Two Over Mars - Mariner 6 and Mariner 7, February - August 1969

8/8/2019 Two Over Mars - Mariner 6 and Mariner 7, February - August 1969

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Prepared Under Contract No. NAS7-100National Aeronautics and Space Administration

This publication available for purchase asNEP-90 from the Superintendent of DocumeU.S. Government Printing Office, Washington,

D.C., 20402.


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The Mariner Mars 1969Project began at the end of 1965,while the results of thefirst mission from Earth to Mars were still being studied. It reached its peak in thesummer of 1969, just after the first manned landing on the Moon. Surrounding andcontemporary with the Project, the total space program evolved and matured. MarinerMars 1969 itself turned a major corner in the exploration of Mars, the Mariner series,and the unmanned planetary program. As far as the near planets were concerned, thepioneering was over, and the detailed work had begun. The target was no longer sim-ply Mars. Now a long and specific list of scientific questions was to be answered.Instruments were to be delivered to specific sites at particular times and pointed inprescribed directions.The technical characteristics, engineering development, flight performance, andscientific results of Mariner Mars 1969 are described at length and in detail in anumber of reports and publications, many of which are listed in the Bibliography.This document is intended to serve as an integrated introduction in narrative form,technically valid but not burdened with detail, to the Mariner Mars 1969 Mission andProject.

The effort was one of many carried out by and for the United States NationalAeronautics and Space Administration, which is generally responsible for flights, tests,development, and research in the fields suggested by its name. The Project was man-aged and conducted by the Caltech Jet Propulsion Laboratory, which was responsiblefor all previous Mariner projects as well as the Ranger and Surveyor lunar missions,several early Explorer and Pioneer flights, the deep-space tracking network, and anextensive research and development program sponsored by NASA.


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Sixty million miles away, on the warm, watery blueworld called Earth, the month of August 1969 has begun.It is high summer on the wide brown continents of thenorthern hemisphere, while south of the equator spring ispoised to begin. Much closer, on the rusty, dusty, bat-tered planet Mars, summer is over in the northern hemi-sphere, and winter is waning to the South. The south-ern polar cap, at this time the most prominent largefeature of the planet, has begun to shrink as the Sunstrack moves South from its northernmost extension. Inthe thin, cold air, the frosty clouds blow away or dissolve.But above Mars, in space, there is no season, no weather,and little sense of time.Earth-style, it is four years and a fortnight since thefirst ship of Earth coasted through these parts. On theMartian calendar, two years and 100 days have passedsince the time of Mariner IV. The same spacecraft isabout 225million miles away, derelict and tumbling, sail-ing its gravity-governed course around the Sun. But itscrew have never left Earth.

InterludeStill closer to Mars are other ships of Earth. Twin

lineal descendants of the first Mariner, built in the sameyards and launched from the same dock, they followclosely in its wake, as Leif Ericson followed Eric theRed across the North Atlantic. Their object is to buildupon that first contact, to extend its scientific explora-tion both in scale and in kind, and to establish a basisfor further investigation, particularly for a search forextraterrestrial life. Their crew also is far behind them,back on Earth. Scientists, engineers, and executives, tech-nicians, secretaries, and craftsmen wait at the end of along radio beam which takes more than five minutes tobring the word home from Mariner VI and Mariner VII.Thus, the mission in fact has brought not two, but morethan two thousand over Mars. It is not machines thatgo out and explore, but people.

Now the first of the two new Mariner machines hasfinished its exploration of the fourth planet, conductedin two stages. As it fell toward Mars for two days, speed-ing up in the far-reaching gravitational pull, Mariner VI

Facing: The planet Mars observed by Mariner VI at a distance of 452,000 miles on August 2 ,1969. Central dark fe ature s are Syrtis Major, a t right, and Meridiani Sinus, projecting to l e f t .


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photographed the swelling globe, recording the picturesand transmitting each days batch back to Earth. Then,at a range of 5000 miles, it began the comprehensiveclose-range survey.

Glass eyes wide, it gazed across the alien land. Thebig metallic head swung and nodded to take in the view,sweeping the horizon twice to be sure of the atmosphericprofile. Then, as it crossed from day to night, the lastpicture was recorded, and the survey was left to theultraviolet and infrared analyzers. A few moments afterhaving passed across the night side, Mariner slippedinto Earth occultation, and the radio signal dimmedout in a unique way, characteristic of Mars atmosphericproperties. Half an hour later, the machine emergedalready playing back from one of its two tape recordersthe evidence collected in its whirlwind visit to the planet.

As it slowly recedes, still jabbering of gas spectra,picture elements, and instrument temperatures, a secondvisitor is on the doorstep of Mars. Mariner VI1 is already

feeling the pull of alien gravity. Soon it will warm up itstelescopic television cameras, swivel its big silvery head,and start taking pictures at long range. Then it will repeatits elder brothers wide-eyed zigzag scan over the planetin the feverish half-hour close to the Martian surface.Finally, it too will duck out of sight of Earth, and comeback spewing out the results of its observations.

Within a few days, all the scientiiic information willhave been returned to Earth. The spacecraft will sail on,their principal voyage done. The giant antennas backon Earth will find other radio voices to listen to; theengineers, other devices to design. The scientists willbe working vigorously to interpret the numbers returnedby the two Mariners and read from them the new, ex-panded story of Mars. They hope for as much newknowledge from this mission as we have ever gathered be-fore. But they will want still more. Each day answers yes-terdays questions, and asks tomorrows. So let it be withthe 1969quest of Mariner VI and Mariner VII.


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The word planet, from a Greek word meaningwandering, was adopted to describe those celestialbodies, including the Sun and Moon, which wanderabout in our skies independently of the self-consistentstars. The ruddy planet Mars is the very paradigm ofwanderers, making its irregular way from West toEast through the constellations of the zodiac aboutevery 23 months, and passing from conjunction to op-position and back to conjunction every 25 to 26months.

For about two to three months surrounding itsopposition to the Sun, Mars reverses its motion againstthe stars, moving from East to West. Combining thisreversal with its North-and-South motion in our sky,the planet traces out a pattern which may resemblethe figure 9 or 6, or the letter S or Z, depending onthe year.

Changing V iew sThis eccentric motion through the sky, which is

performed with variations by all the planets, brought

thabout the ultimate downfall of the Ptolemaic hypothe-sis of the nature of the solar system. The Greek Ptol-emy and his followers held that the Earth is the centerof all things, which orbit variously about our planet.Tycho Brahe carefully measured the positions of Marsduring all its oppositions from 1580 AD to 1600, andhis colleague and successor, Johannes Kepler, workedfor several years to fit the data to a theory which be-came the foundation of our current understanding ofplanetary and celestial motion: the heliocentric ellipseand the nature of motion in it.

Keplers k s t law, propounding the elliptical or-bit, was published in 1609; the same year, Galileoobserved the changing phases of the planet Venus asit traversed its orbit, but could not confirm the moresubtle changes in the shape of Mars disk. Still he ob-served, believed, and taught, in the face of powerfulbut scientifically untenable opposition, that the plan-ets are orbs like our own, moving around the Sun asdoes our Earth,

Before the century was out, other observers,using better telescopes, had noted and drawn distinct


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Map drawn by Giovanni Schiaparelli from bservations between 1877 and 1888. Note that southis at top accor dkg to astronomers convention.

surface features, including the south polar cap, andhad found that Mars rotates on its axis in 24 hours40 minutes, a figure within three minutes of the cur-rently accepted one. Surface changes, seasons, andother similarities to Earth were brought out in theeighteenth century, together with the assumption thatMars was habitable and inhabited. By the nineteenth,a number of observers were compiling maps of theplanet and assigning names to the features. RichardProctor produced one map, calling the dark areas seasand the light areas lands, and naming them after as-tronomers and scientists, with the English observerspredominating. Camille Flammarion drew another,naming the features in the French language and modi-fying Proctors scheme somewhat. Giovanni Schiapa-relli, using the opposition of 1877 as his observationalbase, produced a new map with a new series of names,in Latin, using extant or mythical terrestrial features,which became the foundation of modern areographyor Martian geography.Mars two tiny satellites in August 1877, a very favor-able opposition of Mars. He named them Deimos andPhobos after the two squires (or in other references,

The American Astronomer Asaph

horses) of the antique God of Battle. Using a luckynumerological scheme, Kepler had predicted (and GuZ-livers Travels had suggested) that Mars had twomoons, but their small size postponed actual discoveryuntil observing techniques and equipment had im-proved. Phobos turned out to be so close to the planet,and therefore, so speedy in its near-circular orbit, thatit orbits faster than Mars rotates, rising in the Westand setting in the East twice a day. Deimos, furtherout, completes each orbit in about 30 hours, with theplanet turning under it in 24% hours, so that it ap-pears to move slowly across the sky from East to Westevery5% days.

Analysis at aSpectral study of Mars began in 1862, shortlyafter the invention of the technique, but for 85 years

little more was demonstrated than that Mars shone inreflected sunlight. The last two decades have seenMartian spectroscopy triumph over the dimness ofMars, the brilliant diversity of the solar spectrum, andthe thickness of Earths atmosphere. The first con-stituent of the Martian atmosphere to be identified


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was carbon dioxide, found by 6. P. Kuiper. Discoveryof water vapor followed, when the planet was reced-ing from the Earth, and its spectrum was shiftedslightly off that of Earths atmosphere by the dopplereffect. Careful analysis of carbon dioxide absorptionlines suggested that the total atmospheric pressurewas much less, and the percentage of carbon dioxidemuch greater, than previously believed. A vigoroussearch for spectroscopic evidence of oxygen was notsuccessful, although theoretical models suggest asmall amount of the element in Mars atmosphere.

An improvement in the observation of Mars com-parable to the invention of the telescope and spectro-scope came in the early 1960s with the developmentof a machine to carry remote-reading instrumentsclose to Mars. Mariner IV, the iirst such machine,made possible the investigation of the planets gravi-tational and magnetic fields on the spot, the photogra-phy of a portion of its surface from a distance of afew thousand miles, and the penetration of its atmo-sphere by a precisely measured radio beam. Prior esti-mates of the magnetic field strength and surface-levelatmospheric pressure were revised downward as a re-sult of the 1965 spacecraft encounter. The surface wasobserved to be cratered, much like that of the Moon,a condition which had been predicted in the 1950sby Opik and Tombaugh, but still came as a consider-able surprise to most observers. The general trend inmodeling the conditions on Mars had been in a direc-tion away from the earthly and toward the lunar; inparticular, the estimates of surface atmospheric pres-sure had been falling steadily. But still the idea of an-other terrestrial world died hard.

A Contemporary SurveyThe post-1965 model of the fourth planet was

unearthly and hostile by our standards. Where theEarth is generally thought to have a molten-metal corebounded by a solid mantle of similar composition, witha thin rocky crust, the interior of Mars was now be-lieved to be far simpler. It is probably nearly homo-geneous throughout, with far less free metal, if any. Thenegligible magnetic field (below the threshold of MarinerIvs instrument) was predicted by some planetologists,who thus found their ideas about the planets innerstructure partly confirmed.

he surface of Mars shows greater relief or alti-tude difference than would be expected in comparisonwith the Earth. Craters photographed by Mariner IVshow substantial local relief. Based on the 1965 obser-vation of perhaps 278 craters, at least part of Mars

surface must be topographically very like the ruggedhighlands of the Moon. Unlike our big satellite, how-ever, Mars is relatively brightly colored, with largedark and light features around the globe, and a gen-erally orange cast.

At its mean orbital distance, Mars receives only43% as much solar energy per unit area as the Earth.Its surface temperatures for various locations andtimes of day are therefore colder than those on Earth.In the current southern-springtime season, for example,surface temperatures along the equator are believed torun from -80 to -155F at dawn, rising to an 80Fpeak around noon, and falling to -65 to -100 at sun-set. The polar regions are thought to vary from about20F in local summer to -200F in winter. LackingEarths oceans, which serve as a planetary heat sinkand keep the daily cycle within tolerable limits, Marstemperature is essentially dependent on solar radia-tion, and hence on season and local time of day. Thesurface appears to be a poor heat conductor. It is wellto bear in mind that this part of the model of Marsconditions was based entirely on data received across35 or more million miles of space and through Earthsdeep and active atmosphere, conditions which arguedstrongly in favor of the Mariner Mars 1969 experi-ments.

Having an atmosphere, Mars has weather, andclouds; its poles have white caps of ice and/or frozencarbon dioxide, which shrinks in summer by evapora-tion into the atmosphere. Liquid water could be onlya very temporary feature of the surface; no more thana trace of water vapor could be detected in theatmosphere.As for life on Mars, only the most direct obser-vations could discover it, and only more of the samecould rule it out. Spectrochemical analysis of the at-mosphere, a map of the surface temperature, and ahigh-resolution visible search for habitable terraincould provide strong clues. But all these things werestill in the future.

Earth-based photographs taken during the 1969 opposition ofMars as part of the NASA-sponsored observing program.


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As a concept, Mariner is ten years old. When theunmanned exploration of the Moon and planets wasfirst being planned in the context of a just-demon-strated space flight capability, the idea of an attitude-stabilized, solar-powered planetary fly-by spacecraft wasa major item.

Mariner shares fundamental space flight prin-ciples with many other unmanned spacecraft, includ-ing applications satellites, orbiting observatories, andPioneer deep-space probes. These principles includethe capability to operate in space for months, perhapsyears; the ability to perform at least one thrust maneu-ver after the initial launch phase, in order to refine oraugment the flight path provided by the launch rocketstages; and the maintenance of continuous radio con-tact with Earth, providing for accurate radar tracking,extensive telemetry return, and response to commands.A major difference is that most of the other vehicles arestabilized only in one axis, usually by spinning aboutthat axis, and tend to have a circular or cylindricalsymmetry.

With the other fully attitude-stabilized space-craft, which have included a few Earth satellites andmost of the unmanned lunar investigators, Marinershares the ability to point solar panels at the Sun, an-tennas at the Earth, and scientific sensors or camerasat their targets of interest. These craft tend to be inthe size range of from 500 to 1000 pounds and toresemble large mechanical insects rather than smallrobot rocket ships. Mariner differs from the rest byhaving to travel for months and for hundreds of mil-lions of miles to conduct its primary mission at an in-terplanetary distance from its home base.

Th e InterplanetaryThe 1969 Mariner is the fourth generation of its

family to be designed, and the third to fly a mission.In the early days of space exploration, while Rangermissions and spacecraft were being developed forlunar work, two larger planetary craft called MarinerA and B were designed. Like Ranger, they could notFacing, from op: Mariner I I (1962), Mariner IV (1964-65),Mariner V (1967), Mariner Mars 1969 spacecraft.


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orbit or land on their targets; but to protect possibleMartian or Venusian life from contamination, and forother reasons, they were, unlike Ranger, to miss theirtargets, making observations during a brief close pass.Mariner A was designed for a Venus mission, MarinerB for a Mars flight. They were one size larger thanRanger, and since a larger launch vehicle had not beendeveloped at that time, were not committed to fabri-cation and flight.

Instead, in 1961, the Ranger lunar design wasmodified and combined with elements of Mariner Ato produce a 450-pound planetary spacecraft to go toVenus. After an aborted first launch, Mariner I1 waslaunched on August 26, 1962, and flew by Venus aftera tense three months interplanetary flight, the first ofits kind in history. The spacecraft provided the firstclose-up observation of Venus, as well as a long ex-amination of the interplanetary medium and the firstoperational experience of a planetary mission.

Immediately a Mariner Mars project was set inmotion, for the 1964-65 flight opportunity. The newspace machine was in the Ranger weight class at 550pounds, but hardly resembled its ancestor otherwise.Ranger had been designed to work in the earthly re-gions of the solar system. Its close relative, Mariner11, designed to operate in the warming orbit betweenour world and Venus, had, in fact, suffered greatly inits long, hot journey. The new Mariner had to workin the cold outer regions -the orbit of Mars is halfagain as far from the sun as Earth- nd to operatefor eight months before sighting Mars. It had to carrya record payload of scientific equipment and commu-nicate across record distances (130 million miles whenit reached Mars). Jamming all these capabilities ofcommunication, electrical power, guidance and con-trol, thermal conditioning, mechanical design, andscience, into a quarter ton of mass was a singularachievement of engineering.

The second Mariner effort was also jinxed in thefirst launch, but after a feverish and triumphant re-covery eiTort Mariner IV began the first flight from

Earth to Mars. It fulfilled all its objectives, flying 6000miles above Mars on July 15 , 1965. Then it passedout of communication range, traveled behind the Sun,and reappeared to provide additional interplanetarydata and complete three years as an operating man-made planetoid late in 1967.TheN ew Generation

The spare spacecraft from the 1964 Mars missionwas redesigned, rebuilt, and re-equipped for a Venusflight in the summer and fall of 1967. The Mariner Vmission was conducted at a time of very active plane-tary research, including the Soviet Venus-atmosphereprobe Venera 4 and Earth-based radar examination ofthe planet. Mariner V shed new light on the hot,clouded inner planet, and on the weather of the solarsystem. It also advanced the techniques of buildingand operating planetary spacecraft, as had each Mari-ner in its day.Meanwhile a new, larger Mariner was about tobe born. Formally authorized on December 22, 3965,the Mariner Mars 1969 Project was originally expectedto take a modest next step beyond Mariner IVY mis-sion just concluded at that time, and to set the stagefor later, more ambitious Mars landing missions whichwould undertake the search for extraterrestrial life.

Because of the availability of the Atlas/Centaurrocket, locomotive of the Surveyor lunar landing mis-sion, the 1969 Mariner could be heavier than its fore-bears, somewhat easing the strain imposed on the de-signers of Mariner IV. But the successful 1964 designwould be changed only to improve mission reliability,to reduce cost (by eliminating expensive minimum-weight designs), or to accommodate the differences ofthe 1969 mission or the new payload. As these factorscame into closer focus, the 1969 Mariner evolved froma beefed-up Mariner IV into a mature design with itsown character and qualities. Externally, from a reason-able distance, its similarity to the first Mariner isstriking. But in many critical aspects it is a new spe-cies of spacecraft.


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Mounting and equipping an unmanned interplan-etary expedition is one of the most challenging of tech-nological endeavors, for two reasons: timing and ir-reversibility.

The energy requirements for interplanetary flightare exceedingly high, fluctuating with the differentialrotation of Earth and the target planet between utterlyimpossible and merely difficult. Launches to Mars withan acceptable energy cost can be attempted just aboutevery two years, when Earth is roughly thirty to forty-five degrees behind Mars in orbit around the Sun. Thegreater an energy cost (or spacecraft weight penalty)a mission can afford, the longer this planetary launchopportunity can be stretched. But within a month ortwo, the opportunity runs out, and one must sail atonce or wait two years.

The previous Mariners each started developmentshortly after one launch opportunity, and had to leaveEarth at the next. There was time to do more for the1969 project, in science and spacecraft engineering

Docksidand operations training, but there was no added mar-gin in which to make and correct mistakes. Everythinghad to be very nearly right the first time.

Once an interplanetary vehicle is launched, thereis no turning back. The ship and its mission are com-mitted to a journey which will take months before theobjective is sighted. There can be no pit stops, noheaving to for repairs, in the race to Mars. There canbe no more than the very limited operations of switch-ing to duplicate equipment, automatically or by com-mand, in the event of failure. If the difficulty goes be-yond these options, the mission can only suffer.

Thus the calculation of risks, and covering themwith redundancy whenever possible, takes on a highpriority in a Mariner effort. At the very simplest level,this means sending two spacecraft if it can be done.It was done in the 1962 and 1964 projects, and ineach case the first spacecraft did not survive the launchfor reasons quite beyond its control. There would betwo launches in 1969.


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Concentrating onEarly in the development of Mariner Mars 1969

came a decision which was to determine the characterof the mission in a proliferating way. In selecting thescientific experiments to be performed, NASA delib-erately made this the first truly planetary mission; ofthe six experiments, none would start until the space-craft reached Mars. This contrasted with previousMariner science efforts, which began studying the in-terplanetary medium near the Earth and obtainedmore data en route than on arrival, though the plane-tary objective was always primary. In addition, thescientific instruments carried by the two 1969 space-craft were to be pointed at s p e c k regions of theplanet-instead of making a simple sweep over thedisk- nd were to produce near Mars, for storageand return to Earth, more scientific data than had theprevious Mariners in their combined flight lives.

The six scientific experiments selected by NASAfor Mariner Mars 1969 were to be supported by twotelevision cameras, ultraviolet and infrared spectrom-eters, an infrared radiometer, and the spacecraft track-ing and telemetry link. Astronomers, geologists, physi-cists, chemists, and biologists would study the atmo-sphere and surface of Mars through the various radi-ation-sensing instruments, and physicists and mathe-maticians would use the occultation or extinction of theradio signal as each Mariner passed behind Mars toprobe the atmosphere and d e h e the surface beneath,and the tracking information to study the gravitationalfields through which the spacecraft moved, therebymeasuring the mass and motion of Mars.

The surface-scanning instruments were mountedon a large turret on the antisolar side of the space-craft. Weighing nearly 200 pounds fully loaded, andguided by sensors and electrical angle measurements,this scan platform could move 70 degrees in elevationor cone angle, and 215 degrees in azimuth or clockangle, at a rate in either direction of one degree persecond. The scan platform moved in response to in-structions from the spacecraft computer, which wasalso a new thing with this mission.

The computer developed and flown on MarinerVI and Mariner VI1 had a memory storage of 128 wordsof 22 digital bits each. I t could be programmed and repro-grammed, before launch or in flight, as late as min-utes before the programmed events were to take place.It could read out its memory to the operators on Earthbefore and after changes. Most important, it couldcontrol events as simple as switching off a circuit oras complex as an entire Mars encounter sequence.

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Mariner's heavily-laden scan plat for m is shown ab ove, an d theinstrument passengers are seen separately below . The temperature-control blanket (like that stretched ove r the octagonal spacec raftbo dy) has not yet been installed on the p latfor m. The instrumentsar e ( I ) the infrared radiometer, ( 2 ) the wide-angle TV camera,(3 ) the ultraviolet spectrometer, and (4 ) the narrow-angle TVcamera telesc ope. Planet sensors wh ich help control scientificoperations are marked (5) , an d (6) s the infrared spectrometer.


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The Mariner Mars I969 spacecraft design. Commands are received over the omnidirectionalantenna ( I ) , which also transmits to Earth until, toward Mars, the high-gain reflector antenna(2 ) s lined up by the spacecrafts position. Tiny cold-gas jets (3)on the t i p s of the solar panels (4 )position the spacecraft so that the Sun is overhead in the view shown, Photoelectric sun sensorson the top o f the spacecraft body, invisible in this view, provide the main reference fo r this orien-tation, so that the solar cells on the upper surface o f the panels may convert sunlight into electricpower. The Canopus tracker (5 ) provides an additional reference, keeping the spacecraft fromrolling around the Sun line. The maneuver engines nozzle (6 )points sideways out of the space-craft; the whole machine is turned by the gas jets to point the rocket in the proper direction. Mostequipment is inside the octagonal body; it is kept at the proper operating temperature by insu-lating blankets above and below and by louvers (7), which do not ventilate but change the heatradiation under thermostatic control. The scan platform (8) oints the scientific instruments (seefacing page), rotating or elevating on command of the spacecraft computer.

The InformationProblemThese features gave each spacecraft the ability

to acquire more bits of scientific data near Mars thanall previous Mariner missions had obtained, as wenoted previously. The problem then became one ofhow to return this vast pool of information to the sci-entists on Earth.

The preceding Mariners had used a tape re-corder to store the scientific information acquired atthe planet in a few minutes and return it more slowlyto Earth over a period of several days. But the Mari-ner 1969 system could gather far too much informa-tion about Mars for the tape recorder to hold, even ifthe tape was lengthened, the number of tracks doubled,the density of information put on the tape increasednearly to the bursting point, and a second tape re-corder added to the spacecraft. And the informationhad to go through the tape recorder, for the space-craft could not transmit fast enough to relay directlyto Earth. Or could it?

Space communications have come a long waysince the first simple Earth satellites. In the planetarymissions, the increase in distance had matched the ad-vance in technology, so that in 1965 Mariner N hadtransmitted from Mars to Earth, a distance of 120 mil-lion miles, at a rate of only 8% bits per second, cor-responding to more than eight hours for each TV pic-ture. But after 1965 space communication took a greatleap forward; furthermore, the geometry of the 1969mission reduced the communication distance to 60 mil-lion miles. The spacecraft carried a more powerfultransmitter and a large transmitting antenna, while theadvanced antenna system at Goldstone added a 210-foot-diameter dish antenna to a network of 85-footers, andbacked it with new and very sensitive receivers. Theexperience of the earlier Mars Sgh t had permitted thedesigners to reduce their safety margins as well. Ac-cumulating many factors of improvement allowed thetelecommunications developers to offer Mariner Mars


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Ab ov e, Mariner spacecraft is prepared for a test in the IO-footspace simulator. Below, M ariner VI and Mariner VII undergofinal checkout in the hangar at Cape Kennedy.

1969 an increase in transmission rate of two thousandtimes, limited to the eight to ten hours per day whenthe spacecraft was within range of Goldstone, with abackup mode operating on a slower data rate.

This technique offered Mariners experimenters,especially those concerned with the television surveyof the planet, an exciting hope: instead of eight pic-tures taken during the last day of each approach toMars, they could gather 160, beginning two or threedays out and building an observational bridge fromthe level of Earth-based views to the final closeups.Ultraviolet spectra and planetary temperature mea-surements would be available during this period also.The far-encounter views from each approach could beexamined before the corresponding close pass, provid-ing an opportunity to adjust the near-encounter pro-gram if necessary. Perhaps more exciting was the pro-vision that all of the close-up data except the completeTV pictures would be returned in real time, at the


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speed of light, leaving the spacecraft as soon as theywere gathered and arriving at the Earth about fiveminutes later. Thus some of the limitations on the mis-sion because of restricted recording capacity were eased.

Now the major design features were settled, andit was time to execute them in hardware. About adozen spacecraft subsystem contractors had to buildthe various units, qualify them for flight use, and de-liver them. At JPL, they had to be assembled into fourspacecraft: a proof-test model, two flight craft, andone assembled set of spares. The proof-test modelwould never fly; it and its parts would be tested atlevels simulating an environment even harsher thanthat expected in each aspect of the flight to Mars, toqualify the system design for the mission. The otherthree units would be tested more gently, on the vibra-tion table to rehearse the launch, and in the spacesimulator to practice the spaceflight and Mars en-counter, in such a way as to save some strength forthe real thing.

After many returns to the test bench, and muchrebuilding and repairing, the machines were finished.One would never leave JPL in Pasadena, where itwould live out its life as a test vehicle. The other threewere shipped to Cape Kennedy: two for further ship-ment to Mars, and one to stand by as a supply ofequipment for transplant in the process of diagnosis-and-cure called system test, which would continueright down to the moment of departure.About ten and a half days before the scheduledlaunch of Mariner VI, the Atlas/Centaur/Marinerspace vehicle was standing on the pad, undergoing asimulated launch with the rocket-propellant tanksempty. Suddenly the Atlas began to collapse like apunctured tire. Most of the structural strength of theAtlas is provided by the pressure in its tanks, a bal-loon-like design feature which saves a lot of weight.A worn-out relay in the vehicle had opened the mainvalves, letting out the pressure through six-inch pipes.As the vehicle sagged alarmingly in its gantry, twoground crewmen sprinted for the manual valves insidethe Atlas and shut them off. The pressurizing pumpsrestored tank pressure, and the big rocket resumed itsshape, but a terrible scar was visible in its plating.

Within hours the Centaur and Mariner had beenreturned to their hangars for a quick round of testingwhich would verify their good health after a narrowsqueak. The launch vehicle intended for Mariner

At Cape Kennedy, the spacecraft is installed in the two-pieceCentaur nose fairing, which will protect it on its flight fromEarths surface into sp ace.

was commandeered for the fist launch. The crewfound that they could cut launch preparations fromtwo weeks to one week by working extra hard. A thirdAtlas rocket was shipped out from San Diego by Con-vair, to replace the one borrowed from Mariner VII,while the damaged vehicle went back to be repairedand re-tested, and, months later, to serve another m i s -sion. The Mariner launch schedule, so severely limitedby the motion of the planets, had been saved by redun-dancy and the provision in the plans for a problem ofthis kind.24, the Mariner V flight was ready to begin. The Mar-iner VI1 prelaunch operations had just over a monthto run.

Thus in the late afternoon of


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It was about 8:30 at night when Mariner VIpushed off from the Earth. All stages of the intricatelaunch procedure went off as planned, and about tenminutes after liftoff the spacecraft was in heliocentricorbit, headed for Mars. After another fifteen minutes,it came out of Earths shadow into the sunlight, ori-ented itself, and began operating in the normal cruisemode.

An unusual feature of this launch was that thevehicle maneuvered in two directions. Normally, therocket rises vertically, rolls to the correct heading, andafter lifting for some time begins to pitch over so thatthe final acceleration is nearly parallel to the Earthssurface. Mariner s launch had an additional turn,a yaw to the right after the pitch forward, to permitaiming quite far to the south without crossing landareas. The Centaur guidance system, which controlledboth the Atlas and Centaur stages, carried out thismaneuver.

The Air Force Eastern Test Range tracking net,the Manned Space Flight Network, and the DeepSpace Network received and relayed information onthe spacecrafts condition and operation back to theCape Kennedy control room and the Space Flight Op-erations Facility in Pasadena, where it was duly deter-mined that the mission was going well.

Tracking data showed that the launch was themost accurate in Mariner history. The spent Centaurstage would pass well to the north of Mars, as planned,and Mariner could be shifted down to its flyby ofMars with about 5% of its maneuver capability.The flight path calculations were so good that themaneuver could be conducted four days after liftoff;it was successfully carried out on schedule.

Not quite four weeks later the second bird tookflight. This time it was a daylight launch, with thespacecraft passing into Earths shadow shortly afterseparation. The Sun sensors had a glimpse of the Sun,


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but had to wait half an hour before it reappeared onthe other side of the Earth to drive the spacecraftaround to Sun orientation. Again, the launch accuracywas outstanding.

For a number of reasons, principally the desireto get a very precise pre-maneuver trajectory, the Mar-iner VI1 maneuver was scheduled for 12 days afterlaunch. It proved to be as successful as that performedon Manner VI, moving the spacecraft trajectory intoits appointed niche above Mars.

SurprisesWith both spacecraft now falling ballistically and

inexorably around the Sun toward their planned ren-dezvous with Mars, and with all operations up to andthrough that dual encounter already programmed intothe two spacecraft computers, the rest would seem tobe merely routine. It was the study, review, refinement,possible change, and testing of the complex Mars en-counter sequence that was to be the major activity ofthe period between launch and encounter, occupyingmany engineers, computers, and days. But spacecraftoperators have long since learned to take nothing forgranted. Surprise is always just around the next corner.

The first surprise in flight had to do with the radioaboard Mariner VI, shortly after launch. When theranging channel was on, the radio subsystem tried tolock up on itself rather than on the command andtracking signal transmitted up from the ground; itmight then be unable to acquire the ground signal so

Lef t , Mariner VI1 launch. Below, Mars-target diagram showsthe aiming points and actual destinations of Mariner VI an dMariner VI1 before and after launch and the spacecraft maneu-ver . Both m issions achieved a closest altitude of about 3400kilometers or 2150 miles above the surface of M a rs .THOUSANDS OF

KILOMETERS

LAUNCHT R A J E ~EFFECTOF

SPACECRAFT MANEWER

MARINERV I FLY-BY051750GMT. JULY31, 1969

MARINERVIILAUNCHTARGETAUNCH TRAJECTORY7 / I

MARINERVII FLY-BYENCOUNTER TARGET ZONES

0501 09 GMT, AUGUST 5, 1969


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craft away from its proper orientation. When the par-ticle drifts out of range, Mariner must roll all the wayaround and find its star again. This had happened toMariner IV, and it proceeded also to happen to Mari-ner VI. Bright particles were observed during the mid-course maneuvers of both spacecraft in 1969, but thetracker was not in control at the time. When MarinerVI unlatched its scientific platform from the restraintsthat had protected it during launch, particles wereshaken loose which distracted the tracker and made itlose Canopus lock. Since a similar event was scheduledfor each Mars encounter, and resulting particles mightcause the spacecraft to roll around instead of pointingits cameras at Mars, a solution had to be found.

The solution had been built in as an option longbefore. Autopilots, which keep aircraft, ships, and manyguided missiles on an even keel, derive their sense ofdirection not from the stars but from gyros. Over thelong haul, gyros will drift off position and have to becorrected, but for a few days or hours they are accu-rate enough. Mariner VI and Mariner VI1 had gyrosto hold them steady during the thrust maneuvers car-ried out a few days after launch. So each spacecraftmemory was given something new to remember: gyroson at encounter.The Great Magellanic Chase

Perhaps there is something sinister about the starCanopus. At least, for Mariner VI, it was once morean unlucky star, which led the spacecraft teams intoa frustrating chase through the Greater MagellanicCloud.It started on April 20, when the Canopus trackerwas supposed to change its cone angle. This changeOrbits around the Sun of Mariner VZ, Mariner VII, Earth,and Mars.n

ARINER VI1

was necessary from time to time because, from thepoint of view of Mariner, though there is a single pointin the southern sky that is always ninety degrees awayfrom the Sun-a geometric south pole-Canopus isnot it. As the spacecraft moves in its orbit, alwaysfacing the Sun, Canopus appears to circle in the op-posite direction, bobbing back and forth about 15degrees. The sensor must bob back and forth, changingits angle from the Sun electronically to keep the starin sight.

On April 20, when the computer said Step theangle, the tracker stepped backwards, losing the starentirely. Ground commands could return it one step,but it would go no further forward.

A search was immediately begun for usable starsin the field of view. Earlier, Sirius and Vega had beenavailable, but they were now at the wrong angle. Theplanet Jupiter would soon swim into Mariners ken,but it would not stay there till encounter. There wasindeed nothing that would serve except the GreaterMagellanic Cloud, a nebula hanging above Earthssouth pole. It was dim and widespread, a poor naviga-tional reference, but still worth a try.

For days they tried it. If released in roll search,the spacecraft would simply roll past the cloud withoutstopping. After stepping the roll position- ith gyrosholding at each step, as they were to do at encounter-slowly through the cloud, and mapping its bright-ness with the star sensor, the team carefully led thetracker to the brightest part, and then let it go. Itstayed, but only for a few hours each time. And thestrain of viewing a dim object- ike eyestrain- aswearing out the sensor tube. No go.Next would come another try to change the coneangle with commands, and after that-gyros all theway, perhaps. A barrage of commands was readied-one a minute- o jar the stuck circuit off dead center.Unexpectedly, Mariner VI obediently responded to thefirst command and settled down again, locked securelyon Canopus. Several weeks later, with the last Canopuscone-angle change before Mars encounter, ground com-mands were again required, and once more were imme-diately effective.Alarm of the

One instrument which was not part of the scientificpayload nearly produced a scientific sensation. It was asimple absolute radiometer pointed at the Sun by eachspacecraft to obtain an accurate record of the solar energyfalling on the Mariners as they flew. This informationwould be of great value in designing future spacecraft,especially their temperature-control coatings andblankets, and the solar simulators in which they are


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Johannesburg. Station in South A frica, oldest overseas member of the Dee p Space Ne two rk.

tested before launch. It would also provide a fairly long-term observation of the solar output from space.

After taking the readings for a few months, thetemperature-control engineers concluded with someconsternation that the measurements were falling offconsistently, confirmed by both spacecraft, at a ratefaster than the retreat from the Sun would allow. Eitheran unexpected affliction had clutched both instruments,or they were showing true conditions and the Sun wasslowly going dim. After long sessions with instrumentcalibration and test records, no theory of radiometerbreakdown offered itself. But there was no confirmationoutside the two Mariners of any decline in the sunshine.There was nothing to do but wait.

After the Mars mission was safely over, MarinerVI was turned well away from the Sun, so that the radi-ometer was viewing the cold black sky, and the instru-ment was calibrated. To the relief of all concerned, thecalibration showed that the instrument had drifted.Further analysis led to an understanding and a formula,which, applied to the original measurements, showed afairly steady Sun with good confidence in the numbers.Corrected to the mean Earth-Sun distance, the averagevalue was 0.1353 watts per square centimeter.

By June, most of the surprises were behind thetwo Mariners and their Earthbound crews. Earth hadovertaken Mars in their regular race around the Sun,at a closest approach of 44% million miles, and manyscientists were turning telescopic eyes and instruments

toward Mars on every clear night. It was not an ex-ceptionally close Mars opposition (not as close as in1956 and 1971 by ten million miles), but the compari-son with spacecraft observations would be valuable.

For the spacecraft and the operations teams itwas now a time of testing. All the months of flight toMars were of little value if the few days near theplanet were not perfect. Every act, every ground com-mand, every likely situation was designed, rehearsed,and tested as carefully as the parts of the spacecrafthad been, and the engineering measurements fromMariner VI and Mariner VI1 were carefully assessedto be sure of the capabilities of the hardware.

As June wore on, full-length simulated Mars en-counters were conducted for the Mariner VI and Mari-ner VI1 sequences, using outputs from the spare Marinerspacecraft. The spare was operated in a building a fewhundred yards away from the Space Flight OperationsFacility but acted as if it were out by Mars.

July came, and with it a mighty distraction fromMars. The Apollo 11 mission, like Apollo 10, usedfacilities of the Deep Space Network, including the210-foot Mars Station antenna which would receiveMariner Mars scientific data, as part of its lunar com-munication system. A set of DSN stations around theworld also backed up the Manned Space Flight Net-work. Also in July, Mariner VI and Mariner VII dem-onstrated their ability to play out their tape recordersthrough the high-rate telemetry link, erase the tapes,and reload them.

Now they were ready for Mars.


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It began for Mariner VI some 154 days afterlaunch, with Mars some 780,000 miles ahead. Back onEarth, at the Jet Propulsion Laboratorys Space FlightOperations Facility, it was Monday evening, July 28,1969, when the first far-encounter commands weresent to the spacecraft. The scientific instruments wereturned on, and the scan platform was moved aroundto verify its operation, before pointing at Mars. Thenthe lens cover was removed from the narrow-angle tele-vision camera, and the sequence of recording 33 tele-vision pictures at 37-minute intervals was begun.

After a few hours, the high-rate telemetry linkwas turned on to allow the return of data not beingrecorded. About twelve minutes later, a rough engi-neering-model picture of Mars arrived in Pasadena.Most of the planet was blotted out by a black vertical

arstripe, set aside from this picture format for the trans-mission of ultraviolet and infrared measurements whichwere being processed in another part of the groundcomputer system. The image was coarse and ragged,for (outside the stripe) it was built from only one-seventh of the picture information gathered by thecamera, But it was visible proof that Mariner VI wasdoing its job.

Through the night and the next day, the space-craft tape recorder continued to gather a series of 33television pictures of Mars. I n the afternoon, whenMars and the spacecraft again rose above the horizonof the great 210-foot Deep Space Network antenna atGoldstone, the high-rate telemetry was turned on, and an-other stream of engineering-model pictures and ultra-violet and infrared measurements began to flow acrossalmost sixty million miles of space. By evening, thespacecraft tape recorder was full, and about 6:30 p.m.the controllers in Pasadena instructed the spacecraft,


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now some 450,000 miles from Mars, to play back thetape. Again there was about a twelve-minute wait.Then the first complete picture, recorded the eveningbefore, appeared on the monitors and was transmittedto television networks and local stations. In a roomdeep inside the Space Flight Operations Facility, thePrincipal Investigator of theperiment watched as one view of Mars succeeded an-other, the grey globe against black sky slowly rotatingand growing from picture to picture. Beautiful, hesaid, thats beautiful!

When the three-hour playback was done, and theexperimenters were already studying fresh prints of thepictures with caliper and magnifying glass, the space-craft was commanded to erase the tape, and then tobegin a new series of far-encounter pictures, at about

ariner Television

340,000 miles from Mars. This series would be onlyhalf as long, for it was scheduled to end 71/2 hours be-fore the spacecraft came close to Mars, to allow fulltime for preparation for that important event. Wednes-day evening would see the playback of these seventeenapproach pictures, last-minute preparations for thenear encounter, the close-range scientific pass itself,and the beginning of the data playback. Before Mari-ner VI began its half-hour close pass, the trajectoryhad to be accurately determined so that instrumentpointing angles could be corrected, last-minute checkshad to be made, and ground commands, mostly back-ing up the existing sequence already in the computer,had to be sent. Then the five instruments, includingtwo alternating TV cameras, would automatically start,scanning over the surface in a swath which was zig-


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spectrometer half its information. But theories were forlater. Now there were things to do.

First, the spacecraft computer-the valuable butvulnerable brain of the spacecraft, second of its kind inspace-had to be checked for errors in its memoryand upsets in its logic. A single command produced areadout on Earth of the memory, and a simple se-quence cleared the logic circuitry. The memory wasunperturbed.

Second, attitude control and power had to bechecked, for they would be essential supports of theMars encounter. Telemetry and tests showed them tobe functioning reasonably well, except that the telem-etry channels for Canopus brightness and cone anglewere unreadable-part of the damage to the telemetrysystem. But the spacecraft had successfully acquiredCanopus after the difficulties, and it would be on gyrocontrol at Mars.

Third, the scientific instruments had to be oper-able. Until they were turned on, shortly before farencounter began, three days out from Mars, there couldbe no certainty. There was no evidence of damage.

Fourth, the scan system, which must point the in-struments, had to be checked. The engineers knew thereference values for the first pointing at Mars had beenscrambled in the first storm that swept over the space-craft circuits. The second storm had taken out three ofthe four essential telemetry measurements of those val-ues, so they could not be corrected directly. Also elimi-nated was the telemetry from the long-range Mars sen-sor, so the values could not be corrected by eyeballeither.Therc is always a way for an engineer, if he islucky and clever and stubborn enough. Mariner VISscan platform carried two auxiliary sensors of verycomplicated and expensive design, known popularly astelevision cameras. So, on Friday night, when far en-counter was due to begin, science was turned on early-all instruments working-to allow for an extendedrecalibration operation. The cameras began shootingpictures, and as the reference directions were graduallymoved, the image of Mars edged into the monitors re-ceiving engineering-type video, and approached thecenter. The engineering video has a black stripe downthe center, so it proved necessary to record and playback two frames to complete the calibration. LaterMariner VI1 pictures of Mars showed impressive detail,cratered borders, mysterious and fascinating features.But none was so exciting as the two played back beforethe official start of far encounter, for they showed thatin spite of near disaster, Mariner VI1 far encountercould and would begin.


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Completionof a TriumphOne reason for the dual flight and encounter of

Mariner VI and VI1 was the hope that some informa-tion gathered in the Mariner VI pass could be fed backto change and improve the scientific efforts of MarinerVII. This came to pass when the observers saw indica-tions of slight atmospheric haze and the strikingly irreg-ular border of the south polar cap in the long-rangeMariner VI pictures. They requested additional televi-sion coverage of the polar regions and in the passageonto the limb of the planet. This option-an increasein near-encounter pictures from 25 to 33-bore a risk,for the tape recorders would be full before the night-side data were collected, and the real-time telemetrywould be the only source of dark-side data. But therisk was acceptable; high-rate telemetry had been dra-matically successful on Mariner VI, and the secondspacecraft had performed impeccably in the pre-encounter high-rate test.

Thus in addition to recalibrating the scan systembefore far encounter, Mariner VIIs crew also repro-grammed the near encounter, adding to the first twoswaths across the western and southern reaches ofMars.

In the three days before it drew near Mars, Mari-ner VI1 recorded and played back 93 pictures of theapproaching globe, pictures clearer than those of Mari-ner VI, starting further out and ending closer in-only four and a half hours before closest approach.

On Monday, night of closest approach, the TVteam released a near-marathon of pictures in real time.First there were engineering-model pictures of the ap-proaching surface, at 42-second intervals; then theplayback of the previous days recorded high-qualitypictures, in which the globe of Mars grew until it over-flowed the screen; and then came the engineering-typenear-encounter stream.

In the scientists room in Space Flight Operations,a cold and quiet drama had turned warm and noisy, forone instrument that had not been turned on Fridaynight was only now revealing its condition. The infra-red spectrometer could be turned on only once, andcould operate only once, for its two-channel operationdepended on the expenditure of tanks of nitrogen andhydrogen. These tanks had been eyed askance eversince Mariner VI1 had gone off the air, as possiblebombs. When the rest of the instruments had beenturned on, suspicion had lessened, but still. . . .Theultra-low-temperature refrigerator or cryostat wasturned on, and the instrument engineers and scientistswaited to see the temperatures fall. . .or, as they had

Standing by with an operating spare spacecraft on the ground,this crew backed up spacecraft encounter operations.

on Mariner VI, not fall. They waited. Finally: itscooling!

The television cameras had been taking picturesimpartially, alternating their shutter-and-readout cyclesevery 42 seconds, for three days, whether the out-put was recorded or transmitted or wasted. Now atlast the majestic curve of the edge of Mars appeared onthe monitors, and craters and light and dark featuresand the south pole were visible even in the low resolu-tion of the engineering video.

As it turned out, from there on it was clean sail-ing. All five of Mariner VIIs instruments operatedsmoothly, collecting, by reason of the c h q e in pro-gramming, more information than planned. The secondoccultation passage was as good as that of Mariner VI,or perhaps better in that submergence came in theregion of Hellespontus, an interesting area near theearlier instrument swath.

Picture playback, the next night, was almost anti-climactic-but not quite, because of the quality andinterest of the pictures. The confirming low-rate play-backs of all the scientiilc data, completed for both Mari-ner VI and Mariner VI1 in the weeks after MarinerVIIs encounter, showed mainly that the high-ratetelemetry was as accurate as it was fast-an importantassurance for future missions. The low-rate playback,by the way, was only about 32 times as fast as MarinerIVs playback in 1965. After the dual encounter of1969, the venerable Mariner IV seemed to have takenon the antiquity as well as the stature of a pioneer andtrailblazer.


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An observer studying Mars through an unmannedfly-by spacecraft mission is, in the words of one dedi-cated and well-seasoned planetary scientist, a little likea veterinarian who is watching an unfamiliar species ofelephant, so far away that he can barely see it with high-powered binoculars, trying to find out how the animalfeels by observing the wrinkling of its skin.

Though it was difficult, as implied, the MarinerMars 1969 scientific effort was anything but fruitless.Compared with previous efforts to observe and under-stand the ruddy planet, this project was expected toyield as much new information as mankind had previ-ously possessed about Mars. The Mariner experiment-ers, in turn, revealed that they had gathered more data,

Facing: The great Amazonis-Arcadia-Tempe desert region ofMars viewed by Mariner V l l at a distance of 282,000 miles onAugust 4, 1969. Le ft: Closeup of the south polar cap. The Sun,slanting in from rhe upper left, has melted the snow r o m t henorthern slopes. The largest crater seen is about 49 miles across.


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by and large, than they had expected, both throughexercising various options to increase the scientificvalue of the mission and because of unexpected phe-nomena provided by nature.

Mars had first been described as comparable withEarth; then, after Mariner IV, it was compared with theMoon. The data of 1969 show it to resemble neitherEarth nor Moon, but to have its own distinct and di-verse character. It is a cold, dry,desert world of manyterrain types, clothed in a thin atmosphere of carbondioxide which condenses into polar frost and snow andinto thin, high-altitude dry-ice clouds under appropri-ate conditions. Appearing implacably hostile to com-mon earthly forms of life, Mars may yet be found tosupport its own rugged, unearthly organisms.

Phobos, the inner and larger of Mars two tinysatellites, was spotted in several of the Mariner televi-sion pictures. It appears to be larger than previouslyestimated, and distinctly ellipsoid in shape; its positionis not exactly as predicted.

The atmosphere of Mars was studied physically,chemically, and visually by the Mariner 1969 experi-ments. The S-band occultation experiment producedprofiles of refractivity of the atmosphere at four loca-tions, from which pressure, temperature, and otherproperties were derived from high altitudes down tothe surface. One profile was taken near the north pole,one just north of the south polar cap, one near Merid-iani Sinus close to the equator, and one in the northerndesert near Nix Olympica.

The tropical site, which was examined in mid-afternoon by local time, showed the atmosphere appar-ently warmer than theory would predict, out to analtitude of about 25 miles. In the other sites, the tem-perature profiles were compatible with theory, and overthe north polar regions, it appeared that carbon dioxidecould condense into clouds at virtually all altitudes.Earth-based and far-encounter Mariner pictures ofMars, showing the northern polar area hooded in cloud,match well with this observation. The ionosphere layerwas detected at an altitude of about 85 miles, with apeak density of about 170,000 electrons per cubic cen-timeter on the day side, but no ionization was observedover the night side; these results are consistent withthose obtained from Mariner IV in 1965.Carbon dioxide and its dissociation products werefound by the Mariner spectrometers to dominate thecomposition of Mars atmosphere to an even greaterextent than was the case on Venus. Traces of watervapor and ice, and an assumed but undetected smallquantity of inert gases such as neon and argon, seem tocomplete the list of components of Mars thin, cold air.

Facing: Selected wide-angle closeups of Mars laid in place on aglobe. The Mariner V I pictures make tw o horizontal rows abov e,the Mariner V I1 data slant southward, and sweep over the southpolar cap. Note that these pictures are not fully processed, andshow some residual images of the edge o f the planet, but theprofu sion of craters is obviou s. Outlines show coverage of otheroverlapping frames.

WAVELENGTH, AOne of 1000 pairs of ultraviolet spectra taken by the two space-cra ft. Th ison e was obtained early in the Mariner VI pass, lookingtangentially at the u pper atmosphere. T he sharp spikes representultraviolet emission (glowin g) by carbon dioxide and carb onmonoxide in the atmosphere.

I C 0 2 GAS

C 0 2 GAS C 0 2 GAS C 0 2 GAS

CO GAS1.88-3.68 2.99-6.00

WAVELENGTH (microns)

One of 180 pairs of nfrared spectra taken by Mariner VU showscharacteristic dips caused by absorption of heat radiation bycarbon dioxide, the major constituent of Mars: atmosphere, andcarbon monoxide. Other spectra revealed finely divided dry ice,and traces of water ice.

A very high, extremely thin cloud of atomic hydrogensurrounds the outer atmosphere of Mars as it doesEarth and Venus. Atomic oxygen, carbon monoxide,and various ionized particles, presumably resulting from


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the breakdown of carbon dioxide by solar energy, weredetected in the upper atmosphere. The striking fea-tures of the composition of Martian air were the ab-sences. Nitrogen was not detected, nor were any of itsgaseous compounds (nitrogen is the majority compo-nent of Earths atmosphere, and a minority componenton Venus). Ozone, an unstable form of oxygen whichis formed in a layer high in Earths atmosphere andprotects the surface below from lethal ultraviolet radia-tion, was also absent; the ultraviolet rays appear topenetrate to the surface of Mars.

The clouds of Mars, though neither as impenetra-ble as those of Venus nor as dramatic as those of Earth,have a variety and character of their own. They are dryclouds composed of dry-ice crystals, ice crystals, anddust: it almost certainly never rains on Mars, though itfrosts a lot and may snow. The commonest clouds aresolid carbon dioxide: locally, frozen air. The northernpolar hood observed by the Mariners in the Martiansummer of 1969 was probably of this composition. Thecloud layer observed 10-1 5 miles above the bright limbcertainly was dry ice as were what seemed to be groundfogs just above the edge of the south polar cap. Theclassical W-cloud and other daily formations longobserved in particular locations may not be clouds atall, but ice frost or fog. Spectral indications tentativelyassociated with dust clouds appeared at lower altitudeon the limb, and chemical indications of ice crystals incloud or frost form were seen on or near the surface.The mysterious blue haze, which usually masks theplanet in blue-light photographs taken from Earth, butoccasionally clears up mysteriously, wasnt there. Blue-filter pictures from both spacecraft clearly showed sur-face features, suggesting the Martian haze theory to bean incomplete or incorrect explanation.

The temperature of Mars surface, measured byinfrared radiation in two instruments, registered a high

270

2 2502 230

210zC 190170150

0

vY

Q,

4 LIMBIM B SLEW 1 2 3Temperature trace transmitted by the infrared radiometer aboardMariner VI1 shows varying temperatures along the track of theTV pictures. Water freezes at 273K on Earth; dry ice melts atabout 150K on Mars.

I I I I I I

40

30

*ALTITUDE REFERRED TO ELLIPSOIDWITH a = 3394 A N D e = 0.0052 I

3,Y MARINER V I ENTRYe MARINER VI1 ENTRY5 20ac MARINER V I EXIT

MARINER VI1 EXIT10

0

-1 00 1 2 3 4 5 6 7PRESSURE (mb)

Atmospheric pressures measured at four different locations b yMariners occultation experiment, in which each spacecraft dis-appeared and then reappeared from behind Mars. Mariner VIentered over Meridiani Sinus, an area much photographed;Mariner VI1 entered over Hellespontus, north of the edge ofthe polar cap. For reference, the atmospheric pressure at sea-levelon Earth is about 1,000 millibars.

Facing: The Antarctic of Mars is shown in two versions of thesame set of pictures taken by Mariner V U . The upper group isprocessed to show true relative brightness, so that the snowybrilliance of the cap and the gloom of the polar twilight can beseen; in addition, a geographic grid ha s been added to indicatethe shape of he planet and the site of the pole. The lower grouphas been processed to highlight small crater detail, and the alter-nate narrow-angle pictures are added for more fine structure.

of about 60F in the equatorial regions, on a sultrysummer day, and a low of about -240F in the dry-icesnowdrifts near the south pole. As expected, the darkregions, which absorb more solar energy in the day-time, were warmer than the light areas, which reflectmore. The scientists had also expected, from the obser-vations of Mariner IVY o find Mars covered with irn-pact craters like the Moon, and they did.

Even among the polar snows, the circular featuresstood out, overlapping and interlocking. Near the rimof the polar cap, though, there was an eerie negativeeffect: the shaded slopes were bright, and the sunlitfaces were dark. Here the cap had melted (back intothe air) where the Sun struck, exposing the darker soil,while on the sheltered southern slopes the white dryfrost survived.

Many classical features such as canals and sharpborders of seas do not appear in the Mariner map unlessas ragged chance alignments of craters. But a new


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Th e approaching planet as it appeared to Mariner VI1 during the first 36 hours of fa r encounter.A t left, Syrtis Major and E lysium are seen at a distance of 900,000 miles; in the center, afte r 14%hours, the planet has rotated about 150 degrees and Amazonis, Arcadia, and Solis Lacus arevisible. At right , dfter 9% hours more, Syrtis Major is again visible; it is now only 525,000miles away.mystery appeared as Manner VIIs camera crossedinto Hellas, a circular southern desert that changesfrom pink to white with the seasons. The westernborder was sculptured and terraced with familiar craters,but in the depressed floor of the feature there werefirst a few, and then no craters at all, for hundreds ofmiles. The small high-resolution pictures showed thesame. The few features spotted along the edge of thefloor showed that it was not a cloud layer which blankedout Hellas. What it is that has completely erased thesecharacteristic marks of an ancient bombardment is yetto be discovered. It was as if the sea had flooded in-yetliquid water cant exist on Mars.

From Aurorae Sinus across the southern part ofMargaritifer Sinus, along the equator, another strangeland is visible; the experimenters call it chaotic terrain.In a vast network of strips and blotches, the surfaceappears collapsed and tumbled into a mess of shortridges and depressions that could be called the badlandsof Mars. Though bordered by conventional crateredterritory, these chaotic areas are not cratered them-selves, suggesting that they were formed, like the fea-tureless desert of

The absence of Earth-like features such as moun-tain chains and valleys in the Martian regions examinedby Mariner VI and Mariner VI1 tells us that the forceswhich have shaped our lands have not worked upon thecrust of Mars, at least in the time whose history is por-trayed in its visible features. In addition to the pro-cesses resulting in the featureless and chaotic areas,many of the craters show modification. Analysis of

ellas, after the age of cratering.

craters by size and apparent relative age suggests thatsome have been weathered completely away, and thatthey may have been formed or modified in two or moredistinct waves of activity. Like lunar craters, thetian features may be divided into larger flat-bottringwalls and smaller cup-shaped craters. Members ofboth groups show evidence of erosion or other modifi-cation, especially along the edge of the polar cap where

w enhances contrast.nsofar as the processes which shaped Earthss are associated with the kind of atmosphere wehave, it would seem that Mars never had such an at-

mosphere. There are two alternative theories of theorigin of the atmosphere, either that it was formed atthe same time as the planet, with the common lightgases hydrogen and helium leaking away in the ensuingmillions of years, or that it issued forth from the rocksafter the planet was formed, as is believed to have hap-pened on Earth. Either case would allow a majority ofcarbon dioxide, but both call for a significant minoritycomponent of nitrogen, which remains undetected onMars. What happened to the water is almost as mucha mystery for Mars as it is for Venus, although onit would be likely to remain permanently frozen belowthe surface. The escape or removal of such permafrostin particular areas could well account for the chaoticterrain. The survival of so many craters of great agemakes it clear, if these features are as old as they look,that a dense atmosphere and broad seas, with the rapidand drastic erosion they produce, never existed onMars.


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Mars continues to hurtle towards Mariner VZZ.Above left, 245,000 miles distant, showing thebright ring of Nix Olympica. Above right, 8% hours later and 100,000 miles closer, the darkregion called Mare Cimmerium. Below, viewed in near encounter at a distance of only 5000 miles,the cratered terrain around Meridiani Sinus stretches north (left) to the horizon.


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6N22


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Forever MarinerThe two spacecraft coasted on beyond Mars, their

solar system speed of 50,000 miles per hour relativelyundiminished. It is characteristic of Mariner spaceprobes that they do not stop once they have accom-plished their missions.

Calculation of their fight paths continued, bothrefining the dimensions of the two Mars passes andprojecting into the future the positions and motions ofthe craft. Engineering tests and studies focused in asimilar way on understanding past performance andfuture capability. And the desire to probe scientificperformance and possibly add some new informationprompted several scans of stars and a comet in visible,infrared, and ultraviolet radiation.

Mariner VI and Mariner VI1 would be directlyopposite the Earth in orbit (the position called superiorconjunction by astronomers) in late April and earlyMay 1970, having by that time passed the furthestpoint from the Sun (aphelion) and started back towardwarmer regions. In September and October 1971 they

would come as close as 66 to 65 million miles fromEarth. The nitrogen gas used by Mariner VI to main-tain its orientation was expected to last well into 1971,and Mariner VIIs gas was good nearly as long. For therest, they would operate indefinitely until some crucialpart wore out, or some rare incident of space closed themdown.

In the meantime, they could still be of service toscience, even as bottles tossed overboard by cruisingships on Earth, containing only a note of their startinglocation, can help to chart the currents of the ocean.

Possibly the greatest theoretical achievement ofearly twentieth-century physics, Einsteins GeneralTheory of Relativity had defied efforts to find a sim-ple and precise experimental test. It had predicted anumber of physical effects, most of which were ex-tremely difJicult to verify. One of these effects is simplygravitational, and was attacked by observing the pre-cession of the major axis of the planet Mercurys ellip-tical orbit. This test is complicated by the fact that the

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The fourth effect was analogous to the bending ofstarlight, but paradoxically opposite in degree-atleast apparently. It is called ranging delay. We tend toconsider light as a stream of particles and comparethese to big objects like spacecraft or meteorites, whosepaths are also bent when they come near something asbig as a planet or star. A spacecraft is usually speededup in this process. But in measuring ranging delay, weobserve an apparent slowing down of the round-tripradio signal. Actually there is no speeding up or slow-ing down, for the effect results from the distortion orstretching of the fabric of space by the intense gravita-tional field.

This space distortion proved to be measurable bythe delay method as a byproduct of the existing Mari-ner VI and Mariner VI1 systems. An essential compo-nent of their navigation equipment was a loop of radiosignal stretching from the Earth-based transmitter-receiver to the spacecraft and back. Measuring thechange in the tightly controlled frequency could tell,according to the Doppler effect, the relative velocity ofthe spacecraft; measuring the travel time of the signal, atthe speed of light, could tell the distance. The use ofthese measurements to determine the spacecrafts flightpath (and recommend corrections) is called navigation.Their use to measure gravitational accelerations im-posed on the spacecraft by various members of theSolar System, and thus to measure the position, motion,and mass of such a body as Mars, is called celestial me-chanics, which was one Of the Mariner experiments. Theuse of the ranging measurement to determine the effectof solar gravitation on the two-way radio signal itselfbecame the relativity experiment.

The M ars Stat ion of the Deep Space Network, with 210-ft re-flector, high-power transmitter, and quick-change tri-cone feed,tracks Mariner VI and Mariner VI1 through superior conjunc-tion, beyond the Sun, at ranges up to 240 million miles.

Suns precise oblateness or shape is unknown, and partor even all of the effect measured could be explainedby non-relativity physics.

A second effect, measured through the red shiftin the spectrum of a double star, had to do with timeor frequency. This effect was measured in 1960, butagain it was found that the underlying theory could beseparated from relativity.

The third effect was the bending of light by agravitational field. Unfortunately the effect is verysmall, and the gravitational field must be very large.The bending of starlight by the Sun was observed dur-ing total solar eclipses, but the uncertainty in the mea-surements remained fairly high-about 20%-whichcould c o n h a relativistic effect but not measure itaccurately. And by this time there was at least onemodified theoretical prediction competing with theoriginal.

The same experiment had been attempted usingthe planets Venus and Mercury as passive partners forthe two-way loop, but they are not very good radarreflectors for this purpose, and did not provide a satis-factory return. An active partner was needed.

General relativity predicted a delay of a few thou-sandths of a second in the two-way signal return time ofup to about 40 minutes (the one-way signal time fromMars at encounter had been only about three minutes),which corresponded to a difference in measured rangeof as much as 35 miles out of a total Earth-to-space-craft distance of 250 million miles. This experiment re-quires an extremely precise ranging system, and a verysensitive one, since the uncertainty of range measure-ment must be many times smaller than the maximumdifference, with received signal levels much weakerthan those used before for this purpose. A number ofchanges were made in the Earth-based systems, whichwere already the most sensitive of their kind in the


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world, as the year-long experiment began, and the databegan to roll in.

Thus the Mariner VI and Mariner VI1 spacecraftand the Earth-based components proved once more theadaptability which had brought to a successful conclu-sion a very complex planetary mission by converting inflight to a new scientific effort. In this they joined theMariner V spacecraft, converted on the ground from aspare for the fkst Mars mission to the Venus machineof 1967, and the conceptual Vega spacecraft of a dec-ade before, which was to be readily adaptable for lunaror planetary flights.

Even as the two craft left the Mars mission be-hind and joined earlier Mariners, the Pioneer interplan-etary observers, and a host of other spacecraft as man-made planetoids in permanent solar orbit, Earth was

Mariner Mars 1971 orbiting spacecraft design.

busy making more. Two spacecraft for the MarinerMars 1971 orbiting mission were in the shops, whileparticipating scientists pored over the data from 1969.A 1973 Mariner flight to Venus and Mercury was beingplanned, with one eye on the devices and techniquesused successfully in 1969. And later, larger Mars andinterplanetary flights were being designed and con-ceived, building from the new foundation of the dual

Though it was probably the last mission of itskind, Mariner Mars 1969 turned a corner and made anew beginning. Like the rest of the interplanetary shipswe will never see Mariner VI and Mariner VI1 again,but their cargo of scientific information has been inport since August 1969, and the results have been ap-pearing in the marketplaces of ideas ever since.

Mars fly-by.

Mariner Venus/M ercury 1973 spacecraft design.


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Mariner Mars 1969 Project (Final Report), Technical Report 32-1460, Jet Propul-sion Laboratory, 3 vol., in press.

The Surface of Mars, R. B. Leighton, Scientific Am erican, Vol. 222 No. 5, pp27-41,May 1970.Mariner Mars 1 969 , A Preliminary Report, NASA SP-225, National Aeronautics

and Space Administration, November 3, 1969.The 1969 Mariner Mission to Mars, C . E. Kohlhase, H. W. Norris, J. A. Stall-

kamp, and H. M. Schurmeier. Aeronautics and Astronautics, July 1969, pp.The Book of Mars, Samuel Glasstone, NASA SP-179, National Aeronautics andHandbook of the Physical Properties of the Planet Mars, C . M. Michaux, NASAReturn to Venus (Mariner Venus 1967), J. H. Wilson, Technical MemorandumReport From Mars, Mariner ZV, 1964-65, NASA EP-39, National Aeronautics

80-96.Space Administration, 1968.SP-3030, National Aeronautics and Space Administration, 1966.33-393, Jet Propulsion Laboratory, 1968.and Space Administration, 1966.

Documents

Two Over Mars - Mariner 6 and Mariner 7, February - August 1969