Astronomy in Space

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For Salc by thc Supcrintcndcntof Documents,U.S. Govcrnmcnt Printing office, Washington, D.C. 20402Price 45 centsLibray of Canpss Catalog Card N u m b 66-61933


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FOREWORDThe publication of this book represents an effort

to provide information on present and prospectiveresults of placing astronomical instruments abovethe Earths atmosphere. Interest in such informa-tion has been stimulated by the progress of the spaceastronomy programs of the National Aeronauticsand Space Administration and by the comprehensiveset of recommendations on space astronomy madea t the 1965 Summer Study of the Space ScienceBoard, National Academy of Sciences, a t WoodsHole, Massachusetts, and published in January 1966in Part II .of the report of the Study, ((Space Re-search-Directions for the Future.

The first three of the four papers herein werepresented a t the 121st Meeting of the AmericanAstronomical Society a t Hampton, Virginia, oniMarch 30, 1966. The first paper, by Homer E.Newell, provides a view in perspective. The secondpaper, by Henry J. Smith, covers solar astronomy.The third, by Nancy G. Roman, deds with stellarand galactic astronomy. The fourth paper, byGeorge E. Mueller, was presented a t a meeting ofthe astronomers of the University of California onApril 29, 1966, and deals with the results of themanned space flight program and the opportunitiesprovided by the developing manned flight capability.

Robert C. Seamans, Jr.Deplrty AdministratorNational Aeronautics an d Space Adm inistration

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'.CONTENTS

SPACE ASTRONOMY PROGRAM OF THE NATIONALAERONAUTICS AND SPACE ADMINISTRATION.. ......SOLAR ASTRONOMY.. ....................................Homer E. Newel l

Henry J. SmithNancy G.RomanGeorge E. MueIIer

STELLAR AND GALACTIC ASTRONOMY..................EXPANDING VISTAS IN ASTRONOMY.. .................

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..H67 18731r m9 SPACE ASTRONOMY PROGRAM OF THENATIONAL AERONAUTICS AND SPACEADMINISTRATION cHOMER . NEWELL

Associate Administrator for Space Science and Ap lications1 N A S A iJ /i 4Scope of he Space Program

The expenditcures or the United States Space Program in the 1966fiscal year totaled $5.9 billion. The effort involved over 400 000 peoplein Government, industry, and universities. More than 20 000 in-dustrial concerns took part. Between 20 and 25 percent of thoseengaged in the program were scientists and engineers-only 5.5 percentof the national total.

Of the $5.9 billion, about $719 million was allocated for the SpaceScience and Applications Program. Over 60 000 people were engagedin this part of the NASA effort, about one-fourth of them being scien-tists and engineers. Better than half of the space science effort inFY 1966 was more or less directly related to astronomy.

The space program began with the momentum derived from theInternational Geophysical Year and the impetus imparted by thelaunching of the first Sputnik in the fall of 1957. In those formativedays of our space effort, we committed ourselves to a series of whatmight be called first-generation projects. Included among thesefirst-generation commitments are such well-known projects as Mercury,Explorers, Ranger, Surveyor, Mariner, Tiros, Echo, Relay, Syncom,and many others. Most of these first-generation projects have alreadyborne fruit, and indeed some of them have been completed and re-placed by later more advanced efforts.

A surprisingly large number of these early projects lay in the fieldof astronomy. A small, but appreciable, number of sounding rocketshave been fired each year with astronomical payloads. Explorer XIinitiated satellite gamma ray astronomy in 1961. The Orbiting SolarObservatory, the Orbiting Astronomical Observatory, Ranger,Mariner, and Surveyor fall clearly under the banner of astronomy.

In addition, numerous other projects have had an important bearingon astronomical interests. The Explorers, Geophysical Observatories,1


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2 A STR ON OMY I N SPACE .,. 'nd Pioneers investigating the magnetosphere and interplanetarymedium have extended geophysics and physics into what was once the

undisputed domain of astronomy. Indeed, one of the more excitingoutcomes of space research during the past 10years has been a drawingtogether of the disciplines of geoscience, physics, and astronomy intoa close partnership that is mutually stimulating and beneficial. In-deed, this spectrum of activities, including the Ranger flights to theMoon and the Mariner flights to Venus and Mars, has served togenerate a renewed interest in solar system astronomy, which is all tothe good.

Once the first-generation projects were well underway, there camethe opportunity in good time to consider a second-generation group ofprojects. Some of these have already borne fruit, while the rest arewell underway. Included among these projects are Gemini, Apollo,Biosatellite, and the establishment of operational communicationsand weather satellite systems. These second-generation projectsalso include efforts of primary interest to the astronomy discipline.A Radio Astronomy Explorer is under development for launching inthe years immediately ahead and Lunar Orbiter spacecraft, are de-signed for photographic reconnaissance of the Moon.

And now a third generation of project and program possibilities de-mand attention in our thinking and planning. With the tremendousspace capability that is developing, truly tremendous accomplishmentscan be achieved. The range of possibilities is wide and bewildering invariety and appeal. One may now consider large and complex orbit-ing observatories, orbiting manned stations, large-scale telescopesand antenna arrays in orbit, astronomical facilities on the M oon, nn-manned missions, like Voyager, to various bodies and regions of thesolar system, extended lunar exploration, solar probes, probes out oft'he interplanetary medium into the interstellar regions, advancedinttnned missions to other planets of the solar system, and advancedapplications to practical uses in navigation, data collection and dis-semination, air traffic surveillance and control, long-range weatherforecasting, and so on.

These third-generation possibilities are characterized by complexityand very high cost. For many, the costs must be measured in thebillions of dollars. It is not reasonable to think in terms of doingeverything that one can in principle accomplish in the space arena.Choices must be made, and they will not be easy, from among thevarious opportunities and possibilities in the space field itself. But ,also, choices must be made between space possibilities and othernational interests and needs.

Because these choices can have an important bearing on the develop-ment and growth of science, it is very important for the scientific corn-


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I .. S P A C E ASTRONOMY P R O G R A M O F N A S A 3I munity to give thoughtful consideration to these matters. Because

' ' many of these opportunities lie in the domain of astronomy, the, astronomical community should give careful consideration to therelative roles of space and ground facilities in the development ofthe astronomical discipline.The Past and Current Contributions of the Space Program to Astronomy

Since the past is inevitably prologue to the future, it may be ofvalue to sketch quickly some of the contributions already made bythe space program to the field of astronomy.

When Galileo lifted his telescope to look at the Moon, he was ableto resolve details some 30 times smaller than had been previouslyobserved. With better telescopes, it became possible to resolve stillfiner detail until today we can photograph lunar objects smaller than1 kilometer in diameter and visually observe objects as small as 0.2kilometer. However, we have reached the limit of resolution possiblefrom the surface of the Earth. In fact, a photographic Earth-orbiting telescope of the type of the Orbiting Astronomical Observa-tory could not resolve much more detail. This resolution limitationwas bypassed by the Ranger spacecraft and its program of lunarphotography. While it is true that the t o t a l area of the Moon ob-served in the Ranger program has been very limited, we have observedfeatures as small as 1 meter in diameter. Other regions of the Moonwill be photographed in similar detail as part of the Lunar Orbiterprogram. The Luna 9 spacecraft has transmitted pictures of thelunar surface showing structural details which are below the limitsof resolution of the most detailed Ranger photographs.

Before the advent of the space program, astronomers believed tha tknowledge of the other side of the Moon would be unattainable.Today, as a result of Russian lunar spacecraft, especially Zond 3, weknow that the general appearance of the back side of the Moon isquite different from that of the front side. This new knowledge ofthe Moon has, as is quite often the case in the discovery of newscientific data, raised more questions than it has answered. Studyof the Moon, once the domain of astronomers alone, has now becomeimportant to many disciplines, especially those concerned with theEarth sciences.

This extension to other disciplines of a domain originally exclusiveto the astronomers is even more obvious when the planets are con-sidered. The payload of the Martian flyby spacecraft, Mariner IV,is a good example of the involvement of various disciplines in planetarystudies. I n addition to the well-publicized television pictures of theMartian surface, experiments were flown to detect properties of the

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* .4 A S T R O N O M Y IN S P A C Eplanet's atmosphere, and magnetic field.ments, all developed by nonastronomers, have added to our knowledgeof Mars and will make future astronomical observations moremeaningful.

The television pictures from Mariner, while showing only a smallportion of the planet, have given much information on the nature ofthe Martian surface. While no definite answers were obtained t osuch questions as the existence of life or the existence of the long-debated canals, we now know more about the nature of some majorsurface features. In the approximately 1 percent of the totd plan-etary surface that was observed, we have been able to detect detailsan order of magnitude smaller than ever observed before and twoorders of magnitude smaller than have ever been photographed fronithe Earth. N o tonly did i t produce one of the most exciting discoveries of the spacescience era, but it provided useful observational input to a number oflong-standing problems in astronomy. The use of moderately largeoptical collecting areas and carefully calibrated detectors has led t'oconsistent results for ultraviolet brightnesses of the brightest starsand to a revision of the stellar temperature scale for hot stars.

The developiiieiit of a stabilization system which allows rocketinstrumentation to point to an individual star for IL relittively longperiod of t h e has enabled Dondd Morton of Princetori to obtainphotographic spectrii of several hot stars, which show surprisinglythat material appears to be leaving these stars ut a sigiiificnlil r a t e .The use of relatively short anteniiae o ~ i ounding rocket's i d 1 1satelli tes enabled radio astronoiners t,o metisire the integrated cosmicbackground down t'o frequencies us low tis 0 .75 MHz and t o est.nblislithat the intensity decreases below 3 MHz.

S om e of the most, unexpected discoveries t~1idexciting resrilt,s h ispace astronomy litti-e come froni physicist,s \she hive been tit,triLctetlto the field. 11 1 1962 RictLrdo Giaccoiii tind Herbert Giirsky, lookill:.for X-ray scattering from t,he Moon, observed H bright source ofX-ray emissioii not associated \vitli either t>heMooii or tiny otliei.optically identifiable object. I t is now known ts Scorpio X. Siiiccthen these and other observers, using both spinning and stnbilizedrockets, have nieusured perhaps t\vo dozen ttdditionul sources. Untilrecently, the only X-rtty soiirce \vhich could be identified i v i t h t ipreviously kno\vn object \\t is the Crab Nebula. X-niy eniissionfroin t'his source htkd been expected. The brig1it)nessof the soiircei1i X-rtiys indicat,es t h t it, c,iiii be espltiinect by u11 ex~ensionof thesynchrotron spec.tnini into these short \vaveleiigtlis. Herbert Fried-man ttnd his colleagues tkt . the N I L V I L ~esearch Il thntt ory h v e

Results of these experi- ,

By 1960 rocket astronomy had begun to come into its own.


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.+ S P A C E IGTROX-OMY P R O G R A M O F S A S A 5identified two other radio stars, M87 and Cygnus A , as X-ray sources.Explaining the physical nature of these sources, identifying the X-raystars with optically observed sources, and investigating the possiblerelation, if any, of the X-ray sources to the newly discovered quasars,should present intriguing space-age problems for the Earthboundastronomer.

Other physicists attracted by these exciting discoveries havepushed the spectra of these objects further into the gamma-rayregion with observations from balloons. I n these wavelength regions,techniques are more closely related to the particle observing techniquesof cosmic-ray physicists than to the standard methods of opticalastronomy. Therefore, physicists trained in this field are contributingactively to astronomical research.

Sounding rockets have played an important role in the observationsof the Sun from space. The pioneering effort of space astronomy was,as mentioned previously, the mapping of the near ultraviolet solarspectrum, \vith unguided rockets carrying simple spectrographs andretrievable photographic film records. The need for longer exposuretimes than II spinning rocket could permit led to the early developmentof a ti\-o-axis rocket pointing control, with which an immense amountof fundamental exploratory solar observations have been made.The Suns far iiltraviolet and soft-X-ray spectra have been revealedand explored in progressively finer detail through a continuing seriesof milestone investigations. The new knowledge gained of the Sunsspectrum in the ultraviolet has advanced our understanding of thechromosphere and coronp, to a great degree. In addition these datahave provided a much improved basis for understanding the influenceof the Sun upon the Earths upper atmosphere.

Identifying the solar cause and defining the detailed physicalprocesses in the response of the ionosphere to solar flares was one ofthe outstanding episodes of early space astronomy. We now realizethat the Sun is also a continuous emitter of X-rays, with a verywide range in total X-ray brightness and spectral distribution through-out the solar cycle and also throughout the evolutionary life orindividual uc tive regions. Magnifhen t Lyman-alpha spec trohelio-grams have been made by Dr. Tousey and his colleagues a t the NavalResearch Laboratory.

Fascinating observations have been obtained of the Sun in theionized magnesium H and K lines with birefringent filters, of thechromosphere in neutral and ionized helium resonance lines withslitless spectrographs, and of the corona and active regions in softX-rays, both nith pinhole cameras and the novel grazing-incidenceX-ray telescope. The Orbiting Solar Observatories are being usedto patrol for the unpredickable, transient event, and to monitor

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+ .6 A S T R O N O M Y Ih' S PACEcontinuously the slowly varying phenomena. A more detailed ,description of the solar experiments and their results is given in D r.Smith's paper.

Concluding RemarksSpace science has progressed fastest in new directions for obvious

reasons. Newly opened fields of research naturally a ttrac t a greatdeal of attention by their potential for a quick return of significantdiscoveries. Moreover, for the initial exploration in virgin fields,relatively simple instriimentution and limited scope research proa ramsoften suffice to permit rapid exploitation of ne\\ technologies andbreakthroughs of understanding.

Rather more advanced technology and elaborate programs arerequired when one attempts to apply the technology of space researchto the traditional areas of astronomy, such as making high-resolutionspectrograms of faint sources in the ultraviolet or making high-resolution pictures of the Sun in ultraviolet and X-ray wavelengths.This results basically from the more advanced point of departure,because we must build on the accomplishments of decades of groundobservations in visible wavelengths. Thus, we are forced to aimhigher in our initial uses of space technology in classical investigations.The Orbiting Astronomical Observatory (OAO) and Orbiting SolarObservatory (OSO) already represent major investments in complexspacecraft developments, comparable to relatively large ground-based facilities of not many years ago. However, to match inspace observations the versatility and refinement of modern majorobservatory instruments, we recognize the need even now to planfor the fiiture more advanced facilities in space.

Early in our program, while building the first Orbiting Solar Observ-citory, we recognized the need for an adranced solar satellite, withroughly Len times the pointing and data capacity of OSO. TheAdvanced O S 0 demanded advancement of the state of technologyin some fundamental areas, like thermal stabilization tmd attitudecontrol. Parallel with that development we begttri ttii alternateapproach, in which an astronaut in u manned spwe flight missionwould replace part of the automated functions of AOSO and wouldin addition retrieve photographic film from specially constructedsolar telescopes and spectrographs. This device, the Apollo Tele-scope Mount (ATM), is a step toward the next, generation of spaceastronomical facilities. Similar developments will be lindertaken instellar and radio astronomy.

The future of space clstmiioniy is liniitecl only by the vision ofthe astronomers and their ability to convince their fellow Americamthat astronomy is a worthwhile endeavor and that space astronopiy


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.. S P A C E A STR ON OMY P R O G R A M OF N A S A 7' . is an important component of the whole. At the 1965 SummerStudy at Woods Hole, astronomers indicated that they would like

to have an optical telescope operating between the ultraviolet andthe infrared with an image quality comparable to that of a diffraction-limited 120-inch telescope and a total aperture between 120 and 250inches. They also indicated tha t they would like a radio array 20kilometers on a side, a 100-foot parabolic dish for observations inthe far infrared and submillimeter regions, and large sophisticatedinstrumentation for the gamma- and X-ray regions and for the studyof the Sun. NASA is investigating the technological, the adminis-trative, and the budgetary problems which the construction ofsuch instrumentation will entail.

There are also proposals to send probes to study the magnetizedplasma in the inner and outer reaches of the solar system, in theenvironments of planets and their satellites, and some day even inthe galactic arm field beyond the dominant influence of the Sun.Planetary and lunar orbiters and landers are clearly indispensabletools in the exploitation of the near-solar system.

As mentioned previously, the costs of these programs are substan-tial and choices must be made. The simultaneous accomplishmentsof this whole program would require more resources than we couldbring to bear at one time. It will be necessary to identify the prior-ities of space research effort in astronomy, considering such factorsas the promise of significant accomplishments and the availabilityboth of the scientist to do the job and of the technology to carry i tout. As the program develops and moves into larger and more ex-pensive units of effort, it will be necessary to seek an increasinglybroadened consensus from the scientific community. In the past wehave built o u r program around interested key scientists, with theconcurrence and endorsement of their peers. However, for anythingso large as a telescope comparable in size to o u r largest ground-basedinstruments or a lunar-based radio observatory, the cost to the Nationwould be so great that the enthusiastic support and willing partici-pation of an interested scientific community will be essential. Anenlarged participation is essential not merely in the design, develop-ment, and operation of the space facilities but also in the pursuit oftheoretical and laboratory research and of ground-based astronomicalobservations necessary to support the space program.

Space observations canprovide data that are unobtainable from the ground, that will help ussolve old problems; they will produce information in new fields. How-ever, we at NASA recognize that space astronomy cannot proceedefficiently without the strong cooperation of strengthened ground-based astronomy and theoretical astrophysics. We agree that a

Space astronomy is not a new science.


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.balanced prograni of astronomical research must be maintained, with ..a corresponding augmentation of pound-based observations andtheory necessary to support any enlargement of space astronomy.The next decades could well become the golden age of ast,ronomy.


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..- .r' SOLAR ASTRONOMY L ,

N67 18732-I- HENRY. S M n H %Deputy Director, Physics and AJtronomy Programs II N AS A ,'L 1' '

A study of the Sun plays an important role in o u r program of spaceastronomy. In the very beginning of space science, the Sun offeredthe brightest and most convenient souxe of ultraviolet and gammaradiation. Solar physics represented the scientific discipline in whichone could make the easiest application of the new technologies ofspace research. In addition, the Sun was obviously the driving in-fluence in many of the phenomena encompassed in the general area ofSun-Earth relationships, which then and now occupy an importantpart of geophysical studies conducted with rockets and satellites.

Space vehicles enabled astronomers to carry telescopes, spectro-graphs, and photometers above the absorbing layers of the Earth'satmosphere, opening up to them the ultraviolet, X-ray, and gammaray spectral regions, as well as the very low frequency radio region.It is significant th at the Sun departs most notably from thermal andquasi-thermal equilibrium radiation in these extreme short and longwavelength regions. Thus space techniques enable the scientists tolook with greater emphasis and less confusion of background radiationa t the nonthermal radiative processes which characterize solar activity.

Another significant feature of space research provides an importantrationale to the development of a future program of satellite mis-sions-the expected improvement of observational conditions by theelimination of atmospheric seeing and scattered light sky brightness.Angular resolution with telescopes on the surface of the Earth islimited by atmospheric turbulence t o about one-half arc-second underthe best conditions, regardless of the aperture of the telescope. Sim-ilarly, even a t the best mountain sites yet developed, the brightnessof the sky a few arc-seconds from the limb is still of the order of a few-millionths of the disk brightness of the Sun. The virtual absence ofscattered sky light has stimulated the development of specializedstellar coronagraphs, which can reject disk light by nearly nine ordersof magnitude-provided the contiguous background sky light is notany brighter.

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and the magnetic field associated with them require special vehicles,special measuring devices, and generally a different scientific dis-ciplinary point of departure than telescope and spectrograph opticalobservations of the Sun.

In discussing NASAs space flight program in solar astronomy weshall consider solar observations in three general areas: small vehi-cles; the Orbiting Solar Observatory (OSO) satellite series; and thefuture flight program, including the proposed Advanced OS0 (AOSO)

Small vehicles include scientific research balloons, aircraft, sound-ing rockets, and the small special-purpose Explorer satellites. Thescientific research unit effort in this area may range from a fewthousand dollars and a few man-months up to several man-years.Nevertheless, the distinctive features of these types of missions arelower cost of individual pieces of research, and the shorter proposedreview, administrative approval, and technical development timerequired. The short time scale between conceiving and conductinga piece of research makes small vehicles especially useful for a numberof operations: student training in space research, testing of new tech-nologies, and rapid exploitation of new breakthroughs in knowledge.

Balloons have played an important role in solar research for morethan a decade. The pioneering efforts a t Princeton UniversityObservatory which culminated in the flight of Stratoscope I pro-vided the authoritative observations of solar granulation and the finestructure of sunspots as observed in white light. NASA will par-ticipate in a follow-on program to develop this general area of solarobservations. NASA, in collaboration with the Fraunhofer Inst itutof Freiburg, Germany, will undertake the investigation of fine struc-ture and spectral line structure in the chromosphere in the light ofthe H-alpha line of hydrogen Balmer series, NASA has also par-ticipated in the High Altitude Observatory Project Coronascope,which launched a white light coronagraph on a balloon to observe theouter corona from the scattering-free region above the Earths tropo-pause. Numerous university groups have employed balloons as partof the NASA supporting research and technology program to observehard solar X-ray emission. Though no special effort is made tocultivate balloon techniques in pursuit of the study of the Sun, it isvery likely that use of this method of escaping the handicaps of solarobservation through t,he Earths ntmosphere will continue to findimportant application.

l and Apollo Telescope Mount (ATM).


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SOLAR ASTRONOMY 11.Aircraft have an important role in space astronomy, even though

they do not fly so high as balloons, and they are subjected to severekinematic disturbances. The X-15 aircraft, which flies a t altitudesin excess of 38 000 meters (theoretically has a peak altitude of about91 500 meters), has already been equipped with a stabilized platform(siderostat) which could be used for observations of the Sun. Thoughthe flight profile yields only a few minutes of observing time above thebulk of the atmosphere, it presents a real opportunity for continuousobservations of the Sun down to some wavelength shorter than theground-based ozone absorption limit a t 2910 angstroms. Of moredirect interest to the solar astronomer, however, is the Convair 990 jettransport aircraft, which has been especially equipped to supportscientific observations in the tropopause and tropospheric regions.This aircraft carried an expedition of astronomers to observe the totalsolar eclipse of May 30, 1965. Astronomers have already heard re-sults from the scientific investigations carried out by the 10 researchteams which participated in that expedition. In the future, theConvair 990 will be provided with a heliostat for infrared and high-resolution white light observations, and probably will be used to testairborne coronagraph techniques.

Sounding rockets have enabled pioneering research in space as-tronomy, with early missions providing high-resolution spectroscopicobservations of the ultraviolet and X-ray solar emission. I n theearly days randomly oriented spinning rockets were able to glimpsethe Sun and provide initial exploratory observations on which rocketsolar astronomy was founded. However, the requirement for ac-curately stabilized pointing to form an image of the Sun, as well ascontinuous pointing to increase exposure time, generated the require-ment for an att itude control system. Early in the era of space as-tronomy, a tw-o-axis telescope pointing device was developed for theAerobee rocket. By the maximum of the current solar cycle it isexpected that a three-axis pointing control system mill be available.At the present time the two-axis control system provides nominalangular resolution of about 1 arc minute. The system under develop-ment should improve this performance by roughly one order of magni-tude. Common to all sounding rocket experiments is the opportunityto recover the observations in the form of photographic film. This isim important asset, since photographic data retrieval is unsurpassedus a means of bulk data storage, particularly when a very wide band-width receiver is used; one can increase the bandwidth of a detectormerely by increasing the area of the film in the magazines. Now nosatellite vehicle is capable of matching the sounding rocket or proposedmanned space flight missions in this respect.

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..2 A S T R O N O M Y I N SPACE. .t will be helpful to cite some of the accomplishments of rocketastronomy in the area of studying the Sun. A continuing series of

rocket flights has provided a complete map of the Suns spectrum do\\ 11to ivavelengths shorter than 1 angstrom. Each mission has improvedthe angular and spectroscopic resolution of earlier flights and hasresulted in a continuously iinproved photometric basis. Solar X-rayswere first discovered through sounding rocket observations, and theywere shown to be the fundamental cause of the sudden ionosphericdisturbance (SID).

A sounding rocket flou n above the Earths atmosphere carried ticoronagraph with a scattered light rejection figure of roughly 8 ordersof magnitude. The sounding rockets ha1.e carried instnimentation toproduce spectroheliograms in resonance lines of hydrogen and neutraland ionized helium, in the H and K lines of ionized magnesium, andin soft X-ray wavelengths utilizing both pinhole cameras and grazingincidence compound reflecting telescopes. NASA plans, in futuremissions, to observe the Sun from sounding rockets with (I wriety ofadvanced spectrographs and telescopes. For example, t\\o differentgroups will fly high-resolu tion scanning spectrometers t o observe theLyman-alpha and the Lyman-beta line and contiguous regions. Athird university group is developing a wideband high-resolu tion spec-trograph, based iipon the principle of the Echelle system. A fourthgroup, t~lso iniversity, is developing specialized high-resolii tioii spec-trographs to observe the cen ter-to-limb variation of critical iiltrtivioletwavelengths, especitilly in the contiriiiiini bet\\ een 2000 tind 3000angstroms. Finally, novel wide-aperture X-ray detectors litive beenprepared t o extimine in detail the nont hermd (breiiisstriililrin~) tidia-tion sttribii ted t o certtiin very energetic solar flares. llie tinticipiitedavailnbility of IL 5- to 10-arc-second, three-axis stttbilieed poin t i wplatform for solar observations, coripled \vith the enhanced paylotidcapability of the Aerobee 350 vehicle, certainly \\ill increahe the levelof activity in rocket solar cistronomy.

The earliest unmanned scientific satellites launched by the UnitedStates were the Explorer satellites. The Esplorers continue to beused vigorously by space scientists. In November 1965 and a8 partof the International Quiet Sun Year program, NASA iind the N a v dResearch Laboratory jointly launched IL solw X-ray monitoringsatellite, Explorer X XX (Solrad VIII ). This satellite, the eighth in11 continuing series of solar monitoring satellites produced by thescientists at NRL, included eight X-ray sensors covering the rangefrom 0.1 A to 60 4. This mission was nominally hi support of aninternatiorial program to study the quiet Sun imd assoc-itited geo-magnetic and geophysicd phenomena. NASA arid NRL are plunniiiga second joint launching in this series to take advantage o f the peculitir.


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13. SOLAR ASTRONOMY. development of the limited data magnetic tape recording system andsimple attitude control system which have already been developed.At the maximum of the current solar cycle, NASA, in collaborationwith Harvard College Observatory, plans to launch an Explorersatellite carrying a very low frequency solar burst radio spectrograph.This satellite, called Pilgrim, will be designed to observe every fewseconds the Suns radio frequency spectrum in the range 0.25 to 16M H z . In support of the geophysical research investigations whichare the primary mission objectives of the geophysical observatories,most of the Orbiting Geophysical Observatory (OGO) satellites willcarry either a solar X-ray or a solar ultraviolet photometer. There-fore, important solar data will become available from these continuoussystematic surveys.

Besides the Earth-orbiting Explorer satellites, NASA is planning tolaunch the R4IT-developed Sunblazer, a lightweight low-cost inter-planetary probe. This probe is a 13.6-kilogram spacecraft which isto go into a heliocentric orbit with perihelion a t roughly 0.5 AU. Theinitial Sunblazer flights will carry a two-frequency (100 and 300 MHz)pulsed radio transmitter to study the electron density gradient andinhomogeneities in the solar corona by observing the relative phaseretardation a t two frequencies.Turning now to the current approved program mainstream, theOrbiting Solar Observatory (OSO) is the workhorse of our solarastronomy flight program. The OSO, shown in figure 1, is an Earth-orbital scientific satellite with the following specifications: the satelliteconsists of two basic sections-the wheel and the sail; the wheel sectionrotates u t 30 rpm, with the plane of the rotating wheel constrained towithin about 2 of the solar vector. Most of the time experiments inthe five available wheel compartments point at the sky and then atthe Earth and only sweep past the Sun at the rotational repetitionrate. The sail section of the O S 0 points an instrument package,which is approximately 0.2 meter square and 1.0 meter long and cancarry two, three, or four telescopes, continuously at the Sun duringthe sunlit portion of the orbit, with a precision of about 1 arc-minute.In addition, certain O S 0 missions also provide a capability of scanningout a raster image of the Sun with 40-line resolution.

The currently approved OS0 program calls for eight flights (beforeflying, O S 0 is given a letter designation; after, a Roman numeral desig-nation) , two of which were successfully flown, one in March 1962 andone in February 1965. The third OSO, launched in August 1965,failed to go into orbit because of a vehicle defect. Another set of thescientific experiments onboard this unsuccessful mission has beenrescheduled for flight on the fifth OSO. We are now consideringexperiments for flight on the eighth OSO, which will fly in 1969. We


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14 A STR ON OMY Ih- S P A C E , .

Figure 1 .-Sketch of Orbiting Solar Observatory spacecraft.

are maintaining the option to continue this series beyond the approvedeight, but will nittke that decision only when the evidence is clear thattisefiil scientific results will be obtained by additionti1 flights in thisprogram. The option, however, is particularly att ract ive in the lighto f proposed improvements in the OSO, wherein the pointing capabi1it.ywould be upgraded, heavier and larger experiments could be ttcconi-niodated, tind possibly t t full sunlit polw orbit could be substituted fo rthe low-altitude par tly owtilted orbit .

Ttible T lists the instriinierits ttssigned t o the tir5t five OSOh.Emphasis has been placed upon spectroptplis ttnd image-formingspec,trohelio~r.mphs,which tire the most powerful tools in the tistron-omers workshop. However, II litrge number of experiments aredevoted to monitoring the temporti1 vttristions of the whole-Sriribrightness in ultrtiviolet, X-rttys, ttnd gtimmn rays. In addition. IInumber of experiments, listed under ~liscelliLiieoiis n the tttble, ttrebi~sicallygeophysicd in cbficmwter-for ewtniple, they will nieiisitrceither the particle or proton environment the Earth, or solarneutrons-or ttstronomical in nttture, ILS, for exttrnple, the zodiacallight or the ultrttviolet sky mapping survey.


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S O L A R ASTRONOMY. 15TABLE.-OSO-I to 050-F Experiment Summary

[Parentheses indicate number of flights assigned]Spectrometers: W hit e Light Coronagraph (1)300-1300 A (2) Miscellaneous:60-1300 (1) Neutrons (1)1-400 (3) Earth Albedo (1 )

1-8 ii(2) Emissivity (3)Spectroheliographs: Meteoroid Detector (1)Lyman-alpha (2) Zodiacal Light (2)

2-20 R (3)Helium A 304 (1) Night Sky GlowLyman-alpha (2)Soft X-ray: A> 2 A (4 )A< 2 (2)Hard X-ray (10-100 keV)Low Energy 7-ray (100 MeV) (8)High Energy Tray (100 MeV) (5)

The experiments on a single mission, the seventh OSO, now desig-nated OSO-G, are listed as follows:(1 ) Harvard College Observatory-Spectrometer-Spectroheliometer(2) Naval Research Laboratory-Spectral, Burst and Mapping(3) Rutgers University-Study of the Zodiacal Light.(4) Los Alamos Scientific Laboratories-Solar X-ray Monitoring in( 5 ) University of Bologna-Solar X-ray Monitoring and Gamma(6) University College, London-Study of the He I and He I1(7) University of New Mexico-High Energy Neutron Flux in Space.The X-ray burst photometer-spectrograph package is a refinementof the detector system flown many times by NRL on sounding rockets,Explorer satellites, and earlier O S 0 missions. This experimentpackage has been assigned to OSO-F to obtain, during solar maximum,as much information as possible on the short-lived transient X-raybursts associated with the major flares, with particular emphasisupon their spectral distribution. The HCO 300- to 1300-A scanningspectrometer-spectroheliograph, on its third flight in the O W series,will yield fundamental information on temporal variation and spatialdistribution of principal ultraviolet radiations from the Suns chromo-sphere and corona. Mapping of the Suns disk in selected radiationsin the 300- to 1300-A wavelength range will unfold t.he significantevolutionary development of specific centers of solar activity. Note-

Ultraviolet Sky Spectropho-Photometers: tometer (1 )

(300-1300 A).Measurements of Solar X-rays.

the 1 6 4 0 A Region.Astronomy in the Energy Range 20-200 keV.

Resonance Radiation.


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I ,.6 A B T R O N O M Y I N S P A C Eworthy among the experiments in the wheel section of the OSO-Gmission is Rutgere advanced zodiacal light photometer and polarim-eter, which represent a great advancement over investigations whichmeasured essentially similar phenomena with less detail on OSO-I1and are planned for OSO-F. The University College, London, ex-periment with total Sun flux photometers for the resonance lines ofneutral and ionized helium will monitor the variations of the funda-mental solar input to the E and F regions of the Earths ionosphere.These data will provide the basis for interpreting secular variationsin the structure of the electron density profile. The University ofBolognas low energy gamma ray detector will study not only solarradiations in the 20- to 200-keV wavelength region, but also willdetect and map on the sky cosmic sources of energetic photons. TheUniversity of New Mexicos detector of solar neutrons will look forthese telltale indicators of nucleogenetic processes which are alleged tooccur in the outer layers of the Sun during energetic solar flares.The variety in this payload is typical of that which the O S 0 space-craft is capable of accommodating.

Significant results have already been obtained from the early OS0missions. The first mission, successfully launched in March 1962and designated OSO-I, provided many weeks of continuous observa-tion of the time variation of solar ultraviolet radiation (100 to 400angstroms) and soft X-rays (2 to 20 angstroms). These data providedthe first information on the changes in the intensities of various linesof successive stages of ionization of a given atom; for example, F e XI1through Fe XVI. These data proved especially important in theinterpretation of the slow changes in the density profile of the upperatmosphere, as well as the electron density profile of the ionosphereof the Earth. In addition, they provided valuable insight into thenature of coronal heating by the elevation of activity over chromo-spheric active regions. Besides the ultraviolet observations, OSO-Idata also showed how individual flares as well as whole regions ofsolar activity change the total X-ray flux of the Sun. Previouslyunsuspected short-term variations in X-rays created by solar flareswere detected. Events of only a fraction of a second in durationwere shown to be very frequent and energetically very important.In the middle ultraviolet spectrum, a photometer measured for thefirst time significant fluxes which could be attributed to localizedheating of the chromosphere, the resultant reradiation by Lyman-alpha, and the resonance line of hydrogen.Comparable results from the second Orbiting Solar Observatory,OSO-11, cannot be quoted a t t,he present time, since the data arestill being analyzed. However, it is known that OSO-I1 did yieldthe first spectroheliograms in neutral and ionized helium resonance


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17. S O L A R A S T R O N O M Y.. ines, and did produce some significant maps of the solar corona.

X-ray bursts from a large number of small flares were recorded, andinitial exploratory observations of the zodiacal light and airglow weremade. A projected map of bright sources of ultraviolet radiationfrom the whole sky yielded vast quantitites of data which are stillbeing analyzed by digital computers. OSO-I survived only a shortlifetime (2 months rather than the 6 months programed), and OSO-11,unfortunately, suffered from early failure of its major instruments;nevertheless, these two missions have already provided informationwhich contributed new knowledge about the Sun, and charted vitalnew roads to be pursued in future research undertakings.

Even before the launch of the first OS0 i t was clear to solar astrono-mers that a spacecraft of higher performance capability was requiredto achieve even the most limited goals of solar research by spacetechniques. Following a number of planning meetings by the leadingsolar astronomers in the United States, an advanced solar satellite\vas conceived, called an Advanced Orbiting Solar Observatory(AOSO). An artists conception is shown in figure 2. AOSO was topoint three or more large telescopes at the Sun with quite high pre-cision. These telescopes would be about 0.30 meter in diameter and2.54 meters long, in order to achieve the nominal angular resolutionindicated by the pointing accuracy. The spacecraft was to be de-signed to point at the Sun with a precision of & 5 arc-seconds, locatedwithin a 40 arc-minute square centered a t the Sun, as shown in figure 3.I n addition, AOSO was required to perform a raster scan of the entireSun over the 40 arc-minute square, or a fine scan raster over anytirbitrarily located 5 arc-minute square within the40 rc-minute square.Lo accommodate the vastly increased quantity of data, the AOSO datahandling system was intended to have 20 million bits per orbit capac-ity which is roughly ten times tha t of OSO. AOSO w as to fly in a fullsunlit orbit, as compared with the partly occulted low-inclinationorbits of the early O S 0 missions.

The AOSO program was planned for four flight missions during theperiod 1968 to 1971. Experiments had been selected for the firstt i t o missions (table 11), and significant progress had been made in thedevelopment of the telescopes and spectrographs which had beenselected. Because funds were not available to continue supportof the AOSO spacecraft and experiments, the project was can-celed in December 1965. This unfortunate situation will precludeobtaining high-resolution satellite observations of the Sun duringthe forthcoming solar maximum. Nevertheless, we are taking ad-vantage of this hiatus for critical reevaluation of our solar researchprogrnm goals a n a the technical specifications of the satellite whichwe will recommend to achieve those goals. An example of an alter-


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-.8 ASTRONOMY I N S P A C E

lr in ipal Instrumcntinvcstigntor (cxperi mcn t)Organization___________

1 lnrvnrd Collcgc I,. Goldbcrg Normal Inci-n i n g Spcc-Obscrvntory dencc Scnn-

1 trometcr1- ~- ~

~~~~~ ~

Figure !?.-Artists concept

__---Irirposcl

__-Study of chromo-

Rphcrc iind coron:Lstrricturc

of Advanced Orbiting AstronomicalObservatory.


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S O L A R ASTRONOMY 19.' . TABLE1.-AOSO Experiments-ContinuedInstrument

(experiment)Organization Purpose

Goddard SpaceFlight Center

White LightCoronagraph

UltravioletSpectrohelhgraph

Principalinvestigator

Study of coronalchanges and theircorrelation withsolar activity (spots,flares)-___

Mapping of chro-mcephere andcorona i n ultra-violet

J. C. Lindsay

Naval ResearchLaboratory

High AltitudeObservatory

J. D. Purcell

G. Newkirk, Jr.

7OARSE SCANI20 LINES20 ARC SEC APARTTIME - 35 MIN40ARCMIN

-IHigh Resolution

X-ray Tele-scope

Study of dynamicsof the solar atmos-phere and compari-son with ground-based visible andradio observations

-5 ARC MIN

OFFSET POINTING6 RC SECFINE SCAN--

32ARCMIN 60 LINES5 ARC SEC APARTd ARC SEC ACCURACY OF Q SUN TIME - 6 MIN- 9 MIN

Figure 3.-Stabilization and control capability of the proposed AdvancedOrbiting Solar Observatory spacecraft.


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-.0 ASTRONOMY I N SPA'CEnative to continuing the AOSO project would be to set up special ..missions of the Orbiting Astronomical Observatory (OAO), acceptingas a deliberate sacrifice par t of the functional performance of theplanned AOSO in order to secure continuous satellite observations ofthe Sun a t high resolutioh in an earlier time period than would bepossible otherwise. A t the same time, we are considering the impor-tance and desirability as well as the technical feasibility of upgradingthe performance specifications for an advanced solar satellite. Forexample, astronomers now agree that ideally the stabilization accuracyought to be closer to 1 arc-second than to 5 arc-seconds but that sometradeoff can be accepted in the absolri te coordinate specification orthe coalinement of several experiments relative to the Sun sensor ofthe spacecraft.

Although it was necessary for budgetary reasons to terminatedevelopment of the AOSO spacecraft, it has been possible to maintainsome level of effort in the definition of the scientific investigationsthrough the design and partial development of experimental hardware.Four major investigations are being supported:

(1) The Harvard College Observatory scanning spectrometer(fig. 4 ) , 11 tilizing conventional normal incidence optical techniques,\ d l observe the spectrum within the 500- to 1500-b wavelength rangefrom areas on the Sun as small as 5 arc-seconds square. By stoppingthe spectral scan a t a chosen wavelength and performing a rasterscan of the Sun, the instrument would function as a spectroheliograph,also a t 5-arc-second resolution.

(2) The American Science and Engineering-Goddard SpaceFlight Center high resolution X-ray telescope (fig. 5) utilizes noveldouble reflection grazing incidence compound optics to form an imageof the Sun in soft X-rays, between a wavelength of 3 and 60 b. Forthe AOSO a small version of the same telescope would ac t as a pilotdetector to define the brightest or most interesting X-ray region onthe Sun for detailed analysis with a larger telescope.

(3 ) The High Altitude Observatory White Light Coronagrapll(fig. 6) is a modification of the Evans version of the Lyot coronagraph.In the advanced version an apodized occulting disk external to thefirst objective rejects scattered disk light from the optical system tothe very highest degree. With this instrument i t is possible to studythe polarization and brightness distribiition of coronal structures inthe range 2 to 6 Earth radii above the Sun's limb. For AOSO LLbiplexed data format was available so that one image tube wouldlook at the east limb and another image tube would look a t the westlimb of the Sun.

(4) The Naval Research Laboratory coronal and chromosphericspectroheliographs (fig. 7) were intended to produce 5-arc-second-


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.*-. SOLAR ASTRONOMY 21

COLLECTOR MIRRORAND (3) ADJACENT MOTORS

DETECTOR71 /DIFFRACT1ON GRATING+--J

Figure 4.--Harvard College Observatory scanning spectrometer-spectro-heliometer for AOSO, 500-1 500A.

resolution images of the Sun in the resonance lines 0.f hydrogen andneutral and ionized helium, and, in addition, the 284-A line of Fe XVand the 335-A ine of Fe XVI with a wavelength selection capabilityin the Lyman-alpha line for profile analysis.These instruments, though specifically designed to utilize the capa-bilities of AOSO, are nevertheless the general-purpose instrumentsthat all solar astronomers use in one version or another to conducttheir solar research from the ground, from rockets, and from the smallsatellites like OSO. Whether they fly on an OAO, or are operated


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22 .*A STR ON OMY I N S P A C EBULKHEAD

/- -RAY TELESCOPEFigure 5.-Functional drawing of he American Science and Engineering-

Goddard Space Flight Center high resolutionX-ray telescope.

Figure 6.-Conceptual design of the White Light Coronagraph Experimentfor the Advanced Orbiting Solar Observatory.

I from a lunar-based observatory, they represent the minirnum scientificprogram necessary to carry on our investigations of the striicture andbehavior of the Sitn. l'he contintled development of these experi-inental instruments is justified in the expectation that some means


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1,

.. S O L A R ASTRONOMY 23

Figure 7.-Naval Research Laboratory ultraviolet coronal and chromosphericspectroheliogrophs. 1216 A; 284 A; 304 A; 335 A .

will be found to place these or derivative instruments above theatmosphere and to point them a t the Sun with the necessary precisionto achieve some degree of realization of the initial AOSO investigationobjectives.

As the capability for manned space flight operations moves fromthe developmental to the applications phase it is inevitable thatastronomers will take advantage of peculiar opportunities to do as-tronomy during manned space flight missions. A t the present timevery modest limited-scope experiments have been considered for theApollo command module to operate through an airlock. One of theseis NRLs ultraviolet spectral photography experiment, which essen-tially is a reflight of an instrument which has provided very significantdata in the exploratory analysis of the Suns ultraviolet spectrum.

More important investigations of the Sun would be possible withthe Apollo Telescope Mount (ATM). This device is a spaceborneequivalent of the equatorial solar telescope mounting a t solar observa-tones. One structure would carry several solar telescopes to pointthem at the Sun. The ATM currently under study would carryinstruments up to 3.7meters long, and would, it is hoped, provide stabi-lization as good as 5 arc-seconds. The expected short observing


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. .4 ASTRONOMY I N SPACEperiods in a low-altitude low-inclination Earth orbit, and the require-ment for an astronaut to give time and effort to each individual-'experiment, mean that II single investigation must be satisfied byintermittent operation with alow duty cycle of the order of an houror two per day spread over several hours. The flight of AOSO-typeexperiments on the ATM would provide some of the da ta that couldbe obtained by those same experiments on a full-sunlit continuousdutystandby-mode AOSO flight. There is reason to hope that early flightsof a11 Apollo Telescope Mount would provide some high-resolutiondata on solar activity near the maximum of the current solar cycle in1968 and 1969.

Initial plans call for one ATM mission, primarily to fly the instru-ments derived from those which were under development for AOSO.I t is important to note that these basic instruments must be used forsomewhat different investigations than proposed for AOSO, since themission profile is so drastically different. The successful realizationof this phase of the ATM program very likely will create additionalflight opportunities for other investigations in stellar astronomy aswell as in solar physics.

The ATM, now being designed, will contain a long optical bench tosupport and direct the telescopes a t selected areas of interest. Anastronaut will point the device crudely a t the Sun and then let auto-matic controls take over for final acquisition of the Sun by Sun sensosand servocontrol system. The astronaut will then single out oneexperiment and point i t to a particular part of the Sun for detailedstudy, using the ATM control system. Each experiment would bcserviced in turn; those requiring precise boresighting would be operatedsequentially, while wide-field measuring instruments could operatesimultaneously with others. Beyond the operation of the system asdescribed, the astronaut will be required to recover the filni from thetelescopes at the end of the observation, and prior to the reentrymaneuver.

The further development of space flight solar instrumentatiollbeyond that planned for the early Apollo program occupies an impor-tant place in our planning. NASA realizes that using large opticaltelescopes in orbit is one of the most promising long range goals thathas been suggested for the space science program. The specialrequirements of solar astronomy dictate that peculiar instrumen tationbe provided, such as grazing incidence optics, image-forming X-rayoptics, and imagedifferentiating spectroheliographs for the XUV undX-ray ranges. The ultimate capability during manned space flightL o extend, adjust, oper'ate, and maintain such large telescopes meansthat the ultimate limitation of the Earth's altmosphere need no longerlimit the kind or quantity of scientific information obtainable by


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- I.25. S0 AR A 6 R 0N M Y

I ,telescope techniques required in the study of the Sun. No singleconfiguration of a manned orbiting solar telescope has yet been defined;however, in our long range planning we shall develop conceptualinstrument configurations in order to define performance capabilitiesand better to assess the scientific yield to be expected from the fulfill-ment of such plans.


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. \ PRECEDING PAGE BLAEJR NOTs

STELLAR AND GALACTIC ASTRONOMY c.L , NANCY . ROMAN.;(:

Chief, Astronomy, Physics and Astronomy Programs\ N A S A , b i * 5At the spring meeting of the American Astronomical Society in

1959, I made a brief announcement about the space astronomy pro-gram which the National Aeronautics and Space Administration wasorganizing, and I invited members of the society and their colleaguesto participate in this program. Since that time, w e have made greatprogress. We have successfully launched a gamma-ray telescope andtwo solar observatories. The spinning rockets of 8 years ago havebeen replaced, for the most part, by the stabilized rockets of todaywhich can point not only a t the bright Sun, but also at individualstars or even regions of the sky which do not have an optical target.Since the solar program has been presented in detail in the paper bySmith and since most readers are familiar with the spectacular resultsof the Ranger and Mariner missions, my discussion deals primarilywith NASAs stellar and galactic program.

At the beginning of NASAs astronomy program, several interestedmembers of the astronomical community met to discuss the astrono-mers needs in space instrumentation. We quickly concluded thatwe wanted to have in space as close as possible an analog of ourterrestrial observatories, with the same versatility for making manytypes of observations of numerous celestial objects. We soon recog-nized that the,optics for each satellite package would have to bedesigned separately in order t o provide the highest efficiency and thesimplest automatic operation. Nevertheless, it was obvious thatthere were many needs common t o all astronomical observations whichcould best be handled in a standardized spacecraft. Primary amongthese was the need to direct the optical equipment to a particularstar or region of space on command from the ground.

Since the Orbiting Astronomical Observatory (OAO) was designedto spend only a few minutes per orbit within sight of a ground station,the astronomers first thought of an operation which would be undercomplete ground command and in which a television camera would

27246-016 047-3


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, ,28 A STR ON OMY I N SPA C Ebe used to monitor the pointing was obviously impractical. Instead,it was necessary to provide the satellite with some type of stellarreference system. This has been done by means of six star trackerswhich can be used to orient the spacecraft within 1 minute of arc.To keep the satellite moments of inertia as equal as possible, thusreducing gravitational torques which would tend to disturb the pre-cise pointing, balance booms have been added. A sun shield wasincluded both to protect the experiment before the satellite is fullystabilized and to act as n shade against sunlight scattered into theobserving tube. A central tube, 48 inches (1.2 meters) in diameter,was provided for the optical experiment equipment. By standardizingt,his tube and making it removable, we have attempted to give the

Figure 1.-Orbiting Astronomical Observatory.Instrument weight, k g . . . . . . . . . . . . . . . . . . . . . . . . . . 1710.. . . . . . . . . . . . . . . 450.. . . . . . . . . . . . . . . . . . . . . . A c t i v e 3 axes .Launch veh i c l e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A t l a s A g e n a .

T y p e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Circular.

. . . . . . . . . . . . . . . . . . . . . . . . . . .

O r b i t :A l t i tude , k m . . . . . . . . . . . . . . . . . . . . . . . . .I nc l i na t i on , deg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 .

1.Poin ting-a ccu racy characteristics:Anywhere in celestial sphere, arc-min.Stor target, arc-sec., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1.. . . . . . . . . . . . . . . . . . . . . .


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29. S T E L L A R A N D G A L A C T I C A S T R O ' N O M Y' .

Figure 2.-University of Wisconsin Experiment Package For the first OrbitingAstronomical Observatory.

astronomer the maximum flexibility in the design of his opticalsystems.

The first OAO (fig. l ) ,which will be launched soon, will carry fourexperiments.' An ultraviolet experiment package (fig. 2), providedby Arthur Code and Theodore Houck and their colleagues a t theUniversity of Wisconsin, consists of four 8-inch (0.2meter) diameter,f/4, ff-axis parabolas with photometers a t the prime foci; a 16-inch(0.4-meter) diameter, f /2 , nebular photometer; and two objectivegrating spectrometers, each composed of a 6- by 8-inch (0.15- by 0.2-meter) objective plane grating and a 7- by 10-inch (0.18- by 0.25-meter), f/4, arabolic objective mirror. Each stellar photometer hasa five-position filter wheel: one position permits a dark measurement,ti second exposes the photomultiplier to an ultraviolet Cerenkovcalibration source, and three positions introduce bandpnss filters which

The first OAO W:IS launchcd April 8, 1966. A powcr failurcb which occurrcdafter 2 days prcvcntrd t h e rspc rimc nts from )icing pcrformed: howcvcr, the startracking bystem demonstrated thc ability t o stahilizc th r spncmraft. Th rI-nivcr sity of Wisconsin rxprr imc nt mill br carricd again on the srcond O A O .T hr pnyloads originally schrdiilrtl for the srcond (OAO-R) and third (OAO-A2 )spacrcraft will h e intrrchangcd.


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..30 A S T R O N O M Y I N S P A C Eisolate spectral bands of approximately 300 A in width to cover . .wavelengths longer than about 1000 8,with overlap not only betweenthe longest wavelength photometer and the ground, but also betweenphotometers.The field of view of each photometer can be selected as either 2 or10 minutes of arc, and the exposure time can be varied by commandbetween ) h second and 64 seconds. In addition to the dark shield andthe Cerenkov source, the nebular photometer filter wheel carries f o u rfilters with transmissions centered at 3300 A, 2800 A, 2500 A, and2000 A. The field stop on this photometer may be tidjusted to either10 or 30 minutes of arc. One spectrophotometer covers the wave-length region from 2000 A to 4000 A, using t i 300-line/mm gratinFblazed for 3000 A and an exit slit which can be selected at either 20 Aor 200 8. The other spectrometer, designed to operate between1000 8 and 2000 A, uses a Fa ting blazed for 1500 A and an exit slitcorresponding to either 10 A or 100 8.

The Wisconsin experiment occupies only half of the experimentspace; the other half is occupied by three experiments (fig. 3) in thegamma- and X-ray regions of the spectrum. Phillip Fisher of Lock-heed Missiles and Space Division has prepared two identical arrays of

Figure 3.-Gamma- and X-ray experiments for the Orbiting AstronomicalObservatory.


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.. S T E L L AR A N D G A L AC T I C A S TR O N O MY 31gas proportional counters, each of which has a geometrical apertureof 700 cm2 and is equipped with anticoincidence scintillator shields.Each array can be operated with either a 16' square field for a widefield survey to discover new faint sources of X-rays in the 6 A to 2.4 Aband, or a small 8' square field for more accurate location of brightersources, hopefully, within 20 minutes of arc. An indication of thespectra of the sources will also be measured.A survey of sources in the range of 2 to 150 keV will be conductedby Kenneth Frost and his colleagues a t the Goddard Space FlightCenter, using a detector originally designed for the Orbiting SolarObservatory series. A 78-cm2 scintillating crystal of sodium iodideis surrounded by a large anticoincidence shield. A nine-channel pulse-height analyzer will provide spectral data on the events observed.A third inst,rument to survey the sky for gamma-rays with energiesgreater than approximately 50 to 100 MeV has been built by WilliamKraushaar and his colleagues a t the Massachusetts Institute of Tech-nology. This instrument is actually the prototype for the experimentwhich was flown on the first of the astronomical satellites, Explorer XI.I t consists of a cesium iodide, sodium iodide sandwich-type crystalscintillator followed by a Lucite Cerenkov counter. A large plastican icoincidence scintillator surrounds the telescope to deactivate itduring the passage of charged particles. I t is hoped with this experi-ment both to detect point sources of these gamma-rays and to obtaina better measurement of the diffuse galactic background noted byExplorer XI.

The second Orbiting Astronomical Observatory will carry a 36-inch(0.91-meter) diameter telescope (fig. 4) designed by the Goddard SpaceFlight Center for spectrophotometry of stars as faint as 10th ma nitudein a spectral range from 1050b to 4000 A. A resolution of 2 1,8 b,or 64 f can be used. The optical elements of this telescope are madeof beryllium, an innovation in major optical systems. Six photo-cells mounted approximately 600 A apart will provide both redundancyand data collection efficiencyfor the instrument. The third OAO willcarry a duplicate of the first University of Wisconsin experiment (fig. 5).It will also carry four telescopes designed by Fred Whipple and RobertDavis of the Smithsonian Astrophysical Observatory to map t,he skyin two ultraviolet wavelengths, using specially modified Vidiconsoperating a t the foci of Schwarzschild cameras. The Vidicon has beenmodified for highly sensitive, blemish-free operation in the ultraviolet.The production of a suitable Vidicon, or Uvicon, for this purpose,proved to be sufficiently difficult that this experiment, originally in-tended for the first OAO, had to be delayed.

The fourth of the Orbiting Astronomical Observatories will carry aspectrometer (fig. 6), designed by Lyman Spitzer and John Rogerson

1 .


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-.2 A S T R O N O M Y I N S P A C E

-

i

Figure 4.-Goddard experimental package for the Orbiting AstronomicalObservatorv.

Figure 5.-Smithsonian and the Wisconsin experiment packages for theOrbiting Astronomical Observatory.


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STELLAR AND GALACTIC ASTRONOMY 33.of Princeton University. The 32-inch (0.8 meter), f/3, quartz mirroruses an egg-crate constructiono o reduce its weight. The 2400-line/mm grating is blazed for 2200 A and provides either a 0.1 A o r a 0.4 8resolution. Two carriages which rotate along the Rowland circle withtwo photocells on each carriage cover the wavelength range from 800 8to 3300A . Two monitoring detectors permit corrections of inaccurateguiding. This satellite will also carry a nest of three 8-inch (0.2-meter) parabolic collimators to observe the brighter X-ray sourceswith higher resolution a t 3 to 9 A, 8 to 18 A, and 44 to 60 A. Thisexperiment is being prepared by R. L. F. Boyd of the UniversityCollege, London, and F. A. Stewardson of Leicester University.

Although only four OAO flights have been authorized, NASA con-siders the OAO to be a standard spacecraft which will be usefu,l formany years to come. A fifth OAO may carry a second model of thespectrophotometer being developed by the Goddard Space FlightCenter for OAO-B, modified slightly to provide offset guidance andthe capability of measuring appreciably fainter stars than those whichcan be observed with the OAO-B instrument. This will be primarilya guest-observer instrument; that is, observing time will be madeavailable as far as possible to any qualified astronomer with a soundscientific program who wishes to obtain ultraviolet spectrophotometric

. .

Figure 6.-Princeton experiment package for the Orbiting AstronomicalObservatory.


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..I 34 A STR ON OMY IN S P A C Edata, in much the same way that the telescopes a t the Kit t Peak . -Observatory are made available to guest investigators. We alsohope to have a small, guest-investigator program on the earlier OAOs.In particular, if the ultraviolet experiment on the next OAO operateswell, we shall be inviting proposals for guest investigators for a limitedamount of the time available with this satellite. This policy will befollowed with the later OAOs.

Most of the results in space astronomy have been obtained throughthe use of sounding rockets. Stellar pointing systems for the Aerobee150 are now available; they permit the observer to point his equipmenta t from one to five targets per flight with an accuracy of about 2.These pointing systems, which are still being improved, have not had thedesired reliability, but as their reliability is increased, they are rapidlybecoming almost standard in the stellar sounding rocket program.Broadband ultraviolet photometry results have now been obtainedby a number of observers both in the United States and in Britain.In contrast to the situation several years ago, the results obtained bythe vitriolis observers are in quite good agreement. As a result of therinexpectedly low ultraviolet brightness of the hotter stars, stellarexpectedly low ultraviolet brightness of the hotter stars, stellarmodels have been revised to include a more realistic correction forline blanketing. These changes led, in turn, to a revision of thestellar temperature scale for hot stars, and the observations nowagree well with the models. Sufficient data are now available for areasonable extension of the interstellar reddening curve into theultraviolet (ref. 1). A distinctionbetween scatter that is intrinsic to extinction and the observationalscatter is difficult, biit there is some indication that the variationsi n extinction found for the visible and infrared by Harold Johnsonof the University of Arizona (ref. 5) and others are responsible forsome of the scatter in the ultraviolet as well. The observed ultra-violet extinction curve has lent some support to the recent revivalof the theory that graphite grains are responsible for interstellarextinction. However, the presence of pure graphite grains does notappear to be adequate to explain the detailed course of the extinctionwith wavelength.

Narrowband photometry was introduced into rocket astrononly byTheodore Stecher and James Milligan of the Goddard Space Fl ightCenter (ref. 6). They used objective gratings t o observe the starswith a resolution of 100 hi, in the wavelength range from 1700 %i to4000 h i . The roll of the rocket was used to provide the spectralscan. These spectra also clearly showed the ultraviolet deficiencyevident i i i the broadband photometry. The first published photo-graphic spectni of stiirs were obtained by Donttld Morton wid hisdeiigries ut Princ*eton Universit,.v i n cJ1itw of 1965 (ref. 7). These

The effect isillustrated in figure 7.

_ _ _ _


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6 T E LL AR A ND G AL ACTI C AS TRO NO M Y 35.1 . 8

76

4 .3 -2 -I -0 -

E map

- I-2-3-4

--!; ALEXAN DER ET AL. (REF. 2)- :$ CHUB8 AND BYRAM (REF. 3). O BOGGESS AN D BOR GMA N (REF. 4)

2 1I1 1 1 1 1 41 STECHER(REF. 1) ij;llli(*I f '

.*#

- .-

observers detected weak spectra of Delta and Pi Scorpii a t a d ispetsion of 64 per millimeter in the wavelength region between 1250 Aand 4000 A. Twenty-three lines in the Delta Scorpii spectrum and18 lines in the Pi Scorpii spectrum were identified. ,411 of the lineswere in absorption as had been predicted by theory. Spectral photo-metric scans of stars in Lupis and Canis Major by the Goddard SpaEeFlight Center personnel also showed absorption features near 1500 A .Perhaps the most exciting photographic spectra that have been ob-tained in the ultraviolet are those illustrated in figure 8, also byDonald Morton and his colleagues (ref. 8). Six spectra are visible.The black, nearly vertical lines are zero-order images of field stars.The excit,ing feature is that on the longward edge of the strong linesin Epsilon and Zeta Orionis, emission features are clearly visible.This indicates that these stars are apparently ejecting mass at sub-stantial rate.

One area of space astronomy not even thought of 8 years ago hascome to the forefront of astrophysical importance. In 1962 RicardoGiacconi, Herbert Gursky, and F.R. Paolini of the American Scienceand Engineering, Inc., and Bruno Rossi of the Massachusetts Insti tuteof Technology first detected X-rays originating outside the solarsystem (ref. 9). Wide-field Geiger counters revealed the existence ofa strong source in the constellation Scorpius, not far from the galacticcenter, and a possible secondary source in the constellation Cy, nus.The Geiger counters also revealed R diffuse X-ray background which


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36 A S T R O N O M Y IX S P A C E/----. .L 4 . 1

Figure 8.-ultraviolet stellar spectrograph recovered from attitude-controlledA e r o b e rocket flight (ref. 8).

W ~ L S ppnrently dso of celestial origin. The realit,y of these sources,whose locations w e indicated in figure 9, hns been confirrned t)y theseinvestigators ttnd others. Litter flights showed seven or eight tkddi-tionnl X-ray soiirces, inclnding the Crah Nebula. These lat ter flightsalso proved conclusively that the gdnctic center w t ~ s ot II soilrce ofX-rays. In tin ingenious observation in mid-July 1964, Friedrnttnnnd his colleagues (ref. 10) showed tlitLt the X-ray source in Tt~u11is indeed coincident with the Crab Nebula tmd that it is npproxinlately

Figure 9.-Sounding rocket celestial map of X-ray sources.


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390 340-. SEC SEC

CENTERSOURCEOF X-RAY

150SEC

Figure 10.-Method used to locate the X-ray source in the Crab NebulaDashed lines indicate the position of edge ofuring a lunar occulation.M oon at time shown; arrow indicates direction of motion of Moon.1 arc-minute in diameter. The size was determined by measuringthe decrease in the X-ray emission from Crab Nebula as the Moonmo\-ed ticross the Nebulii (occultation indicated in fig. 10). Fried-in i in (ref. 11) hiis recently discovered approximately iL dozen nddi-tional X-ray sources and has identified two of these with the radiostars 1487 and Cygnus A . Increasingly well collimated detectorshave been used for X-ray sky surveys, and positions of the brightersources are now known t o within a fraction of a degree. However,iis yet, optical identification has proved impossible for most of thesources. Most are well concentrated in the galactic plane, particularlythe galactic center region.

Rockets and satellites do riot provide the only platforms for spacetLstronomy. For many purposes, balloons carry instrumentation toiLn alti tude high enough for useful astronomical results. For someyeiirh, NASA hiis joined with the Office of Niival Resenrch iind theNational Science Foundation in supporting the Strntoscope I1 balloonflights. Not only do we expect, this project to produce valuableresults, biit the techniques being used to obtain high-resolutioiiphotogriiphs remotely will nlso be iipplicable to high-resolution


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* .$8 A S T R O N O M Y I N S P A C Ephotography in satellites. NASA encouraged and supported modifica- , .tion of the Stratoscope I1 instrumentation for use with an infraredspectrometer in 1963. The first flight encountered many difficultieswhich limited the scientific results to those which were obtained ntapproximately the same time by other means. However, a secondflight successfully recorded new infrared spectra. of the Moon, Jupiter,Mu Cephei, and six other red giant stars (ref. 12). Water vaporabsorption bands a t 1.4 and 1 .9 microns were found in five of the sixstars (clearly shown in fig. 1 1 for Alpha Orionis). The sixth, AlphnTaurus, shows practically no water vapor bands but an intensitypeak near 1.6 microns. The long-period variable, Omicron Ceti,also showed a water vapor band at 2.7 microns. Jupiter clearlyshowed the expected bands of CH,, molecular hydrogen, and NH,.The highest balloons also attain an altitude sufficient for at leastpreliminary surveys in the soft gamma-ray region. George Clarkand his colleagues a t the Massachusetts Institute of Technology,Laurence Peterson a t the University of California, and others at theGoddard Space Flight Center and elsewhere have flown gamma-raydetectors in balloons. Clark first observed the Crab Nebula in theenergy range between 15 and 60 keV in July 1964 (ref. 13). Peterson

-. * ST EL L ARo. aiM2-3 lab- . "" ST EL L AR*:

(Detector A )?o. 1 . 1 3 ~.. .+.' ' 1 . ~ ~NKNOWN

ST EL L AR

''-T EL L AR HzO

*. UNKNOWN'. ST EL L AR*\ .* 2 . 3 5 r'..

I . , . , I . , , , I . . , I . , . I .0' 1.0 15 2 o 2.5 3.0Wavelength (Microns

Figure 11 .-Infrared spectrum of Alpha Orionis obtained by StratoscopeI I (ref. 1I).

II


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S T E L L A R A N D G A L A C T I C A S T R O N O M Y 39.also observed the Crab Nebula in the energy range between 16 and120 keV in September 1965 (ref. 14) with a somewhat better spectralresolution (fig. 12). It is interesting to note that Crab Nebulapresents a smooth spectrum for frequencies ranging from 10' Hz to10" H z, with a gradual increase in slope in the infrared and visiblcregions. Upper limits have been established at frequencies up tolozBHz. These also fall near a smooth curve extrapolated from theX-ray and soft gamma-ray regions, indicating that improvements insensitivity by a factor of 10 to 100 may be adequate to permit detec-tion at these higher frequencies. The index of differential energyflus in the gamma- and X-ray regions appears to be near 0.9. Severalgroups are flying spark chambers in balloons to detect higher energygamma-rays. As yet, I am unaware of any positive results fromthese flights, but the sources must be nearing the margin of detect-ability, tl margin that should be crossed fairly readily at satellitea1 i tudes.

As has been mentioned, the Explorer XI satellite, shown in figure13, was designed to detect high-energy gamma-rays. The satellite,which was launched in 1961 by a Juno I1 rocket, was placed in toohigh an orbit. Because the anticoincidence shield deactivated thecounter whenever a charged particle was detected, the instrumentwas turned off most of i ts time in the Van Allen belts, and consequentlythe amount of usable observing time was severely limited. No pointsources were detected. However, an average gamma-ray backgroundintensity of 3X 10-4/cm2/sec/sterad was observed (ref. 15). Althoughno asymmetry which might be related to galactic concentration wasobserved, it seems highly probable that this flux did arise from out-side of the solar system. This intensity of gamma-rays can beaccounted for if a moderate intensity of high-energy electrons inintergalactic space is assumed. As mentioned previously for theCrab Nebula, the upper limits placed on the gamma-ray emissionfrom the Andromeda Nebula and Magellanic clouds, the galacticcenter, and seven of the stronger radio sources may not be far abovethe actual value.A t the other end of the electromagnetic spectrum, significantprogress has been made in the measurement of the spectrum of thecosmic background at frequencies between 1 and 5 MHz. Rocketshave been flown by Fred Haddock and his colleagues at the Universityof Michigan (ref. 16) and by Robert Stone and Joseph Alexanderat the Goddard Space Flight Center (ref. 1 7 ) ; Richard Hugueninand his colleagues at Harvard University (ref. 18) have designedexperiments which have flown on several satellites; Theodore Hartzof the C'unadian Defence Research Telecommunications Establisli-ment (ref. 19) analyzed the results of the Alouette topside sounders

* .


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-20 ,.A S T R O N O M Y I N S P A C El t I I I l l l " 1 I I I I I 1 1 ' 1 I I I I I l lN R L

\ i\

CLARKbT

FROST, ETAL.

I I I I I I I I I I 1 I I I 1 1 1 1 I I I l l l l L l1 10 100 1000

0-5~ ENERGY (keV)

Figure 19.-Photon flux from the Crab Nebula versus photon energy in theX- and soft gamma-ray spectral region (ref. 14).


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STE LLA R .I ND G.\L.\C;TIC: A ST RO NO M P 41.

Figure 13.-Explorer XI gamma-ray satellite.for intensity of galactic background emission; and Graham Smithand Haddock have flown receivers on Ariel I1 (ref. 20) and OrbitingGeophysical Observatories T and 11, respectively. Each of theseexperimenters used short dipoles with almost no directivity. Asfigure 14 shows, the measurements agree reasonably well (ref. 21)in indicating t hat the maximum intensity of the galactic backgroundoccurs between 5 and 10 MHz, with a decided decrease a t lowerfrequencies. Some of these experiments have also indicated thatthe ionosphere may be n significant source of noise, possibly emittingbursts of energy in a mode analogous to that of the Jupiter ionosphere.Th e first a ttempt to obtain even limited directivity at these fre-quencies will be made in 1967 with a Radio Astronomy Explorer(fig. 15) designed by Stone (ref. 22). This satellite will carry twoV-antennas, each leg of which is 750 feet (230 meters) in length.These antennas will produce R beam of about 25O by 45' a t 3:i MHz,with a front-to-back ratio near 17 dB. Several Ryle-Vombergmultifrequency radiometers will be used in the frequency range of250 kHz to 1@ MHz. The satellite will carry also several multi-frequency h r s t receivers and an antenna impedance probe. I t wasoriginally hoped to iise 1000-foot (300-meter) antennas, but problemsof grnvit,y and thermal bending prevented the use of such long struc-


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..2 A S T R O N O N Y I N S P A C EI I I 1 1 1 1 1 1 I I I

n CHAPMAN 8 M OL O ZZ I (1961) . .0 SMITH (1961)TURTLE ET AL. (1962)8 ELLIS ETAL. (1962)0 WALSH ET AL. !(1963)A HU GU EN IN ETAL. (1963)A PAPAGIANNIS ET AL. (1964)HARTZ (1963)? ,

0 II+10 50 100 50010-*:.5 1

FREQUENCY IN MHzFigure 14.-Measured values reported for the mean cosmic backgroundAll points represent measurements from rockets of satellitesoise.

except those of Ellis (ref. 21).I tires. A T V Vidicon cninerii (wried on tlie spncecraft will wILt(*llthc tips o f the antennas to correct the gain for the misalinernent.

However, 11s theiiitlritied space program matures, plans we being made to incorporatethe flexibility of mnnned operidon into the progrilm to tm increasingextent. We hope to obtain both direct photographs, using inter-ference filters, and objective prism spectra with a dispersion of 200%, per millimeter down to 2000 %L, using an ultraviolet modification ofthe Mauer camera carried by the This wrangernent isshown in figure 16. Only the very brightest stars will be ohservedspectroscopically. Karl B. Henize, of Northwestern University, whohas been responsible for the design of this experiment, is plannitig umore sophisticated version to be flown 011 Apollo. A special 6-inch(0.1&meter) Ritchey-Chretien, f/3, camera with an objective prist11will be inserted into mi airlock utid will be used to obtain II nutnber ofX I I .

So fur, all space astronomy has been automated.

2 I':xcoIl(~nt tcslI:ir photographs WVIY- obt:iiiicvI oil ( h i i i i i i I I I ~ R H ~ O ~ L S, SI, t l l ( l


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STELLLIT:L I N D GALLICTIC A S T R O N O 3 f T 43-

Figure 15.-Radio Astronomy Explorer.objective prism spectra of early type stars with a dispersion near370 b per mm at 2000 A. In addition, a low-dispersion prism willbc used to observe a large number of stars with a resolution of 10 000b per mm at 2000 A. An X-ray instrument is being designed byGiacconi, Waters, and their colleagues a t American Science andEngineering, Inc., for flight on Apollo. It will be used to determineaccurate positions of known X-ray sources and angular diameters ofthe litrger X-ray sources, and for a general sky survey to find newX-ray sources in wavelengths between 5 b and 50 A. Two broadbanddetectors with fields of view of 20' by 4' will be flown as will a narrow-band detector, variable in angular resolution between 40 minutes and3s seconds.

Any discussion of the NASA astronomy program would be in-complete witshotit nention of laboratory, theoretical, and ground-basedobservational work. A detniled description of the sixty-odd projectsin stellar and galactic astronomy alone is beyond the scope of thispaper., but a few project,s are mentioned here to give an idea of thertinge of these activities. NASA is, of course, supporting the develop-inrn t of inwe sophisticated experiments in the ultraviolet and X-rayregions and exciting experirnent,s t80 est various aspects of the theory

2.13-016 0-67--4


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. I44 A S T R O N O M Y IN S P A C E

Figure 16.-Mauer camera with objective prism in place to be used b y theastronauts during the Gemini X and XI flights to obtain direct and l o wresolution stellar photographs and spectra in the ultraviolet.


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.\ S T E L L A R X N D G A L A C T I C A S T R O N O M Y 45instruments. Arno Landolt, a t Louisiana State University, is studyingthe spectra of OB stars; Gerry Neugebauer at the California Instituteof Technology is surveying the sky in the infrared and cataloging themany infrared stars he has observed. At the University of Indiana,Hollis Johnson is studying the details of stellar atmospheres andMorton, at Princeton, is doing theoretical astrophysical research inareas in which the ultrnviolet observations planned for the PrincetonOAO experiment wiil be most directly concerned. We are supportingthc construction of a 105-inch (2.7-meter) telescope at the McDonaldObservatory and an S4-inch (2.1-meter) telescope a t the University ofHawaii. Moreover, NASA has also supported construction of :Lnumber of smaller ground-based instruments for both optical andradio observations. A s an outgrowth of satellite tracking, theSmithsonian Astrophysical Observatory has compiled a cataloguecombining numerous star catalogues and containing many newlycomputed proper motions. Obviously, space astronomy is not a newfield, but simply n new tool for attacking traditional astronomicalproblems. Therefore, space astronomy cannot be conducted as anentity, but must be part of a coordinated program of research, mostof which is conducted on the solid earth.

In 1959, we stood at the dawn of a new space astronomy program.Today we have accomplished many of the things we were planning a tthat time and have opened exciting new fields, but we may be a t thedawn of an even more important space astronomy program. I t is ob-vious that if we are going to exploit the possibilities of space astrono-my, instruments such us those recommended in the National Academyof Sciences Woods Hole St8udyof 1965 will be needed, and we aremaking the first tentative plans for such instrumentation. At thcpresent time, if II transistor burns out or a battery dies, an unmannedspacecraft such as the OAO has taken an irremediable step towarddeath. Moreover, no changcs can be made to update the instru-inentntion with which the spacecraft is launched. We recognize thateven simple operations by man could greatly extend both the lifetimeand the versatility of an OAO; therefore, we have been looking in to thepossibilities of using man in such maintenance activities. Such anoperation is illustrated in figure 17. Looking further ahead, we alsorealize that eventually astronomers will want larger telescopes inspace than are possible with the current series of OAOs. Therefore,we have had u study conducted on the possibility of building, mount-ing, urid using in space a telescope of approximately 120 inches( 3 meters) in diameter. The problems are far from trivial, but theyalso do not appear insurmountable, given the resources necessary forthe job. Following the recommendation of the Woods Hole Summer


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. -

Figure 17.-The use of man to repair and maintain an Orbiting AstronomicalObservatory.

St,iidy, w e tire tils0 investigttt,ing the possibility of flying low-frequencyrridio tistronomy experiments with higher directivity thiiti thnt of theH i i d i o Ast,ronorny Esplorer. Studies tire being cwndiicted t i t t h e ITni-versil,y of Michignn m d tit TRW Systems on tierinl systems which c r ~ nbe spiin out, t o sizes of t8hcorder of kilometers. Perhttps ndditiondsmdler imtenntis will be placed on the periphery of siich structures.The Goddnrd Space Flight, Center is considering t8he possibilit,y ofusing 11 separnte nutneuvertible sritellite, t,oget,Iier wit,h ILn itritennciotirried o n tt synchronous orbiting Apollo, t o Innp the celestial skieswit'li reiisonIible dirrc-t,ivit,yusing tipert,rire syrit81iesis. We w e tils0siipport'ing Htirvwd for IL ground-btised t,est, of ot)t,ttiiiing directivityiising sophisticnt8eddnttt cindysis t,ec,hriiqiies. W e hive done less sofw tjo iniplenien