America in Space - The First Decade. Exploring the Moon and Planets

8/8/2019 America in Space - The First Decade. Exploring the Moon and Planets

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Ten years ago when the National Aerona utics an d Space presents the findings ma de in the first ten years of theAdministration was brought into being to explore space, existence of NASA It is one of the avenues selected byit began immediately to give thought to exploring the the Agency to disseminate the knowledge that it hasMoo n and the planets. F rom its beginning NAS A has gained in its program in keeping with its responsibilities.been interested in the scientific studies of these bodies.Since 1961 it has had the add ed responsibility of John E. Nauglesecuring information concerning the M oon th at would Associate Administrator forbe needed for a m anned expedition to the Moon. In Space Science and Applicationsthe ten years of NASA's existence our knowledge of theMoon has increased immeasurably; we know the f ar sideas well as the front. We have seen a portio n of the surfa ceof Mars from nearby. We have made measurem entsclose to Venus. "Exploring the Moon and Planets"


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Exploring theMoon andPlanetsAnEarth Dweller'sviewThe positions of the planets at your birth fix yourdestiny--so said the ancient astrologers. Many peoplestill believe this to be true. Others know for certainthat sleeping in the moonlight courts lunacy.Not content with superstitious speculations,scientists have watched and measured the motions ofthe Moon and planets for thousands of years. First,they used the naked eye; then, the telescope; and,more recently, spectroscopes and other mcderninstruments. Only a decade or so ago, astronomerswere well satisfied with the apparent order theyhad found in the solar system. To be sure, the Earth'sMoon was indecently large to be a satellite, and theaxis of Uranus pointed the wrong way. But, by andlarge, only a few problems remained to be ironed outbefore the origin, evolution, and future of the Sun'slittle group of planets could be described withassurance.Space probes and radar astronomy have changed allthat. We know now that Mercury rotates withunconscionable rapidity; while its cloud-shroudedneighbor, Venus, turns hardly at all and the wrongway at that. Mars, which seemed an older version ofEarth to scientists of 1900,has been found pock-markedwith great craters. Frozen Jupiter has a magnetic field

v many th e s that of Earth and radiation belts to match.

In just a decade, our comfortable view of thesolar system has been shaken to its foundations.Solar system astronomy is in ferment.

The MoonThrough theTelescopeThe first thing man wanted to know about the Moonwas where it was going to be and when. Eclipses,especially, were events of great religious concern.Even primitive man was able to discover a great dealof order in the Moon's motions. Visualize the precisionof the huge circles of stone markers at Stonehenge.How many centuries did Stonehenge's builders watchthe Moon and mark its progress before they discoveredits lengthy eclipse oycle?Today, we are more interested in knowing wherethe Moon came from and what it's made of. Surprisingly,just by watching it through the telescope we can


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1 Some lunar local hot spots. Dots indicate hot spots;shaded areas, widespread thermal anomalies.

narrow down thc range of possible answers. First,the Moon is big as natura l satellites go-the Ea rthbeing only 80 times heavier. It is also qu ite c!ose-about a quarter million miles awiy. Astronomers,in fact, tend to think of the Earth-Moon corporationas a double-planet. The two spheres revolve like alop-sided dumbbell about rheir com m m center of mass,located some 3000 miles from the Earth's center towardthe Moon. No other solar system planet can claimsuch an out-sized moon.The Moon is so big and so close that its gravitationpulls our atmosphere, our oceans, and even the Earth'srocky mantle out of shape , although only slightlyin the latter instance. The energy absorbed bythe friction of these tides comes ultimately from themotion of the Earth and Moon. As a result, the Earth'sday increases about 33 seconds per century and theMoon is slowing down in its orbit. As the Moonslows down, it slowly recedes from the Earth. Hereis where we get some insight into the past historyof the Earth-Moon system. y r k i n g the tidalfriction theory backwards, scikntists such as GordonJ . F. MacDonald arrive at the conclusion that the Moonwould have receded to its present distance in 1.3billion years. However, the Earth's age is apparentlysome 4.5 billion years, inferring that thc Earth-Moonpartnership may not have been created when theEarth was born.

Well, then, whence the Moon? Perhaps the Moon wasonce an asteroid that wandered in from the beltbetween Mars and Jupiter and was somehowgravitationally snared by the Earth. (One can imaginethe ter-iestrial geologic up heavals accompany ing sucha union.) Or, as George H. Darwin, Charles' son,calculated around 18 80: The Earth might havespun off a glob of Moon stuff eons ago when it wasrotating much faster on its axis. Some believe thatthe Pacific Ocean basin is the scar left by this planetaryfission.Another intriguing obscrvation: knowing the Earth'smass and the Moon's distance, astronomers can inferthe Moon's mass and measure its diameter through thetelescope. The a verage density of the M oon is onlyaround 3.4, much less than the Earth's average of 5.5;yet it is close to the density of the lighter rock formingthe Earth's crust. The advantage of the lunar probeis now obv ious; it can analyze directly the compositionof the sur face of the Moon an d see if it resem bles thatof the crust of the Earth.'The most obvious feature of the Moon through thetelescope--even good binoculars-is its crateredsurface. Fo r the first half of this century, just abod teveryone believed that ltlnar craters were meteorcraters. Th e Moon was s u p p s e d to be a cold,dead world "where nothing ever happened," so, thecraters could not be of volcanic origin. Besides, theEarth, too, is pock-marked by huge craters. Photos


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from aircraft (and more lately satellites) have revealedmany well-weathered craters right here on Earth,particularly in Canada and the Carolinas. It could bethat bo th sets of crate rs were born during and afterthe Moon's bilth pangs or its cataclysmic capture.But is the Moon really dead? On the night of A pril 18,1787, the great G erman-English astronomer WilliamHergchel saw a red area near the crater Aristarchusglowing like "slowly burning charcoal thinly coveredwith ashes." Over the centuries, many Moonwatch ers have seen flashes of light and g lowing redspots. Some old craters even seem to have disappeared.Despite the imperious pronouncement that theMoon was dead, over 400 "transient lunar events"have been recorded over the past five centuries.Apparently, the Moon's pulse still beats-thoughperhaps weakly.Astronomers can also watch the Moon tl~roughour atmosphere's infrared window.* Duringthe lunar eclipse of March 13, 1960, for example,

ZRichard W. Shorthill and his associates noticed that. , the infrared temperatures of the floors of some ofthe lunar craters did not fall @'rapidly as the rest ofthe shadowed surface. More recently, infrared mapsof the Moon have revealed many hot spots. Theerratic visual red spots seen by Herschel and

Our rtmosphere blocks out most radraiion from space; the"window$" are open to visible, w m r infrared and most radiofreuuencles.

many others since may be lava flows or some othersurface manifesta tion of lunar volcanic activity.Nonthermal explanations also exist; areas of the Moonmight glow when stimulated by Sunligh t--or possiblythe solar wind.Th e telescope has obviously been a p owerful tool inlearning about the Moon. But there is a limit towhat we can do with Earth-bged tools. A well-equipped scientist-astronaut on the spot could bestfind out what is really transpiring on the Moon.First, though, there is an active lunar experiment wecan carry out from Earth.RadarAstronomyOf the MoonThe only kind of experipent we can perform on theMoon across a quarter million miles poses its questionselectrom ag~eticd ly. To d o this, we aim a radartransmitter at the Moon, send out short pulses of radiowaves, and wait'for the echoes to arrive some 2%seconds later. In radar p arlance, the echo gives usthe Moon's rad ar signature, which can be mostrevealing to the radar graphologist. For example,a smeared echo that is strong in the beginning andthen trails off in a few microseconds reveals that theradar signal is being reflected from a rough


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Summary of NASA's Lunar And Planetary Programs

PioneerRanger (phase 1)Ranger (phase 2)Lunar OrbiterSurveyorApolloMariner IIMariner IVMariner V

Number ofFlights55457-111

LaunchDates58-5951-6264-6566-676-68+626467

TargetMoonMoonMoonMoonMoonMoonVenusM a nVenus

FlybyHard landersHard landersOrb~tersSoft landersManned landersFlybyFlybyFlyby

In order of marksmanship and technical sophisticationrequired, the feasible space probe missions are:(1 ) a near miss or flyby: (2 ) a hard landing o runbraked impact; (3 ) a swing around the Moon,coming back toward Earth; (4 ) an orbit around theMoon; and (5) a soft landing. As mission difficultyincreases so does the scientific potential of the p robe.Both the United S tates and Russia were interestedin the Moon early in their space programs. TheU.S. Pioneer space probes flew toward the Moon; theywere instrumented to explore space between theEarth and Moon (called cislunar space ). Theonly Pioneer instrument that m ight have providedinsight abou t the Moon itself was a magnetometer. ThePioneer that came closest to the M oon was Pioneer IV(launched March 3, 1959), which passed 37,300 milesfrom the Moon on its way into orbit about theSun. Those tarly Pioneers contributed greatly to ourunderstanding of the Earth? Van Allen Regionand the solar wind.Russia's Luna 1 was launched on January 2, 19 59and passed within 3728 miles of the Moon. Lu na 2

on Lu na 2 indicated that the M oon possesses a veryweak magnetic field. Lun a 3 did even better;launched on O ctober 4, 1959, it was temporarilycaptu red by the Moon's grav itation al field onOctober 6, swung around the Moon, and headed backtoward Earth, where it went into orbit. Whileswinging around the Moon, Luna 3 took 4 0 minutesof pictures on 35-mm film. When the film wasautomatically developed and its images relayed backto Earth, the world had the first photographs of theMoon's back side, although the pictures did nothave high resolution.The United S tates undertook its first lunar explorationin the Ranger Program . Originally, the first Rangerspacecraft , the Block I models, were to fly by the Moon ,making measurements of m icrometeoroids andradiation in the space near the Moon like the Pioneers.Toward the end of the program, Block I1 Rangerswere to drop packages of in struments onto the lu narsurface. Rangers I and 11, the two Block I spa cecra ft,were launched in the latter half of 1961 . Neither wasinjected successfully into a Moon-bound trajectory.

scored a direct hit a few m onths later. A magnetometer


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3 Photograph of a Block I Ranger.4 Ptcture taken by Range r IX frcrn 25 8 mrles, show;:rgthe c ra te r A l~honsus

,-

RUBIDIUM V4POfi - EHO c t ANTENNAMAGNETOMETER

GEIGER TUBE 6 ' c rSEUICONDL'CTOR DFTECTORS

ELECTROSTATIC ANALYZER ,ALPtiF TFLESCOPE,660 N . W 4

EL

' . SOLPQ 04NE.PITCH & ROLL JETS /SUN SENSOSDuring 1902, the t h r ~ e angers in Block I1 departedfrom Ea rth satisfactorily. But, because of laun chvehicle proble ms , Ran gers 111 and V missed the M oon(only by 4 50 miles in the case of R acge r V). Ranger IVlanded on the back side of the Moon , but its instrumentpackage was destroyed when a time r failed tofire a retrorocket.Block 111 was created when President K ennedydeclared that landing a man on the moon by 1970was a national goal. The mission of Rangers VTthrough IX was to obtain close-up pictures of theMoon's surface during approach to a hard land ing insupport of the man-on-the-Moon goal.On January 30, 1964,Ranger V I left the launch padfor the Moon. Everything looked great up to a fewseconds before impact on the Moon, but n o picturesarrived back at Earth. It was later discovered that thetelevision cam era had been accide ntally turned on whilethe probe was ascending through the Earth's atmos-phere. Electrical arcing had occurre d, damaging thecamera. The remainder of the Ranger program wasoutstandingly successful. Rangers VII, V III. and IX

returned over 17.000 excellent photographs of thelunar wrf ace in 1964 and 1965.N'ith visual resolution a thougand times those of thebest Earth-based telescopes. scientists hoped th at theRang er pictures might settle some of the lun arcontrovers ies. Details less than two feet across couldbe seen. But the photos coulu not tell what the Moo nwas made of. nor was obviou s volcanic activity seen.What science did gain was a tremendo us mass ofdetailed terrain inform ation. plus a few new p erplexities.In the Rang er p ictures, countless craters of a11 sizesdominate the lunar maria (dark areas) that look sosmooth through the telescope. The sn alle r the craterdiameter, the m ore there ar e. Most of the crziters smallerthan I00 0 feet in diameter have smoothiy rounded rims.giving them a w indblown appearan ce, which is quitecontrary to the impression given by the larger, muchsharper craters seen through the telescope.The smaller lunar craters are n onuniformly distributed.Many seem to be second ary craters occupying thebright "rays" we see around large craters through thetelescope. Presum ably, these swarm s of secondarycraters are formed by debris or ejecta thrown out fromthe primary crater when it was formed .The Ranger photos show.few rocks and little rubble.Numerous collapse features. such as crevasses andcrater slumpings, were photographed.Rang er IX obtained some excellent close-ups of theprominent crater Alphonsus (diameter about 5 0 miles)and its surroun dings. Many of the transient lunarevents noted by earlier observers had occurred aroundthe crater . Do the Rang er photos sustain the viewthat internal thermal activity still transpires in andaround it? The answ er is a fairly definite yes, althoughAlphonsus seems no different from m any oth er largelunar craters. It boasts a striking central peak standingincongruously in the center of the re l~ti vel ylat crater %+floor. Detailed Ranger photos show P is peak to be vtnearly featureless and twice as bright as its surrou nd- ...ings-almost as if it we re covered with snow . Snowis manifestly impossible on the cloudless, airless Mo on.


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13 Typical flight profde for a Lunar Orbiter. 7 heflight to the Moon takes about 90 hours

The fact that fresh craters exist in the company ofold craters infers the ex iste xe of some sort ofweathering process on the Mo on. Since the Mo on hasno weather as we know it, perhaps a stead y rain ofmicrometeo roids knocks the fresh edges off c ratero intime. Only the fresh craters display angu lar, block-likerubble; will astronauts find similar blocks buried aroundold crater sites? If so , how did they get buried ? Andby what? Quite obviously, our closer look at the M oonraises questions we never thought of while lookingthrough telescopes.Some of the M con's hot spo ts have been identifiedwith f resh craters. I t seems as if the impac t of ameteoroid, or possibly a subsurface exp losion, blastedoff a layer of thermal insulation while making the crate r.Fresh craters appear hotter than the surroundingsbecause we see exposed the uninsulated, hot rocksurface of the Moon. If this view is correct, we canimagine that today's hot spo ts will slowly be coveredby dust- -or whatever comprises lunar prccipitation-and fade from our infrared pictures. After somenlillions of years. fresh craters may take on th at worn ,windblown look of the mare cratersThe subject of lunar crater s is not as far removedfrom Earthly concerns as .r . : might like to think. Thereare enough fresh craters on thc Moon to tell us that' large meteoroids ate still colliding with the Moon(assuming they a re meteoroid craters of cou rse) . Ifl a r g meteoroids collide with the M oon, they can also

, hit the Earth. Is Meteor Crater, in Ar no na , a fresh:: terrestrial crater correspond ing in time to thcse fresh% craters we see on the Moon? Perhaps !he Ear th andMoon survived the sam e celestial cannonades and the(i Moon can give us hints about the Earth's past andfuture.

An intriguing fcnture frcln the O rbiter photographs:several of the pictures provide strong ind ications thatmater ial has flowed down the sides of some crate rs.There is clear evidence of what terrestrial geologists call"mass wastingw-that is, dirt, rock, or somethingflowing down hill. On Earth, both lava and water-

saturated soil flow down hill. We might expect lavaon the Moon-but water? Unlikely as it may seem,some scientists feel that ice may still survive under theinsulating dust that seems to covcr most of the M oon.Lun ar mass wasting might occur as dust-covered icemoves glacier-like down slopes.ThePlanets

Beyond the Moon, eight planets and numberlezsplanetoids and asteroids ply their elliptical pathsaround the Sun. The plmets, in particular, attractus. Through the telescopic eyepiece, they seem tofloat tantalizing ly in space-so near yet so far.Though we know now that these distant worlds arein5ospitable to o ur form of life, they are our closestneighbors in a un iverse that otherwise seems veryempty. Perhaps they harbo r life forms adapted totheir extreme environments; or we may find remnantsor prec ursor s of life.Mars has always been a favorite target of theastronomer's telescopes. Rich surface detail swimsfrustratingly in and out of the observer's ken. Someastronomers see a Martian surface covered with agr ~d wo rk f artificial-looking lines; others see no thingor a few smudges at best. Most agree, however, thatd peculiar wave of darkening sweeps toward theequato r following the shrinking of the po lar caps in theMartian springs. Clouds are also observed, indicatingthe presence of a n atmosphere. In fact, Mars lookedso obviously inh abitable to early qbservers, especiallyBoston's Percival Lowell, that they scarcely doubtedthat it was inhabited. Modem telescopic studiesreveal a much less inviting planet-a frigid sphe roidwhere temperatures at the equator barely get abovefreezing during the hottest summers, and a thinatmosphere, n~ostly arbon dioxide, only one-fortieththe pressure at the Earth's surface.Mars is a little too far aw ay for today's radar to be


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of much use in probing its surface. It is the planetVenus that yields to radar interrogation. This isfortunate because Venus is so shrouded by clouds soheavy that astronomers never catch even fleetingglimpses of anything below. By choosing the properfrequency, radio pulses can be reflected d irectly fromVenus' solid surfac e. Venus also emits rad io wavesthat help us diagnose its atm ospheric structure andtemperature; however, these radiations are subject towildly varying interpretations.Beneath all the clouds , Venus is abou t the size of theEar th, but since we cannot see the surfa ce, we canno tguess its topography nor even ded uce from visual dataalone how fast it rotstes on its axis. The Irishplanetologist, Patrick Moore, once counted all theguesses of the rotationa l period of Venus made from1666 to 196 1 and came up with 85, ranging from ab out22 hours to 225 days.Ra dar offers a way to determine the rate of ro tationof a pla net even if the. surface cannot be seen . If apulsed radar signal of a given frequen cy is transmittedtoward a planet, the echo will be spread ou t both intime and in frequency. The first part of the echo wasreflected back from the nearest part of the planet andthe tail of th e echo from more distan t parts of the planet.The spread in frequency is a D oppler effect producedby the relative motions of the plane t and the Earth,including the rotation of the p lanet. I f, for a givenportion of the tail of the echo, the spread in frequencyof the echo is plotted for various positions of the plan etin its orbit, both the direc tion and rate of rotationof the planet can be calculated.

%

The first radar co ntacts with Venus were made in 1961.These echoes were too weak to de k~ m in ehe planet'srotation. More powerful radar systems were used duringsubsequent years By watching the shape of the echosignal, radar astrono mers decided that Venus rotateson its axis only once every 240 to 25 0 days an d that itrotstes the wrong way; that is, its rotation is retrograde,opposite to the rotations of all the other planets. Th eexact rate of rotation has k e n determ ined by anotheruse of radar. Today's radar is good enough to distinguishrough spots on the planet's d isk. It is assumed thatthese rough spots are true surface features, such asmountains, and not atmospheric phenomen a. Bywatzhing the movement of these feature s over longperiods of tim e, the rate of ro tation of Venus is knownto within an ho ur o r so.* A micron Is o w mllllonth ot r m t e r .

The temperature of Venus' unseen surface is also asubject of controversy . The tempera ture of a planetcan be deduced from the electromagnetic radiation itemits. In the case of Mars, surface temperature iseasy to measure because we can !w k directly at the.da ce with an infrared detector. The temperatureof th e invisible surface of Venus ca n be i.derred intwo ways: (1) by direct measurement of surfaceradiatio n in the microwave region of th: spec trumwhich may pass throueh the cloud -over unalter~d;and (2) by measuring the temperature of the top ofthe cloud layer and making assum ptions abouthow temperatules vary below. The first supposedtemperature m easuremen ts of Venus' surface were madein 1924 in the 8-13 micron* infrared band whichrepresents a spectral window in the Earth's atmosphere .The result was a chilly -28 F. But the clouds of;'?nus are apparen tly not transparent in the infrared,and the -28 F turned out to be only the temperatureof the atmosphere far above the surface. Whentemperature m easureme nts were made in the microwaveregion in 1956, a startling ? '0Fwas measured.More microwave mea surements confirmed a hoti.atherthan cold Venus. Visions of a life-sustaining plane tvanished as scientisrs pictured a baked surface,possibly spotted with lakes of liqcid m etal.Venus did not stay hot for long. In thick atmospherecontinuously stirred and activated by the Sun,nonthermal radiation is quite possible in the microcvavcportion of the radio spectrum; glow discharges andsimilar electrical activity might radiate microwavesthat would make Venus appear hotter than it is. Someexperimenters rneosuring Venus' radio emissions in1966 estimated that 30% of the radio energy theyreceived was probably nonthermal. This viewpointbrought the surface temperatures or Venus down sofar that seas of ordinary water and even polar icecould exist. T oday , however, most observers ofVenus fe d that nonthermal microwave radiationis negligible. Th e temperature q uestion is still notsettled.FlyingByVenurWhy fly by Venus instead of going straight in throughits atmosphere for a hard or soft landing? Surveyor-type soft landings have been out of the question untilrecently because operational booster rockets were bd


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Documents

America in Space - The First Decade. Exploring the Moon and Planets