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Home , Ashen light, Atmosphere of Mercury, Orbit of Venus

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ED 093 662 SE 017 716 TITLE The Now Frontier. Linking Earth and Planets. Issue No. 1-4. INSTITUTION Jet Propulsion Lab., Pasadena, Calif. SPONS AGENCY National Aeronautics and Space Administration, Washington, D.C. PUB DATE 74 NOTE 16p. AVAILABLE FROM Jet Propulsion Laboratory, Public Educational Services, 4800 Oak Grove Drive, Pasadena, California 91103 (no price quoted)

EDRS PRICE MF-$0.75 HC-$1.50 PLUS POSTAGE DESCRIPTORS Aerospace Technology; *Earth Science; Instruction; *Instructional. Materials; *Science Activities; Science Education; Technology IDENTIFIERS NASA Program

ABSTRACT This publication includes four pamphlets providing background material for understanding the NASA program of planetary flights. Each issue presents student involvement activities as well as suggested reading lists. Issue 1 describes the innermost planets of the solar system. Issue 2 gives information about the evolution of the planetary system as well as specific notes cn the planets Venus and Mercury. "Mission to the Inner Planets" is the title given to Issue 3. The Mariner 10 mission is described in some detail. Issue 4, "Venus and Mercury Encounters," describes Mariner 101s encounter with Mercury and Venus.(EB) S DEPARTMENT OF HEALTH. EDUCATION & WELFARE NATIONAL INSTITUTE OF EDUCATION DOC NI Pf N CAJCF D Ar. F D r,11 PC P SON uk.',,:".%, 7 poN I0; ,f 0, t D ,Al,! oA) EDO) AT por,,: 0,, NATIONAL AERONAUTICS AND SPACE ADMINISTRATION THE NOW FRONTIER LINKING EARTH AND PLA\ETS

Innermost Planets of the Solar System

JET PROPULSION LABORATORY ISSUE NUMBER ONE CALIFORNIA INSTITUTE OF TECHNOLOGY \ PASADENA, CALIFORNIA PUBLIC EDUCATIONAL SERVICES OFFICE THE INNER PLANETS ingly associated with the Roman goddess of much more massive atmosphere composed beauty, Venus. mainly of carbon dioxide, a much higher The dull-white star shone steadily across the surface temperature, about 700°K, a much Planets of the solar system consist of two Aegean Sea in the warm glow of dawn. The slower backwards rotation period of 243.1 distinct types:mall, dense, inner planets Ancient Greeks called this star Apollo. And Earth clays, and no moon or oceans. they thought it different from another dull- with solid surfacesMercury, Venus, Earth and its Moon, and Marsand large, predom- white star that often lingered for a few weeks Spacecraft traveling to Venus in the past in inantly gaseous outer planetsJupiter, Sat- the sunset glow across the Ionian Sea. haveriotdetecteda significant magnetic urn, Uranus,andNeptune.Pluto,the This star they called Mercury, the messenger field(the field being less than 1/5000 of outermost known planet, cannot beob- of the gods. But by the time of Plato, the the Earth's). Greeks had discovered that the two stars are served well enough to be accurately classi- one, and are not stars, but a wandering planet fied, thoughitis believed to be similar to the inner planets. The closeness of the mean density of Venus always moving close to the Sun like a moth to that of the Earth might imply that the around a candle flame. Also, between the orbits of Mars and Jupiter chemical composition of the two planets is is a zone of minor planets called asteroids, almost the same.A question often asked Mercury is the innermost planet of the solar the largest of which, Ceres, is only about 50C is whether Venus is at a stage earlier or later system. Venus orbits the Sun between the in evolution than the Earth, or follows an orbits of Earth and Mercury. Both planets milesindiameter,while most are much smaller. entirelydifferent evolutionary path from are now targets for a Mariner 10 spacecraft Earth. launched November 1973 and scheduled to Indistance outwards from the Sun,the fly past Venus in February 1974, and Mer- asteroids divide the inner from the outer Radar inspection of Venus from Earth re- cury in March 1974. Venus has been reached planets, Jupiter being the first of the outer vealsa surface thatisgenerally smoother several times by spacecraft from the U.S. and planets, Mars the most distant of the inner than the Moon and gently undulating. There the U.S.S.R., but Mercury has never before planets. are, however, several regions that appear to been reached. be much rougher than their s...iTroundings, ANALOGY WITH EARTH AND and some large shallow crater:. Mercury averages 36 million miles from the MOON Sun, about 40 percent of Earth's distance, while Venus, at 67 million miles, is about 75 Venus is approximately the same size and SOLAR ORBITS AND APPEARANCE percent of Earth's distance from the Sun. mass as Earth, Mercury somewhat larger than OF INNER PLANETS IN THE SKY Earth's Moon. While Earth and Venus both (APPARITIONS) Since the planet Mercury is so close to the have atmospheres, the Moon, and, apparently, Sun and moves rapidly in its orbit to 2 Mercury, are airless bodies.Venus and Mer- It is instructive to look at Mercury and Venus times faster than Earth) it flits from side to cury might have been a twin system like from the standpoint of the early astrono- side of the Sun to be seen only ju :t before Earth and Moon, except that the closeness of mers. Earthbound, they watched the motions sunrise and just after sunset. Its rapid motion Venus to the Sun probably prevented Mer- of the planets against the background of and brief appearances and disappearances pro- cury from being a satelli;e of Venus and pre- stars and deduced that the planets, including bably caused the ancients to associate it with vented Venus from possessing a terrestrial- the Earth, move around the Sun in almost the winged-foot messenger of mythology. type atmosphere and oceans. circular orbits.Because Mercury and Venus orbit the Sun inside the Earth's orbit, they By contrast, Venus is closer to Earth, moves From the point of view of planetary dyna- are termed inferior planets. further away from the Sun in the evening mics, Mercury is perhaps the most important and morning skies, appears placid and bril- object in the solar system. Being the closest As seen from the Earth, inferior planets ap- liant and is perhaps the most beautiful object planet to the Sun,itis the most sensitive pear to move close to the ecliptic (the ap- in the skies"Mistress of the Heavens" said detector of departures from the laws pro- parent yearly path of the Sun relative to the the Babylonians. This planet was accord- posed to account for planetary orbital mo- stars, which is the plane of the Earth's orbit tion. An unusual period of spin and an un- projected againstthestars), and to move usually high density make Mercury of gr-sat backwards and forwards, oscillating to either Cover:Mercury and Venus the innermost interest to astronomers. side of the Sun and never far trom it in the planets of the solar system are shown here sky (Figure 1).The maximum distance to in their orbits around the Sun:first, Mer- A space probe flying by Mercury can pro- east or west of the Sun is termed elongation. cury (seen here as a crescent), and then vide important information about both the At eastern elongation, Mercury and Venus are Venus; the Earth is next, with our Moon dynamics of Mercury and its interior. The seen in the evening sky as evening stars be- close to it. Planets farther from the Sun are radius, mass, and orbit can be refined. cause they appear to follow the Sun inits Mars, Jupiter (shown with some of its satel- daily Tlotion across Earth's sky due to the ro- lites), and Saturn, withits rings. Uranus, Venus is the planet most similar to the Earth tation of the Earth.At western elongation, Neptune, and Pluto are the most distant in size, density, and distance from the Sun. they are ahead of the Sun and are seen as planets. However, it differs significantly in having a morning stars before sunrise. Table 1. Venus Picture Sequences looking back on the gradually diminishing disc of the planet. This 2-week series of pictures Date and time (PST) Sequence is very important. Scientists have observed rapid rotations of ultraviolet mark- Feb. 5, 8.28 to 8.58 a.m. Limb, cusps, and terminator scans ings on Venus and major up-and-down pulsations of the atmosphere. So as Mariner 9.02 and 9.46 a.m. Point directly beneath the Sun on the limb recedes from Venus, emphasis is placed on 10.13 a.m. 163 photo strips of Venus time-lapse photographs of ultraviolet and any visible light markings to measure their 12.18 p.m. 238 ultraviolet for mosaics period of rotation about the planet, which is believed to be about 4 days from Earth- 2.12 p.m. to 4.51 a.m., 1207 photos for various mosaics based observations. Feb. 6

Feb. 6, 9.35 a.m. This task over, Mariner reverts to observa- through 408 pictures, one each hour tions of interplanetary space until it begins Feb. 22, 10.30 a.m. to approach Mercury towards the end of March 1974 (Figure 2).

About 4 weeks elapse from the last pictures of Venus to the first pictures of Mercury. The infrared radiometer can also be used the size of particles in the clouds, composi- Mariner 10's prime target is Mercury, since during the approach to sweep across the tion of the clouds, pressures in the atmo- other spacecraft have flown to Venus but darkened planet and measure the temper- sphere, and small- and large-scale cloud struc- none previously have gone to Mercury. The atures of the clouds on the night side of ture. Scientists hope to determine whether sequences of pictures of Mercury are the most important part of the mission, and as Venus. Also, the magnetic field and particles the clouds are hazy, foglike sheets or possess many pictures as Possiblewillbe taken instruments will observe the "tail" of Venus turbulent billowing tops like Earth's cumulus (Table 2). The sequence is divided into five and the interaction of the planet and its elec- clouds. The cameras also photograph the distinct parts: incoming far encounter, in- tricallycharged upper atmosphere(iono- limb edge of the planet to look at the cloud coming near encounter, encounter, outgoing sphere) with the solar wind. The ultraviolet tops as they appear silhouetted against the near encounter, and outgoing far encounter. instrument will check on the auroras on the dark background of space. night side of Venus. These have been de- Mariner 10 approaches Mercury almost on a As Mariner flies around the daylight hemi- tected by earlier spacecraft and may be tangent to the planet's orbit near its furthest sphere of Venus it becomes hidden from responsible for the ashen light of darkened point from the Sun (aphelion). Each day in Earth on the far side of the planet. Com- Venus as sear' by astronomers from Earth. the final week before Mercury encounter, munications are temporarily interrupted. But Mariner sends back to Earth sequences of At the closest approach of Mariner to within Mariner 10 continues with allits scientific pictures taken through several different color 3100 miles of the cloud tops, the sunlit side missions, storing the information, including filters. Project scientists will inspect these to of the planet has become visible and the photographs, in its memory. Then when it search for features of interest which can later cameras are busily photographing the brilliant emerges again from behind the planet and be photographed in more detail as the en- cloud tops.Filters are changed to inspect radio waves again can get hack to Earth, the counter proceeds. Since no one has seen any the clouds inlight of different colors and information is transmitted. real detail on Mercury from Earth, this pre- different planes of vibration (polarization). encounter series of photographs is awaited Some of the pictures are along the terminator As well astaking many photographs of expectantly. As with most earlier planetary fine, the boundary between night and clay Venus, Mariner 10 scans the clouds in ultra- explorations, the photographs will probably on the planet. Because the Sun shines at a violet and infraredfor clues asto their be filled with many surprises. very low angle along the terminator it will composition. reveal structural details of the clouds if they Man's concepts of Mars were completely re- are present, just as an automobile's head- But quickly Mariner flies on, its rendezvous vised as a result of close looks by earlier lamps will show up small detail in the road with Venus over. As it heads back towards spacecraft. And Mars shows quite extensive surface to a pedestrian standing some way interplanetary space its instruments look at detail when observed by telescope from in front of the automobile and looking down the solar wind and magoetic fields and par- th. By contrast, Mercury shows littleif at the road. ticularly the effects of the bow shock where any detail when seen through an Earth-based the planet plows into the environment of telescope, so it has remained very much a A few minutes afterclosest approach to space like a supersonic airplane shocking the planet of mystery. Venus, Mariner sees the nearly fully illumi- the Earth's atmosphere. nated disc of Venus. Series of pictures are As Mariner bears clown on Mercury, the taken in ultraviolet and polarized visible light And for several days (17) after encounter, ultraviolet instruments will search for evi- of several colors to obtain information about planet photography continues, the cameras dence of an atmosphere and the infrared in- In the following months, project scientists and engineers inspect the wealth of information returned from Mercury and confer on a plan for a second encounter with this innermost planet of the solar system. And during this period, too, while Mariner inspects the interplanetary medium, it will also look around with its cameras to see whether there are any small planets within the orbit of Mercury. There are plans for more than 7500 pictures of Venus and Mercury during the encounters. These willprovide the first photographic survey of both planets and present information impossible to obtain from Earth. Many of the pictures will be made available to national television and the newspapers within Figure 2. An artist's conception of the Mariner 10 encounter with Mercury. hoursafterbeing received,pictures that generations of astronomers would have given strument will be checking on the temperature 600 miles of the barren siirface and then almost anything to be able to see. Thus of the planet. starts away from the planet. But this close Mariner 10 will complete much of the jigsaw approach is made over the night hemisphere puzzle of the terrestrial-type planets of the Just before encounter, the cameras will be of the planet and the surface cannot be solar system and allow better understanding programmed to sweep space around Mercury photographed. There are opportunities to of how these planets were formed and to see whether the planet possesses any small observe and photograph the pointed ends of evolved to their present states. satellites that would be invisible from Earth. the crescent of Mercury, as with Venus, at STUDENT INVOLVEMENT the beginning and end of the night side pass. On the day of closest approach, March 29, Student Project One These may be inspected to check for atmo- 1974, detailed photographic surveys are made sphere and for surface irregularities. Then, Take the various events listed in the tables of the surface, particularlythe boundary as Mariner moves away from the planet and in this leaflet and make a combined listing between light and darkness where detail will over towards the daytime hemisphere, the of the sequential happening when Mariner 10 be thrown into sharp relief by the low angle picture taking will start again.It will con- makes its encounters with its interplanetary of sunlight shining on the planet's surface. tinue until the image of Mercury no longer targets. List the date, time, what happens, Objects only 350 feet across are expected to reveals any detail. and leave a column for your remarks about be revealed on these pictures. Rapidly the each event. Remember that the times given illuminated shape of Mercury changes from As with the Venus flyby, instruments on here are for Pacific Standard 'lime (PST). a half-moon to a crescent as the spacecraft the spacecraft will check the interaction of (This couldbe anindividualor aclass hurtles towards close approach. About 12.00 the planet with the solar wind and the inter- project.) noon PST on March 29, Mariner flies within planetary medium. Classroom Project Table 2. Mercury Near-Encounter Picture Sequences Study the results of the Mercury encounter. Discuss any unknowns that have still not been clarified by the first flyby. With the Date and time (PST) Sequence knowledge that sufficient propellant exists Satellite search, March 23, 5.00 to 8.30 a.m. 45 pictures to make another flyby, make a plan for this next flyby: whether it should fly on the March 27, 3.30 to 6.30 a.m. 45 pictures day side or the night side, what it should photograph, and Why. Then later, when the Near encounter, March 28, 8.24 p.m. to 8.30 a.m. (March 29) 162 pictures (9 mosaics) next leaflet of this series is issued, check to Encounter, March 29, 8.30 to 11.24 a.m. 225 pictures see whether your mission plan agrees with that selected by the project scientists and 11.24 to 11.37 a.m. 18 pictures compare your reasons with theirs.

12.10 to 12.22 p.m. 17 pictures READING LIST Watch TV newscasts and special program. 12.22 to 4.20 p.m. 329 pictures Listen to radio newscasts. 5.30 p.m. to 3.30 a.m. (March 30) 144 pictures Read yourlocalnewspaper L. news-magazines,

P73-219 JPL 654 4.5M 1-74 PRINTEL NATIONAL AERONAUTICS AND SPACE ADMINISTRATION THE NOW FR 1 NTIER LINKING EARTH AND PLANETS

Venus and MercuryasPlanets

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S JET PROPULSION LABORATORY ISSUE NUMBER TWO CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA PUBLIC EDUCATIONAL SERVICES OFFICE EVOLUTION OF THE PLANETARY into Earth's atmosphere to break water vapor SYSTEM into oxygen and hydrogen. This is important because the hydrogen would bubble off into Planets of the solar system probably formed space like steam from a saucepan and grad- four to five billion years ago when hosts of ually Earth would have lost its n-;eans in this smallrockyparticles and clouds of gases way. collected together bytheir own gravity. Gravity appears to be a universal property of A photographic mosaic of the Earth, taken matter, as a result of which every particle, by Mariner 10, is shown in Figure no matter how small, attracts every other. Thus, left to themselves in space, individual THE CHARACTERISTICS OF particles (and a gas consists of particles: VENUS molecules) tend to collect together into large masses. Venus is very similar to Earth in size and mass; its diameter is 7,520 miles compared So after the Sun condensed from the pri- with Earth's 7,920, its mass and density are meval nebula, planets of different sizes and slightly less than those of Earth. Physically probablydifferent composition originated the two planets are almost twins, but they from concentrations of matter present at seem to have grown up quite differently. various distances from the Sun. And electric and magnetic fields in the gas forced these Venus, too,isEarth's nearest neighbor in condensing planets into orbits around the space after the Moon. Its closest approach is central Sun and spun them on their own axes 26 millio;i miles compared with 34 million like tops. miles for Mars. Figure 1.A mosaic view of the Earth taken The larger craters on Mars, the Moon, and But a telescope reveals virtual y no details on by Mariner 10 on its way to Venus and Venus are thought to be gouged by falling the bright disc of the planet. Some obser- Mercury. Such pictures were used by the bodies during the final stages of planetary vers have recorded faint and elusive markings, engineers and scientists to test the space- accretion, as the process of falling together is visible in the near ultraviolet, ill-defined dark craft's cameras in space. These pictures were termed. Smaller ones reprLsent a continuing shadows and bright patches seemingly be- taken when Mariner 10 was 120,000 miles but much lower rate of bombardment by having very much as cloud systems might from Earth. solar system debris. behave. quantities of heat that might have been the The absence of surface features or persis- While Mariner 10, now on its way to Venus, cause of extensive volcanism to generate the tent cloud features made it difficult to deter- is not designed to find out anything directly dense atmosphere of Venus. mine the period of rotation of Venus on its about life on Venus, the scientific informa- axis.Wildly varying estimates were made, tion about conditions on Venus may be im- Venus has been the target for several earlier ranging from a 24-hour day like that of Earth portant to biologists seeking understanding space missions: two successful flybys were to a day equal to the Venusian year. The of why Earth spawned life and Venus did made by earlier Mariners, and Soviet Venera question was not answered untilrecently not.It was life in the primitive atmosphere spacecraft flew by, orbited, and landed cap- when radio waves penetrated the thick clouds ofEarth which helpedto produce the sules on the surface. These probes confirmed of the brilliant planet.Surprisingly, it was oxygen-rich atrnosohere that in turn keeps a a high surface temperature of around 475°C found that Venus rotates in 243 days in the lid on the oceans and stops them leaking and a pressure at the base of Venus' atmos- opposite direction to that of the Earth. This off into space. Oxygen, converted to ozone phere about equal to that at a depth of 400 isslightly longer than the Venus yearthe at high altitude by the Sun's rays, prevents fathoms in Earth's oceans. Generally Venus time the planet takes to revolve around the solar ultraviolet from penetrating low enough is a hot dry planet with only slight traces of Sun, which is 225 Earth days.Because of water vapor inits atmosphere of 95 per- the rotation and revolution in opposite di- cent carbon dioxide. There are also traces of Cover: The planet Venus as it has been rections, the day on Venus is only 127 Earth oxygen,nitrogen, andinert gases,itis observed from Earth through the technique days. But these figures pose the question of believed. of radar mapping, in which a radar signal is how Venus might have slowed down from bounced off the surface of the planet. The rotating as the other planets do and started The clouds of Venus, which, according to re- three light areas marked with Greek letters to rotate in the opposite direction. One of cent spectroscopic observations, may have a (a, 0, and 5) are rough sections that may be several theories is that Venus captured a large topmost layer of concentrated sulfuric acid mountains, but could also be craters or Moon-like body moving in the opposite di- droplets, are about 18 to 25 miles thick, and boulder fields. The mapping was done by rection to Venus' spin and that this body their tops may be about 90 miles above the the Jet Propulsion Laboratory (JPL), using crashed onto the surface of Venus. The surface, cornp9red with Earth's highest clouds the NASA/JPL deep space antennas at Gold- impact would have stopped Venus from rota- of six miles or so. Below the Venus clouds stone, California. ting and would also have released tremendous is a clear atmosphere, while the clouds themselves probably consist of stacked layers of atmosphere build up such high pressures? One theory suggests that water is trapped in different composition; for example, carbon How could bodies plunge through this dense voluminous polar caps of Venus; another, dioxide ice at some levels and water ice at atmosphere to produce craters detected by thatallthe water was lost because water others. radar? vapor could rise in the atmosphere of Venus and be broken down by sunlight into oxygen Cloud features seen in ultraviolet light appear Answers to these and other questions about and hydrogen. The hydrogen boiled off into to move around the planet in only four clays. Venus will help us understand how planets space. But what happened to the oxygen? It There are also some large scale up-and-down evolve and why there is an Eden-like Earth does not appear to be left in the atmosphere. pulsations of the cloud layers. while nearby planets are so inhospitable. Understanding how the atriosphere of Venus On the other hand, the major differences An interesting theoretical aspect of the dense became, or stayed, inimical to life may help but clear atmosphere below the clouds of among the terrestrial planets may have arisen us prevent our own atmosphere from going because these planets formed at different dis- Venus is that if a student could stand on the wrong too,either fromnaturalor man- tances from the Sun and thus consisted of surface and look around, he would appear to made causes. An important question today be standing in the bottom of a vast bowl. different materials in the first instance. For is whether pollution might trigger drastic example, Mercury might have formed from Looking into the distance in each direction, changes in Earth's atmosphere which could materials rich in iron, whereas Venus formed he would see a blurred, ruddy red landscape change our planet into another Venus. curving upwards towards the cloud layers as from silicate-rich materials. And Earth may have accreted in a region in which there were though he were inside a hollow planet:the At the distance of Venus from the Sun its water-containing substances while Venus did atmosphere, acting like a giant lens, would temperature would be expected to be about not. bend thelightrays upwards in an effect 60°C on the average ifit were an airless Calculations have shown that the pri- meval nebula might have separated in this similar to a mirage appearing on a hot road body.Why then isit so much hotter, es- way at various distances from the Sur. surface in summer. pecially when its clouds reflect a large part of the solar radiation falling upon them? This strong bending of light might turn night The answer seems to lie in the atmosphere of THE CHARACTERISTICS OF into a dull day on the night side of the planet, carbon dioxide.This gas acts, with water MERCURY and this could explain the ashen light of vapor, as a one-way transmitter of incoming Mercury, 3000 miles in diameter, is probably Venus. Astronomers have claimed that they heat energy from the Sun.It opens a door see this faint glow on the dark side of the the smallest planet in the solar system (Fig- to let the energy in and slams it shut when ure 2). The outermost planet, Pluto, might planet when it is turned towards Earth. But the energy tries to go out again. The planet this ashen light, first seen in 1643, is nowa- possibly besmaller. Mercuryishalfway heats up like the inside of an automobile in between the Moon and Mars in size. Even days believed to he an auroral glow, like the sunshine with its windows closed. northern and southern lights over Earth's two satellites of the planet Jupiter are as polar region.Since Venus does not have a By contrast, the atmosphere of Earth has large as Mercury, though much less dense. strong magnetic field, its auroras can occur only 0.03 percent of carbon dioxide.On allover the planet, whereas on Earth the Venus, carbon dioxide has remained free in strongmagneticfieldcauses the aurora- the atmosphere, whereas on Earth in the producing particles from the Sun to stream presence of much water it has reacted with towards the polar regions. One of the space- minerals to form large deposits of carbonates. craft did observe the glow over the night side Photosynthesis in plants has also extracted of Venus, and the existence of an electrically carbon from the atmosphere. excited upper atmosphere, the ionosphere, has also been confirmed by spacecraft. Another mystery about Venus is why it does not have much water. One suggestion is that Study of the atmosphere of Venus isim- the water isstill entrapped in molten rock portant because the whole balance between beneath a plastic surface which resists frac- heat coming in and heat going out from the tures.In the formation of Venus, high tem- planet is hound up with atmospheric struc- peratures drove off carbon dioxide but al- ture and composition.This heat budget of lowed water to remainin solution in the ti energy income and spending could explain rocks.As the planet cooled, a plastic crust why Venus is today such a vastly different formed of sufficient thickness to prevent planet from Earth despite the similarities in fractures and the consequent escape of water size. The great mystery about Venus is how to form oceans. a planet about the same size as Earth, which Figure 2. A photograph of Mercury taken at might have supported oceans in the past as However, the fact that the planet has not the Pic du Midi observatory in France. This Earth does today, developed so differently outgassed suggests little differentiation or, at is the best type of picture that can be taken during its evolution.Why did the planet the least, an inactive mantle.If the planet from Earth, but Mariner 10 should send loseallits water?Why didthe surface were to outgas, the crust would not be likely back Mercury photographs as good as the temperatureriseso high? How didthe to keep the lid on this outgassing. picture of Earth in Figure 1. At one time it was thought that the closeness a kind, as itis temporarily held by Mercury Student Project Two of the planet to the Sun caused it to turn one before streaming off again into space. hemisphere eternally sunwards, just as the From the textbooks now make a list of the Moon turns one hemisphere towards Earth. Thesurfacetemperatureof Mercury is unknowns about Mercury and Venus, their However, radio astronomers discovered very thought to range from about 325°C at local surfacefeatures,theiratmosphere,their recently (1965) that Mercury rotates on its noon to -125°C at local midnight, but these physicalcharacteristics, their origin,their axis in 58 days. Coupled with the planet's temperatures vary considerably with the po- evolution.Check with a later pamphlet in 88-day period of revolution around the Sun, sition of Mercury on its orbit as the planet this series as to what instruments are car- this gives Mercury a solar day of 176 Earth moves in and out from the Sun. ried by Mariner 10 and what they might reveal.Identify those present unknowns that days. So one day of Mercury occupies two Mercury's surface is exposed to the fierce might he solved by Mariner 10. Later check years of Mercury time.And because the erosion of the solar wind as well as to solar the actual results from the flybys and see planet follows an elliptical path around the heat and light.As with Earth's Moon, the which of the problems are solved. Sun withits speed in orbit changing as a solar wind probably changes the composition consequence, the path of the Sun through of the outer surcaces of rocks and surface Student Project Three Mercury's sky is quite erratic. Some parts of soils.Solar radiation can vary between five Mercury experience a double dawn, for ex- and ten times that received by Earth be- For individual student or as a classroom pro- ample, in which the Sun rises, then slips back tween aphelion and perihelion of Mercury ject.Using the map of the orbits of Earth, below the horizon, to rise again later. (most distant and closest parts of its orbit to Venus, and Mercury prepared as a project in the Sun).Imagine a day in Earth's desert Through alarge telescope the planet pre- connection with the first pamphlet of this with ten suns shining at once in the noon sents a yellowish color broken by some gray- series, mark a position for Earth with a cir- sky.That approaches what it may be like ishpatches. Mercuryisbelieved to be cle onitsorbit. Then on the orbits of cratered and much like Earth's Moon. The on Mercury when the planet is at perihelion. Venus and Mercury draw circles, approxi- grayish areas may be the same as the lunar Key questions about this tiny planet concern mately to scale (that is, the circle represent- maria, the great gray plains of the Moon. its rotation, density, and surface molding. ing Venus should be about the same size as Mercury, like the Moon, does not appear to Why does Mercury rotate three times while that for Earth and about 21/2 times the di- have an appreciabl? atmosphere, and it is a making two revolutions around the Sun? ameter of Mercury) at the correct positions poor reflector of light.But its density is Might this have produced some unusual sur- for eastern and western elongation, superior believed to be much more than that of the face features? One theory suggests that the conjunction (far side of the Sun) and inferior Moon, possibly a littlegreaterthan the heat from the Sun at perihelion may have conjunction (between Earth and Sun) and Earth's.This is unexplained.It leads to the produced two opposing bulges on the planet half way between each of these positions, anomaly that Mercury's surface gravityis which now keep Mercury locked into this that is, eight positions for each orbit. greater than that of the bicger planet Mars. peculiar rhythm of rotation and revolution. A 100-pound student would weigh 40 pounds Now draw a set of circles showing the rela- on Mercury, compared with 38 pounds on Why does Mercury have the highest density tive sizes of each planet as seen from Earth Mars and 17 pounds on the Moon. On among the planets and their satellites? Does for each of the eight positions. On each of Venus the same student would weigh 91 it have a central core of iron?And what these comparative circles draw the phase of pounds. processes have shaped the surface of the in- the planet, that is, the part that is in dark- nermost planet? It is unlikely that Mercury can possess an at- ness as seen from Earth and the part illuminated; for example, fully illuminated like the mosphere, though some astronomers say they Mariner 10is expected to provide much fullMoon, half illuminated like a quarter have seen veilings of surface detail at times, valuableinformationbearing upon these Moon, gibbous, and crescent shaped.You in the form of a whitish haze. Despite Mer- questions. cury's greater surface gravity c-Per that of the will have to calculate the size of the planet Moon, Mercury is closer to the Sun and there- STUDENT INVOLVEMENT at each of these phases from the rule that fore much hotter than the Moon: molecules if the planet is twice as far from the Earth can be heated to move at greater velocities Student Project One it will appear half as large (in diameter) and so on. and thereby reach escape velocity. Thus, From reading astronomical textbooks make gases that may have come from the rocks as four lists as follows: a result of volcanic activity will, over mil- Note how much smaller Mercury always ap- lions of years, have escaped from the planet's (1) Those things about Venus that are pears to an observer on Earth even when gravitational grasp and rocketed as molecules similar to those on Earth. close to the Earth, and how both planets into space. (2) Those things that are different. turn their dark hemispheres towards us when (3) Those things about Mercury that are they are at the very closest. Mercury does, however, collect plasma from similar to those on the Moon. the Sun, the steady stream of particles flung (4) Those things that are different. Thus you understand why we know so little out by the Sun into interplanetary space and about these planets and why a spacecraft referred to as the solar wind. Some of this Make a drawing or painting of your impres- flying by them can add greatly to man's plasma might indeed form an atmosphere of sion of the surface of Mercury. knowledge of these neighboring worlds.

P73-219 JPL 654 4.5M 1-74 PRINTED IN U.S.A. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION THE NOW FRONTIER LINKING EARTH AND PLANETS

Mission to the Inner Planets

JET PROPULSION LABORATORY ISSUE NUMBER THREE CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA PUBLIC EDUCATIONAL SERVICES OFFICE THE MARINER 10 MISSION Mariner 10 arrives at Venus on February 5, 1974, after a 3-month, 150 million mile trip halfway around the Sun.The gravity of Venus changes the speed of the spacecraft to plungeitdeeper into the gravitational field of the Sun to reach the orbit of Mer- cury and rendezvous with this innermost planet on March 29, 1974.

Mariner 10 sped from Cape Canaveral, Flor- ida,November 2,1973, atop anAtlas/ Centaur multistage rocket launch vehicle. And, in contrast vith earlier planetary spacecraft,ittried outitsinstruments On the Ea-th and Moon as it sped at 25,255 miles per hour on its way to Venus. The day following launch, scientists directed Mariner's twotelevisioncamerastoproduce four mosaics of 88 pictures each of Earth and Moon. Since Earthis cloud-covered like Venus, and the Moon airless and optically similar to Mercury, these picture series gave 1. The Mariner 10 spacecraft. valuable early information on how the cameras might return information about the two target planets.Thus the instruments were calibrated. Other instruments too were tested For example, at Mariner's closest approach to Two solar panels extend on,ther side of the on the Earth and Moon. the Sun, it recPives five times as much light basic structure for almost 9 feet. There are and heat asit does on leaving Earth. This also two antennas for communicating with Experimenters were delighted at the clarity requires that the solar radiation must be Earth. A low-gain antenna projects almost of the returned pictures. The Earth photo- screened from the spacecraft and its scien- 10 feet from the top of the basic structure. graphs revealed clear details of clouds; some tific instruments to pri,,ent them from over- A high-gain parabolic antenna, 54 inches provided three-dimensional views.And the heating. A sunshade ofbeta cloththe across, extends to one side and is motor- pictures of the Moon showed north polar material used for astronauts' suitsis ex- driven to point towards Earth. tended as the spacecraft leaves Earth. regions not seen by astronauts, photographs There arealso sensors to track the star that recorded objects only 2 miles across. Canopus and the Sun, and abattery of The solar panels, too, which extend into scientific instruments. Two identical televi- Later the cameras photographed star fields sunlight to collect and convert solar radia- sion cameras peer like twin eyes from the top tion into electrical energy for the spacecraft's and further confirmed the high quality of the of the structure.Together with an ultra- images being returned over the radio link to instruments and controls, are designed to be violet spectrometer, they can be pointed by tilted more and more from the sunlight as Earth. Earth command. Other instruments are Mariner 10 approaches closer and closer to affixed directly to the basic structure and THE SPACECRAFT AND ITS PATH the Sun. are fixed relative to the spacecraft.Also, Mariner 10 is the first complex spacecraft to booms extend from the basic structure, car- 1 shows the main features of the be designed to travel into the inner reaches Figure rying instruments to measure magnetic fields Mariner spacecraft.There is a basic eight- of the solar system as close to the Sun as 43 and theinterplanetary plasma of charged sided framework which encloses eight bays million miles. Developed from earlier space- particles. of electronics. This structure is 54'/2 inches craft in the Mariner series that flew by Venus across and 18 inches deep.Its center cavity Mariner 10 carries reliable communication and Mars and orbited Mars, Mariner 10 faces is almost filled by a spherical tank of rocket radio systems new challenges of the interplanetary environ- to accept commands from propellantand a rocket motor used to Earth and return scientific and engineering ment. maneuver the spacecraft. The nozzle of this information.The spacecraft also carries a rocket projects through the base of the struc- computer so that commands can be accepted Cover:Mariner 10 on its path from Earth ture, which is on the sunwards side during by Mariner that will trigger a whole sequence (in the upper left part of the picture, with the flight.Below this structure is the heat of events, as well as direct commands to per- the Moon close by). The spacecraft has just shade, through a hole in which the rocket form specific tasks, such as switching on a passed Venus and is on its way to Mercury. nozzle projects. piece of apparatus. VENUS above or below the Sun at closest approach. Itis also clearly visible in daylight if an observer knows where to look; for example, when the planet appears close to the Moon

MERCURsr in the sky. Venus passes from greatest elongation as an evening star to greatest western elongation as a morning star in about 140 days, and from a morning star back to an evening star in about 430 days.

SIGHTINGS AND MOTIONS IN THE 0 SKY SUN HORIZON BE LO`:: HORIZON During December 1973, Venus was a brilliant evening star close to the planet Jupiter. Figure 1.The appearance of Mercury and Venus as evening stars as they travel around Venus passed between Earth and Sun on the Sun, inside the orbit of Earth. January 24, appearing above the Sun as seen from Earth's northern hemisphere. Then Venus becomes a brightening morning star through February and March, gradually ris- Because the orbits of these interior planets mately every 584 days. Mercury repeats ing earlier and earlier before sunrise (Fig- are completely contained within the Earth's approximately every 116 days. But since ure 2). orbit, both Mercury and Venus pass between Mercury's orbit varies much more from a Earth and Sun. This is termed inferior con- true circle than does that of Venus, the re- Mercury is an evening star observable from junction. When the planets are on the far side petition of Mercury's positions relative to the the beginning of February through about the of the Sun from Earth, they pass through Sun in Earth's skies varies too. third week of that month, then passes above superior conjunction. And because the orbits the Sun early in March and becomes a morn- of the Earth and the two planets are not The distance of Mercury from the Sun in ing star with good elongation from the Sun exactly in the same plane, that is, they are the sky at elongation also varies, from only towards the end of March.Unfortunately, tilted slightly with respect to each other like 18 degrees to as much as 27 degrees, and Mercury as a morning star in the spring is not crossed hoops, Mercury and Venus normally thus affects its visibility.Mercury is a rela- well suited to observation since itis very Pass through conjunction above or below the tively dull object. Like the Moon, it does not close to the horizon south of the point of Sun.Infrequently the orbits line up so that reflect much of the sunlight falling on it, so sunrise (Figure 3).It will be in the constella- the planets pass across the face of the Sun in it does not appear very bright in the sky. tion Aquarius, where there are not many a transit or behind the Sun in occultation. Moreover, Mercury can rise before the Sun bright stars to help identify the planet. Occultations are not observable because of or set after the Sun by only 21/4 hours, so it the brilliance of the Sun, but transits are. is rarely seen in a dark sky, but usually only Venus, too, will be in Aquarius at this time (It was Captain Cook's voyage to observe a in the twilight glow, low down near the hori- butwillbe unmistakable because ofits transit of Venus where it was visible in the zon, competing with the sunset. And the brightness, even though itisalso close to South Pacificthat lead to his discovering planet cannot be seen for much longer than the horizon. Tahiti.) two weeks close to the time of its elongation. There is an average interval of 44 days Transits of Venus occur very rarely: the most between Mercury's appearance as an evening CYGNUS recent occurred in 1882, the next are not due and a morning star. until the beginning of the next centuryJune 7, 2004, and June 5, 2012 (they occur in By contrast, Venus can move as much as 47 close pairs). Transits of Mercury occur much degrees from the Sun and can be seen in the so o ALTAIR more frequently.One was visible from the late evening or early morning skies as the DELPri INUS East Coast of the United States on Novem- brightest object after the Moon. Venus re- see ber 11, 1973. The next transit will take place flects a large proportion of the Sun's light on November 12, 1986. falling upon it and it appears very bright in VENUS the skies of Earth. Venus is brightest about CAN; ICORNUS Mercury revolves around the Sun in a period one month before and after inferior con- of 88 days, Venus in a period of 225 clays. junction, when a telescope shows it as a fat EAST HORIZON SOUTHEAST But their visibility in Earth's skies must also crescent shape.It can then become so bright take account of Earth's movement around as to cast distinct shadows. The planet can Figure 2. Venus at the day of Mariner 10 the Sun.So Venus repeats its apparitions be observed for many months and has even flyby (February 5, about 6:30 a.m., (elongations and conjunctions)approxi- been observed through binoculars as it passes Pacific Standard Time). SQUARE OF PEGASUS the way. Have these two students hold a Around mid-March, start looking for Mer- white styrofoam ball, big enough for other cury, averyfaint star-like object inthe students to see (about 4 inches in diameter morning sky between Venus and the point of for a medium-sized class). the sky where the Sun will rise.Try to see Mercury on March 29, the date Mariner flies The student representing the Sun shines a *- VENUS past Mercury, and you will be able to com- flashlight on the globe representing Mercury, pare what you see in the sky, a faint star- MERCURY and the Earth observer, standing close to a like object, with the close-ups of a possibly HORIZON EAST SOUTHEAST blackboard, draws what he sees,a dark Moon-like cratered world seen on the tele- globe. The student Mercury then moves part vision newscasts that evening. Figure 3. Mercury at the day of Mariner 10 way round to the side of the class as though flyby (March 29, about 6:00 a.m., orbiting the Sun.Again the flashlightis On this date, Mercury will be about one-third Pacific Standard Time). shone on the globe from the Sun, and the of the way between Venus and the Sun but Earth student now draws the half-lit planet below the direct line between Venus and Sun. he sees. Of course you will not be able to see either when the Sun is in the sky unless you ob- ihe comet Kohoutek, visible in late Decem- The Venus student demonstrates likewise, serve with a precisely pointed astronomical )er and early January close to Venus and then the classis asked to draw the visible telescope, so you will have to estimate where lupiter in the evening sky, will still be a faint illuminated shapes of Venus and Mercury as the Sun is below the horizon. )bject in the evening sky when Venus and these planets move around the Sun as seen Mercury have become morning stars during from Earth, at closest approach to Earth, Student Project Two :ebruary. at greatest elongations, and when most distant from Earth. )HASES OF THE INNER PLANETS This can also be a classroom project on the Student Project One blackboard.Refer to an astronomical text- ks the inferior planets move around the Sun book and draw a plan map of the solar sys- hey display phases, as seen from Earth, corn- Earlyin January, look at the evening sky tem snowing the orbits of Mercury, Venus, iarable to those of the Moon. When Mercury when it starts to become dark after sunset. and Earth.Try to draw them to scale, and rid Venus are on the far side of the Sun The Sun set.,in the southwest.Look left note that Mercury's orbit seems offset to one hey appearfullyilluminatedlikea full from the sunset boirt, and close to the hori- side of the Sun.Than try to work out /loon, but because of their great distances zon there will be a fairly bright star, which where the Earth and the two planets must be hey are then unfavorably placed for obser- .s the planet Venus.If you have access to a to be seen in the sky as they are. Remember, ation anshow only very small discs.At small telescope or very good field glasses you as seen from above (north), Earth rotates on asternal.clwestern elongations, Mercury will be able to see that itis shaped like a its axis and all the planets revolve around the nd Venus appear half illuminated, like half- tiny, very fine crescent Moon.If Comet Sun in a counterclockwise direction. Re- noons.Then as they swing between Earth Kohoutek is still a bright object, Venus will member, too, that Venus moves between nd Sun, the planets display a narrowing be between the head or the cornet and the Earth and Sun on January 24 and Mercury rescent phase to Earth until,if they cross point on the horizon where the Sun set.In between Earth and Sun around February 25. he disc of the Sun, they appear as black the following days, observe how Venus moves Plot points on the orbits every ten days pots upon it.Most times they pass either deeper and deeper into the sunset glow until working backwards and forwards from the lightly above or below the Sun as seen from it finally disappears. times of conjunction.Save your solar sys- :arth and thus, in a telescope, can he oh- tem plan map for a later exercise. erved as a fine crescent all the way through About the first week inFebruary, get up nferior conjunction. before sunrise and look for Venus inthe morning skies before the Sun comes up and ;TUDENT INVOLVEMENT while the sky is still fairly dark. Try to see READING LIST Venus on February 5, since this will he the iassroom Project day when the Mariner spacecraft flies past G. JAbell, Exploration of the Universe, the planet. Holt, Rinehart and Winston, New York, lave one student stand in the center of the 1964, rp. 23-26 (orbits). lassroom (which is preferably semi- Continue to observe Venus in the following J. H. Wilson, Mariner Venus Mercury 1973, arkened) to represent the Sun. Have another weeks arid watch how it moves further from Technical Memorandum 33-657, Jet Pro- tudent stand at the front of the class to re- the sunset glare and becomes brighter day pulsion Laboratory, Pasadena, California, resent the observer on Earth. Position two IT, day.If you have access to a telescope oh- October 15, 1973. ther studentsto represent Mercury and serve, too, that itis now a crescent racing L. Rudaux and G. De Vaucouleurs, Larousse 'enus,the first almost half way between opposite to the way it faced when it was an Encyclopedia of Astronomy, Prometheus un and Earth. the second three quarters of evening star. Press, New York, 1959.

P73-219 JP1. 654 4.5M 1-74 PRINTED IN U.S.A. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION THE NOW FRONTIER LINKING EARTH AND PLANETS

Venus and Mercury Encounters

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JET PROPULSION LABORATORY ISSUE NUMBER FOUR CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA PUBLIC EDUCATIONAL SERVICES OFFICE ENCOUNTERING THE PLANETS mission by keeping the spacecraft performing photographs, when looked at togetherone as it should and directing it to make the ex- with the right eye and the other with the At 8.57 a.m. PST on February 5, 1974, periments required by the project scientists. left in a special viewer will let scientists see Mariner 10 will speed past Venus only 3000 an unprecedented three-dimensional view of miles above the brilliant cloud tops and six The men and women of the project have to the comet. minutes later disappear behind Venus. Com- be on the alert every second of the encounter, munication with the spacecraft will then be ready to issue correcting commands to right Comet Kohoutekwillbe observed from lost for 21 minutes until Mariner emerges on any problems that might arise in the space- Mariner during the period January 17 through the other side of the planet. craft and around the worldwide data gather- 25, 1974. The ultraviolet airglow instrument For several weeks around that date, teams ing net. Communications are maintained with of Mariner 10 inspects the coma (the hazy of spacecraft controllers, engineers, analysts, the spacecraft at all times, 24 hours each head of the comet) and the comet's long tail about January17.Picture and scientists will be running a 24-hour-a-day, day. As the spacecraft sets over the tracking mosaics of Kohoutek will be obtained on January 19, three-shift operation, inspecting engineering station at Goldstone in California's Mojave 22, and 25. Four television pictures will be and scientific information pouring in from Desert, it is rising for the station at Canberra, taken every hour for 9 hours each of the spacecraft and commanding and con- Australia. And asitsets at that station it these days. trolling its actions. The solar panels are tilted comes into view at the next station, near at increasing angles from the Sun to protect Madrid, Spain. Then itis handed back to The second encounter is with Venus (Figure them from the greater intensity of the sun- Goldstone. Antennas thesizeof football 1). Mariner approaches Venus from the night light, and the necessary corrections are made fields gather the incoming signals and pass side of the planet. Viewing is thus unfavor- in the trajectory. the information to the control center at the able for photographs, since the cloud tops JetPropulsionLaboratory inPasadena, are shrouded in darkness. Moreover, since MissionOperations Systemactivitiesare California. the spacecraft is in an attitude to screen its centered in the Mission Control and Com- instruments from the intense radiation from puting Center at the Jet Propulsion Lab- ENCOUNTER SEQUENCES the Sun, its sunshade obstructs a view of oratory and make use of a widespread ground The first encounter is with a comet, but this Venus. The cameras on the spacecraft do data system consisting of facilities spread is not a closeencounter,since Comet not get a glimpse of Venus until about 28 around the world as a part of the tracking Kohoutek has swung around the Sun away minutesbeforeclosestapproachtothe and data system network. from Mariner. Nevertheless Mariner has the planet (Table 1). This opportunity may be Information from the spacecraft is presented opportunity of inspecting the comet from taken to obtain photographs of the pointed to flight controllers as quickly asitis re- space, photographing its head and probing ends of the crescent shape seen from Mariner, ceived and processed by the computers; this it with the ultraviolet instrument. And be- the cusps of Venus. These ends are impor- isalmost as the events happen. The only cause Mariner 10 looks at the comet at a tant because the atmosphere causes them to real delay is the timeseveral minutestaken differentangle from observers on Earth, be illuminated into the dark hemisphere, and for the radio waves to travel from Venus and photographs can be obtained simultaneously from the observations scientists can obtain Mercury to Earth. Television pictures of the from Earth and Mariner that provide two more information about the composition and planets are displayed on monitors similar to views of the comet, a stereo pair. These two structure of the Venusian atmosphere. home television sets so that the scientists can quickly see the quality of the information coming from the spacecraft. Computers back up the men and women at the Mission Control and Computing Center. Many computer programs are available for quick instructions to the spacecraft. For example, suppose ascientist sees something interesting on a picture returned from Mer- cury, an unusual crater or a volcanic cinder cone or a sinuous valley, he can call for a computer program that will generate instructions to the spacecraft camera pointing system to photograph this area again despite the rapid movement of the spacecraft and the rotation of the planet. About 120 persons are involved in operating the spacecraft to ensure the success of the

Cover: An artist's conception of Mariner 10 Figure 1.An artist's conception of the Mariner 10 encounter with Venus. The antennas as it approaches the planet Mercury. on the Earth, of course, are not drawn to scale. Some information collected by the spacecraft mand the spacecraft into other maneuvers Arrival times and distances have to be ar- can be stored on magnetic tape within it and after it passes Mercury, the spacecraft has to ranged precisely so that instruments can not transmitted later. This permits the spacecraft be aimed as accurately as possible in the only photograph the target planets atre- to collect data when it is hidden from Earth early stages of flight.Even so, four correc- quired angles and covering specified areas, behind a planet and later to send this data tive thrust periods are planned:the first but also look at the planets at infrared and when it emerges. just after leaving Earth and Moon, the sec- ultraviolet wavelengths and make other ex- ond just before arrival at Venus, the third periments. The Mariner 10 spacecraft weighed 1042 after flying by Venus, and the fourth before pounds at launch, including 64 pounds of arrival at Mercury. For all these tasks, timing is extremely criti- rocket fuel and nitrogen gas. cal because of the short time that the space- Three further maneuvers are needed to bring craft is near the planet. A requirement for this new spacecraft is to the spacecraft back to Mercury again (Sep- transmit much more information back to tember 1974) 6 months after the first pass. Mariner 10 follows a curved path to Venus Earth than earlier flyby spacecraft during And if there is propellant left, a third pass (Figure 2), going about one-quarter of the the brief periods of encounter with the two by Mercury could be attempted 6 months way around the Sun beforeit meets with target planets.So the rate at which infor- later (March 1975). the planet.When this path is bent by the mation is transmitted over the radio links gravityof Venus,itcurves more steeply has been increased.This is like talking fas- The success ofallthese maneuvers relies towards Sun.The spacecraft then fol- ter on the telephone if you want to describe upon controllers having accurate knowledge lows the new path for about another quarter as much as possible about an event asit about the speed of the spacecraft and its of the way around the Sun untilit meets actually happens. position along its path, as well as the direc- with Mercury. tion of its path. Such information is obtained This higher date .ate, as itis termed, per- by tracking the spacecraft from ground sta- Ifthe spacecraft were not slowed by the gravity of Venusby the way in which it is mits Mariner to send back more live pictures tions at Goldstone, inthe Mojave Desert, directed to fly past that planetits speed of the planets as it flies past them. California; in Spain; and in Australia.The three tracking stations pass information to a would be too high to allow the gravitational Another requirement for the mission was to control center at the Jet Propulsion Labora- field of the Sun to pull Mariner to the orbit ensure very accurate flight. Because the tory in California where navigation special- of Mercury. Without the aid from the gra- spacecraft uses the gravity of Venus to swing ists, aided by powerful computers, deter- vity of Venus, the path of Mariner would it on a path to Mercury, small errors in its mine the exact position and speed of the dip only slightly inside the orbit of Venus, approach to Venus would be magnified a spacecraft. Computers predict where the and to reach Mercury by a direct launch thousand times at Mercury unless corrected spacecraft is headed.Navigators decide on without a gravity assist from Venus would by the rocket engine of the spacecraft. But changes, and mission controllers send corn- require a larger, more expensive launch ve- since project scientists want to conserve as mands to the spacecraft to make the neces- hicle or a lighter payload that could include much fuel as possible so that they can corn- sary change:, in its path. only a few scientific experiments. After encounter with Mercury, Mariner 10 zooms outwards again and crosses the orbit MERCURY ENCC,JNIER MAT 20, 1974 of Venus, but Venus is far away on its orbit. 0 Later, Mariner 10 moves back to Mercury's orbit and now it is lucky. Mercury has made two circuits of the Sun while the spacecraft EARTH MAR 29, made one.Mariner meets Mercury for the VENUS AT LAUNCH 1974 second time. And if sufficient propellant re- mains in the spacecraft, a third rendezvous can be made with Mercury the next time i around the spacecraft's orbit. MERCURY AT LAUNCH AND JAN 28, 1?74 WHAT THE SPACECRAFT DOES VENUS ,I) eNcour, TER FEB 5, 1974 Mariner 10 is designed to investigate Mer- cury and Venus in several ways: r (1) Measure particles, fields, and radiation EARTH AT from the planets and their environs. LAUNCH ABOUT EARTH NOY 1, 1973 FEB 5, 1974 (2) Televise pictures of the planets with great detail to show cloud structure on Venus and the surface structure Figure 2. The Mariner 10 flight path to Venus and Mercury. of Mercury (Figure 3). (3) By ground-based observations of the form, the complex is directed by command goes around the Sun in 3651/4 days, Venus path of the spacecraft, measure the from Earth.As well as taking pictures in in 225 days, and Mercury in 68 days, Mark gravity field of Venus and Mercury, different colors of light, these cameras also positions on allorbits at 10-day intervals. their sizes, and their masses. reveal how the light is vibrating (its polari- Allow forthefactthat Mercury moves (4) By observations of the radio signals zation).Such observations reveal informa- slightly faster when on the parts of its orbit coming from the spacecraft asit tion on the composition of the ciouds of closer to the Sun than when most distant goes behind Venus and Mercury as Venus and the surface of Mercury. from the Sun. The most distant part of Mer- seenfromEarth, determine the cury's orbit is approximately where Mariner structure of the Venus atmosphere A radio experiment uses the signals trans- encounters the planet.Venus' speed is al- and check whether Mercury has a mitted from the spacecraft to Earth. By most constant, and so is the Earth's, since tenuous atmosphere of any kind. tracking the spacecraft's signals, experimen- both move on almost circular orbits. ters determine how the spacecraft is affected In addition, the spacecraft also provides in- by the gravitational fields of the planets. Student Project Two formation on the interplanetary space from From this information they may determine Earth's to Mercury's orbit, and on how the the shape of each planet and determine In an earlier pamphlet it was stated that an planets interact with this interplanetary en- whether there are any concentrations of observer on Venus would appear to be stand- vironment. mass that distort the gravitational field, like ing in a huge bowl as though in a hollow the mascons (mass concentrations) discovered planet with clouds in the center. Use your To perform these tasks, Mariner 10 uses a beneath the maria of the Moon. By analysis imagination and produce an illustration, a battery of scientific in:trurnents. A mag- of what happens to the radio signals as they drawing or painting, of what it would be netometer measures magnetic fields, a plas- pass close to the edge of the planet when the like if you could stand on Venus in some ma analyzer measures the ions and electrons spacecraft goes behind the planet as seen kind of space suit that would protect you flowing through space from the Sun (the from Earth, experimenters probe the atmos- from the enormous pressure at the bottom solar wind), cosmic ray telescopes study phere of Venus to determine its characteris- of the carbon dioxide atmosphere. Radar solar and galactic cosmic rays.The main tics and constituents and to check for any indicates that the surface is gently undulat- objective of allof these instruments is to tenuous atmosphere of Mercury.If one is ing in parts with some rough surface and learn about the planets by studying their detected,itscharacteristicswillalso be some large shallow craters. effects on the interplanetary environment, determined. such as magnetic tails and bow shocks con- nected with them. To take full advantage of the Venus occultation, which willbend theradio signals An infrared radiometer (heat measurer) mea- appreciably, an antenna on the spacecraft is sures temperatures of the clouds of Venus steered so as to partially compensate for the and the surface of Mercury. Two indepen- bending.In this manner, information can be dent ultraviolet instruments (measuring light obtained to much deeper levels of the Venu- beyond the violet of the spectrum) analyze sian atmosphere. the planetary atmospheres. One instrument A project of NASA's Office of Space Science, is fixed to the body of the spacecraft and the Mariner 10 project is managed by the Jet is used at Mercury to search for traces of Propulsion Laboratory of the California In- atmosphere along the edge of the planet's stitute of Technology, Pasadena, California. disc. A scanning instrument can be pointed The spacecraft was built by The Boeing on command.This is used to scan the en- Company, Seattle, Washington. Tracking is tire discs of the planets, searching for evi- by NASA's Deep Space Network, operated dence of hydrogen, helium, argon, neon, bytheJetPropulsion Laboratory. The oxygen, and carbon.Close to Earth, this scientific instruments are supplied by NASA same instrument measured the hydrogen co- centers, universities, and private industry. rona of the Earth and the reflective proper- Figure3. A photographic mosaic of ties of the Moon.At Venus both instru- STUDENT INVOLVEMENT Mercury is expected to be obtained from ments search for specific gases, and during overlapping photographs of the planet. the cruise phase they look for sources of Student Project One radiation coming from hot stars and nebula (or gas clouds) in the galaxy. On the map of the solar system, made as a project for leaflet No. 1 of this series, draw READING LIST A complex of two television cameras with the path of Mariner 10.Work backwards eight filters, capable of taking both narrow from the encounter with Mercury and Venus J. H. Wilson, Mariner Venus Mercury 1973, and wide angle pictures, photographs Earth on March 29 and February 5 respectively, Technical Memorandum 33-657, Jet Pro- and Moon (for comparison purposes) and using Figure 2 as a guide to find the posi- pulsion Laboratory, Pasadena, California, Venus and Mercury.Mounted on a plat- tion of the Earth at launch. Remember Earth October 15, 1973.

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