Space Pioneers and Where They Are Now

8/8/2019 Space Pioneers and Where They Are Now

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Space Pioneers.--I- - NASA EP-264

And Where They Are Now

EARL J. MONTOYAPr-ogram ManagerHeadquarters, Washington D.C.RICHARD 0.FIMMELManager, Pioneer MissionsAmes Research Center

Prepared at Ames Research Center

NASAEducational Affairs Division 1987National Aeronautics andSpace AdministrationWashington, D.C.


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Table of ContentsPage

Pioneers In Space To Understand Our Earth . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . IFirst Pioneers ................................................................................. 3The Interplanetary Pioneers . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . .. . . . . 5Planetary Pioneers ............................................................................ 7Outer Solar System Pioneers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 1The Pioneers Now and In The Future .......................................................... 15The Pioneer Spacecraft ....................................................................... 20Pioneer Firsts ................................................................................ 20

Facing PagePioneer 10Spacecraft

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ORlGCOLORPioneers In SpaceTo UnderstandOur Earth

JNAU PAG-EPHOTOGRAPH Th e great excitement of an y age has been created bypioneers - hose who so ught o ut new lands, new ideas,new social systems, new forms of go vernance, newtechnologies, and new goals for hu mank ind.

In our time we have been privileged to w itness anoutstanding hu man achievement of pioneers probingspace; the new frontier. Th is fron tier initially b eginswith the space environment beyond the fringes ofEarths atmosphere, then extends into th e spacebetween E arth and the orbi t of the M o o n , next into theinner Solar System, and w ith curren t explorat ion, intothe oute r So la r Sys tem and beyond.As sailing ships characterized Columbus pioneeringthe discovery of the New W orld, as the covered wagonprairie schooners characterized the exploration of th eWest , we now have spacecraft to characterize th e explo-rat ion of space front iers. This mod ern explorat ioninvolved large team efforts of ma ny people; includ ingclerks, secretaries, technicians, engineers, scientists, andmanagers. Engineers became deeply involved in thechallenging developmen t of these highly reliable space-

craft . New techniques fo r remote co ntro l of spacecraftoperat ions and for comm unicat ing over many mil l ionsof miles of space were devised to achieve the o bjectives.Scientists devised new ways to make exper iments atenorm ous distances from Earth.

View of Earth from space centered o n California

Objectives of these pion eering activities hav e been t obroaden hum an knowledge of the environment of theSolar Sys tem of which o ur E ar th i s a mem ber planet,and t o t ry to unders tand more abou t how thi s environ-ment affects life on Earth, i ts past and i ts future. To d oso, scient ists needed to know how the S u n affects th eenvironm ent in the space containing the planets (theinterplanetary environment), how energetic particles -protons an d electrons - nd ra d ia t ion f rom t he Su nspread thro ugh space to the planets, how th e planetsare affected by these solar influences and what happensat different t imes dur ing the 1 I-year solar activity cyclesand the occurrence of individua l s torms on t he Sun . a nd

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ORIGINAL PA-=MInR PHOTOGRAPH

Terrestrial aurora as seen from Skylabhow the oth er planets are affected compared with Earth.Scientists also want t o know how far the influence ofthe S u n extend s into space as a heliosphere and howand where the interstellar environment - he characterof space containing the s tars - ecomes predominant.This is important to ou r understanding of how we onEarth might be affected by events occurring in the gal-axy of s tars and du st and gas clouds beyond ou r SolarSystem.Four groups of pioneering spacecraft have made, andcontinue t o make, significant progress in the exploratio nof the near space frontie r and th e achievement of theseobjectives. These spacecraft starte d with Pioneers 1through 5 , which made the first thrusts into spacetoward the M oon and into interplanetary orbit . Theycontinued with Pioneers 6 th rough 9, which exploredinward and outward fro m Earth's orbit . The PioneerVenus Multiprobe (Pioneer 13) pushed t o the incrediblyhot surface of Venus, the erran t twin of Ear th , and thePioneer Venus Orb iter (Pioneer 12) surveyed thatintriguing planet from orbit for many years. Pioneers10 and 1 1 , which blazed a trail through th e asteroid beltand were f i rs t to explore Jupi ter , Saturn , and the ou terSo lar System, are currently seeking the borders of theheliosphere, an d will ultimately journey to th e distantstars.Man y of these Pioneers have op erated far beyondthe period expected of them in their original conceptan d design. Th e initial relatively small investments havebrought a priceless wealth of new information to ournatio nal resource of basic knowledge. Several of thespacecraft can productively continue gathering moreimportant d ata for m any years to come. All we need tod o to harvest benefit fro m this continued pioneering isto keep w atch over som e of these spacecraft, direct their

atten tion periodically t o new targets of opportunity,and receive and process the information being sent backto us.

Cornparafive sizes of the planers and the Sun

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First Pioneers ORIGINAL PAGECOLOR PHOTOGRAPH

Th e first prog ram t o initially ex plore the nea r spacefrontier consisted of a group of Pioneer spacecraftinitiated by the Advanced Research Projects A gencyof the D epar tme nt of Defense in 1958 when it auth o-rized five launches toward the M oo n. Three spacecraftwere the responsibility of the Air Fo rce, an d two th eresponsibility of the Army . As part of the nations con-tribut ion to the Intern at ional Geophysical Year, theselunar prob e Pioneers were intended to gather data fr ombeyond the fringes of Earths atmosphere. Additionally,they were intended to develop a technology to reachescape velocity fro m the gravitational pull of Ea rth an dto navigate a spacecraft to the vicinity of ano the r world- amely, the Moo n - s a prelude to interplanetarymissions.By 1958 a few Earth-orbiting spacecraft had beensuccessfully launched: S putn iks I a nd I1 and E xplorer.They showed tha t surroun ding Earth is a very complexregion of magnetic fields and high-energy charged par-ticles. As a result there was great interest in exploringthis dynamically changing environment to much greaterdistances from Earth. Earlier concepts of an uninterest-ing vacuum of space had rapidly changed to accept aninvolved and dynamic structure of the space environ-ment; an extremely active region of particles an d elec-tromagnetic fields interacting with each othe r and withthe planets in then poorly understood ways.These Pioneers were launched with varying measuresof success. On O ctober 11 , 1958, Pioneer 1 at tained adistance of 72,765 miles from Ea rth b efore falling backto E arth. A magnetometer carried by th e spacecraftrevealed for the first time com plex geo magn etic effectsthousan ds of miles from E arth. Pioneer 2 reached only963 miles from Earth on November 8, 1958. Pioneer 3,under the management of the newly formed NationalAeronautics and Space Administration, reached 63,580miles on Decem ber 6, 1958, before falling back t oEarth. Pioneer 4, launched o n M arch 3, 1959, reachedescape velocity, passed within 37,300 miles of theMo on, and went into orbi t a round the Su n. Anotherlunar probe, P ioneer 5, was launched o n M arch 11,1960, and entered an orbi t arou nd the S un o n a missionto exp lore space to the m aximu m range of communica-tions available at tha t time.

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First Pioneers; op to Doffom,Pioneer 1. Pioneer 4, qna Pioneer 3

Pathway into space

All these spacecraft made important discoveriesabo ut the extent an d form of the Earths m agneto-sphere, the region relatively close to E arth w here theEarths magnetic field deflects the solar winds blizzardof electrons and protons and traps some of these ener-getic particles to create radiation belts. These dis-coveries led to intriguing new questions which requireda m ore advanced type of spacecraft capable of exploringspace to considerable distances within and beyondEarths orbit. There followed the Interplanetary Pio-neers 6 th rough 9, designed t o explore the environmentof Earths orbit, to investigate the sola r wind and itseffects on E arth, to observe the interplanetary m ediumcloser to, and furthe r from, the Su n, and to define theimp ortan t role of particles and fields in their interac-tions with the planets.

\ROBE ORBIT

EARTH~MOONRELATIONSHIPAT FIRING TIME m#w= PAGF

COLOR PHOTOGRAPH4


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ORIGINAL PAGECOLOR PHOTOGRAPHThe InterplanetaryPioneersFive spacecraft were planned, all virtually identical

and spin-stabilized, and each designed to operate inspace for at least 180 days; i .e., app rox ima tely half a norbi t aro und the S un . Pioneer 6 left Ea rth successfullyon Decem ber 16, 1965, Pioneer 7 on August 17 , 1966,Pioneer 8 on December 13, 1967 and Pioneer 9 onNov ember 8, 1968. Th e fifth spacecraft was lost becauseof a Delta launch vehicle failure.These Pioneers dem onstrated the practicality of spin-ning the spacecraft to stabilize it and to simplify contro lof i ts orien tat ion. Measurements mad e by these space-craft greatly increased ou r knowledge of the interplane-tary environm ent an d the effects of solar activity onEarth. New inform ation was gathered abo ut the solarwind, sola r cosmic rays, the structu re of the Sun'splasma and magnetic fields, the physics of particles inspace, and the n ature of storm s on the Su n which pro-duce solar flares. Important incidental results were amore acc ura te measurem ent of the mean distance ofEar th f rom the Sun , referred to as the as t ronomica lunit (AU ), and more accu rate measurement of themasses of the Earth an d M oon . Addit ional ly, thespacecraft demonstrated high reliability by operatingsuccessfully for m any y ears beyond their designlifetime.them inward to a distance of 0.8 AU from the S un ,Pioneers 7 and 8 moved away from the Su n to a dis-tance of 1.1 A U , to ex plo re the region of the E arth's

Pioneers 6 and 9 traveled alo ng orbits which carried

magnetotail and wake, streaming like a pennant awayfrom the Su n. When Pioneers 7 and 8 explored themagnetic tail of the Ea rth, they discovered well-orderedcharacteristics but w ith discontinuities a t great distancesfrom Earth .At the beginning of the space age there ha d been abelief tha t cosmic dust could present a serious hazard tospacecraft. The Pioneers proved this fear to be unwar-ranted. Also, these spacecraft recorded the num bers ofdifferent types of cosmic rays and sh owed how theyvary with the solar cycle.

Th e structu re of the interplanetary magnetic field wasdiscovered, pulses of solar protons from the outburst offlares on the S un w ere recorded, and short-termchanges in the Earth's magnetic field in space were dis-covered. Th e spacecraft also identified cosmic rays ofGalactic origin and found a striking anisotropy (direc-tional organization) of cosmic rays of low energiescompared with those of high energies. It was found thatthe low-energy cosmic rays come mainly from the Suneven at the minimum of solar activity.

PIONEER 6-9(1965-1968)

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ORIGINAL PAGF ,OocOR PHOTOGRAPHBefore Pioneer 6, few spacecraft were capab le ofmaking detailed measurements of the solar wind. ThePioneers discovered a n east-west asymmetry of thesolar wind. The wind was described a s spiraling fromthe Sun, analogous to a rotating water sprinkler. ThePioneers were first to show th at its direction and speedcould be recorded and studied.

checked the propagation of radio waves through spaceand how this propagation is affected by storms on theSun . Detailed studies were made on the plasma cloudsejected from the S un by solar flares. The shapes andextent of plasma clouds passing by the E arth weredetermined. Passage of radio waves throu gh the solarcoron a was also investigated for the first time a s Pio-neer spacecraft were occulted (passed behind the S un) .

Radio s ignals sent from E arth t o the spacecraft

NOV 8.1968

MA Y 7,1969

NOV 3,1969

Interplanetary Pioneer 6- 9 spacecraft

Pioneers mapped the solar wind patterns6


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Planet Venus imaged from Pioneer VenusIn size Venus is the closest planet to being Earthstwin; but it is an erra nt twin that has evolved into avery inimical world from the s tand poin t of livingthings. The study of Venus has deep implications forunders tanding our own planet. We need to know whyand how Ea rth evolved benignly from the prim ordialsolar nebula for l ife to evolve upo n i t and continue tobe supported by it. With the growing awareness oflong-term environmental effects on Ea rth, scientistswanted t o know much m ore abo ut Venus and i ts mean-ings for E arth ; why Venus is so different, why it shouldhave such a strange atmosph ere - 00 times as denseas Earths - hat shields the planets surface f romdirect Earth-based observation a t wavelengths of visiblelight, why its surface is hot enou gh to m elt lead, andwhy the planet lacks significant am ou nts of oxygen andwater. If Venus had oceans, where did they go?The Pioneer Venus program was designed to investi-gate this mysterious planet in detail. The P ioneer 13multiprobe spacecraft dispatched heat-resisting probesto penetrate the atmosph ere and radio back to E arth in-si tu measurements from the uppe r atmosph ere, through

Topographic map of Venus obtained by Pioneer Venus Orbfter. Pioneer 72


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ORIGINAL PAGECOLOR PHOTOGRAPH

Artists rendering of Pioneer probe entering the atmosphere of Venus

the dense c louds , and down to the 900 F surface. AnOrb iter (Pioneer 12) circles Venus to this day and hasmapped its hidden surface by radar, imaged its chang-ing cloud systems, explored its unique ma gnetic envi-ronment, an d observed the interactions of the solarwind with a p lanet th at ha s no intrinsic magnetic field.Th e four probes plunged into the atmosphere a twidely separated loc ations in day and night hemi-spheres . They m easured temp erature, pressure, and den-sity dow n to the planets surface. The y discovered diur-nal changes in the upper atmosp here and found t hat a thigh altitudes Venus atm osph ere is cooler and a t lowaltitudes much hotter th an Earths. T he temperaturebelow the clouds was found t o be relatively constant a t

a given height everywhere on the planet. Instead of tur-bulence, unexpectedly stable atmosph eric layers werefound below the clouds.chemical composition of the atmosphere. It is mainlyTh e probes charted vertical winds and determined the

carbon dioxide and nitrogen but, unlike Earth, it ha svery little oxygen or water vapor. Rare atmosphericgases and their isotopes were detected and their concen -trations were measured. Unexpectedly, there w ere highabundances of neon and of argon 36 and 38 isotopes,which leads us to conclude that Venus and Earthreceived different original volatiles from the solar neb-ula out of which the planets are believed to haveformed.The ratio of deuterium to hydrogen is 100 times aslarge as that in Earths oceans, which suggests thatthere has been a preferential escape of a large am ou ntof hydrogen from Venus. This is evidence that V enusprobably had a large quantity of water, perhaps as

much as a n ocean, sometime during its history. Theocean m ay have been lost because a greenhouse effecttrapped solar heat in Venus atmosp here so that theplanet became too hot to retain water on its surface.

A Pioneer Probe spacecraft on the surface of Venus

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Regions of V enusatmosphere explored by spacecraft

The Orbiters ra dar provided altimetry maps fo rnearly all of the surface of Venus, resolving featuresdown to abou t 50 miles across. On these maps, scientistsidentified volcanoes th at m ay still be active; contine ntalareas with great plains and mountain ranges in whichone peak rises 35,500 feet; island masses rising fromglobal plains; rift valleys such as D iana C has ma , whichis 9,500 feet deep; some craters; and extensive area s ofglobal plains with relatively flat and monotonous sur-faces. Key features on Venus have been an d will con-tinue to be named after women; e.g., Ap hrodite, Freyja,Asteria, Atla, Ish tar, Atalanta, Phoebe, Themis, andLeda. Venus was found to be a planet tha t is closer tobeing a sphere than are Earth a nd o ther p lanets , asmight be expected from its slow rotation, which is 242times slower than Earths.Pioneer Venus rada r al t imetry data were used toselect targ ets for subsequent U.S.S.R.-Venus probes.Important international cooperation has been enhancedby exchanges of information ab ou t Venus.Sensitive and detailed measurements of spacecraftaccelerations in orbit show that m ountainou s forma -tions are less dense than the global crust , and that , a son Earth , they appear to be supported on a liquidinterior.Movements of masses of the atmosph ere, charted inobservations made from orbit, revealed global winds of220 mp h a t the cloud tops. Th ese winds generally blowwestward, with virtually n o mo tions north or south .

Diagram of global winds in the atmosphere of Venus as charted by PioneerspacecraffElectrical signals from Venus detected by Pioneer arepossibly caused by lightning flashes in gas clouds aris-ing fro m active volcanoes. If these emissions are n otcaused by lightning in such clouds they present a prob-lem because there is no other terrestrial counterpart to

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IONOPAUSE

WAKE REGlCO P " A -.-... ESCAPEko\ --nN \

IONOSHEATH

EUV PRODUCTIONAND HEATING.

/' / LOWER ATMOSPHERESOLARWIND

HEATING

I \- OBSERVATIONVET

Diagram of the magnetosphere of Venus

Th e Orbiter travels in an elliptical o rbit aroun d theplanet. This enables i ts instrumen ts to sample variousregions of the planet's iono sphe re (th e region ofcharged particles in the upp er atm osph ere) which is athinner ionosphere than th at of E arth. Venus has nomagnetic field to shield it from th e solar wind, but thewind itself generates a magnetic field.Th e induced magnetic field deflects the solar windand gives r ise to a bow sh ock and an ionopause (whichhas n o terrestrial cou nterpart) . T he iono pause is theboundary between the ionosp here and the solar wind.Th e spacecraft showed th at the io nopause exists on the

night side as well as the da y side of the planet a nd itidentified and mapped specific ionospheric regions: anionosheath, a mantle, and a w ake. Changes i n the ion-osphere and the particles and field environment sur-rounding Venus were recorded as solar activity, and thestrength of the solar wind changed during the solarcycle. A new understanding of how a planet without amagnetic field interacts w ith the sola r wind is beingdeveloped, a nd c ontributions to the possible explan a-tions for some of the differences between Earth andVenus are being m ade.

ORIGINAL PAGEC W R PHOTOGRAPH

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Outer SolarSystem Pioneers ORIGINAL PAGECOLOR PHOTOGRAPH

Scient ists gave high priority to extending the explora-t ion of the outer space front iers. Early in the 1970sscientific descriptions of the ou ter parts of the So larSystem still differed enor mo usly because de tails werelacking from observat ions made from Ear th. Therewere many unkno wns a bo ut the far reaches of inter-planetary space beyond the orb i t of Ma rs and a bo ut thevery large an d distan t planets of the o uter So lar Systemin which most of the mat te r and the angular m omentumof the total system are con centrated.Between the orbi ts of M ars and giant Jupiter, theAsteroid Belt was known to contain man y thousan ds ofsmall rocky bodies, nearly all less tha n a few tens ofmiles in diameter and possibly including untoldnumb ers of very small part icles. There were doubts thatspacecraft could safely cross this region. To d o so wasimportant to ou r f inding out more a bou t the oute rgiants, particularly so because to reach the more distantplanets requires full use of Earths orb ital velocityaroun d the S un an d a spacecraft t rajectory direct lythrough the heart of the asteroid bel t. Th e gravity andorbi tal motion of Jup iter are then used to urge a space-craft flying by th at planet to the high velocities neededto reach other m ore distant planets - a turn, Uranus ,and Neptune - ithin reasonable times and with rea-sona ble scientific payload s.The fi rst outer-planet target for the Pioneers wasJupiter, giant of the system and almost a proto star,with its retinue of giant satellites. Th e design challengesfor Pioneers 10 and 1 1 - recursors to later missionsby larger spacecraft - ere to explore the Jovian sys-tem and obtain the first closeup spin-scan images of theplanet and to investigate i ts enorm ous m agnetosphere,the composi t ion of the planets turbulent atmosphere,and i ts internal physical st ructure. R adio signals hadbeen traced as o riginat ing from Ju piter, implying that i thad a large magnetosphere with intense radiation belts;the spacecraft had to survive this radiat ion env ironment .Despite its enormous size, 11 t imes the diameter ofEarth, Jupi ter is the most rapidly rotat ing bod y in theSolar System. From Earth we see light and d ark ban dscrossing Jupiters disk, an enormo us Red S po t that hasbeen observed for centuries, and rapidly changingweather features. We recognized the presence of hydro-gen, hel ium, water vapor, am mon ia, and m ethane in theplanets atmosph ere, and we believed Jup iter t o be a gasgiant consisting predominantly of hydrogen in gaseous,l iquid, and metal lic forms.Th e successful enco unter of P ionee r 10 with Jupite r,and th e meeting of all its scientific ob jectives, p erm ittedPioneer 1 1 to be retargeted in flight to fl y by Jupiter

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ORIGINAL PAGFCQLOR PHOTOGRAPHin such a way that it was hurled on to intercept the m a-jestically ringed planet, Saturn. Pioneer l ls survivalattested to the outs tanding engineering and o perationalefforts that had gone into its design. The redirectionand acceleration of Pioneer I 1 toward S atur n requireda tightly curved passage from south to north around -Jupiter, enabling that second flyby to penetrate thedeepest ever into Jupiters radia tion belt and to p rovidean unparalleled perspective of the g iant planets polarregions. This retargeting was aimed toward a Saturnflyby that w ould exp lore the feasibility of Voyager latermaking a safe passage by S atur n on its way to Uranusand N eptune.Saturn, slightly smaller than Jupiter and nearly twiceas far from the Sun, is another giant consisting of somuch gas tha t its density is less than that of water. Th eplanet also has a retinue of satellites and a n enorm ousring system whose dynamics had intrigued mathemati-cians for centuries . Information a bou t Satu rn was evenmore sketchy than that abo ut Jupiter . I t , too, is a rapidrotator. It, too, has belts and spots. But these featuresare very much less distinct than those of Jupite r. Andbeyond Sa turn is the outer S olar System, s tretching bil-lions of miles into space; the realms of Uranus, Nep-tune, and Pluto, and of the solar wind blowing outtoward a boundary referred to as the heliopause wherethe influence of the Su n ends and a spacecraft travelingfast enough would e nter interstellar space and reach fo rthe s tars .

The planet Jupiter and it s Great Red Spot

Important discoveries were made by Pioneers 10 and11, discoveries with far-reachin g consequences forfuture space exploration and for ou r understanding ofthe Solar System and of vast magnetospheres contain-ing energetic plasmas that we cannot duplicate inlaboratories on E arth. F irs t , the myth of a hazardousasteroid belt was dispelled. The anticipated concentra-tion of small particles did not exist. Spacecraft couldsafely reach Jupite r an d use the Jovian gravitationals lingshot to hurtle them to more dis tant planets .

?planet Saturn and its majestic ring system

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. ORlGfNAL PAGECOLOR PHOTOGRAPH

COMPLETE SPECTRUMOF PROTONSAND ELECTRONS

WIDE SPECTRU3 LOWER ENERGY ELI\IMOFECTRONS

Diagram of the enormous magnetosphere of Jupi ter as revealed by the Pioneer 10an d 11 Spacecraft

RISINGATMOSPHERE

SOUTHPOLE

DIRECTION OF ROTATIONDiagram of the cloud bel ts and zones of Jupiter

Th e Pioneers m ade discoveries abo ut the d istribut ionof particles causing the Zodiacal Light and the Gegen-schein, glows in the night sky observed fr om Earth.They also mapped the backgro und of starl ight .At Jup iter, the Pioneers explored the g iant planetsmagnetosphere and found that i t is disk shaped andbigger than the S un i tself. Th e magnetic field of Jupiter,

10,000 times stronger than Earths field, is opposite indirection to E arths field, its dipole mo me nt is offsetand tilted so as to cause a wobbling of the huge mag-netosphere a s the planet spins on its axis.

CONCENTRATIONOF HIGH ENERGY

ELECTRONS

Th e Pioneers discovered a ring current within themagnetosphere and radiation belts whose trapped elec-trons have a n intensity 10,000 times Earths trappe delectrons, an d its proton s, 1000 times Earths. Jupitersmagnetosphere was also revealed as the source of high-energy electrons observed everywh ere that spacecrafthave traveled in the heliosphere.Th e ratio of hel ium to hydrogen in the Jov iana tmosphere was found t o be 0.14, fairly close to the0.1 I rat io of the Su n. Close-up pictures were obtainedof the belts and zones, of the Grea t Red Sp ot , and ofthe polar regions. A new understan ding was obtained ofthe weather pat terns on a giant , rapidly rotat ing planetwith no solid surface.The hea t ba lance was measured and show ed tha t Jup-iter emits 1.7 times the heat the planet receives from theSu n, indicating th at heat is still being generated in itsvast interior.At Saturn, Pioneer 1 1 found and explored ano thermagnetosphere and made equal ly impor tant discover-ies. The suspected presence of a m agnetic field was con -firmed. It h as a very small tilt and possesses the sam epolarity as the Jo vian field, with a dipole mom enta bou t 540 times that of Earths field. The m agneto-sphere contains corotat ing plasma a nd radiat ion bel tscomparable in intensity to Earths but much larger insize. Rings and satellites were foun d to a bs orb particlesand sweep them from the radiation belts.Close-up pictures obtained of the belts and zonesshowed them as narrower and m ore numerous thanJupiters. C yclonic spots were also recorded on theimages, and high-velocity zonal winds w ere discovered.The atmosphere has thicker clouds having less contrastthan those of Jupiter, presumably because Saturnsatm osp her e is colder because of its greater distancefrom the Sun.

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The heat balance was measured as a t Jupi ter a i d themeasurement showed that Sa turn emits 2.8 times theheat i t receives from the S un. O ne explanation is thathelium is gravitationally sepa rating within the atm o-sphere and the interior of S atu rn and is releasing energyas it falls toward a rocky core an d solidifies under theintense internal pressure.Th e vast ring system was imaged an d m ore detailswere revealed tha n ca n be seen from Earth . The r ingswere also observed from behind the p lanet to revealeven more structure when back-lighted. An extremelynarrow F ring was discovered, visible outside the o uter-most ring seen from Earth, and other extremely tenu-ous rings were detected by their a bsorp tion of particlesfrom the m agnetosphere. Before making its closestappro ach to the cloud tops, Pioneer showed th at aspacecraft could pe netra te the ring plane fairly close tothe planet without being dam aged. This was an essen-tial precursor t o later flybys of S atu rn by a largerspacecraft which would be deflected to Uranus.New small satellites of Sat ur n were discovered. T he

ORIGINAL PAGEPHOTOGRAPH

imaging photopolarim eter discovered one optically and Diagram of the magnetosphere of Saturn discovered by Pioneer I Iano the r was discovered by its distinctive sweepingeffects on charged particles. Thi s latter was the first dis-covery of a satellite by nono ptica l means.

A Pioneer spacecraft Nies by Saturn14


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The Pioneers Now AndIn The Future ORIGINAL PAGECOLOR PHOTOGRAPHAt the end of 1986, Pioneers 6, 7 , 8 , 10, and 11 , a n d

Pioneer 12, the Venus O rbiter, are all still operatingand g athering data . Pionee r 9's signal was lost in 1983,up t o which t ime, 15 years after laun ch, it still providedgood d ata. Pioneers 6 and 7 continue to gather solarwind and cosmic ray data inward and outward ofEarth's orbit, while Pioneer 8 provides data ab ou t elec-tric fields in space. Pioneers I O and 1 I , heading out ofthe Sola r System in almost op posite directions, are stillpioneering as they explore the remotest region s of theSolar System for the fi rst t ime in h uman history. Theyseek evidence of perturbation s from possible remo teplanets beyond Neptune and Pluto, and from gravi tywaves originating outside the S olar System. They con-tinue to m ap the heliosphere, observing the changes andcharacteristics of the solar wind, and a re graduallyaccum ulating data over a complete cycle of solar activ-it y in the far reaches of space.

These programs have produced a number of engi-neering and operation al highlights as well as majorscientific results. Special encoding of d ata permittedinformation to be t ransmit ted over enormo us inter-planetary distances. Power generators were designed toprovide lifetimes very much longer tha n hithertoachieved. The spacecraft were mad e magn etically cleanso they could measure low-level magnetic fields inspace. Entry probes were developed to pen etrate thehot , dense atmosphere of Venus and to sample thatatmosphere, sending information direct ly back toEarth. An adv anced tracking system arranged aroun dthe Pacific basin accurately followed the prob es to m ea-sure windspeeds in the atmosphere of the distant planet.An antenna despin drive kept an antenn a on the spin-ning Pioneer Venus spacecraft facing Earth for over 8years without appa ran t wear. Comm unicat ions weremaintained over many years and great distances withround-trip light time reaching hours for the outer SolarSystem spacecraft. Scientists and engineers dedicated adecade or m ore of their professional lives to teamskeeping in touch with these long-lived spacecraft.and accum ulat ing a wealth of information ab ou t theplanet and nearby space, gathering much inform ationabout how the solar wind interacts with a planet lack-ing an intrinsic magn etic field. In the past 2 years, Pio-neer 12 has made significant co ntribution s to the studyof comets by its obser vations of comets Enck e,Giacaboni-Zinner, and Halley. It provided entirely newestimates of the rates at w hich water is released fromcomet nuclei by solar heating, and spin-scan images ofthe hydrogen corona of Comet Halley when it was clos-est to the S u n (i.e., dur ing its perihelion passage). From

Pioneer 12 (Pioneer Venus) continues orbi t ing Venus

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QRlGlNAC PAGE.COLOR PHOTOGRAPH

I

EQUINOXE G Z L V L W WOV27NOV 27 \VENUSCLINATION /EARTHS

ORBIT

Halleys Comet, its path around the Sun in 1985186 and its close approach to PioneerVenus

the vantage of Venus orbit, this P ioneer spacecraft w asable to obta in informat ion about comets that could notbe obtained fro m a terrestrial viewpoint; in particular,i t provided an opportunity to observe Comet Halleywhen it was most active. These unique observations ofthree different com ets by one spacecraft benefitted fromusing the same instrument, a n im portant capability formaking co mparison s am ong comets . Pioneer V enus willhave the opportunity of observing two comets in 1987:Com et Wilson, a new comet discovered in August 1986,and Comet Encke, one of the most evolved cometsknown. Both will be observed for many weeks durin gthe comets most active periods. Together with theobservations of Halley, a middle-aged com et, thesemeasuremen ts will provide a unique set of cometportraits from youth to old age.

Comet Halley

The primary scientific objectives of the extended mis-sion of Pioneer Venus ar e to gain increased coverage inspace and time as the orb it around Venus changes. Th eincreased spatial coverage is unique to Pioneer V enussince most planetary spacecraft ar e locked in to oneorbit around their target planet. Initially, PioneerVenus orbit was held .in a controlled configuration but,since the spacecraft com pleted its low-altitude samplingof the upper atmosphere, the gravity of the Sun isgradually changing the orientation of the orbit an dproviding a dditio nal viewpoints for observing Venusand its near-space e nvironm ent.

24 hr ORBIT ATTITUDE

Ultraviolet image of Halleys Comet obtained by Pioneer Venus when the comet wasclose to perihelion

Changing Orientation of Pioneer orbiters path16


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ORIGINAL PAGETh e nose of the bow shock an d the near wake in thecavity behind the planet is where our understanding ofthe interaction of the solar wind with Venus can best beincreased. All regions of this interaction can be investi-gated dur ing the expected lifetime of Pioneer Venus.This extended mission will provide the first con tinuo usmeasurem ent of solar-wind interactions with a planetover a complete cycle of solar activity. Pioneer Venusalso describes concentrations of electrons and ions inthe ionosphere of Venus, their energy distribution, andtheir motions over the sola r cycle. Th e changing orb itwill permit in-depth studies of the nighttime ionosphere.Periodic mapping of cloud-top features permits long-term studies of weather patterns on Venus. We c anobserve the formation and decay of major cloud sys-tems, variations in the haze layers above them, an d thechanges to the amo unt of sulfur dioxide in the atmo-sphere of the planet. A possible volcanic eru ption push-ing clouds of sulfur dioxide into the atm osp here ofVenus just before the arrival of the Pioneer Orbiterthere in 1978 has im port ant imp lications regarding the

curr ent state of Venus a nd whether it is still volcanicallyactive.From such observations we look forward to gaining abetter und erstanding of th e photochemical and aerosolprocesses occurring at Venus, an d how the zona l circu-lation is generated - nd a better unde rstanding ofplanetary m eteorology in general.Early in 1992, Pioneer Venus will enter a final phaseof operations. Exhausted of the propellant needed tooffset solar gravitat ional influence upon the o rbital tra-jectory, the spacecraft will enter the atmosphere ofVenus and be destroyed. For a brief period there will besufficient propellant to keep the spacecraft orbitingalm ost as it was during the early pa rt of its mission.The important difference will be that the periapsis -the closest appro ach to Venus in each orbit - ill belocated in the southern, instead of the northern, hemi-sphere. Th e radar ca n be turned o n again to get mappingdat a abo ut regions closer to the so uth pole, and con-firm data fr om are as that were distant earlier in themission. In-situ measurements of the atmosphere can bema de at altitud es below 140 kilometers as the periapsisdescends lower into the atmosphere. The ionospherecan be sampled in the southern hemisphere. And whilethe nominal mission in 1978 corr esp ond ed with a min-

imu m of sol ar activity, the reentry phase will be close toa solar maximum.Information being gathered by Pioneers 10 and 1 1can be compa red with that gathered by the Voyagerspacecraft as they broach the outer regions of the SolarSystem an d in different directions fro m those of the Pio-neers. Th e magnetic field in the ou ter So lar System hasbeen found t o corre spon d fairly closely with a spiralfield model developed in the late 1950s. Th e effects ofsolar maxim um a nd minimum activit ies are beingobserved, with a minimum expected in 1987; also, theeffects of the recent 22-year cyclical reversal of the solarmagnetic field can be studied.

COLOR PHOTOGRAPH

MAGNETIC FIE1!llOLAR WIND

VENUS

I MAGNETOSHEATH

Diagram of interaction of the solar wmd with Venus

The magnetometer and charged-particle in strumenta-tion carried by the two spacecraft are recording the dis-tribution of cosmic ray particles and have revealed thatlow-energy particles are trap ped in a coro tating struc-ture, a kind of containment bottle. Four experimentsare investigating cosmic rays in the h eliosphere an dhow the various types and energies are affected by thesolar cycle.Most cosmic ray part icles from the Ga laxy areexcluded fro m the Solar System a t periods of solarmaxim um activity, and the anoma lous compon ent ofthe cosmic rays is extremely sensitive to the sola r cycle.Recovery of low-energy Galactic cosmic ray particles bythe instruments a bo ard Pioneer began in 1984 as thesolar activity declined. It is expected that the region ofthe heliosphere where m odu lation of the incoming cos-mic rays occurs is at a distance of between 40 and 90 AU.It is highly probable that by 1989 Pioneer 10 will bethe first spacecraft to reach the boundary of cosmic raymo dula tion. The question is whether there w ill be aclean shock between the heliosphere and interstellarspace, or if there will be only a turbule nt region. Manyof the experiments carried by Pioneer 10 are capable of

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.IOPAUSE

PIONEER10PMOCK FRONT

PIONEER 11 INTERSTELLAR MEDIUM

VOYAGER 2

rarns or me n one er spacecrarr rnrougn me neiiospnere ana out or me Solar system

VOYAGER 1

- - . . .

PIONEER 10

/ \ 1 yrVOYAGER 2

Relative paths of four spacecraft leaving the Solar System for intarstellar space

1

detecting this boundary. Finding out how cosmic raysenter our Solar System from the galaxy and how theyare modulated is important to designing experiments toseek anti-protons so as to gain a better understandingof the building blocks of matter and their origins.Th e Doppler tracking data a s the Pioneers are pass-ing beyond the previously know n limits of the Sola rSystem have been used t o check for gravity waves andfor perturbations that might result from trans-Plutonian planets or from a companion dark s tar . Sofar the results a re negative, implying that if there is aplanet beyond those presently known, or a dark s tarresponsible for dislodging comets and causing periodicextinction s of life from E arth, it must be extremely dis-tant a t present, possibly far out alon g an elliptical orb it.While past influences upon the orbital paths of Neptuneand Uranus app ear to have been real, a companiondar k s tar is ruled ou t by Pioneer 10 data a s being thecause of such perturbations.

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Th e concept of pioneering spacecraft has paid eno r-mo us dividends ov er the three decades of their activitiesand explor at ions beyond the Earth. Th ese relat ivelyinexpensive spacecraft have opened up the Solar Sys-tem to exp lorat ion by more complex m achines,machines to carry cost ly experiments that we could notwisely send into space before we knew the nature of theques t ions to be asked an d the n a ture of the da ta to besoug ht . The P ioneers provided the essential precursormissions. M oreov er, they have proved over the yearsthe value of relatively simple and highly reliable space-craft capable of pushing with minim um risk into newand unexplored frontiers. With Pioneer-type missionswe prepa re the way for further in-depth studies, moredetailed exploration, and hopefully increased wisdomconcerning how we as a species can harm onize with theprocesses of nature. In this way we may discov er new

ways to apply ou r increased knowledge and unders tand-ing of planets, their evolution, and their interactionswith the l ife-supporting Su n to solve cur rent hum anproblems affect ing the terrestrial environ men t and t oavoid inadvertent d egrada t ion of our futur e habitation.We should salute the people, past and present , of th ePioneer t eam and those who supported them. Thei refforts have revealed new perspectives of our Solar Sys-tem. These modern Pioneers have told us stories of dis-covery that wil l continue to stimulate o ur ima ginat ionrelat ive to the new frontiers of space explorat ion. Th eirefforts have presented hum ankind with an intriguingPando ras Box of discoveries, results, an d m ysteries,whose interpretation will challenge the technical andscient ific com mu nity and you ng students for m anyyears to come.

c

ORIGINAL PAGECOLOR PHOfOGRAPH

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The Pioneer SpacecraftName Launch Date Mission Status

Pioneer 1P i o n e e r 2Pioneer 3P i o n e e r 4P i o n e e r 5P i o n e e r 6P i o n e e r 7Pioneer 8Pioneer 9P i o n e e r EPioneer 10Pioneer 1 1P i o n e e r 12P i o n e e r 13

1 1 Oct.8 Nov.6 Dec.3 Mar .11 Mar .16 Dec .17 Aug.13 Dec.8 Nov.7 Aug.2 Mar .5 Apr.20 May8 Aug.

19581958195819591960196519661967196819691972197319781978

MoonMoonMoonMoonSolar orbitSolar orbitSolar orbitSolar orbitSolar orbitSolar orbitJupiterJupiterVenusVenus

72,765 mi. altitude only963 mi. altitude only63,580 mi. altitude only37,300 mi. from Moon, entered solar orbitEntered solar orbitStill ope ratingStill operatingStill operatingSignal lost in 1983Launch failureStill operating in outer Solar SystemContinued to Sa turn and s til l operating in outer S olar SystemStill operating in orbit around VenusSuccessful entry of four probes and b us into atmosphere of Venus

Pioneer FirstsFirst U . S . spacecraft to pass the M oon and enter solarorbit (Pioneer 4)First spacecraft to travel throug h the asteroid belt(Pioneer 1 1 )First spacecraft to measure integrated starlight from theGalaxy free of Zodiacal Light (Pioneer 10)Discovery that the G egenschein is not associated withEarth (Pioneer 10)First spacecraft to fly by Jupiter (P ioneer 10) andSa turn (Pioneer 11)First spacecraft images of the large Jovian satellites(Pioneer 10)Discovery of an ionosph ere on Jupiters satellite Io(Pioneer 10)Discovery of a hydrogen torus surrounding Jupiter(Pioneer 10)Discovery t ha t Satu rns rings- an d satellites sw eepparticles from the magnetosphere (Pieneer 11)Discovery of add itiona l rings and satellites of S atu rn(Pioneer 1 1 )First observatio n of back-lighted rings of Satu rn(Pioneer 11)Discovery of particles in the ring gaps seen from Ear th(Pioneer 1 1 )First image showing the featureless atmosp here ofTitan, Saturns largest satellite (Pionee r 11)

J

First polarization measurement of Titans atmosphereover a wide range of phase angles (Pioneer 11)First m apping of outer planet m agnetospheres (Pioneer10, Pionee r 11 )

Discovery of Saturns m agnetic field andmagnetosphere (Pionee r 1 1 )Mapping of corotating streams of particles from theSu n into the outer Solar System and identif ication ofinterplanetary acceleration processes (Pioneer 10 andPioneer 1 1 )Mapping of the tightly wound solar magne tic field anddiscovery of a warped current sheet (Pioneer 10 andPioneer 11)First detection of helium atom s entering Solar Systemfrom the Galaxy (Pioneer 10 and Pioneer 1 1 )First spacecraft to leave the known Solar System(Pioneer 10)First spacecraft to carry messages to extraterrestrials(Pioneer 10 and Pioneer 1 I )First simultaneous multiprobe entries of Venusatmosphere (Pioneer 13)Firs t topograph ic maps of cloud-shrouded Venusidentifying continental masses, high and low points,and general shape of the planet (Pioneer 12)First synoptic observations of cloud patterns on Venusover a Venus year (Pioneer 12)Discovery of polar collar and polar dipole feature onVenus (Pioneer 12)Observations of solar wind interactions with Venusover a complete solar cycle (Pio neer 12)First spacecraft observations of comet near perihelion(Pioneer 12)First observations of three comets by one spacecraft(Pioneer 12)

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Space Pioneers and Where They Are Now

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