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The Now Frontier. Linking Earth and Planets. IssueNo. 1-4.Jet Propulsion Lab., Pasadena, Calif.National Aeronautics and Space Administration,Washington, D.C.7416p.Jet Propulsion Laboratory, Public EducationalServices, 4800 Oak Grove Drive, Pasadena, California91103 (no price quoted)

MF-$0.75 HC-$1.50 PLUS POSTAGEAerospace Technology; *Earth Science; Instruction;*Instructional. Materials; *Science Activities;Science Education; TechnologyNASA Program

ABSTRACTThis publication includes four pamphlets providing

background material for understanding the NASA program of planetaryflights. Each issue presents student involvement activities as wellas suggested reading lists. Issue 1 describes the innermost planetsof the solar system. Issue 2 gives information about the evolution ofthe planetary system as well as specific notes cn the planets Venusand Mercury. "Mission to the Inner Planets" is the title given toIssue 3. The Mariner 10 mission is described in some detail. Issue 4,"Venus and Mercury Encounters," describes Mariner 101s encounter withMercury and Venus. (EB)

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THE NOW FRONTIERLINKING EARTH AND PLA\ETS

Innermost Planets of the Solar System

JET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGY

\ PASADENA, CALIFORNIA

ISSUE NUMBER ONE

PUBLIC EDUCATIONAL SERVICES OFFICE

THE INNER PLANETS

The dull-white star shone steadily across theAegean Sea in the warm glow of dawn. TheAncient Greeks called this star Apollo. Andthey thought it different from another dull-white star that often lingered for a few weeksin the sunset glow across the Ionian Sea.This star they called Mercury, the messengerof the gods. But by the time of Plato, theGreeks had discovered that the two stars areone, and are not stars, but a wandering planetalways moving close to the Sun like a motharound a candle flame.

Mercury is the innermost planet of the solarsystem. Venus orbits the Sun between theorbits of Earth and Mercury. Both planetsare now targets for a Mariner 10 spacecraftlaunched November 1973 and scheduled tofly past Venus in February 1974, and Mer-cury in March 1974. Venus has been reachedseveral times by spacecraft from the U.S. andthe U.S.S.R., but Mercury has never beforebeen reached.

Mercury averages 36 million miles from theSun, about 40 percent of Earth's distance,while Venus, at 67 million miles, is about 75percent of Earth's distance from the Sun.

Since the planet Mercury is so close to theSun and moves rapidly in its orbit to 2times faster than Earth) it flits from side toside of the Sun to be seen only ju :t beforesunrise and just after sunset. Its rapid motionand brief appearances and disappearances pro-bably caused the ancients to associate it withthe winged-foot messenger of mythology.

By contrast, Venus is closer to Earth, movesfurther away from the Sun in the eveningand morning skies, appears placid and bril-liant and is perhaps the most beautiful objectin the skies "Mistress of the Heavens" saidthe Babylonians. This planet was accord-

Cover: Mercury and Venus the innermostplanets of the solar system are shown herein their orbits around the Sun: first, Mer-cury (seen here as a crescent), and thenVenus; the Earth is next, with our Moonclose to it. Planets farther from the Sun areMars, Jupiter (shown with some of its satel-lites), and Saturn, with its rings. Uranus,Neptune, and Pluto are the most distantplanets.

ingly associated with the Roman goddess ofbeauty, Venus.

Planets of the solar system consist of twodistinct types: mall, dense, inner planetswith solid surfacesMercury, Venus, Earthand its Moon, and Marsand large, predom-inantly gaseous outer planetsJupiter, Sat-urn, Uranus, and Neptune. Pluto, the

outermost known planet, cannot be ob-served well enough to be accurately classi-fied, though it is believed to be similar tothe inner planets.

Also, between the orbits of Mars and Jupiteris a zone of minor planets called asteroids,the largest of which, Ceres, is only about 50Cmiles in diameter, while most are muchsmaller.

In distance outwards from the Sun, theasteroids divide the inner from the outerplanets, Jupiter being the first of the outerplanets, Mars the most distant of the innerplanets.

ANALOGY WITH EARTH ANDMOON

Venus is approximately the same size andmass as Earth, Mercury somewhat larger thanEarth's Moon. While Earth and Venus bothhave atmospheres, the Moon, and, apparently,Mercury, are airless bodies. Venus and Mer-cury might have been a twin system likeEarth and Moon, except that the closeness ofVenus to the Sun probably prevented Mer-cury from being a satelli;e of Venus and pre-vented Venus from possessing a terrestrial-type atmosphere and oceans.

From the point of view of planetary dyna-mics, Mercury is perhaps the most importantobject in the solar system. Being the closestplanet to the Sun, it is the most sensitivedetector of departures from the laws pro-posed to account for planetary orbital mo-tion. An unusual period of spin and an un-usually high density make Mercury of gr-satinterest to astronomers.

A space probe flying by Mercury can pro-vide important information about both thedynamics of Mercury and its interior. Theradius, mass, and orbit can be refined.

Venus is the planet most similar to the Earthin size, density, and distance from the Sun.However, it differs significantly in having a

much more massive atmosphere composedmainly of carbon dioxide, a much highersurface temperature, about 700°K, a muchslower backwards rotation period of 243.1Earth clays, and no moon or oceans.

Spacecraft traveling to Venus in the pasthave riot detected a significant magneticfield (the field being less than 1/5000 ofthe Earth's).

The closeness of the mean density of Venusto that of the Earth might imply that thechemical composition of the two planets isalmost the same. A question often askedis whether Venus is at a stage earlier or laterin evolution than the Earth, or follows anentirely different evolutionary path fromEarth.

Radar inspection of Venus from Earth re-veals a surface that is generally smootherthan the Moon and gently undulating. Thereare, however, several regions that appear tobe much rougher than their s...iTroundings,and some large shallow crater:.

SOLAR ORBITS AND APPEARANCEOF INNER PLANETS IN THE SKY(APPARITIONS)

It is instructive to look at Mercury and Venusfrom the standpoint of the early astrono-mers. Earthbound, they watched the motionsof the planets against the background ofstars and deduced that the planets, includingthe Earth, move around the Sun in almostcircular orbits. Because Mercury and Venusorbit the Sun inside the Earth's orbit, theyare termed inferior planets.

As seen from the Earth, inferior planets ap-pear to move close to the ecliptic (the ap-parent yearly path of the Sun relative to thestars, which is the plane of the Earth's orbitprojected against the stars), and to movebackwards and forwards, oscillating to eitherside of the Sun and never far trom it in thesky (Figure 1). The maximum distance toeast or west of the Sun is termed elongation.At eastern elongation, Mercury and Venus areseen in the evening sky as evening stars be-cause they appear to follow the Sun in its

daily Tlotion across Earth's sky due to the ro-tation of the Earth. At western elongation,they are ahead of the Sun and are seen asmorning stars before sunrise.

Table 1. Venus Picture Sequences

Date and time (PST) Sequence

Feb. 5, 8.28 to 8.58 a.m. Limb, cusps, and terminator scans

9.02 and 9.46 a.m. Point directly beneath the Sun on the limb

10.13 a.m. 163 photo strips of Venus

12.18 p.m. 238 ultraviolet for mosaics

2.12 p.m. to 4.51 a.m., 1207 photos for various mosaicsFeb. 6

Feb. 6, 9.35 a.m.through 408 pictures, one each hour

Feb. 22, 10.30 a.m.

The infrared radiometer can also be usedduring the approach to sweep across thedarkened planet and measure the temper-atures of the clouds on the night side ofVenus. Also, the magnetic field and particlesinstruments will observe the "tail" of Venusand the interaction of the planet and its elec-trically charged upper atmosphere (iono-sphere) with the solar wind. The ultravioletinstrument will check on the auroras on thenight side of Venus. These have been de-tected by earlier spacecraft and may beresponsible for the ashen light of darkenedVenus as sear' by astronomers from Earth.

At the closest approach of Mariner to within3100 miles of the cloud tops, the sunlit sideof the planet has become visible and thecameras are busily photographing the brilliantcloud tops. Filters are changed to inspectthe clouds in light of different colors anddifferent planes of vibration (polarization).Some of the pictures are along the terminatorfine, the boundary between night and clayon the planet. Because the Sun shines at avery low angle along the terminator it willreveal structural details of the clouds if theyare present, just as an automobile's head-lamps will show up small detail in the roadsurface to a pedestrian standing some wayin front of the automobile and looking downat the road.

A few minutes after closest approach toVenus, Mariner sees the nearly fully illumi-nated disc of Venus. Series of pictures aretaken in ultraviolet and polarized visible lightof several colors to obtain information about

the size of particles in the clouds, composi-tion of the clouds, pressures in the atmo-sphere, and small- and large-scale cloud struc-ture. Scientists hope to determine whetherthe clouds are hazy, foglike sheets or possessturbulent billowing tops like Earth's cumulusclouds. The cameras also photograph thelimb edge of the planet to look at the cloudtops as they appear silhouetted against thedark background of space.

As Mariner flies around the daylight hemi-sphere of Venus it becomes hidden fromEarth on the far side of the planet. Com-munications are temporarily interrupted. ButMariner 10 continues with all its scientificmissions, storing the information, includingphotographs, in its memory. Then when itemerges again from behind the planet andradio waves again can get hack to Earth, theinformation is transmitted.

As well as taking many photographs ofVenus, Mariner 10 scans the clouds in ultra-violet and infrared for clues as to theircomposition.

But quickly Mariner flies on, its rendezvouswith Venus over. As it heads back towardsinterplanetary space its instruments look atthe solar wind and magoetic fields and par-ticularly the effects of the bow shock wherethe planet plows into the environment ofspace like a supersonic airplane shocking thethe Earth's atmosphere.

And for several days (17) after encounter,planet photography continues, the cameras

looking back on the gradually diminishingdisc of the planet. This 2-week series ofpictures is very important. Scientists haveobserved rapid rotations of ultraviolet mark-ings on Venus and major up-and-down pulsa-tions of the atmosphere. So as Marinerrecedes from Venus, emphasis is placed ontime-lapse photographs of ultraviolet andany visible light markings to measure theirperiod of rotation about the planet, whichis believed to be about 4 days from Earth-based observations.

This task over, Mariner reverts to observa-tions of interplanetary space until it beginsto approach Mercury towards the end ofMarch 1974 (Figure 2).

About 4 weeks elapse from the last picturesof Venus to the first pictures of Mercury.Mariner 10's prime target is Mercury, sinceother spacecraft have flown to Venus butnone previously have gone to Mercury. Thesequences of pictures of Mercury are themost important part of the mission, and asmany pictures as Possible will be taken(Table 2). The sequence is divided into fivedistinct parts: incoming far encounter, in-coming near encounter, encounter, outgoingnear encounter, and outgoing far encounter.

Mariner 10 approaches Mercury almost on atangent to the planet's orbit near its furthestpoint from the Sun (aphelion). Each day inthe final week before Mercury encounter,Mariner sends back to Earth sequences ofpictures taken through several different colorfilters. Project scientists will inspect these tosearch for features of interest which can laterbe photographed in more detail as the en-counter proceeds. Since no one has seen anyreal detail on Mercury from Earth, this pre-encounter series of photographs is awaitedexpectantly. As with most earlier planetaryexplorations, the photographs will probablybe filled with many surprises.

Man's concepts of Mars were completely re-vised as a result of close looks by earlierspacecraft. And Mars shows quite extensivedetail when observed by telescope from

th. By contrast, Mercury shows little ifany detail when seen through an Earth-basedtelescope, so it has remained very much aplanet of mystery.

As Mariner bears clown on Mercury, theultraviolet instruments will search for evi-dence of an atmosphere and the infrared in-

Figure 2. An artist's conception of the Mariner 10 encounter with Mercury.

strument will be checking on the temperatureof the planet.

Just before encounter, the cameras will beprogrammed to sweep space around Mercuryto see whether the planet possesses any smallsatellites that would be invisible from Earth.

On the day of closest approach, March 29,1974, detailed photographic surveys are madeof the surface, particularly the boundarybetween light and darkness where detail willbe thrown into sharp relief by the low angleof sunlight shining on the planet's surface.Objects only 350 feet across are expected tobe revealed on these pictures. Rapidly theilluminated shape of Mercury changes froma half-moon to a crescent as the spacecrafthurtles towards close approach. About 12.00noon PST on March 29, Mariner flies within

600 miles of the barren siirface and thenstarts away from the planet. But this closeapproach is made over the night hemisphereof the planet and the surface cannot bephotographed. There are opportunities toobserve and photograph the pointed ends ofthe crescent of Mercury, as with Venus, atthe beginning and end of the night side pass.These may be inspected to check for atmo-sphere and for surface irregularities. Then,as Mariner moves away from the planet andover towards the daytime hemisphere, thepicture taking will start again. It will con-tinue until the image of Mercury no longerreveals any detail.

As with the Venus flyby, instruments onthe spacecraft will check the interaction ofthe planet with the solar wind and the inter-planetary medium.

Table 2. Mercury Near-Encounter Picture Sequences

Date and time (PST) Sequence

Satellite search, March 23, 5.00 to 8.30 a.m. 45 pictures

March 27, 3.30 to 6.30 a.m. 45 pictures

Near encounter, March 28, 8.24 p.m. to 8.30 a.m. (March 29) 162 pictures (9 mosaics)

Encounter, March 29, 8.30 to 11.24 a.m. 225 pictures

11.24 to 11.37 a.m. 18 pictures

12.10 to 12.22 p.m. 17 pictures

12.22 to 4.20 p.m. 329 pictures

5.30 p.m. to 3.30 a.m. (March 30) 144 pictures

In the following months, project scientistsand engineers inspect the wealth of informa-tion returned from Mercury and confer on aplan for a second encounter with this inner-most planet of the solar system.

And during this period, too, while Marinerinspects the interplanetary medium, it willalso look around with its cameras to seewhether there are any small planets withinthe orbit of Mercury.

There are plans for more than 7500 picturesof Venus and Mercury during the encounters.These will provide the first photographicsurvey of both planets and present informa-tion impossible to obtain from Earth. Manyof the pictures will be made available tonational television and the newspapers withinhours after being received, pictures thatgenerations of astronomers would have givenalmost anything to be able to see. ThusMariner 10 will complete much of the jigsawpuzzle of the terrestrial-type planets of thesolar system and allow better understandingof how these planets were formed andevolved to their present states.

STUDENT INVOLVEMENTStudent Project One

Take the various events listed in the tablesin this leaflet and make a combined listingof the sequential happening when Mariner 10makes its encounters with its interplanetarytargets. List the date, time, what happens,and leave a column for your remarks abouteach event. Remember that the times givenhere are for Pacific Standard 'lime (PST).(This could be an individual or a class

project.)

Classroom Project

Study the results of the Mercury encounter.Discuss any unknowns that have still notbeen clarified by the first flyby. With theknowledge that sufficient propellant existsto make another flyby, make a plan for thisnext flyby: whether it should fly on theday side or the night side, what it shouldphotograph, and Why. Then later, when thenext leaflet of this series is issued, check tosee whether your mission plan agrees withthat selected by the project scientists andcompare your reasons with theirs.

READING LISTWatch TV newscasts and special program.Listen to radio newscasts.Read your local newspaper L.

news-magazines,

P73-219 JPL 654 4.5M 1-74 PRINTEL

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

THE NOW FR 1 NTIERLINKING EARTH AND PLANETS

Venus and Mercury as Planets

JET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGYPASADENA, CALIFORNIA

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ISSUE NUMBER TWO

PUBLIC EDUCATIONAL SERVICES OFFICE

EVOLUTION OF THE PLANETARYSYSTEM

Planets of the solar system probably formedfour to five billion years ago when hosts ofsmall rocky particles and clouds of gasescollected together by their own gravity.Gravity appears to be a universal property ofmatter, as a result of which every particle,no matter how small, attracts every other.Thus, left to themselves in space, individualparticles (and a gas consists of particles:molecules) tend to collect together into largemasses.

So after the Sun condensed from the pri-meval nebula, planets of different sizes andprobably different composition originatedfrom concentrations of matter present atvarious distances from the Sun. And electricand magnetic fields in the gas forced thesecondensing planets into orbits around thecentral Sun and spun them on their own axeslike tops.

The larger craters on Mars, the Moon, andVenus are thought to be gouged by fallingbodies during the final stages of planetaryaccretion, as the process of falling together istermed. Smaller ones reprLsent a continuingbut much lower rate of bombardment bysolar system debris.

While Mariner 10, now on its way to Venus,is not designed to find out anything directlyabout life on Venus, the scientific informa-tion about conditions on Venus may be im-portant to biologists seeking understandingof why Earth spawned life and Venus didnot. It was life in the primitive atmosphereof Earth which helped to produce theoxygen-rich atrnosohere that in turn keeps alid on the oceans and stops them leakingoff into space. Oxygen, converted to ozoneat high altitude by the Sun's rays, preventssolar ultraviolet from penetrating low enough

Cover: The planet Venus as it has beenobserved from Earth through the techniqueof radar mapping, in which a radar signal isbounced off the surface of the planet. Thethree light areas marked with Greek letters(a, 0, and 5) are rough sections that may bemountains, but could also be craters orboulder fields. The mapping was done bythe Jet Propulsion Laboratory (JPL), usingthe NASA/JPL deep space antennas at Gold-stone, California.

into Earth's atmosphere to break water vaporinto oxygen and hydrogen. This is importantbecause the hydrogen would bubble off intospace like steam from a saucepan and grad-ually Earth would have lost its n-;eans in thisway.

A photographic mosaic of the Earth, takenby Mariner 10, is shown in Figure

THE CHARACTERISTICS OFVENUS

Venus is very similar to Earth in size andmass; its diameter is 7,520 miles comparedwith Earth's 7,920, its mass and density areslightly less than those of Earth. Physicallythe two planets are almost twins, but theyseem to have grown up quite differently.

Venus, too, is Earth's nearest neighbor inspace after the Moon. Its closest approach is26 millio;i miles compared with 34 millionmiles for Mars.

But a telescope reveals virtual y no details onthe bright disc of the planet. Some obser-vers have recorded faint and elusive markings,visible in the near ultraviolet, ill-defined darkshadows and bright patches seemingly be-having very much as cloud systems mightbehave.

The absence of surface features or persis-tent cloud features made it difficult to deter-mine the period of rotation of Venus on itsaxis. Wildly varying estimates were made,ranging from a 24-hour day like that of Earthto a day equal to the Venusian year. Thequestion was not answered until recentlywhen radio waves penetrated the thick cloudsof the brilliant planet. Surprisingly, it wasfound that Venus rotates in 243 days in theopposite direction to that of the Earth. Thisis slightly longer than the Venus yearthetime the planet takes to revolve around theSun, which is 225 Earth days. Because of

the rotation and revolution in opposite di-rections, the day on Venus is only 127 Earthdays. But these figures pose the question ofhow Venus might have slowed down fromrotating as the other planets do and startedto rotate in the opposite direction. One ofseveral theories is that Venus captured a large

Moon-like body moving in the opposite di-rection to Venus' spin and that this bodycrashed onto the surface of Venus. Theimpact would have stopped Venus from rota-ting and would also have released tremendous

Figure 1. A mosaic view of the Earth takenby Mariner 10 on its way to Venus andMercury. Such pictures were used by theengineers and scientists to test the space-craft's cameras in space. These pictures weretaken when Mariner 10 was 120,000 miles

from Earth.

quantities of heat that might have been thecause of extensive volcanism to generate thedense atmosphere of Venus.

Venus has been the target for several earlierspace missions: two successful flybys weremade by earlier Mariners, and Soviet Veneraspacecraft flew by, orbited, and landed cap-sules on the surface. These probes confirmeda high surface temperature of around 475°Cand a pressure at the base of Venus' atmos-phere about equal to that at a depth of 400fathoms in Earth's oceans. Generally Venusis a hot dry planet with only slight traces ofwater vapor in its atmosphere of 95 per-cent carbon dioxide. There are also traces ofoxygen, nitrogen, and inert gases, it is

believed.

The clouds of Venus, which, according to re-cent spectroscopic observations, may have atopmost layer of concentrated sulfuric aciddroplets, are about 18 to 25 miles thick, andtheir tops may be about 90 miles above thesurface, cornp9red with Earth's highest cloudsof six miles or so. Below the Venus cloudsis a clear atmosphere, while the clouds them-

selves probably consist of stacked layers ofdifferent composition; for example, carbondioxide ice at some levels and water ice atothers.

Cloud features seen in ultraviolet light appearto move around the planet in only four clays.There are also some large scale up-and-downpulsations of the cloud layers.

An interesting theoretical aspect of the densebut clear atmosphere below the clouds ofVenus is that if a student could stand on thesurface and look around, he would appear tobe standing in the bottom of a vast bowl.Looking into the distance in each direction,he would see a blurred, ruddy red landscapecurving upwards towards the cloud layers asthough he were inside a hollow planet: theatmosphere, acting like a giant lens, wouldbend the light rays upwards in an effectsimilar to a mirage appearing on a hot roadsurface in summer.

This strong bending of light might turn nightinto a dull day on the night side of the planet,and this could explain the ashen light ofVenus. Astronomers have claimed that theysee this faint glow on the dark side of theplanet when it is turned towards Earth. Butthis ashen light, first seen in 1643, is nowa-days believed to he an auroral glow, like thenorthern and southern lights over Earth'spolar region. Since Venus does not have astrong magnetic field, its auroras can occurall over the planet, whereas on Earth thestrong magnetic field causes the aurora-producing particles from the Sun to streamtowards the polar regions. One of the space-craft did observe the glow over the night sideof Venus, and the existence of an electricallyexcited upper atmosphere, the ionosphere,has also been confirmed by spacecraft.

Study of the atmosphere of Venus is im-portant because the whole balance betweenheat coming in and heat going out from theplanet is hound up with atmospheric struc-ture and composition. This heat budget ofenergy income and spending could explainwhy Venus is today such a vastly differentplanet from Earth despite the similarities insize. The great mystery about Venus is howa planet about the same size as Earth, whichmight have supported oceans in the past asEarth does today, developed so differentlyduring its evolution. Why did the planetlose all its water? Why did the surfacetemperature rise so high? How did the

atmosphere build up such high pressures?How could bodies plunge through this denseatmosphere to produce craters detected byradar?

Answers to these and other questions aboutVenus will help us understand how planetsevolve and why there is an Eden-like Earthwhile nearby planets are so inhospitable.Understanding how the atriosphere of Venusbecame, or stayed, inimical to life may helpus prevent our own atmosphere from goingwrong too, either from natural or man-made causes. An important question todayis whether pollution might trigger drasticchanges in Earth's atmosphere which couldchange our planet into another Venus.

At the distance of Venus from the Sun itstemperature would be expected to be about60°C on the average if it were an airlessbody. Why then is it so much hotter, es-pecially when its clouds reflect a large partof the solar radiation falling upon them?The answer seems to lie in the atmosphere ofcarbon dioxide. This gas acts, with watervapor, as a one-way transmitter of incomingheat energy from the Sun. It opens a doorto let the energy in and slams it shut whenthe energy tries to go out again. The planetheats up like the inside of an automobile insunshine with its windows closed.

By contrast, the atmosphere of Earth hasonly 0.03 percent of carbon dioxide. OnVenus, carbon dioxide has remained free inthe atmosphere, whereas on Earth in thepresence of much water it has reacted withminerals to form large deposits of carbonates.Photosynthesis in plants has also extractedcarbon from the atmosphere.

Another mystery about Venus is why it doesnot have much water. One suggestion is thatthe water is still entrapped in molten rockbeneath a plastic surface which resists frac-tures. In the formation of Venus, high tem-peratures drove off carbon dioxide but al-lowed water to remain in solution in therocks. As the planet cooled, a plastic crustformed of sufficient thickness to preventfractures and the consequent escape of waterto form oceans.

However, the fact that the planet has notoutgassed suggests little differentiation or, atthe least, an inactive mantle. If the planetwere to outgas, the crust would not be likelyto keep the lid on this outgassing.

One theory suggests that water is trapped involuminous polar caps of Venus; another,that all the water was lost because watervapor could rise in the atmosphere of Venusand be broken down by sunlight into oxygenand hydrogen. The hydrogen boiled off intospace. But what happened to the oxygen? Itdoes not appear to be left in the atmosphere.

On the other hand, the major differencesamong the terrestrial planets may have arisenbecause these planets formed at different dis-tances from the Sun and thus consisted ofdifferent materials in the first instance. Forexample, Mercury might have formed frommaterials rich in iron, whereas Venus formedfrom silicate-rich materials. And Earth mayhave accreted in a region in which there werewater-containing substances while Venus didnot. Calculations have shown that the pri-meval nebula might have separated in thisway at various distances from the Sur.

THE CHARACTERISTICS OFMERCURY

Mercury, 3000 miles in diameter, is probablythe smallest planet in the solar system (Fig-ure 2). The outermost planet, Pluto, mightpossibly be smaller. Mercury is halfwaybetween the Moon and Mars in size. Eventwo satellites of the planet Jupiter are aslarge as Mercury, though much less dense.

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Figure 2. A photograph of Mercury taken atthe Pic du Midi observatory in France. Thisis the best type of picture that can be takenfrom Earth, but Mariner 10 should sendback Mercury photographs as good as the

picture of Earth in Figure 1.

At one time it was thought that the closenessof the planet to the Sun caused it to turn onehemisphere eternally sunwards, just as theMoon turns one hemisphere towards Earth.However, radio astronomers discovered veryrecently (1965) that Mercury rotates on itsaxis in 58 days. Coupled with the planet's88-day period of revolution around the Sun,this gives Mercury a solar day of 176 Earthdays. So one day of Mercury occupies twoyears of Mercury time. And because theplanet follows an elliptical path around theSun with its speed in orbit changing as aconsequence, the path of the Sun throughMercury's sky is quite erratic. Some parts ofMercury experience a double dawn, for ex-ample, in which the Sun rises, then slips backbelow the horizon, to rise again later.

Through a large telescope the planet pre-sents a yellowish color broken by some gray-ish patches. Mercury is believed to becratered and much like Earth's Moon. Thegrayish areas may be the same as the lunarmaria, the great gray plains of the Moon.Mercury, like the Moon, does not appear tohave an appreciabl? atmosphere, and it is apoor reflector of light. But its density isbelieved to be much more than that of theMoon, possibly a little greater than theEarth's. This is unexplained. It leads to theanomaly that Mercury's surface gravity is

greater than that of the bicger planet Mars.A 100-pound student would weigh 40 poundson Mercury, compared with 38 pounds onMars and 17 pounds on the Moon. On

Venus the same student would weigh 91pounds.

It is unlikely that Mercury can possess an at-mosphere, though some astronomers say theyhave seen veilings of surface detail at times,in the form of a whitish haze. Despite Mer-cury's greater surface gravity c-Per that of theMoon, Mercury is closer to the Sun and there-fore much hotter than the Moon: moleculescan be heated to move at greater velocitiesand thereby reach escape velocity. Thus,gases that may have come from the rocks asa result of volcanic activity will, over mil-lions of years, have escaped from the planet'sgravitational grasp and rocketed as moleculesinto space.

Mercury does, however, collect plasma fromthe Sun, the steady stream of particles flungout by the Sun into interplanetary space andreferred to as the solar wind. Some of thisplasma might indeed form an atmosphere of

a kind, as it is temporarily held by Mercurybefore streaming off again into space.

The surface temperature of Mercury is

thought to range from about 325°C at localnoon to -125°C at local midnight, but thesetemperatures vary considerably with the po-sition of Mercury on its orbit as the planetmoves in and out from the Sun.

Mercury's surface is exposed to the fierceerosion of the solar wind as well as to solarheat and light. As with Earth's Moon, thesolar wind probably changes the compositionof the outer surcaces of rocks and surfacesoils. Solar radiation can vary between fiveand ten times that received by Earth be-tween aphelion and perihelion of Mercury(most distant and closest parts of its orbit tothe Sun). Imagine a day in Earth's desertwith ten suns shining at once in the noonsky. That approaches what it may be likeon Mercury when the planet is at perihelion.

Key questions about this tiny planet concernits rotation, density, and surface molding.Why does Mercury rotate three times whilemaking two revolutions around the Sun?Might this have produced some unusual sur-face features? One theory suggests that theheat from the Sun at perihelion may haveproduced two opposing bulges on the planetwhich now keep Mercury locked into thispeculiar rhythm of rotation and revolution.

Why does Mercury have the highest densityamong the planets and their satellites? Doesit have a central core of iron? And whatprocesses have shaped the surface of the in-nermost planet?

Mariner 10 is expected to provide muchvaluable information bearing upon thesequestions.

STUDENT INVOLVEMENT

Student Project One

From reading astronomical textbooks makefour lists as follows:

Student Project Two

From the textbooks now make a list of theunknowns about Mercury and Venus, theirsurface features, their atmosphere, theirphysical characteristics, their origin, theirevolution. Check with a later pamphlet inthis series as to what instruments are car-ried by Mariner 10 and what they might re-veal. Identify those present unknowns thatmight he solved by Mariner 10. Later checkthe actual results from the flybys and seewhich of the problems are solved.

Student Project Three

For individual student or as a classroom pro-ject. Using the map of the orbits of Earth,Venus, and Mercury prepared as a project inconnection with the first pamphlet of thisseries, mark a position for Earth with a cir-cle on its orbit. Then on the orbits ofVenus and Mercury draw circles, approxi-mately to scale (that is, the circle represent-ing Venus should be about the same size asthat for Earth and about 21/2 times the di-ameter of Mercury) at the correct positionsfor eastern and western elongation, superiorconjunction (far side of the Sun) and inferiorconjunction (between Earth and Sun) andhalf way between each of these positions,that is, eight positions for each orbit.

Now draw a set of circles showing the rela-tive sizes of each planet as seen from Earthfor each of the eight positions. On each ofthese comparative circles draw the phase ofthe planet, that is, the part that is in dark-ness as seen from Earth and the part illumi-nated; for example, fully illuminated like thefull Moon, half illuminated like a quarterMoon, gibbous, and crescent shaped. Youwill have to calculate the size of the planetat each of these phases from the rule thatif the planet is twice as far from the Earthit will appear half as large (in diameter) andso on.

Note how much smaller Mercury always ap-

(1) Those things about Venus that are pears to an observer on Earth even when

similar to those on Earth. close to the Earth, and how both planets

(2) Those things that are different. turn their dark hemispheres towards us when

(3) Those things about Mercury that aresimilar to those on the Moon.

they are at the very closest.

(4) Those things that are different. Thus you understand why we know so little

Make a drawing or painting of your impres-sion of the surface of Mercury.

about these planets and why a spacecraftflying by them can add greatly to man'sknowledge of these neighboring worlds.

P73-219 JPL 654 4.5M 1-74 PRINTED IN U.S.A.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

THE NOW FRONTIERLINKING EARTH AND PLANETS

JET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGYPASADENA, CALIFORNIA

Mission to the Inner Planets

r

ISSUE NUMBER THREE

PUBLIC EDUCATIONAL SERVICES OFFICE

THE MARINER 10 MISSION

Mariner 10 arrives at Venus on February 5,1974, after a 3-month, 150 million mile triphalfway around the Sun. The gravity ofVenus changes the speed of the spacecraftto plunge it deeper into the gravitationalfield of the Sun to reach the orbit of Mer-cury and rendezvous with this innermostplanet on March 29, 1974.

Mariner 10 sped from Cape Canaveral, Flor-ida, November 2, 1973, atop an Atlas/Centaur multistage rocket launch vehicle.And, in contrast vith earlier planetary space-craft, it tried out its instruments On theEa-th and Moon as it sped at 25,255 milesper hour on its way to Venus. The day fol-lowing launch, scientists directed Mariner'stwo television cameras to produce fourmosaics of 88 pictures each of Earth andMoon. Since Earth is cloud-covered likeVenus, and the Moon airless and opticallysimilar to Mercury, these picture series gavevaluable early information on how the cam-eras might return information about the twotarget planets. Thus the instruments werecalibrated. Other instruments too were testedon the Earth and Moon.

Experimenters were delighted at the clarityof the returned pictures. The Earth photo-graphs revealed clear details of clouds; someprovided three-dimensional views. And thepictures of the Moon showed north polarregions not seen by astronauts, photographsthat recorded objects only 2 miles across.

Later the cameras photographed star fieldsand further confirmed the high quality of theimages being returned over the radio link toEarth.

THE SPACECRAFT AND ITS PATH

Mariner 10 is the first complex spacecraft tobe designed to travel into the inner reachesof the solar system as close to the Sun as 43million miles. Developed from earlier space-craft in the Mariner series that flew by Venusand Mars and orbited Mars, Mariner 10 facesnew challenges of the interplanetary environ-ment.

Cover: Mariner 10 on its path from Earth(in the upper left part of the picture, withthe Moon close by). The spacecraft has justpassed Venus and is on its way to Mercury.

1. The Mariner 10 spacecraft.

For example, at Mariner's closest approach tothe Sun, it recPives five times as much lightand heat as it does on leaving Earth. Thisrequires that the solar radiation must bescreened from the spacecraft and its scien-tific instruments to pri,,ent them from over-heating. A sunshade of beta cloththematerial used for astronauts' suitsis ex-tended as the spacecraft leaves Earth.

The solar panels, too, which extend intosunlight to collect and convert solar radia-tion into electrical energy for the spacecraft'sinstruments and controls, are designed to betilted more and more from the sunlight asMariner 10 approaches closer and closer tothe Sun.

Figure 1 shows the main features of theMariner spacecraft. There is a basic eight-sided framework which encloses eight baysof electronics. This structure is 54'/2 inchesacross and 18 inches deep. Its center cavityis almost filled by a spherical tank of rocketpropellant and a rocket motor used tomaneuver the spacecraft. The nozzle of thisrocket projects through the base of the struc-ture, which is on the sunwards side duringthe flight. Below this structure is the heatshade, through a hole in which the rocketnozzle projects.

Two solar panels extend on ,ther side of thebasic structure for almost 9 feet. There arealso two antennas for communicating withEarth. A low-gain antenna projects almost10 feet from the top of the basic structure.A high-gain parabolic antenna, 54 inchesacross, extends to one side and is motor-driven to point towards Earth.

There are also sensors to track the starCanopus and the Sun, and a battery ofscientific instruments. Two identical televi-sion cameras peer like twin eyes from the topof the structure. Together with an ultra-violet spectrometer, they can be pointed byEarth command. Other instruments areaffixed directly to the basic structure andare fixed relative to the spacecraft. Also,booms extend from the basic structure, car-rying instruments to measure magnetic fieldsand the interplanetary plasma of chargedparticles.

Mariner 10 carries reliable communicationradio systems to accept commands fromEarth and return scientific and engineeringinformation. The spacecraft also carries acomputer so that commands can be acceptedby Mariner that will trigger a whole sequenceof events, as well as direct commands to per-form specific tasks, such as switching on apiece of apparatus.

VENUS

MERCURsr

HORIZON

0SUNBE LO`::HORIZON

Figure 1. The appearance of Mercury and Venus as evening stars as they travel aroundthe Sun, inside the orbit of Earth.

Because the orbits of these interior planetsare completely contained within the Earth'sorbit, both Mercury and Venus pass betweenEarth and Sun. This is termed inferior con-junction. When the planets are on the far sideof the Sun from Earth, they pass throughsuperior conjunction. And because the orbitsof the Earth and the two planets are notexactly in the same plane, that is, they aretilted slightly with respect to each other likecrossed hoops, Mercury and Venus normallyPass through conjunction above or below theSun. Infrequently the orbits line up so thatthe planets pass across the face of the Sun ina transit or behind the Sun in occultation.Occultations are not observable because ofthe brilliance of the Sun, but transits are.(It was Captain Cook's voyage to observe atransit of Venus where it was visible in theSouth Pacific that lead to his discoveringTahiti.)

Transits of Venus occur very rarely: the mostrecent occurred in 1882, the next are not dueuntil the beginning of the next centuryJune7, 2004, and June 5, 2012 (they occur inclose pairs). Transits of Mercury occur muchmore frequently. One was visible from theEast Coast of the United States on Novem-ber 11, 1973. The next transit will take placeon November 12, 1986.

Mercury revolves around the Sun in a periodof 88 days, Venus in a period of 225 clays.But their visibility in Earth's skies must alsotake account of Earth's movement aroundthe Sun. So Venus repeats its apparitions(elongations and conjunctions) approxi-

mately every 584 days. Mercury repeatsapproximately every 116 days. But sinceMercury's orbit varies much more from atrue circle than does that of Venus, the re-petition of Mercury's positions relative to theSun in Earth's skies varies too.

The distance of Mercury from the Sun inthe sky at elongation also varies, from only18 degrees to as much as 27 degrees, andthus affects its visibility. Mercury is a rela-tively dull object. Like the Moon, it does notreflect much of the sunlight falling on it, soit does not appear very bright in the sky.Moreover, Mercury can rise before the Sunor set after the Sun by only 21/4 hours, so itis rarely seen in a dark sky, but usually onlyin the twilight glow, low down near the hori-zon, competing with the sunset. And theplanet cannot be seen for much longer thantwo weeks close to the time of its elonga-tion. There is an average interval of 44 daysbetween Mercury's appearance as an eveningand a morning star.

By contrast, Venus can move as much as 47degrees from the Sun and can be seen in thelate evening or early morning skies as thebrightest object after the Moon. Venus re-flects a large proportion of the Sun's lightfalling upon it and it appears very bright inthe skies of Earth. Venus is brightest aboutone month before and after inferior con-junction, when a telescope shows it as a fatcrescent shape. It can then become so brightas to cast distinct shadows. The planet canbe observed for many months and has evenbeen observed through binoculars as it passes

above or below the Sun at closest approach.It is also clearly visible in daylight if an ob-server knows where to look; for example,when the planet appears close to the Moonin the sky. Venus passes from greatest elon-gation as an evening star to greatest westernelongation as a morning star in about 140days, and from a morning star back to anevening star in about 430 days.

SIGHTINGS AND MOTIONS IN THESKY

During December 1973, Venus was a bril-liant evening star close to the planet Jupiter.Venus passed between Earth and Sun onJanuary 24, appearing above the Sun as seenfrom Earth's northern hemisphere. ThenVenus becomes a brightening morning starthrough February and March, gradually ris-ing earlier and earlier before sunrise (Fig-ure 2).

Mercury is an evening star observable fromthe beginning of February through about thethird week of that month, then passes abovethe Sun early in March and becomes a morn-ing star with good elongation from the Suntowards the end of March. Unfortunately,Mercury as a morning star in the spring is notwell suited to observation since it is veryclose to the horizon south of the point ofsunrise (Figure 3). It will be in the constella-tion Aquarius, where there are not manybright stars to help identify the planet.

Venus, too, will be in Aquarius at this timebut will be unmistakable because of itsbrightness, even though it is also close tothe horizon.

CYGNUS

soo ALTAIR

DELPri INUS

see

VENUS

CAN; ICORNUS

EAST HORIZON SOUTHEAST

Figure 2. Venus at the day of Mariner 10flyby (February 5, about 6:30 a.m.,

Pacific Standard Time).

SQUARE OFPEGASUS

*- VENUS

MERCURY

EASTHORIZON SOUTHEAST

Figure 3. Mercury at the day of Mariner 10flyby (March 29, about 6:00 a.m.,

Pacific Standard Time).

ihe comet Kohoutek, visible in late Decem-)er and early January close to Venus andlupiter in the evening sky, will still be a faint)bject in the evening sky when Venus andMercury have become morning stars during:ebruary.

)HASES OF THE INNER PLANETS

ks the inferior planets move around the Sunhey display phases, as seen from Earth, corn-iarable to those of the Moon. When Mercuryrid Venus are on the far side of the Sunhey appear fully illuminated like a full/loon, but because of their great distanceshey are then unfavorably placed for obser-ation an show only very small discs. Atastern al.cl western elongations, Mercurynd Venus appear half illuminated, like half-noons. Then as they swing between Earthnd Sun, the planets display a narrowingrescent phase to Earth until, if they crosshe disc of the Sun, they appear as blackpots upon it. Most times they pass eitherlightly above or below the Sun as seen from:arth and thus, in a telescope, can he oh-erved as a fine crescent all the way throughnferior conjunction.

;TUDENT INVOLVEMENT

iassroom Project

lave one student stand in the center of thelassroom (which is preferably semi-arkened) to represent the Sun. Have anothertudent stand at the front of the class to re-resent the observer on Earth. Position twother students to represent Mercury and'enus, the first almost half way betweenun and Earth. the second three quarters of

the way. Have these two students hold awhite styrofoam ball, big enough for otherstudents to see (about 4 inches in diameterfor a medium-sized class).

The student representing the Sun shines aflashlight on the globe representing Mercury,and the Earth observer, standing close to ablackboard, draws what he sees, a darkglobe. The student Mercury then moves partway round to the side of the class as thoughorbiting the Sun. Again the flashlight is

shone on the globe from the Sun, and theEarth student now draws the half-lit planethe sees.

The Venus student demonstrates likewise,then the class is asked to draw the visibleilluminated shapes of Venus and Mercury asthese planets move around the Sun as seenfrom Earth, at closest approach to Earth,at greatest elongations, and when most dis-tant from Earth.

Student Project One

Early in January, look at the evening skywhen it starts to become dark after sunset.The Sun set., in the southwest. Look leftfrom the sunset boirt, and close to the hori-zon there will be a fairly bright star, which.s the planet Venus. If you have access to asmall telescope or very good field glasses youwill be able to see that it is shaped like atiny, very fine crescent Moon. If CometKohoutek is still a bright object, Venus willbe between the head or the cornet and thepoint on the horizon where the Sun set. In

the following days, observe how Venus movesdeeper and deeper into the sunset glow untilit finally disappears.

About the first week in February, get upbefore sunrise and look for Venus in themorning skies before the Sun comes up andwhile the sky is still fairly dark. Try to seeVenus on February 5, since this will he theday when the Mariner spacecraft flies pastthe planet.

Continue to observe Venus in the followingweeks arid watch how it moves further fromthe sunset glare and becomes brighter dayIT, day. If you have access to a telescope oh-serve, too, that it is now a crescent racingopposite to the way it faced when it was anevening star.

Around mid-March, start looking for Mer-cury, a very faint star-like object in themorning sky between Venus and the point ofthe sky where the Sun will rise. Try to seeMercury on March 29, the date Mariner fliespast Mercury, and you will be able to com-pare what you see in the sky, a faint star-like object, with the close-ups of a possiblyMoon-like cratered world seen on the tele-vision newscasts that evening.

On this date, Mercury will be about one-thirdof the way between Venus and the Sun butbelow the direct line between Venus and Sun.Of course you will not be able to see eitherwhen the Sun is in the sky unless you ob-serve with a precisely pointed astronomicaltelescope, so you will have to estimate wherethe Sun is below the horizon.

Student Project Two

This can also be a classroom project on theblackboard. Refer to an astronomical text-book and draw a plan map of the solar sys-tem snowing the orbits of Mercury, Venus,and Earth. Try to draw them to scale, andnote that Mercury's orbit seems offset to oneside of the Sun. Than try to work outwhere the Earth and the two planets must beto be seen in the sky as they are. Remember,as seen from above (north), Earth rotates onits axis and all the planets revolve around theSun in a counterclockwise direction. Re-

member, too, that Venus moves betweenEarth and Sun on January 24 and Mercurybetween Earth and Sun around February 25.Plot points on the orbits every ten daysworking backwards and forwards from thetimes of conjunction. Save your solar sys-tem plan map for a later exercise.

READING LIST

G. J Abell, Exploration of the Universe,Holt, Rinehart and Winston, New York,1964, rp. 23-26 (orbits).

J. H. Wilson, Mariner Venus Mercury 1973,Technical Memorandum 33-657, Jet Pro-pulsion Laboratory, Pasadena, California,October 15, 1973.

L. Rudaux and G. De Vaucouleurs, LarousseEncyclopedia of Astronomy, PrometheusPress, New York, 1959.

P73-219 JP1. 654 4.5M 1-74 PRINTED IN U.S.A.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

THE NOW FRONTIERLINKING EARTH AND PLANETS

Venus and Mercury Encounters

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JET PROPULSION LABORATORYCALIFORNIA INSTITUTE OF TECHNOLOGYPASADENA, CALIFORNIA

1

ISSUE NUMBER FOURPUBLIC EDUCATIONAL SERVICES OFFICE

ENCOUNTERING THE PLANETS

At 8.57 a.m. PST on February 5, 1974,Mariner 10 will speed past Venus only 3000miles above the brilliant cloud tops and sixminutes later disappear behind Venus. Com-munication with the spacecraft will then belost for 21 minutes until Mariner emerges onthe other side of the planet.

For several weeks around that date, teamsof spacecraft controllers, engineers, analysts,and scientists will be running a 24-hour-a-day,three-shift operation, inspecting engineeringand scientific information pouring in fromthe spacecraft and commanding and con-trolling its actions. The solar panels are tiltedat increasing angles from the Sun to protectthem from the greater intensity of the sun-light, and the necessary corrections are madein the trajectory.

Mission Operations System activities are

centered in the Mission Control and Com-puting Center at the Jet Propulsion Lab-oratory and make use of a widespread grounddata system consisting of facilities spreadaround the world as a part of the trackingand data system network.

Information from the spacecraft is presentedto flight controllers as quickly as it is re-ceived and processed by the computers; thisis almost as the events happen. The onlyreal delay is the timeseveral minutestakenfor the radio waves to travel from Venus andMercury to Earth. Television pictures of theplanets are displayed on monitors similar tohome television sets so that the scientists canquickly see the quality of the informationcoming from the spacecraft.

Computers back up the men and women atthe Mission Control and Computing Center.Many computer programs are available forquick instructions to the spacecraft. For ex-ample, suppose a scientist sees somethinginteresting on a picture returned from Mer-cury, an unusual crater or a volcanic cindercone or a sinuous valley, he can call for acomputer program that will generate instruc-tions to the spacecraft camera pointing sys-tem to photograph this area again despitethe rapid movement of the spacecraft andthe rotation of the planet.

About 120 persons are involved in operatingthe spacecraft to ensure the success of the

Cover: An artist's conception of Mariner 10as it approaches the planet Mercury.

mission by keeping the spacecraft performingas it should and directing it to make the ex-periments required by the project scientists.

The men and women of the project have tobe on the alert every second of the encounter,ready to issue correcting commands to rightany problems that might arise in the space-craft and around the worldwide data gather-ing net. Communications are maintained withthe spacecraft at all times, 24 hours eachday. As the spacecraft sets over the trackingstation at Goldstone in California's MojaveDesert, it is rising for the station at Canberra,Australia. And as it sets at that station itcomes into view at the next station, nearMadrid, Spain. Then it is handed back toGoldstone. Antennas the size of footballfields gather the incoming signals and passthe information to the control center at theJet Propulsion Laboratory in Pasadena,

California.

ENCOUNTER SEQUENCES

The first encounter is with a comet, but thisis not a close encounter, since CometKohoutek has swung around the Sun awayfrom Mariner. Nevertheless Mariner has theopportunity of inspecting the comet fromspace, photographing its head and probingit with the ultraviolet instrument. And be-cause Mariner 10 looks at the comet at adifferent angle from observers on Earth,photographs can be obtained simultaneouslyfrom Earth and Mariner that provide twoviews of the comet, a stereo pair. These two

photographs, when looked at togetheronewith the right eye and the other with theleft in a special viewer will let scientists seean unprecedented three-dimensional view ofthe comet.

Comet Kohoutek will be observed fromMariner during the period January 17 through25, 1974. The ultraviolet airglow instrumentof Mariner 10 inspects the coma (the hazyhead of the comet) and the comet's long tailabout January 17. Picture mosaics ofKohoutek will be obtained on January 19,22, and 25. Four television pictures will betaken every hour for 9 hours each ofthese days.

The second encounter is with Venus (Figure1). Mariner approaches Venus from the nightside of the planet. Viewing is thus unfavor-able for photographs, since the cloud topsare shrouded in darkness. Moreover, sincethe spacecraft is in an attitude to screen itsinstruments from the intense radiation fromthe Sun, its sunshade obstructs a view ofVenus. The cameras on the spacecraft donot get a glimpse of Venus until about 28minutes before closest approach to theplanet (Table 1). This opportunity may betaken to obtain photographs of the pointedends of the crescent shape seen from Mariner,the cusps of Venus. These ends are impor-tant because the atmosphere causes them tobe illuminated into the dark hemisphere, andfrom the observations scientists can obtainmore information about the composition andstructure of the Venusian atmosphere.

Figure 1. An artist's conception of the Mariner 10 encounter with Venus. The antennason the Earth, of course, are not drawn to scale.

Some information collected by the spacecraftcan be stored on magnetic tape within it andtransmitted later. This permits the spacecraftto collect data when it is hidden from Earthbehind a planet and later to send this datawhen it emerges.

The Mariner 10 spacecraft weighed 1042pounds at launch, including 64 pounds ofrocket fuel and nitrogen gas.

A requirement for this new spacecraft is totransmit much more information back toEarth than earlier flyby spacecraft duringthe brief periods of encounter with the twotarget planets. So the rate at which infor-mation is transmitted over the radio linkshas been increased. This is like talking fas-ter on the telephone if you want to describeas much as possible about an event as itactually happens.

This higher date .ate, as it is termed, per-mits Mariner to send back more live picturesof the planets as it flies past them.

Another requirement for the mission was toensure very accurate flight. Because thespacecraft uses the gravity of Venus to swingit on a path to Mercury, small errors in itsapproach to Venus would be magnified athousand times at Mercury unless correctedby the rocket engine of the spacecraft. Butsince project scientists want to conserve asmuch fuel as possible so that they can corn-

VENUS ATLAUNCH

EARTH ATLAUNCH ABOUTNOY 1, 1973

mand the spacecraft into other maneuversafter it passes Mercury, the spacecraft has tobe aimed as accurately as possible in theearly stages of flight. Even so, four correc-tive thrust periods are planned: the firstjust after leaving Earth and Moon, the sec-ond just before arrival at Venus, the thirdafter flying by Venus, and the fourth beforearrival at Mercury.

Three further maneuvers are needed to bringthe spacecraft back to Mercury again (Sep-tember 1974) 6 months after the first pass.And if there is propellant left, a third passby Mercury could be attempted 6 monthslater (March 1975).

The success of all these maneuvers reliesupon controllers having accurate knowledgeabout the speed of the spacecraft and itsposition along its path, as well as the direc-tion of its path. Such information is obtainedby tracking the spacecraft from ground sta-tions at Goldstone, in the Mojave Desert,California; in Spain; and in Australia. Thethree tracking stations pass information to acontrol center at the Jet Propulsion Labora-tory in California where navigation special-ists, aided by powerful computers, deter-mine the exact position and speed of thespacecraft. Computers predict where thespacecraft is headed. Navigators decide onchanges, and mission controllers send corn-mands to the spacecraft to make the neces-sary change:, in its path.

MERCURY ENCC,JNIERMAT 20, 1974

0

MERCURY AT LAUNCHAND JAN 28, 1?74

VENUS

i

,I)eNcour, TERFEB 5, 1974

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Figure 2. The Mariner 10 flight path to Venus and Mercury.

EARTHMAR 29,1974

EARTHFEB 5, 1974

Arrival times and distances have to be ar-ranged precisely so that instruments can notonly photograph the target planets at re-

quired angles and covering specified areas,but also look at the planets at infrared andultraviolet wavelengths and make other ex-periments.

For all these tasks, timing is extremely criti-cal because of the short time that the space-craft is near the planet.

Mariner 10 follows a curved path to Venus(Figure 2), going about one-quarter of theway around the Sun before it meets withthe planet. When this path is bent by thegravity of Venus, it curves more steeplytowards Sun. The spacecraft then fol-lows the new path for about another quarterof the way around the Sun until it meetswith Mercury.

If the spacecraft were not slowed by thegravity of Venusby the way in which it isdirected to fly past that planetits speedwould be too high to allow the gravitationalfield of the Sun to pull Mariner to the orbitof Mercury. Without the aid from the gra-vity of Venus, the path of Mariner woulddip only slightly inside the orbit of Venus,and to reach Mercury by a direct launchwithout a gravity assist from Venus wouldrequire a larger, more expensive launch ve-hicle or a lighter payload that could includeonly a few scientific experiments.

After encounter with Mercury, Mariner 10zooms outwards again and crosses the orbitof Venus, but Venus is far away on its orbit.Later, Mariner 10 moves back to Mercury'sorbit and now it is lucky. Mercury has madetwo circuits of the Sun while the spacecraftmade one. Mariner meets Mercury for thesecond time. And if sufficient propellant re-mains in the spacecraft, a third rendezvouscan be made with Mercury the next timearound the spacecraft's orbit.

WHAT THE SPACECRAFT DOES

Mariner 10 is designed to investigate Mer-cury and Venus in several ways:

(1) Measure particles, fields, and radiationfrom the planets and their environs.

(2) Televise pictures of the planets withgreat detail to show cloud structureon Venus and the surface structureof Mercury (Figure 3).

(3) By ground-based observations of thepath of the spacecraft, measure thegravity field of Venus and Mercury,their sizes, and their masses.

(4) By observations of the radio signalscoming from the spacecraft as itgoes behind Venus and Mercury asseen from Earth, determine thestructure of the Venus atmosphereand check whether Mercury has atenuous atmosphere of any kind.

In addition, the spacecraft also provides in-formation on the interplanetary space fromEarth's to Mercury's orbit, and on how theplanets interact with this interplanetary en-vironment.

To perform these tasks, Mariner 10 uses abattery of scientific in:trurnents. A mag-netometer measures magnetic fields, a plas-ma analyzer measures the ions and electronsflowing through space from the Sun (thesolar wind), cosmic ray telescopes studysolar and galactic cosmic rays. The mainobjective of all of these instruments is tolearn about the planets by studying theireffects on the interplanetary environment,such as magnetic tails and bow shocks con-nected with them.

An infrared radiometer (heat measurer) mea-sures temperatures of the clouds of Venusand the surface of Mercury. Two indepen-dent ultraviolet instruments (measuring lightbeyond the violet of the spectrum) analyzethe planetary atmospheres. One instrumentis fixed to the body of the spacecraft andis used at Mercury to search for traces ofatmosphere along the edge of the planet'sdisc. A scanning instrument can be pointedon command. This is used to scan the en-tire discs of the planets, searching for evi-dence of hydrogen, helium, argon, neon,oxygen, and carbon. Close to Earth, thissame instrument measured the hydrogen co-rona of the Earth and the reflective proper-ties of the Moon. At Venus both instru-ments search for specific gases, and duringthe cruise phase they look for sources ofradiation coming from hot stars and nebula(or gas clouds) in the galaxy.

A complex of two television cameras witheight filters, capable of taking both narrowand wide angle pictures, photographs Earthand Moon (for comparison purposes) andVenus and Mercury. Mounted on a plat-

form, the complex is directed by commandfrom Earth. As well as taking pictures indifferent colors of light, these cameras alsoreveal how the light is vibrating (its polari-zation). Such observations reveal informa-tion on the composition of the ciouds ofVenus and the surface of Mercury.

A radio experiment uses the signals trans-mitted from the spacecraft to Earth. Bytracking the spacecraft's signals, experimen-ters determine how the spacecraft is affectedby the gravitational fields of the planets.From this information they may determinethe shape of each planet and determinewhether there are any concentrations ofmass that distort the gravitational field, likethe mascons (mass concentrations) discoveredbeneath the maria of the Moon. By analysisof what happens to the radio signals as theypass close to the edge of the planet when thespacecraft goes behind the planet as seenfrom Earth, experimenters probe the atmos-phere of Venus to determine its characteris-tics and constituents and to check for anytenuous atmosphere of Mercury. If one isdetected, its characteristics will also bedetermined.

To take full advantage of the Venus occul-tation, which will bend the radio signalsappreciably, an antenna on the spacecraft issteered so as to partially compensate for thebending. In this manner, information can beobtained to much deeper levels of the Venu-sian atmosphere.

A project of NASA's Office of Space Science,the Mariner 10 project is managed by the JetPropulsion Laboratory of the California In-stitute of Technology, Pasadena, California.The spacecraft was built by The BoeingCompany, Seattle, Washington. Tracking isby NASA's Deep Space Network, operatedby the Jet Propulsion Laboratory. Thescientific instruments are supplied by NASAcenters, universities, and private industry.

STUDENT INVOLVEMENT

Student Project One

On the map of the solar system, made as aproject for leaflet No. 1 of this series, drawthe path of Mariner 10. Work backwardsfrom the encounter with Mercury and Venuson March 29 and February 5 respectively,using Figure 2 as a guide to find the posi-tion of the Earth at launch. Remember Earth

goes around the Sun in 3651/4 days, Venusin 225 days, and Mercury in 68 days, Markpositions on all orbits at 10-day intervals.Allow for the fact that Mercury movesslightly faster when on the parts of its orbitcloser to the Sun than when most distantfrom the Sun. The most distant part of Mer-cury's orbit is approximately where Marinerencounters the planet. Venus' speed is al-most constant, and so is the Earth's, sinceboth move on almost circular orbits.

Student Project Two

In an earlier pamphlet it was stated that anobserver on Venus would appear to be stand-ing in a huge bowl as though in a hollowplanet with clouds in the center. Use yourimagination and produce an illustration, adrawing or painting, of what it would belike if you could stand on Venus in somekind of space suit that would protect youfrom the enormous pressure at the bottomof the carbon dioxide atmosphere. Radarindicates that the surface is gently undulat-ing in parts with some rough surface andsome large shallow craters.

Figure 3. A photographic mosaic ofMercury is expected to be obtained from

overlapping photographs of the planet.

READING LIST

J. H. Wilson, Mariner Venus Mercury 1973,Technical Memorandum 33-657, Jet Pro-pulsion Laboratory, Pasadena, California,October 15, 1973.

P73-219 JPL 654 4.5M 1-74 PRINTED IN U.S.A.