Astronomy and Astrophysics for the 1970s: Volume 1: Report of the Astronomy Survey Committee (1972)

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CHAPTER FIVE The High-Priority Program The first II Sections of this Chapter describe in detail the programs and facilities recommended as being of highest priority. Many more suggestions, which may be justified as of great urgency now or in the future, will be found in the individual panel reports of Volume 2. We describe here, in brief, scientific justifications and content of the programs we now recommend. The first four we view as of the very highest urgency and priority. The next seven are also essential to the health and balance of the total astronomical enterprise. The costs over a decade are ap- proximately S600 million for the first four and 51200 million for the entire program. The rate of growth, as has been mentioned previously, is not large, and the manpower available or at present being trained is sufficient. In the final Section of this Chapter, we discuss a program of further new starts that we would have recommended if we bad had only scientific goals in mind with no financial restrictions. VERY LARGE ARRAY The Committee recommends construction of a very large radio telescope array with the ability to observe the u.niverse to great depth with un- precedented clarity. Such an instrument can break through existing observational barriers on a broad front and reveal important new lines of enquiry. Radio telescopes have demonstrated their value by their involvement in 76 

Tltt Hl#t·Prlorlry Prov•m 77 an extraordinary number of disa>veries in astronomy. These iJK"Iude the quasars, objec1.s of unbelievable energy production and visibility at great distance: the pulsars: the universal blackbody radiation : and the detection of the vast en.s embk of complex intemellu mole<:uks. 10ese discoveries owe mueh to the union ofenJineerlng and electronies, ,.bicb has produced large radio telescopes capable ofdeteetlna lncrodibly faint signals. Indeed, all the radio-sianal en~ detected in our radio·astronomical history is linle more than the energy released by the silent impact of a few snowflakes on the ground. Our te!Hropes can today detect easily the radiations of quasars to what ,..., believe to be the edge of the observabk unin:rse. It is not surpming that there has been a flood of remukabk discoveries. However, te<:hnlques that produce great signal Knsilivily could noc as readily give us an ability to see clearly. In fac1, the limit on our ability to KC bas been the d illkulty In distinguishing from one another the numerous objec1s that we can now detect In the sky: a blurred radio picture of the sky has been normal. Great effon has been invested in finding ways to..,. the sky clearly. Following the development of a new instrumental concept for high resolving po'""r In Australia and England, KVeral obK<Vatories in the United States have developed to a highly successful state a technique that can provide the resolving power so long sought after. This is the method called "apenure synthesis." The basic technique of apenure synthesis involves the combining of signals received at two individual telescopes. retaining all the electrical characteristics of the sianals, including the slana) phase information. Suc.h a pair of telescopes can resolve two point sou roes as ...,u as can a single large telescope whose diameter l' equal to the separation of the two an· tennas. ObKrvations with a radio interferometer in which the separation of the antennas is increased from zero to some large dimension, perhaps miles, can produce as detailed a picture of the object as that produced by a single prohibiti>·ely expensive telescope of the same large dimension. A large number of geographical orientations of the line between the two antennas must be used for the method to succeed. Very high resolving pov."ers can be achieved by this approach at relatively low cost. Indeed, KVeral obK<Vatories have uKd this technique to achieve hiah-quality radio pictures of the sky with resolutions only ten times less than that achieved by optical tekscopes. The method is, howeYer, slow, and satisfactory progress requires simultaneous UK of many antennas. Many astrOnomical probkms require a radio resolving power that approaches that of ground·based npticaltelescoJ!e- - 1 sec of arc. The National Radio Astronomy Observatory has carried out extensi..., and detailed studies of aperture synthesis systems to achieve this goal. The 

78 ASTRONOMY AND ASTRDPIIYS ICS FOR Til E 1910"o result is a design that can achieve high·quality radio pictures of the required resolution at a rate of about two pictures of new regions per day. This ingenious design achie"\'es this speed and r601ution with a minimum cost by u1ilizing 27 antennas of 85-ft aperture, d•ploycd in a arelltlly Calculated pattern 0\""C-T an a~a 26 miles in diameter. The rotation Of the earth 0\·er severa1 hours causes the geometric separation of the antennas as seen from the sky to be altered to produt"e the ~uired antenna orientations and separatjons. The antennas are conrrolled. and the in· formation from them processed. by a central large computtr system. This antenna systcn1 is called the Ver)' Large Array tvLM and will be by far the largest ttnd most advanced radio.as tronomical instrument ever con· structcd. It will produce the equivalent of a radio "eye" 20 miles in din. meter. It is estimated that five years will be required to construct it at a COSI or $62 million. Although s uch a giant step in capability will certainly produce major discoveries and surprises that cannm now be predicted, there is an ex· tensive ensemble of new results that can be foreseen. Particularly revealing will be the dttaUtd pictures of radio galaxies and quasars. pictures that will show the distribution of high-.:nergy particles and magnetic 6elds. allowing us to trace tht t\'Oiution of these vast radiating region.s as they art created by th• violent uplosh-e evenu in these objects. There will be high· resolution radio pictures of normal galaxies to compaft' with tbe radio galaxies and with our theories of the radio emission or nonnaJ gaJuies and of the objects in them. Th• \' LA will be a major new tool for cosmology by virtue of its ability to distinguish large numbers or point sources one from another. A key cosmological problem is to plot a number-flux relation to very faint limiting nu.xes, so one is sure to be including sources that are distant enough to distinguish different cosmological models. The VLA can count such source$ because of its narrow beam and large collecting area. Howe\ler, a more subtle. problem is to eliminate from the count the numerous, but uninteresting, near-by sources that ore intrinsically faint. At present , we :a not sure how numerous such sources are. The Vt.A can rc determine this by observing all sources at a known distance, s uch as in a cluster of galaxies. The narrow bea.m will be decisive in distinguishing indiv·idua1 sources in such crowded regions. There is some hope that spectral or other characteristks can be used to distinguish between intrinsically bright and faint sourus: the multifre· qu•ncy and polariution capabilities of the vu will be impocunt in this regard. Furthermore, if sourus can be found which ha•• a definite distri· but ion of linear sizes. the high angular resolution of the YLA may be able to d•termine the angular sizes of such objects at large distances and there· 

The Hlglr·l'rioriry Program 79 fore study the angular diameter-flux relation, which should be sensitive to cosmological effects. In summary, the VLA will be able to approach the solution to the cosmological problem by a variety of avenues. The vu will also open a new method for study ofthe stars-by provid· ing information on the continuum radio emission of many normal stars. Just as radio telescopes have revealed imponant new information about high-energy envelopes of the sun, particularly about the solar corona, the VLA will give us our first opportunity to observe these phenomena in other stars, opening the door to important advances in stellar and plasma physics and perhaps providing clues to unsolved mysteries of the sun itself. Galactic novae have been observed with interferometers. and the VLA will give the detailed evolution of the clouds of plasma and gas ejected violently in the nova outburst. Perhaps emission from Wolf-Rayet, P Cygni, and magnetic stars will be detectable. Prototypes of the VLA have measured the astonishing changes in the emission of x·ray stars in only hours. Nevertheless. the searches for x-ray star radio emission have been panicularly frustrating. contributing little data toward the solution of the enigma of x-ray st.ars. The great im· provement in sensitivity offered by the vu may well remove a barrier to the understanding of these intriguing objects. The VLA will give us for the first time a clear picture of the bean of our galaxy. where there is a complex ensemble of radio-emitting regions, concealed from optical telescopes by the dense dust clouds of the Milky Way. There is evidence that violent events in the nucleus of the galaxy have strongly influenced galatic evolution. Indeed, one object in the center may be the same type of structure that produces the quasar phenomenon. By measuring the radiation of individual radio spectral lines, such as that of atomic hydrogen at 21-cm wavelength, the VLA will be able to give pictures of the gas clouds of our galaxy in such detail that we will see the processes taking place in them; the effects of heating, cooling, and supersonic collisions should all be discernible. The structure of the gas system of nearby galaxies will be sharply defined, testing theories of galactic dynamics and evolution. The vu will be able to distinguish detail in t.h e radio emission of all the planets but Pluto, enabling the temperatures of the planets at various latitudes, seasons, and times of day to be established. The radiation belts of other planets could be measured in detail, and the atmospheric structure and nature of the planetary surface, be it rock. soil, or water· containing material, could be studied. The VLA, and some other radio-astronomy facilities. will require a new site. It is possible that the large steerable dish or millimeter-wave dish could be located in the same area. Site development economies are 

80 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970'o possible in radio astronomy, since the major common requirement for all these innrun\cnu is a large area. free from industrial and radar elex1:rical interference and direct aircraft routes. They all require highly develoP«! technical suppon fOT retth-ers, computers, data analysis. and control. A dry, high -altitude site is preferable for the millimeter-wa•·e dish, although nCM so important for the 01ber devices. Whh 1he program for the vu. which -.;u rome into operation only near the c:nd of this decade. we recomme-nd expansion of research suppon and funding of' moderate·si:zed instruments at university or consot1ium- operated r3dio observatories at a rate of S2.5 million per year. This will permit smaller groups to probe new areas of technology: new concepts in antenna and receiver design, ultra·high·frequency detectors. small millimeter-wave antennas and interferometers, centimeter-wave inter· fcromcters and receivers, adaptable to the new atomic nnd molecular lines disc.'O\'tred, and vcry-fong-bascline interferometric terminals and arrays. A balanced program in radio astronomy requires a variety of less expensh·e racHides and innovative, Oexible research projecrs, in addition to the large national facilhy described. The oosts over ttn years for university facilities would be S2S million, and S62 million for the VLA _ About S6 million per year (10 percent of the capital c»st) will be required to operate t.he vu . The full operating costs " 'ill not oecur until the last half of the deeade. OPTICAL ASTRONOMY-ELECTRONIC TECHNOLOGY AND LIGHT- GATHERING POWER We have witnessed a decade of remarkable discoveries in astronomy, including qua~ars, x-ray s tars. and infrared galaxies. Most of these discoveries resulted from the expansion of astronomy into new regions of the electromagnetic spectrum. but obsen•ations a t visual wavelengths have remained central in astronomy because they provide basic information about di.stancc, mass. temperature, pressure. and chemical composition. Funhcrmore. through comparisons with well-esublishod theories. optical astronomy i.s the basic tool for studying stellar C'\'olution and nucleosyn· or thesis. the: ages stars and clusters. the distances and stellar content of g:ala.xies. and the scale of the unh·erse. Moreover. optical astronomy has provided data that challenge established theories. For eaamplc. r=nt photographic advances have re\'ealed puzzling phenomena in highly distorted galuies. For optical astronomy to fulfill all these roles. we must have te.l acopes co collect the photons and detectors to record them. Progress in astronomy 

The Hlgh·Prlorlry Program 81 has depended heavily on our ability to build larger telescopes and more efficient detectors. Introduction of refracting telescopes more than three centuries ago led gradually to a SOO-fold improvement in angular res· olution and permitted objects to be seen that are 10.000 times fainter than those that could be seen with the eye alone. These refracto"' "'ere adequate for finding new planets and charting the stellar unive"'e in the nearer parts of our Milky Way. but the astronomer was still left with only the memory of his pe=nal visual perception. Photography. beginning about a century ago. brought modern as- tronomy into being. Not only could each astronomer now share his vision with the world. but. equally important. he cou ld extend it to objects a hundred times fainter. due to the ability of photographic emulsions to store light during long exposures. Photography unveiled the extragalactic universe. but the full appreciation of its size and grandeur depended on the parallel development of large reflecting telescopes through a progression culminating in the 200-in. rellecting telescope on Palomar Mountain, with its ability to study objects 10 million times fainter than can be seen with the unaided human eye. This great instrument. after nearly 25 yea"' of use. still serves as the spearhead of world astronomy. It is worth noting that the 200-in. telescope was funded and designed during the presidency of Calvin Coolidge. before the space age and even before the first nuclear accelerators or radio telescopes. Some of the smaller telescopes still in active use in the country are nearly 100 years old. Since there has been only modest improvement in the efficiency of photographic emulsions during the last 50 years. the building of ever· larger telescopes was aimed almost entirely toward collecting more light. The cost of conventional telescopes increases nearly with the cube of the aperture, making this an expensive. although necessary. pursuit. Con· sequently, astronomers began to investigate techniques that would detect photons more effectively than the photographic plate. which nt best can record I out of every 100 photons collected by the telescope. The in· troduction of photomultiplie"' with quantum efliciencies up to 25 percent was a major improvement. but they were limited to view a single resolution element of an image at a time. Detectors were needed that would combine the high sensitivity of the photocathode with the ability of the photograph to record all parts ofa large two-dimensional picture at the same time. The first objective has been accomplished in the last few yea"' by developments that include (I) image intensifiers in which photoelectrons , from a cathode excite a phosphor screen that is then photographed. (2) eleetronographlc cameras in which the photoelectrons strike a photo- graphic emulsion directly. and (3) integrating television cameras in which the photoelectrons are stored in a target that con be read out with 

82 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's an electron beam. These techniques have in tum pointed to ultimate systems that will count individual photoelectrons focused onto a two- dimensional array of sensitive elements. In some of these systems. as the data are obtained, they can be read into a computer for immediate processing so that the astronomer can watch the image build and optimize the exposure. 11>e impact of these developments on astronomy has been enormous. In many situations they render present telescopes up to 25 times more effective than before. This is equivalent to scaling each existing 40·in. telescope into a 200-in. and the 200-in. into a 1000-in. If a 1000-in. telescope cou ld be built. it would cost S2 bill ion: the replacement cost of the 200-in. is now near $25 million. The equivalent cost of such a fivefold transformation, assuming it could be done in the old way by actually rebuilding existing telescopes, would be a t least SS billion, whereas the cost of equipping all major American telescopes with such devices will be much less than I percent of this. These factors amply account for the unanimity of astronomers in giving high priority to the development of these electrooptical detectors and their installation on large telescopes. Additional improvements can come from the more efficient use of telescope time through various controls for automatic setting and guiding and television cameras for finding and tracking objects too faint (or too red) for the eye alone. At present. work on invisible objects requires the time-consuming procedure of offsetting the telescope from objects that can be seen. The major effecl of the new detectors will not be to observe the same objecls in shorter time but rather to study much fainter objects and to use higher spectral resolution. This will permit critical investigations not thought possible 10 years ago, such as analyzing individual stars in nearby galaxies for element abundances. studying the absorption lines in the faintest quasars. and measuring red shifts of the most distant galaxies. However, even with these impressive advances in detectors and controls, we still need more large telescopes. Some of our major reflectors are near growing urban areas whose lights make the sky too bright for work on the fainter objects. and even the Palomar telescopes are already threatened. While we make all possible elfons to improve the efficiency of present telescopes. we must also build new ones at safe dark sites where there is good seeing. The cost of a ,·ery large single-mirror instrument is so high that we recommend experiments with the concept of an optical telescope array. In order to achieve a large coUecting area at a moderate cost. initial efforts should be directed toward developing a multiple-mirror telescope with either an array of mirrors on a common mount or a system of separate telescopes feeding the same detector. If prototype tests prove 

The HI,M'rlcr/1)1 Pto,.m 8J these concepts feasible. an operating telescope of high optical quality equivalent In area to a ISO. or ~in. dlould be buill, follcr..,d by !he daip and con.wvction or • much lafJCf system in !he 4()(). to 60Q.in. dass. if uperience ,.;lh the smaller one iodieates that the next step will succeed. Ho,.ever. If the multiple-mirror telescope don not fulfill ex· pectations, another conventional reflector of the 200·in. class should be built as soon as possible. While the multiple system is being designed and tested. we must proceed with the construction of at least one standard telcscope 90 in. or larger. at a dark site. In order to begin to compensate for those in· struments that no longer can be used on the faintest objects because of lhe lights from e•pandlng cities. Funding of at least SIO million will be needed for the development of the new elcctrooptkal detectors and installation of the bc>l •ystems on all major U.S. telescopes. There are at least nine e•i>tina telescopes large enough to use one or more of lhese detectors profitably. three more under construction. and three proposed. Outfitting these telescopes with tel· evision cameras and automatic controls for serting and guiding as ,.ell as with small computers for immediate data reduction ,.;11 cost anolher SS million. An operatina multimirror telescope equivalent to a ISO. to 200-in. single mirror is estimated to cost about S.S million. Further funding up to S25 million should then be provided to build the largest possible telescope within that budget-ither a multiple-mirror one with an elfective aperture of 400 to 600 in. if the concept proves to be feasible or a con· ventional 200-in. telescope. An additional SS million is for the urgently needed intcrn>ediatc·sizcd telescope at a dnrk site. The well -rounded program in optical astronomy requires (I) advanced sensors and controls-S IS million. (2) test of array concept- SS million. (3) a 100-in.·class telescope-55 million. (4) construction of a large optical array or another 200-in.·dass telescope-S2S million. Operatina costs for the new optical facUlties ,.ill reach S3..S million per year by the end of !he decade. INFRARED ASTRONOMY Although Herschel detected infrared radiation from the sun "ilh a thermometer more than 170 years ago. it is only in the past decade that infrared observations have become important to the mainstream of uuonomkal research. Only recently have solid-state and low·ttmperaturo technotoaies developed to the point where available infrared detectors are 

84 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's sensitive enough to study objects other than the sun in any detail. Low- temperature techniques are especially important, because the earth's atmosphere and the telescope are strong sources of background radiation in the infrared and are thus seen by the detector. Infrared detectors must be cooled. often to temperatures as low as 2 K. Ideally, the entire telescope should also be cooled and then lifted into space to avoid contamination by atmospheric radiation. Going high in the atmoshere or into space would also extend the available range of wavelengths, because water vapor makes the atmosphere opaque in large portions of the infrared region of the spectrum. Unlike ultraviolet or x-ray astronomy, which can be con· ducted only from space. some infrared astronomy can be carried out through the atmosphere by large ground-based telescopes. At other wavelengths. the absorption by water vapor. if not the background radiation. can be overcome by observing from an aircraft or balloon above the tropopause. The infrared has great potential for astronomical research. This part of the spectrum begins at the long-wavelength end of the visible spectrum. at about ll'fll. and stretches over a range of more than ten octaves to about I mm. where it overlaps the short-wavelength end of the radio region of the spectrum. Within this range lies the characteristic blackbody radiation of the moon and planets, cool stars. and prestellar clouds. as well as the background radiation of the expanding universe. The infrared is useful for observing any object with a temperature between 3 and JOOO K. The infrared is the realm of molecular spectroscopy. the range wherein lie the vibrational-rotational bands and lines of many cosmically im- portant molecules. Theoretical studies of the interstellar medium also indicate that many of the important heating and cooling mechanisms involve infrared radiations from atoms and ions. But as always. it is the unexpected and surprising that is the most in- teresting. Photometric studies aimed initially at improving temperature and luminosity determinations for cool stars led to the discovery of excess infrared radiation from circumstellar dust shells. A ground-based sky survey found some enormously luminous "infrared stars" that are barely detectable with optical telescopes. Exploratory observations of peculiar galaxies and qua.sars in the near infrared soon led to the realization that some of these objects emit more energy in the infrared than in all other wavelength regions combined. an unexpected and still unexplained result. Rocket observations of the cosmic background radiation. initiated mainly as a check on what had already been learned in the radio region of the spectrum, found a much greater flux than had been expected. and the resolution of the discrepancy may have profound implications for cosmology. 

Thelllgh·Priorlty Program 85 The new technology and the new exciting problems uncovered attract a large number of astronomers. particularly young experimenters. into the field . We recommend ex.pansion of support for this vigorous activity in all areas. including development programs for more sensitive detectors. exploration of new high-altitude dry sites for infra~ telescopes. and exploitation of multiplex spectroscopic techniques. as well as increased funding of ongoing ground -based. airborne. and rocket programs. So much has been done with so little money (les.s than S2 million per year) that a large payoff is almost sure to follow from n doubling of this effort. As port of this expansion, we recommend an imnuxliate start on a program of surveying the sky for objects bright in the far infrared. This is extremely important for understanding the nature of exploding galaxies and may uncover new and unexpected phenomena. The first step. a balloon survey down to a relatively bright limit. can be done immediately for less than 5200.000. We also foresee the future need for a telescope with a large collecting area and high angular resolution in the far infrared. Such an instrument must of necessity operate in the stratosphere. and we recommend that a design study be initiated soon to determine the most suitable and econo- mic platform. The growth of infrared astronomy is creating large demands on existing telescopes. most of which are neither at the best sites nor optimally designed for infrared work. We therefore recommend as one item in the i.ncreased infrared program. construction of moderate-sized infrared tele- scopes. particularly in the southern hemisphere. We also recommend con- struction of a large (3 to 4 m) infrared telescope (at a cost ofSS million) at the best available high-altitude site in the northern hcmi.sphcre. Such a combined program of ground ·based, airborne. and rocket in- frared astronomy is sure to lead to many exciting discoveries in this new and expanding field . The total budget is estimated to be S25 million. HIG H-ENERGY ASTRONOMICAL PROGRA M During the first half of the last decade. the total "observing time" in x-ray astronomy had accumulated only to about one hour. through many rocket flights. During that hour it had become apparent that the • · ray sky is extraordinarily rich in new phenomena, and that vast and vital aspects of many optical and radio objects had not been appreciated from observa- tions in those wavelengths. The Crab nebula is not only one of the brightest objects in the x-ray sky. 

86 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'1 but it is also extraordinarily complex. A s1eady x··ray glow is emitted by electrons spiraling in tho magnetic fidds of tho nobula. Pulm! x rays aro emitted from the pulsar created in the spectacular supernova explosion of A.. D. 1054. one of only two radio pulsars known to emit x rays. 11:te x-ray spectNm ex·tends up into the gamma-ray ftgion. &c><pius X· l. the bright.,t x·ray object most of the lime. is auociated whh a blue starlike object with strong optical emission lines. X rays are emitted from a hot plasma in the vicinity of the blue object whose nature n:mains a mystery. It appears likely thai many of the celestial x-ray sources in our galaxy are generally similar to Sco X-1. Occasionally, a new x-ray source appears in the sky, is more: intense than Sco X-I for a few months. then declines until it is no longer detect- able. We do not have good enough position measurements of these sources to attempt to identify them with optical objects. One of the first major discoveries of the Uhuru x·ray satellite has been " new class of x·ray sources that undergo regular (pulsarlike) and irregular fluctuations on a rime scale between 0. 1 and 10 sec. No optical identifications are yet available. Many unusual galaxies are X·ray sources. These include strong radio galaxios (M87J. quasars (3C273). Seyfen galaxies. and ordinary galaxios (Jhe Magellanie Clouds shaa· a c:omplex x-ray structure~ Tromendous amounts or energy are rele.ased in the lt·ray reeion in some or these souru.. po5ing serious challenges to our understanding of high-a~ergy I.Sirophysic:s. Underlying all th""' sources is a diffu>e x·ray glow that appears to be featureless. Many astronomers believe that the background x rays were created far away and long ago in the early cosmological history of our universe. This brief and incomplete list of important discoveries in x·ray at· tronomy is reminiscent of the early exciting years of radio a.scronomy. A wide range of new phenomena had been found. but understanding of these phenomena was minimal. The search for understanding required much larger instruments. new techniques. bener detectors. better spectral coverage or the sources. polarization measurements. and the ability to repeac observations for variability. a common featu~ of ··compact" objects. A similar pattern of devdopment is needed in a:·ray astronomy. Much J.arger·an:a dctecton than have been flown are required in order to find and study faint sources. For the lower-<nergy x rays. focusing optical techniques, involving gruing·incidence instruments. should be Down. The>e will allow detailed pic:turos with high angular rosolution to be obtained. Thoy will also act as photon collectors. concentrating • ·ray photons from weak sources on Bragg crystal spccnomcten and on 

The High-Priority Program 81 polarimeters so that the detailed spectral properties of the sources can be measured. Because tbe detectors used with focusing optics can be made very small. the unwanted detector background counting rate can be greatly reduced, facilitating measurements of extended sources and of the apparently isotropic x·ray background. With this major instrumentation. very large numbers of x-ray sources should be discovered. Many new examples of the various classes of x-ray sources in our galaxy should be found. so that the full range of properties of these sources can be studied. Positional determinations of these sources should be greatly improved. thus allowing large numbers of them to be identified with optical objects. With the resulting ability to study the sources in many different wavelength ranges, our theoretical understand· ing of the character and structure of the sources should improve rapidly. Of great importance will be the ability to point at x·ray sources steadily for hours at a time. Not only will this allow a major improvement in the statistics of tbe spectral measurements. but it will also permit studies of the time variations of the total x·ray emission and of individual spectral features. One of the principal striking characteristics of the galactic •·ray sources that have so far been found has been the temporal variability of tbe x-ray Dux. ranging from rapid Ouctuations to long·term changes. This characteristic is more frequently found in x-ray sources than in optical and radio sources. The major instrumentation should also have extreme importance for studies of extragalactic x-ray sources. It should permit detection of in· dividual sources in nearby galaxies and of emission from active galaxies and quasars to very great depths in space. More definitive measurements of hot plasma concentrated in clusters of galaxies will be possible. allowing a determination of whether sufficient masses of such plasma exist in the clusters to bind the galaxies gravitationally. Much more definitive measurements of the spectrum and isotropy (or lack of isotropy) of the background x rays will improve our understanding of the cosmology and early history of our universe. The National Aeronautics and Space Administration ti<ASAl has recogniud the richness and promise of this field of research by requesting congressional authorization for two large rotating High Energy As· tronomical Observatories ( KEAO 's). These are to be large spacecraft in orbit about the earth, slowly rotating so that the instruments scan across tbe sky. These will be survey spacecraft. with a large collecting area in· tended to discover new faint x·ray sources. to measure their positions accurately. and to measure spectral properties. Combined with the x·ray instrumentation would be gamma-ray and cosmic-ray instruments. The spacecraft will play an essential role in the future of astronomy. X· ray astronomy will increasingly become a partner to ortical and radio 

88 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970'1 astronomy as more J[-ray sources are identified and their propc:nies a~ correlated with thost in other wavelength band$. h is possible that some typt:s of x-ray source may ne\-er be optically identified. in -...hich case •--e • ·ill be entlrt-ly dependent on H £AO techniques to s rudy them. NAS pla nning also calls for two pointable HEAO 's. 1nc:sc will be el-cn A more imponant to the future or x-ray astronomy chan the rotating HEAO 's. They will permit short-time· scale Ouctualions in intensity to be followed continuously and to be correlated whh s imultaneous optical. radio, and perhaps infrared observations from the ground. They will take ttdvanttlge of focusing x-ray optics to concentrate the x·ray photons onto small detectors. where background problems can be reduced and angular structural information and positions can be obtained with high accuracy. 11 ls important that NASA also seek authorb·.alion for the pointable 1-H!.AO 's as soon as possible. in order that there not be too g.rcat a time delay berween the discovery of new x-ray objects by the first rotating H EAO and Ihe del ailed study of them by the fi"'t pointable II EAO. A measure of the importance attached to x-ray astronomy by astronomers is that they have scheduled large blocks of lime on major optical instruments to exploit the discoveries and positional measu~ments of new x-ray sources by the UhuN x-ray satellite. This rdl<tt~ their expectation that a number of optical identifications will be possible of the newlydiscoVtted x-ray sour=. If this is the case. the HEAO program will make lar~ demands on optical astrOnomy and probably also on infrared astronomy. There should be an expansion in major optical facilities to satisfY the requirements of x·ray astronomy. Extragalactic objects in which a major portion of the energy emi.ssJon i.s in the infra.red are also proving to be x·ray objects: it is possible that a similar correlation may exist among some classes of galacttc x·ray objecu. Thu.s an expansion in infrared facilities may also be required for support or X ·ray astronomy. The high·encrgy astronomical program given extremely high priority by the Committee includes the four HE-AO 's in the NASA planning program. two rotating and two pointed. together with an associated expansion in oplical and infrared facilities to provide the ground support required for lhe development of x-ray astronomy. The esdmated rost of the four HEAO missions is SJ80 million. In ad· dition. at least one intermediate·sized optical celescope to support the program should be constructed at a eos1 of SS million. MILLIMETER-WAVE ANTENNA One of the dramatic discoveries of the recent past was the detccdon in tbe clouds of interstellar space of an astonishing variety of molecular spc:ctes. 

The High-Priority Program 89 The- e findings contradicted our expectations that the formation of such s molecules was a rare event and that their destruction was rapid bec.ause of the flood of ultraviolet light in the galaxy. The species found range from the sma11. diatomic molecules. such as CO. CS. and CN, to such complex substances as cyanoacetylene. methyl alcohol. formaldehyde. and formam ide. containing as many as six atoms. Carbon monoxide is present in an abundance some thousand times greater than other molecules. probably reflecting lts resis~ance to dissociation by ultraviolet light. The molecules of greatest abundance are those found in our laboratories to form the basic constituents of biochemical systems. For instance. formaldehyde is a precursor of both amino acids and sugars in experiments simulating conditions on t he primitive earth. Thus the molecules observed seem to indicate that the chemistry of life on earth is closely paratteled in interstellar space. The diatomic molecules are almost always best observed at relatively short radio wavelengths of a few millimeters. They form the basic building blocks for the larger molecules, and the physical interpretation of their spectra is much simpler than for the larger molecules. The larger molecules have great significance. however. since they often possess a rich spectrum. both at cent-imeter and millimeter wa\'elengths, and form a particularly powerful tool for probing the physical conditions in the interstellar medium. High resolution is necessary to define the distribution of the molecules from which the-modes oflheir formation and destruction can be studied. High sensitivity is necessary to discO\·er large molecules. which may have low abundances, and other low-abundance substances such as molecules containing rare isotopes. High resolution and high sensitivity require a very large stecrablc tele- scope with a very precise reflecting surface. Such a telescope has many other important uses. particularly for the study of variations of quasar spectra and intensities and planetary emissions. Such a telescope is not easy to build, because it must n1aintain its geometry to accuracies of tenths of millimeters under the influence of changing gravity forces, wind. and thermal stresses. A great deal of research has been carried out at the National Radio Astronomy Ob- servatory on such precise and stable telescopes. A new approach to telescope design, called the "homology telescope"' has b<-.:n developed. which appears capable of attaining the desired performance. Indeed. some of the principles of this approach have been applied successfully in the new 100-m radio telescope of t he Ma.x Planck lnstitut fiir Radio-- astronomie in Gennany. The very large radio telescope recommended for observations a t milli- meter wavelengths would very likely be a fully steerable parabolic reflector with an aperture of 215ft. performing satisfactorily at wavelengths of 3 

90 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'1 mm and longer. The cost of this instrument is not as well determined as that of the vu but is estimated to be SIO million. The ronstruction of this telescope will provide a major capability in a particularly promising area of astronomical research and will capitalize on our receiver technology. momentum, and d<sign capabilities in a field developed in the United States and in which the rountry is pre-eminent. AIRCRAFT, BALLOONS, AND ROCKETS An essential part of space research is carried out using small vehicles- aircraft, balloons, and rockets. They are relatively inexpensive and ideally suited for programs of observation with specialized instrumentation where a few minutes or hours of data-taking will accomplish the research ob· jective. They have also been essential for testing astronomical in- strumentation for use in space. These vehicles have proved invaluable in the past; their utility in the future is assured by t.h e steadily increasing requirements for their use. At a time of severe fiscal ronstraints, the reduction of the number and variety of large astronomical missions in space can, in part, be balanced by the initiation of much less costly programs utilizing small vehicles. These may be able to carry out some of the research contemplated in the abandoned missions, thus maintaining a degree of flexibility and vitality in the affected field of research. The scientifically sensible rourse of action is to increase funding for aircraft, balloons, and rockets when fewer major satellite experiments are planned. If satellite programs are increased, an accompanying increase in rocket research, with smaller but innovative goals, will lead to optimum satellite design and therefore be of high value. Until recently, x-ray astronomy depended entirely upon rocket research. The x-ray sources were discovered by rockets. and quite ac- curate positions were measured for some of them with ingenious rocket instrumentation. Rocket measurements made duri. g a lunar eclipse of the n Crab nebula revealed that the x rays were not a point source. At the present time, rockets are proving essential to the further study of some x· ray phenomena discovered by the UhuTV x-ray satellite. Unexpectedly rapid x-ray fluctuations of the Cyg X-1 source were discovered utilizing the satellite, but since the satellite rotates, it is not suitable for following the fluctuations. Rockets are capable of pointing at a source like this for several minutes at a time, and missions can now be Instrumented to provide the data essential to a better understanding of Cyg X-1. Since this source appears to be but one of several classes of strange x-ray objects, it is clear that there will be a pressing need for more x-ray astronomy 

The Higlt·Prlorfty Program 91 rockets for the next several years- and most certainly through the era of the Kigh Energy As1ronomical Observatories. Ultraviolet astronomy also began with rockets, first for studies of the sun and then for studies of the stars. Differences were found between theoretically calculated ultraviolet stellar spectra and the rocket ob· servations. Rapid rates of mass loss from hot supergiant stars were discovered by spectroscopic observation in the ultraviolet. Perhaps one of the most important of the ultraviolet astronomical discoveries was that of molecular hydrogen in interstellar space. Today the bulk of the ultraviolet astronomical observations are carried out with an Orbiting Astronomical Observatory. but the instrumentation in this vehicle is relatively infiexible, even though it returns a great amount of data. It is necessary to sup· plement and enrich these data with selective rocket measurements using a wider range of instrumentation. The loss of OAO·B has been a severe setback for ultraviolet astronomy. The authorized program will conclude with the launching of OAO·C in fiscal year 1973. For many years, the program of ultraviolet astronomy from spacecraft is likely to be modest even if new satellites such as the proposed SAS· D are authorized. In these circumstances, it will be all the more important that a supplementary program of rocket observations in the ultraviolet be provided to maintain vigor in this field of research. The instruments carried in these rockets may provide some of the measurements that would have been made by OAO· B. They also will provide an opportunity to exploit the discoveries made by OAO·A and OAO.C and will provide an important survey of certain classes of ultraviolet phenomena. There will undoubtedly be many celestial objects found in these ultraviolet studies that will turn out to pose important scientific puzzles. many of which can be further studied and elucidated by resea.rch using rockets. Infrared astronomy now relies heavily upon aircraft and balloons. While a few infrared windows can be exploited from the ground, most of the wavelength region, and especially the far infrared, requires an ob· serving platform above the bulk of the atmospheric water vapor. Observations from balloons and aircraft have given important new spectroscOpic information in the infrared about the sun and planetary atmospheres. Observations from aircraft have detected high Ouxes of radiation in the infrared from the cores of active galaxies and quasars. Large numbers of strong infrared sources near the center of the galaxy have been discovered during surveys made from aircraft and balloons. NASA is providing an aircraft platform for a 36-in. infrared telescope, which should produce important new results. The Committee recom· mends that a first, crude, long-wavelength infrared sky survey be carried 

92 ASTRONOMY AND ASTROPIIYSJCS I' OR Till! 1910 's out from balloons in the near furure. In the longer·range future. a deep- sky SUf\'ey in the infrared will probably require satellite techniques, but these will require a prior ro<ket de..elopment program. Hen~. in&aml astronomy will be a major user of aircraft. balloons, and rockets in the next few )'tars. Solar rescareh has been heavily dependent on ro<kets as well as on satellites in the Orbiting Solar Observatory series. These have produ~ detailed ultraviolet spectra and x-ray pictures. They have been Oown on command at times of solar Oares. There is a continuins need to sup- plemenc che sacellite coverage or che sun with special, Oexible. quick· response rocket instrumentation. Thus essentially all che major are.as of space astronomy have an ex· panding need f'or small researeh vehicles: aircraft, balloons, or rockets. The expenditure on these research vehicles for astronomical research presenlly amounts 10$12 million to SIJ m illion per year. The Committee slrongly recommends that the expend iture for chis type or researeh be doubled os rapidly as possible. cenainly within the next ch~ years. SOLAR PROGRAM The Opening up of the extreme ultraviolet and X•ray region of the solar spectrum by ro<ket and satellite observations has pro¥ided many im· ponant new advances in solar research in the lase decade. l.n this region or the spectrum occur the dominant emissions from the solar corona, where mechanical energy, generated in the solar outer convection zone, is deposited both in the form of steady heating and in violenl even IS such as solar nares. Apan from tcoching us more about eoronal heating and the origin of flares and cosmic rays. euv and x-ray observations of the sun, as the brlghcest astronomical object, also play a role in leading the way to the understanding of similar observations elsewhere in the universe. The Orbiting Solar Observatory tOSOI program was started in the beginning or the last decade. The oso 's provide • platform ror studying both raptd events and slow variations of radiation over time intervals up to one year. There has been steady improvement in the capabilicies or these sacellites. Early oso 's bad vinually no spatial resolution and carried only small payloads. Rapid tecllnologi<:al development will ma.k e it possible for the eighth oso . to be Oown in 1973, to cany instrumeniS that attxin a spacial resolution or - 1 sec or are. comparable with chat obcained with the better around-based telescopes. This proaram or continuous development and gradual impro¥ement has J 

The High·Priority Program 93 made the oso program among the most successful and productive of all astronomical satellite programs. We recommend the continuation of this program beyond the present oso series. through oSO·L, . ., , and ·N (at a cost of S30 million each), to be Oown during the next solar maximum (1977-1931 ). These oso 'swill probably provide for the first time a spatial resolution equal to or better than that of the very best observations ob- tained from the ground or balloons. This improved spatial resolution is of utmost importance, since we know from ground-based observations that the energy transfer to the chromosphere, to flares and cosmic rays, and perhaps to the corona, occurs on scales probably less than or equal to 1 sec of arc. oso. L., ·M, and ·N will fly during the next period of maximum solar activity, with a spatial resolution 10 to SO times better than was possible in the last period. They will carry instruments capable of analyzing the properties of flares and active regions in the spectral region from 3000 A down to the very energetic x rays below 0.1 A. It is entirely reasonable to expect that these observations will result in a significant increase of our understanding of the layers of the sun above the photosphere, of solar activity, and of solar Hares. We envisage this continued oso program, together with the expanded solar rocket program discussed in the space astronomy recommendation, as the bac.kbone of the solar space program. It is of the greatest im· portance, however, that improved observations from space go hand in hand with the improvement and extension of observations from the ground. The solar photosphere, best observed in visible and near infrared radiaHon, reveaJs most of the sources of the energy input in the chromosphere. and eorona in the form of granulation, magnetic structures, and mechanical motions. Coronagraphs, eclipse experiments, anticipated observations of far infrared recombination lines, and radio observations provide relatively inexpensive ways to observe other aspects of the sun's upper atmosphere. We therefore recommend the continuous updating of existing ground·based and aircraft facilities and the construction of small specialized telescopes for the visible and infrared spectral regions (at a cost of approximately Sl.O million per year). This updating includes improved image detection, storage, and analysis, as well as improvement of image quality by telescope refinement and site selection. For the study of the interaction of solar·Oare plasma with the magnetic field and plasma of the outer solar corona, we suggest the construction of a relatively inexpensive multifrequency metric and decametric radioheliograph with moderate (I -5 min of arc) spatial resolution (at a cost of approximately $1.5 million). The cost of the program over the next decade will be S90 million for oso.L. ·M. and .Nand SIO.Omillion for ground-based facilities. 

94 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o THEORETICAL ASTROPHYSICS AND COMPUTING REQUIREMENTS Physical theory has always played a crucial role in astronomy-from the period when Newton's theory of gravitation provided the uplanation of planetary orbits to the present time. when nuclear reattion theory promises to el<plain the synthesis of chemical elements in supernova explosions. Any balanced program for progress in astronomy wiU necessarily contain a vital, if relatively inexpensive, program of theoretical research. Much theoretical astrophysics today is concerned with model building. In this type of activity, physical principles substantiated in the laboratory, including those of quantum theory, nuclear physics, and plasma physics, are used to construct a mathematical model of an observable astronomical object, such as a star, a galaxy, or even the whole universe. The relevant equations are. usually complex and nonHnear and must be solved on a computer. The resulting models are then compared with observations to fix parameters of the model, such as the mass of the star or the random velocities of stars in a galaxy, and to show how the model should be im- proved to attain agreement with observations. Model building is essen- tially the only way known to convert the stream of photons entering a telescope into a physical picture of what is going on. The theoretical astrophysicist thus stands astride physics and as· tronomy. Oose contact with physkists is essential if current developments there are to be properly included in the model. Constant interaction with observen is essential if theoretical work is to be aimed in the most productive directions for interpreting nature and if observational work is to be focused on the most theoretically significant questions. In the recent past there has been increasing exploration of dynamic states. The theory of stellar evolution can be largely constructed from a sequence of static stellar models, but in the final stage of a star's life-in some ways the most interesting one-events occur very rapidly, with gravitational collapse and outgoing shock waves playing a vital role. To reconstruct these phenomena, it is vital to simulate the dynamics in a computer. Dynamical modeling is playing an ever-increasing role, from stellar explosions to interstellar shock waves to the spiral structure of galuies. Such modeling is orders of magnitude more time-consuming than static modeling, so fasttr computers with larger memories are required. A prime example of the success of this apptoKh is the modeling of a supernova el<plosion, in which the progress of a shock wave is followed in detail, and a netWOrk of about 100 nuclear reactions is followed at each time step. The result is a prediction of the abundances of the chemical elements. which seems to agree remarkably well with observation. 

The High-1+/orlty Ptogram 95 A related activity is theoretical work in dynamical astronomy-the application of Newton's equations of motion (with small relativistic corrections) to the positions of planets and satellites of the solar system. Here the problem is to compute the orbits using interactions between all bodies to extract precise values for the parameters of the system, including the masses of the bodies involved. Recently, such work has demonstrated its vitality by providing extremely ae<:urate motions of the earth for use in reduction of optical observations of pulsars. Without these precise positions (about J0·8 of the distance to the sun), it would have been im· possible to utilize the precise optical timing measurements, which require correction for light-travel time within the solar system. It would thus have been impossible to infer the existence of abrupt changes in the period of the Crab pulsar, which have been interpreted as due to starquakes in the crust of a neutron star. Such is the unity of astronomy, of the old and the new. We believe that increasing the effort in the universities, where there is strong interaction of theoretical astrophysicists with both observers and physicists, is the best way to optimize results in theoretical research. We suggest particular emphasis on relativistic astrophysics, stellar evolution (particularly early and late phases), derivation of physical data needed to construct precise stellar models (including opacity sources, nuclear·energy generation rates, convection theory, and equations of state), and theoretical interstellar physics and chemistry (including the solid-state theory of grains, molecular and atomic cross sections and transition probabilities, the theory of masers, and the plasma physics of interstellar gas and magnetic fields). Interaction between relatively isolated theoretical groups should be increased wherever possible, for example, between groups working on stellar interiors, stellar atmospheres, and observational stellar spec.. troscopy, between plasma theorists and astrophysicists working on stellar and interstellar plasma processes, and between chemists and astronomers working on molecular astronomy. Support should be increased for both theoretical and experimental study of atomic and nuclear collis-ion cross sections and transition probabilities, taking care to locate this work in several independent groups to increase the effectiveness of cross checking. By and large, this can be accomplished by supporting physicists in universities where there is an active astrophysics group that can be helpful in establishing priorities for experimentation and calculation. We recommend that in the specific areas of beam-foil spectroscopy and low-energy nuclear cross sections, the U.S. Atomic Energy Commission lAEC) consider support of groups utilizing existing facilities for this work. Funds are needed for individual university investigators to increase 

96 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970's their efforts using suclt university computers as are available. The fund.s available for computation generally need to be increased. Theoretieal astrophysicists and dynamical astronomers are moving into an era when the maximum speed and storage capacity available will be needed to solve dynamical problems, but many university and national center computers are not equal to this task; selected ones should be upgraded. In addition, state-of-the-art computers in mission-oriented agencies such as the AEC and I<ASA would be extremely useful if means for using them part-time can be worked out. The additional funds needed for first-rate activity in this area are not trivial-perhaps SS million per year. The theoretical etfort at the national observatories needs to be fostered. Research output would be optimized by increasing the availability of theoreticians at the national centers. To succeed, it is essential to find highly quali6ed versatile individuals as visitors or on the staff. Such a goal involves enhancing the computer facilities, as required, to make the observatory attractive both to resident and visiting theorists. Joint activities between physics and astronomy programs in universities should be encouraged. Because of the close relationship of theoretical ast.rophysies to both physics and observational astronomy. productivity is served by every possible mode of cooperation. including. in some cases. merged departments, joint academic programs. and shared facilities. It is most important that astronomy PhD students receive as thorough training as possible in physics. and to this end. special seminars should be designed. A National Institute of Theoretical Astrophysics has been suggested. to provide a focus for theoretical research, to promote interchange between astrophysicists from different suhfields and between astrophysicists and other scientists, and to provide a stimulating atmosphere for postdoctoral fellows before they accept permanent appointments. A proposal by the Panel on Theoretical Astronomy would fund an institute at an annual rate of approximately $750.000 for a fixed period of seven years. The institute would have some six permanent statf members. with an outstanding scientist as director. and would be located in an anractive place close to a researclt university and close to a group of observational astronomers. There would be particular emphasis on p015tdoctoral and visiting ap- pointments. and in keeping with the need to keep administrative and other expenses low. the support statf and computarional facilities would be strictly limited . The Committee concurs with the panel in the thrust of its recom- mendation for an institute. Nevertheless. it believes that for both pragmatic and historical reasons. the main strength of theoretical astrophysics is likely to remain in the universities. There It can have the 

The High-Priority Program 97 greatest impa£1 on the educational process and on young men from a wide diversity of backgrounds and fields of interest. The institute. if it is set up. should strengthen. not compete with. university groups. Emphasis on interaction b«ween groups. on funding of young people. and on a moderate budget. whi<:h will suffice if the staff and computer facilities are limited, is consistent with this goal. We recommend. to this end, that if the institute postdoctoral fellowship program is established. it be used also for purposes not immediately related to a"endance at the institute. including travel funds for visits to other institutions and the cost of computing at home institutions or other facilities. While there are advantages in such a permanent institute. we recom- mend that, as a first step. consideration be given to smaller funding for a summer institute. Such an Institute would have no permanent staff beyond the director and would occupy rented space at one of a number of possible sites that may prove attractive. No computation facilities would be provided; the entire funds beyond rental and minimal administrative expen.ses would be expended on travel and subsistence for a few senior and a larger number of junior people. We believe that the final plans for a possible permanent institute would be beneficially affected by one or two years• experience with such a summer institute. Both the Theoretical Astrophysics Panel and the Commi"ee wrestled at length with a problem that theoretical astrophysicists, along with others in all areas of theory, now face in their needs for a very large computer. Our conclusion may be viewed as suggesting something for evel')-one. We are probably in a state of transition from a stage in which large general· purpose university centers were optimum to a stage when the needs of many different research groups will share much larger computers through sophisticated data-communication links. We understand that quantum chemists have considered a national center with high -power computers. comprehensive software library. and staff of computer-oriented theoretical chemists. able to do large-scale service-type calculations for others. The needs of the Global Atmospheric Research Program suggest that an international network of large computers would be desirable. It will ultimately be necessary for scientists to assess these requirements and discuss the problems of a national computing system. making maximum use of facilities already in place, or needed, for calculations in industry. the space program, weather forecasting. and reactor design, among othen. The needs of astronomy should be considered when such an over- all national computing system is discussed . Theoretical astrophysics is a growing field rhat aruacts young astronomers and physicists with a broad range of interests. The speed of modern computers makes it possible to construct models of atoms. stars. 

98 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970's and galaxies and to study the dynamics of the solar system or the universe. The tools of the theoretician, excq>t for the large computers. are inex- pensive. The pa!!C1'n for the bes! range of computing racili!ies. national and local. muse still be •'Orked out. We recommend an inCTeased program of abouc SJ million a year. For the theoretician. travel. co make new contacts and co anend summer institutes. performs a spcc:ial function. Interdisciplinary research is particularly elfec!h'O and nor erpensive. Theoreticians ean work at small institutions. often at colleges or u_ ivcrs1lies without large facilities. n OPTICAL SPACE ASTRONOMY-LEADING TO THE LARGE SPACE TELESCOPE Some of the- most far·reaching additions to our kn~·ledge of the universe occurred during the first half of this century with the development of asuonomkal speetroscopy and its utilization with large telescopes. During thls time, spec!roscopic analysis of planetary atmospheres. the sun, the stan. and the intersce11ar medium brought about clarifications in our understanding or these objects. Of equal significance was the spee· troscopic StUdy or extC1'nal galaxies, leading to the discovery Of the in- CTCUO of Spec!r()SCOpic red shift with distance and the realiurion that we live in an exptnding universe. Throughout this development. ucronomers have been acutely conscious of the fact that their analyses ..'eft inromplete and tentative. since much of the information that they would have liked to have obtained was in the inaccessible ultraviolet r-ange of wavelengths. The mlssing spectroscopic information oonsists of two classes: one is the spectral lines in the ultraviolet due to elements and stages or ionization of elements that do not have lines in the visible region of the spectrum: the other is the general shape of the spectrum ln the ultraviolet and the relation of this to the distribution of emitted energy In the visible and Infrared wavelength regions. Ultraviolet observations c.an be made only above the atmosphere. During the last IS years, the technological barriers against such ob- servations have progressively been broken. Rockel obsC1'Vations of the sun and the stars have resulted in a numb« of important discoveries con· cerning the ultraviolet spec!rum of the brightest objects risible in space. At the same time. the discovery of quasars. some of them with large Spec!r()Scopic red shifts. has pi'OYided a means .. •hereby the ulttariol<t emission frun a limited doss of objects can be studied frun the ground. because the light originally emitted in the ultraviolet has been red-shifted into the risible region of the spec!rum. 

Tilt H/tlt-I'Worlty Pro,.m 99 ll«au• obj«ts emitting ultraviold lipt are also likely 10 emil visible li&lll. il has not been expected thai completely n..., classes of obj«ts would be di-ed. NevertMlas. there ha... been a number of important discoveries made concunlng IM properties in IM ultraviolet of some of the obj«ts that had previously been studied in tM viJiblt: I. 'The ultraviolet resonance lints in oa-tain early·lype nellar aianiS have shown that manor is 01reaming out from lheoe stan with velocities of the order of 1000 km per sec. wilh total mass loss raltS of the order of to·' solar mass per year. 2. 'The extinction of ultraviolet light by the interstellar medium has turned out to be dllferent from that predicted on the basis of observations made In the visual region. There is a prominent absorption feature ncar 2200 J. and a gradual increase in the extinction toward shorter wavelenat)ls. These results are leading to extensive m-\sions of our ideas concemina tM character of interstellar grains. and the prest!KC of considerable variations of these features In difl'erent pans of the in· terstellar medium l ndleates that individual stan can mndify tMlr in· terstdlar environments. 3. MOOiplaxles have been found to emit more radiation in tM shorter ultraviolet wavelengths than would have been eapected on the basis of tMir apparent c:olor temperatures in the visible rqion. 4, Loree hydroeen clouds have been found •urTOUndlng tM recent bright c:ome1S Tago-Sato-Kosaka and Bennett. Such laree clouds appear to c:onstitute a fourth ma}o< structural component of the c:omet. S. A broad absorption feature at ). 2550 has been discovered in the spectrum of Man, possibly due to ozone. The Orbiting Astronomical Observatory program is becomlna a true national facility for astronomers. On the firs~ OAO. about ten groups of astronomers have been observing approximately 100 objects. 'The OAO·C is upected to have a c:onsiderably greater obsenlna capability. and c:onnquently il should be of great service to the astronomical c:ommunhy throup IM pcst·obsene< program. 'The Orbiting Astroncmical Observatory proeram has. unfortunately, been marked by tragedy. Tbe first and third launcbcs were failures. the fintthroup troubles •ith tbe bancry. and the third through a failure in the launch vehicle. Afier the launch of o•O·C. tMr< are no further authorlz.cd proerams in space ultraviolet astronomy. At the present time, no satellite capable of carrying on intermediate spectral and spatial observations in the ultraviolet is funded. 'The ultimate objective of the ultraviolet astronomy program should be 

100 ASTRONOMY AND ASTRO PHYS ICS I'OR TilE 1910'• the: development of a National Space Observatory containing a large diffrattion ·limited telescope capable of operating in the near·infrared and visual rcaions as well as in the ultraviolet: . The exciting role that such a large space lclcscopc(LST)could play in astronomy during the decades to rome is disaJsscd in 1he final Section of this Chapter. The nominal apcrlure lhal has been utilized in stud;.. of the UT is 120 in. Such an instrument roukl anack problems that arc of 'he: most fundamental astronomical significance and that are unlikely ever 10 be solved using ground~based instruments. Perhaps of even greater importance than its ultraviolet capability would be the high angular resolulion of s uch a telescope. Turbulence in the atmosphere limits the angulor resolution obtainable with large telescopes to the equivalenl ol' that obtainable with a 12·in.·aperture telescope, although the light·galhering power of a larger instrument is superior. In the visible region, the L would have an ST angular resolulion better by a factor of 10. whic.h means that one resolution element observed with a ground·ba.• ed telescope could be divided into 100 resoturion elements with the uT. The angular resolu1ion in the ultraviolet would be still better by a factor ncar 2. One result of this high a ngular resolu1ion should be the capability of observing stars and stellar-appearing objo:ts at nearly ten times the di>tancc at which such objects can now be studied with the 200-in. telesropc. Tt.e LST should lead to a much improved understanding of the most fundamental problems in cosmology. as well as of the broad range of astronomical problems pmenlly being in,·estigated by ground·based astronomers. A great deal of technological dcvclapment will be required before such an LST can be launched. It will be desirable to test the new 1cchnology, not only Chrough rocket instrumentation for ultraviolet studies but also through the construction and flight of intermediate instruments. For example. a diffraction·limited space telescope of about(>() in. would have a tremendously useful versatility and capability beginning to approach that of the cs·r itself. It is now technically feasible t o build s uch an instrument, and it would be useful to incorporate into its design che results of new technological developments intended foc the LST. Yet no high ·quality large telescope is in the current planning stagt:. The Committee recommends very strongly that a vigorous program be maint· ined in uhraviolet astronomy. This program should be directed a toward the ultimate use of an 1ST'. One or more intennedi.ate instruments, designed to test the technology of the ur and to return large amounts of data of immense value to the astronomical community. should be launched. If there is to be an extended delay between the launch of OAO.C and the first of these intermediate in.struments. then it is most desirable that an interim ultraviolet telescope be launched. perhaps a replacement for the OAO· B or a smaller instrument in a Small Ascronon'y Satellite. 

The Hlgh·Prlorlty Progrom 101 The program for ulrraviolet astronomy that "'e have outlined is a large one. leading, as it eventually should. toward a large spaee telescope as a majoc prosram for the next two decades of astroncmy. Within il there is enoogh Ouibility to provide ample trade-off pclOSibilities t>eno·een small· scale acdvities and larger instruments. If we cannot alford the largest diffraction·limited instrUment soon. then a much more vigorous rocket and lntermediate·size ultraviolet and infrared telescope program is needed to avoid losing all opportunities in this aru. If, as appears likely. the 120·in. must be delayed to the mid·l980's. the 6Q.in. diffraction· limited tcleseope is an important prototype. giving both valuable ex· perience and important scientific results. The cost of continuing the ultraviolet satellite program throughout the next decade at a pproximately the current level of expenditure (SJS million per ycor) Is SJSO million. LARGE CENTIMETER·WAVE PARABOLOID large stcerable paraboloids have been the basic instrument of radio astronomy. Within minutes. a modern radio dish can be converted from one frequency band to anOiher. and its mode of operation can change from polarimetry to spcctroseopy at the Dick of a switch. Even major changes: in receiving equipment. suth as the installation of masers aod other refrigerated amplifiers or the installation of radar transmitters. take only a few hoors. This versatility has paid rich scientific dividends. especially in the study oftime variation of radio sourees. In spectrographic studies of the interstellar medium. and in studying the polariurion of radio sources. Large steerable paraboloids have been esstntial elements in the recent developments of ''ery-long-baseline interferometry CVLBU. in which the study of radio-source struccure to angular resolutions of better than 0.001 sec of arc has been possible. They have geodetic applications. Each larger instrument has, in its first few years of operation. produced new discoveries. Even a modest increase i_n size gives a surprising ad- vant:lge. bt(ause the etrective sensitivity, for observation in a given period of time. varies as the fourth power of the diameter. An add itional ad · vantaae is the freedom, with a Oexible instrument. to pursue occasional speculative programs. The recent explosive growth of diseovery of new molecules in the intcntellar medium provides an ex«.llent example, IU a new subbranch of astronomy- the chemistry of spac&-has staned to grow. The choice of instrument size. and of its wavelength capability (determined by the precision of its eonstruction). has been carefully considered. An instrument whose diameter is approximately 440 ft v.'Ould represent a significant nep beyond any existing or planned steerablc 

102 ASTRONOMY AND ASTROPHYSICS FOR THe 1970's paraboloid. and it appean that a dish that performs well at 2 em and is usable with somewhat reduced efficiency to 1-cm wavelength is well within present ongineering pract~. Tho largost comparablo antonna. tho 100-m telescope of Cormany's Max Planck lnstitut. is actually only an SS.m telescope at wavolengths shorter than 6 em. Thus the projtcted instrument has three times grtater observing capability at all wavtlengths, and at wavelengths of 6 em and smalkr o-·er six times grtater observing capability. An especially attractivo feature of the new paraboloid is its com- plementary role with our proposed millimeter-wave telescope. The simple basic molecules such as CO, CN. and CS have spectra that lie in the millimeter·wave region, while the larger. quasi-organic compounds such as methyl alcohol, formaldehyde, cyanoacetylene. and formic acid have spectral lines in the band from 2 to 30 em. Many of the larger molecules, and ammonia. possess lines that could be observed with either system, although tho grcater angular rtsolving powor of the 440-fi telescope would give it an advantage for certain problems. The large centimotor-wave paraboloid would certainly servo as the hub of many VLII observing programs, and its large area would inertase ononnously the classes of objtct accessible to study. In conjunction with the other large paraboloids of the world. stntcbing from Australia to the Soviet Union. the present observations of the closer, bri&ht objtcts would be extended to quasan and radio galaxies that are far more distant and faint. The radar capability of the new instrument would also be impressive. With the exception of Pluto. all the planets and the larger moons of Jupiter and Saturn would be within range of its 6-cm radar, while the greatly enhanced signal-to-noise ratio would enable the radar astronomers to study the surfaces of Venus and Mars in great detail, enhancing the effectiveness of space missions to those planets. The estimated cost of such an installation. including the telcsonpe. land acquisition, site development, controls, computers, radiometers, and radar, would be approximately SJS million. Some economies could be effected by s haring common support facilities with other instruments such as the very large array or the large millimeter-wave tele=pe. Operating costs would be S3.5 million per year following its completion. ASTROMETRY The establishment of a system of star positions based on an absolute inertial system is essential, and the system of proper motions should be detennined with respect to such an inertial frame. 

1M HiiJt·l+forlty ,.,..m 103 The mean propu modons of faint stars are of fundamental importance to the study of unusual stars found in the galutic halo. Many interesting objects in the halo are between I and 5 lqx from the plactk plane. and ..en with the rapid spac:e m(l(ions of elttrmle halo nars, thdr angular proper m(l(ions are small-approximately 0.25 sec of arc: pu year. The modons must be determined ..;th high indmdual accuracy. This requires that the inertial frame be determined to an oa:uracy of at least 0.005 sec of arc per year; Ideally, the accuracy should be several times hlgber. Ooe type of fundamental data that astronomers mu$1 have is the distan<e to the object studied. Interesting objects are at great distances, which can be calibrated in successive steps if nearby objects of similar characteristics have accurate distance measurements. The m0$1 fun· damental method uses accurate trigonometric parallax-the anaular displacement of a star caused by the earth's motion about the sun. These parallues are the backbone of the stellar distance scale. They are oeeded for faint stars near the sun and for bri&ht stars at ereater dittanc:es. An insufficient number of trigonometric parallaxes in the southern hemis· phere will reduce the beoefiu of the laraer facilities built there by the United States and Europe.a n countries. Stars morina parallel in space appear to converac. becauJe of per· spective effects; this method provides individual distances for nearby star clusters. Ou$ter parallaxes should be extended to the southern hemi· sphere and to fainter clu5ters in the northern hemisphere. For other distant types of stars, we mU51 take advantage of the accumulated drift provided by the moe ion oft he sun through space, which cauJeS the 5tars to drift bacb•ard at angular speeds proportional to their parallax. Such group or secular parallaxes are often the only possible distance measure for the moSII.nteresting stars of high luminosity. They depend directly on the accuracy of the fundamental system of proper motions. Theories of stellar interiors would have a sounder basis If a sufficient number of parallaxes and masKs of nearby stars and clusters could be provided. These should include interestlna and important objects like rapid variables. hiably luminous B Slars. plaoeta. y ncbulu, hoi sub- r dwarfs, bright white dwarfs, and cool red d~nerate stars. The establishment of the actual luminosity-temperature diaaram for stars like the sun and fainter is CSJeDtial for the determination of the distances to the &lobular ciU5ters and the luminosities of the RR Lyrae stars. For these important determinations, a combination of trigono- metric. clu5ter. secular parallaxes. and all (~(her pa~sible methods must be used. Recently. the possibility has appeared of detecting companions of low mass by the nonlinearity of the motion of a nearby star throut~h space. Several companions have been announced that have masses like that of 

104 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o Jupiter-or even lower values. These astrometric binaries have been studied ....,ntially in very few institutions. take a long time to give results, and yet will provide us witb our only diRe! proof of the existence of other planetary systems until radio communication from some of these may eventually be rurived. The changes of period detected in pulsan are fundamental to the theory of neutron stan. Yet the lint observations of these changes ,...,,. com· promised by uncertainties in such suppusedly well-known subjects as tbe orbits of the planets around tbe sun and the masses of t.he planets. The motion oft he earth around the center of gravity or the earth-moon S)'1tem is detectable in the accurate observations of the radio pulsars. Jmpro'lcd planetary orbits are necessary to take full advantage or this technique. Similarly, the very-long-baseline-interferometry technique requires ac- curate geodesy anq accurate timekeeping. The improvement and e,xte,nsion of astrOmetric measurements nee· essary to Interpret the problems mentioned above rests ultimately on ob- serva1ions by small astrometric instruments. We 1herefore recommend const.ruction of two automatic transit circles. three photographic zenith tubes, three astrolabes. and three automatic measuring engines, as weU as modernitation or several existing long-focus telescopes. the equipment to be located geographically so as to provide systematic observations in both the northern and southern hemispheres. The precision attained by these rundamentalastrometri<: instrumenu bas hardly been affected by modem electronic technology (u..,pt for tbe timekeeping funetlon). However, the modem technology or automatic measu_ement is in fact successful. and r we recommend it. together with some or the classical smaller telescopes mentioned above ... part or our fundamental program. The estimated cost of these small instruments is 56.4 million. BEYOND THE RECOMMENDATIONS After concluding a detailed study of the state or our science and making our recommendations within the framework of recent available funding, we feel that it is important to discuss. in certain areas, what additional programs our science requires to meet fully the seientilic challenges tbat -..'e face. We have therefore re-examined the manpo--er raourees that will be available in the decade and tbe technologk:ally feasible and desirable projects studied by tbe panels. What areas have ""'omitted, disearded, or redueed in siu mostly becau.e or financial constraints? How much have we failed to recommend or the urgent needs pressed by our technical panels? 

The H/xh·l'rlorlty Pro,.m lOS Larg<' Space Telescope Without any doubt. the largest and most exciting area is the ronnruction and launch of a large space telescope u..sn . for high·rcsolution nudics in the normal and ultraviolet spectral regions. possibly with manned resupply and maintenance (e.g•• by the space shuttle). This development can be underuken in a vigorous way only at budget levels for astronomy and physics that represent c:onsiderable growth over the nut decade. The LST concept is based on two major exploitation.s of the orbital environment. First, the mirror-from 60 to 120 in. in diameter. depending on available fundo-will c:over completely the wavelength interval from 1000 A (the eutoH' imposed by interstellar attenuation) to 10.000 A(or 1 pm). with considerable utility out to 1 mm. thereby covering the entire ultraviolet and Infrared range not accessible from the ground. as well as the optical window. The large collecting area and high angular resolution over this entire range would provide unmatched versatility. But a more important dimension of the LST is the precis,ton of its image in the ultraviolet and optical ranges. On the ground, the d<let<rious effects of atmospheric seeing smear the image to one or m6re seconds of an: even at an excellent she. This means that the obse"er is in eft"tc1 comparing the image ohhe tarect object with tbat of tbe night sky (including ba<kground galactic light. zodiacal light. and airglow) in a comparable solid angle. lf a 120-in. t<lcs<ope can be designed to achieve diffrocrion limitation at SOOO A. an image as small as 0.04 sec of arc in diamet<r would result. If an image ofO. l sec of arc can be achieved in practitt, the night·sky radiation. which tends to obscure the imago of a faint object, is effectively redu<cd by a ractor of 1()0-a five-magnitude gain in sensitivity over ground-based instrumcnt,sofcomparable aperture. There is an additiona.l gain from the fact that the tel<scope operates above the alrglow layer and, of course. dots not sulfer from atmospheric attenuation. 11 should be possibl< to observe to apparent magnitude 29 in several hours of int<gration. Th< implications of such a capability for all branches of astronomy are great. The Committee feels that the LST has extraordinary potential for a wid< variety of astronomical uses and believes that it should be a major goal in any Vo'ell·pla.n ned program of ground· and spacc·bascd as1ronomy. The Committee recognizes that th< large <ost involved can be ac· commodated only "'·itbin a vigorously growing prosram. Therefore. it has adopted the view that, w;lhio the main program. the emphasis on the LST is at a moderate level of som< SJS million per year. enough to fund tcchnoloaioal development of smaller.apenure telescopes aod an LST in the following de<ad<. A much more expensive program is required if the LST is to become a 

106 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'1 reality in the 1980-1985 period. This Committee sees the l.ST as a natural program goal to follow the High Energy Astronomical Observatories I HEAOI mission. To achieve this will require budgets for diffraction· limited missions that grow from a level of t.h e order of S20 million per year in 1970 to the order of $200 million per year in 1980, with launch scheduled for the early 1980's. Total cost of the program leading to the final fabricat.ion of a 120-in. telescope will be of the order of Sl billion over 10 years. A program of this magnitude requires the highest quality scientific leadership a.n d the most advanced space engineering available. The highest quality scientific leadership in this field can be found in the academic community. and the highest degree of space engineering talent exists in the centers of the National Aeronautics and Space Ad· ministration. Therefore, the best chance for success lies in a merging of academic talent with that in the NASA centers. We suggest that NASA select one or more centers to carry out the engineering phases of the program and that the National Academy of Sciences encourage the formation of a new corporate entity representing universities with strong programs in space astronomy. The latter should be limited to less than eight members in the interests of efficiency. This corporation would be responsible for establishing a National Ultraviolet Space Observatory I NUSOI - a working scientific laboratory under con· tract to NASA and the National Science Foundation. The Director of the NUSO should be a scientist of top rank in space astronomy. The NUSO would be responsible for the planning and utilization of a series of satellite ultraviolet observatories, including the LST , and for administeri.ng them on behalf of the entire scientific community, as is done for the ground-based national observatories. To achieve this mission, the NUSO would work closely with the responsible NASA centers. Effective control of the engineering task ofthe Nuso would be exercised by NASA ; effective control of the scientific direction would rest in the Director and in the Board to which he would report. Optical· and Radio-Astronomy Instruments Certain major facilities in optical and radio astronomy were omitted from our program, for reasons of economy. Optical astronomers could make effective use of two more telescopes in the 200-in. class, with modern electronic auxiliaries. The pressures generated by space and radio astronomy have so overcrowded the few large instruments that even the two ISO-in. telescopes under construction fail to match the present needs. In addition to our recommended optical program, it would be desirable to 

Thtlllgh·Prlority Program 107 double the effeclive collecling area of existing large telescopes. To ac· complish this, at least two additional 200-in. telescopes or two equivalent· cost larger arrays or possibly one even larger array would have to be built in addition to those that we have recommended. Such a program would cost SS() million (whh site development) plus the modem instrumentation described earlier in this chapter under Optical Astronomy-Electronic Teehnology and Ught·Gathering Power. or the radio telescope systems planned. studied. and repeatedly recommended. one major ite,m is omitted from our list of new starts. It is the only large. university-based plan that goes bac.k to the Whitford report-the completion of the Owens Valley aperture-synthesis inter- ferometer of 130-ft radio telescopes. The original plan required five ad· ditlonal antennas. tracks. receiver. and computer. The high quality of the mechanical design makes the present 130-ft good at 2 em and possibly usable at I em. An aperture-synthesis array working at high frequencies, usable for molecular and atomic lines, can be constructed for SlS million. Its beam, at 2 em, will give 2 to 4 sec of arc resolution; its collecting area and sensitivity is about half that of the VLA. One advantage of the relat.ivdy small number of IJO.ft antennas is the Oexlbility, ability to change rapidly, and reduced cost of the receivers needed to permit aperture synthesis, at high resolution, in moleeular and atomic emission and absorption lines. In addil.ion, the interferometer could be used for extragalactic astronomy at higher frequencies, providing data on the time- f&riable radio sources ,.;th Oat or rising speetra. Vtry·Long-Baseline Interferometry A tremendous breakthrough in our ability to perceive fine details in radio sources has come in the last four years as the result of the development of very-long-baseline Interferometry <VLBil. By using highly stable atomic clocks. high·speed magnetic recording. and modern computing techniques. antennas distributed over the entire world can now be used as elements or a single radio telescope. If we were to extrapolate from our present pioneering observations of the brightest sources and construct a vision of future developments. we could confidently sketch a technically feasible system that could construct complete maps of the details of quasars and interstellar masers. The present network of large antennas gives a sketchy view because there is a lack of intermediate spacings and north-south baselines. The situation could be remedied by the development of a mobile vLao terminal, consisting of two dishes, one large and one small, plus the necessary atomic clocks and recording apparatus. The large dish would be designed to permit rapid assembly and 

108 ASTR ONOMY AND ASTROPHYSICS FOR THE 1970 '1 disassembly. so that it could be transported 10 new locations. The small antenna would constantly monitor one of the stronger sources, to provide constant updating of the station clock. S.Venl terminals would be needed, cutainly 01 least two on each continent, although the best disposition would have t o be determined by a cueful study. The resulting network, if operated at 1~ wavelen81h (which recent observation of H20 masers at 1.35 em have shown to be feasible) could give us a complete pictun: of the radio structure of quasars. with 0.0001 sec of arc resolution. If our ideas of the distances or quasan are correct. we could see structures appro:Umately I light-year in siu and could follow the development of dynamic events from year to year, seeing the details of these enormously energetic events. There are other. more speculative areas that one can also foresee-the study of the coronas of other scars, the observation of their sunspots and flares, the study of supernova shell developments in other galaxies, and the analysis of the mysterious nuclei of Seyfert galulcs. In add ilion to the Yl.BI program at radio wavelengths, we foresee the development of interferometer techniques at both infrared and opti<al waveltngtbs. Bec-ause the angular resolving power or an intttferometer vari~ inversely with the wa\·ele.ngth, one can anticipate remarkable di.sccweries by such systems. rivaling the recent radio vu 1 demonstration of motions appan:ntly faster than light in a quasar explosion . The ultimate instrument would be a 10-pm YLBI having a global baseline oo• kml. Such a devWe would have a n:solution of JO·' sec of are, pennitdng one to peer deep into a quasar, perhaps to see explosive events on the surface of a superma.sslve star, which, some say. powers a quasar. The surface features of exotic: stan that sporadic-ally shoot dust and molecules into Interstellar space could also be studied. The choice of 10· .urn wavelength IJ dictated partly by the fact that atmospheric phase shifts are small there. permitting the use of large apertures, and partly by the fact that quasars and n:d giants-key objects in relativistic astrophysics and molecular astronomy--radiate a major f-raction of their energy there. The 10-~o~m VLBI might use a superheterodyne system, which mixes the incoming infrared signal with a stabilized CO, laser to produce a microwave signal that can he recorded at each telescope. The bandwidth of available tape recorders (100Hz) should be sufficient to detect at least the brighter sources. A f~nner of this device is nov.• under C-'Onstruc:don, usina Hne-of· sight transmission of a w t.Hz bandwidth microwave signal to a common point to form an interference pattern. Following tests of the system with a 0. 1-km baseline (lo·> sec of an:), it wiD be expanded to 10 km (10 .. sec of arc). It will be sensitive enough to study nearby Seyfert galuies and bright 

The lligh-Prlorlry Progrom 109 galactic objects. but a version sensitive enough to study quasars (where the resolution will be I light-year) will require larger telescopes and better detectors. Of course, most astronomical objects emit more powerfully with visible light so that there also is need for devices that can work in that spectral range. Fundamental studies of angular sizes are possible with both the intensity interferometer, which conelates the intensities in the two signal$, and the Michelson interferometer. wbich brings togethor the raw signals to fonn fringes. A large-intensity interforometer could be built im· mediately with a l ·km baseline to givo tO"' sec of arc resolution, but perfection of the Michelson system requires dovelopment of an optical delay line and techniques of fringe detection. The Optical Facilities Panel believes th3l both the delay line and fringe detection should be studied immediately with funding up to $200.000. Beyond these preliminary investigations, worthy goals of a teo-year program include a sensitive 10-km infrared interferometer, and perhaps a IO'·km infrared VLBt, and for visible wavelengths a J. or 2-km intensity interferometer and a Michelson interferometer with a similar baseline. The sensitive IO.km infrared interferometer is estimated to cost SIO million over the decado. and the large intensity interferometer S4 million. Further studies are needed before the cost of the infrared vut or the Michelson interferometer can be estimated. Infrared Astronomy The growth of infrared technology resulted in discovery of quite unex· pected objects that radiated most of thoir energy in the infrared. The energy maximum atiSOO K is at 2,um and is observable from the ground. A survey with a 62·in. light collector discovered 20,000 cool stellar and prestellar objects. Observations in the far infrared are needed to study objects near 500 K. most of whose radiation falls in regions of high at· mospheric absorption; to study objects at 50 K, observations above the atmosphere are needed. The Infrared Panel put highest priority on a large stratospheric telescope, about 120-in. in diameter, in a large, high-Oying aircraft or possibly supported by balloons, gliders, or kites. We recom· mended funds only for study of the most economical modo of operating suc.h a large infrared telescope. but the scienti6c goals of the large stratospheric telescope are extremely important. No realistic financial estimate can yet be made; both the study and e"J)Crience with the NA SA C· 141 airplane (with a 36-in. telescope) will determine the best course of ac1ion. The infrared groups are small at many universities. in both astronomy and physics. The changes in technology, the availability of new 

110 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'a detedOI'$, and the revelations of new types of objects make this an un- predictable but challenging field. lntenlisclplinary grants to physics and asuophyslcs departments will enlist the aid of low-temperature physicists for astrophysical applications. Solar Phy•ics Solar physics has benefited enormously from the oso seri.. of solar observations. oso· s are rapidly becoming more sophisticated and more reliable. However, a large diffraction-limited solar telescope (about 40-in. diameter) is needed, carrying a heavy payload (over 1000 lb) and capable of accurate pointing and 0. 1 sec of arc gu iding. This will provide high spectral resolution in the optical and near ultraviolet a nd will permit very fi ne-scale study of the rapidly fluctuating solar plasma, iu excitation temperature, velocity, and magnetic field . This is a large project, of the order of S200 million, but it is one that will both provide experience use~ ful for the UT and be a nearly ultimate solar space telescope. High-resolution observations of the radio sun provide information on the energetic partido acceleration ptOC0$5, as revealed by the gyrosyn- chrotron radiation. The relativistic electrons are studied near the site of the acceleration of solar cosmic-ray baryons. This s1udy requires a high- resolution radio telescope with about S sec of are resolution, which works on a short time scale and ..sentially giv.. a radio picture. A radio spec- troheliograph in Australia has already demons1rated iu usefulness in the study of the interaction of fast particles and the hot solar plasma and has shown that Hares are triggered across the sun as disturbances run out through the corona or return to other activecentel'$ on the disk. Theoretical Astrophysics Facilities should not monopolize our attention. The present a nd planned facilities, the space astronomy program, and the importance of the fie ld for itself justify a strong case for theoretical as1rophyslcs, over the wides1 possible range of topic$-$tudy of neutron Sill'$, the quieter phases of stellar evolution, planetary dynamics. galact ic structure, supernovae, collapse nuclcosynthesls. explosions in galaxies, black holes. relativity, and cosmology. A test of the concept and viability of an Institute of Theoretical Astrophysics is an inexpensive recommendation. Also linked to theoretlcal needs is a fourth- or fifth-generation computer at a single National Computing Center. The total cost of an Institute and Computer Center ror 10 years might be S40 million. About JO percent of our recent PhD's in astronomy have their degrees in, and wish to work in, theoretical 

The High-Priority Program I II astrophysics or dynamical astronomy. The issue of a National Computing Center is not clear-cut. since the efficiency and costs of high-speed long- distance lines are not yet known, but the 't'ery large computer is at the heart of much theoretical model building in astrophysics. To take ad- vantage of the presently available theoretical talent among young astronomers and physicists, we also urge that an expanded postdoctoral and senior postdoctoral program be considered. The goal would be to provide a number of theoreticians with at least a summer's or, preferably, a year's visit to other universities, national, ooo, or NASA centers, by direct fellowship grants. with freedom to travel, or by small research grants covering their salaries and expenses.