DOCUMENT RESUME

ED 310 938 SE 050 884

AUTHOR Montoya, Earl J.; Fimmel, Richard 0.TITLE Space Pioneers and Where They Are Now.INSTITUTION National Aeronautics and Space Administration,

Mountain View, Calif. Ames Research Center.; NationalAeronautics and Space Administration, Washington, DC.Educational Affairs Div.

REPORT NO NASA-EP-264PUB DATE 87

NOTE 27p.; Colored photographs will not reproduce well.PUB TYPE Reports Descriptive (141)

EDRS PRICE MF01/PCO2 Plus Postage.DESCRIPTORS *Astronomy; Satellites (Aerospace); Science History;

*Science Materials; *Science Programs; ScientificResearch; *Space Exploration; *Space Sciences

IDENTIFIERS National Aeronautics and Space Administration;*Pioneer Program (NASA); *Solar System

ABSTRACTThis booklet describes the Pioneer Program and its

role in exploring the solar system. Sections include: (1) "Pioneersin Space to Understand Our Earth" (describing the background of theprogram); (2) "First Pioneers"; (3) "The Interplanetary Pioneers";(4) "Planetary Pioneers"; (5) "Outer Solar System Pioneers"; (6) "ThePioneers Now and In the Future"; (7) "The Pioneer Spacecraft"(listing the launch date, mission, and status of 13 Pioneers); and(8) "Pioneer Firsts" (describing the first discovery and performanceby the Pioneer Program). Many diagrams and photographs are provided.(YP)

****1:******************************************************************Reproductions supplied by EDRS are the best that can be made

from the original document.***********************************************************************

Space PioneersAnd Where They Are Now

EARL J. MONTOYAProgram ManagerHeadquarters, Washington D.C.

RICHARD 0. FIMMELManager, Pioneer MissionsAmes Research Center

Prepared at Ames Research Center

NASAEducational Affairs Division 1987National Aeronautics and Space Administration

Washington, D.C.

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NASA EP-264

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Table of Contents

Page

Pioneers In Space To Understand Our Earth

First Pioneers 3

The Interplanetary Pioneers 5

Planetary Pioneers 7

Outer Solar System Pioneers 11

The Pioneers Now and In The Future 15

The Pioneer Spacecraft 20

Pioneer Firsts 20

Facing Page

Pioneer 10 Spacecraft

111

5

Pioneers In SpaceTo UnderstandOur Earth

1

The great excitement of any age has been created bypioneers -- those who sought out new lands, new ideas,new social systems, new forms of governance, newtechnologies, and new goals for humankind.

In our time we have been privileged to witness anoutstanding human achievement of pioneers probingspace; the new frontier. This frontier initially beginswith the space environment beyond the fringes ofEarth's atmosphere, then extends into the spacebetween Earth and the orbit of the Moon, next into theinner Solar System, and with current exploration, intothe outer Solar System and beyond.

As sailing ships characterized Columbus pioneeringthe discovery of the New World, as the covered wagon"prairie schooners" characterized the exploration of theWest, we now have spacecraft to characterize the explo-ration of space frontiers. This modern explorationinvolved large team efforts of many people; includingclerks, secretaries, technicians, engineers, scientists, andmanagers. Engineers became deeply involved in thechallenging development of these highly reliable space-craft. New techniques for remote control of spacecraftoperations and for communicating over many millionsof miles of space were devised to achieve the objectives.Scientists devised new ways to make experiments atenormous distances from Earth.

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NView of Earth from space centered on California

Objectives of these pioneering activities have been tobroaden human knowledge of the environment of theSolar System of which our Earth is a member planet,and to try to understand more about how this environ-ment affects life on Earth, its past and its future. To doso, scientists needed to know how the Sun affects theenvironment in the space containing the planets (theinterplanetary environment), how energetic particlesprotons and electrons and radiation from the Sunspread through space to the planets, how the planetsare affected by these solar influences and what happensat different times during the 11-year solar activity cyclesand the occurrence of individual storms on the Sun, and

6

Terrestrial aurora as seen from Skylab

how the other planets are affected compared with Earth.Scientists also want to know how far the influence ofthe Sun extends into space as a heliosphere and howand where the interstellar environment the characterof space containing the stars becomes predominant.This is important to our understanding of now w e onEarth might be affected by events occurring in the gal-axy of stars and dust and gas clouds beyond our SolarSystem.

Four groups of pioneering spacecraft have made, andcontinue to make, significant progress in the explorationof the near space frontier and the achievement of theseobjectives. These spacecraft started with Pioneers 1through 5, which made the first thrusts into spacetoward the Moon and into interplanetary orbit. Theycontinued with Pioneers 6 through 9, which exploredinward and outward from Earth's orbit. The PioneerVenus Multiprobe (Pioneer 13) pushed to the incrediblyhot surface of Venus, the errant twin of Earth, and thePioneer Venus Orbiter (Pioneer 12) surveyed thatintriguing planet from orbit for many years. Pioneers10 and 11, which blazed a trail through the asteroid beltand were first to explore Jupiter, Saturn, and the outerSolar System, are currently seeking the borders of theheliosphere, and will ultimately journey to the distantstars.

Many of these Pioneers have operated far beyondthe period expected of them in their original conceptand design. The initial relatively small investments havebrought a priceless wealth of new information to ournational resource of basic knowledge. Several of thespacecraft can productively continue gathering moreimportant data for many years to come. All we need todo to harvest benefit from this continued pioneering isto keep watch over some of these spacecraft, direct their

attention periodically to new targets of opportunity,and receive and process the information being sent backto us.

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Comparative sizes of the plants and the Sun

'1.

First Pioneers

3

The first program to initiaily explore the "near" spacefrontier consisted of a group of Pioneer spacecraftinitiated by the Advanced Research Projects Agencyof the Department of Defense in 1958 when it autho-rized five launches toward the Moon. Three spacecraftwere the responsibility of the Air Force, and two theresponsibility of the Army. As part of the nation's con-tribution to the International Geophysical Year, theselunar probe Pioneers were intended to gather data frombeyond the fringes of Earth's atmosphere. Additionally,they were intended to develop a technology to reachescape velocity' from the gravitational pull of Earth andto navigate a spacecraft to the vicinity of another world

namely, the Moon as a preiuie to interplanetarymissions.

By 1958 a few Earth-orbiting spz cecraft had beensuccessfully launched. Sputniks 1 and 11 and Explorer.They showed that surrounding Ea:th is a very complexregion of magnetic fields and higli-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 and elec-tromagnetic fields interacting with each other and withthe planets in then poorly understood ways.

These Pioneers were launched with varying measuresof success. On October 11, 1958, Pioneer 1 attained adistance of 72,765 miles from Earth before falling backto Earth. A magnetometer carried by the spacecraftrevealed for the first time complex geomagnetic effectsthousa.ids of miles from Earth. 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 December 6, 1958, before falling back toEarth. Pioneer 4, launched on March 3, 1959, reachedescape velocity, passed within 37,300 miles of theMoon, and went into orbit around the Sun. Anotherlunar probe, Pioneer 5, was launched on March 11,1960, and entered an orbit around the Sun on a missionto explore space to the maximum range of communica-tions available at that time.

First Pioneers; top to bottom, Pioneer 1, Pioneer 4, and Pioneer 5

Pathway Into space

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All these spacecraft made important discoveriesabout the extent and form of the Earth's magneto-sphere, the region relatively close to Earth %%here theEarth's magnetic field deflects the solar wind's blizzardof electrons and protons and traps some of these ener-getic particles to create radiation belts. These dis-coveries led to intriguing new questions V6hich requireda more advanced type of spacecraft capable of exploringspace to considerable distances within and beyondEarth's orbit. There followed the Interplanetary Pio-neers 6 through 9, designed to explore the environmentof Earth's orbit, to investigate the solar wind and itseffects on Earth, to observe the interplanetary mediumcloser to, and further from, the Sun, and to define theimportant role of particles and fields in their interac-tions with the planets.

4 9

PROBE ORBIT

EARTH/MOONRELATIONSHIPAT FIRING TIME

EARTH ORBIT

The InterplanetaryPioneers

Five spacecraft were planned, all virtually identicaland spin-.;tabilized, and each designed to operate inspace for at least 180 days: i.e., approximately half anorbit around the Sun. Pioneer 6 left Earth successfullyon December 16, 1965, Pioneer 7 on August 17, 1966.Pioneer 8 on December 13, 1967 and Pioneer 9 onNovember 8, 1968. The fifth spacecraft was lost becauseof a Delta launch vehicle failure.

These Pioneers demonstrated the practicality of spin-ning the spacecraft to stabilize it and to simplify controlof its orientation. Measurements made by these space-craft greatly increased our knowledge of the interplane-tary environment and the effects of solar activity onEarth. New information was gathered about the solarwind, solar cosmic rays, the structure of the Sun'splasma and magnetic field.), the physics of particles inspace, and the nature of storms on the Sun which pro-duce solar flares. Important incidental results were amore accurate measurement of the mean distance ofEarth from the Sun, referred to as the astronomicalunit (AU), and more accurate measurement of themasses of the Earth and Moon. Additionally. thespacecraft demonstrated high reliability by operatingsuccessfully for many years beyond their designlifetime.

Pioneers 6 and 9 traveled along orbits which carriedthem inward to a distance of 0.8 AU from the Sun.Pioneers 7 and 8 moved away from the Sun to a dis-tance of 1.1 AU, to explore the region of the Earth's

magnetotail and wake, streaming like a pennant awa)from the Sun. When Pioneers 7 and 8 explored themagnetic tail of the Earth, they discovered well-orderedcharacteristics but with discontinuities at great distancesfrom Earth.

At the beginning of the space age there had been abelief that cosmic dust could present a serious hazard tospacecraft. The Pioneers proved this fear to be unwar-ranted. Also, these spacecraft recorded the numbers ofdifferent types of cosmic rays and showed how theyvary with the solar cycle.

The structure of the interplanetary magnetic field wasdiscovered, pulses of solar protons from the outburst offlares on the Sun were recorded, and short-termchanges in the Earth's magnetic field in space were dis-covered. The 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.

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Before Pioneer 6. few spacecraft were capable ofmaking detailed measurements of the solar wind. ThePioneers discovered an east-west as)mmetr) of thesolar wind. The wind was described as spiraling fromthe Sun. analogous to a rotating water sprinkler. ThePioneers were first to show that its direction and speedcould be recorded and studied.

Radio signals sent from Earth to the spacecraftchecked the propagation of radio waves through spaceand how this propagation is affected b), storms on theSun. Detailed studies were made on the plasma cloudsejected from the Sun by solar flares. The shapes andextent of plasma clouds passing b!, the Earth weredetermined. Passage of radio waxes through the solarcorona was also investigated for the first time as Pio-neer spacecraft were occulted (passed behind the Sun).

Interplanotety Pioneer 6.9 spacecraft

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Planetary Pioneers

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Planet Vonus imaged from Pionear Vonus

In size Venus is the closest planet to being Earth'stwin; out it is an errant twin that has evolved into avery inimical world from the standpoint of livingthings. The study of Venus has deep implications forunderstanding our own planet. We need to know whyand how Earth inoked benignly from the primordialsolar nebula for life to inolve upon it and continue tobe supported by it. With the growing awareness oflong -term environmental effects on Earth, scientistswanted to know much more about Venus and its mean-ings for Earth, why Venus is so different, why it shouldhave such a strange atmosphere 100 times as denseas Earth's that shields the planet's surface fromdirect Earth-based observation at wavelengths of visiblelight, why its surface is hot enough to melt lead, andwhy the planet lacks significant amounts 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 Pioneer 13multiprobe spacecraft dispatched heat-resisting probesto penetrate the atmosphere and radio back to Earth in-situ measurements from the upper atmosphere, through

Topographic mop of Venus obtained by Pioneer Venus Orbiter. Pioneer 12

12

Artars rendering of Pioneer probe entering the atmosphere of Venus

the dense clouds, and down to the 900° F surface. AnOrbiter (Pioneer 12) circles Venus to this day and hasmapped its hidden surface by radar, imaged its chang-ing cloud systems, explored its unique magnetic envi-ronment, and observed the interactions of the solarwind with a planet that has no intrinsic magnetic field.

The four probes plunged into the atmosphere atwidely separated locations in day and night hemi-spheres. They measured temperature, pressure, and den-sity down to the planet's surface. They discovered diur-nal changes in the upper atmosphere and found that athigh altitudes Venus' atmosphere is cooler and at lowaltitudes much hotter than Earth's. The temperaturebelow the clouds was found to be relatively constant ata given height everywhere on the planet. Instead of tur-bulence, unexpectedly stable atmospheric layers werefound below the clouds.

The probes charted vertical winds and determined thechemical composition of the atmosphere. It is mainly.

carbon dioxide and nitrogen but, unlike Earth, it hasvery little oxygen or water vap'r. Rare atmosphericgases and their isotopes were detected and their concen-trations were measured. Unexpectedly, there were 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 Earth's oceans, which suggests thatthere has been a preferential escape of a large amountof hydrogen from Venus. This is evidence that Venusprobably had a large quantity of water, perhaps asmuch as an ocean, sometime during its history. Theocean may have been lost because a "greenhouse" effecttrapped solar heat in Venus' atmosphere 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 Venue atmosphere explored by spacecraft

The Orbiter's radar provided altimetry maps fornearly all of the surface of Venus, resolving featuresdoss n to about 50 miles across. On these maps, scientistsidentified olcanoes that may still be active, continentalareas with great plains and mountain ranges in whichone peak rises 35,500 feet, island masses rising fromglobal plains; rift valleys such as Diana Chasma, whichis 9,500 feet deep; some craters, and extensive areas ofglobal plains with relatively flat and monotonous sur-faces. Key features on Venus have been and will con-tinue to be named after w omen, e.g., Aphrodite, Frey ja,Asteria, Atla, Ishtar, Atalanta, Phoebe, Themis, andLeda. Venus was found to be a planet that is closer tobeing a sphere than are Earth and other planets, asmight be expected from its slow rotation, which is 242times slower than Earth's.

Pioneer Venus' radar altimetry data were used toselect targets for subsequent U.S.S.R. Venus probes.Important international cooperation has been enhancedby exchanges of information about Venus.

Sensitive and detailed measurements of spacecraftaccelerations in orbit show that mountainous forma-tions are less dense than the global crust, and that, ason Earth, they appear to be supported on a liquidinterior.

Movements of masses of the atmosphere, charted inobservations made from orbit, resealed global winds of220 mph at the cloud tops. These winds generally blowwestward, with virtually no motions north or south.

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Diagram of global winds in the atmosphere of Venus as charted by Pioneerspacecraft

Electrical signals from Venus detected by Pioneer arepossibly caused by lightning flashes in gas clouds aris-ing from active olcanoes. If these emissions are not

used by lightning in such clouds they present a prob-lem because there is no other terrestrial counterpart toexplain them.

14

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The Orbiter travels in an elliptical orbit around theplanet. This enables its instruments to san ale %ariousregions of the planet's ionosphere (the region ofcharged particles in the upper atmosphere) which is athinner ionosphere than that of Earth. Venus has nomagnetic field to shield it from the solar wind, but thewind itself generates a magnetic field.

The induced magnetic field deflects the solar windand gives rise to a bow shock and an ionopause (whichhas no terrestrial counterpart). The ionopause is theboundary between the ionosphere and the solar wind.The spacecraft showed that the ionopause exists on the

night side as well as the day side of the planet and itidentified and mapped specific ionospheric regions. anionosheath, a mantle, and a wake. Changes in the ion-osphere and the particles and field ernironment sur-rounding Venus were recorded as solar actRity, and thestrength of the solar wind changed during the solarcycle. A .;%% understanding of how a planet without amagnetic field interacts with the solar wind is beingde%eloped, and contributions to the possible explana-tions for some of the differences between Earth andVenus are being made.

1510

Outer SolarSystem Pioneers

I I

Scientists gave high priority to extending the explora-tion of the "outer" space frontiers. Early in the 1970sscientific descriptions of the outer parts of the SolarSystem still differed enormously because details werelacking from observations made from Earth. Therewere many unknowns about the far reaches of inter-planetary space beyond the orbit of Mars and about thevery large and distant planets of the outer Solar Systemin which most of the matter and the angular momentumof the total system are concentrated.

Between the orbits of Mars and giant Jupiter, theAsteroid Belt was known to contain many thousands ofsmall rocky bodies, nearly all less than a few tens ofmiles in diameter and possibly including untoldnumbers of very small particLs. There were doubts thatspacecraft could safely cross this region. To do so wasimportant to our finding out more about the outergiants, particularly so because to mach the more distantplanets requires full use of Earth's orbital velocityaround the Sun and a spacecraft trajectory directlythrough the heart of the asteroid belt. The gravity andorbital motion of Jupiter are then used to urge a space-craft flying by that planet to the high velocities neededto reach other more distant planets Saturn, Uranus,and Neptune within reasonable times and with rea-sonable scientific payloads.

The first outer-planet target for the Pioneers wasJupiter, giant of the system and almost a protostar,with its retinue of giant satellites. The design challengesfor Pioneers 10 and 11 precursors to later missionsby larger spacecraft were to explore the Jovian sys-tem and obtain the first closeup spin-scan images of theplanet and to investigate its enormous magnetosphere,the composition of the planet's turbulent atmosphere,and its internal physical structure. Radio signals hadbeen traced as originating from Jupiter, implying that ithad a large magnetosphere with intense radiation belts,the spacecraft had to survive this radiation environment.

Despite its enormous size, 11 times the diameter ofEarth, Jupiter is the most rapidly rotating body in theSolar System. From Earth we see light and dark bandscrossing Jupiter's disk, an enormous Red Spot that hasbeen observed for centuries, and rapidly changingweather features. We recognized the presence of hydro-gen, helium, water vapor, ammonia, and methane in theplanet's atmosphere, and we believed Jupiter to be a gasgiant consisting predominantly of hydrogen in gaseous,liquid, and metallic forms.

The successful encounter of Pioneer 10 with Jupiter,and the meeting of all its scientific objectives, permittedPioneer 11 to be retargeted in flight to fly by Jupiter

16

in such a way that it was hurled on to intercept the ma-jestically ringed planet, Saturn. Pioneer I l's survivalattested to the outstanding engineering and operationalefforts that had gone into its design. The redirectionand acceleration of Pioneer II toward Saturn requireda tightly curved passage from south to north aroundJupiter, enabling that second flyby to penetrate thedeepest ever into Jupiter's radiation belt and to providean unparalleled perspective of the giant planet's polarregions. This retargeting was aimed toward a Saturnflyby that would explore the feasibility of Voyager latermaking a safe passage by Saturn on its way to Uranusand Neptune.

Saturn, slightly smaller than Jupiter and nearly twiceas far from the Sun, is another giant consisting of somuch gas that its density is less than that of water. Theplanet also has a retinue of satellites and an enormousring system whose dynamics had intrigued mathemati-cians for centuries. Information about Saturn was evenmore sketchy than that about Jupiter. It, too, is a rapidrotator. It, too, has belts and spots. But these featuresare very much less distinct than those of Jupiter. Andbeyond Saturn is the outer Solar System, stretching 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 Sun ends and a spacecraft travelingfast enough would enter interstellar space and reach forthe stars.

The planet Saturn and Its majestic ringsystem

The planet Jupiter and its Great Red Spot

Important discoveries were made by Pioneers 10 and11, discoveries with far-reaching consequences forfuture space exploration and for our understanding ofthe Solar System and of vast magnetospheres contain-ing energetic plasmas that we cannot duplicate inlaboratories on Earth. First, the myth of a hazardousasteroid belt was dispelled. The anticipated concentra-tion of small particles did not exist. Spacecraft couldsafely reach Jupiter and use the Jovian gravitationalslingshot to hurtle them to more distant planets.

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Diagram of the enormous magnetosphere of Jupiter as revealed by the Pioneer 10 and 11 spacecraft

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Diagram of the cloud belts and zones of Jupiter

The Pioneers made discos cries about the distributionof particles causing the Zodiacal Light and the Gegen-schein, glows in the night sky observed from Earth.They also mapped the background of starlight.

At Jupiter, the Pioneers explored the giant planet'smagnetosphere and found that it is disk shaped andbigger than the Sun itself. The magnetic field of Jupiter,10,000 times stronger than Earth's field, is opposite indirection to Earth's field, its dipole moment is offsetand tilted so as to cause a wobbling of the huge mag-netosphere as the planet spins on its axis.

13

CONCENTRATIONOF HIGH ENERGY

ELECTRONS

The Pioneers discovered a ring current within themagnetosphere and radiation belts whose trapped elec-trons have an intensity 10,000 times Earth's trappedelectrons, and its protons, 1000 times Earth's. Jupiter'smagnetosphere was also revealed as the source of high-energy electrons observed everywhere that spacecrafthave traveled in the heliosphere.

The ratio of helium to hydrogen in the Jovianatmosphere was found to be 0.14, fairly close to the0.11 ratio of the Sun. Close-up pictures were obtainedof the belts and zones, of the Great Red Spot, and ofthe polar regions. A new understanding was obtained ofthe weather patterns on a giant, rapidly rotating planetwith no solid surface.

The heat balance was measured and showed that Jup-iter emits 1.7 times the heat the planet receives from theSun, indicating that heat is still being generated in itsvast interior.

At Saturn, ?ioneer 11 found and explored anothermagnetosphere and made equally important discover-ies. The suspected presence of a magnetic field was con-firmed. It has a very small tilt and possesses the samepolarity as the Jovian field, with a dipole momentabout 540 times that of Earth's field. The magneto-sphere contains corotating plasma and radiation beltscomparable in intensity to Earth's but much larger insize. Rings and satellites were found to absorb particlesand sweep them from the radiation belts.

Close-up pictures obtained of the belts and zonesshowed them as narrower and more numerous thanJupiter's. Cyclonic spots were also recorded on theimages, and high-velocity zonal winds were discovered.The atmosphere has thicker clouds having less contrastthan those of Jupiter, presumably because Saturn'satmosphere is colder because of its greater distancefrom the Sun.

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The heat balance was measured as at Jupiter and themeasurement showed that Saturn emits 2.8 times theheat it receives from the Sun. One explanation is thathelium is gravitationally separating within the atmo-sphere and the interior of Saturn and is releasing energyas it falls toward a rocky core and solidifies under theintense internal pressure.

The vast ring system was imaged and more detailswere revealed than can be seen from Earth. The ringswere also observed from behind the planet to revealeNen more structure when back-lighted. An extremel}narrow F ring was discovered, visible outside the outer-most ring seen from Earth, and other extremely tenu-ous rings were detected by their absorption of particlesfrom the magnetosphere. Before making its closestapproach to the cloud tops, Pioneer showed that aspacecraft could penetrate the ring plane fairly dose tothe planet without being damaged. This was an essen-tial precursor to later flybys of Saturn by a largerspacecraft which would be deflected to Uranus.

New small satellites of Saturn were discovered. Theimaging photopolarimeter discovered one opticall} andanother was discovered by its distinctive sweepingeffects on charged particles. This latter was the first dis-covery of a satellite by nonoptical means.

A Pioneer spacecraft files by Saturn

ag, am or the magnetosphere of Saturn discovered by Pioneer 11

1 14 ! 9

The Pioneers Now AndIn The Future

At the end of 1986, Pioneers 6, 7, 8, 10, and 11, andPioneer 12, the Venus Orbiter, are all still operatingand gathering data. Pioneer 9's signal was lost in 1983,up to which time, 15 years after launch, it still providedgood data. Pioneers 6 and 7 continue to gather solarwind and cosmic ray data inward and outward ofEarth's orbit, while Pioneer 8 provides data about elec-tric fields in space. Pioneers 10 and 11, heading out ofthe Solar System in almost opposite directions, are stillpioneering as they explore the remotest regions of theSolar System for the first time in human history. Theyseek evidence of perturbations from possible remoteplanets beyond Neptune and Pluto, and from gravitywaves originating outside the Solar System. They con-tirrte to map the heliosphere, observing the changes andcharacteristics of the solar wind, and are graduallyaccumulating data over a complete cycle of solar activ-ity in the far reaches of space.

These programs have produced a number of engi-neering and operational highlights as well as majorscientific results. Special encoding of data permittedinformation t, be transmitted over enormous inter-planetary distances. Power generators were designed toprovide lifetimes very much longer than hithertoachieved. The spacecraft were made magnetically cleanso they could measure low-level magnetic fields inspace. Entry probes were developed to penetrate thehot, dense atmosphere of Venus and to sample thatatmosphere, sending information directly back toEarth. An advanced tracking system arranged aroundthe Pacific basin accurately followed the probes to mea-sure windspeeds in the atmosphere of the distant planet.An antenna despin drive kept an antenna on the spin-ning Pioneer Venus spacecraft facing Earth for over 8years without apparant wear. Communications weremaintained over many years and great distances withround-trip light time reaching hours for the outer SolarSystem spacecraft. Scientists and engineers dedicated adecade or more of their professional lives to teamskeeping in touch with these long-lived spacecraft.

Pioneer 12 (Pioneer Venus) continues orbiting Venusand accumulating a wealth of information about theplanet and nearby space, gathering much informationabout how the solar wind interacts with a planet lack-ing an intrinsic magnetic field. In the past 2 years, Pio-neer 12 has made significant contributions to the studyof comets by its observations of comets Encke,Giacaboni-Zinner, and Halley. It provided entirely newestimates of the rates at which 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 Sun (i.e., during its perihelion passage). From

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Halley Comet, its path around the Sun in 1985,86 and its close approach to PioneerVenus

the vantage of Venus' orbit, this Pioneer spacecraft wasable to obtain information about comets that could notbe obtained from a terrestrial viewpoint; in particular,it provided an opportunity to observe Comet Halleywhen it was most active. These unique observations ofthree different cumets by one spacecraft benefitted fromusing the same instrument, an important capability furmaking comparisons among comets. Pioneer Venus willhave the opportunity of observing two comets in 1987.Comet Wilson, a new comet discovered in August 1986,and Comet Encke, one of the most evolved cometsknown. Both will be observed for many weeks duringthe comets' most active periods. Together with theobservations of Halley, a "middle-aged" comet, thesemeasurements will provide a unique set of cometportraits from youth to old age.

Ultraviolet Image of Halley Comet obtained by Pioneer Venus when the comet wasclose to perihelion

k 16

Comet Halley

The primary scientific objectives of the extended mis-sion of Pioneer Venus are to gain increased coverage inspace and time as the orbit around Venus changes. Theincreased spatial coverage is unique to Pioneer Venussince most planetary spacecraft are locked into oneorbit around their target planet. Initially, PioneerVenus' orbit was held .in a controlled configuration but,since the spacecraft completed its low-altitude samplingof the upper atmosphere, the gravity of the Sun isgradually changing the orientation of the orbit andproviding additional viewpoints for observing Venusand its near-space environment.

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1878.80

ORBITATTITUDE

The nose of the bow shock and 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 during the expected li:-time of Pioneer Venus.This extended mission will pi ovide the first continuousmeasurement 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 solar cycle. The changing orbitwill permit in-depth studies of the nighttime ionosphere.

Periodic mapping of cloud-top features permits long-term studies of weather patterns on Venus. We canobserve the formation and decay of major cloud sys-tems, variations in the haze layers above them, and thechanges to the amount of sulfur dioxide in the atmo-sphere of the planet. A possible volcanic eruption push-ing clouds of sulfur dioxide into the atmosphere ofVenus just before the arrival of the Pioneer Orbiterthere in 1978 has important implications regarding thecurrent state of Venus and whether it is still volcanicallyactive.

From such observations we look forward to gaining abetter understanding of the photochemical and aerosolprocesses occurring at Venus, and how the zonal circu-lation is generated and a better understanding ofplanetary meteorology in general.

Early in 1992, Pioneer Venus will enter a final phaseof operations. Exhausted of the propellant needed tooffset solar gravitational influence upon the orbital tra-jectory, the spacecraft will enter the atmosphere ofVenus and be destroyed. For a brief period there will besufficient propellant to keep the spacecraft orbitingalmost as it was during the early part of its mission.The important difference will be that the periapsisthe closest approach to Venus in each orbit will belocated in the southern, instead of the northern, hemi-sphere. The radar can be turned on again to get mappingdata about regions closer to the south pole, and Lon-firm data from areas that were distant earlier in themission. In -situ measurements of the atmosphere can bemade at altitudes below 140 kilometers as the periapsisdescends lower into the atmosphere. The ionospherecan be sampled in the southern hemisphere. And w hilethe nominal mission in 1978 corresponded with a min-imum of solar activity, the reentry phase will be dose toa solar maximum.

Information being gathered by Pioneers 10 and 11can be compared with that gathered by the Voyagerspacecraft as they broach the outer regions of the SolarSystem and in different directions from those of the Pio-neers. The magnetic field in the outer Solar System hasbeen found to correspond fairly closely with a spiralfield model developed in the late 1950s. The effects ofsolar maximum and minimum activities are beingobserved, with a minimum expected in 1987, also, theeffects of the recent 22-year cyclical reNersal of the solarmagnetic field can be studied.

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BOW SHOCK

INTERPLANETARYMAGNETIC FIELD

SOLAR WIND

MAGNETOSHEATH IONOSPHERE

Diagram of interaction of the sola wind with Venus

The magnetometer and charged particle instrumenta-tion carried by the two spacecraft are recording the dis-tribution of Losmic ray particles and have reealed thatkm-energy particles are trapped in d corotating struc-ture, a kind of containment bottle. Four experimentsare inestigating Losmic rays in the heliosphere andhom, the carious types and energieb are affected by thesolar cycle.

Most cosmic ray particles from the Galaxy areexcluded frem the Solar System at periods of solarmaximum activity, end the anomalous component ofthe cosmic rays is extremely sensitive to the solar cycle.Recoery of low-energy Galactic cosmic ray particles bythe instruments aboard Pioneer began in 1984 as thesolar activity declined. It is expected that the region ofthe heliosphere where modulation 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 cosmii, raymodulation. The question is whether there will be aclean shock between the heliosphere and interstellarspace, or if :here will be only a turbulent region. Manyof the experiments carried by Pioneer 10 are capable of

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POSSIBLEBOW SHOCK (?)

VOYAGER 1

.INTERSTELLARMEDIUM

HELIOPAUSE

SHOCK FRONT

c2 AHELIOSHEATH

MAGNETIC f /'-----FIELD LINES

.----..._----,-_

PIONEER 11

VOYAGER 2

INTERSTELLAR MEDIUM

s' INTERSTELLAR SUPERSONIC TURBULENTWIND :s-' SOLAR WIND FLOW

Paths of the Pioneer spacecraft through the hehosphere and out of the Solar System

VOYAGER 1

Relative paths of four spacecraft leaving the Solar System for interstellar spaco

P38

PIONEER 10

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.

The Doppler tracking data as the Pioneers are pass-ing beyond the previously known limits of the SolarSystem have been used to check for gravity waves andfor perturbations that might result from trans-Plutonian planets or from a companion dark star. Sofar the results are negative, implying that if there is aplanet beyond those presently known, or a dark starresponsible for dislodging comets and causing periodicextinctions of life from Earth, it must be extremely dis-tant at present, possibly far out along an elliptical orbitWhile past influences upon the orbital paths of Neptuneand Uranus appear to have been real, a companiondark star is ruled out by Pioneer 10 data as being thecause of such perturbations.

The concept of pioneering spacecraft has paid enor-mous dividends over the three decades of their actiitiesand explorations beyond the Earth. These relativelyinexpensive spacecraft have opened up the Solar Sys-tem to exploration by more complex machines,machines to carry costly experiments that we could nu,wisely send into space before we knew the nature of thequestions to be asked and the nature of the data to besought. The Pioneers provided the essential precursormissions. Moreover, they have proved over the yearsthe value of relatively simple and highly reliable space-craft capable of pushing with minimum risk into newand unexplored frontiers. With Pioneer -type missionswe prepare the way for further in -depth studies, moredetailed exploration, and hopefully increased wisdomconcerning how we as a species can harmonize with theprocesses of nature. In this way we may discover new

as to apply our increased knowledge and understand-ing of planets, their eolution, and their interactionswith the life-supporting Sun to solve current humanprobleath affecting the terrestrial emironment and toa old inadvertent degradation of our future habitation.

We should salute the people, past and present, of thePioneer team and those who supported them. Theirefforts ha c revealed new perspectives of our Solar Sys-tem. The modern Pioneers have told us stories of dis-eovery that will continue to stimulate our imaginationrelative to the new frontiers of space exploration. Theirefforts have presented humankind with an intriguing"Pandora's Box" of distAncries, results, and mysteries,whose interpretation will challenge the technical andscientific community And young students for manyyears to come.

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19

The Pioneer SpacecraftName I aunch Date Mission Status

Pioneer 1 11 Oct. 1958 Moon 72,765 mi. altitude onlyPioneer 2 8 Nov. 1958 Moon 963 mi, altitude onlyPioneer 3 6 Dec. 1958 Moon 63,580 mi. altitude onlyPioneer 4 3 Mar. 1959 Moon 37300 mi. from Moon, entered solar orbitPioneer 5 11 Mar. 1960 Solar orbit Entered solar orbitPioneer 6 16 Dec. 1965 Solar orbit Still operatingPioneer 7 17 Aug. 1966 Solar orbit Still operatingPioneer 8 13 Dec. 1967 Solar orbit Still operatingPioneer 9 8 Nov. 1968 Solar orbit Signal lost in 1983Pioneer E 7 Aug. 1969 Solar orbit Launch failurePioneer 10 2 Mar. 1972 Jupiter Still operating in outer Solar SystemPioneer 11 5 Apr. 1973 Jupiter Continued to Saturn and still operating in outer Solar SystemPioneer 12 20 May 1978 Venus Still operating in orbit around VenusPioneer 13 8 Aug. 1978 Venus Successful entry of four probes and bus into atmosphere of Venus

Pioneer FirstsFirst (:.S. spacecraft to pass the Moon and enter solarorbii (Pioneer 4)

First spacecraft to travel through the asteroid bk.:,(Pioneer 11)

First spacecraft to measure integrated starlight from theGalaxy free of Zodiacal Light (Pioneer 10)

Discovery that the Gegenschein is not associated withEarth (Pioneer 10)

First spacecraft to fly by Jupiter (Pioneer 10) andSaturn (Pioneer 11)

First spacecraft images of the large Jovian satellites(Pioneer 10)

Discovery of an ionosphere on Jupiter's satellite lo(Pioneer 10)

Discovery of a hydrogen torus surrounding Jupiter(Pioneer 10)

Discovery that Saturn's rings and satellites sweepparticles from the magnetosphere (Pioneer II

Discovery of additional rings and satel:ites of Saturn(Pioneer 11)

First observation of back-lighted rings of Saturn(Pioneer 11)

Discovery of particles in the ring gaps seen from Earth(Pioneer II)

First image showing the featureless atmosphere ofTitan, Saturn's largest satellite (Pioneer 11)

First polarization measurement of Titan's atmosphereover a wide range of phase angles (Pioneer 11)

First mapping of outer planet magnetospheres (Pioneer10, Pioneer 11)

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Discovery of Saturn's magnetic field andmagnetosphere (Pioneer 11)

Mapping of corotating streams of particles from theSun into the outer Solar System and identification ofinto- planetary acceleration processes (Pioneer 10 andPioneer I I )

Mapping of the tightl!, %%round Nola magnetic field anddiscovery of a warped current sheet (Pioneer 10 andPioneer 11)

First detection of helium atoms entering Solar Systemfrom the Galaxy (Pioneer 10 and Pioneer 111

First spacecraft to leave the known Solar System(Pioneer 10)

First spacecraft to carry messages to extraterrestrials(Pioneer 16 and Pioneer 11)

First simu'taneous multiprobe entries of Venus'atmosphere (Pioneer 13)

First topographic maps of cloud-shrouded Venusidentifying continental masses, high and low points.and general shape of the planet (Pioneer 2)

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 (Pioneer 12)

First spacecraft observations of comet near perihelion(Pioneer 12)

First observations of three comets by one spacecraft(Pioneer 12)

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M AC_ ALtaw Ili.avNational Aeronautics andSpace Administration

Ames Research Center27