GRAVITY ASTROPHYSICS a Plan for the 1990S

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ULTRAVIOLET, VISIBLE, and GRAVITY ASTROPHYSICS A Plan for the 1990s

(NASA-NP-I52) ULTRAVIOLET, N94-24973 VISI3LE, ANO GRAVITY ASTROPHYSICS: A PLAN FOR THE 1990'S (NASA) 76 p Unclas

HI190 0207794 ORIGINAL PAGE COLOR PHOTOGRAPH

National Aeronautics and Space Administration

I-oreword

N ASA'sprioritiesOfficefrom oftheSpaceU.S. NationalScience Academyand Applicationsof Sciences.(OSSA)Guidancereceivesto theadviceOSSAon Astrophysicsscientific strategyDivision,and in particular, is provided by dedicated Academy committees, ad hoc study groups and, at 10-year intervals, by broadly mandated astronomy and astrophysics survey committees charged with making recommendations for the coming decade.

Many of the Academy's recommendations have important implications for the conduct of ultraviolet and visible-light astronomy from space. Moreover, these areas are now poised for an era of rapid growth. Through technological progress, ultraviolet astronomy has already risen from a novel observational technique four decades ago to the mainstream of astronomical research today. Recent developments in space technology and instrumentation have the potential to generate comparably dramatic strides in observational astronomy within the next 10 years.

In 1989, the Ultraviolet and Visible Astrophysics Branch of the OSSA Astrophysics Division recognized the need for a new, long-range plan that would implement the Academy's recommendations in a way that yielded the most advantageous use of new technology. NASA's Ultraviolet, Visible, and Gravity Astrophysics Management Operations Working Group was asked to develop such a plan for the 1990s. Since the Branch holds programmatic responsibility for space research in gravitational physics and relativity, as well as for ultraviolet and visible-light astrophysics, missions in those areas were also included.

The Working Group met throughout 1989 and 1990 to survey current astrophysical problems, assess the potential of new technologies, examine prior Academy recommendations, and develop the implementation plan. The present report is the product of those deliberations. A companion report by the Ad Hoc Committee on Gravity Astrophysics Technology describes the technology developments needed to support future space missions in gravitational physics and relativity.

THESPIRAL GALAXY MESSIER 81 is about 12 million light years away in the constellation Ursa Major. The best ground- based image of M 81, recorded in red light (excluding Ha) by a CCD camera at the Kitt Peak National Observatory (left), is dominated by old, red stars in the nuclear region. A satellite image of the same galaxy was recorded in ultraviolet light by the ASTRO-1 Ultraviolet Imaging Telescope aboard the Space Shuttle in December 1990(right, also shown on cover); extremely hot, young stars, invisible on the ground-based image, gleam from star-forming regions in the spiral arms.

Contents

EXECUTIVE SUMMARY ...... 1 III. PROGRAM CONCERNS ...... 45

Implementation Plan for the 1990s, 3 A. Program Balance, 46 I. Access to Space, 46 Overview Chart: Plan for the 1990s, 4 2. Commitment to Infrastructure and Continuity, 46 Some Key Astrophysical Questions, 9 3. Access to the Electromagnetic Spectrum, 46 4. Project Support to Completion, 47

B. Community Support and the Training of New Scientists, 48 INTRODUCTION ...... 11 C. Technology, 48 Approach, 12 I. Deleclors, 48 Perspective on the Plan, 13 2. Optics, 49 3. Structures, 49 4. Pointing, 50 5. Command, Control, and Data Management, 50

II. ASTROPHYSICAL QUESTIONS ...... 15 D. International Cooperation, 51 A. Structure of the Universe, 16 I. Scale of the Universe, 16 2. Deuterium, 16 IV. THE RECOMMENDED PROGRAM ...... 53 3. Density of the Universe, 17 4. Distribution of Galaxies, 19 A. Approved Missions, 54 B. Structure of Galaxies and Quasars, 21 1. Hubble Space Telescope (HST). 54 2. HST Second-Generation Instrumcms, 54 I. Formation and Evolution of Galaxies, 21 3. HST Third-Generation Instruments, 56 2. Active Galaxies, 23 4. International Uhraviolel Explorer (IUE), 57 C. Structure of the Milky Way, 25 5. Extreme Ultraviolet Explorer (EUVE), 57 1. Interstellar Dust, 25 6. Orbiting and Retrievable Far and Extreme Uhraviolel Spectrometer (ORFEUS), 58 2. Interstellar Gas, 26 7. Interstellar Medium Absorption Profile Spectrograph 3. Complex ISM Structure, 28 (IMAPS), 58 4. Diffuse Ultraviolet Emission, 29 8. Astronomy ObservaloD,-2 (ASTRO-2), 59 5. Stellar Populations, 29 9. Far Ultraviolet Spectroscopic Explorer (FUSE), 50 D. Structure of Stellar Systems, 32 10. Shuttle Test of Relativity Experiment (STORE), 60 1. Formation of Stars, Planets, and Protoplanetary Disks: B. Future Missions, 61 Origin of Solar Systems, 32 1. Augmentation to Explorer Program, 6 I 2. Stars and Stellar Atmospheres, 34 2. Additional Small-Class Explorer: 3. End Products of Stellar Evolution, 36 Deep Uhr_wiolet Survey, 61 4. Sources Powered by Accretion, 37 3. Gravity Probe-B (,GP-B), 62 5. Positions and Motions of Astronomical Objects, 39 4. Laser Geodynamics Satellite-3 (I,AGEOS-3), 62 E. Verification of Relativistic Theories of 5. New Flagship and Intermediate Space-Based Missions, 63 Gravitation, 41 6. Lunar Outpost Astrophysics Program, 64

F. The Unknown: Serendipity, 44 C. The Research and Analysis Program, 67

1. Background, 67 2. R&A Funding, 67 3. Sounding Rockets, 68

APPENDICES

A. Participation, 71

B. Acronyms and Abbreviations, 72 D. Acknowledgments, 73

C. References, 73 E. OSSA Management, 73

Executive Summary

Figure 1. THE INTERNATIONAL ULTRA VIOLET EXPL ORER (IUE) satellite, launched in 1978 to address a broad range of astrophysical questions, is well into its second decade of operation. The geosynchronous orbit, providing continuous, 24-hour contact with ground stations in the United States and Spain, permits an unusually efficient observing schedule.

concepts of hot stars and the inter- these observations to much fainter, been built upon mil- stellar medium. In only four de- and hence more distant, objects than odernlennia astronomyof ground-has cades, we have moved from these was previously possible, allowing us, based observations primitive beginnings of UV as- for the first time, to examine the UV made at visible wavelengths of light-- tronomy to the sophistication and properties of large numbers of extra- the wavelengths transmitted through scientific power of the Hubble Space galactic objects. the Earth's atmosphere. Telescope (HST), launched in April t990 as the first of NASA's "Great Studies at visible and infrared Direct observations of astro- Observatories." wavelengths also benefit dramatically nomical objects from space, by con- from satellite observations. In the trast, are less than 40 years old. They One reason that UV astronomy visible region of the spectrum, space began with the flight of instruments has been so scientifically productive telescopes can provide much higher sensitive to ultraviolet (UV) radia- is that it can be used to diagnose the angular resolution than ground-based tion carried aboard suborbital sound- physical conditions of astronomical telescopes, since the blurring effect ing rockets. Since nearly all UV phenomena whose temperatures of atmospheric turbulence is elimi- radiation is blocked by the Earth's range from a few degrees Kelvin (K) nated. Moreover, a telescope in space atmosphere, UV observations rnust to nearly l0 TK. The UV is the only can study fainter objects than the be made from space. portion of the electromagnetic spec- same telescope on the ground because trum for which this is true. of the reduction in sky-background These pioneering rocket flights noise. provided only brief, 5-minute It is therefore not surprising that glimpses of how the sky would look the range of scientific discoveries The Hubble Space Telescope, if we ourselves had eyes sensitive to attributed to UV observations has for example, was designed to achieve ultraviolet light--yet they were suf- been so broad. With the advent of a tenfold improvement in angular ficient to force major revisions in our HST, it is now possible to extend resolution by comparison with a Figure 2. THE YOUNG STAR CLUSTER Rt36, as imaged by a large ground-based telescope (left) and the HS T Faint Object Camera after computer processing (right). R136 lies within the 30 Doradus nebula in a rich, star-forming region of the Large Magelfanic Cloud. ground-based telescope and to detect been discussed in 2,083 articles pub- foresee the contributions of new gen- objects twenty-five times fainter than lished in refereed scientific journals erations of instrumentation to many can be observed from the ground. and had in addition provided the ba- fundamental and exciting areas of When equipped with second-genera- sis for 107 Ph.D. dissertations in the current research. tion instruments that correct for the United States alone. The Hubble effects of spherical aberration, HST Space Telescope can carry on this The NASA Ultraviolet, Visible, will greatly enlarge the observable tradition for a much broader range of and Gravity Astrophysics Manage- Universe at visible and UV wave- the electromagnetic spectrum, in- ment Operations Working Group lengths, increasing by orders of mag- cluding the UV, visible, and infrared (MOWG) recognized in 1989 the nitude the number of extragalactic spectral regions (Figure 2). need to systematically plan for the objects that can be studied. implementation of a space- astronomy The past decade brought a tre- program that builds on our current Advances in ultraviolet, visible, mendous increase in the sophistica- missions and knowledge to exploit and infrared space astronomy have tion of space technology and instru- this new technology. gone hand in hand with advances in mentation. Applied to space as- space technology. Some have ar- tronomy, these increases will lead to The goal of the MOWG is to assist gued that the International Ultravio- telescopes and interferometers of NASA in defining an implementation let Explorer (IUE), launched in 1978 unprecedented capability. The sci- plan that will assure the continuation and still operating in 1991, has al- entific gains to be brought about by of the Ultraviolet, Visible, and Grav- ready surpassed any previous satel- increasing the size of the observable ity Astrophysics program at the fore- lite observatory in scientific produc- Universe and the detail with which front of science, and to aid NASA in tivity and may in fact be the most we can observe it will be profound. its implementation. The plan pre- productive single astronomical in- Although the most important discov- sented in this report, the outcome of strument ever built (Figure Ij. As of eries may be those we cannot now deliberations of the MOWG over December 31, 1990, IUE data had anticipate, astronomers can already nearly tw O years, embodies that goal. EXECUTIVE SUMMARY Implementation Plan for the 1990s

Space-based observations at recommendations of the National The actions required to carry out visible and UV wavelengths will Academy of Sciences. Carried out the implementation plan are discussed make possible major advances to- over the next 10 years, it will produce below (see next page for summary). ward answering the fundamental major growth in our scientific knowl- Data from both approved and future questions of astrophysics. edge of astrophysical sources and space missions are essential. conditions. The plan maintains bal- The evolution and large-scale ance among large observatories, small structure of the Universe, the ener- and moderate missions, advanced (1) Hubble Space getics of active galactic nuclei, the technology research and develop- details of stellar evolution, the com- ment, and community support and Telescope (HST) position of the interstellar medium, participation. It also provides for the the study of planets in our own Solar training of a new generation of sci- The Hubble Space Telescope, System, and the search for planets in entists. The major components of the the first of four planned Great Obser- other solar systems are but a few of plan are as follows: vatories, will be the centerpiece of the many astrophysical questions that the NASA Ultraviolet, Visible, and (1) Hubble Space Telescope, will be addressed by space-based Gravity Astrophysics program for the observations at UV and visible wave- (2) A commitment to the existing the 1990s (Figure 3). HST marks the lengths. The fine-scale structure of program, beginning of an era of true observa- stars, galaxies, and active galactic tory-class space telescopes. nuclei can also be explored with in- (3) A vigorous program of struments of high angular resolution, Explorer missions, In tandem with the other three and the rapid motions of massive (4) A strong research base to Great Observatories--the Gamma objects in binary systems can be support future missions, probed with gravity astrophysics Ray Observatory (GRO), the Ad- (5) Advances in General Relativity experiments now under study. vanced X-Ray Astrophysics Facility and gravity astrophysics, and (AXAF), and the Space Infrared The plan presented in this report (6) A large space interferometer or Telescope Facility (SIRTF)--HST is ambitious, and fully consistent with telescope beyond HST. will provide crucial coverage of key

Figure 3. RING OF LIGHT AROUND THE REMNANT OF SUPERNOVA 1987A arises from illumination of previously ejected material by radiation from the supernova blast. This HST Faint Object Camera image, recorded in 1990, shows the ring structure in detail. Subsequent observations by HST and IUE have permitted measurement of the progression of the illuminated region and thus an estimate of the distance to the supernova. ULTRAVIOLET, VISIBLE, AND GRAVITY ASTROPHYSICS

Plan for the 1990s

(1) Hubble Space Telescope:

• Rapid correction of effects of spherical aberration

• Timely installation of second-generation instruments (within 5 years) and technology development leading to installation of third-generation instruments within 10 years

(2) Commitment to the existing program:

• Timely launch of EUVE, ORFEUS/IMAPS/AstroSPAS, and ASTRO-2

• Support of the operating missions

(3) Vigorous program of Explorer missions:

• Increased resources for Explorer missions

• Timely launch of FUSE

• A Small Explorer mission for a high-sensitivity, all-sky survey at UV wavelengths

(4) Strong research base to support future missions:

• An enhanced commitment to a research base that includes detector development, theory, data analysis, and laboratory astrophysics to keep pace with the explosive increase in observational capabilities

• An energetic program of astronomical research using sounding rockets

• Augmented resources for the training of new scientists

(5) Advances in General Relativity and gravity astrophysics:

• Continued program of advanced redshift and dynamical experiments aboard deep-space probes

• Timely launch of Gravity Probe-B and LAGEOS-3

• Support of current technical developments leading to future missions

(6) Large space interferometer or telescope beyond HST:

° A program leading to the construction of a powerful UV and visible-wavelength space interferometer or large-aperture telescope

• As an important step in this program, with its own strong scientific rationale, development of an Astrometric Interferometry Mission (AIM) in space for astrometric measurements to an accuracy of 3 to 30 micro-arcseconds

4 EXECUTIVE SUMMARY portions of the electromagnetic spec- In addition: It is crucial to the health of as- trum. Lesser spectral gaps will be tronomy that NASA maintain its filled by such smaller missions as the NASA release of an Announcement commitment to these missions. Extreme Ultraviolet Explorer of Opportunity within the next 2 to 4 (EUVE), the Far Ultraviolet Spectro- years for design and construction of The currently operating set of scopic Explorer (FUSE), and HST one or more third- generation HST missions consists of HST, IUE, and second-generation instrumentation: instruments will ensure the telescope's the Ultraviolet Spectrometers (UVS) scientific pre-eminence. on the Voyager-I and Voyager-2 NA Sit is committed to ensuring that deep-space probes. The Astronomy HST is restored to its full capabili- As more sophisticated and powerful Observatory-I (ASTRO-I) Space Shuttle payload was flown in De- ties and then supported at the level detectors are developed that exploit cember [990; an ASTRO-2 mission required to attain its full scientific the currently unused potential of liST, we will witness a dramatic increase is now in planning. productivity. in the range and variety of new astronomical knowledge from this fa- The Extreme Ultraviolet Explorer Beyond e tic lent operation of the (EUVE) is scheduled for launch in cility. HST observatory, it is also of para- 1992. Through a cooperative program mount importance to exploit the great with the Federal Republic of Germany, potential of this telescope with the two additional instruments will be orders-of-magnitude improvements (2) Commitment to placed into orbit in 1993: the Orbiting in performance available through the Existing Program and Retrievable Far and Extreme Ul- approved second-generation and traviolet Spectrometer (ORFEUS) and third-generation instrumentation: During the past decade, NASA the Interstellar Medium Absorption has created a research program in Profile Spectrograph (IMAPS). The Installation of the second-genera- Ultraviolet, Visible, and Gravity As- ORFEUS/IMAPS mission will use the tion lIST instruments" within 5years trophysics founded on a powerful AstroSPAS platform provided by is of paramount importance. complement of flight missions: Germany.

ULTRAVIOLET, VISIBLE, AND GRAVITY ASTROPHYSICS Approved Missions

Hubble Space Telescope Extreme Ultraviolet Explorer (HST)--launched 1990 (EUVE)--1992 launch

HST Second-Generation Orbiting and Retrievable Far and Instruments: Extreme Ultraviolet Spectrometer (ORFEUS)m1993 flight aboard the • Wide Field/Planetary Camera II AstroSPAS platform (WF/PC II) Interstellar Medium ° Space Telescope Imaging Spectrograph (STIS) Absorption Profile Spectrograph (IMAPS)D1993 flight aboard the ° Near Infrared Camera (NIC) AstroSPAS platform

HST Third-Generation Astronomy Observatory-2 Instruments (ASTRO-2)D1994 flight

Voyager Ultraviolet Spectrometers Shuttle Test of Relativity Experiment (UVS)--Iaunched 1977 (STORE)--1994 flight

International Ultraviolet Explorer Far Ultraviolet Spectroscopic Explorer (IUE)--Iaunched 1978 (FUSE)--2000 launch

5 Wavelength (A) 6000 3000 2000 1500 1200 1000 900

u I' FUSE ,--3

! IUE I i ...... I I I I I I ...... I i i e- HST ra r- I °< 300

250 O.

G) c 200 ..I o_ •s.__ 150 O G) O. Or) 100 N-,, O

-Q 5O E ::3 Z

2 4 6 8 10 12 14 Energy (eV)

Figure 4. RICH CONCENTRATION OF RESONANCE ABSORPTION LINES in the far ultraviolet, inaccessible to tUE or HST, will be studied by NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) mission. One half of the lines arise from molecular deuterium (HD) and molecular hydrogen (H2).

(3) Vigorous Program of At least six times as many astrophysically important lines are Explorer Missions present in the 300-_ wide far-ultraviolet (FUV) spectral region as in the The Far Ultraviolet Spectro- 2,000-._ wide traditional UV region scopic Explorer (FUSE) is a far-ul- extending from 1,200 ,_ to 3,200 traviolet astronomy mission that will (Figure 4). Many important ioniza- give astronomers continuous access tion states of astrophysically domi- to a vital region of the spectrum that nant atoms (e.g., hydrogen, oxygen, is currently observable only during sulphur) radiate in the FUV. Knowl- 5-minute rocket flights. edge gained from observations of these lines is essential to the analysis of non-thermal ionization processes The National A cadem y of Sciences' in astrophysical plasmas and to de- Astronomy Survey Committee for terminations of the relevant atomic the 1980s, as well as numerous advi- abundances. sory and peer-review committees, have ranked FUSE as number one Among larger Explorer-class mis- in its class of flight missions. sions, highest priority is placed on the timely launch of FUSE.

6 EXECUTIVE SUMMARY

Historically, Explorer missions characterized to the greatest sensitiv- tinuity and stability to NASA's astro- have been the backbone of our ity possible. From the survey, as- physical research program. discipline's scientific program. Ex- tronomers will be able to determine To the detriment of the nation's citing new scientific investigations the classes of sources that emit at UV in astrometry, very high resolution wavelengths, the morphology of the capabilities in science and technol- UV and EUV astronomy, inter- emission, and the range of luminosi- ogy, support for work in these areas ferometry, multispectral observa- ties in each class. Such measure- has not kept pace with the need for fundamental data and instrument de- tions, stellar seismology, and long- ments are critical to initiate an under- term studies of variability are all well- standing of the emission processes velopment generated by rapid advances in forefront scientific research suited to larger Explorer-class mis- that radiate at these wavelengths. sions: from space. This component of the program continues to advance the resources of our nation in scientific We support the high priority NASA (4) Strong Research and technical areas, and to train new gives to augmentation of the Ex- Base to Support generations of scientists. plorer Program. Future Missions The R&A program needs continued attention to augment its resources The capabilities provided by the Underpinning the scientific vi- and to prevent erosion of its pur- Small Explorer (SMEX) program are tality of the discipline is the strength chasing power. well matched to at least one category of our supporting programs, includ- of experiment in our discipline. A ing research with new detectors, We strongly endorse the planned deep UV survey with spectroscopic laboratory astrophysics techniques, augmentation of the Astrophysics capability would provide rich scien- sounding rockets, and pursuit of as- Division Theory Program. We also tific results and would represent an trophysical theory. These programs, support the Origins of Solar Systems important resource for the future ex- collectively grouped in the Research Program and other Office of Space ploration of this region of the spec- and Analysis (,R&A) Program, pro- Science and Applications (OSSA) trum. Both point and extended vide support to develop new tech- programs that serve to strengthen the sources need to be cataloged and nologies and theories, and lend con- nation's research base.

ULTRAVIOLET, VISIBLE, AND GRAVITY ASTROPHYSICS

Future Missions New Flagship and Intermediate Augmentation to Explorer Space-Based Missions: Program: • Astrometric Interferometry Mission (AIM) • Timely launch of FUSE • Imaging Optical Interfer.ometer in Space • Additional opportunities for Astrophysics missions • 16-Meter Telescope in Space • Laser Gravitational Wave Antenna in Space

Additional Small-Class Explorer: Lunar Outpost Astrophysics Program:

• Deep Ultraviolet Survey • Lunar Transit Telescope (LTT)

• Optical Interferometer • Filled Aperture Segmented Optical Telescope

Laser Geodynamics Satellite-3 • Laser Interferometric Gravity (LAGEOS-3) Wave Observatory (5) Advances in (6) Large Space General Relativity and Interferometer or Gravity Astrophysics Telescope beyond HST Even before imaging capa- The technology needed for next- The bold human vision that led bilities become feasible, however, century missions and programs in to HST recognized the benefits to optical interferometric technology General Relativity and gravity astro- science from space observatories that can be applied to high-precision physics has been reviewed in detail took advantage of ongoing techno- astrometry from space. In its March in the Repor( of Ad Hoc Committee logical advances. Consistent with 1991 report, entitled The Decade of on Gravity Astrophysics Technology the view of exploiting new technolo- Discovery in Astronomy and Astro- to the Ultraviolet and Visible Astro- gies for the benefit of science, educa- physics, the Academy's Astronomy physics Branch of the OSSA Astro- tion, and exploration, we support a and Astrophysics Survey Committee physics Division. The report of the plan for a powerful UV and visible- for the 1990s notes that astrometry Ad Hoc Committee, now being pre- wavelength interferometer or large- now lies on the verge of a technologi- pared for publication by NASA, thus aperture telescope that would use the cal revoIution and recommends a complements the present MOWG full range of technologies developed space-based Astrometric Inter- rcport, which highlights mission and in the 20 years since the inception of ferometry Mission (AIM) with an programmatic opportunities for the HST. astrometric accuracy in the range of 1990s (e.g., Gravity Probe-B and the 3 to 30 micro-areseconds. Such an Laser Geodynamics Satellite-3). A National Academy of Sciences instrument could vastly reduce the NASA's growing commitment to study, Space Science in the Twenty- current uncertainty in the cosmic dis- gravitational-physics programs is First Century. published in 1988, tance scale and could detect Jupiter- demonstrated by the institution of a concludes that imaging interferom- sized planets around hundreds of stars university Gravitational Physics etry in space will ultimately play a up to 500 light-years away. component of the R&A program. central role in astrophysics and recommends the construction, through a An Astrometric lnterferometry well directed technology-develop- Mission, designed to carry out high- ment program, of a large, passively precision measurements at the fore- cooled space telescope operating at front of science, is an important step UV, vi sible, and infrared wavelengths in a long-range program of space as a successor to HST. interferometry and should be developed as the first dedicated optical The design of the instrument to interferometer in space. follow HST remains to be determined. It might be either a filled-aperture, The next steps will involve a segmented-mirror telescope in the 16- serious evaluation of the scientific, meter class (or larger), or a large, technical, and cost questions and is- diluted-aperture interferometer con- sues, with the goal of defining a suc- sisting of widely scparatcd telescopes cessor to HST within the near future. phased to produce images in much The Lunar Outpost segment of the the same manner as in radio as- Bush Administration's Space Ex- tronomy. ploration Initiative (SEI), announced in 1989. provides an ideal site for It is crucial that we start non, to very-long-baseline optical inter- de velop the strategies and tech nolo- ferometers and for large telescopes gies required to implement this ad- that otherwise would have to be as- vanced instrument. sembled in orbit. EXECUTIVE SUMMARY Some Key Astrophysical Questions

The MOWG recommended plan Ultraviolet Spectroscopic Explorer stars allows the physics of accretion and programs address many of the (FUSE) will place more stringent to be probed in otherwise well-un- most exciting questions of modem limits on both the size and motions of derstood systems. The accretion- astrophysics. A brief overview is the central sources. However, in or- flow geometry, the structure and lo- presented below; further detail is der to view the actual details of these cation of the boundary layer (where provided in Part II of this report. mysterious objects, we will require angular momentum is deposited), the instruments with even higher spatial composition and outflow geometry resolution. associated with outbursts, and mass Structure of transfer in close binary systems in- the Early Universe volving degenerate stars all represent Stellar Winds current problems that can be explored It has become increasingly clear in Massive Stars directly with HST, EUVE, and FUSE. over the past decade that galaxies in the observable Universe are not dis- One of the first achievements of Moreover, knowledge of the in- tributed smoothly, but instead are UV astronomy was the confirmation teraction between stellar magnetic "clumped" into structures of enor- of the existence of winds from hot, fields and accretion flow obtained mous extent. Studies of the nature massive stars and the discovery that through space-based measurements and dynamics of these structures will such winds are ubiquitous. These is crucial to an understanding of the give us a better understanding of the winds carry away so much mass that angular-momentum evolution of physical processes that shaped the they have a profound impact upon magnetic systems. Such knowledge early Universe, the period of initial the star's subsequent evolution. This, is also needed to address problems as condensation from the primordial in turn, affects the chemical compo- diverse as the origin of millisecond fireball. sition of the material which they re- pulsars and quasi-periodic oscilla- turn to the interstellar medium to tions in accreting compact objects. In order to see how these struc- become available for the fomlation tures evolved, we must look further of subsequent generations of stars. back into time. This is equivalent to Composition looking at more distant and hence Stellar winds played a key role and State of the fainter objects. Such observations in the process by which the Galaxy Interstellar Medium are best carried out in space. Until evolved from a massive cloud of the advent of even larger and more nearly pure hydrogen and helium to a The composition of the inter- sensitive telescopes, we will use the composition containing the rich va- stellar medium (ISM) contains a second-generation Near Infrared riety of elements we observe today. record of Galactic evolution. Fur- Camera (NIC) on HST to observe the HST's increased resolution of spec- thermore, the present state of the ISM most distant galaxies, permitting new tral lines formed in the winds of hot and its response to such external in- insights into critical cosmological stars will help determine the vari- fluences as star formation and super- parameters, including the overall ability and clumpiness of the wind nova explosions determine the dy- density of baryons in the Universe. flows. The capability of STIS to namics of subsequent star formation observe many lines in the wind si- and, therefore, the course of future multaneously will permit identifica- Galactic evolution. Energetics of tion of the energetics of the flow. The Active Galactic Nuclei additional spectral lines available to This complicated interplay FUSE in the far ultraviolet will allow among the ISM, the stars to which it To date, the intemaI conditions us to probe the high-temperature gives birth, and its response to these suggested by spectral analysis of these structure of these regions, providing stars must be understood in order to powerful sources of energy strongly further details about the dynamics of reveal the intricacies of Galactic indicate that they are powered by the instabilities. evoIution. The analysis of different black holes with masses on the order interstellar spectral lines provides of a million to a billion times the mass information about the motions of Sources of the Sun. The increased spatial and material at a variety of temperatures spectral resolution of liST (especially Powered by Accretion and allows us to uncover the kine- when equipped with the second-gen- matical and geometrical relationships eration Space Telescope Imaging Accretion processes pervade as- between ISM phases at different Spectrograph, or STIS) and the Far trophysics. Study of close binary ternperatures. EXECUTIVE SUMMARY

Nature of the nostic of conditions in the Universe frared spectral regions will permit Galactic Halo and the course of nucleosynthesis nearly direct observation of the col- immediately after the Big Bang. Only lapse of the gas and dust involved in Another component of the ISM far-ultraviolet measurements of the star formation. A comprehensive, is the Galactic halo, a high- tempera- relevant spectral lines from space can multispectral approach is required to ture plasma that surrounds the Gal- provide a reliable empirical estimate understand the complicated, sequen- axy. The existence of such a halo was of this ratio--an important future task tial processes responsible for star for- predicted in the 1950s but remained for FUSE. mation. unobservable until IUE was launched in 1978. Planetary systems will form Chromospheric around some of these newborn stars; The plasma component of the Activity in Cool Stars visible and near-infrared observations halo is an interface between our Gal- will be used to investigate those axy and the intergalactic environ- By studying chromospheric ac- systems that could be creating plan- ment, and may be driven by circula- tivity in late-type stars, HST ets. Near-infrared observations, in tion (possibly in the form of gigantic (equipped with second-generation particular, allow the greatest possible instruments) and FUSE will work in convection cells) caused by explo- contrast between planets and the sive events occurring in the Galactic tandem to place our Sun and its mag- parent stars, yielding the best chance plane. The study of the Galactic halo netic activity in the context of the of successfully discovering and in- can reveaI conditions both within our total stellar population and general vestigating such evolving systems. own Galaxy and within the interga- stellar properties. Signatures of high- Visible-light astrometric measure- lactic medium that surrounds it. The temperature plasmas will define the ments carried out by telescopes in the highly ionized material of the halo evolution and decay of magnetic ac- HST class could also provide im- can only be observed by space-based tivity and identify the poorly under- portant indirect evidence of the ex- observatories, and only FUSE will be stood processes that determine the istence of other planets. However, capable of sampling its highest-en- mass loss and evolutionary state of truly giant instruments, such as an cool stars. ergy components. interferometer or a 16-meter class telescope in space, will be required to directly image planetary systems and Evolution of Formation of Stars search for signatures of life. the Early Universe and Planetary Systems

The ratio of hydrogen to its stable The merging of observations obtained in the UV, visible, and in- isotope deuterium is a sensitive diag-

DEFINITIONS

Ultraviolet: Electromagnetic radia- Angstrom: A unit of length equal to (UV) tion of wavelength shorter (A) 10-" cm. Frequently used as than the short-wavelength the unit of measurement for (violet) end of the visible visible and ultraviolet wave- spectrum, roughly 1,200 to lengths of electromagnetic 4,000 A. The lower limit marks radmtion. the cutoff in the reflectivity of normal-incidence optics; Micron: A unit of lenclth equal to 10 .4 shortward of 1,200 A, grazing- (lum) cm, or 10' A. Often used as incidence optics must be used. the unit of measurement for infrared wavelengths. Far Ultraviolet: Electromagnetic (FUV) radiation in the range from Near Infrared: Electromagnetic 912 A (the hydrogen ioniza- (Near IR) radiation of wavelength tion limit) to 1,200 A. This longer than the long- small region of the spectrum wavelength (red) end of the is rich in astrophysically _i visible spectrum, approxi- Important absorption lines. mately in the range 7,000 A (0.7 _tm) to 30,000 A (3 _m). Extreme Ultraviolet: Electromagnetic (EUV) radiation in the range from Visible: Range of the electromag- the low-energy end of the soft z (VIS) netic spectrum visible to the X-ray spectrum at about 100 A human eye, approximately to the beginning of the FUV 4,000 to 7,000 A. spectrum at 912A.

10 Part I. Introduction

he spectral range from the short-wavelength end of the extreme ultraviolet (about 100 A) to the long-wavelength end of the near T infrared (about 3 [am) can provide vital insights into all of the important questions that face astronomers today.

To achieve these insights, a number of complementary diagnostic tools must be employed to study this radiation, including imaging, spectroscopy, photometry, polarimetry, astrometry, interferometry, and gravitational-wave detection.

Advances in space astronomy have always gone hand in hand with advances in space technology. The NASA Ultraviolet, Visible, and Gravity Astrophysics Management Operations Working Group (MOWG) saw the need to systematically plan for the irnplementation of a space-astronomy program that builds on past discoveries, assimilates currently approved missions, and exploits emerging new technologies.

TOOLS OF MODERN SPACE ASTRONOMY

Imaging: Formation of an image respect to carefully established to determine the location and reference frames; used to deter- structure of the emitting source. mine the distance to, and motion of, the source. Spectroscopy: Dispersal of light emitted by a source into its basic Interferometry: Use of two or spectral elements (absorption more telescopes or radiation and emission lines, and con- collectors to sample different tinuum). Used to determine regions of the wavefront from composition, temperature, den- a source, so that information on sity, and dynamics of a source. source direction and structure can be inferred from the interfer- Photometry: Accurate measurement ence patterns obtained; can of the flux of light emitted by a be used to reconstruct an image source, often used in studies of of the source. source variability. Gravitational-wave detection: Mea- Polarimetry: Measurement of the surement of the motions of extent, sense, and orientation massive bodies in response to of the polarization of the light the extremely weak forces exerted emitted by a source, as clues to by the gravitational waves pre- the source emission mechanisms dicted by Einstein's General and the nature of intervening Theory of Relativity and other matter. theories of gravity. Detection systems have been proposed for Astrometry: Accurate measurement placement on the Earth, on the of the position of a source with Moon, and in space.

11 Approach

After surveying the most press- ics, together with NASA's approved technologies, build upon current pro- ing questions in astrophysics today, and proposed future missions in these gram strength, and take into account the Working Group assessed the po- areas. other important concerns (e.g., pro- tential of new and emerging tech- gram balance, the training of new nologies for providing answers to Finally, the Working Group de- scientists, and opportunities for in- these questions and for making fur- veloped a plan for the implementa- ternational cooperation). The ap- ther astrophysical discoveries. The tion of the Academy recommenda- proved and future missions included group next examined the National tions that will address major scien- in the plan are discussed in Part IV of Academy of Sciences' recommenda- tific questions, capitalize on new this report. tions for Astronomy and Astrophys-

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Figure 5. MAJOR ADVANCE IN ULTRA V/OLET SPECTRAL RESOLUT/ON FURN/SHED BY HST is illustrated in this comparison of spectra of the chemically peculiar star Chi Lupi recorded by/UE (top) and the liST Goddard High Resolution Spectrograph (bottom). The atmosphere of Chi Lupi is rich in several unusual chemical elements, containing t00,000 time more mercury and 10,000 time more platinum per unit volume than that of the Sun. FIST ultraviolet spectra/observations are expected to help clarify the origin of such composition anomalies.

12 INTRODUCTION ii Perspective on the Plan

The International Ultraviolet plorer (FUSE, in planning), will be eas of current research. The more Explorer (IUE), launched in 1978 dedicated to ultraviolet astronomy. advanced ("almost-state-of-the-art") and still operating in 1991, made use These missions will employ new sen- technologies will form the basis for of new ultraviolet detector technolo- sor technologies to extend observa- the preliminary designs for the next gies to open a new window on the tions into the extreme ultraviolet generation of major Ultraviolet, Vis- Universe. Some have argued that (EUV). ible, and Gravity Astrophysics facili- IUE is the single most productive ties. Such facilities will provide or- astronomical instrument ever built. The past decade has brought a ders-of-magnitude improvements in tremendous increase in the sophisti- sensitivity and in spatial, spectral, Today we stand on the threshold cation of space technology and in- and temporal resolution by compari- of a new era in space astronomy. The strumentation. When applied to space son with the missions contained or launch of Hubble Space Telescope astronomy, these advances will lead planned in the current program. (HST) has placed into space an ob- to telescopes of unprecedented sensi- servatory whose size, pointing accu- tivity and to significantly improved Technologies that promise the racy, and stability will eventually capabilities to probe and diagnose most significant gains in capability enable it to outperform all of its observed objects through studies of will be developed under NASA ground-based counterparts, as well their temperature, density, dynam- sponsorship: these may be proven on as provide new and vastly improved ics, composition, and structure. sounding rockets prior to being de- observing capabilities in the ultra- ployed as the next generation of Ex- violet and near-infrared spectral re- These technologies will, first of plorers----or as even more advanced gions (Figure 5). all, be used to enhance the capabili- space-astronorny facilities that lie ties of existing missions. In the case beyond the Great Observatories. Smaller missions will also make of HST, second- and third-genera- major contributions. The ASTRO-I tion focal-plane instruments will The implementation plan for Ul- Spacelab payload, flown in Decem- make full use of the substantial col- traviolet, Visible, and Gravity Astro- ber 1990, returned important new lecting area and imaging capability physics, while ambitious, neverthe- ultraviolet observations that are now of the HST optical telescope. In less maintains a good balance among being analyzed; a reflight of the addition, these advanced technolo- large observatories, small and rnod- ASTRO ultraviolet telescopes is now gies will be applied to the develop- crate missions, advanced technology being planned as the ASTRO-2 mis- ment of new, special-purpose Ex- research and development, and com- sion. The ORFEUS and IMAPS in- plorer and Spacelab missions, such munity support and participation. The struments are scheduled for 1993 as the Deep Ultraviolet Survey. plan also provides for the training of Space Shuttle launch and operation a new generation of scientists and aboard the AstroSPAS platform. Two Although we cannot know what engineers. The program is designed Delta-class Explorer missions, the important discoveries lie ahead, we to ensure that each element comple- Extreme Ultraviolet Explorer (EUVE, can foresee how new generations of ments the others and, at the same scheduled for launch in 1991) and the instrumentation will contribute to time, produces growth at the forefront Far Ultraviolet Spectroscopic Ex- many fundamental and exciting ar- of astrophysical knowledge.

13 ! Part II. Astrophysical Questions

Astrophysics must be the provision of answers to ,,fignificant T hescientificprimary questionsgoals of anda plantheforpotentialUltraviolet,for newVisible,scientificand Gravitydiscov- eries.

In this part of the report, we examine the most pressing scientific questions now under discussion in astrophysics_questions that have been raised in no small measure by the achievements of past and currently operating NASA space missions. These questions are grouped under five main headings:

A. Structure of the Universe;

B. Structure of galaxies and quasars;

C. Structure of the Milky Way;

D. Structure of stellar systems; and

E. Verification of relativistic theories of gravitation.

For each question, we provide examples of the major contributions to be made by the missions included in the recommended plan. Finally, recogr'iz- ing the potential for serendipitous discovery that characterizes each of the missions that we have identified, we present a sixth category:

F. The unknown: serendipity.

_PI_CEM)tNG PAGE BLANK NOT FILMED

15 A. Structure of the Universe

Current Program Contributors:

• Hubble Space Telescope (HST) • HST Second- and Third-Generation Instruments • Far Ultraviolet Spectroscopic Explorer (FUSE) • Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS)

Future Program Contributors:

• Optical lnterferometers (space-based or lunar-based) • 16-Meter Telescope (space-based or lunar-based) • Lunar Transit Telescope

1. Scale of the Universe nosities of Cepheid variables. A sec- though developed within the com- ond-generation instrument, the Wide mon framework of Einstein's Gen- Even six decades after Hubble's Field/Planetary Camera lI, wilt be eral Theory of Relativity, typically discovery of the expansion of the able to extend this work out to the yield different abundance predic- Universe and recognition of the cos- distance of the Virgo cluster of gal- tions. Observations of light-element mic distance scale, observational axies. abundances can therefore be used to cosmology remains seriously data- discriminate among these competing deficient. Because of the faintness of Further improvements can also theories. However, current observa- objects at such immense distances, be expected from new initiatives in tions are inadequate to do so. observations typically press the lim- the next I 0 years. The P-L relation of its of current techniques. Cepheids at visible wavelengths has The Far Ultraviolet Spectro- considerabIe scatter and is sensitive scopic Explorer (FUSE) will observe The need for new, high-quality both to metallicity effects in the stars and analyze the absorption of radia- observations is underscored by the and to the effects of extinction in tion by deuterium that was formed continuing controversy over the value their neighborhoods. Near-infrared during the first 3 minutes after the of H 0, the Hubble constant, which photometry provides a more accurate Big Bang (Figure 6). Different cos- speci ties the velocity of recession of index of luminosity than measure- mological models predict different a galaxy through a proportionality ments in the visible and yields a P-L ratios of the abundances of deute- with the distance. An overall uncer- relation with considerably less scat- rium (D), helium (_He and 4He), and tainty of about a factor of two in the ter. In the crowded fields of these lithium (¢Li) to hydrogen (H) in the value ofH 0 results from the accumu- relatively remote galaxies, however, early Universe. These abundances lation of contributing uncertainties at such observations can be made only can then be extrapolated to present- every step of the distance ladder. with the second-generation NIC in- day values. However, since some of strument for HST. these elements are also products of A significant step towards im- stellar evolution, one must sort out proving this situation, based on the original component of the abun- Henrietta Leavitt's famous period- 2. Deuterium dances produced by the Big Bang luminosity (P-L) relationship for from the contributions subsequently Cepheid variable stars, will be taken The cosmic abundances of light made by stars. % by HST. A designated HST "key elements place a powerful constraint project" using current, first-genera- on cosmological theories. Different Nuclear fusion within stellar tion instruments will carry out mea- explanations of the "Big Bang" and cores converts deuterium to helium surements of the periods and lumi- subsequent cosmic evolution, al- (3He) and synthesizes other light ele-

16 ASTROPHYSICALQUESTIONS ments. Since the interstellar abun- measured in distant (and apparently The simplest cosmologies are dances are modified by continuous younger) external galaxies by HST to parametrized by the Hubble constant injection of new material processed determine the primordial ratio. H0and the deceleration parameter q0, in stars, we need a means of extrapo- or equivalently the mean density of lating the modifications of abun- the Universe at the current epoch. A dances back to their values before 3. Density reliable estimate of the critical den- stellar processing. of the Universe sity requires an accurate value for the Hubble constant. FUSE holds the key to this ex- We want to know the current trapolation. By studying differences density of the Universe so we can Estimates of the density param- in D/H ratios in many locations in the predict its future. To distinguish eter fall into two categories. First are local interstellar medium (i.e., within whether or not the Universe will con- dynamical estimates, which are sen- 50 pc) and in many infalling gas tinue to expand forever, or whether it sitive to the presence of both lumi- clouds in our own Galaxy, astrono- will eventually collapse upon itself, nous and dark matter. Various esti- mers will learn how the D/H ratio we must know its density. The den- mates have been made from indi- varies with degree of stellar develop- sity at which there is just enough vidual galaxy-rotation curves, stud- ment and mixing with the surround- mass to allow gravity to stop the ies of binary galaxies, cluster and ing medium. These D/H ratios may expansion and eventuaIly initiate the supercluster dynamics, and so on. then be compared with D/H ratios collapse is called the critical density. Most of these yield values for the

CEPHEID VARIABLES AS DISTANCE INDICATORS

Cepheid variable stars can be from the Earth's orbital baseline (the used to measure distances to stellar method of trigonometric parallax). groups outside our Galaxy because However, even the nearest Cepheids there is an observed connection in our Galaxy are so distant that between the period (P) of a Cepheid's measurements from space are light variation and its luminosity (L): required. the longer the period, the higher the luminosity. This remarkable period- New initiatives will provide other luminosity (P-L) relationship was basic information necessary for a discovered by Henrietta Leavitt in the truly accurate value for Ho. The best course of studying Cepheids in the current P-L calibration relies on data Small Magellanic Cloud, a nearby from Galactic clusters whose dis- dwarf galaxy. tances can be estimated from cluster main-sequence fitting and from The P-L relationship has been stellar-pulsation theory. Future refined since Leavitt's time, but its space-astrometry missions will application to the extragalactic provide direct parallax measurements distance scale remains the same. By to nearby Cepheids, while space measuring the apparent magnitude interferometers will make possible and period of a Cepheid in a remote direct measurements of their radii. A galaxy, we can determine the dis- very large optical and infrared tele- tance to the remote galaxy in terms of scope in space can detect Cepheids the distance to the calibrating galaxy more distant than those accessible to (e.g., the Small Magellanic Cloud). HST, thereby reducing the need for Calibration of the P-L relation re- tertiary distance indicators, which are quires the measurement, by other currently required to reach distances means, of the actual distance to a where velocity perturbations to the Cepheid. The only direct way to do Hubble flow are negligible. this is by triangulation--for example,

17 "standard candles" from the Local 10 lo - _...i BigBang I Group of galaxies to more distant galaxies (e.g., the Virgo Cluster). Surveys to detect supernovae from the ground have begun and will pro- g" duce a reasonable number of candidates at low redshifts for spectroscopic observations using HST. 10s & These measurements will be very E difficult, and substantial effort will also be needed to advance our theoretical understanding of supernovae, but the potential reward is great. It may prove possible to measure directly the curvature of space. A 10o project such as the Lunar Transit Telescope would play a key role in 100 10lo 1( the detection of cosmologically dis- Time since Big Bang (sec) tant supernovae. Comparisons of their light curves with those of the Figure 6. THERMAL HISTORY OF THE UN/VERSE can be reconstructed current epoch place direct constraints with the aid of data from NASA space missions, The Far Ultraviolet on the geometry of the Universe. Spectroscopic Explorer (FUSE) will observe absorption lines arising from deuterium that was formed during the first 3 minutes after the Big Bang. The standard Big Bang model current mass density that lie in the important to recognize that other agrees in a general way with avail- range from 10% to 30% of the critical forms of matter (e.g., neutrinos) are able observations. However, diffi- density, although some estimates up known to exist, and that still other culties emerge when observations of to 100% have been recently pub- forms (e.g., postulated but undiscov- Big Bang remnants are attempted. lished. ered particles like axions) may also For example, the recent Cosmic exist. The density of mass in these Background Explorer (COBE) mi- A second sensitive estimate of other forms could conceivably ex- crowave-background observations the density parameter can be obtained ceed the density of baryons and could appear to be too isotropic on large from comparisons of the abundances even close the Universe. angular scales by comparison with of the light chemical elements with results now emerging from studies at the prcdictions of hot Big Bang mod- A direct attack on the geometry visible wavelengths. Such discover- els, as discussed above. These esti- of the Universe--H 0 and q0--should ies have stimulated a new cross-fer- mates will constrain the fraction of prove feasible during this decade us- tilization between fundamental the critical density which is present ing supernova observations, since physics and observational cosmology. in the familiar"baryonic" form. It is these objects currently serve "as Since the early Universe is the only

HUBBLE SPACE TELESCOPE AND THE AGE OF THE UNIVERSE

However, a designated HST "key The Hubble Space Telescope is project" will finally finish Hubble's named after Edwin Hubble, who pioneering work after more than half discovered, in the 1920s, that the a century, using one of the same Universe is expanding. Hubble then distance indicatorsDCepheid vari- initiated a program to estimate the able starsDthat he used. When age of the Universe through system- equipped with second-generation atic observations of the distances instruments that correct for spherical and recessional velocities of external aberration, HST will be able to ob- galaxies. It has not been possible to serve Cepheids seven times more complete this program with tele- distant than those studied so far with scopes on the ground. the largest ground-based telescopes.

18 ASTROPHYSICALQUESTIONS

DEUTERIUM Measurements of the ratios of the During the first 3 minutes after numbers of these atoms relative to the Big Bang, conditions in the hydrogen reveal the density of the Universe were similar to those that Universe at this very early epoch. In now exist at the center of the Sun. general, the higher the density, the The temperature and density were so more hydrogen is ultimately con- high that protons and neutrons fused verted to helium, and the less deute- to yield chemical elements slightly rium remains. Space observations, more complicated than the initial particularly those to be made by the Far Ultraviolet Spectroscopic Ex- hydrogen (H), deuterium (2H), and the plorer (FUSE), offer the best means of two forms of helium (3He and 4He). obtaining this information. Present The deuterium Is produced by an estimates suggest that the early intermediate step in the fusion of Universe had a very low density, but hydrogen into helium. the uncertainties in the data are still large.

environment energetic enough to al- verse. No satisfactory picture of the induced perturbations in the Hubble low observation of the most basic formation of both galaxies and their flow. Unambiguous detection of large physical interactions, both cosmolo- large-scale distribution has emerged. peculiar velocities, such as those at- gists and physicists are now extremely Model universes whose density is tributed to the "Great Attractor," have interested in obtaining a more accu- dominated by cold dark matter are proved difficult with current extraga- rate observational understanding of successful in producing galaxies with lactic distance indicators. However, the evolution of the Universe. many of the observed properties, but something must be responsible for are unable to produce the observed the anisotropy in microwave radia- pattern of large voids. Hot universes tion seen in the Local Group refer- 4. Distribution of dominated by dark matter are able to ence frame. Galaxies yield the large-scale pattern but have difficulty producing the individual Redshift measurements are only The large-scale structure in the galaxies. sensitive to the radial dimension of distribution of galaxies found from galactic motion. To fully map the extensive redshift surveys (Figure 7) Another test of models for the flow pattern of the luminous matter is difficult to reconcile with the stan- origin of structure in the Universe in the nearby parts of the Universe, dard Big Bang model of the Uni- involves detection of gravitationally proper motions are also necessary.

COSMIC DENSITY AND THE EXPANSION OF THE UNIVERSE The total density of matter of all kinds is a crucial number. Low When we look out at the Universe density means that the Universe will through our telescopes, we see only expand forever. But if the density is luminous matter. However, we also sufficiently large, gravitational effects observe indirectly the gravitational will not only slow the expansion, but effects of matter that we cannot see-- eventually halt and reverse it-- the so-called "dark matter." Mea- leading to a future collapse of the surements of dynamical interactions Universe. Although the density of among galaxies suggest that there is luminous matter is now fairly well approximately 10-100 times as much determined, the final answer on our dark matter as luminous matter in the cosmic fate awaits better information Universe. on the ratio of dark to luminous matter.

19 Figure 7. DISTRIBUTION OF GALAXIES in the Universe is shown in this Harvard-Smithsonian Center for Astrophysics map of 4,000 galaxies within 450 million light years of our own Milky Way (at origin of coordinate system, bottom center). The so-called "Great Wall" of galaxies, some 500 million light years long, stretches horizontally across the middle of the diagram.

An astrometric interferometer, lo- Imaging observations of galac- ated with the peak of the galactic cated in space or on the Moon, would tic structure and morphology are pos- emitted-energy profile, are redshifted be capable, in principle, of measur- sible out to moderate distances with into a near-infrared region of rela- ing the proper motions of galaxies HST. But even with second-genera- tively high sky background. A new, with compact nuclei out to a distance tion visible and near-infrared spec- very large telescope in space, operat- of approximately 100 megaparsecs. trographs, HST is too small to pro- ing in the visible and near infrared vide the necessary dynamical data on and equipped for both imaging and Much work remains robe done if these very faint sources. Even the spectroscopy over a broad range of we are to connect the present observ- largest ground-based telescopes lack these wavelengths, is required if we able Universe with the physics of the the spatial resolution to carry out are to successfully attack the most early Universe. "Lookback observa- such observations; moreover, the basic questions remaining in cosmol- tions" of galaxies and their distribu- spectral features of interest, associ- ogy. tions are required out to the largest possible redshifts.

LARGE-SCALE DISTRIBUTION OF GALAXIES

Distances to galaxies can be No satisfactory explanation has estimated by using the Hubble yet been given for this sponge-like relationship, which establishes a geometry. If the measured sizes and proportionality between galaxy velocities of the galaxies are correct, redshift and distance. Large-scale there has not been sufficient time for surveys of galaxy redshifts now in the normal motions of galaxies to progress show that the three-dimen- have cleared out the voids. Perhaps sional distribution of matter in the they were swept out by colossal Universe is not smooth, but filamen- explosions--or perhaps, on the tary. Large voids, sometimes as largest scales, the matter in the much as 200 million light-years Universe was highly filamentary or across, appear to be surrounded by clumped from the beginning. sheets and filaments containing most of the galaxies and galaxy clusters.

2O ASTROPHYSICAL QUESTIONS i B. Structure of Galaxies and Quasars

Current Program Contributors:

• Hubble Space Telescope (HST) • HST Second- and Third-Generation Instruments • International Ultraviolet Explorer (IUE) • Far Ultraviolet Spectroscopic Explorer (FUSE)

Future Program Contributors:

• Deep Ultraviolet Survey • Optical lnterferometers (space-based or lunar-based) • 16-Meter Telescope (space-based or lunar-based) • Lunar Transit Telescope

1. Formation and sophisticated, systematic, and broadly erage will allow detailed study of key physical processes simultaneously Evolution of Galaxies ranging ground-based observational studies. Nevertheless, progress has throughout the electromagnetic been slow. While there remains much spectrum. Particularly interesting questions to be accomplished with ground- in galactic formation concern the based telescopes, their inherent limi- One of the most exciting discov- manner in which protogalactic gas- tations are now clear. eries of the last decade was the recog- eous clouds collapsed to form galax- nition that galaxies undergo signifi- ies, the frequency of these collapses, The high spatial resolution, high cant evolution on limescales much and the frequency of interactions and spectral resolution, and wideband less than the age of the Universe, and mergers among galaxies or spectral coverage (X-ray to infrared) that some evolution has occurred quite protogalactic condensations of stars offered by substantial telescopes in recently The resolving power of liST, and gas. The epoch at which galaxies space are critical for significant fur- potentially greater than that of ground- formed in the early Universe also ther progress. High spatial resolu- based telescopes for galactic studies, remains unclear, as does the total tion will permit detailed observations will have a major impact on our un- duration of the formation process. within the crowded fields typical of derstanding of why some galaxies remote galaxy clusters. High spec- have undergone such recent evolu- The complex problem of galaxy tral resolution and wide spectral cov- tion. formation has been attacked through

HOW DID GALAXIES FORM?

The question of galaxy formation that brought the galaxy to its present is a wide-ranging one. It deals not state. This question also leads only with the physical processes at astronomers to seek to determine the work during the collapse of the amount of dark matter associated protogalactic clouds, together with with galaxies, the size of any dark- initial star formation and establish- matter envelopes around galaxies, ment of a stable dynamical structure and the ratio of dark matter to lumi- for the galaxy, but also with the nous matter in different types of subsequent and less rapid stellar, galaxies. chemical, and dynamical evolution

21 MILLI-ARCSECOND (mas) AND MICRO-ARCSECOND (pas)

The angular height of a 6-foot Space Telescope astrometric obser- astronaut on the Moon, as seen from vations is a few milli-arcsec (mas). the Earth, is 1 milli-arcsecond (1 mas, The accuracy of astrometric mea- or 0.001 arcsec). The angular thick- surements from Earth orbit and from ness of a 5-cent piece (a nickel) held the Moon is expected eventually to by such an astronaut is 1 micro- approach the micro-arcsecond (pas) arcsecond (1 _tas, or 0.001 mas). level. The accuracy expected from Hubble

this probe of the intergalactic medium and gaseous clumping from the nearest to the most distant quasars.

One of the primary objectives of the recommended plan is to probe the Universe at distances (and hence eras) associated with the primary episodes Of galaxy formation. This objective places stringent demands upon the capabilities of even space-based telescopes.

A particular requirement is the resolution of low-surface-brightness structures with a spatial scale of 102- 102pc at redshifts greater than unity. These are the spatial scales typical of the structures that reveal what is hap- pening in galaxies during their formation and evolution, e.g., star- forming complexes, spiral arms, and merger tails and structures. Their observation and measurement require angular resolutions on the order of i0-50 milli-arcseconds (mas) at wavelengths ranging from the ultraviolet well into the infrared.

Figure 8. CENTRAL BLACK HOLE surrounded by accretion disk of high- While HST can meet the less temperature matter is believed to supply the power for radiation emitted demanding end of this requirement, a by active galactic nucleL The width of the spectral lines depends on the major advance in observational ca- distance of the emitting region from the center; linewidth diminishes with pability is actually needed. This could distance. be provided by a large telescope that combines light-gathering power over For nearby objects, the high spa- even more powerful instruments will a broad (UV to IR) spectral region tial and spectral resolution of HST be required to study the most remote with spatial resolution that matches will further our understanding of the sources. the scales of the relevant structures in contribution of evolutionary activity high-redshift galaxies. High in galactic nuclei relative to that in Observations of the absorption throughput and" wide-field spectro- galactic disks and envelopes. The lines in quasar spectra, arising from scopic instruments are also needed in role played by interactions and merg- intervening matter, have proved to be such a telescope. Given these capa- ers will be greatly clarified by such an invaluable tool for diagnosing bilities, astronomers will for the first studies. However, the sensitivity and conditions in the post-formation time be able to unveil the early Uni- resolving power of HST will still be Universe. High-resolution ultravio- verse and to witness directly the pro- a limitation at large distances, and let spectroscopy is essential to extend cess of galaxy formation.

22 ASTROPHYSICAL QUESTIONS

2. Active Galaxies Space astronomy has already also produce major strides in our un- played a rnajor role in expanding our derstanding of gravitational lensing; The central engines of active ga- knowledge of active galaxies. Satel- the characteristic angular scale of the lactic nuclei are thought to be very lite observations were required to image formed by a galactic gravita- massive black holes that produce discover that such galaxies are sub- tional lens is I arcsec or less, well power by accreting material into their stantial sources of power in the infra- suited to HST's angular resolution deep gravitational wells (Figure 8). red, ultraviolet, X-ray, and gamma- but very difficult to measure from the But even if this speculation is correct, ray regions of the electromagnetic ground. many fundamental questions about spectrum, and not only in the optical active galactic nuclei remain to be and radio bands. The identification When applied to studies ofhigh- answered. How do they manage to of possible thermal accretion-disk redshift quasars, the HST ultraviolet radiate photons with equal facility in radiation was brought about largely sensitivity (which extends down to so many wavebands? What is the through observations made by the 1,200/_) will also provide a first look source of their accretion fuel? Why International Ultraviolet Explorer at the truly hard ultraviolet spectrum do they occur in some galaxies and (IUE). Future missions should bring of active galactic nuclei. This por- not others? What is it about young further advances in our understand- tion of the spectrum should be par- galaxies that amplifies their power? ing of the nuclei of active galaxies. ticularly useful in revealing extreme What determines their choices from relativistic phenomena at the very the "menu" of activities? Why are Within the next few years, HST edge of the central energy engine. some strong emitters in the radio re- should be able to tell us a great deal gion, while others are weak? Why about the characteristics of the galax- The Far Ultraviolet Spectro- ies that host active nuclei. This mis- are some highly variable on short scopic Explorer (FUSE) will also timescales, while others are quiet? sion wilt provide the first close look contribute significantly to studies of at the interactions between relativis- Why are some strongly polarized, active galactic nuclei by measuring and others hardly polarized at all? tic jets and galactic material. It should the hard-ultraviolet spectrum of

ACTIVE GALACTIC NUCLEI (AGNs)

At one time it was thought that delivered in either of two remarkable galaxies were little more than self- ways. In most cases, the power is gravitating collections of stars spread roughly equally over the producing optical light by thermal entire electromagnetic spectrum, radiation. Over the past several from the mid-infrared through the decades, however, we have learned gamma-ray region. Sometimes, that they are much more complicated however, the power is emitted prima- and surprising objects. In addition to rily in relativistic jets of matter, with containing significant quantities of comparatively little going into any gas and dust unattached to stars, kind of electromagnetic radiation. galaxies seem also to harbor large amounts of dark matter, and, in many Because the relative strengths of cases, non-stellar nuclei of extraordi- these different sorts of non-stellar nary luminosity. One or two percent processes vary from case to case, of present-epoch galaxies produce as numerous subclasses of these active much power by means of non-stellar galaxies have been identified; among processes in their nuclei as is pro- the most common are Seyfert galax- duced by all their stars combined. ies, radio galaxies, "BL Lac Objects," and quasars. However, one of the In the early Universe, the same achievements of the past decade is fraction of galaxies housed active the recognition that all these different nuclei, producing a hundred to a forms are at heart the same sort of thousand times as much power as system--hence the portmanteau their surrounding stars. Moreover, name, "active galactic nuclei" this extraordinary power output is (AGNs).

23 nearby active galaxies. This mission's where interruptions arising from resolve the stronger emission-line EUV sensitivity will also permit the weather, lunar phase, etc., do not region, the so-called "broad line re- first observations of active-galaxy Occur. gion," even in the nearest objects. continuum emission in the neighbor- The same resolution is required to hood of 100 _--the long-wavelength Another reason to study AGN resolve the inner core of active galax- cutoffofthe Advanced X-Ray Astro- variability from space is the opportu- ies at cosmological distances to dis- physics Facility (AXAF), the third nity to bring absolute flux measure- cern the boundary between those Great Observatory. ments to bear on the the extraordinar- portions of the galaxy dominated by ily wide spectral range characteristic stellar processes and those truly con- Other possible missions can of AGN emissions. Coordination of trolled by the nucleus. Such high contribute in major ways as well. An variability studies in many spectral angular resolution can only be all-sky ultraviolet survey (e.g., the regions is an important tool for the achieved by optical interferometry in Deep Ultraviolet Survey) should be study of the central engine. space. an ideal way to compile very large, homogeneous samples of active ga- In the long term, further obser- Collecting area is complemen- lactic nuclei because the contrast with vational progress depends on the ex- tary to angular resolution. HST-level ordinary stars and galaxies is greatest ploitation of new technology. The resolution is, in principle, high enough in the UV. In order to truly under- two technological frontiers in visible to allow imaging of even the most stand their dramatic cosmological and ultraviolet astronomy are higher distant galaxies, but a practical limit evolution, such large well-defined angular resolution and larger collect- is set by the rapid cosmological dim- samples are essential. ing area. A resolution of 0.1 arcsec ming of surface brightness. Consid- (100 mas), available for the first time erably greater throughput will be Space observations also hold with the second-generation Wide necessary for efficient observations unique promise for studies of vari- Field Planetary Camera II (WF/PC of host galaxies at high redshift. ability. Such studies can be used both I1) on HST, will open up the possibil- Because the efficiency of present to infer the structure of the inner ity of spatially-resolved studies (in detectors for much of the visible and emission-line region of galaxies and the nearer active galaxies) of the larger ultraviolet approaches I00%, only to provide a new window for study of of the two principal emission-line greatly enlarged apertures will solve the central engine. Regularity of regions, the so-called "narrow line this problem. Thus, to achieve the sampling is a prerequisite for vari- region" (Figure 9). full benefits of high angular resolu- ability studies. However, on tion, a large collecting area is also timescales of days to years, regular- However, it will take an angular necessary. ity can best be achieved from space, resolution of 0.I mas (100 laas) to

12 Angular Resolution: .001 .01 .1 1 AGN Broad- 10 Line Region

_8 8 ¢56 .i..a _v _:_ 4

8 12 16 20 24 28 log (size), cm Figure 9. MAJOR ADVANCES IN ANGULAR RESOLUTION will be required for detailed studies of active galactic nuclei. The second-generation Wide Field/Planetary Camera II on HST will bring l O0-milli-arcsecond resolution to bear on the AGN narrow-line region, but micro-arcsecond capability is needed to probe central AGN features and many other astronomical objects.

24 ASTROPHYSICAL QUESTIONS C. Structure of the Milky Way

Current Program Contributors:

• Hubble Space Telescope (HST) • HST Second- and Third-Generation Instruments • Extreme Ultraviolet Explorer (EUVE) • International Ultraviolet Explorer (IUE) • Far Ultraviolet Spectroscopic Explorer (FUSE) • ORFEUS/IMAPS/AstroSPAS

Future Program Contributors:

• Deep Ultraviolet Survey • Optical Interferometers (space-based or lunar-based) • 16-Meter Telescope (space-based or lunar-based) • Lunar Transit Telescope

1. Interstellar Dust size of the absorbing grain. In addi- therefore provides clues to grain size tion, there are "features" in the varia- and composition. tion of absorption strength with An interstellar dust grain absorbs wavelength that depend upon the Shape effects can also be impor- light most strongly when the wave- composition of the dust. Analysis of tant, particularly at long wavelengths. length of the light approximates the the absorption spectrum of dust It seems likely that dust grains with

INTERSTELLAR DUST

Although interstellar dust is a composed of metals that have "fro- minor constituent of the interstellar zen out" of the interstellar gas, and medium (ISM), it has a profound since (under most interstellar condi- impact on several fundamental tions) the primary formation site for astrophysical processes and issues molecules is grain surfaces, the (Figure 10). These include the forma- importance of interstellar dust in this tion and collapse of interstellar fundamental process is clear. clouds into stars and planetary systems, the regulation of the rate at Furthermore, the rate at which which galaxies evolve, and the interstellar clouds collapse to form determination of chemical composi- succeeding generations of stars tion of the ISM in our Galaxy and its regulates both the rate of new-star evolution. To address these issues, a formation and the types of stars detailed understanding of the dimen- which can be formed. These two sions and composition of interstellar factors are instrumental in determin- dust grains is required. ing how a galaxy evolves. Dust plays critical roles at every stage of these The ability of a gaseous plasma processes. In addition, because dust to radiate away heat, and thereby to has been detected in the earliest contract and collapse, is strongly known galaxies and QSOs, dust affected by the presence of metallic must have played these roles since ions and molecules. Since dust is the first epochs of galaxy formation.

25 in space. For the first time, astronomers could investigate a variety of ionization states for different elements.

Determinations of the abundances of many elements (relative to those of atomic and molecular hydrogen) furnished broad new insights into the fundamental processes of depletion, caused by the condensation of free atoms onto dust grains. Comprehensive surveys of molecular hydrogen (H 2) were carried out for many lines of sight. The rota- tional excitation of this molecule provided estimates of local kinetic temperatures, densities, and ultraviolet starlight fluxes inside diffuse clouds of gas. The existence of per- vasive, low-density, very hot gas in Figure I0+ DARK LANES OF INTERSTELLAR DUST bisect the image of the disk of our Galaxy was revealed the "Sombrero Galaxy" (M 104) in the constellation Virgo. This flattened by the ubiquitous absorption features spiral system is inclined only 6 degrees to the line of sight; clouds of dust of five-times ionized oxygen (O VI). pervading the galactic plane are silhouetted against a background of millions of stars in the nuclear region. In the decade that followed Copernicus, ultrav iolet spectroscopy elongated shapes or "fluffy" struc- Because of IUE's limited sensitivity, was carried out by the International tures can account for the 100-tJ.m however, we have not yet been able Ultraviolet Explorer. Although this cirrus emission observed by the In- to probe grain properties in the dens- satellite provided several key ISM frared Astronomy Satellite (IRAS). est regions of dust clouds, where much findings, IUE was not designed to This dust may also be observed in the of the important cloud chemistry oc- provide the high spectral resolution curs. ItST will enable us to probe far-UV diffuse background emission. and photometric precision necessary these environments far more deeply for forefront ISM research. Studies both from the ground than before. and from space have shown that the It will therefore be the task of the Several important interstellar mean size of an absorbing grain is first-generation Goddard High Reso- small, on the scale of ultraviolet molecules are disrupted by radiation lution Spectrograph (GHRS) cur- wavelengths. The ultraviolet is thus between the hydrogen ionization limit rently on HST to build upon the ear- one of the most useful wavebands for (912 ]k) and the effective cutoff of the lier accomplishments of Copernicus. IUE and HST optics (about 1,200 ,_). the investigation of dust properties. A vast increase in the capability to Furthermore, because the absorption Only FUSE can provide studies of record a broad wavelength range in a dust absorption at these wavelengths, by dust effectively "shields" the in- single exposure will be provided by ner regions of interstellar clouds from permitting assessments of the role of the Space Telescope Imaging Spec- the ambient stellar ultraviolet radia- dust in the chemistry that triggers the trograph (STIS), a second-generation, which could disrupt the forma- collapse of interstellar clouds. tion instrument now being designed tion of molecules in the cloud core, for installation on HST in the mid- understanding its absorption at ultra- 1990s. violet wavelengths is particularly 2. Interstellar Gas important. The Far Ultraviolet Spectro- The appearance of interstellar scopic Explorer (FUSE) will comple- The International Ultraviolet ultraviolet absorption lines in stellar ment the HST observing program by Explorer (IUE) has contributed spectra recorded by the Copernicus extending spectroscopic observations greatly to our understanding of inter- satellite during the 1970s triggered a to wavelengths below the HST sensi- stellar dust, and has even allowed us vigorous era of research on the tivity cutoffnear 1,200,_. This region to sample the ultraviolet extinction chemical composition and physical contains all but a few of the Lyman by extragalactic dust for the first time. state of atomic and molecular gases bands ofH 2and HD; the entire Wemer

26 ASTROPHYSICAL QUESTIONS

or radio wavelengths. Of particular importance is the ability of FUSE to detect molecular hydrogen, the most abundant molecule in the Universe.

The cold component of the ISM has an appreciable amount of H_,. However, Copernicus observed this component only along sightlines toward brighter stars in the solar neigh- borhood; such lightly obscured regions cannot be typical of star-forming environments generally (.Figure 11). The much more sensitive FUSE instruments will record spectra of stars that are more heavily obscured by interstellar dust, allowing us to sample the major constituent of molecular clouds to greater depths.

Many additional questions can be addressed only by space ultraviolet spectroscopy. For example, ygung hot stars, old giant stars, and supernovae inject enriched gas into the ISM, heating the surrounding matter Figure 11. TRIFID NEBULA (M 20) in the constellation Sagittarius glows from the radiation of hot, internal stars. Dense lanes of interstellar dust and accelerating it to supersonic obscure light from surrounding clouds of hydrogen and helium, driven speeds (Figure 12). Ultraviolet ab- outward by radiation pressure and stellar winds from stars in the center. sorption lines yield important insights into the effects of these processes and system of H,; weaker members of the these ions are produced only in high- into the subsequent dynamical dis- Lyman series of hydrogen, which are temperature, collisionally ionized turbances. In many circumstances, crucial for measurements of the plasmas. shock-heated gas can rise high above atomic D/H ratio; and all the reso- the galactic plane, forming a highly nance lines of certain key atoms and These ultraviolet observations ionized, dynamic halo that can be ions. The spectra of O VI and S VI will sample a wider range of physical studied in the ultraviolet; this mate- are of particular importance, since conditions than is possible at visible rial could circulate in a "fountain

THE INTERSTELLAR MEDIUM (ISM)

In our Milky Way Galaxy, the gas as that in a similar tube only average density of gas between the 1 meter long containing air at sea- stars is only about one atom per level pressure. cubic centimeter--about ten billion times less dense than the "empty The ISM is continually recycled space" produced by a good labora- as it coalesces under the influence of tory vacuum pump. This interstellar gravity to form new stars, only to be gas consists almost entirely of replenished as the stars evolve and hydrogen (about 90%) and helium die. Explosions from supernovae (about 10%), with small traces of violently disturb and heat some of the heavier elements, if we were to look gas, raising temperatures from about through a tube from a platform above 50 K to 106 K or more. The contrast in the Earth's atmosphere toward the density between the cold and hot most distant star visible to the naked interstellar matter is about the same eye, we would view this star through as the ratio of the densities of water approximately the same amount of and airmabout 1,000 to 1.

27 (.The velocily channel separations of radio receivers are often fine enough to resolve the separate domains, but we are then defeated by the finite beam widths of the telescopes!)

With the power to isolate and study small parcels of gas, we could address still further questions about how different phases of the ISM in- teract-specifically, how the gas within individual clouds responds to external dynamical disturbances. What happens internally, when a cloud is overtaken by a shock wave in the surrounding medium? How does a cloud respond to the ram pressure .L'J I' i I' from a smooth, external flow of ! I I | 1 kpc intercloud gas? Do magnetic fields Multiple / / / t, I modify' this reaction? Atoms with excited fine-structure levels allow us Supern°vae J/ _kM + to correlate kinematics with local

., ./ _ . Metal- pressures. High- _'- "_ Enhanced Velocity Supernovae It is important to investigate in- Clouds Ejecta terstellar shock waves at high spectral resolution to determine their Figure I2. SUPERNOVA DETONATIONS IN THE GALACTIC PLANE chemical and ionization structures, shock-heat surrounding hydrogen gas to million-degree temperatures both for structures that are softened and eject high-velocity clouds of material enriched in heavy elements. Ultraviolet observations from space are essential for the study of these by coupling to magnetic pressures dynamic processes. and those that are not. The stratifica-

flow" as it cools or, alternatively, escape from the galaxy in a wind. Both HST and FUSE can provide | significant insights into these processes. i

! 3. Complex ISM Structure 7

The complex, filamentary character of the ISM is conspicuous in =_ photographs of absorption and emission nebulae. With very." few excep- tions, observations of interstellar absorption lines merge the contributions from individual gas parcels as they sample a complex, heteroge- neous environment. Even HST and = Figure 13. SHOCK-HEATED INTERSTELLAR GAS CLOUDS m the FUSE will have insufficient velocity z constellation Cygnus glow with the ultraviolet radiation emitted by triply resolution to always differentiate ionized carbon atoms. This image, recorded by the ASTRO- 1 Ultraviolet contributions from regions with Imaging Telescope aboard the Space Shuttle in December I990, shows a widely disparate physical conditions portion of the "Cygnus Loop, "shaped by shock waves from a supernova or modest differences in location. explosion some 20,000 years ago.

28 ASTROPHYSICAL QUESTIONS tion of cooling and recombining rna- terial in post-shock regions can be resolved only if we measure components separated by about l kin/see. Cygnus Loop (Some chemical species are predicted Ly o_ to exhibit displacements of less than 1km/sec from others, even if they are OI c tv viewed perpendicular to the shock fronl.) The GHRS and STIS instru- I ments for HST will provide the first steps, supplemented by observations HeII with the Interstellar Medium Ab- sorption Profile Spectrograph (IMAPS) in the far ultraviolet.

Theories of the ISM predict the existence of other interesting processes, such as conductive and evaporative interfaces between cool, dense clouds and the much hotter, low-density medium. We need to 1000 1200 1400 i600 1800 explore the interface structure, the Wavelength (A) exchange of material between the two media, and the effects of mag- Figure 14. ULTRAVIOLET SPECTRUM OF GLOWING GAS IN THE netic fields on the stucture. We also CYGNUS LOOP, recorded by the ASTRO- 1 Hopkins Ultraviolet Telescope need to know the spectrum of hydro- during a 9-minute exposure from the Space Shuttle, displays more exten- magnetic disturbances in the ISM. sive wavelength coverage and higher spectral resolution than can be At very high resolutions, we may provided by IUE. The nebular spectrum is dominated by emission lines from such highly ionized species as C IV and 0 VI; features marked with examine the clumping in velocity and an Earth symbol are airglow lines originating in the Earth's atmosphere. carry out an inventory of subsonic motions that pervade individual do- simple instruments on sounding the same directions. Within certain mains. rockets and Space Shuttle missions temperature regimes, the absorption (figures 13, 151). lines reveal the integrated electron Finally, we can determine the density along a line of sight, whereas temperatures of regions that pre- Measurements ofotheremission the emission lines measure the mean dominantly contain atomic gas. features have also been made, but square of the electron density'. Previ- Comparisons of line widths for spe- low signal-to-noise ratios and spec- ous attempts to compare ultraviolet cies having different masses but tral resolutions have prevented us absorptions and emission fluxes in similar ionization properties would from identifying plausible origins soft X-ray bands have been muddled allow us to differentiate turbulence (e.g., H a fluorescence, collisionally by' the difference in temperature re- from thermal Doppler broadening. excited or recombining gases, or sponses for the two sampling meth- starlight scattered from dust grains). ods. 4. Diffuse We do not know if any of the background emission detected at high Ultraviolet Emission Galactic latitudes is extragalactic. 5. Stellar Populations

Investigations of ultraviolet The most controversial but theo- The observable structures and emissions from the ISM remain retically important parameter char- substructures of galaxies in general, primitive. IUE has recorded good acterizing the vet T hot ISM is its and of our own Galaxy in particular, spectra of planetary nebulae and the relative filling factor in the plane of have intrigued astronomers since shocked clouds within supernova the Galaxy. Our understanding of galaxies were first recognized as remnants, but only the brightest wisps this key topic would be significantly massive stellar systems similar to (but of emitting material can be detected. advanced by comparisons of emis- external to) our Milky Way. Tantalizing glimpses of diffuse sions from certain highly ionized ions emission from highly ionized atoms (e.g., 0 VI, S VI, N V) with their Evidence suggests that the Gal- in the hot gas of the Galactic plane corresponding absorption features in axy collapsed from an initial agglom- and halo have been obtained with

29 OUR MILKY WAY GALAXY

The Milky Way is flat, in a state of and evolved into its current state. differential rotation, and composed of Ultraviolet and visible-wavelength a variety of stellar and gaseous sub- instruments in space will make major systems, it is the birthplace of the contributions to our understanding of Sun and our Solar System. One of the Milky Way and its evolution in two the biggest challenges to modern areas: studies of stellar populations, astrophysics is understanding how and studies of the properties of the the Galaxy fragmented out of the interstellar medium. primordial matter of the Big Bang

eration of primordial gas. During been accompanied by the formation Several populations of stars this collapse, the protogalaxy began of a variety of objects whose physical within the Galaxy have been identi- forming stars with a wide range of and dynamical properties derive from fied. First formed was a spheroidal masses, and the most massive of these the material out of which they were component of old-population stars stars enriched the primordial cloud in formed, as well as from their initial and clusters which developed in the the chemical elements found today in locations and motions. These char- early stages of the collapse of the the Solar System. The earliest gen- acteristics are the clues to the myster- Galaxy. More recently, young stars erations of stars were formed in a ies of the structure and evolution of were (and arc still being) formed in large, spherical halo; most stars, the objects themselves and of our the collapsed disk. Accurate three- however, were formed after the Gal- Galaxy as a whole. Here we give a dimensional stellar locations and axy had partially collapsed to a disk. few examples of the questions that space motions for large samples of may be addressed by the ultraviolet often distant stars will require space The evolution of the Galaxy and visible space missions of the fu- astrometry. The measurement of through these different stages has ture in this important area. chemical abundances in the atmo-

THE COLOR-MAGNITUDE DIAGRAM

Two easily derived characteris- magnitude diagram; if spectral types tics of stars of known distances are are used instead of color indices, their absolute magnitudes (or lumi- one obtains an essentially similar nosities) and their surface tempera- Hertzsprung-Russell (H-R) diagram tures. The absolute magnitudes can (Figure 15). be found from the known distances and the observed apparent magni- The most significant feature of tudes. The surface temperature of a the color-magnitude (or H-R) diagram star is indicated either by its color or is that the stars are not distributed its spectral type. Before the develop- over the diagram at random, but ment of yellow- and red-sensitive rather are clustered into certain photographic emulsions--and, of regions. Most of the stars are aligned course, photoelectric techniques-- along a narrow sequence running the spectral types of stars were from the upper left (hot, highly usually used to indicate their tem- luminous) part of the diagram to the peratures. Now that stellar colors lower right (cool, less luminous) part. can be measured with precision, the This band of points is called the main color index is more often employed, sequence. Three other populations even though the use of spectral are also obvious. These are the types is still of great value. giants and supergiants, which lie along the top and right of the dia- A plot of absolute photographic gram, and the white dwarfs, which magnitudes versus color indices for lie to the bottom and left. a given group of stars yields a color-

3O ASTROPHYSICAL QUESTIONS

-10

® -5

¢:: O') =E .2 0

¢L

03 o "6 +5 ¢L

I 0

,.CI _: +10

+15 O B A F G K M Spectral Type

25,000 10,000 5,000 3,000 Temperature (K) -0.6 0.0 +0.6 +2.0 Color Index

Figure 15. HERTZSPRUNG-RUSSELL DIAGRAM, or color-magnitude diagram, displays the correlation between stellar absolute magnitudes (a measure of luminosity) and spectral types or colors (a measure of temperature). Stars on the "main sequence" are in the prolonged, hydrogen-burning phase of evo!ution. spheres of globular-cluster stars, or part of the research to be attempted differences between the evolutions the members of close binaries, for with future imaging space telescopes. of these different populations. which the rnasses can be determined empirically, will require both the col- Nearby and Local Group galax- The establishment of reliable lecting area and the resolving power ies will be resolvable into individual population models for the Milky Way of large space-based telescopes. stars by HST and subsequent large and neighboring galaxies is a prereq- imaging space telescopes. Members uisite for interpreting the properties In the cases of the globular clus- of different populations within these of more remote galaxies; for these, ters in the Galactic halo, and the open galaxies will be studied to determine resolution into individual stars is not clusters and associations in the Ga- the similarities and differences be- possible, and only integrated star- lactic disk, ages can be derived from tween the main population groups in light can be studied. The ultraviolet studies of color-magnitude diagrams. these galaxies and our own. For spectrum below 3,000/_ is quite sen- These require high-resolution imag- example, photometry of individual sitive to the presence of young stars ing to permit accurate photometry in stars in the M31 and M33 globular within an otherwise ancient popula- crowded stellar fields, and complete- clusters will provide color-magnitude tion. Space telescopes will provide ness in star counts down to faint mag- diagrams that will determine the rea- spectroscopy of distant galaxies in nitudes. Such color-magnitude stud- sons for the differences in the popu- this spectral region. Such studies will reveal the duration and nature of ies are among the highest-priority lations of the cluster systems already observations scheduled for HST, and detected from the ground. The obser- star formation within galaxies at different lookback times. they are likely to form a significant vations will provide insight into the

31 D. Structure of Stellar Systems

Current Program Contributors:

• Hubble Space Telescope (HST) • HST Second- and Third-Generation Instruments • International Ultraviolet Explorer (IUE) • Extreme Ultraviolet Explorer (EUVE) • Far Ultraviolet Spectroscopic Explorer (FUSE) • Voyager Ultraviolet Spectrometers (UVS) • Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS)

Future Program Contributors:

• Deep Ultraviolet Survey • Advanced Explorer Missions • Optical Interferometers (space-based or lunar-based) • 16-Meter Telescope (space-based or lunar-based)

Knowledge of our Solar System of atomic and molecular emissions already been observed (Figure 19), has undergone a revolution over the common to planets and other astro- and these observations can be greatly last two decades as a result of fly-by physical sources. One example is the improved and expanded with HST's missions to every planet except Pluto fluorescence ofH 2that occurs in both high angular resolution, low scat- (Figures 16, 17). This scientific pro- outer planetary atmospheres and cir- tered light, and high sensitivity. There gram has been greatly augmented by cumstellar molecular clouds. In ad- is a good chance for the detection of remote observalions from the ground dition, the growing emphasis on planets around other stars within the and Earth orbit. studies of the origin and evolution of next decade. These observations will planetary systems around other stars help to decipher the origin and evolu- These combined measurements is blurring the distinction between tion of our own Solar System and will have enjoyed a productive relation- "solar system" and "astrophysics" provide information for estimating ship, with each data set identifying investigations. Some of the specific the frequency of planetary-system new phenomena for the other to study. areas of interest in the structure of formation around other stars. Planetary atmospheres and plasmas stellar systems, including the Solar with temperatures ranging from less System, are addressed below. Both HST and FUSE will pro- than 100 K up to some 10_'K or more vide unique insights into these ques- have been observed remotely and tions. HST, in particular, will help sampled directly during these fly-bys 1. Formation of define the mechanisms governing (Figure 18). Such knowledge, gained Stars, Planets, and the development of stars and from a comparison of the remotely protoplanetary systems at the earliest Protoplanetary Disks: sensed and planetary-probe data, is phases of their formation (Figure 20). applicable to other astrophysical Origin of Solar Systems HST observations of star-forming problems for which direct measure- regions at high spatial and spectral ments are not possible. A fundamental goal of as- resolution can help to determine the tronomy is to understand the forma- detailed chemical composition of A complementary relationship tion of our Solar System, and to de- young stellar neighborhoods. This is also exists between remote planetary termine whether such systems are of great importance, since the evolu- observations and astrophysics obser- commonplace. Protoplanetary disks tion of a star is determined by its mass vations because of the large number around stars other than the Sun have and its chemical composition. By

32 ASTROPHYSICAL QUESTIONS i

planetesimal accumulation, the radial variation of temperature within the disk, and the timescale for disap- pearance of the accretion disk. HST and FUSE will provide valuable observational data to better understand these processes. Moreover, spectroscopic observations at visible wavelengths will detect small variations in stellar radial velocities caused by the presence of planets in orbit around a normal star. |nterferometric observations, either from space or from the surface of the Moon, may lead to the first direct detection of planetary systems around nearby stars.

The detection of other solar systems would provide evidence that our own solar system is not an anomaly, but rather, came into existence as part of a normal set of pro- Figure 16. VOYAGER II DEEP-SPACE PROBE, shown leaving Saturn cesses by which all stars form. This, (1981) after flying by Jupiter (1979), heads toward Uranus (1986) and in turn, would mean that observa- Neptune (1989). With the completion of the Voyager I and II missions to tions of the dynamics and composi- the outer Solar System, NASA had sent probes to all the planets except Pluto. tional variations in our own solar examining the high-temperature plasmas surrounding young stars, FUSE will help determine the nature of the dissipative forces that cause the equatorial disks observed around very young stars to break up and collapse into planetary systems.

Important features of stellar collapse can be investigated through observations in the visible and ultraviolet during the later stages of the process. Interactions of the newly fomled star and the accretion disk, mediated by the stellar magnetic field, lead to bipolar outflows of matter and energy and to a copious amount of ultraviolet radiation (Figure 21). Under- standing the energetics, extent, and timescale of these phenomena will help astronomers understand how material can be captured by the star through the dissipation of angular momentum by the accretion disk.

Figure 17. THE SPLENDOR OF JUPITER is revealed by Voyager I The differences between terres- cameras. Visible here are complex cloud-band structure and the famous trial planets and giant gaseous plan- Great Red Spot (lower left), a long-lived feature of the stormy Jovian ets may well hinge on a delicate bal- atmosphere. Images from HST will permit continuation of high-resolution ance between particle coagulation, planetary studies from space,

33 ORIGINAL PAGE COLOR PHOTOGRAPH 80

300 I I 1 I I I I IIitt II IIISll I IIII I I I 225 8

15O

0 6O0 800

Wavelength (/_)

980 1000 1020 1040 1060 1080 11O0 1120

Wavelength (_)

Figure 18. ULTRA VIOLET SPECTRUM OF PLASMA TORUS associated with Jupiter's moon Io was recorded in 1979 by Voyager ultraviolet spectrometers (upper left) and in 1990 by the AS TRO- 1 Hopkins Ultraviolet Telescope aboard the Space Shuttle (lower right). The ASTRO- 1 observations probed an unresolved feature of the Voyager spectrum to reveal a rich concentration of emission lines.

system can be used to refine and test models for the formation of stars with masses similar to that of the Sun.

2. Stars and Stellar Atmospheres

Stars and stellar systems, in our own Galaxy and others, challenge us to understand their never-ending cycle of formation and evolution. Stars form from the stuffofthe interstellar medium, then live their lives replenishing the interstellar medium with reprocessed stellar material, which, in turn, is used to form new stars.

How these processes work is still a puzzle. Space observations can Figure 19. IMAGE OF BETA PICTORIS IN VISIBLE LIGHTshows uniquely investigate the outer atmo- circumstellar disk of material (seen edge-on) detected earlier at infrared wavelengths by NASA's Infrared Astronomical Satellite (IRAS). This spheres of stars and the structure of Southern Hemisphere star is one of several stars that appear to show their atmospheres. Such observa- evidence of current planetary formation, tions can determine whether strong

34 ASTROPHYSICAL QUESTIONS

magnetic fields capture the hot plasma in the star's atmosphere and bind it to Photospheric Absorption the surface or whether this material 2500 flows smoothly and gently, or in er- I ratic puffs, into the interstellar medium. With space observations we 2000 may determine how quickly stars lose mass and investigate the underlying energy source driving the hot atmo- 1500

spheres and the mass loss. c-

O The diversity in mass-loss rates 1000

from different stars appears to be Z enormous, ranging over ten orders of magnitude. The rate of mass loss can 500 l- profoundly effect the evolutionary history of a star. For example, some hot, massive stars can lose an entire 0 solar mass in a million years, a sig- 2598 2598.5 2599 2599.5 2600 2600,5 nificant time frame with respect to Uncorrected Wavelength (_) the lifetime of these stars and the time Figure 20. ULTRA VIOLET SPECTRUM OF BETA PICTORIS recorded required for star formation. by the HST Goddard High Resolution Spectrograph in 1990 shows strong circumstellar (CS) absorption features and variable spectral asymmetries Space observations in the ultra- that arise from infalling gas. Such spectra strengthen the evidence for violet spectral region are sensitive to protoplanetary collapse. the outermost layers of a star's atmosphere (see again Figure 21). In these detected. Spectra taken with HST ionized carbon, nitrogen, and oxygen layers the acceleration and eventual and FUSE will reveal the important (C IV, N V, and O VI) that will allow escape of matter from a star can be sequence of resonance lines of highly us to derive meaningful rates for mass

THE ORIGIN OF SOLAR SYSTEMS

It is believed that our Solar typically be 10,000 to 100,000 times System was formed about 4.5 billion brighter than any orbiting planet, years ago through a process that and since an Earth-like planet would gave rise to a protostar surrounded typically be separated from the star by a rotating disk of material. Accre- by an angle of 0.1 arcsec or less at tion within this disk produced the the distances in question. planets and moons that we know today, while the central star, our However, advances in astro- Sun, evolved rapidly toward the nomical instrumentation have now main sequence. permitted detection of protoplanetary material in the stellar systems Although we have only the Beta Pictoris and Vega, and we geologic record on comets, the major may expect to detect the first planet and minor planets, and planetary orbiting about another star during satellites from which to deduce how this decade. We anticipate that our Solar System formed, we can observations with HST will reveal learn much more about the process the next level of material and giant by studying other, nearby stellar planets around nearby stars, and that systems in the early stages of col- the next generation of large space- lapse, star formation, and accretion. based telescopes and interferometers Circumstellar material is very difficult beyond the Great Observatories will to detect amid the glare of the light of continue the quest for Earth-like the parent star, since the star would planets about other stars.

35 t High-Velocity Massloss ILow-Ve!ocityI

Infrared-Emitting Disk

H 2 Fluorescence

Boundary Layer

Figure 2 t. EARL Y "T TAURI" PHASE OF STELLAR EVOLUTION marks emergence of newborn stars from dusty, infrared-emitting disks into optical visibility. Details of subsequent mass loss are revealed by far-ultraviolet emissions from neutral and molecular hydrogen and from highly ionized heavier species (e.g., 0 Vl).

loss. Because metal abundances can include magnetic activity in late-type pable of resolving stellar disks and vary from star to star, the atmospheric stars (and possibly also early-type imaging accretion disks and circum- energy balance of stars in other gal- stars), radiative instabilities and non- stellar shells would profoundly ad- axies can differ from that of stars of radial pulsations in early-type stars, vance our knowledge of the distribu- our own Galaxy. HST will let us and the determination of densities tion of activity on stellar surfaces and study stars in different galactic envi- and energy balance of the wind in characteristics of the stellar environ- ronments. both types of stars. ment. By resolving stellar disks, it would also be possible to locate mag- To determine accurate rates of It will be possible to construct a netic activity or "starspots" and test mass loss, particularly for massive, detailed picture of the stellar-wind the predictions of magnetic-dynamo early-type stars, as well as the energy structure and its relationship to the theories. mechanism responsible, it is impor- atmospheric emitting region through tant to determine the degree to which the use of simultaneous measure- the stellar wind is ionized. HST will ments of hot and cool ions at many 3. End Products provide only partial information, be- stages of ionization. The question of Stellar Evolution cause it can only observe saturated that remains open is whether mass ultraviolet lines; however, with the loss comes from a fairly steady pro- Although considerable progress addition of FUSE, the degree of the cess occurring over the entire surface has been made toward defining the ionization of the wind can be ad- of a star, or instead, from material final stages of stellar evolution, the dressed directly. ejected from smaller regions, as in details of the transition from post- the case of solar coronal holes. The red-giant stars to white dwarfs re- The high sensitivity of both HST rate and character of mass loss is mains poorly understood. and FUSE will allow us to study needed to build a physical theory of the evolution of stars. various steady-state and time-depen- Observations with IUE have dent behaviors that relate to mass revealed a few very hot white dwarfs loss and the radiative energy budget Development of a very-long- (temperatures in the range 60,000 K of the star. These studies would baseline optical interferometer ca-

36 ASTROPHYSICAL QUESTIONS to 80,000 K) that exhibit a wide range mal, photospheric radiation or by estimated to lie in the range 105-106K. of surface-layer compositions. The some other mechanism (e.g., accre- These temperatures produce radia- high temperatures of white dwarfs, tion). Sensitive spectroscopic mea- tion at wavelengths that lie predomi- and the fact that many are nearby, surements covering a broad range of nantly in the unexplored far-ultravio- make them ideal candidates for study lines will permit detailed study of let and extreme-ultraviolet spectral in the ultraviolet, far ultraviolet, and such objects and a better understand- regions (Figure 22). extreme ultraviolet with HST, FUSE, ing of the apparent outflow of mate- and EUVE, respectively. The in- rial. The Far Ultraviolet Spectro- creased sensitivity and spectral reso- scopic Explorer (FUSE) and the Ex- lution of these missions will allow treme Ultraviolet Explorer (EUVE) temperature and composition mea- 4. Sources are ideally suited to can'5, out spectroscopy in these regions. These surements to be obtained for a large Powered by Accretion sample of white dwarfs, subdwarfs, missions can provide the accurate and nuclei of planetismal accumula- flux distributions and spectral-line tions. Such measurements will es- Emission from interacting binary measurements needed to discrimi- tablish accurate temperature limits systerns arises from the conversion nate among increasingly complex and permit the detection of trace ele- of gravitational potential energy into models for the disk emission. Such ments, which will, in turn, allow a radiative energy, either through dis- models must include surface and at- more exact evolutionary history to be sipative forces associated with vis- mospheric effects as well as hydro- developed. cous accretion disks or through shock dynamic and magnetic processes. waves generated by the impact of Some white dwarfs exhibit other material on the stellar surface. Ac- The Hubble Space Telescope peculiarities that appear to result from cretion is important in X-ray binaries (HST), offering high signal-to-noise the properties of matter in a high- containing neutron stars (pulsars), in ratio and good temporal resolution in gravity environment. Soft X rays cataclysmic variables containing the ultraviolet, will be able to observe have been observed from several white dwarfs (dominated by disk ac- moderate-temperature plasmas rather low-temperature (less than cretion or magnetic accretion), and in throughout an orbit. These observa- 25,000 K) white dwarfs. Still others symbiotic stars and Algol-type sys- tions can be used to construct de- display high-temperature lines in- tems with high-temperature compo- tailed maps of disk structure, includ- dicative of mass outflow. Obse_'a- nents. ing locations of the "hot spots" and tions with FUSE and EUVE will obscuration zones suggested by pre- clarify whether the high-energy The temperatures of the inner vious studies of the brightest, low- emission is powered by purely ther- portions of the accretion disks are mass X-ray binaries and cataclysmic

THE NORMAL LIFE OF A STAR

Birth: A star begins its life as a our Sun. After hydrogen fusion ends, contracting mass of gas and dust the outer layers of the star expand within a giant molecular cloud. enormously, marking the beginning Compression and subsequent heat- of the red-giant phase; the density ing of the interior continue until the and temperature of the core continue core reaches a temperature sufficient to increase, leading to the fusion of (greater than 10 T K) to ignite the elements heavier than hydrogen. fusion of hydrogen into helium. Death: When this final episode of Middle Age: During the longest fusion ends, the outer layers of the period of its life---the "main se- star are ejected into space through quence" stage--the stability of the formation of a planetary nebula or in star is maintained by a balance a supernova explosion. The fate of between the inward pull of gravity the remaining core region depends and the outward pressure of radia- on its mass. Stellar remnants of less tion. This stage persists until all the than 1.4 solar masses collapse into hydrogen in the hot core is con- white dwarfs, whereas more massive sumedmabout ten billion years for a remnants collapse into neutron stars star with mass comparable to that of or black holes.

37 log (frequency" Hz) variables carried out by IUE and the 13 14 15 16 17 European Space Agency's X-Ray I I Observatory Satellite (EXOSAT).

Studies with HST can also help "V= determine the properties of two kinds of observed disk variations: rapid, random variability ("flickering"), believed to be associated with accre- -E tion at hot spots or at the inner disk

x boundary layer; and quasi-periodic

I oscillations (QPOs), thought to arise o from thermal or magnetic instabilities. 41) ¢/1 v In the cases of cataclysmic bina- o ries displaying outbursts (i.e., novae and dwarf novae), studies with HST, FUSE, and EUVE will investigate how the outburst modifies the temperature of the underlying white Infrared Visible Ultraviolet X-Ray dwarf, as well as determine the composition of the ejected material (Fig- Figure 22. PREDICTED ACCRETION-DISK EMISSION SPECTRUM (top curve) extends over many decades of frequency into the extreme ultraviolet, ure 23). In addition to providing whereas emission from an ordinary hot star (e.g., Vega, bottom curve) is constraints on outburst models, these normally restricted to a decade or two. These are predictions of theoretical results will also help explain the dif- models; only ultraviolet observations from space can confirm them. ferences between (or possible con-

Z Cam

Si III NIV NIII II OIV NV 0 VI

o 1000 1200 1400 1600 1800 Wavelength (k)

Figure 23. ULTRA VIOLET SPECTRUM OF THE CA TACL YSM/C VARIABLE STAR Z CAMELOPARDAL/S, I, recorded by the ASTRO-1 Hopkins Ultraviolet Telescope in December 1990, shows numerous broad absorption lines against a complex continuum. This star is the prototype of a class of dwarf novae that exhibit irregular outbursts of fight.

38 ASTROPHYSICAL QUESTIONS nections between) novae and dwarf emission components. In many cases, Kinematic and dynamical stud- novae. the high-temperature lines observed ies must be referred to a non-rotating cannot be produced by the stars alone. reference frame, classically defined Studies of accretion flows in To understand the nature of the hot by the rotation axis of the Earth and cataclysmic binaries with magnetic component will require detailed study the orbital motion of the Earth around white dwarfs (AM Her and DQ Her of density and temperature param- the Sun. The best current optical systems) will particularly benefit from eters. These parameters will be de- reference frame is known with an extreme-ultraviolet observations. termined from HST, FUSE, and overall accuracy of about 0.1 arcsec, The deposition of accreted material EUVE observations of ultraviolet, but the accuracy is less than this in a small area at the magnetic pole, far-ultraviolet, and extreme-ultra- in several regions, especially in rather than overa disk boundary layer, violet emission-line fluxes. Some of the Southern Hemisphere. The produces an increased X-ray flux that the accreting binaries also show evi- European astrometry satellite heats the white dwarf. This flux has dence of highly ionized outflowing HIPPARCOS may produce an opti- a component in the extreme ultravio- winds. Time-resolved studies of the cal reference frame with milli- Iet. Concurrent, time-resolved ob- changing shapes of the spectral lines arcsecond (mas) accuracy. At this servations of this component over are crucial diagnostics of the ioniza- level of accuracy, however, the many wavelength regions will pro- tion and velocity properties of these HIPPARCOS reference frame must vide direct knowledge of the pro- winds. still be tied to extragalactic objects-- cesses occurring at the white-dwarf a job ideally suited to HST. surface. The temperature, flux level, and geometry are important con- 5. Positions and Motions Until milli-arcsecond (mas) straints on the totaI accretion sce- of Astronomical Objects astrometric accuracies are achieved, nario. Extreme-ultraviolet observa- we may assume that extragalactic tions will permit the resolution of High-accuracy measurements of objects are moving so slowly that current discrepancies between the the positions and motions of astro- their transverse angular motions will observed ratios of soft and hard X- nomical objects--the province of not be detected with respect to a truly ray fluxes and those predicted by astrometry--lie at the foundations of inertial frame--i.e., one defined by accretion models. astronomy and astrophysics. local dynamics and the "true" laws of Astrometric knowledge is needed gravitation. This assumption can be Several symbiotic binaries, as particularly for Solar System and tested by observation. On the other well as Algol-type and W UMa-type stellar-mass studies, and for calibra- hand, radio observations of quasars systems, also have high-temperature tion of the extragalactic distance scale. at the mas-accuracy level have re-

ASTROMETRY

Astrometry, the measurement of own century, astrometry has played a the positions and motions of celestial key role in confirming the predictions bodies, is one of the oldest and most of Einstein's General Theory of fundamental branches of astronomy. Relativity. Astrometric data remain It has made momentous contribu- essential for direct determinations of tions to science at least since the stellar and planetary masses and for time of Hipparchus of Rhodes, who calibration of the extragalactic dis- compiled the first comprehensive tance scale. catalog of stellar positions more than 2,000 years ago. Astrometry now lies on the verge of a technical revolution. Within the Systematic measurements of next decade or two, optical inter- planetary positions made by Tycho ferometers in space will be able to Brahe in the late 16th century permit- achieve positional accuracies of 3 to ted Johann Kepler to deduce his 30 micro-arcseconds, sufficient to famous three laws of planetary reduce dramatically the uncertainty in motion; together with Galileo's the cosmic distance scale. Such telescopic observations of 1609, instruments will also vastly expand Kepler's work set the stage for the numbers of stars that can be Newton's synthesis of calculus, searched for Jupiter-sized planetary dynamics, and gravitation. In our companions.

39 vealed apparent motions on the trigonometric-parallax range ofttST. vide astrometric accuracy in the range timescale of a year that have been However, astrometric missions with 3 to 30 micro-arcseconds. It will interpreted as actual, internal mo- near-laas accuracy would also yield provide this accuracy for sources tions. We must therefore begin to precise distances to globular clusters separated by 30 or more degrees of consider the effects of possible tem- in our Galaxy and could detect the arc. poral changes within the extragalac- parallaxes of the nearest extragalactic objects (e.g., quasars) used to detic objects, the Magellanic Clouds. The HIPPARCOS astrometric fine the reference frame. Such measurements would greatly satellite, currently in operation, is reduce the uncertainty in the cosmic capable of milli-arcsecond accuracy; Every order-of-magnitude in- distance scale. it will extend direct parallax mea- crease in astrometric accuracy re- surements to many thousands of stars veals many new effects that had not Masses in tile Solar System. beyond the reach of ground-based earlier been known or did not previ- Astrometry, combined with the laws parallax techniques. However, AIM ously need to be considered. (At the of dynamics, could provide more ac- will provide a hundredfold improve- Iaas level, for example, we may ask curate measurements of the masses ment in angular resolution over intriguing questions about the nature of the planets and the mass distribu- HIPPARCOS, permitting distance of gravity itself.) At present, it ap- tions of the minor bodies, thus im- determinations for many Cepheids in pears that only optical interferom- proving our knowledge of perturbed our Galaxy and hence a dramatic eters in space could provide a dra- motions. These advances would yield reduction in the uncertainty of the matic improvement in the optical a better understanding of the gravita- cosmic distance scale--an achieve- coordinate reference frame. Such tional field structure in the Solar Sys- ment made possible both by higher instruments would yield dynamical tem and permit more sensitive tests accuracy and by the skipping of measurements that are uncon- of the General Theory of Relativity. several rungs on the distance ladder. taminated by reference-frame mo- AIM will also permit major strides in tions, permit a reconciliation at the Detection of planets around our knowledge of Solar-System mas-to-laas level of reference frames other stars. Astrometric detection of masses and of possible extrasolar defined at various wavelengths, and the "wobble" of a star's path across planetary systems. help to determine whether the Uni- the sky would provide the best evi- verse is rotating. dence for a planetary companion. In Two candidate AIM design combination with radial-velocity concepts are already under study; both Astrometry can also provide measurements, astrometric observa- make use of Michelson interferom- values of astrophysical quantities that tions would also permit a thorough eters, and both can achieve the AIM cannot be measured directly in any investigation of the dynamics of such mission objectives. In addition to other way. For example: a planetary system. providing the promise of outstanding scientific return, the Astrometric ln- Distances to the nearest "stan- Major advances in all of these terferometry Mission represents an dard('andles." A few RR Lyrae stars, research areas will be made possible important first step in a long-range some Cepheid variables, and the by NASA's Astrometric Interferom- plan for UV and visible-light inter- Hyades cluster already lie within the etry Mission (AIM), designed to pro- ferometry from space.

40 ASTROPHYSICAL QUESTIONS E. Verification of Relativistic Theories of Gravitation

Current Program Contributors:

• Shuttle Test of Relativity Experiment (STORE) • High-precision dynamical tests using advanced transponders aboard deep-space probes • Lunar laser ranging

Future Program Contributors:

• Gravity Probe-B (GP-B) • Laser Geodynamics Satellite-3 (LAGEOS-3) • Precision clock experiments • Laser Gravitational Wave Antenna in Space

Gravitation, one of the four fun- motion, published in 1687, have been In the General Theory, Einstein damental forces of Nature, is long- verified by increasingly precise So- treated gravity as a distortion of the range and couples universally to all lar System observations over several otherwise "flat" four-dimensional matter. On the cosmic scale, gravity centuries. Buoyed by this achieve- geometry of space-time. In this dis- dominates the other three fundamen- ment, physicists extended Newton's totted geometry, particles move along tal forces (electromagnetism, the theory to the binding and interactions geodesics--paths of shortest four- weak nuclear interaction, and the of stars and galaxies. Astronomy had dimensional distance. In the limit of strong nuclear interaction). Gravita- become astrophysics. small velocities and weak gravitation controlled the way in which the tional fields, the relativistic laws initial density perturbations follow- By the beginning of this century. governing the motions of such par- ing the Big Bang evolved into galax- however, precise measurements had ticles reduce to Newton's laws. ies and stars, and it has controlled the revealed a discrepancy between the dynamics of these objects ever since. observed rateofadvance of Mercury's Einstein's new theory ("General periheIion and the rate predicted by Relativity") immediately explained In the form of massive rotating Newtonian physics. The discrep- the observed discrepancy in the mo- black holes, gravitation is also thought ancy was slight: only 43 arcsec per tion of Mercury's perihelion. It also to provide the "central engine" that century. Nevertheless, attempts to predicted that light passing through a powers tad io gal ax ies and collimated explain it within the Newtonian gravitational field would be bent and radio jets. Relativistic theories of frameworkIfor example, by postu- delayed; wavelengths of light escap- gravitation are essential to an under- lating the existence of one or more ing from a gravitating mass would be standing of supernovae, quasars, X- additional, perturbing planetsIwere shifted toward the red; gravitational ray binaries, and the expansion and unsuccessful. interactions propagate at finite speed large-scale structure of the Universe (the speed of light); there exist itself. In 1915, Albert Einstein an- "gravitomagnetic" forces between nounced the General Theory of Rela- moving bodies; and gravitational en- As the oldest mathematically tivity, the first comprehensive new ergy may, under certain conditions, rigorous and experimentally testable theory of gravitation since that of be radiated from a system in the form physical theory, Isaac Newton's Newton. The name was chosen to of gravitational waves. classical theory of gravitation, in mark the contrast with the Special combination with his laws of motion, Theory of Relativity, announced in The deflection of light by a provided the first application of 1905, which included electrodynamic gravitational field was first observed physics to objects beyond the Earth. interactions but excluded gravita- during a solar eclipse in 1919; the Newton's predictions of planetary tional effects. displacements of apparent stellar po-

41 sitions away from the Sun agreed affected by it. Stars, galaxies, clus- to the direction to a guide star (Rigel) with those predicted by General ters of galaxies, and even the origin of known proper motion. This mea- Relativity within about 20%. More of the chemical elements are now surement will determine both the precise ray-bending experiments and placed in this context. geodetic and the "frame-dragging" tests of the gravitational redshift have precessions of the gyroscopes. Geo- come with space-age improvements Two space missions currently detic precession is the gyroscope spin in measurement technology. How- under study are specifically intended change caused by motion along a ever, gravitational waves have yet to to provide more precise tests of the path in curved space-time. Frame be detected directly. General Theory of Relativity than dragging, by contrast, is caused by have previously been carried out: the "gravitomagnetic" effect of mat- Einstein's General Theory of Gravity Probe-B (GP-B) and a Laser ter in motion--in this case, the ro- Relativity has also provided the theo- Gravitational Wave Antenna in tating Earth. Its effect is to rotate the retical basis for modem cosmology. Space. It may also be possible to use inertial frame in which the GP-B gy- Plausible evolutionary models of the the proposed Laser Geodynamics roscopes spin. entire observable Universe became Satellite-3 (LAGEOS-3) for a rela- available for the first time, and the tivity test. Further tests of the Gen- For the proposed orbit of GP-B, theory soon became essential for an eral Theory can be carried out through General Relativity predicts a geo- understanding of the cosmic expan- the use of highly accurate satellite detic precession of about 6 arcsec/ sion. This revolution in our world clocks and the incorporation of tran- year and a frame-dragging preces- view is perhaps the most significant sponders on deep-space probes. sion of about .045 arcsec/year. For single contribution of General Rela- one year of tracking, the anticipated tivity. All of our concepts about the Gravity Probe-B will measure accuracy of the experiment is about age and development of the Universe, the orientations of the axes of four 1% of the frame-dragging effect, a and of its constituents, have been superconducting gyroscopes relative precision limited mainly by uncer-

WHAT ARE GRAVITATIONAL WAVES?

Gravitational waves are propagat- (i.e., astrophysical phenomena), and ing, polarized gravitational fields, (2) These waves propagate un- "ripples" in the curvature of changed from their sources, unaf- spacetime. Like electromagnetic fected by scattering or absorption by waves, they are transverse to the intervening matter. Gravitational direction of propagation, have two waves thus offer observational independent polarizations, travel at astronomy a new window on the the speed of light, and carry energy Universe, one that contains informa- and momentum. The absorption of tion fundamentally different from that gravitational waves will slightly contained in the electromagnetic change the separations of test window. masses or the rates at which separated clocks keep time---effects that All theoretical models of violent can be used to design detectors for activity in supernovae, galactic these waves. nuclei, and quasars presuppose bulk motions of matter at relativistic Unlike electromagnetic waves, speeds within strong gravitational however, gravitational waves couple fields. Detection of gravitational universally to all matter, not just to waves from these objects would charged matter. They are also much therefore provide our first observa- weaker: the ratio of gravitational forces tions directly into the high-gravity, to electrical forces is about 10 -z9 relativistic-velocity interiors of such (1/1,000,000,000,000,000,000,000,000,000,000,000.000,000).systems. Moreover, since these waves would be unaffected by inter- This extraordinary weakness has vening matter, important information two consequences: (1) Gravitational about source structure and time waves are generated at detectable variation would not be compromised levels only by very massive sources by subsequent absorption or scatter- undergoing very violent dynamics ing.

42 ASTROPHYSICAL QUESTIONS

-12

CO CO

t- O "_ -14 (1)t- E '13

1o -16

E o "_ -18

o_- -20

-22 -6 -5 -4 , -3 -2 -1 0 1 2 3 4

log (characteristic frequency), Hz

Figure 24. PROSPECTS FOR DETECTION OF GRAVITATIONAL WAVES are limited by current techniques to the cases of large amplitude or high frequency. Microwave and optical interferometers in space will extend the search to small amplitudes and low frequencies, vastly expanding the numbers of radiating systems that can be studied.

tainty in the proper motion of Rigel. launch in 1992. These satellites are racy. One possibility is to place a With improvements in proper-mo- intended for measurements of the hydrogen-maser "clock" in an eccen- tion measurements, the anticipated geoid through accurate laser ranging. tric orbit around the Earth and com- accuracy will improve to about 0.2%. However, a suitable orientation of pare its readings with those of ground- the LAGEOS-3 orbit relative to that based clocks as the maser clock A Laser Gravitational Wave of LAGEOS-1 makes it feasible to traverses regions of higher and lower Antenna in Space, incorporating test measure the slight non-Newtonian field strength. By placing such a masses separated by distances of contribution to the orbital precession clock on a spacecraft that will pass about 10v km, would be capable of rates of the satellites. within four solar radii of the center of observing bursts, periodic sources, the Sun, such as NASA's proposed and stochastic backgrounds of gravi- In addition, it is important to Solar Probe mission, the redshift pre- tational waves with unprecedented continue to test one of the most fun- diction could be tested with a preci- sensitivity (Figure 24). Time-re- damental predictions of General sion nearly five orders of magnitude solved periodic signals would be Relativity: the variation in the rate of greater than that achieved by GP-A. expected from roughly 104binary stars flow of time with position in a gravi- of different types in our Galaxy. Other tational field. In a radial field, for Finally, it is important to con- possible sources of gravitational example, a clock nearer the field tinue the tests of General Relativity waves include bursts from white source should run more slowly than made possible by the incorporation dwarfs and from neutron stars coa- an identical clock farther from the of transponders on deep-space probes, lescing into black holes within active source. In the case of the Earth's which permit radio ranging over So- galactic nuclei, and stochastic waves field, this prediction was confirmed lar System spatial scales. The most arising from amplified relic gravi- to high accuracy in 1976 by NASA's accurate confirmation of the geodetic tons from the Big Bang. rocket-borne Gravity Probe-A (GP- bending and time-delay of light to A). date was obtained by radio ranging to The LAGEOS-3 mission is pro- the two Viking spacecraft that landed posed as the third in a series of It is now technically feasible to on Mars in 1976. It is essential that geodynamics satellites that began conduct an advanced gravitational transponders be incorporated on fu- with the launch of LAGEOS-I in redshift experiment that will mea- ture NASA deep-space missions for 1976; LAGEOS-2 is scheduled for sure this effect to much greater accu- such purposes. = = 43 ASTROPHYSICAL QUESTIONS R The Unknown: Serendipity

Current Program Contributors:

• All Missions and Programs

Future Program Contributors:

• All Missions and Programs

Every time a completely new The program we have detailed the flexibility to respond to new ideas means of looking at the heavens is systematically in this document ex- and innovative theories. Other ex- found, exciting new and unexpected tends our study of the sky as we amples of such "externally induced" astrophysical phenomena are dis- currently understand it. A lively and serendipity include rare celestial covered. These discoveries can arise vigorous program must, however, events. In the last decade the Super- when a previously unexplored region also be able to respond to new dis- nova (SN) 1987A offered an unprec- of the electromagnetic spectrum is coveries as they are made. edented opportunity to study the death investigated, or when celestial ob- throes of a star in its final stages of jects are viewed with vastly increased We can fully expect, for instance, evolution. Further serendipitous sensitivity in spatial, spectral, or tem- that HST will make discoveries that opportunities can be expected when poral resolution (Figure 25). will require new kinds of investiga- new technologies arise and permit tions. We can also project that the new kinds of more powerful instru- The implementation plan rec- HST observing program will have ments to be developed. ommended by the Working Group encompasses both types of opportunity for discovery. The Extreme Ul- • Unidentified Radio traviolet Explorer (EUVE) and the SOUrCeS Far Ultraviolet Spectroscopic Ex- • Gamma-Ray Bursts _i plorer (FUSE) will open up new win- 10 dows in the electromagnetic spectrum. An all-sky survey at ultraviolet wavelengths will yield hundreds of thousands of objects to study.

On the other hand, the Hubble Space Telescope (HST) will provide the high sensitivity and spectral resolution needed to study objects already known, as well as those that will be discovered by HST itself and 7 other missions. An optical inter- 1960 1965 1970 1975 1980 ferometer in space will provide spa- Year of Discovery tial resolutions long thought to be unattainable. Finally, a Laser Gravi- Figure 25. SERENDIPITOUS COSMIC DISCOVERIES made between 1960 and 1979 illustrate the rewards of opening up new channels of tational Wave Antenna in space will cosmic information--in particular, previously unexplored regions of the offer us new opportunities to study electromagnetic spectrum. Major advances in angular, spectral and the rapid motion of massive objects temporal resolution can also produce new discoveries. in binary systems.

44 Part III. Program Cc)ncerns

hequestionsprecedingin Ultraviolet,part of this reportVisible,hasanddiscussedGravity theAstrophysicsmajor scientificiden- tified by the Management Operations Working Group. Part III treats four concerns that were additionally considered by the Working Group in developing the implementation plan for the 1990s. They are:

A. Program balance and the realities of the budget process,

B. The need for continuing support of the scientific community and for opportunities to train new scientists,

C. The availability of near-term critical technologies and the promise of emerging technologies, and

D. The implications and desirability of continued international cooperation.

45 A. Program Balance

In a discipline such as space as- 1. Access to Space 2. Commitment to trophysics, where projects have in- Infrastructure and herently long lead times, directions This report presents a vigorous set for the next decade will determine Continuity scientific program that can be the nature and vitality of the science achieved with a variety of instru- Our community needs both in- into the next century. With this in ments using a number of paths to frastructure and continuity to capital- mind, it is necessary to address the space. For success, scientific re- ize on these opportunities and others balance and mix of projects and pro- searchers need frequent access to that derive from new initiatives. grams within the NASA Astrophys- space. Missions in the Flagship or ics program. People for these activities will be "Great Observatory" class may oc- trained in the universities. There must cur only once per quarter century in he a strong component of university Thc Working Group has, for any one field. But forefront science support within the NASA Astro- convenience, grouped future projects can also be done with moderate-class physics Program during the coming into categories on the basis of antici- missions and small programs that can decade. The new NASA Fellowship pated cost. It must be understood, be initiated more frequently and however, that it is the priority as- Program, which we applaud, is one implemented more rapidly than mis- element to address these issues. Fur- signed to a project within a category, sions in that class. and not the category itself, that de- ther initiatives are needed to encourage and sustain the work at universi- termines its importance to a vital As- Modest efforts are, however, ties needed for programs in the next trophysics program for the next de- frequently overlooked and un- cade. century. derfunded when the focus is on large programs and big new initiatives. A There are now greater opportu- focus on big missions can be coun- nities than ever before. These include 3. Access to the terproductive in the long term because a decade of use of HST, including the the moderate and small missions, with Electromagnetic upgrade of its instruments to the their multiple opportunities for flight, Spectrum cutting edge of technology. We shall provide the way to challenge schol- also have available FUSE and other ars and to train each generation of This report describes a program spacecraft complementary to HST, students and instrumentalists. that is a stable and vital scientific permitting unprecedented observa- endeavor. Our plan calls for a num- tions ranging from the far-ultraviolet In addition, the ability to put an ber of projects to be initiated within through the visible and into the near- experiment quickly into space is a the next decade. A strength of this infrared regions of the spectrum. requirement for a vital astrophysics program lies in its abiIity to explore program. NASA's rapid response to the Universe in wavebands that span A balanced, evolutionary astro- Supernova 1987A is an excellent two decades of the electromagnetic physics program is needed to turn example. The opportunity to respond spectrum containing unusually large these dreams into realities. The many quickly to a new scientific discovery numbers of astrophysically impor- aspects of our program--small ex- or a theoretical insight can most eas- tant absorption and emission lines. periments, sounding rockets, moder- ily be accommodated with small and Through the recommended mix of ate missions, theoretical studies, moderate-sized experiments. current and future projects, we seek gravity physics, laboratory astro- to ensure our scientific goals with physics, and analysis of observa- balanced access to the spectrum. tions-must receive balanced support in the overall program to encourage students and scientists to continue in our exploration of the Universe.

We must take care that our program remains in balance among these varied activities. Four important themes illustrate the balance we seek: access to space, commitment to infrastructure and continuity, access to the electromagnetic spectrum, and project support to completion.

46 PROGRAM CONCERNS

4. Project SuppoH view of the limitations now imposed by spherical aberration in the HST to Completion primary mirror. Second- and third- generation instruments fitted with Projects must be supported to corrective optics will permit most or completion to ensure that the scien- nearly all of the scientific potential of tific objectives are met. The nature of HST to be realized over the projected the funding process makes it attrac- 15-year mission lifetime. tive to argue for funding for new initiatives rather than for completing Data reduction and analysis. existing ones. Throughout this re- Data from existing space projects port we have sought to achieve a must be fully reduced and analyzed balance between new initiatives and in order to yield definitive scientific obtaining the scientific benefits from results. The Astrophysics Data Pro- current projects. There are three spe- gram and the NASA Fellowship cific examples of this balancing of Program recognize this need. The needs that illustrate the importance impact of these programs must not be of supporting projects to completion: diluted or diverted.

Hubble Space Telescope. HST Productivio' of _'urrent missions. was designed to be a national facility HST, IUE, and UVS are the only with a 15-year observing lifetime. missions currently active in gather- The timely refurbishment and re- ing data for this discipline. There is placement of HST focal-plane in- a strong rationale to continue opera- strumentation is critical to achieving tion of IUE until it can no longer the scientific goals originally envi- provide unique and highly signifi- sioned for this facility, particularly in cant scientific results.

47 B. Community Support and the Training of New Scientists

A stable community of space NASA Fellowship Program is one of small, medium, and large mis- astrophysicists, with appropriate element that addresses these issues, sions, combined with the suborbital numbers of members ranging from but further initiatives to encourage and grants programs, are all neces- interested undergraduates through and sustain the work at universities sary components. leaders of major research groups, is are needed to provide the scientists to crucial. These people must represent implement and productively use the Data from existing space projects and maintain expertise in the tech- Astrophysics missions planned for must be fully analyzed and reduced niques needed to allow exploration the next century. to yield scientific results. The Astro- of all accessible wavelength regions physics Data Program and the NASA of the electromagnetic spectrum. As NASA's interactions with our Fellowship program are steps in the in the past, it will fall to the universi- universities must remain broad and right direction to provide support for ties to recruit and train these people. diverse. The universities must be in scientific research. The needs-based a position to train the next genera- budget for analysis of HST data is There must therefore be a strong ti on s of theori sts, space -data analysts, also central to the health of the disci- component of university support and hardware Principal Investigators pline. The resources for these pro- within the NASA Astrophysics Pro- (PIs). No single program will ac- grams must not be diluted or diverted. gram during the coming decade. The complish this. A balanced program

C. Technology

As has been stressed earlier, a in several technological areas, in- zation of MCP resistivity and stack- vigorous program in Ultraviolet, cluding photocathodes, windows, ing of multiple MCPs, the develop- Visible, and Gravity Astrophysics quality microchannel plates (MCPs), ment of discrete-dynode MCPs, and depends upon the use of advanced readout systems for photon counters, the development of intensified-ana- technology. Here we discuss the and ultraviolet sensitization and par- log CCDs. importance of detectors, optics, ticle damage protection for charge structures, pointing, and command, coupled devices (CCDs). Critical The use of CCDs may be a solu- control, and data management. Ad- technologies require systematic long- tion for visible and extreme-ultra- ditional technical challenges, such as term development to insure their use violet bands in orbital environments the provision of radiation shielding, in future astronomical instruments. of low particle radiation, but caution will have to be rnet in the course of is required in high-radiation environ- establishing an astronomical obser- Desirable improvements for ments. Currently, direct-illuminated vatory on the Moon. missions in their early stages or un- CCDs are less suitable for astronomi- der consideration include the exten- cal detectors in orbits above the sion of pixel formats (particularly for Earth's geomagnetic shield because 1. Detectors FUSE spectroscopy), the reduction the cosmic-ray impact rate increases of pixel size to control the overall dramatically. CCDs with low read- Several instruments for missions instrument size, the reduction of cost out noise could use the combination already in progress (e.g., HST, EUVE, of detectors, and accommodation of of many frames or coincidence tech- FUSE) will provide detectors con- curved focal planes. niques to eliminate cosmic-ray hits. siderably more advanced than their In the case of lunar observatories, predecessors in terms of number of There is a particular need for a shielding below the lunar surface is pixels, quantum efficiency, out-of- large-format, high-count-rate UV and possible. band rejection, and noise. visible-light detector for camera use to obtain simultaneous measurements The development of 8- to 16- To ensure full performance ca- of faint and bright sources. Current meter telescopes and large inter- pabilities for these missions and in- approaches to the development of ferometers in space will place further struments, continued effort is required this type of detector include optimi- demands on detectors. Larger pixel

48 PROGRAM CONCERNS formats, including mosaics of detec- roughly one milli-eV. Such band- tive optics to space missions. Adap- tors (about 10,000 x I0,000 pixels), gaps are achievable in superconduct- tive optics has been most vigorously will allow imaging of large sections ing materials. pursued in connection with ground- of the sky to extremely faint limiting based observations, since these tech- magnitudes, when combined with the niques can help to overcome the blur- angular resolving power of the tele- 2. Optics ring effects of the Earth's atmosphere scopes. This is especially true in the on timescales of a few hundredths of ultraviolet, where the zodiacal-light The development of the primary a second and thus increase spatial background is much fainter than at mirror for the HST represented a resolution. However, through simi- visible-light and infrared wave-, major milestone in ultraviolet optics, lar approaches to wavefront sensing lengths. spherical aberration notwithstanding. and restoration, the performance of Optics for future missions will need large, segmented radiation collectors The decade of the 1990s may to meet even more stringent require- in space can also be optimized through also see the development of rudimen- ments. Among the goals to be active control of the shape of the tary "3-D" detectors for ultraviolet achieved are larger and much reflecting surface. and visible-light photons. Gamma- smoother mirror surfaces, lighter- ray and X-ray detectors have long weight mirrors, lower thermal coef- been able to record simultaneously ficient mirror and mirror structure 3. Structures both the two-dimensional incident materials, higher-efficiency ultra- location and the energy of each pho- violet reflective coatings, and the Large 8- to 16-m telescopes in ton (the "third dimension") to rea- maintenance of extreme cleanliness space, and particularly space inter- sonable accuracy. With this capabil- during manufacture, system integra- ferometers, present a number of ma- ity at visible wavelengths we could, tion, and deployment in space. Good jor technological challenges in the for example, map the large- scale materials and optical designs are hard area of structures. For all interferom- structure of galaxies by obtaining their to find in the UV and, especially, the eter configurations, understanding of redshifts while taking images. To EUV spectral regions. the behavior of the underlying struc- detect a visible photon and resolve its ture is essential to designing a sound energy to a significant degree re- It will also be important to apply optical configuration and control quires a detector with a band-gap of the emerging technologies of adap- system. Because variations in the

TECHNOLOGY DEVELOPMENT

Detectors: In order to increase with increasingly sensitive future the field of view and improve spatial instrumentation. resolution and spectral coverage, detectors must have a large format, Structures: In order to support high dynamic range, geometric stabil- these optical systems, particularly for ity, particle-radiation insensitivity, interferometry, structures must be high quantum efficiency, and low very stable, with low thermal-expan- noise. For isolation of the ultraviolet sion coefficients. They must not portion of the spectrum, rejection of contaminate the optics and detectors. visible-wavelength radiation is also Although fitting into a launch vehicle required. Furthermore, it is possible smaller than the structure, they must in principle to measure the energy as also be deployable in space to well as the time of arrival of every provide long baselines. photon detected; array detectors with Pointing: For interferometry, this capability would revolutionize pointing-control systems must be the fields of ultraviolet and visible- further improved beyond HST. light astronomy. Command, Control, and Data Optics: Optical systems with Management: Data storage capacity large collecting area, high spectral and transmission rates must be transmission, extreme smoothness, enlarged to read images sent down low thermal-expansion coefficient, from detectors of larger format. and low weight are required for use

49 baselines of only 50 _, are important, A third area where new develop- understanding of mutual dependen- materials and structural properties ments are necessary is in low thermal cies among structural, optical, detec- will have to be well understood to expansion, low-contamination struc- tor, and control subsystems is critical that level. tures and materials. This is of par- for optimization. ticular importance for ultraviolet To generate a database of infor- optical performance, since many mation, a combination of ground and materials currently in use can cause 5. Command, Control, flight experiments is needed. Nei- catastrophic degradation in ultravio- and Data Management ther test methodology nor analytical let reflectivity if they are deposited modeling are currently designed to on the mirror surface. Current spacecraft, particularly address these submicron-level ques- HST, seriously tax the capabilities of tions. Structural-motion damping available spacecraft computer and mechanisms are poorly understood 4. Pointing when the deflections are in the tens of general bus systems. A program to develop a stable of low-cost, stan- Angstroms (nanometer) range. Ma- The HST pointing-control sys- dard spacecraft at various different jor efforts to define laboratory tests tem is a major advance in precision levels of weight, power, and onboard and modeling programs to this level over previous spacecraft. The com- control capability would greatly will be required. bination of careful structural design, benefit science, by enabling more low mechanical noise gyroscopes and future missions to be implemented Another area requiring develop- reaction wheels, and an advanced within a given budget. ment for large future missions is de- guidance sensor and control system ployment. These instruments are too are responsible for this gain. A large increase in the amount of large to launch with any foreseen onboard processing capability is also capability, so that partial assembly in Large space telescopes and, es- necessary for the major new missions space will be required. Concepts and pecially, large interferometers require discussed in this report. Artificial in- mechanisms that allow a spacecraft pointing stabilization one or two or- telligence applied to flight systems to be packaged in a small launch ders of magnitude better than the could greatly increase the efficiency volume, but which could achieve 10- HST design goals. Much further of science observations. Ground to 100-m dimensions on orbit, must work is therefore necessary in these systems will also need to become be developed. The deployment areas. A major goal of the technol- more powerful and sophisticated to methods and mechanisms must re- ogy-development effort for the con- manage the operations and accept the sult in small setup errors, must ex- trol systems is achieving simplicity, data flow from these new initiatives. hibit linear dynamic behavior, and or at least avoiding debilitating com- must not be a source of disturbance plexity and extreme expense. An once deployed.

5O PROGRAM CONCERNS D. International Cooperation

Astronomy and astrophysics are international cooperation could pro- lion of experiments, and to encour- inherently international activities. duce a better NASA mission. In age coordination of missions and Scientists from all countries can ob- other cases, cooperation might make sharing of results. serve the Universe without regard to the mission possible by reducing the national boundaries. Many scientific cost to each participant. To ensure that the total program problems require continuous, multi- is optimized for science, opportuni- site observations, or access to spe- Additional opportunities may ties for participation in international cialized instruments or specific sites. arise for NASA investigators to par- missions must fall within the general Thus, there exists a long history of ticipate in the programs of other na- strategy of the Ultraviolet, Visible, international collaboration in as- tions. Either way, more scientific and Gravity Astrophysics disciplines, tronomy that predates access to space results may accrue, and each partner and must be evaluated in the context for scientific purposes. will benefit when a better mission is of the NASA Astrophysics Division's achieved. Cooperation must be a science plan for these disciplines. NASA and many other nations bargain in which each partner per- NASA also needs to be a responsible and agencies have substantial ac- ceives a benefit. At the minimum partner in these arrangements. Some complishments in space science. level, conversations and correspon- Congressional action may be required These nations can be attractive part- dence must take place with other to maintain the continuity of funding ners for space science efforts. For spacefaring nations to be aware of that is required for international mis- some experiments, the synergy of planned programs, to avoid repeti- sions to proceed smoothly.

51 ,v I _ I _ Part IV. The Recommended

byhis thepart Managementof the report describesOperationsthe Workingelements Groupof the planin accordancedeveloped with the scientific strategy and priorities furnished by the Na- tional Academy of Sciences. The missions included in the plan have been divided into two categories:

A. Approved missions, and

B. Future missions.

We conclude by discussing the vital underpinnings of these missions:

C. The Research and Analysis (R&A) program, including the sounding-rocket program.

PII_?,,FIDtNG PAGE BLANK NOT FILMED 53

THE RECOMMENDED PROGRAM

HUBBLE SPACE TELESCOPE SCIENTIFIC INSTRUMENTS

Figure 26. HUBBLE SPACE TELESCOPE SCIENTIFIC INSTRUMENTS have been designed to carry out an enormous variety of investigations. Illustrated here are actual data from WF/PC I (Saturn), FOC (Pluto and Charon), GHRS (Chi Lupi spectrum), FOS (quasar spectrum), HSP (calibration test), and FGS (study of visual binary star). as modules to be repaired or replaced (a) Wide FieldPlanetary Camera II with a 1% passband from 4,000 ,& to in orbit as required. I0,000 ,_. Finally, the addition of The Wide Field/Planetary on-board calibration lamps will pro- In addition, all of the original Camera II (WF/PC II) will be the first duce flat fields of higher fidelity and HST scientific instruments were de- instrument to be installed aboard HST will reduce the amount of observa- signed for eventual replacement by in orbit. In an engineering sense, it is tory time and resources needed to second- and third-generation instru- a copy of the first-generation WF] calibrate the camera. ments, so that new advances in PC. However, in addition to provid- technology (e.g., larger and more ing corrective optics to bring the HST sensitive detectors) could be brought image into sharp focus, WF/PC II (b) Space Telescope to bear on the HST observing pro- will incorporate a number of impor- Imaging Spectrograph gram. Over the past 15 to 20 years, tant scientific improvements. while HST was being designed and The Space Telescope Imaging developed, these advances have been The detectors in the new camera Spectrograph (STIS) is a multi- particularly important for uhravio[et will be much more stable in their resolution, high-sensitivity spectro- and infrared astronomy. performance, thereby increasing the graph that will incorporate several overall efficiency of the HST obser- advances over first-generation HST With the discovery of spherical vatory. In addition, a Woods-type spectrographs and allow HST to be aberration in the HST primary mirror, filter wilI be included to provide an much more produ6tive. The use of the need for such second- and third- ultraviolet imaging capability below large-format detectors (2,048 x 2,048 generation instruments becomes even 2,000 ,_ (while rejecting undesirable pixels) allows improvement by a fac- more critical since the effects of the visible light) at the high spatial tor of 40 to 130 to be obtained for aberration can be removed through resolution afforded by HST. A set of echelle-spectrograph formats with use of suitably designed correcting linear variable filters will also be resolving powers in the range 15,000- optics on each of the new instru- added to provide narrow-band imag- 140,000, and up to 1,000 elements to ments. ing over a 13-arcsec field of view be imaged simultaneously along the

Pl_i,e,,ll)l'N_ PAGE BLANK NOT F_MED 55 slitwithresolvingpowersintherange prolific stellar nurseries, among other the 0.1 arcsec level by a properly 1,000-20,000. exotic phenomena. No other instru- designed instrument. This is particu- ment is capable of giving images of larly true at the short-wavelength end Bothphoton-countingandCCD these regions with the clarity afforded of the spectrum, where the HST tele- technologieswill be usedin the by NIC to HST. In addition, NIC will scope is not diffraction-limited. spectrometer,sothattheinstrumentstudy the organic compounds of So- will besensitivein theultraviolet, lar System objects, such as water, High-sensitivity spectroscopy. visible,andnear-infraredwave- which are blocked from ground-based The current generation of spectro- lengthregions,coveringtherange view by the same compounds in the scopic instruments leaves significant 1,050,_ to 11,000 A in four bands. Earth's atmosphere. room for improvements in through- Two multi-anode microchannel ar- put, particularly for high resolution. rays (MAMAs) will bc the ultraviolet Similarly, low internal background photon-counting devices in STIS be- 3. HST instruments might allow significant cause of their quantum efficiency, Third-Generation gains in limiting magnitude through longer observations. low noise, geometric and photomet- Instruments ric stability, resolution, and insensitivity to visible wavelengths. Two Programmable-slit imaging/ CCD detectors will be used for the Examples of the kinds of instru- spectroscopy. One current limitation visible and near-infrared because of mentation that are being considered that might be overcome is the restric- for future HST generations are as their high quantum efficiency. tion to observation of one target at a follows: time. A device with a remotely The two-dimensional ultraviolet openable, arbitrarily shaped entrance Advanced camera. A camera spectra obtained by STIS will be par- slit for multi-object or long-slit more sensitive in both ultraviolet and ticularly useful for studies of inter- spectroscopy could greatly enhance visible wavelengths would be a ma- stellar ultraviolet absorption lines and the productivity of the HST mission. for investigations of such extended jor improvement over current capability. Higher performance in the objects as galaxies, supernovae, and Wide-band spectroscopy. It is ultraviolet requires a visible-blind accretion-powered sources. possible to design spectrographs that large array detector with good dy- cover the entire four decades of the namic range. Higher performance in HST spectral band simultaneously visible wavelengths requires CCDs (c) Near h_frared Camera without lowering sensitivity. Such with efficient coatings over broad an instrument could enhance produc- The Near Infrared Camera (NIC) wavelength ranges and lower readout tivity and provide new science and dark noise. will open up a whole new vista to through simultaneous monitoring of HST. Operating at near-infrared the target over a broader band. Ultra-high-resolution spectros- wavelengths ( 1.0-3.0 p_m), and using copy. There is evidence that much extremely sensitive detectors devel- Spectropolarimetry. It is pos- information about the structure of sible to build a general spectrograph oped specifically for this project, NIC interstellar clouds is available at ve- will more than triple the wavelength with linear and circular polarimetric locity separations under I km/sec. A range of the electromagnetic spec- capability, together with far-ultra- spectrograph that could perform trum observable by HST. violet capability, that would use the spectroscopy with a resolution in high angular resolution of liST in the excess of 1,000,000 would therefore NIC uses dispersive spectros- ultraviolet and visible regions of the be invaluable. copy in four resolution modes-- 100, spectrum. 1,000, 6,000 and 10,000_to achieve Narrow-band imaging. A de- the highest possible sensitivity. It is These are just a few of the scheduled for insertion into HST in vice that could use the high-resolution promising ideas now under discus- the late 1990s. NIC will then use its imaging of liST across a wide field in sion. The astronomical community a very narrow spectral band could infrared array sensors to see deeply is full of new and powerful observing provide new insights into compli- into the dust-obscured regions in our techniques waiting to be harnessed cated fields. This might be accom- Galaxy that are the birthplaces of by HST. NASA should continue to plished with a Fabry-Perot setup or newly forming stars. It also will support research into the best use of an acousto-optical filter. penetrate the centers of external gal- HST through study of these oppor- axies that are so obscured that virtu- tunities. Improved imaging. It is possible ally no visible light escapes, but which that imaging could be pushed below house prodigious energy sources and

56 ORIGINAL PAGE COLOR PHOTOGRAPH THERECOMMENDED PROGRAM

4. International Ultraviolet Explorer (IUE)

The International Ultraviolet Explorer (IUE) has been observing astronomical spectra almost continu- ously since its launch into a geosynchronous orbit in January 1978. For more than 13 years, HIE has been the only orbiting instrument capable of obtaining ultraviolet spectra in the region from !,200 A to 3,000 A.

The prodigious scientific output of IUE is attributable to the unique- ness of this research opportunity and to IUE's capability for measuring a very wide region of the spectrum in a single exposure. Its two spectrographs cover the ranges 1,150- i,950 A and 1,900-3,200 ,_. With Figure 2Z THE EUVE DEEP SURVEY SPECTROMETER, shown SEC Vidicons as detectors, quantita- here undergoing final laboratory testing, will obtain spectra of extreme- tive measures of the full spectrum ultraviolet sources detected during the initial, all-sky survey phase of the within either of these regions are EUVE mission. obtained either at high resolution (0.1- 0.3 ,_) or at low resolution tional flexibility, and its unique capa- Limited exploration of the EUV (6-7/1,). The high observing effi- bilities in ultraviolet spectroscopy, region has been carried out by instru- ciency possible in geosynchronous this mission will continue to provide ments on the Apollo-Soyuz, orbit also contributes to IUE's ex- significant science throughout the EXOSAT, and Voyager missions. ceptional scientific productivity. next decade--and at extremely low These missions gave hints of the cost. wealth of new scientific insights that HST will be vastly superior for will be obtained on classes of objects many kinds of science formerly re- such as white dwarfs, cataclysmic stricted to IUE, and will open up 5. Extreme variables, cool stars and their coro- whole new areas impossible for IUE Ultraviolet Explorer nae, and planetary emissions. Addi- to approach. However, there are spe- tional EUV data on about a thousand cial categories of observations for (EUVE) sources have now been obtained by which IUE will remain unique. These instruments on the Federal Republic include many-object surveys, vari- The Extreme Ultraviolet Ex- of Germany's Roentgensatellit ability monitoring, extensive wave- plorer (EUVE) consists of four (ROSAT) mission, in which the length coverage, and other projects grazing-incidence, EUV-sensitive United States is participating. As requiring large numbers of observa- telescopes housed on an Explorer with all exploratory missions like tions or large blocks of time--all of platform. During the early months of EUVE, the discovery of as yet un- which are worthy science but do not its 30-month design lifetime, EUVE known sources of extreme ultraviolet require the greater sensitivity of liST. will make the first map of the sky emission may prove to be the most throughout the 80-800/_ EUV portion significant scientific return. Fo'r example, continuous obser- of the spectrum. Approximately 95% vation of planets in our Solar System, of the sky will be mapped in 0.1- Once new EUV stars are discov- detection and characterization of degree increments. From this map, a ered by EUVE, the next scientific stellar variability, and regular moni- catalogue of EUV sources will be task undertaken by the mission will toring of active galactic nuclei are obtained, providing motivation for be to obtain spectra of these stars to future missions in the EUV band. f but a few of the projects ideally suited understand the processes and condi- to IUE. By capitalizing on IUE's EUVE is currently scheduled for a tions leading to the observed emis- Delta rocket launch in 1992. continued "good health," its opera- sions. The EUVE science payload

57 includes a spectrometer that will be able to obtain spectra with a resolution of 0. 1-3/_, (Figure 27).

Scientific studies to be carried out on nearby stars by EUVE Guest Investigators will extend our understanding of stellar chromospheres and coronae. Combined with similar studies carried out using IUE, astronomers will be able to study stellar chromospheres and coronae over a very wide range of conditions and stellar types.

6. Orbiting and Retrievable Far and \ Extreme Ultraviolet Spectrometer (ORFEUS)

ORFEUS is a joint project of NASA and the German government designed to obtain medium (3x 10_) to high (about 104) resolution spectra of a wide range of sources at FUV and EUV wavelengths (Figure 28). Figure 28. ULTRA VIOLET SPECTROSCOPY A T MEDIUM AND HIGH RESOLUTION will be carried out by the Orbiting and Retrievable Far and The science payload is a 1-m Extreme Ultraviolet Spectrometer (ORFEUS) mission, a joint U.S.-German project scheduled for Shuttle launch in 1993. Instruments on the German- class normal-incidence telescope with built AstroSPAS platform will probe properties of the interstellar medium. two spectrographs that alternately share the beam. An echelle spectro- of these elements lie in the FUV and graph, operating at high dispersion in ORFEUS is capable of observ- cannot be observed with IUE or HST. the range 900-1,200 ,_, is to be pro- ing up to 150 sources with a spectral vided by Germany; a system of four resolution comparable to the very high resolution of the earlier Rowland spectrometers covering the 7. Interstellar Medium range 400-1,200 ,_, will be provided Copernicus satellite but with a 100- fold increase in sensitivity. The high Absorption Profile by the United States. The echelle resolution and signal-to-noise ratio spectrograph will have a resolution Spectrograph of 0.14 _, and the Rowland spec- provided by ORFEUS will permit the (IMAPS) study of narrow interstellar lines to trographs will have a resolution of determine the ionization state of the 0.40 A. The pointing accuracy will IMAPS is an objective-grating, interstellar medium. Of equal impor- be 5 arcsec. The telescope will be ultraviolet echelle spectrograph tance is the reliable determination of carried on the German AstroSPAS (Figure 29) that can record stellar the ratio of isotopes of the cosmo- platform, which will be deployed by spectra over a 200-A wavelength in- logically important light elements the Space Shuttle for several days of terval between the Lyman limit of deuterium and hydrogen in the inter- science data gathering. The flight is hydrogen at 912 ._ and the wave- stellar medium. The absorption lines scheduled for early 1993. length at which HSTloses sensitivity (about 1,100 ,_).

58 THE RECOMMENDED PROGRAM

The principal mission oflMAPS 3,200/_. Its principal application is 9. Far Ultraviolet is to study absorption lines produced to investigations of hot stars and gal- Spectroscopic Explorer axies in broad ultraviolet passbands by many different atoms, ions, and (FUSE) molecules in space. IMAPS pro- with a wide field of view. vides exceptionally spectral resolu- The newest addition to NASA's tion-it can distinguish wavelength differences of 1 part in 200,000, per- (c) Wisconsin Ultraviolet approved observatories is the Far mitting a very fine differentiation of Photo-Polarimeter Experiment Ultraviolet Spectroscopic Explorer (FUSE; see Figure 30). This mission Doppler velocities for different parcels of gas and the study of special The Wisconsin Ultraviolet will conduct high-resolution spec- phenomcna. IMAPS was developed Photo-Polarimeter Experiment troscopy of faint sources at wavelengths from 912 A to 1,200 ]k and for use on sounding rockets and is (WUPPE) is a 0.5-m telescope with a moderate-resolution spectroscopy now being converted to operate in spectropolarimeter (6-_ resolution) down to 100 A. It will consist of a orbit on the German AstroSPAS plat- sensitive between 1,400 _ and 0.7-m aperture telescope, grazing- form in early 1993. 3,200 ,_. It is used to explore the polarization characteristics of hot incidence spectrographs, and associated detectors that will achieve an 8. Astronomy stars, galactic nuclei, and quasars. angular resolution of about 1 arcsec, Observatory-2 (A fourth telescope designed for a spectral resolving power of ap- (ASTRO-2) X-ray spectroscopic observations-- proximately 30,000, and a sensitivity the Broad Band X-Ray Telescope, or of about 100 cm 2for wavelengths in The ASTRO-2 mission is a Space BBXRT--was also flown on the the previously unobservable FUV Shuttle reflight of the ASTRO ASTRO-I mission; however, band from 900 ]k to 1,200 A. Spacelab payload flown aboard the BBXRT will not be included in ASTRO-1 mission in December ASTRO-2.) The FUV region of the spectrum 1990. The ASTRO payload consists contains unique diagnostics that will of three separate bui complementary Flight of ASTRO-2 is tenta- allow us to measure the physical state and evolution of some of the most UV telescopes, aligned with each lively scheduled for 1994. other on a single pointing system so that all three may observe the same object simultaneously:

Photocathode (a) Hopkins Ultravioh't Telescope

The Hopkins Ultraviolet Tele- scope (HUT) is a 0.9-m telescope Echelle Grating with a low-dispersion spectrograph (3-/_ resolution) optimized for observations in the range 900-1,200 _, _CD Detector but able to observe between 425 ,_ Multi-Grid and 1,850 _,. It is intended primarily for studies ofthc behavior of quasars, galaxies, and active galactic nuclei in the far ultraviolet.

Off-Axis Parabolic (b) Ultraviolet Imaging Teh'scope Cross Disperser

The Ultraviolet Imaging Tele- Figure 29. ECHELLE-GRA TING SPECTROMETER will be used by the scope (UIT) is a 0.38-m telescope Interstellar Medium Absorption Profile Spectrograph (IMAPS) mission to that images over a40-arcmin field of study absorption lines in the interstellar medium between 912,4 and about view (2-arcsec resolution) with 1,100 A. The exceptionally high spectral resolution of IMAPS will permit various filters over the range 1,200- detailed dynamical investigations.

59 measure the ionization and heating source of stellar winds and flares and can study the accretion processes that power the high-energy sources in collapsed stars and active galactic nuclei.

10. Shuttle Test of Relativity Experiment (STORE)

The Gravity Probe-B (GP-B) mission (discussed in Section B to follow) is intended to provide novel, high-precision tests of Einstein's General Theory of Relativity as well as geodesy and other co-experiments. The GP-B science payload represents over 25 years of research and embodies state-of-the-art technologies in many fields, including gyroscope fabrication, suspension, and readout; cryogenics (superfluid helium cooling to 1.8 K); magnetic shielding; superconductivity; precision optics and alignment methods; and satellite drag compensation far more accurate than any yet flown.

An engineering test of portions of this advanced technology, as- Figure 30. FAR ULTRA VIOLET SPECTROSCOPIC EXPLORER (FUSE) sembled into a Shuttle Test Unit mission will carry out UV spectroscopy at high resolution from 912 A to (STU), will be carried out on a Space 1,200,4 and at moderate resolution down to 100,4. The widely ranging Shuttle flight through the Shuttle Test research program of this satellite is central to the MOWG plan for the of Relativity Experiment (STORE). 1990s, and a timely FUSE launch commands high priority. The STU will incorporate a quartz telescope block assembly, test gyroscopes, computer control and sus- basic material of the Universe. Hy- high-temperature, ionized states of drogen, and its related forms of deu- atoms, FUSE wilt also allow us to pension electronics, a flight suspension system, and other components terium and molecular hydrogen, can constrain models for the production operating at ambient temperatures. be studied effectively only in the far of light elements in the early Uni- ultraviolet. With its ability to study verse. Using FUSE, we can also

60 ,THE RECOMMENDED PROGRAM B. Future Missions

The missions that were identi- charges, most of which were a direct Explorers permit a flexible re- fied by the Working Group as ele- result of the delay in Shuttle launch sponse to scientific problems and ments of the Ultraviolet, Visible, and opportunities during the late 1980s. complement the capabilities of both Gravity Astrophysics plan, but which small payloads and the Great Obser- have not yet been approved, are dis- The Office of Space Science and vatories. We believe an augmenta- cussed below. Each has been se- Applications subsequently sought and tion of the Explorer budget line is lected to make major and unique con- received an Explorer funding aug- necessary now to recapture scientific tributions to the scientific questions mentation for a new Small Explorer leadership in space astronomy and raised in Part II of this report. (SMEX) mission class designed to astrophysics. provide a "fast track" to space. In 1991, in response to the recommendations of the Bahcall Committee, 2. Additional 1. Augmentation to OSSA furthermore created two new Small-Class Explorer: Explorer Program mission classes--Middle and Univer- Deep Ultraviolet Survey sity--as successors to the "Delta- The Explorer program has been class" missions, such as FUSE, cur- A high-sensitivity, all-sky sur- vital to the development of space rently in the Explorer queue. How- vey in the 1,200-2,000 t_ spectral astronomy and astrophysics. Ex- ever, no funding augmentation has region is important to the develop- plorers of the past, such as the Small yet been provided for these larger ment of ultraviolet astronomy. This Astronomy Satellite (SAS) series that Explorer-class missions. included the Uhuru X-ray mission, is an extremely "dark" region of the as well as the International Ultravio- Consider, in particular, one of electromagnetic spectrum--a hundred times fainter than the visible- let Explorer (IUE), the Infrared As- the most recently selected Delta-class tronomical Satellite (IRAS), and the Explorers--FUSE, the mission of light background--and the potential recent Cosmic Background Explorer highest priority in its category in the for unexpected discoveries is great. (COBE) have left indelible imprints 1982 report of the National Academy upon contemporary astrophysics. of Sciences' Astronomy Survey Moreover, the fact that a survey IUE, IRAS, and COBE were all larger Committee (the "Field Report"). so potentially rich in scientific con- Explorer missions. FUSE has been studied since 1982 tent can be undertaken by a Small and is now schedulcd for launch in Explorer (SMEX-class) instrument The scientific goals that can be 2000. However, three other Explor- lends it a special attractiveness. Both accomplished with Explorers are rich ers in this class--EUVE, XTE, and diffuse and point-source surveys are and varied. However, serious prob- the Advanced Composition Explorer desirable and, while the optimal in- lems exist in the Explorer budget (ACE)--are in line ahead of FUSE, strument design for the two survey line. Funding for this level-of-effort and experience shows that initial types may differ, it is possible that program has not kept pace with infla- launch dates are optimistic. Thus the both surveys can be accommodated tion, and extraordinarily long times situation that galvanized the CSAA adequately by the same mission. now pass between mission selection into action in 1986 has, in fact, wors- and flight. Fresh ideas and scientific ened considerably during the 5 years A diffuse-source survey in this goats are abundant, but the rate of since their report. region of the spectrum will benefit new scientific results from Explorers from the coincidence that a number is diminishing as flight programs are Much outstanding science that of different emission mechanisms stretched out. needs to be done is well matched to contribute to the background. These the Explorers. The three-axis stabi- emission mechanisms convey infor- In 1986, the National Academy lization and fine pointing needed for mation on media characterized by a of Sciences' Committee on Space astronomy Explorers generally de- wide range of physical conditions. Astronomy and Astrophysics mand the capabilities of larger space- They include probable back-scatter- (CSAA) examined the deplorable craft buses. Some examples of sci- ing of Galact it-plane starlight by dust, situation in the Explorer program. ence in this class include multispec- including interstellar cirrus clouds, Even 5 years ago, Explorer opportu- tral observations, astrometry, high- H 2 fluorescence arising in the cold, nities were scarce, and the time from resolution UV and EUV spectros- neutral phase of the interstellar me- Phase A studies to launch of an Ex- copy, stellar seismology, and long- dium (ISM), and radiation from plorerexceeded 15 years. Moreover, term studies of variability. Peer collisionally-ionized gas at high tem- the Explorer budget had to accom- review will decide the next selection peratures. With some spectral infor- modate substantial additional for all Explorer categories. mation, the various ISM components

61 will be separable and the spatial dis- So,a, lributions of several different components will become evident. I1

A point-source survey can increase by two or three orders of magnitude the number of known active galactic nuclei (AGNs), quasars, e,ec,ron,cs white dwarfs, cataclysmic variables, and evolved stars. The richness of ( the resulting data set will substan ,ooe- lially increase our insight into such issues as the luminoshy function and evolution of AGNs and quasars; the correlation of AGNs with galaxies, clusters, and large-scale structures; the nature, history, and global behavior of star formation in normal galaxies; and the evolutionary paths to I+_i "- Equipment cataclysmic wlriables, Type I supernovae, and X-ray binaries of low Module mass. With the number of newly detected sources approaching one Figure 31. GRAVITY PROBE-B will test predictions of Einstein's General million, there is potential for such a Theory of Relativity by measuring the precession of cryogenically cooled mission to become the ultraviolet superconducting gyroscopes in the gravitational field of the rotating Earth. equivalent of the Palomar Sky Sur- The experiment is designed to assess both the curvature of space-time and the gravitomagnetic or "frame-dragging" effect of matter in motion. vey.

star. Improvements in proper-mo- geoid). The success of LAGEOS-I 3. Gravity Probe-B tion measurements will improve the for such measurements has prompted anticipated accuracy to about 0.2%. successors in other orbits chosen to (GP-B) permit the sampling of different combinations of the Earth's multi- Gravity Probc-B (Figure 31) is 4. Laser Geodynamics pole moments. proposed for launch into a polar Earth Satellite-3 (LAGEOS-3) orbit for a one-year missirm follow- The orbit of LAGEOS-2 has al- ing the Shuffle Test of Relativity ready been fixed by geodynamical The Laser Geodynamics Satel- Experiment (STORE). research requirements; however, the lite (LAGEOS) series was initiated orbit of the proposed LAGEOS-3 GP-B will measure the orienta- with the launch of LAGEOS-1 in satellite remains to be chosen. By 1976. The launch of LAGEOS-2 is tion of four cryogenically cooled su- giving LAGEOS-3 an orbital incli- perconducting gyroscopes re[alive to projected for 1992, and a third mem- nation supplementary to that of ben" of the series, LAGEOS-3, is now a guide star {Rigel/of known proper LAGEOS-I, the large Newtonian molten. This measuremenl will de- tinder study. LAGEOS is a joint contributions to the precession rates project of NASA and the Italian gov- termine both the geodetic and frame- of the two satellite orbits can be made ernment. dragging precessions of the gyro- to cancel. It then becomes feasible in scopes. For thc orbit chosen for GP- principle to measure the General The LAGEOS satellites are B, General Rclalivily predicts a geo- Relativity contribution to the pre- detic precession of about 6 arcsec/ dense, inert spheres covered with recession rates with an accuracy of trorefleciors for laser ranging. Be- year and a frame-dragging preces- perhaps 20e/, over the course of a sion of about .045 arcsec/year. For cause of their high ratio of mass to few years. Since the supplementary froula] area, these passive, long-lived one year of tracking, the anticipated LAGEOS-3 orbit would also be ac- satellites are relatively insensitive to accuracy of the experiment is about ceptable for geodynamic research, I_ of the frame-draggi,lg preces- atmospheric perturbations and thus this choice would appear to be an permit highly accurate determinations sion, lhnilcd primarily by uncerlain- attractive one. tics in the proper motion of the guide of the Earth's gravitational field (the

62 THE RECOMMENDED PROGRAM

5. New Flagship Interferometer in Space (POINTS). (c) 16-Meter Telescope and Intermediate Both make use of Michelson interferometers, and both can achieve the The 16-Meter Telescope, or Next Space-Based Missions mission objectives. Mission develop- Generation Space Telescope, has ment will be accompanied by an ad- been conceived as a vastly more Future, H'agship and intermedi- vanced technology program designed powerful successor to HST that will ate-class missions in space offer not only to support an AIM new start, build upon the HST scientific base. It prospects for dramatic advances in but also to further a more general, will fully exploit the potential of space angular resolution and in the imaging long-range plan for UV and visible- observatories by having precision of faint objects. We discuss below light interferometry from space. In optics that will perform at the optical the missions that have been proposed particular, AIM will represent an diffraction limit allowed by physics. as free-flying satellites; in several important step toward the develop- It will be cooled to temperatures only cases, however, these instruments or ment of an Imaging Optical Inter- 100 degrees above absolute zero so their analogues could also be placed ferometer in Space. that it can work in the infrared with a on the Moon (see No. 6: "Lunar Out- background less than one-millionth post Astrophysics Program"). of the background that limits ground- (b) Imaging Optical lntetferometer based infrared telescopes. in Space. (a) Astrometric lnteJferometry The !6-Meter Telescope will also Mission (AIM) The primary goal of the first have wide-field, state-of-the-art Imaging Optical Interferometer in cameras and spectrographs that will NASA's Astrometric Inter- Space will be to achieve an order-of- work over the entire ultraviolet, vis- ferometry Mission (AIM), planned magnitude improvement in angular ible, and mid-infrared spectral range. resolution in the ultraviolet and vis- for launch early in the 21st century, Its clarity, wide bandwidth, and dra- will be the first dedicated optical in- ible spectral regions by comparison matically lower background radia- with HST. The imaging interferom- terferometer in space. It will be cation will allow us to go to the heart of cter shouId also have significant light- pable of astrometric measurements some of the most fundamental ques- collecting area, since many of the having an accuracy of 3 to 30 micro- tions of astrophysics--for example, arcseconds. objects to be studied will be quite how stars like our Sun formed, and faint. A location either in Earth orbit how galaxies like our Milky Way or at a lunar base would be accept- AIM will make highly accurate formed and changed as the Universe able. measurements of the positions and evolved. It will even allow us to motions of distant stars, star clusters, address the question of whether plan- quasars, and other faint astronomical For an orbiting interferometer, ets like our own, with atmospheres two approaches are feasible. One objects. These fundamental new data like our own, exist around nearby will furnish precise distances to ob- employs a rather stiff structure, about stars. jects throughout our Galaxy, vastly 30 meters in size, with sources of improving the calibration of the cos- mechanical and thermal disturbances mic distance scale and permitting de- minimized. The other is based on a (d) Laser Gravitational finitive estimates of the size of the more flexible structure and makes Wave Antenna in Space. Universe. Using quasars as common extensive use of developments from points of reference, AIM will forge a the field of control-structure interac- A laser gravitational wave an- vital link between the radio and opti- tions. A lunar-based interferometer, tenna in space, with test masses cal reference frames. Observations by contrast, wouId be more like a separated by distances of about 107 of the dynamics of globular star clus- ground-based instrument. It would kilometers, has been proposed. One ters in our Galaxy will shed new light have the advantage that additional possible configuration, called the on galactic formation and evolution. telescopes could be added after the Laser Gravitational Wave Observa- AIM will also be capable of detecting initial configuration is established. tory in Space (LAGOS), consists of planets around remote stars by mea- an L-shaped array of three spacecraft suring perturbations in stellar mo- The development of an Imaging located 60 degrees behind the Earth tions caused by orbiting companions. Optical lnterferometer in Space will in solar orbit. benefit strongly from work on ground- Two promising candidate design based interferometry. This work will The orbits of the two outer concepts for AIM are already under test different instrument designs and spacecraft are somewhat eccentric study: an Orbiting Space Interferom- will provide complementary obser- and inclined in order to keep their eter (OSI) and a Precision Optical vations at longer wavelengths. distances to the central spacecraft

63 Figure 32. ASTRONOMICAL INSTRUMENTS ON THE MOON could provide powerful imaging and spectroscopic capabilities together with extremely high angular resolution at ultraviolet, visible, and infrared wavelengths, opening up new research areas at the frontiers of astrophysics. Shown in this artist's concept are the proposed Lunar Transit Telescope, the Optical Interferometer, and the Filled Aperture Segmented Optical Telescope, together with an interferometer for submillimeter and far-infrared observations. constant within l% over 10 years. ground of relic gravitons from the The low surface gravity, high- Each spacecraft is held centered on Big Bang amplified by inflation vacuum environment, and surface test masses within 10 _cm to reduce would also be sought. stability of the Moon make it a prom- gravitationaI-force changes. The ising astronomical base. It is attrac- passage of gravitational waves across tive for the emplacement of high- the antenna changes the difference in 6. Lunar Outpost resolution, long-baseline inter- length of the two long antenna arms. Astrophysics Program ferometers and for large-area, high- Lasers with I-watt beam power are sensitivity collectors (Figure 32). used to measure the arm lengths. On July 20, 1989, President Bush Moreover, the lunar rego[ith offers Phase-locked lasers at the two outer announced the Space Exploration virtually unlimited amounts of radia- spacecraft provide the return beams. Initiative (SEI), embodying the goal tion shielding for sensitive detectors. of accelerating human exploration of This ultra-sensitive interfero- the solar system. Eleven days later, By comparison with Earth orbit, metric detector would be capable of NASA Administrator R. H. Truly the lunar surface presents the disad- observing bursts, periodic sources, commissioned a study of ways to vantages of extreme thermal excur- and stochastic backgrounds of grav- implement this initiative; the study sions, opportunity for contamination ity waves in the 10-1aHz to [-Hz report was delivered to the National by dust, and potential for damage frequency range with unprecedented Space Council in November 1989. from micrometeorites, cosmic rays, sensitivity. Unless General Relativ- The NASA/OSSA Astrophysics Di- and magnetotail electrons. It is nev- ity is fundamentally in error, obser- vision was assigned responsibility for ertheless relatively straightforward vations would "guarantee" detection producing the astrophysics portion to counter these effects and to exploit the favorable attributes of the lunar of gravitational radiation from known of the report, and the present Work- sources, e.g., cataclysmic variables, ing Group reviewed and identified surface for forefront astrophysical research. neutron-star binaries, and white- several important opportunities for dwarf binaries in our Galaxy. Sig- Ultraviolet, Visible, and Gravity As- nals associated with supermassive trophysics in this connection. The primary criterion for the se- black holes and a stochastic back- lection of potential instruments for

64 THERECOMMENDED PROGRAM emplacement on the lunar surface ground-based observatories to obtain 0.1 to 10p+m. The interferometer will was scientific merit and high priority much more accurate angular mea- be particularly useful for resolving within the NASA Astrophysics pro- surements of source positions than both the broad-line and narrow-line gram. Each instrument selected had could be obtained from ground-based regions in active galactic nuclei and to possess capabilities beyond those measurements alone. will provide data that can be used to already planned, and the lunar sur- study the isotropy and uniformity of face had to provide an advantageous the Hubble flow to an accuracy of site for an investigation. (a) Lunar Transit Telescope 1%. Investigators will also be able to use this facility to measure the paral- The NASA science-discipline The Lunar Transit Telescope laxes of objects out to tens of MOWGs were used as informal peer- (LTT) is a 1- to 2-meter stationary megaparsecs, to image white dwarfs, review committees to review the sci- telescope that will use the motion of and to image accretion disks around entific credibility of recommended the Moon to scan the sky. The charge steltar objects, neutron stars, and black investigations, to define in greater coupled device (CCD) radiation de- holes. detail the candidate instruments and tectors will be clocked at the lunar facilities, and to review the program rotation rate, permitting superposition The Optical lnterferometer is strategy, instrument priorities, and of successive views of the same sky planned as a modular instrument in- technology requirements. Individual area. The lunar motion makes it corporating individual collectors with missions were prioritized by the As- possible to carry out a long- integra- apertures of about 1.5 meters. It will trophysics Subcommittee of NASA's tion, deep-sky survey at the telescope consist initially of four such collec- Space Science and Applications diffraction limit over a bandwidth tors, together with a signal combiner. Advisory Committee in September that is not possible from the Earth's Later, the initial 4-meter element of 1989. surface. the 16-meter Filled Aperture Seg- mented Optical Telescope (see be- Three ultraviolet and visible- LTT will be an imaging survey low) will be deployed: the 4-meter light missions were selected by the instrument for observations within telescope will thus serve as the fifth Astrophysics Subcommittee: a Lu- five broad bands spanning the ultra- element of the Optical Interferom- nar Transit Telescope (LTT), an Op- violet, visible, and near-infrared eter in addition to performing its im- tical Interferometer, and a Filled spectral regions (0.1 to 2 p.m) with a aging objectives. A final, sixth ele- Segmented Aperture Optical Tele- spatial resolution of 0.1 arcsec. The ment of the interferometer will even- scope. objective of the LTT is to conduct a tually be added. deep-sky survey down to 28th visual In developing the present plan magnitude, equivalent to the sensi- When completed, the inter- for Ultraviolet, Visible, and Gravity tivity that HST was designed to ferometer will have a total collecting Astrophysics, the Working Group has achieve; the LTT survey will thus area of about 21 square meters. The considered two of these missions-- complement HST measurements. projected baseline is 1-10 kin. The the lunar-based Optical lnterferom- low-seismic-noise environment of the eter and the Filled Aperture Seg- The lunar orbit allows the tele- Moon provides an ideal base for the mented Optical Telescope--to be scope to be operated with no moving Optical lnterferometer because the programmatic alternatives to the parts. The lunar site also offers long lunar surface serves as a highly stable Imaging Optical Interferometer in integration times, passive cooling, optical bench for the placement of Space and the 16-Meter Telescope and a high-vacuum environment. the interferometer components. discussed above, since it appears Either lunar-surface material or man- unlikely that the nation and NASA made shielding can be used to shield will fund both the lunar-based and the CCD detectors, which can be (c) Filled Aperture the space-based versions of these passively cooled on the lunar surface Segmented Optical Telescope instruments. down to 100 K. The attractive possibility of making LTT a soft-lander is The Filled Aperture Segmented As the lunar base matures, a La- worthy of study. Optical Telescope is a passively ser Interferometric Gravity Wave cooled, diffraction-limited telescope Observatory will be considered for with a diameter of 16 meters and a placement on the Moon. This facility (b) Optical lnte_ferometer field of view of approximately 1 is not considered to be a program- arcmin (Figure 33). It will have a matic alternative to a Laser Gravita- The lunar-based Optical Inter- pointing accuracy of about 10 mas tional Wave Antenna in Space be- ferometer is an extremely high angu- and be able to track aq object within cause it functions at a much shorter lar resolution instrument that will 1 mas. The telescope's greatest wavelength. It would work with operate in the wavelength range from strength will be spectroscopic obser-

65 Figure 33. 16-METER TELESCOPE ON THE MOON would be ideally suited to spectroscopic studies of faint sources or objects of low surface brightness. A lunar base provides the stability needed for accurate pointing, tracking, and control. The telescope would be assembled from modular segments, beginning with a 4-m instrument. vations of faint or low-surface- gram (see also discussion of the Op- surface is the Laser lnterferometric brightness objects; it will be able to tical Interferometer above). The re- Gravity Wave Observatory, which search for Earth-like planets around maining modules, representing an would use a laser interferometer of nearby stars, study the structure of additional mass of 27,000 kg, will be up to 50-kin baseline to search for high-redshift galaxies, determine added to the 4-meter telescope as gravity waves. Seismic test masses stellar populations, and study star increased weight-carrying capability connected by laser interferometers in formation. It will represent a major becomes available later in the pro- an "L" configuration are capable of step beyond HST. gram. measuring fractional changes in the separation of the masses to better The completed 16-meter tele- The lunar site provides a stable than 10 2_and are sensitive to gravity scope will have a mass estimated to base for high-precision pointing, waves with frequencies ranging from be 42,000 kg. The full instrument tracking and control. The high- about 1 Hz to a few kHz. cannot, therefore, be accommodated vacuum environment of the Moon in the early phases of the Lunar Out- enables broadband spectral observa- The low-seismic-noise environ- post Astrophysics Program even tions to be made, and the availability ment of the Moon is ideal for a gravity- though it has been recommended by of astronauts and work facilities will wave detector of this type. A lunar several scientific groups. make the in-situ assembly of the gravity-wave observatory, operated telescope vastly simpler than in orbit. in conjunction with similar instru- The telescope can, however, be ments on Earth, could locate gravity- constructed in modules. Because of wave sources through triangulation. the scientific importance of the in- (d) Additional Instrument However, the feasibility of the tech- strument, NASA has adopted a strat- Considered for the Lunar Base nique must first be proven on Earth egy in which a 4-meter central mod- before the instrument will be con- ule will be deployed and put into An additional instrument considered for emplacement on the operation relatively early in the pro- sidered for placement on the lunar Moon.

66 THE RECOMMENDED PROGRAM C. The Research and Analysis Program

NASA's primary mode of support for space astrophysics is the Ultraviolet and Visible Astrophysics Branch implementation of missions and the provision of both data and the re- Research and Analysis Program, FY 1990 sources to analyze these data. How- ever, the Research and Analysis Research Area Number $K (R&A) program is the agency's rec- of Grants ognition of the need to maintain "a healthy research base from which new and innovative ideas can arise. Sounding Rockets 9 3,917 Space Detector Development 5 841 Instrumentation and Technology 8 610 1. Background Theory and Data Analysis 13 523 Laboratory Astrophysics 9 510 The purpose of the R&A program is to insure the continued intellectual Gravitational Physics 6 490 and technical vibrancy of the field. Ground-Based Work/NASA Centers 6 295 R&A funds support modest experi- Space Data Analysis 5 203 ments in which high-risk, high-payoff Data Cataloging 3 137 ideas can be tested. The R&A program also supports development of Other 1 29 otherwise untested prototype instruments, fundamental laboratory stud- Totals 65 7,555 ies necessary for the interpretation of astrophysical data, theoretical and analytical studies that could not oth- Moreover, for their fullest un- puting at the current state-of-the-art erwise be funded in so timely a way, derstanding and interpretation, both level. Advanced computational ca- and the involvement of graduate stu- observational and experimental re- pabilities are also essential for effec- dents in forefront research. sults require complementary theo- tive data analysis and interpretation. retical work. New theoretical ideas Historically, NASA planning has are stimulated by new observations been oriented toward flight projects and experimental results; theoretical 2. R&A Funding and has therefore been focused on predictions, in turn, help to focus the hardware. In the case of Ultraviolet, planning of new observations in the The R&A component of the Visible, and Gravity Astrophysics, near term as well as to provoke ideas OSSA Astrophysics Division budget this emphasis is reflected in substan- for future missions. The increasingly is the portion that most directly affects tial R&A support for sounding important field of gravitational phys- many scientists. Of the Division's rockets, detector development, and ics, among many others, will benefit $52 million Fiscal Year 1990 R&A instrumentation and technology. All from the interplay of observation, budget, $24 million was devoted to of these efforts are essential for a experiment, and theory during the the funding of small and moderate- vigorous research program. 1990s and beyond. sized grants. The remainder was used to support early mission studies Astrophysics, however, depends We therefore applaud NASA's and larger-scale Advanced Technol- on the interplay between observa- expansion of support for astrophysi- ogy Development efforts. tion, experiment, and theory, and an cal theory during the second half of optimally productive program must the 1980s and call for a further Nearly $7.6 million in grants support all three. Laboratory astro- strengthening of this effort. In par- funding was awarded in FY 1990 physics, for example, is often the ticular, the continuing rapid growth through the Ultraviolet and Visible only source of data on atomic and in computational power has had, and Astrophysics Branch (seebox). These molecular transition probabilities, will continue to have, a strong influ- funds supported 65 grants in a num- properties of materials under unusual ence on the power of astrophysical ber of areas, including sounding conditions, reaction rates, and other theory. A complete program of sup- rockets, detector development, htbo- information crucial to the interpreta- port for astrophysical theory must ratory astrophysics, gravitational as- tion of astronomical observations. therefore also include funds for corn- trophysics, space data analysis, and

67 ORIGINAL PAGE COLOR PHOTOGRAPH

Figure 34. SOUNDING ROCKETS like this stalwart Black Brandt launcher will continue to play a substantial role in the NASA astrophysics program, particularly for observations in the ultraviolet spectral region. Although restricted in duration, rocket flights can provide rapid and comparatively inexpensive access to space for a wide variety of exploratory payloads. theory. Sounding rockets account for conducting research in astronomy. since the payloads typically had small for a large fraction of the budget Depending on the observing require- collecting areas and were relatively because of their vital role in provid- ments, sounding rockets, balloons, simple. A consequence of this reap- ing ready access to space for modest and high-flying aircraft were the es- praisal was a shift in NASA support experiments. tablished routes through which as- from sounding-rocket experiments to tronomers could address the observ- small payloads launched by the na- The Working Group believes that ing challenges that were out of reach tional Space Transportation System the present division of funds among from the ground (Figure 34). (i.e., the Space Shuttle). various research areas of the R&A program is reasonable; the compara- In the decade that followed, The delay in Shuttle launch op- tively high level of support for both launch vehicles and spacecraft sup- portunities during the late 1980s, to- sounding rockets and detector de- port systems for experiments in orbit gether with a reaction against reli- velopment reflects the high cost of matured and became able to sustain a ance on a single launch vehicle, pre- hardware programs. It is the strong sequence of large and small observa- cipitated an abrupt reversal of belief of members of the Working tories that could operate for long in- NASA's drift away from sounding Group that R&A support, which has tervals of time. On the heels of such rockets for space astronomy. At about remained at an essentially constant advances, some questioned the need the same time, the temporary de- level over the past 5 years, is in urgent for suborbital astronomy missions emphasis of sounding rockets restored need of augmentation. and, in particular, for sounding rock- an awareness of the program's special ets. contributions, which reach beyond the simple calculation of the amount 3. Sounding Rockets It was difficult, for instance, to of scientific data generated per mis- defend the cost-effectiveness of sion. These higher-order benefits are In the 1960s, suborbital missions sounding rockets in the simplistic as follows: provided the broadest access to space terms of observing time per dollar,

68 THE RECOMMENDED PROGRAM

SOUNDING ROCKETS

Space astronomy began when short observing times (typically simple instruments were installed on about 5 minutes). German military V-2 rockets captured at the end of World War II and flown Even in an era when orbital on test flights at the White Sands missions have become common- Missile Range in New Mexico. (Rock- place, these suborbital rocket flights ets in this general size and weight are still of special value, since they class are popularly known as "sound- (a) can be scheduled on short notice, ing rockets" because they have been (b) are well suited to highly special- used extensively to sample or ized observations or the testing of "sound" the upper atmosphere.) payloads featuring new and risky Since that time, suborbital rocket technologies, and (c) serve as an flights for scientific research have entry mode and training ground for provided inexpensive access to new individuals or groups working space for small instruments (usually on space hardware. up to several hundred pounds) and

Opportunities to fly specialized there is some doubt that there will be Many experiments that have been or high-risk e._periments. Given the an opportunity to use it? The expec- flown on Shuttle flights or are now infrequent opportunities within a tation that an instrument can be flown manifested for such flights were given discipline for access to space, on a rocket within a few years of its originally flown on sounding rock- there is an understandable pressure to initial development is a powerful in- ets. In several cases, valuable les- develop highly reliable, general-pur- centive, even if the scientific return sons were learned which influenced pose instruments that can be applied will be modest because of the short the designs of the instruments. by a broad community of investiga- observing time. tors to a variety of general problems. Training of young investigators. By the same token, there is a reluc- An important corollary is that a A vigorous program in space as- tance to fly a technologically or sci- sounding-rocket mission is a simple tronomy must be sustained by an entifically risky experiment even if enough undertaking that its launch influx of young astronomers who are the potential payoff is very high. It schedule can be relatively flexible. proficient in designing, building, and follows that sounding rockets, by When an experimenter encounters flying space hardware. Suborbital virtue of their simplicity and low unforeseen difficulties, as is often the missions are an ideal training ground cost, are an attractive alternative for case with a new instrument, it is a for graduate students and post-doc- much more specialized experiments simple matter to reschedule the flight toral fellows. People in such posi- or those that carry high risk. These to a later time. Furthermore, a scien- tions must be able to fly their ex- vehicles provide an ideal option to tist often has access to the payload periments within a few years of the pursue objectives that have narrow, right up to the day before launch. time they begin building them. The but still worthwhile applications in These alternatives are not provided low cost of sounding-rocket research astronomy. by the Space Shuttle or major ex- is an important characteristic which pendable-launch-vehicle missions. permits the support of a reasonable Encouragement of new instru- number of young investigators. A ments and technology. Sounding Realistic testing of prototype in- high proportion of the scientists who rockets encourage the development struments. Sounding rockets pro- are now developing major space- of new and innovative instruments, vide realistic tests of prototype in- astronomy instruments "cut their often ones that incorporate enterpris- struments intended for longer-term teeth" on sounding rockets early in ing technological concepts. Who will space missions. In fact, the envi- their careers. spend extensive time creating and ronment of a rocket flight is, in sev- perfecting an instrument, especially eral respects, more brutal than that Entry of new scientific groups. one with special design challenges, if associated with an orbital mission. An additional consideration, related

69 THERECOMMENDED PROGRAM

to that mentioned directly above, is In summary, the Working Group Space Fright Center at the Wallops the fact that sounding-rocket grant urges that NASA continue an ener- Flight Facility to support the required programs provide an excellent entry getic program of astronomical re- frequency of flights for astronomy mode for new space-astronomy search using suborbital vehicles. and assure good mission reliability. groups with fresh approaches to re- Sounding rockets are ideal for those Under most circumstances, a fre- search. Some turnoverand evolution experiments that must operate above quency of one operationally suc- in the complexion of the institutions virtually all of the atmosphere but cessful flight per year for a research that carry out space astronomy is an that can achieve their technical or group of average size is appropriate. important prerequisite for avoiding scientific goals within a brief flight. stagnation and continuing true It is vitally important that sufficient progress in science. funding be restored to the Goddard

"2_

7O APPENDICES Appendix A: Participation

A.K. Dupree (Chairperson), Harvard-Smithsonian Center for Astrophysics (1988-1991) J.W. Armstrong, Jet Propulsion Laboratory (1987-1990) P. Bender, JILA/University of Colorado (1989-1992) C.S. Bowyer, University of California at Berkeley (1988-1991) W.C. Cash, Jr., University of Colorado (1988-1991) J.T. Clarke, University of Michigan (1989-1992) F.B. Estabrook, Jet Propulsion Laboratory (1990-1993) R.J. Harms, Applied Research Corporation (1985-1988) P.D. Hemenway, University of Texas at Austin (1989-1992) J.G. Hoessel, University of Wisconsin at Madison (1989-1992) G.D. Illingworth, University of California at Santa Cruz (1986-1989) E.B. Jenkins, Princeton University (1988-1991) J.H. Krolik, The Johns Hopkins University (1988-1991) R.F. Malina (Chairperson Elect, 1991), University of California at Berkeley (1990-1993) J.T. McGraw, University of Arizona ( 1988-1991 ) J.R. Mould, California Institute of Technology ( 1990-1993) M. Shara, Space Telescope Science Institute (1988-1991) P. Szkody, University of Washington (1989-1992) A. Tokunaga, University of Hawaii ( 1986-1989) B. Woodgate, NASA Goddard Space Flight Center (1988-1991)

Management Operations Working Group--NASA Members ex officio:

G.C. Clayton, NASA Headquarters (1988-1990) D.P. Huenemoerder, NASA Headquarters (1990-1992) J.M. Mead, NASA Goddard Space Flight Center R.V. Stachnik, NASA Headquarters E.J. Weiler, NASA Headquarters

Liaisons:

M.L. Aizenmann, National Science Foundation P.B. Boyce, American Astronomical Society

71 Appendix B: Acronyms and Abbreviations

ACE: Advanced Composition Explorer ISM: Interstellar medium

AGN: Active galactic nucleus LAGEOS: Laser Geodynamics Satellite

AIM: Astrometric Interferometry Mission LAGOS: Laser Gravitational Wave Observatory in Space

ASTRO: Space Shuttle Astronomy Observatory LTT: Lunar Transit Telescope

AstroSPAS: German space platform MAMA: Multi-anode microchannel array

AXAF: Advanced X-Ray Astrophysics Facility MCP: Microchannel plate

BBXRT: Broad Band X-Ray Telescope MOWG: Management Operations Working Group

CCD: Charge coupled device NASA: National Aeronautics and Space Administration

COBE: Cosmic Background Explorer NIC: Near Infrared Camera

Copernicus: Third OAO satellite NSF: National Science Foundation

CSAA: Committee on Space Astronomy OAO: Orbiting Astronomical Observatory and Astrophysics ORFEUS: Orbiting and Retrievable Far and ESA: European Space Agency Extreme Ultraviolet Spectrometer EUV: Extreme ultraviolet OSI: Orbiting Stellar lnterferometer

EUVE: Extreme Ultraviolet Explorer OSSA: Office of Space Science and Applications

EXOSAT: ESA X-Ray Observatory Satellite P-L: Period-luminosity

FGS: Fine Guidance Sensors PI: Principal Investigator

FOC: Faint Object Camera POINTS: Precision Optical lnterferometry in Space

FOS: Faint Object Spectrograph QPO: Quasi-periodic oscillation

FUSE: Far Ultraviolet Spectroscopic Explorer R&A: Research and Analysis

FUV: Far ultraviolet SAS: Small Astronomy Satellite

GHRS: Goddard High Resolution Spectrograph SEI: Space Exploration Initiative

GP-A: Gravity Probe-A SMEX: Small Explorer

GP-B: Gravity Probe-B SN: Supernova

GRO: Gamma Ray Observatory STIS: Space Telescope Imaging Spectrograph

H-R: Hertzsprung-Russell STORE: Shuttle Test of Relativity Experiment

HIPPARCOS: ESA astrometric satellite STU: Shuttle Test Unit

HSP: High Speed Photometer UIT: Ultraviolet Imaging Telescope

HST: Hubble Space Telescope UV: Ultraviolet

HUT: Hopkins Ultraviolet Telescope UVS: Ultraviolet Spectrometers on Voyager spacecraft

IMAPS: Interstellar Medium Absorption VLBI: Very long baseline interferometry Profile Spectrograph WF/PC: Wide Field/Planetary Camera IR: Infrared WUPPE: Wisconsin Ultraviolet IRAS: Infrared Astronomical Satellite Photo-Polarimeter Experiment IUE: International Ultraviolet Explorer XTE: X-Ray Timing Explorer

72 APPENDICES Appendix C: References

Astronomy and Astrophysics for the I980s. Volume 1: Report of the Astronomy Survey Committee (National Academy Press, Washington, D.C., 1982).

The Explorer Program fi_r Astronomy and Astrophysics. Committee on Space Astronomy and Astrophysics (National Academy Press, Washington, D.C., 1986).

Space Science in the Tweno'-First Century: lmperatives fi)r the Decades 1995 to 2015--Astronomy and Astrophysics (National Academy Press, Washington, D.C., 1988).

Report of Ad Hoc Committee on Gravity Astrophysics Technology (Ultraviolet and Visible Astrophysics Branch, Astrophysics Division, NASA Headquarters, Washington, D.C.).

The Decade of Discovet 3, in Astronomy and Astrophysics. Astronomy and Astrophysics Survey Committee (National Academy Press, Washington, D.C., 1991).

Appendix D: Acknowledgments

E.J. Weiler: Chief, Ultraviolet and Visible Astrophysics Branch, Astrophysics Division, NASA Office of Space Science and Applications

R.V. Stachnik: MOWG Report Manager

P.A. Blanchard and B. Geldzahler: MOWG Report Editors

Appendix E: OSSA Management

L.A. Fisk: Associate Administrator for Space Science and Applications

C.J. Pellerin: Director, Astrophysics Division

73 Credits

Ground-based image of M 81 beneath Foreword: National Optical Astronomy Observatories.

Figures 2,3: Space Telescope Science Institute. 7: M. Geller, Harvard-Smithsonian Center for Astrophysics. 10, 11: National Optical Astronomy Observatories. 15: adapted from a diagram by G. Abell in E._ploration of the Universe (1969). t9: B. Smith, University of Arizona and R. Terrile, Jet Propulsion Laboratory. 22: J. Krolik, The Johns Hopkins University. 25: adapted from a diagram by M. Harwit in Cosmic Discoveo' (1984). 26: Space Telescope Science Inslitute (WF/PC and FOC images), University of Wisconsin (HSP data), and Lowell Observatory (FGS data). 27: University of California at Berkeley.

Other images and illustrations provided by National Aeronautics and Space Administration or adapted from publications of NASA-sponsored research.