Planetary Exploration Space in the Seventies

8/8/2019 Planetary Exploration Space in the Seventies

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PLANETARYEXPLORATIONSpace in the Seventies

National Aeronautics and Space Administration


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SPACE IN TH E SEVENTIES

Man has walked on the Moon, made scientificobservations there, and brought back to Earthsamples of the lunar surface,

Unmanned scientific spacecraft have probed ffacts abo ut matter, riadiatian and m agnetism inspace, and have collected data relating to theMoon, Venus, Mars, the Sun and some of the stars,and reported their findings to ground stationson Earth,

Spacecraft have been put into orbit around theEarth as weather observation stations, ascommunications relay statians for a world-widetelephone and television network, and as aids tonavigation,

In addition, the space program has acceleratedthe advance of technology for science and industrcontributing many new ideas, processes andma aria s.All this took place in The decade of the Sixties.

What ne xt? What may be expected of spaceexploration in the Seventies?

NASA has prepa~ed series of p ~ b li ~ a ti o n sndmotion pictures to provide a l ~ o korward toSPACE IN THE SEVENTIES. The topics coveradthis series iaclude: Earth orbital s~ ien ce ; lanetarexploratiqn; prac tical applications of satelliteo;technology utilization; manin space; an daeronautics. SPACE IN THE SEYENTIES presentsthe planned programs of NASA for the comingdecade.

I

June, 1971

COVER: A concept fo r a spacecraft to explore th e outer solar system i s th eThermoelectric Outer Planets Spacecraft (TOPS). (See page 23)

ONational Geographic Society


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NETARY

XPLORATION


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TABLE OF CONTENTS

4 INTRODUCTION4 A STRATEGY FOR PLANETARY EXPLORATION5 ORBITING THE RED PLANET-MARINER-MARS 71

11 MACHINES ON MARS-VIKING 75

TRAILBLAZING BEYOND MARS-

PIONEER-JUPITER 72, 7322 OUTER PLANETS MISSION

24 AN INNER TOUR-MARINER-MERCURYJVENUS 726 CLOSEST TO THE SUN-HELIOS 74, 7528 IN THE DECADE OF THE 19703


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Seven was a mystical nu mber to the ancients.From Babylon to Alexandria, astronomer-priestssaw only seven points of light wandering acrossthe fie ld of fixed stars. The so-called "Sacred Seven"became a very unm ystical eight when Neptune wasdiscovered through the telescope by J. G. Gallein 1846. Now we know that there are at leastnine planets, with a very slight chance that one ortwo more small ones are swinging slowly,undetected as yet, around the Sun at the fringesof th e solar system.

Planet discovery was a passion with astronomersduring the 1800s and early i n this century; bu ttoday they want close-up pictures of the nineplanets we already know, followed by analyses oftheir atmospheres and surfaces. In short, modernastronomers want to dissect and analyze the otherplanets just as they have dissected and analyzedthe ~ a r t hver the past centuries. Whereas h l e m y ,Tycho Brahe, Kepler, and the other astronomicalpioneers devoted their lives to describing planetarymotion accurately, the o bjectives of mod ernplanetary exploration are to:

1. Reconstruct accurately the origin and evolutionof the nine planets, the asterolids, the comets,and the interplanetary medium.

2. Recount accurately the origin and evolution oflife within the solar system.3. Apply new-found knowledge of the other

planets to the Earth so that we can u nderstandit better.

The objectives of planetary explorationoriginate in our curiosity about the heavens and lifein general. I t is one portion of NASA's programthat cannot be evaluated readily in dollars-and-centsterms. What would it be worth, fo r example, todiscover extraterrestrial life? Despite the profound

1 effect this discovery would have upon olur outlookand concept of the universe, it transcends ourcommon scheme of values. Economicjustificationofplanetary exploration,is like trying to justify thepainting of the Mona Lisa or th e Curies1 discovery

1 of radium. Both are priceless, ye t both cost money.NASA's goal is to explore the planets, aiming a t

I targets with high potential scientific payoff withthe least consumption of national resources.

A STRATEGY FOR PLANETARY EXPLORATIONThrough the telescope, the planets are fuzzy dis

of light with many details swimming tantalizingjust out of reach. The space program, however,has provided science with instrument carriersthat can take close-up looks with N ameras,thermometers, life de tectors, and otherinstruments. Already NASA's Mariner planetaryfly-bys have revamped many of our ideas aboutMars and Venus. The surface of Venus is an 80O(427C.) nferno instead of a primitive, expectant"twin" of the Earth as astronomers once thoughtMars is pocked with craters and possesses a highvaried terrain. The planets all seem to haveperso nalities of their own, and the solar systemseems a more m ysterious place than it did a decaago. To un derstand our own origin and the o riginour planet, we must know the solar system's origand evolution.

We cannot afford to launch spacecraft willy-ntowards al l p lanetary targets. A strategy is needeto m aximize the scientific payoff within theresources available to NASA. Which kinds ofspacecraft shall we send to which planets?

In order of increasing difficulty and expense,types of planetary missions are:

1. "Fly-by" missions, in which spacecraft, suas the NASA Mariners, pass close to aplanetary target and scan it with instrume

2. A tmospheric probe missions, in whichspacecraft penetrate a planet's atmospherebut are destroyed upon impact. Example: t

' Russian Venera probes.3. Orbiter missions, where spacecraft survey t

planet from orbit. NASA's Lunar Orbiterstypify this class.

4. Lander missions, where the spa cecraftsoftlands on the surface and radios data bacto Earth. Example: America's lunar Survespacecraft.

5. Sample-returning, unmanned missions, duwhich atmospheric and surface samples areacquired and brought back to Earth.

6. Returning, manned lander missions, such athe Apollo flights to the Moon. These aredifficult and costly.


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Figure 1 NASA's balancedof planetary ex~loqtion.

strategy

Only the first four types of missions fal l within th e Lowellwas mapping through his telescope a tlimitationsof NASA's planetary exploration budget Flagstaff, Arizona. Newspapers were fu l l ofand technical capability during the decade of speculation that Mars w as i,nhabited by anth e 1970s. intelligent race that was slowly perishing as its

meager supplies of water dwindled. ~ r o murGiven th e above mission types, here should we superior technical v a ~ t a g e oint, we no longer

aim th e spacecraft? Because of it s nearness and expect s ign i f ican t mounts f water on M~~~andth e still lingering possibilitythat life may exist there, probably no intelligent ife; ut M~~~remainsMars is th e primary target of NASA.But spacecraft enigmatic n many ways,will also be dispatched inward toward Mercuryand outward toward Jupiter. NASA terms thisa "balanced strategy" because it focusses on thetarget of greatest interest, Mars, yet it still bringstwo new planets under terrestrial surveillance,Mercury and Jupiter. (Fig. 1) The approved missionsare summarized in Table 1.

Three Mariner spacecraft have flown past Mars:Mariner 4, i n 1964, and Mariners 6 and 7, in 1 x 9 .They radioed back pictures, n ot of great artificialwaterways b ut rather of a highly diverse landscapchaotic here, cratered there, featureless elsewhere.Mariners 6 and 7 sent back over 2001 ~ h o t o s long

ORBITING THE RED PLANET with more than 5000 ultraviolet and infraredMARINER-MARS 71 spectrograms of Mars. To say th e least, th e geology

of Mars was hardly what astronomers ha d expectedOne of th e most popular books ever written about Mars ha s some terrestrial features, some lunar

Mars was Percival Lowell's MARSAN D ITS CANALS. similarities, too, but we must consider it a uniqueIt was published in 1906, when th e whole world was planet, like nothing else in the solar system. Th eagog over th e intricate network of canals that following "Martian Mysteries" support this view.

T A R E 1. Appwvetl NASA Planetary M P s $ I P ~ 970*'

TargetFlamrtMarsMarsVenus/M~rcury

I Jupiter

S ~ f t c m f t ypeMariner orbitersViking orbiters and wftlmdemMariner fly-by spacecraftPioneer fly-by spi;loaomTt

Launch kiteWl

19751m

19JZ-1373

NASACanterJet P~populsi~naboratoryLangley Research CenterJet Propulsisn Laboratorydmes Research Center

I *MASA elm sUD~orts H ~ I O Bqlac probe project of West Germany -as ~ t 1s a &@yarn trf tmestrial planetary astronomy.


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Some Unsolved Martian Mysteries1. How was the chaotic Martian terrain created?

(Fig. 2) Some small earthquake-ravaged areas of

the Earth show similar signs of violent upheaval.2. There seem to be two distinct, ye t intermixed,populations of craters; one group, old an d eroded;the other, young and sharp. What could havecaused the two bombardments? (Fig. 3 )

3. How did th e wide stretches of featurelessterrain escape the meteoric cannonades?

4. The Hellas region-300,000 square miles of th eMartiansurface-is now one of th e brightestfeatures of Mars. In 1954, it was very dark. Why?

5. Dark-flooredcraters an d dark patches often giveterrestrial astronomers the impression ofstraight lines (canals), which are so~metirnesdoubled and often thousands of miles long. Howcould seemingly random phenomena create suchneat, geometrical networks?

6. What causes the "waves of darkening7'thatsweep toward the Martian equator as summersmelt th e carbon-dioxide polar caps?Is this a manifestation of some form sf lifeor just a chemical reaction on the surface?Or is it neither?

7. What causes the great yellow clouds? In 1877,1909, 1922, and 1956, much of the face of Marswas olbscured by these clouds while the planetwas closest to th e Sun.

8. Many terrestrial microorganisms do quit@well i na simulated Martian environment. Is there lifeon Mars?

Figure 2. The Mariner 6 flyby took this picture of chaoticMartian terrain. Areas on Earth hard hi t by earthquakessometimes s h w the same kind of slumping and upheaval.

Figure 3. This mosaic af photos snapped by Mariner 6A Martian Strategy. Any one of the above questions indicates that cratered terrain dominates on Mars.is intriguingenough to warrant sending morespacecraft to Mars, The question o f life in particular,has far-reaching philasaphical sonnotatiolns.Three unmanned Mariners have flown by Mars;what should be the next step? NASA's basicstrategy is similar to that it employed sosuccessfully in lunar exploration; namely, thefollowingsequence of ever-more-sophisticatedspacecraft: fly-by, orbiter, unmanned lander, andmanned lander.

In keeping with this philosophy, NASAscheduledtwo orbiters, Mariners H an d I*, o followthe 1965an d 1969 flybysuccesses. But MarinerH fellvictim

*NASA spacecraft are designated by letters before launch.If the launches are successful, they are assigned numbers.


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targets for the V iking landers. The sequence isessentially the same as the Lunar Orbiter-SurveyorLander interlockin g missions to th e Moon.

The M ariner-9 orbiter is more than just a trail-bIazer for the Vikings. I t will carry out a m appingmi=ion of the Martian surface and will also studyfeatures of M ars tha t vary with time, such as thoseareas engulfed by the annual wave of darkening,

The plan i s logical, b ut we should bear in m indtha t terrestrial logic may be superseded once thespacecraft arrives at Mars. For example, earlyphotos may reveal "targets of o pportunity1' hat c allfo r special attention. For thi s reason Martianmissions are flexible in design.

Mariner Sketch. The long line of successfulMariner spacecraft began at the Je t P ropulsionLaboratory (JPLI in the late 1950s. In addition t othe Mars fly-by missions already mentioned,Mariners 2 and 5 reached Venus i n 1962 an d 1967.(Mariners 1 nd 3 were victims of a launch vehiclefailure and a shroud-jettison failure, respectively.When one considers that the Mariners have tocruise for about six months through space beforereaching their targets and then, i n a fewsupercharged hours, scan the planet with severalinstruments, they have done very well indeed.

to a failure of the Centaur stage of th e Atlas-Centaur Superficially,th e Mariners a ll look alike.launch vehicle; although it s designation became [Figs. 4 and 5) Wing-likesolar paddles extendingMariner8 it did nat*achievea trajectorytowards from a central equipment compartment characterizeMars. Mariner9 (Mariner I) was successfully th e species. Another common element is thelaunched on May30, 1971an d scheduled to arrive dish-shaped, high-gain radio antenna that must beat Mars N6vember 13,1971. pointed a t the Earth focr telemetry transmission,

The Vikinglanders willfollowin 1975 according command reception, an d radio tracking. At th e top

to current plans. Manned flightssimilar to th e of th e spacecraft, a maneuver engine pokes its,

Apollo lunar missians may be attempted in the nozzle along th e spacecraft axis. The sides of th e1980s. Mariner9 willgive scientists a long looka t spacecraft are usually occupied by star trackers,the Martian surface an d help them select propitious auxiliary antennas, pressure tanks, and the

window-blind-like louvers that cantrol thetemperature inside the spacecraft electroniccompartments. The scie ntific "eves" of thespaiecraft are located on the bottom on amavable platform. As the spacecraft appraachesitis target, th e scan pla tform keeps the scien tificinstruments pointed toward the planet.


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Figure 4. Top view of Mariner-Mars 71.


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Down the years, the Mariners have been structure, and zones conducive to life. Astronomersrefined bit by bit. The important features of the have tried to map Mars' tiny disc through the1971 Mariners are presented in Table 2. telescope fo r decades at a ll wavelengths. Now,

at long last, most of the Martian surface will be aStudying Mars from Orbit. The Mariner television wide panorama only 1000 miles below theircameras which will sweep the still-enigmatic instruments. These instruments are describedMartian terrain will be the primary instruments briefly in Table 3.aboard the orbiter. Scientists and laymen alikewant a good close-up view of Mars. However,a great deal can also be learned about the planet'satmosphere and surface by analyzing theelectromagnetic radiation reflected from andemitted by the planet below. Infrared radiationfrom the Martian surface, fo r example, is a measureof surface temperature at a distance; solarultraviolet rays are strongly affected by the differentgases in the Martian atmosphere. By mapping Marsin the infrared an d ultraviolet as well as the visibleportion of the spectrum, scientists hope to learnmore about possible volcanic activity, atmospheric

TABLE 2. ueslgn tearures ana viral statlstlcs, ~a r i ne r-Mar s 1

I Spacecraft FunctionsCommunication an ddata handling

Power supply

Att i tude control

Propulsion

Structure

Launch vehicle

Tracking an d dataacquisition network

Design Features

Forty-inch paraboloidal, high-gain antenna pointed a t Earth. Taperecorder used when instruments acquire data faster than transmitterca n send them.

Four solar panels with total area of about 83 square feet provides800 watts of power near th e Earth an d 450-500 watts near Mars.Battery with 600 watt-hour capacity provides for cr i t ical spacecraftmaneuvers and shadow periods.

Su n sensor an d star tracker (using Canopus as guide star) t e l lspacecraft i t s orientation in space. Two systems of gas jets can:(1) Point th e solar panels toward the Su n (cruise phase of flight);(2) Point th e maneuver engine in th e proper direction to make mid-course corrections an d insert the spacecraft into Martian orbit; an d(3) Orient th e spacecraft fo r surveying th e Martian surface.

A 300-pound-thrust, restartable maneuvering engine. Fuel: monomethylhydrazine. Oxidizer: nitrogen tetroxide.

Central compartment (Figs. 4 an d 5) s a magnesium framework witheight equipment bays. Solar panels are hinged to th e framework an dare unfolded once i n space. With solar panels extended, spacecraftmeasures 22 feet 7.5 inches across. Total weight a t launch: about2150 pounds; weight at Mars: about 1200 pounds du e to maneuverengine fuel consumption.

The Atlas-Centaur.The Deep Space Network, with 85-foot and 210-foot paraboloids fo rtracking, data acquisition, an d sending commands from Earth-basedcontrollers.


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r

-ABLE 3. Scientific Instrumentation, Mariner-Mars 71

InstrumentTwo N cameras; onenarrow angle, highresolution (0.1 km!;one wide angle, with1.0 km resolution.

Ultraviolet spectrom-eter; two channels,1700-3400 and 1100-1900 Angstrom units.

lnfrared interferometerspectrometer; range6-50 microns.

lnfrared radiometer;tw o channels, 8-12an d 18-25 microns.

Radio occultationexperiment (nospecific instrument)

Celestial mechanicsexperiment (nospecific instrument)

Scientific ObjectivesDetailed mapping of the planet'ssurface, especially the wave ofdarkening, the polar caps, thenightside atmosphere, surfacefluorescence, haze, cloud cover,and dust clouds.

Measure local atmosphericpressures. Pinpoint ozoneconcentrations (as possibleindicators of biological activity).Map the structure and compositionof the upper atmosphere andionosphere.

Measure vertical temperatureprofiles, composition, and dynamicsof atmosphere. Measure thetemperature, composition, andthermal 'properties of surfacematerials, with special emphasison potential biological materials,such as vegetation.

Map the temperature of the surfaceas a function of local time, withspecial attention to "hot spots"indicative of internal heat sources.

Measure variations in t he atmosphereand ionosphere by noting howthe spacecraft transmitter signalsare affected as the spacecraft swingsbehind the planet.

Improve our knowledge of solarsystem distances, astronomicalconstants, the mass of Mars,and relativistic effects.

1Principal Investigator(s)H. Mas ursky (U.S. Geolo gical1 Survey); G. de Vaucouleurs (U.

Texas); J. Lederberg (Stanford);B. Smith (N. Mex. State U.); W.

, Thompson (Bellcomm Inc.)

C. Ba rth (U. Colorado)

R. Hanel (Goddard Space FlightCenter)

G. Neugebauer (Californ ia Ins ti-tute of Technology)

A. Klio re (Jet PropulsionLaboratory)

J. Lor ell (Jet Propu lsion Labora-tory), I. Shapiro (M.I.T.)

Th e Long Cruise to Mars. he famous Martian canalswere first pub licized by Giovanni Schiaparelli whilehe was mak ing a high precision map of M ars dur ing1877, when Mars approached to within 34.8 mill ionmiles o f Earth. These favorable approaches or"oppositions" occur about every 2 6 months, butsome, like that of 1877, are much better thanothers. In 1971, the distance of closest approach

w ill be 34.9 mill ion miles-almost as good as tha tin 1877. Shortly after the 19 71 opposition Mariner9 should be in orbit around Mars to complementtelescopic observations.

In the original plan, two M ariners wereto havebeen launched from Cape Kennedy dur ing a 28-daylaunch window which began in early May 1971. Thesecond would have followed th e first after about10 days. But M ariner 8, although successfullylaunched from t he pad, landed in the Atlan tic Oceanwhen the launc h vehicle failed. L aunch of the

second sbacecraft was delayed pe nding investiga-tion of the launch failure and the revision of theMariner 9 mission.

Scientists na turally wished to rearrange prioritiesand accomp lish some of Mariner 8's objectives withMariner 9. The new plan accommoda ted al l of theoriginally planne d experiments from both flights,but with a reduction in the amoun t of data trans-mit ted back to Earth from each experiment.

Mariner 9 left the launc h pad at Cape Kennedy at6:23 p.m. E.D.T. on May 30, 19 71; it was scheduledto go into orbit around Mars on November 13.Already headed towards Mars at the tim e were twoSoviet spacecraft-each weig hing abou t five ton sas cornpared with 2,200 pounds for M ariner 9.There was no announcement from th e Soviets abou tthe character of the m issions undertaken.

The revised mission of Mariner 9 called for anorbit around M ars with a period of 1 2 hours,


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p r e 6. Mariner 9 will be placed in a Martian orbit rnatrmits systematic camera mapping of much of the

surface. The four passes above illustrate how photographsshould overlap.

MACHINES ON MARS-VIKING 75

periapsis (closest approach) of 7 5 0 miles, apoapsisof 10,700 miles and inclinationof 65 degrees to theMart ian equator. This orbit was selected to enablethe spacecraft's television cameras to obtain a mapcovering about 8 0 percent of th e Martian surface.

Th e basic mission for Mariner9 is t o obtain datafrom orbit around Mars fo r a t least 9 0 days. Thiscontrasts with the brief glimpses a fe w hours longobtained from the three preceding Mariner missionsto Mars. With thousands of new pictures plus

infrared and ultravioletmaps of Mars, scientistswil lbe able to refine their theories about theplanet-this is the least we ca n expect. Actually,we should "expect th e unexpectedlVor we reallyknow very littleabout Mars; fa r less than we d idin the case of the nearby Moon and our astronautshave brought back many surprises from there. Mars,with i ts puzzlingterrain, th e planet of innumerablescience fictionstories, s t i l l ha s many secrets.

After th e Mariner orbiter ha s mapped th eMartian surface i n 1971, Mars wi l l receive nofurther mechanical visitors from Earth unti lmid-1976 when the first Viking spacecraft closeswith th e planet. The next logical step i n NASA'sMartian strategy, th e Vikings wil l be combinationorbiter-landers; tha t is, double spacecraft, one partremaining in orbit while th e other descends ontothe Martian surface.

Can th e Viking landers help us answer the otherquestions science poses about Mars (see l is t above)Undoubtedly they ca n because they wi l lcarry experiments r ight down to th e surface whereinstruments can make direct analyses of th eatmosphere and surface materials. Like theSurveyor lunar landers of the late 1 9 6 0 ~ ~he Vikingwil l carry mechanical arms which ca n retrieve andmanipulate Martian soil and rocks, or whatever elsehappens to be on th e surface-possibly smallorganisms or simple plants. Frankly, scientists donot know exactly what to expect. Mars doesresemble ou r cratered Moon from 10,000 miles outBut, a t this distance, the Earth itself appears

lifeless, watery, and a displayer of perplexing cloudand color changes. The Viking landers, neverthelesswil l carry several biological experiments-just i ncase.


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I '

The Double Spacecraft. The Viking orbiter an dlander work together, just like th e Apollo CommandModule and Lunar Module have done, except, ofcourse, that there is no orbital rendezvous andreturn t o Earth. Viking orbiter-lander teamworktakes three forms: (1) The orbiter cameras an dother instruments help select th e landing sites;(2) The same instruments maintain surveil ance of

th e planet and help the lander synchronizeexperiments with variable planetary phenomena,such as an onrushing wave of darkness or adust cloud; and (3) The orbiter i s a vitalcommunication relay point fo r th e lander unti l itestablishes an independent communication l inkwith th e Earth.

By th e time th e Vikings are launched in 1975,th e orbiter portion should have proven itself duringth e 1971 Mariner flights. The Mariner 71 design isa good basic building block, bu t changes wil l besubstantial. Th e orbiter's diameter wi l l have to growbecause it must be capable of carrying th e lander Figure 8. The Viking orbiter has the basic Marinerand inserting it into Martian orbit. (Fig. 7) Further, configuration; however, two solar panels have been addedelectrical power loads an d data streams wi l l be to provide the extra power needed for the more complexlarger due to th e heavy dependence of th e lander Viking mission.upon th e orbiter. The major changes made fromth e basic Mariner 71 design include:

capacity fo r orbital insertion a t Mars du e torn Approximatelythree t imes as much propulsive mass of lander and th e increased mass of

th e orbiterrn Larger structure to carry th e lander and th e

large amount of fuel required fo r planetaryorbit insertion.

rn Solar-cell panels increased from four to six(Fig. 8)

rn Battery capacity uppedrn Equipment compartment enlarged; 16 instead o8 baysrn New receiver tot pick up lander transmissions

fo r relay to Earth

The Viking lander is an entirely new spacecraft,although technology has been borrowed from th eSurveyor and Apollo lunar landers. The lander'sconfiguration has been shaped by th e tasks it

SPACECRAFT must perform. Th e deorbit and landing phasesar e very influential. (Fig. 9) Followingseparation from th e orbiter, deorbiting engines slowth e lander down so that it falls toward Mars alongit s entry trajectory. Mars possesses an atmospherealbeit a thin one, and a t an altitude of about 150- 64 IN. DIA. miles aerodynamic braking begins. Nearer th esurface, a parachute wil l slow th e craft s t i l l furtherAt about 3900 feet from th e surface, the parachute

Figure 7. The Viking spacecraft inside the launch vehicleshroud. The "canned" lander is on top of the orbiter.


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Figure 9. The iiking entry profile, showing events from lander separation'through soft landing.

is cut loose a td the terminal descent engines through the thin Martian atmosphere and, brakedease the spacecraft down to the surface, where by parachutes, settles to the surface withoutshock absorbers cushion the impact of landing. a cargo of microscopic terrestrial invaders. BecauseThese maneuvers dictate much of the lander's the Martian environment may harbor life of its own,equipment-eigines, guidance and control devices, NASA wishes to avoid infecting it with Earthlanding gear, k c . microorganisms which would confuse the search for

indigenous life. NASA's planetary quarantineAnother big factor in lander design is the guidelines for the first Viking landing mission state

geometry of the "can" (bioshield) that sealsthe heat-sterilized lander and prevents laterreinfection from the orbiter and rocket gases. Thecan consists of two lens-shaped shells (Fig. 10),which are shed prior to deorbiting and remainin orbit. The sterilized lander then plunges

Figure 10. The lander is encapsulated by the bioshield andaeroshell during the long flight to Mars.


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SURFACE SAMPLEF

TERMINALDESCENT ENGINE

that the probability of infecting Mars with terrestriallife must be less than three chances in one hundredthousand. 1

Basically, the Viking lander is a triangular "box"about 8 feet 4 inches from center to center of thetripod-like landing "feet." (Fig. 11) Many spacecraftcomponents are packed inside the box, but fueltanks, cameras, power supplies, and antennas aremounted externally. The major subsystems aredescribed briefly in Table 4.

Scientific Outpost on Mars. The scientific interestin Mars is so high that NASA received 165 proposalsfor Viking experiments. After rigorous evaluation,25 were selected for the payload. As the spacecraftand instruments are designed in detail, minorchanges will undoubtedly be made.

The payload selected is large, even larger thanmost Earth satellites. Only NASA's OrbitingGeophysical Observatories can match this total. The

rgure 11. The Viking lander wrih all apperlaagew aeproyedon the Martian surface.

scientific value of a large, diverse payload ofinstruments is obvious, but there are two soberingpractical considerations: (1) the data streamdirected toward the Earth is broad and deep (about8000 bits per second near opposition, or about1000 times that of Mariner 2 in 1962); and (2) themany instruments make the spacecraft morecomplex. However, NASA has had considerableexperience in both areas with its orbitingobservatories and Apollo spacecraft.


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TABLE 4. Design Features and Vital Statistics, Viking 75

Lander Functions 1 Design FeaturesCommunication an ddata handling

Power supply

Propulsion an dat t i tude control

Guidance and control

Structure

Launch vehicle

Tracking and dataacquisition network

Ultra High Frequency (UHF) ransmitter andomnidirectional antenna fo r relaying datato orbiter overhead. S-band (about 2300 MHz)transmitter an d 30-inch paraboloidal antennafo r direct telemetry l ink to Earth after com-pletion of landing phase. Tape recorder an dmagnetic-core memory store data duringentry blackout an d when Earth cannot bereached. Commands from Earth ma y be re-ceived by either th e omni antenna or th eparaboloid.

Two radioisotope thermoelectric generators(RTGs) using th e heat from decaying plu-tonium-238 deliver about 50 watts of constantelectric power. The RTGs also supply heatduring th e cold Martian night. Batteriesprovide power during entry an d landing an dthereafter during peak loads.

Eight small hydrazine jets around r im o faeroshell orient lander fo r deorbit maneuver.Each je t delivers abo ut 10 pounds of thrust.Four of these jets provide th e deorbiting im -pulse. Six different hydrazine jets on th ebottom of th e lander control it s at t i tudeduring terminal descent. Three 600-pound-thrust hydrazine engines slow th e cra f t to asoft landing. (Figure 12.)

Radars measure th e al t i tude an d rate ofdescent. Gyroscopes and accelerometers helpdetermine lander attitude. These data are fe dt o a computer which controls th e engines andjets described above. (Control from Earth i simpossible because radio signals require al -most 40 minutes fo r a round trip.)

"Canned" lander measures about 12 fee t i n

diameter an d 5 feet 9 inches high.It

weighsabout 2200 pounds. As described i n text, th elander is a triangular box about 8 feet 4inches on a side.

The Titan-Centaur

Th e Deep Space Network, with 85-foot an d210-foot paraboloids fo r tracking, data ac -quisition, an d sending commands fromEarth-based controllers.

*Some of these data, particularly weights, may changeslightly as design and fabrication proceed.


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TABLE5. Scientific Instrumentation, Viking 75

Lander Typical ExperimentsInstrument Group an d Instruments 1 Scientific ObjectivesEntry science

Meteorology

Seismology

Imagery

Organicanalysis

Biology

Water mapping

1, Thermal mapping

Radio Science

Mass spectrometerPlasma analyzer

Measure th e composition of th e upper atmosphere.Measure th e ion an d electron densities an d energydistribution i n th e upper atmosphere.

Accelerometers, pressuresensors, temperaturesensors

Pressure sensors andtemperature sensors

Wind speed an d directionsensors

Water-vapor detectors

Seismograph

Camera (two perspacecraft) (Fig. 13)

Gas chromatograph, massspectrometer

Carbon-dioxide fixationexperiment

Labeled releaseexperiment

Light-scatteringexperiment

Gas-exc hangeexperiment

lnfrared monochromometer

lnfrared radiometer

Orbiter and lander radios an dground tracking stations

Measure th e density, pressure, an d temperaturedistribution in th e lower atmosphere.

Measure atmospheric pressure an d temperature a tth e surface.

Measure wind speed an d direction.

Measure Martian humidity.

Measure seismic activity of Mars.

Transmit pictures of th e Martian surface to Earth,includingmeteorological, geological, an d biologicalphenomena. Monitor soil sampler operations. (Fig.14)

Measure th e molecular weights of compounds re-trieved by soil sampler

Detect photosynthetic an d respiratory fixation ofisotopic carbon dioxide by soil sample.

Detect th e release of volatile tagged compoundsfrom incubated soil samples.

Detect changes in turbidity (cloudiness) of aqueoussuspension of soil sample due to biologicalactivity.

Detect changes in gas composition over incubatedsoil sample.

Measure th e water abundance distribution i n th eatmosphere

Determine variation i n surface temperature an dthermal balanceMeasure solar system constants more accurately.Perform General Relativity experiment. Atmospheredetermination by occultation. Determine surfacereflection.

,LAUNCHAUGUST '75 AT KENNEDY

I 1

I , AZIMUTH= 90-108"

2ND MIDCOURSE30 DAYS

1ST MIDCOURSE5 DAYS

LANDER DEBREIT

Figure. 12. A typical Vikingmission profile.


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Figure 13. Schematic showing how pictures taketh e Viking N amera are transmitted to Earth.

I t is premature to attempt a definitivedescription of th e lander's instruments because

most are st i l l in the development stage. In Table 5,some of the broad features of th e experimentalpayload are sketched out.

1975 Is Harder Than 1971. The opposition of 1975will bring Mars to within 52.4 million miles of Earth,considerably more than the 34.9 million miles fo rthe 1971 opposition. The propulsive energyrequirements pe r pound of spacecraft weight arealmost doubled. So as not to exceed the capabilityof the launch vehicle, a new trajectory willbe used.In the new trajectory, the leg of the transfer ellipseconnecting th e orbits of Earth and Mars willrequire almost a year of flight time rather than th e

192 days or so fo r th e 1971 Mariner orbiters.The planned Viking mission profile (Fig. 12) calls

fo r a launch using the Titan-Centaur in August 1975.The Centaur upper stage will propelth eorbiter-lander combination ou t of an Earth parkingorbit into the interplanetary transfer ellipse fo rMars. In July 1976, the Viking willencounter Mars,an d the orbiter's engine wil l insert the craft into anellipse around Mars. After orbit t r imming byth e maneuver engine an d the selection of alanding site using data taken by the orbiter'sinstruments, the lander will be separated from the

y Figure 14. The Viking surface sampler w i l l be able t oretrieve soil an d rock samples between 3 and 10 feet awayover an azimuth of about 120".

orbiter in preparation fo r its descent to the surface.

(Fig. 9)Plans call fo r tw o Vikings to be launched in 1975.Both th e orbiters an d landers are being designedfo r three months of operation after th e landingon Mars.

TRAILBLAZING BEYOND MARSPIONEER-JUPITER 72, 73

To a spacecraft heading away from the Sun, Marsis the last of the terrestrial planets, so-calledbecause Mercury, Venus, and Mars have certainthings in common with th e Earth, such as smallsize an d high density, l u s t beyond Mars lies th easteroid belt with it s uncounted millions of bitsand chunks of matter varying from dust-size specksto the planetoid Ceres, 480 miles in diameter.This curtain of debris separates the terrestrialplanets from the giants of the solar system,Jupiter, Saturn, Uranus, and Neptune. The ninthplanet, Pluto, is so small an d fa r away that wehardly know how to classify it . Possibly it is an

escaped moon of Neptune. Beyond Pluto, whoknows what we shall find? (Incidentally, Neptune isnow th e most distant planet from the Sun an d wil lremain so until 2009. Pluto's orbit is rather eccentricand when it is closest to th e Sun, it is actuallyinside Neptune's orbit.)

Jupiter is the nearest and th e biggest of the giantplanets. Much of what we learn about Jupiter'sstructure and composition will probably also applyto Saturn, Uranus, an d Neptune. It i s a fittingtargetfor our first probings beyond Mars.

Through the telescope, Jupiter appearsponderous, almost bizarre. It is markedly flattened


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at the poles, with slate-blue and pink bands, awholly alien object compared to Mars. (Fig. 15)Plying it s own course in Jupiter's southern latitudes

is the Great Red Spot,30,000

miles long, severaltimes the size of the Earth. The Spot waxes andwanes and changes shape as it rotates aboutJupiter's axis faster than the other relativelypermanent features of the planet. The Great RedSpot is just the most obvious Jovian enigma. Someadditional questions follow:

Some Unsolve,d Jovian Mysteries1. Does Jupiter have a solid surface? We see only

clouds through the telescope. Jupiter's averagedensity is about equal to 1.3 times that of water.

2. Hydrogen and helium seem to be the majorconstituents, although helium has not actually

been observed, with smaller quantities ofammonia, methane, and other gases. No oneknows the percentages accurately.

3. Might not Jupiter be a very primitive planet thatwill eventually lose most of its lighter elements,revealing beneath a small terrestrial-type core?

4. Why does Jupiter change hues? Are some ofthese changes synchronized with the 11-yearsolar cycle as some astronomers assert?

5. What is the nature, strength, and extent ofJupiter's belt of trapped radiation? Someestimates place the strength at one million timesthat of Earth's belt.

6. Jupiter emits radio signals that seem to be

correlated with the motio'ns of its larger satellites.Why?7. Some of Jupiter's moons are as large as Mercury.

Do they have atmospheres and other features ofthe Earth-like planets?

8. Jupiter possesses a magnetic field many timesstronger than Earth's. Earth and Jupiter are sodifferent one wonders whether the fields mighthave different origins.

9. Jupiter radiates approximately twice as muchenergy as it receives from the Sun. Where doesthis energy come from? Is Jupiter, as some think,a very cold, miniature star?

The above list could be extended, but there is noneed, Jupiter's alien nature is only too apparent.No wonder it is a prime astronautical target.

Reliability Is the Key. Exploration of the outer solarsystem differs in one important factor from visitingthe other members of our cozy little group ofterrestrial planets. That factor is time. The roadto Jupiter is a long one-a half bill ion miles andalmost two years long. Furthermore, the propulsiverequirements are so high (ten times as muchenergy per pound of spacecraft as Mariner-Mars 71)

that only a small spacecraft can be economicallyinjected into the long trajectory leading to Jupiter.Therefore, the Jupiter mission should be based

on the technology of small, highly reliablespacecraft. NASA has just such a spacecraft family:the Pioneer deep space probes. Four of these crafthave been placed in heliocentric o rb it A11 arestill working. Pioneer 6 has been sending telemetryback to Earth since December 1965.

The Jupiter-Pioneer Program has been underwayat NASA's Ames Research Center, a t Mountain View,California, since 1969. The main target is, of course,Jupiter, but three other objectives helpshape spacecraft design and instrument selection:

Study the interplanetary medium between theEarth and Jupiter.

Determine the nature of the asteroid belt.Asteroids, by the way, present a hazard tothe spacecraft that we cannot evaluate throughterrestrial telescopes.Develop technology for subsequent flights tothe outer planets. In other wolrds, the JupiterPioneers are precursors for more ambitiousflights to follow.

Using the Mariner-Mars flights to establish abenchmark, the Jupiter-Pioneers must accommodateto two! facts of interplanetary life: thecommunication distance from Jupiter at encounterwill be almost three times that at Mars encounter,while the solar power available will be down by a

factor of ten. Including the requirements of smallsize and extremely high reliability, one sees thatthe Jupiter Pioneers will have to be remarkablemachines.

The Jupiter-Pioneers (Pioneers F and G) retainfew of the external characteristics of the Pioneersnow in heliocentric orbit (Pioneers 6-9). They are,in fact, four times heavier and carry twice as manyexperiments. Rather than simple, cylindricalspacecraft, like their predecessors, the Jupiter-Pioneers are box-like equipment compartments,each dominated by a 9-foot paraboloidal high-gainantenna and two long booms, each holding tworadioisotope thermoelectric generators (RTGs).(Fig. 16) The resemblance to previous Pioneers isprimarily internal; that is, electronic. The majorfeatures of the Jupiter-Pioneers are summarizedin Table 6.


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zure 15. Conceptual view of Jupiter, showing i ts postulated radiation belts.

-

TABLE 6. Design Features and Vital Statistics, Jupiter-Pioneers *

Spacecraft Functions

Communication an ddata handling

Power supply

Propulsion

Attitude control

Thermal control

Structure

Launch vehicle

Tracking an d dataacquisition network

*Some of these data, particul

Design Features

Redundant travelling wave tubes (for reliability) transmittelemetry t o Earth via th e 9-foot, S-band, high-gain parab-oloid. With 8 watts of power an d a 210-foot paraboloid onEarth, a Jupiter-Pioneer can send about 1000 informationbits pe r second from Jupiter a t encounter (roughly 440 mil-lion miles away). Commands are transmitted "uplink" throughth e same antennas. Closer t o Earth medium/low gain an -tennas on th e spacecraft may be used fo r communicationboth ways. When data accumulate faster than they ca nbe transmitted, they are stored i n a 49,152-bit memory. The"sequential coding" technique proven on Pioneer 9 wil l beused.

Solar panels were originallyproposed, bu t two pairs of radio-isotope thermoelectric generators (RTGs) were substitutedlater. Mounted away from the spacecraft on two booms, theygenerate an average of 120 watts. These wil l be th e firstNASA spacecraft to depend completely on RTGs. (Fig. 17)Three hydrazine engines developing about 1 pound of thrusteach wi l l produce th e impulses necessary fo r midcoursecorrections.

Th e same hydrazine engines wil l also partly despin th espacecraft after launch an d provide attitude control torques.The spacecraft is spin-stabilized-like a rifle bullet--at about5 rpm. The high-gain paraboloid always points toward Earthalong th e spacecraft spin axis, which is fixed in space.Th e orientation of th e spacecraft will b e determined by aSun sensor an d a star tracker se t on Canopus.

Louvers on th e spacecraft bottom maintain internal temper-atures between -20" an d 90F.

Irregular box made of aluminum honeycomb sandwich. Ap-

pendages mounted as shown in Fig. 16.The Atlas-Centaur.

The Deep Space Network, with 85-foot an d 210-foot parab-oloids fo r tracking, data acquisition, an d sending com-mands from Earth-based controllers.

weights, may change slightly as design and fabrication proceed.


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ONational Geographic Society

Figure 16. The Jupiter Pioneer spacecraft. (Pioneers F and GI.

Pioneer's Diverse Sensors. Thirteen experimenhave been selected for the Jupiter-Pioneers.(Table 7) Because the scientific mission inclustudies of the interplanetary medium and theasteroid belt on the way to Jupiter, the instrum

payload i s varied indeed. Roughly half of theexperiments will make measurements of spacephenomena in transit. Even if the spa cecraftshould m iss Jupiter by a wide m argin, the flighcould still be highly significant scientifically

The Longest Interplanet ary Flight. Jupiter's perof rotation around the Sun is so lon g (nearly 12years) that th e Earth catches up to it and passeonce every 13 months. As a consequence, Jupilaunch opportunities occur every 13 months. Tlaunch windows selected for the Jupiter-Pioneeare:

Pioneer F Feb. 26 to M arch 26, 1972Pioneer G March 30 to May 1, 1973.

Figure 17. Cutaway view of the RTG to be used on PioF and G. Heat from the central capsule of plutonium-2is partially converted in to elec tricity by the small cyliof thermoelectric material.


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TABLE 7. Scientific Instrumentation, Jupiter Pioneers

Experiment/ Instrum ent Scie ntific Objectives

Magnetic fields: tri - Measurement of the interplane taryaxial heliu m magnetometer and Jovian magnetic fields

Plasma: 2 quadraspherical Measurement of the interplane taryplasma analyzers solar wind and the boundaries of

the Jovian magnetosphereCharged particles: solid-state telescope, high-energy electron andproton detectors

Jovian charged p articles:Geiger telescope, triangu-lar array of detectors,and low-energy Geiger.

Cosmic rays: 3 radiationtelescopes

Jovian trapped radiation:Cerenkov counter, electronscatter detector, scint-illator, and other detectors.

Ultraviolet photometry:single-channel ultravioletphotometer

Imaging photopolarimeter:a small scanning telescopewith a Jupiter resolutionof 120 miles.

Measurement of interplanetary solar-generated protons, electrons, andhelium nuclei. Measurement ofJovian trapped radiation.

Measurement of the energies andspecies of charged particlestrapped by Jupiter's magneticfield.

Measurement of the energies anddistributions of galactic andsolar protons, electrons, andlight nuclei through neon.

Measurement of the energies andspecies of charged particlestrapped by Jupiter's magneticfield.

Mapping of ultraviolet "heliosphere.'"Determination of Jupiter hydrogen-helium ratio, presence of auroras,and atmosphere scale height.

Mapp ing of zodiacal light. S tudyof asteroids and Jovian satellites.Imaging of Jupiter in two colors.

Infrared thermal structure: I Measurement of the net thermal energy2-channel radiometer flux emitted by Jupiter.Asteroid-meteoroidastronomy: 3 Ritchey-Cretein telescopes

Meteoroid detection:216 pressurized cells

S-Band occultation: usesspacecraft transmittersignals

Celestrial mechanics:uses spacecraft trackingdata

Measurements of the distributionof particles and cometary matter.

Measurement of meteoroid populationand spacecraft hazards withinasteroid belt.

Measureme nt of effect of solarcorona on radio waves and therefractivity of Jovian ionosphere

1 and atmosphere.More accurate determination ofJupiter's mass and ephemeris andthe motion of its satellites.

Principal Investigators

E. J. Smith (JetPropulsion Laboratory)

J. H. Wolfe (AmesResearch Center)

J. A. Sim pson (Unive rsityof Chicago)

J. A. Van Allen (Uni-versity of Iowa)

F. B. McDonald (GoddardSpace Flight Center)

R. W. Fillius (Universityof California, SanDiego)

D. L. Judge (Universityof Southern California)

T. Gehrels (Universityof Arizona)

G. Munch (CaliforniaInstitute of Technology)

R. K. Soberman (GeneralElectric Co.)

W. H. Kinard (LangleyResearch Center,NASA)

A. J. Kliore (JetPropulsion Laboratory)

J. D. Anderson (JetPropulsion Laboratory)


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Figure 20. The tw o proposed three-planet tours of the outer planets.

While th e opportunity for th e four-planetmission is a very rare event, three-planet an dtwo-planet Mini-Tours could be flown morefrequently. (Fig. 20) One of th e more interesting ofthese is th e Jupiter-Saturn tour, which is possibleevery 20 years. Jupiter and Uranus can be probedby spacecraft every 14 years.

Despite th e rarity of a four-planet opportunity,NASA currently favors taking advantage of twothree planet opportunities with singleJupiter-Saturn-Pluto launches in 1976 an d 1977 an ddual Jupiter-Uranus-Neptune launches in 1979.The resultant flyby data from all of th e outerplanets would be returned in t ime forknowledgeable planning of second-generationmissions an d the establishment of relativepriorities among th e outer planets before th enext two-planet opportunities occur in the 1990s.

Many types an d sizes of spacecraft have beenevaluated fo r accomplishing the exploration of th eouter solar system. The Thermoelectric OuterPlanets Spacecraft (TOPS) concept of the Je tPropulsion Laboratory fo r a basic multi-purposespacecraft has been selected as th e mosteconomical approach consistent with th e highest

priority scientific objectives an d the long-lifereliability required fo r a ll outer-planets missions.A stabilized scan platform, such as that used onthe Mariners will permit sensitive measurementsto be made on the planets an d satellites. TOPSwill also take advantage of recent high frequencycommunications developments to achieve highreal-time data rates from Jupiter and scientificallyuseful data rates over even the three-billion miledistances to Neptune an d Pluto. The TOPS conceptalso incorporates adaptive control an d self-test


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and repair features believed essential to achieving spacecraft and throughout the aerospace industry,long-life reliability with a high degree of but also in general commercial applications.confidence. (Fig. 21) STAR is a step toward truly automatic spacecraft

that will "think for them~elves."~ -, T1 -A 150-watt radioisotope thermoelectric generator(RTG) is being developed by the Atomic Energy AN INNER TOURCommission to provide a steady source of power MARINER-MERCURYIVENUS73independent of the spacecraft's distance fromthe Sun. Mars and the solar system giants pose perplexing

I scientific problems, but Venus and Mercury areThe self-test and repair (STAR) computer, hardly devoid of mystery. Let us look, then, toward

already well along in development, is capable of the Sun, where distances are shorter but wheremanaging and controlling all spacecraft functions searing solar rays replace the deep freeze of theindependent of any commands from Earth. The outer solar system.necessity for this capability becomes apparent ifone realizes that when the outer-planets spacecraft , Venus has already seen five space probes:approaches Neptune and Pluto, a radio signal Mariners 2 and 5 plus three in Russia's Veneratraveling at the speed of light will take eight series. Nevertheless, the major questions raisedhours to make the round trip from Earth. Results about Venus remain unsolved; in fact, there seemsto date in the development of the STAR computer to be more controversy than ever. In particular, weand of its highly miniaturized electronic know little of what lies below the thick, opaquemicro-circuits already promise an unprecedented Venusian atmosphere that except for tantalizingadvance in operating reliability. This highly glimpses of "something" below the clouds, hidescompact and competent STAR computer, weighing the planet's visible surface. Special radars based ononly 30 pounds and "guaranteed" to last at least Earth have penetrated the cloud blanket withten years without servicing, is expected to see long-wavelength pulses of radio energy. The echoeswidespread application, not only in many future returned to Earth reveal surface relief, perhaps

Figure 21. TOPS is th e Thermoelectric Outer Planets Spacecraft, a conceptfo r a basic multi-purpose spacecraft fo r exploration of th e outer planets. 0 ational Geographic Sociecy


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mountainous areas. What is more, the planet'sdirection of rotation is very slow andopposite that of the other terrestrial planets.Some of the most intriguing questions aboutVenus are listed below:

Some Unsolved Venusian Mysteries1. What does the surface look like? Orbiting probes

will have to draw maps by radar because th esurface receives little or no sunlight.

2. Past probes show the Venusian atmosphere tobe over 90% carbon dioxide with only 0.1 to0.7% water vapor. I f Venus is so dry what makesup the clouds? Doubtless, they are totally unliketerrestrial clouds.

3. The Soviet probe, Venera 4, abruptly ceasedtransmitting before scheduled touchdown on theplanet's surface. Was it crushed by the highpressures, which are estimated to be 100 timesthose on the Earth's surface? Our model of theVenusian atmosphere dlepends heavily upon datafrom Veneras 4, 5, an d 6 and Mariner 5; butmany questions raised by these flights remainunanswered.

4. The magnetic field of Venus is apparently lessthan 0.1% that of the Earth's field. This wassurprising initiallybecause Venus has long beencharacterized as Earth's "twin." Now, it is thoughtthat Venus' small field is a natural consequenceof it s slow rate of rotation. More magnetic fieldmeasurements are needed to study this problem.

Mercury is always located close to the Sun;astronomers have despaired of ever seeing muchdetail on th e planet's surface. There are vague

markings to be sure, but this planet confounds thebest observers. For example, until recently, theclassical figure for Mercury's period of rotation was88 days. The planet was supposed to be chainedgravitationally to the Sun so that it always kept thesame side pointed toward the Sun-just as theMoon does to the Earth. Radar observations fromEarth followed by some new visual measurementsindicate that Mercury turns on it s axis onceevery 59 days. This figure is exactly 2h of one ofMercury's years, leading some astronomers tosuspect some sort of resonance action betweenMercury and the Sun. Just why any resonanceshould exist, no one knows. Mercury also presentsus with other puzzles.

Some Unsolved Mercury Mysteries1. What does it look like? Is it cratered like the

Moon, Mars, and the Earth?2. The average density of Mercury is significantly

higher than that of the Earth. Did Mercury havedifferent origin or have i ts lighter elements beenvolatilized by the Sun-perhaps a much hotterSun?

3. Often Mercury's already indistinct features aresuddenly veiled. Is there an atmosphere thatsomehow survives despite the Sun's heat? Arethere dust storms on Mercury?

Another Member of the Mariner Family. The 1973mission to Venus and Mercury, is a late addition tNASA's program of planetary exploration. For thisreason, the description that follows does no t havethe detail of the other approved NASAmissions, andis more subject to rethinking.

The Venus-Mercury Mariner will stronglyresemble the Mariners that flew past Mars in 1969This similarity is to be expected because th emissions are similar and, because of the short leatime involved, NASA had to make heavy use of thewell-proven Mariner technology. The novel aspect othe Venus-Mercury mission is that it i s a doubleflyby. There is also the problem of solar heating,but the Mariners sent to Venus in 1962 an d 1967 alshad to solve this problem. The Mariner designsummarized in Table 2 can be used as a referencprovidingthe followingchanges are made:

Only tw o solar panels are required to captureenough power from the much nearer Sun. Thesolar panels will be tiltable to reduce the Sun'sheating effects.

The maneuver engine will be so mounted thatthe entire spacecraft and its equipment bays cabe tilted with respect to the Sun to reducethe heat load.Temperature-controlling louvers will be added tomore of the equipment bays to help keep theircontents coal.

The overall spacecraft wil lweigh just over 900pounds. The Atlas-Centaur will launch this Marinfrom Cape Kennedy. As usual with planetarymissions, the Deep Space Networkwill track,command, an d acquire data from the spacecraft.

NASAsolicited the scientific community fo rVenus-Mercury experiments in March 1970. OnJuly 28, 1970, NASA announced that the sevenexperiments listed in Table 8 had been selectedfo r flight. All of the instruments have been provenin previous space flights.


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The "outer tour" described earlier is a rarephenomenon because favorable arrangements ofall four giant planets repeat only after long intervals.Mercury and Venus, however, rotate around theSun much more quickly-as described byIsaac Newton. Opportunities for "inner" Tours tothese planets thus arrive much more frequently.The 1973 opportunity requires a launch from

Cape Kennedy between October 12, 1973, andNovember 20, 1973. The spacecraft is first placedin a parking orbit around the Earth by theAtlas-Centaur launch vehicle. When the propermoment arrives, the Centaur upper stage injectsthe spacecraft into the heliocentric orbit shown inFig. 22.

CLOSEST TO TH E SU NHELlOS 7 4 7 5

The Sun is the mainspring of the solar system.It dominates many planetary phenomena and isthus a factor in our exploration of the planets.NASA is building no solar probes of its own, but it isupporting the West German Helios Program. Thespacecraft is being designed and built by a group oGerman firms headed by Messerschmitt-Bolkow-Blohm GmbH. The General Electric Co., in theUnited States, is a consultant on the project. NASAwill supply the launch vehicle, a pad and facilitiesat Cape Kennedy, and the services of theDeep Space Network.

The heliocentric orbit will, after midcoursecorrections, possess just the right eccentricity forthe spacecraft to intercept Venus between Feb. 3and Feb. 6, 1974. The distance of closest approachshould be 2500-3000 miles. The gravitational pullof Venus will deflect the spacecraft into a neworbit that will lead to the Mercury encounterbetween March 19 and April 3,1974. Using additionalmidcourse maneuvers, the trajectory should comewithin 700 miles of the surface. At the time of theMercury flyby, the Earth will be between 801 and100 million miles away. Following this encounter,the Mariner will orbit the Sun with a period ofabout 176 days.

The objective of Helios is the measurement ofthe structure and time variation of the plasma,cosmic rays, and magnetic fields in interplanetaryspace as they are controlled by solar processes anevents. This objective is essentially the same asthat of America's heliocentric probes, Pioneers 6-9.Pioneers 6-9, however, ranged only between 0.8and 1.2 A.U.*, while the two Helios probes willpenetrate to 0.3 A.U

Helios will utilize many of the design features oPioneers 6-9. I t will be a cylindrical spacecraft,

*A.U. = astronomical unit. 1 A.U. is equalbetween the Sun and the Earth, about

the average distancemillion miles.

Figure 22. The Mariner-MercuryIVenus mission calls for an"inner tour" past th e planets between th e Earth an d th e Sun

1


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TABLE 8. Scientific Instrumentation, MARINER-VENUSIMERCURY73

Instrum ent Scie ntific Objectives Scientific Team Leader

Two television cameraswith 1500-mm telescopes

Spacecraft transmitter andterrestrial receivers

Scanning ElectronAnalyzer (SEA)

Two triaxial fluxgatemagnetometers

Two ultraviolet gratingspectrometers

Infrared radiometer

Charged particledetector

Study Venus' dense cloud blanket and theultraviolet clouds that appear to circlethe planet. Map and identify Mercury'slandmarks, such as craters; determine

spin 'axis orientation. Search forsatellites of both planets.

Provide radio propagation data on inter-planetary phenomena during flight and,at planetary encounters, information onatmospheres, ionospheres, radii, andsurface characteristics.

Determine how solar wind interacts withVenus and Mercury and the characteristicsof the solar wind between Earth andMercury. Compare latter data with similardata from Pioneers F an d G between Earthand Jupiter.

Measure the interplanetary magnetic field andthe fields near Venus and Mercury. Studythe interaction of the solar wind withthese planets.

Search for an atmosphere surroundingMercury and, if it exists, determine itsstructure and composition. Obtain similardata for the atmosphere of Venus.

At Venus, measure cloud-top and limb-darken-ing temperatures and search for holes in thecloud cover. At Mercury, measure surface-brightness temperatures and correlate infra-red features with visible features.

Study the charged particle bombardmentof Mercury and the properties ofcharged particles reaching Mercuryfrom solar flares.

I

'

with a narrow waist and flared ends. (Fig. 23)Four radial booms will protrude from th e waist;on top, another boom and th e antenna lie along hespin axis. The spacecraft will be spin-stabilized inspace so that it s spin axis is always perpendicularto th e plane of th e ecliptic. So far, th e descriptiongiven resembles that of Pioneers fS9. Helios,though, will weigh about 425 pounds, more thantwice as much as th e early Pioneers.

Two problems faced by Helios require drasticdepartures from Pioneer design philosophy.

B. C. Murray (CaliforniaInstitute of Technology)

H. T. Howard (StanfordUniversity)

H. S. Brid ge (M.I.T.)

N. F. Ness (Gaddard SpaceFlight Center)

A. L. Broadfoot (Kitt PeakNational Observatory)

S. S. Chase, Jr. (SantaBarbara Research Center)

J. A. S impso n (Un ivers ityof Chicago)

First, the solar heat necessitates a new thermalprotection strategy for the sensitive solar cells. The I

igure 23. The West German Helios solar probe. Solar ce lls an dmirrors cover the flared sides. The antenna reflector on to^ of the Ispacecraft is spun m echanically so tha t it always points at t he Earth.


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TABLE9. Helios Scientific Experiments

Instrument Scientific ObjectivesPlasma analyzer ( Measurement of th e solar wind velocityFluxgate magnetometers Measurement of th e interplanetary(2 separate experiments) magnetic field close to the Su n

Search-coil magnetometer Measurement of low frequency fluctuationsI in th e interplanetary fieldPlasma an d radio-wave Measurement of radio waves from 50 kH z toexperiment 2 MHa; measurement of plasma from 10 Hz

to 100 kH z

Cosmic-ray detector

Cosmic-ray detector

Electron detector

IMeasurement of th e energies of solar an dgalactic cosmic raysMeasurement of th e flux an d energies ofsolar and galactic cosmic rays;measurement of solar X-rays

I Counting of solar electronsZodiacal l igh t IObservation of zodiacal l ight wavelengthphotometer an d polarizationMicrometeoroid analyzer Measurement of th e masses an d energies of

cosmic dust particles

solar-cell surfaces are slanted (Fig. 23) sol thatthe sunlight does no t hit them directly. In addition,the solar cells are interspersed checkerboardfashion with mirrors having the same dimensionsas the cells. Heat from the solar cells is conductedto adjacent metal-backed mirrors and radiatedaway.

The second problem involves th e much greatercommunication distances. To be able to reach theEarth with it s telemetry, Helios' transmitter powermust be concentrated in a narrow beam. To thisend, a parabolic reflector is mounted behind theantenna on top of the spacecraft. However, theHelios spacecraft, being spin-stabilized, rotatescontinuously. The reflector, therefore, must be spunin the opposite direction so that it pointsperpetually at the Earth. This is called a"mechanically despun" antenna, and Helios is thefirst interplanetary probe to use one.

The ten experiments scheduled fo r flightonHelios are listed in Table 9.

Two Helios spacecraft are being built. Theplanned launch dates are in July 1974 and

October 1975. (Note that one does not have towait fo r just the right arrangement of planets tolaunch sola r probes.) From Cape Kennedy, Helioswillbe injected into a heliocentric orbit having aperihelion of 0.3 A.U. and an aphelion of 1.0 A.USince the orbital period will be roughly 200 days,the closest approaches to the Sun willoccur 100;300, 500, etc. days followingth e launch.

IN THE DECADEOF THE 1970s

I f only one planet-the Earth-circled the Sun, andit had no Moon, space travel would be animpractical dream. Lonely in the light years ofemptiness, we would have no convenientintermediate astronautical targets to land uponBut, in reality, we have stepping stones to the edgeof the so~lar ystem. And these planets are sodifferent from the Earth that our innate curiosityinsists that we explore them. What are oursister planets like? Do they harbor life? In thepreceding pages th e Mariners, Vikings, andPioneers of the immediate future have beendescribed. Their destinations are known; but theyare only the precursors.


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A D D I T I O N A LR E A D I N G

For title s of books and teaching aids related to thesubjects discussed in this booklet, see NASA'seducational publication EP-48, AerospaceBibliography, Fif th Edition.

Produced by the Office of Pu blic AffairsNation al Aeronautics and Space Adm inistration

* .S. GOVERNMENTPRINTINGOFFICE: 1971 0-440-917For sale by t h e Superintendent of Documents , U.S. Government Printing Office, Washington, D.C. 20402 Price 75 centsStock N u m b e r 3300-0399


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Planetary Exploration Space in the Seventies

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