some scientists suggest that the planet may be like ahuge factory, turning out vast amounts of life-supportingchemicals (complex carbon-based compounds) from these rawmaterials, using its own internal energy. If so, life couldexist without photosynthesis. Any solar photosynthesis wouldhave to be at a very low level since Jupiter receives only1/27th of Earth's solar energy. It would probably be lowenergy life forms at most (plants and microorganisms) becausethere is belived to be no free oxygen. Life firms would floator swim because a solid surface, if any, would be deep withinJupiter at very high pressures.

Planet Structure. While there are wide differencesamong

scientists on planet structure, most proposed models containelements like the following:Going dowr., it is believed that tmeperature risessteadily. The cloud tops may consist of super-cold ammoniacrystals, underlain by a layer of ammonia droplets, underwhich may be a region of ammonia vapor. Below this may belayers of ice crystals, water droplets, and water vapor.Below this is either the planet's solid surface, or liqaidhydrogen oceans. Still lower is a region of metallic hydrogencreated by high Jovian gravity with perhaps a core of rockysilicates and metallic elements. The core might be ten timesthe mass of the Earth by one estimate.Some theorists doubt that the planet has any solid materialat all, but is entirely liquid. Others propose a qradualthickening from slush to more rigid material.Most planetologists think the Great Red Spot may be acolumn of gas, the center of an enormous vortex, rising fromJupiter's interior to the top of the atmosphere. Known inphysics as a Taylor column, such a vortex would have to beanchored by a prominent surface feature, either a high spotor huge depression. The Red Spot has circled the planetmore than once relative to visible cloud features in the pasthundred years.Some scientists suggest that Jupiter's surface itselfmay be rotating at varying rates of speed relative to theatmosphere, thus moving the Spot. The rotation of the planet'smagnetic field, tracked by the timing of its radio emissions,is believed the best measure of rotation rate of the planet.

A Hot Planet. Theorists suggest that Jupiter is almost largeenough to be a small star. Because of measurements of excessheat radiated by the planet, current Jupiter theories call fora relatively hot core, compared with earlier ideas of a super-cold interior.

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MODEL OF JUPITER INTERIOR

CLOUD TOPSf AMMONIA CRYSTALSAMMONIA DROPLETSUPPER AMMONIA VAPORATMOSPHERE ICE CRYSTALSWATER DROPLETS r-WATER VAPOR

LIOUID AND/OR SOLID HYDROGENMETALLIC HYDROGEN

INTERNAL ENERGYSOURCE GRAVITATIONALOR RADIOACTIVEROCKY SILICATES-METALLIC ELEMENTS


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One hypothesis holds that despite the five billion yearssince formation of the planets, Jupiter has not completed itsgravitational condensation. This continued settling towardthe center (as little as one millimeter per year) could pro-duce the required heat energy. If the planet has a rocky core,some of the heating could be from decay of radioactive materialin the core.Magnetic Field. Internal heat also could explain the magneticfieldT A hot core might be a fluid core. Convective and ro-tational motion of electrically conducting fluids at tempera-tures of 10,000 degrees C.(18,000 degrees F.) may generate thefield. Or it could be generated by conductive atmosphere layersbelow the clouds which would store and later release energy.

With knowledge of Jupiter's magnetic field, scientistsshould be able to make inferences about the planet's internalstructure, particularly its fluid component.

Jupiter's MoonsJupiter's 12 natural satellites have some odd characteris-

tics. The second moon, Io, appears to be brighter for 10minutes after emerging from Jupiter's shadow. If so, thesimplest explanation, supported by recent stellar occultationobservations, is that Io has an atmosphere (probably nitrogenor methane) which "snows out" on the surface when Io is onthe cold, dark side and revaporizes when back in sunlight. Ioalso is distinctly orange in color and has odd reflecting prop-erties. Most of the sunlight reaching the moon's surface isstrongly scattered back, making Io extremely bright. Thisreflection is believed to be more pronounced for Io than forany other known object in the solar system. Infrared measure-ments show Europa and Ganymede to have water ice on their sur-faces.The inner moons in order of distance from the planet are:tiny Amalthea, diameter 160 kilometers (100 miles), whichorbits Jupiter twice a day at only ..5 planet diameters, 106,000kilometers (66,000 miles), above the cloud tops; the four

large moons Io, Europa, Ganymede, andCallisto, whose orbits

lie between 422,000 kilometers (262,000 miles) and 1,882,000kilometers (1,169,000 miles) from Jupiter.Beyond these are the seven tiny outer moons. The innerthree of these, Hestia, Hera, and Demeter, have orbits whichlie between 11.5 and 11.7 million kilometers (7.2 and 7.3million miles) from Jupiter.

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Orbits of the four outermost, Andrastea, Pan, Poseidon,and Hades, lie between 20.7 million and 23.7 million kilo-meters (12.9 and 14.7 million miles) of Jupiter. All are inretrograde orbits, moving counter to the usual direction ofplanet rotation. This suggest that they may be asteroidscaptured by Jupiter's powerful gravity. Diameters of six ofthe outer moons range from 15 to 40 kilometers (9 to 24 miles),with Hestia, the seventh, having a diameter of about 130kilometers (81 miles).Orbital periods of the four large inner moons range from1.7 days (for Io) to 16.7 days. Orbital periods of the innerthree of the outer seven moons are around 250 days. While

the four farthest-out, backward-orbiting moons complete theircircuits of Jupiter in around 700 days."V The backward orbit of the far outer moon, Poseidon, isJ 2highly inclined to the equator, and wanders so much due tovarious gravitational pulls that astronomers have a difficulttime finding it.

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THE HELIOSPHERE

Pioneer 0 may follow Pioneer 10 in exploring much ofthe heliosphere, the atmosphere of the Sun. The spacecraftmay survey a region of space 2.4 billion km (1.5 billionmiles) wide between the orbits of Earth and Uranus. It mayalso learn something about the interstellar space outsidethe heliosphere.The thinly diffused solar atmosphere is hundreds oftimes less dense than the best vacuums on Earth. Yet it is

important because it contains:The ionized gas known as the solar wind, roughly a

50-50 mixture Of protons Mhydrogenuclei) and electrons.It flows out from the 3,600,000 degrees F. (2,000,000degrees C.) corona or the Sun in all directions at averagespeeds of 1.6 million km/hr (one million mph).Where doesi the solar wind stop blowing? Where is theboundary between the solar atmosphere and the interstellargas? Estimates range from 560 million to 16 billionkilometers (350 million to 10 billion miles) from the Sun.

Complex magnetic and electric fields, carried out fromthe Sun by the solar win.Solar cosmic rays, high energy particles thrown outby the huge explosions on the Sun's surface at up to 480million km/hr (300 million mph).The heliosphere also encompasses planets, moons,asteroids, comets and dust. It is traversed by electro-magnetic radiation from the Sun: radio w.ves, infrared,ultraviolet, and visible light. Earth gets most of itsenergy from this radiation.The heliosphere further contains cosmic ray particles

from within and beyond our galaxy, travelingat nearly the

speed Qf light. This meanb enormous particle energies, upto ll0 million electron volts (MEV). The expression OlNis 1 followed by 14 i.eros. There are also neutral hydrogenatoms, believed to be part of the interstellar gas fromwhich the Sun and planets were formed.

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Study of these phenomena has a variety of applications.For example, storms of solar particles striking Earthinterrupt radio communications and sometimes electric powertransmission.The heliosphere can be thought of as a huge laboratorywhere phenomena occur that cannot be simulated on Earth.For example, man cannot accelerate particles in Eartnlaboratories to the near-light speeds reached by galacticcosmic ray particles. These particles are observed byPioneer instruments.Solar wind particles are so thinly spread that theyrarely collide, and only the magnetic field and electro-static fields link them together as a collisionless ionizedgas (collisionless plasma). Even though collisionless,these plasmas transmit waves of many kinds because of thicmagnetic field linkage.Knowledge of these plasmas can shed light on an areaof current high interest--hcw to contain plasmas inmagnetic fields. Accomplishing this would allow controlof the hydrogen thermonuclear reaction continuously on asmall scale to generate virtually unlimited electric powerby a clean process.All this involves such relationships as that of thetemperatures of solar wind protons and electrons nearEarth: they are almost the same, even though theory pre-dicts that the electrons should be several hundred timeshotter.How is energy transferred between them to equalizethe temperature? How far out from the Sun do thesetemperatures remain equal? Why?Solar Wind, Magnetic Field, and Solar Cosmic Rays.Near the Earth, the speed or the solar wind varies fromone to three million km/hr (600,000 to 2,000,000 mph),

depending or. activity of the Sun. Its temperature variesfrom 10,000 to 1,000,000 degrees C. (18,000 to 1,800,000degrees F.). Near the Earth, collisions between streamsof the solar wind use up about 25 percent of its energy.The wind also fluctuates due to features of the rotatingsolar corona, where it originates, and because of variouswave phenomena.

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The fastest solar cosmic ray particles jet out fromthe Sun in streams which can cover the 150 million Im (93million miles) to Earth in as little as 20 minutes. Slowerparticle streams take one or more hours to reach Earth andtend to follow the curving interplanetary magnetic field.The positive ions are 90 percent protons and ten per-cent helium nuclei, with occasional nuclei of heavierelements.There are from none to 20 flare events on the Sun eachyear which produce high energy solar particles, with the

largest number of flares at the peak of the 11-yearcycle.

Planetary Interactions and the Interstellar Gas.Since Jupiter is expected to have a magnetic field likethat of Earth, a bow shock wave should form in the solarwind in front of the planet. There should be a magneticenvelope around the planet, shutting out the solar wind,and trailing magnetic tail.Pioneer G instruments should easily characterizeionized particles swept from the atmospheres of Jupiter,its Moons, or the asteroids because their properties willgreatly differ from those of particles of solar origin.Pioneer 0 will measure helium, as well as interstellarneutral hydrogen. Both hydrogen and helium atoms arebelieved to be part of the interstellar gas which hasforced its way into the heliosphere as the solar sy stemmoves through interstellar space at 72,000 km/hr (45,000mph). Pioneer G also will measure the ultraviolet glow ofthe interstellar gas which can be seen out beyond theheliosphere boundary.Galactic Cosmic Rays. Galactic cosmic ray particlesusually have far higher energies (velocities) than solarcosmic rays. These particles may get their tremendousenergies from the explosion of stars (supernovas), the

collapse of stars (pulsars), or accelerationin the

colliding magnetic fields of two stars. Pioneer studiesof these particles may settle questions of their origin inour Galaxy and important features of the origin and evolu-tion of the Galaxy itself. These studier should answersuch questions as the chemical composition of stellarsources of cosmic ray particles in the Galaxy.

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Near Earth, numbers of these Galactic cosmic rayparticles can vary up to 50 percent as !ncreased solaractivity (a raster solar wind and stronger magnetic field)pushes them out of the inner solar system at the peak ofthe 11-year solar cycle. Cosmic ray particle intensityvaries up to 30 percent during individual solar flares.Galactic cosmic rays consist of protons (hydrogennuclei), 85 percent; helium nuclei, 13 percent; nuclei ofother elements, 2 percent; and high energy electrons, 1percent.Particles usually randy in energy from 100 millionelectron volts (MEV) to 10 MEV.Near Earth the average flux of these particles is fourper square centimeter per second, with most particles inthe 1,000 MEV range. Presumably as the spacecraft movestoward the edge of the blocking solar atmosphere, more ofthese medium-energy particles will be observed.

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PIONEER 10 EXPERIMENT RLSUIiS

Pioneer 10 so far has crossed the region lying between160 million and 544 million km (100 million and 340 millionmiles) from the Sun over a flight path roughly 696 millionkm (435 million miles) long. In crossing this 384-million-km (240-million-mile-)-wide region of space, it has surveyedthe 243-million-km (152-million-mile-)-wide Asteroid Belt,and has measured the outward gradient of several phenomena.The principal result so far is that the Asteroid Belt

appears to contain somewhat less materialthan many

scientists had believed, and does not offer a serioushazard to spacecraft. Pioneer 10's instruments reportedthis, and the spacecraft itself served as an instrument,receiving no damaging hits by an asteroid particle on itsseven-month, 338-million-km (205 million-mile-)-longflight through the belt.Other major findings are that the distribution of dustparticles between the Earth's orbit and the far side of theAsteroid Belt seems to depend on particle size. Thereappear to be rio more of the very small particles in thebelt than outside it, and there may be fewer of the smallestparticles in the belt than near the Earth.Researchers found that out to 560 million km (350million miles) from the Sun, solar magnetic field strength,solar wind density, and numbers of solar high energy par-ticles all decline roughly as the square of the distancefrom the Sun.Because turbulence of the solar wind and magneticfield do not decline with solar distance, large parts ofthe low energy cosmic ray particles from the galaxy appearto be shut out of the solar system perhaps as far as beyondJupiter.The gigantic solar storm of August 2, 1972, showedclearly that as it moves out the solar wind tends to slowdown while its gases heat up.Among solar high energy particles, experimenters havefound the elements sodium and aluminum fo r the first time.

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Researchers found that the neutral hydrogen of theinterstellar wind appears to enter the heliosphere in theplane of the Earth's orbit, about 60 degrees away from thedirection of travel of the sola, system through space, andmeasured interstellar helium for the first time.

Asteroid Belt and Interplanetary mustDistributions with particle size were as follows:Going out from the Earth's orbit, the smallest

particles (arourd 1/1000 mm diameter) actually appear todecline in numbers. Somewhat larger particles (1/100 to1/10 mm diameter) seemed to be even'.y distributed all theway from the Earth's orbit through the far side of the Beltwith no inc-ease in ,he Belt. (Tnere are 25 mm to an inch).Still larger particles (1/10 to 1 mm diameter) werefound all the way out but were almost three times asfrequent in the Belt as outside it.Particles larger than 1 mm diameter appear to be verythinly spread (as many scientists expected). Preliminaryanalyses of observelsons by Pioneer 10 asteroid telescopes

have not produced certain identification of any particleslarger than 1 mm diameter, ;hough further analysis may wellshow sor.e.One explanation for the absence of very small particlesin the Asteroid Belt woul.3 be that solar radiation mayreduce the orbital speed of chese particles. The largerparticles with more mess would be less affected by solarradiation and would maintain their orbits.Other explanations would be that original conditionsof solar system formation led to this dust distribution,or that comets tend to break up relatively close to theSun, leaving more dust there. Other findings:For the smallest particles (around 1/1000 mm diameter)--measurements of the zodiacal light reflected from theseparticles as one looks away from the Sun, have shown thatbetween th&, orbits of the Earth and Mars ;otng away fromthe Sur. zodiacal light declines steadily. If these verysmall reflective dust particles were mostly in the AsteroidBelt, the reflected sunlight would have remained relativelyconstant until the belt was reached, and then droppedrapidly as the spacecraft passed through the belt.

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The rate of decline of this reflected light suggeststhat concentrations of these tiny particles may decline asthe square of the distance from the Sun.The gegenschein or counter glow seen from the Earthin the night sky at the point opposite the Sun wasbelieved by some to be sunlight reflected from a "dusttail" of the Earth pointed away from the Sun. Since theglow remained the same when Pioneer 10 was 9 million kmaway, the gegenschein is now known to be an interplanetary

phenomenon.In the very small asteroid particles 1/100 to 1/10mi iameter (one billionth to one millionth of a gram)--there was no increase in the Asteroid Belt and theexperimenter believes the particles measured are cometary.For these partitcles, the one-quarter square meter of gas-cell detector surface received 25 penetrations between theEarth's orbit and the Belt's near side, and 17 penetra-tions in the Belt.For the larger asteroid particles, 1/10 mm to 1 mmdiameter (one ten millionth to one ten thousandth gram)--incomplete data analyses suggest that there are almostthree times more particles in the Belt than outside it.

Reflectivity of these particles is believed to bethree times higher than expected, perhaps because of icecoating or solar wind polishing. The four Asteroid tele-scoRes counted almost 200 particles in this size range.No 'large" bodies (one meter diaw-ter, for example)appear to have been seen.

The Solar WindBetween 100 million and 350 million miles from theSun:

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Density dropped by approximately the square of thedistance from the Sun.Variations (turbulence) were about the same at 350

million miles as at 100 million miles.Apparently the wind slows down while increasing intemperature as some of its outward motion energy is con-verted to thermal motion energy.During the huge storm on the Sun of August 2, 1972,largest of the solar cycle, Pioneers 9 and 10 happened tobe on the same line going out from the Sun. Pioneer 9 wasnear Earth's orbit, Pioneer 10,213 million km (132 millionmiles) farther out.Over this distance, solar wind speed dropped from

over 3,600,0oo km/hr (2,240,000 mph) at Pioneer 9, thehighest speed ever measured, to less than 2,520,000 km/hr(1,565,000 mph). But temperature went up from less than1 million degrees K (about 1.8 million degrees F.) to over2 million degrees K (about 3.6 million degrees F.) betweenPioneers 9 and 10.This temperature increase Is believed to have occurred

because the outward motion of the ionized solar windparticles was changed into thermal motion as the fast solarwind plasma bumped into slower plasma already there.

Magnetic FieldThe field (carried by the solar wind) decreased instrength by roughly the square of the distance going outfrom the Sun. The field was measured over 9 solarrotations.Changes in large-scale properties were consistent

with extrapolation from near-Earthconditions outward.

Discontinuties (turbulence) were the same at 560million Ian (350 million miles) as at 160 million km (100million miles).

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During the big storm, August 2, 1972, the fieldincreased its strength three times in one minute (100 timesover normal) at Pioneer 10. The shock wave in the solarwind shown by the field slowed down between Pioneers 9 and10.Galactic Cosmic Rays (high energy particles from the starsof the Galaxy)

Because of the expected decrease in blockage by thesolar wind and magnetic field from the Sun, numbers of lowenergy (O to 100 million electron volts) cosmic

rayparticles were expected to increase out to 350 millionmiles from the Sun. This did not occur apparently becausedespite the decline of solar wind density and magneticfield strength, solar wind and magneti' field turbulenceremained as high as near Earth. This turbulence apparentlycan shut out a large part of the low energy galacticparticles from the inner solar system, perhaps as far outas beyond Jupiter.

The increases in the high energy galactic particlesfor all energy ranges were less than 6 percent per 160million km (100 million miles) from the Sun, far less th&nthe expected two to three time increase.

High-Energy Solar ParticlesThe best resolution yet seen of elements making upthe solar high energy particles has been obtained by theNASA-Goddard instrument. It identified sodium and aluminumfor the first time, determined relative abundances ofhelium, carbon, nitrogen, oxygen, fluorine, neon, sodium,magnesium, aluminum, and silicon nuclei coming from theSun.During the solar storm of August 2, 1972, the highest

numbers of solar high energy particlesever measured were

recorded on Pioneers 6 and 9 and on several earth and lunarsatellites. Measurements of these solar particles were notas high on Pioneer 10.

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Interstellar GasThe University of Southern California ultravioletinstrument has been able to locate concentrations ofneutral hydrogen and helium and to make some tentativeseparations between gas originating in the Sun and thatpenetrating the solar system from interstellar space.The experimenters have measured the ultraviolet lightglow emitted by interplanetary neutral hydrogen and heliumatoms. Preliminary findings from this data are that theinterstellar neutral hydrogen appears to be entering the

heliosphere in the plane of the Earth'sorbit (the ecliptic)

at around 100,000 'Ar (62,000 mph). The ecliptic is alsoroughly the plane of the orbits of all the planets.The direction of movement of the solar system throughspace is about 60 degrees above the ecliptic, and theinterstellar hydrogen might well have been expected toenter directly from this direction.These findings are similar to those of three experi-menters on the 0G0 V Earth satellite several years ago.Small differences with the 000 findings in incomingvelocities of the interstellar gas might suggest somevariations with time, and hence turbulence in the "inter-stellar wind." The ultraviolet experiment also has madethe first measurements in interplanetary space of theultraviolet glow of interplanetary helium atoms. Con-clusions on properties of this interstellar helium willrequire further data analysis.

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THE SPACECRAFT

Pioneers 10 and G are the first spacecraft designedto travel into the outer solar system and operate there,possibly fo r as long as seven years and as fa r from theSun as 2.4 billion kilometers (1.5 billion miles).

For these missions, the spacecraft must have extremereliability, be of very light weight because of launchvehicle limitations, have communications systems fo r extremedistances, and employ non-solar power sources.Pioneer G is identical to Pioneer 10 except that a

12th on-board experiment has been added, a fluxgatemagnetometer, to measure high fields very close to Jupiter.Pioneer G is stabilized in space like a gyroscope by itsfive-rpm rotation, so that its scientific instrumnents scana full circle about five times a minute. Designers chosespin stabilization for its simplicity and effectiveness.Since the orbit planes of Earth and Jupiter coincide toabout one degree, the spacecraft will be in or near Earth'sorbit plane (the ecliptic) throughout its flight. Tomaintain a known orientation in this plane, controllersregularly adjust spacecraft position so that the spin axispoints constantly at Earth. The spin axis coincides withthe center line of the radio beam, which also points

constantly at Earth.Spacecraft navigation is handled on Earth using two-wayDoppler tracking and by angle-tracking.For mid-course corrections, the Pioneer propulsionsystem can make changes in velocity totaling 720 km/hr(420 mph).The spacecraft can return a maximum of 2,048 data bitsper second (bps) when relatively close to the Earth. AtJupiter distance, the 64-meter (210-foot) antennas of theDeep Space Network can hear a data rate of 1,024 bps. Itcan store up to 49,152 data bits while other data is beingtransmitted.Pioneer G is controlled largely from the Earth ratherthan by sequences of commands stored in on-board computers.Launch energy requirements to reach Jupiter are fa rhigher than for shorter missions, so the spacecraft is verylight. Pioneer G weighs only 270 kilograms (570 pounds).This includes 30 kg (65 lbs.) of scientific instruments,and 27 kg (60 lbs.) of propellant fo r attitude changes andmid-course corrections.

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Because solar energy at Jupiter is only4 percent of

energy received at the Earth and grows steadily weakerbeyond the planet, designers selected a nuclear power sourceover solar -ells.The mission's 14 experiments are carried out by 12onboard scientific instruments. Two experiments use thespacecraft ard its radio signal as instruments.For reliability, spacecraft builders have employedan intensive screening and testing program for parts andmaterials. They have selected components designed towithstand radiation from the spacecraft's nuclear powersource, and from Jupiter's radiation belts. In addition,key systems are redundant. (That is, two of the samecomponent or subsystem are provided in case one fails.)Communications, command and data return systems, propulsionelectronics, thrusters, and attitude sensors are largelyredundant.Virtually all spacecraft systems reflect the need tosurvive and return data for many years a long way from theSun and the Earth.

Pioneer G DescripItionThe spacecraft fits within the 3-meter (10-foot) diametershroud of the Atlas-Centaur launch vehicle with booms retracted,and with its dish antenna facing forward (upward). The Earth-facing dish antenna is designated the forward end of thespacecraft. Pioneer G is 2.9 meters (9.5 feet) long, measuringfrom its farthei ' forward component, the medium-gain antennahorn, to its farthest rearward point, the tip of the aft-facingomni-directional antenna. Exclusive of booms, its widestcrosswise dimension is the 2.7-meter (9-foot) diameter of thedish antenna.The axis of spacecraft rotation and the center-line ofthe dish antenna are parallel, and Pioneer spins constantly

for stability.The spacecraft equipment compartment consists of a flatbox, top and bottom of whicb are regular hexagons. Thishexagonal box is roughly 35.5 cm (14 in.) deep and each of itssix sides is 71 vm (28 in.) long. One side joins to asmaller box also 35.5 cm (14 in.) deep, whose top and bottomare irregular hexagons.

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MAJOR SUBSYSTEMS( RADIOISOTOPETHERMOELECTRICGENERATORS (2)D THRUSTERS- MEDIUM-GAINANTENNA( HIGH-GAIN ANTENNA- COMMAND

DISTRIBUTION UNIT- STELLAR REFERENCE~~ASSEMiIBLY LOW-GAIN ANTENNA * .A TRAVEUNG WAVE TUBES (2)


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PIONEER G SPACECRAFTRADIOISOTOPE THERMOrLECTRIC UV PHOTOMETERGENERATOR (RTG) - . SPIN/DESPIN THRUSTEPRTG DEPLOYMENT I HELIUM VECTORDAMPING CABLE - IMAGING PHOTOPOLARIMETER MAGNETOMETER

LOW GAIN ANTENNA r / ATTITUDE THRUSTERSEPARATION RING GEIGER TUBE TELLSCOPE

ASTEROID- METEOROIDDETECTOR SENSOR

THERMAL CONTROL PLSMAAAYELOUVERS --- METEOROID DETECTOR SENSOR PANELRADIATION DETECTOR ISTELLAR REFERENCE --- HIGH GAIN ANTENNA REFLECTOR

LIGHASHIEMLY HIGH GAIN ANTENNA FEED ASSEMBLYUMEDIUM GAIN ANTENNA

RTG POWER T PRO\ECAALE POSMICY HGH GAIN ANTENNA REFLECTOR/TELESCOP THUER MjEDIUMI GAIN ANTENNA

> >s/||LUX GATE HIGH GAIN/ / ,|MAG NETO ME T ER A ;]NTFENEND

RTG'S. INFRARED r*SUN SENSORCHARGED PARTICLE INSTRUMENT jpLASMA ASSEMBLY

3 /ATTITUDE THRUSTERS } PETROETHUERSUN SENSOR


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This smaller compartment contains most of the 12 onboardscientific experiments. However, 12./ kg (28 lbs.) of the30 kg (65 lbs.) of scientific instruments (the plasmaanalyzer, cosmic ray telescope, four asteroid-meteroidtelescopes, meteoroid sensors, and the magnetometer sensors)are mounted outside the instrument compartment. The otherexperiments have openings cut for their sensors to look out.Together both compartments provide 1.4 square meters (16 squarefeet) of platform area.

Attached to the hexagonal front face of the equipmentcompartment is the 2.7-m (9-ft.) diameter 46-cm (18-in.) deepdish antenna.The high-gain antenna feed and the medium-gain antenna

horn are mounted at the focal point of the antenna dish onthree struts projecting about 1.2 m (4 ft.) forward of therim of the dish. The low-gain, omni-directional spiral antennaextends about .76 m (2.5 ft.) behind the equipment compartment.Two three-rod trusses, 120 degrees apart, project from

two sides of the equipment compartment, deploying thespacecraft's nuclear electric power generator about 3 m (10 ft.)from the center of the spacecraft. A boom, 120 degrees from eachof the two trusses, projects from the experimnrt compartmentand positions the helium vector magnetometer sensor 6.6 m(21.5 ft.) from the spacecraft center. The boors are extendedafter launch.At the rim of the antenna dish, two Sun sensors aremounted. A star sensor looks through an opening in the equipmentcompartment and is protected from sunlight by a hood.Both compartments have aluminum frames with bottoms and

side walls of aluminum honeycomb. The dish antenna is made ofaluminum honeycomb.Rigid external tubular trusswork supports the dish antenna,three pairs of thrusters located near the rim of the dish,radioisotope thermoelectric generator trusses, and launchvehicle attachment ring. The message plaque also is attachedto this trusswork.

Orientation and NavigationThe spacecraft communications system also is used to orientthe Pioneer in space.Heart of the communications system is the spacecraft'sfixed dish antenna. This antenna is as large as diameter ofthe launch vehicle permits. It focuses the radio signal onEarth in a narrow beam.

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NAVIGATIONJUPITERAT LAUNCH AT L~S:H iEA.RTH OREITIONEER EA"s ORM

SPIN AXIS__ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _

PLANE CNTNa ll \ EARTH-POINT K\- L |RADIO SIGNAL ERHA

JUPITER AT-

-EARTH WILL ORBIT SUN TWnCE JUPITER POSSSLE PIONE BGCOMPLETES 1/6 REVOLUTION DUAIMG MISSION ESAP TRJCTR25.000 MPHCANOPUSREFERENCE


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The spaceraft spin axis is aligned with the centerlines of its disi, antenna and of the spacecraft radio beam.Except during the early part of the mission and coursechanges, all three are pointed toward Earth throughoutthe mission. Pioneer G maintains a known attitude in spaceas a result of this continuous Earth point.For navigation, analysts will use the Doppler shift infrequency of the Pioneer radio signal and angle-tracking byDSN antennas to calculate continuously the speed, distance,and direction of the spacecraft from Earth. Motion of thespacecraft away from Earth causes the frequency of thespacecraft radio signals to drop and wavelength to increase.

This is known as the Doppler shift.Propulsion ana Attitude Control

The propulsion and attitude control system providesthree types of maneuvers.It c n change velocity, thus altering course to adjustthe place and time of arrival at Jupiter.It can change the attitude of the spacecraft in space,either to point thrusters in the right direction forvelocity-change thrusts or to keep the spacecraft narrow-beam antenna pointed precisely at the Earth. Controllerswill command about 150 of these Earth-point adjustments asPioneer G and Earth constantly change positions in space.The system also maintains spacecraft spin at 4.8 rpm.Over the entire mission, the propulsion and attitudecontrol system can make changes in spacecraft velocity totaling720 km per hour (420 miles per hour), attitude changes totaling1200 degrees (almost four full rotations of the spin axis), andtotal spin-rate adjustments of 50 rpm. These capacities shouldbe substantially more than needed.

The system employs six thruster nozzleswhich can be

fired steadily or pulsed and have 0.4 to 1.4 pounds of thrusteach. Electronics and attitude sensors are fully redundant.For both attitude and velocity changes, two thruster pairshave been placed on opposite sides of the dish antenna rim.One thruster of each pair points forward, the other aft.

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ATTATNDE ATDHITEROCT ATTITUDE CONTROL AND PROPULSIONSPIN ASi=-

CONSTANT EARTH-

RADIO SIGNAL PATTERN TFROM GOLDSTONETACKIN~ s /STATION, CAlLIF-ORNIA\

CHANGE IN SPIN-RATE CHANGE IN THRUSTERS CHANGE IN ATTITUDEVELCITY | FIRE IN UNISON ATTITUDE THRUSTERS

EAT-PolN EA~-POINT


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To change attitude, the spacecraft spin axis can berotated in any desired direction. This is done using twonozzles, one on each side of the dish antenna. One nozzle isfired forward, one aft, in momentary thrust pulses onceper spacecraft rotation. Thrusts are made at two fixedpoints on the circle of spacecraft rotation. Pulses aretimed by a signal which originates either from the starsensor which sees the star Canopus, once per rotation, orfrom one of the two Sun-sensois which see the San once perrotation. Each pair of thrust pulses turns the spacecraftand its spin axis a few tenths of a degree, until the desiredattitude is reached.For velocity changes, the spin axis is first rotateduntil it points in the direction of the desired velocitychange. Then two thruster nozzles, one on each side of thedish, hut both facing forward or both facing aft, are firedcontinuously.Flight directors can command these velocity changemaneuvers directly in real time. Or they can put commandsfor the maneuvers into the system's attitude-control storageregister. Rotation of the spin axis, velocity change thrust,and rotation back to Earth-p

Documents

Pioneer G Press Kit