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4 High-Energy Astrophysics I. INTRODUCTION High-energy astrophysics involves a wide range of observed phe- nomena, physical processes, instrumental techniques, and mission requirements. The field includes x rays to the low-energy limit determined by interstellar absorption, gamma rays to the highest energies, and cosmic-ray particles of all varieties and energies. The blossoming of this field and its close relationship with radio and optical astronomy have led to an entirely new view of a universe dominated in large degree by high-energy particles and processes. Recent discoveries have revolutionized astrophysical thought: the remarkable periodic and pulsed x-ray sources such as Hercules X-l, which can only be explained in terms of compact objects such as neutron stars or black holes revolving in close contact with massive stars; x rays from galaxies and from the intergalactic medium of clusters; a multicomponent diffuse, nearly isotropic background extending over the entire x- and gamma-ray range, which clearly involves the large-scale structure of the universe; gamma rays from point sources and the galactic plane; and, finally, the detection of extremely high-Z cosmic rays, extremely high-energy electrons, the isotopic composition of the lightest elements, and spectral dif- ferences of the various components. In the early period of Shuttle use the opportunity will exist to make detailed measurements of the charge and isotopic composition of cosmic rays, the fine details of the energy spectra to 10' 4 eV, and the streaming patterns of low-energy particles outside the sphere of solar influence. We will be able to determine the spectrum and spatial structure of gamma-ray sources in considerable detail, and we believe that detection of nuclear gamma-ray lines from outside the solar system will be possible. The High Energy Astronomical Observatory (HEAO) program should extend the number of x-ray sources from 40

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Scientific Uses of the Space Shuttle 41 the presently known 160 to over 1000 and obtain detailed measure- ments on the spectra, structure, and time variability of these sources. Focusing x-ray devices on HEAO-B and the larger telescopes of the 1980's will operate as a facility much like ground-based optical telescopes and obtain analogous information arising from entirely different physical processes than those giving rise to optical and radio radiation. We identify a requirement for an x-ray focusing telescope with an aperture of at least 2 m, during the last half of the 1980's, capable of accommodating a number of instruments at the focus and operated as a national facility. Such a facility will allow us to observe and study high-energy process in the faintest extragalactic objects that will become observable with the Large Space Telescope. Cosmic-ray and gamma-ray research during this period will also require major instruments to determine the spectrum of particles with energies beyond 1014 eV and to measure weak fluxes of photons beyond 1011 eV. These objectives will require greater resources devoted to this discipline, as well as an increased number of missions, since the field is totally dependent on observations from space. Although we envisage a range of opportunities, only a continuing program of unmanned, long-lived automated spacecraft can provide the con- tinuity of observations required to develop the field and to ensure a succession of new discoveries. The pallet on the Shuttle sortie missions provides opportunities for short observing programs, for development and test of instruments before commitment to long- term flight, and for involvement of many participants in the program, as in the present balloon and rocket efforts. We believe that the program presented here can only be realized if costs are minimized through standardized interfaces, more tolerance of risks, standard spacecraft systems built in quantities, and the operation of large, long-lived instruments as national facilities. II. SCIENTIFIC OBJECTIVES A. X-Ray Astronomy Discoveries in the past few years have clearly established that x-ray observations are an essential tool in the study of many of the objects of greatest current astrophysical interest such as pulsars, quasars,

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42 HIGH-ENERGY ASTROPHYSICS Seyfert galaxies, clusters of galaxies, and the intergalactic medium. The study of compact x-ray emitting objects in binary systems permits investigations of the properties of stars near the end point of stellar evolution and of the physics of matter at extreme pressures, densities, and magnetic fields. In the coming decade, x-ray obser- vations will likely be extended to the coronas of main-sequence and giant late-type stars, as well as to peculiar stars such as flare stars. It will also be possible to detect and resolve clusters of galaxies at extreme distances (Z = 3) and study their evolution over times comparable with the age of the universe. X-ray emission from clusters of galaxies is likely to originate in the heretofore unobserved intergalactic medium, which may contain a large fraction of the total observable mass of the universe. These studies will profoundly influence our understanding of the dynamics and evolution of the cosmos. 1. SCIENTIFIC OBJECTIVES The scientific objectives of x-ray astronomy can be broadly grouped under the following headings: (a) STELLAR STRUCTURE AND EVOLUTION There is convincing evi- dence that many of the galactic x-ray sources are binary systems in which one member is a collapsed star, either a neutron star or a black hole. Far from being an oddity, such an x-ray emitting phase appears to be a necessary consequence of present theories in the evolution of stars in close binary systems. (Half of all stars occur in binary systems.) The study of these systems, in which very large amounts of mass are transferred from one member to the other, is essential to the understanding of stellar evolution occurring in these conditions. The presence of neutron stars and black holes in binary systems permits us to obtain a vast amount of information on the physics of highly compressed and nuclear matter. Furthermore, we can for the first time examine dynamical properties in a very intense gravita- tional field—one in which general relativity effects predominate. Thus we are provided with the equivalent of a general relativity astrophysical laboratory. The importance of x-ray observations of stellar structure is not limited to objects that are primarily x-ray emitters. We can extend to a large range of stars the type of detailed study of stellar atmospheres previously limited to the sun.

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Scientific Uses of the Space Shuttle 43 Finally, supernova remnants, for example the Crab nebula, in which most of the electromagnetic energy dissipation occurs via high-energy photons, give us an invaluable astrophysics plasma laboratory for which x-ray observations can be carried out with techniques similar to the ones used in the study of solar plasmas. The study of the generation, containment, and dissipation of the high-energy particles at the pulsar and of the mechanisms of energy transfer to the interstellar medium is of great astrophysical signifi- cance. (b) LARGE-SCALE GALACTIC PHENOMENA X-ray observations pro- vide unique capabilities for studying the interstellar medium. The column density of elements such as oxygen, neon, and sulfur, and possibly the state of ionization of the gas, can be measured directly by observation of the appropriate K-shell absorption edges in the spectra of discrete sources. X-ray observations of the soft back- ground (~0.25keV), which we believe to be at least in part of galactic origin, can yield information on the structure and distribu- tions of clouds of interstellar material. Such observations can be carried out both of our own galaxy and of galaxies of the local group, such as M31, where we can map the entire galaxy in soft x rays. (c) NATURE OF ACTIVE GALAXIES The study of the spatial distribution, spectral characteristics, and time variations of the x-ray emissions from the nuclear regions of galaxies could yield the key to the understanding of the fundamental processes that give rise to the enormous production of energy occurring there. In addition, the inverse Compton reaction between cosmic rays and the microwave background, which result in high-energy photons, will allow study of the extended radio regions associated with these objects. Extending the observations to earlier epochs, i.e., greater distances, would allow the study of the evolution of active galaxies. It is also possible that a new type of extragalactic object has already been discovered, whose detailed properties are as yet unknown, that emits most of its energy in the x-ray region of the electromagnetic spectrum. Approximately 40 of the 160 Uhuru sources appear to fall in this class of "x-ray galaxies." The study of their nature may well turn out to lead to results as startling as the study of quasars.

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44 , HIGH-ENERGY ASTROPHYSICS (d) RICH CLUSTERS OF GALAXIES—COSMOLOGY The recently discovered extended x-ray emitting regions in rich clusters of galaxies are likely to be a manifestation of a complex intercluster medium. The structure of this region and its relation to the dynamical parameters of the cluster must now be investigated. Observation of very distant members of this class of objects can allow us to study the evolution of the emission regions and of clusters with obvious consequences on cosmological theories. Other x-ray observations have direct bearing on cosmological theories, in particular the study of the extragalactic diffused x-ray background. The important question to be resolved is whether the background is due to a large number of individual sources or is truly diffused. In any event the background is a probe into a region of red shift > 3 and can provide data on the nature and structure of the cosmos on this very large scale. 2. OBSERVATIONAL OBJECTIVES The above scientific objectives can be translated into observational objectives as follows: (a) HIGH-SENSITIVITY SURVEYS The present limit of 10t4 Sco X-l should be extended to 10t8 Sco X-l with a survey divided into three energy ranges, -0.1-2 keV, 2-20 keV, and 20-200 keV. In at least one of these energy ranges, the surveys must have the following capability: location of point sources to 1 sec of arc, structure of extended sources to 0.1 sec of arc, and broadband spectra (X/AX~5) over the entire range. (b) HIGH RESOLUTION OF SPECTROSCOPY OF SELECTED SOURCES The sensitivity should be extended to sources of 10t4 Sco X-l intensity with X/AX~ 10 . (c) POLARIMETRY OF SELECTED SOURCES One percent polarization measurements should be made on sources to ~10t3 Sco X-l intensity. (d) STUDY OF TIME STRUCTURE High-time-resolution studies should be made of aperiodic pulsating sources such as Cyg X-l with 1-jusec resolution. In addition, studies of binary periods and changes of the pulsation periods and orbital parameter over 2-4 years should be carried out. Such studies to be conducted on sources of Lx = 103 6 erg/sec to distances of 30 Mpc.

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Scientific Uses of the Space Shuttle 45 B. Gamma-Ray Astronomy Gamma-ray astronomy provides information that can be obtained in no other way on the high-energy particles and processes occurring in the universe, both currently and in the remote past. Of all parts of the electromagnetic spectrum, only this one measures directly the presence and effects of energetic nuclei and antiparticles, while also preserving the directional and time features of the sources. Nuclear de-excitation gamma rays can uniquely identify places and events in which element synthesis is occurring and give detailed information about what happens in supernova explosions. Furthermore, high- energy electrons, wherever they exist, signal their presence by emitting gamma rays via scattering of the lower-energy radiation present in the same places. This gives knowledge of the electrons independent of assumptions about the magnetic field, on which the radio emission from these electrons depends, and indirectly about the magnetic fields as well—in supernovae, radio galaxies, galactic nuclei or jets, and intergalactic space. A most exciting recent development has been the discovery from OSO-7 measurements of nuclear gamma rays at 0.51, 2.2, 4.4, and possibly even 6.12 MeV due to accelerated protons interacting in the solar atmosphere or its surface during intense solar flares. Similar emissions may be expected from objects that exhibit flaring phenomena many orders of magnitude more energetic than that of the sun. These observations herald a major breakthrough in nuclear gamma-ray spectroscopy. Gamma rays result from quite different mechanisms than those that produce most of the cosmic x rays, hence they convey different types of information. Moreover, the present universe is extremely transparent to gamma rays, hence they retain the detailed imprint of spectral, directional, and temporal features imposed at their birth, even if they were born deep in regions opaque to visible light or at times far back in the evolutionary history of the universe. Emission-line spectroscopy will undoubtedly be as significant to gamma-ray astronomy as it has been to astronomical research in the optical and radio regions. In this area, experimental work has been far outpaced by theoretical nuclear astrophysics. Recently, however, gamma-ray line emission during solar flares has been measured, and an indication of monochromatic 0.47-MeV gamma ray from the galactic center has been obtained. Supernova and blast nucleosynthesis theories have been especially fruitful in posing questions to be answered by nuclear gamma-ray

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46 HIGH-ENERGY ASTROPHYSICS spectroscopy. Several researchers have suggested that the exponen- tially decaying light curve of the type I supernova is related to the radioactive decay of isotopes synthesized in the explosion either by the r-process or by silicon burning. Either process will leave quantities of radioactive materials in the debris, and determination of the presence and constituency of these materials could decide between the mechanisms. In addition to the residual radiation, prompt nuclear emissions from interactions taking place during the explosion could yield information about the supernova processes themselves. Prompt and secondary nuclear emissions from extragalactic supernova explosions should also be a significant component of the diffuse cosmic gamma-ray background. Since radiation at early epochs will be red shifted, one should see a line profile whose shape is a historical record of the rate of nucleosynthesis in the universe. Thus, it is no surprise that the gamma-ray sky looks very different from the x-ray sky, which in turn is different from the optical and radio skies. An intensive effort to determine the nature and detailed features of the discrete sources of gamma-ray continua, to measure the spectral structure and understand the origin of the diffuse background and to detect nuclear line radiation from galactic and extragalactic sources, will not only solve or sharply delineate many present astrophysical questions but will set the stage for exciting new discoveries. It helps in deciphering the origins of the high-energy gamma radiation that the most interesting and likely processes leave characteristic signatures on the spectrum. The interaction of cosmic- ray particles with gas, producing gammas by TT° decay, yields a spectrum with a broad peak at 68 MeV, tailing off gradually to a spectral slope paralleling that of the cosmic rays. Matter-antimatter annihilation also produces gammas via ir° decay, hence again with a peak at 68 MeV; but since annihilation favors nucleons of low velocity, the spectrum falls sharply at energies above a few hundred MeV, instead of following the cosmic-ray spectrum. Scattering of lower-energy photons by high-energy electrons yields a power-law gamma-ray spectrum (if that is the character of the electron spectrum) with a spectral index equal to that of the synchrotron radiation from the same electrons—namely, half the spectral index of the electrons. Other features may be imposed on the radiation by opacity of the universe, produced in the present epoch for gammas above 1014 eV by the 2.7° background and marginally for gammas above 101' eV by the optical background. In the cosmological past,

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Scientific Uses of the Space Shuttle 47 the opacities were much higher and the cutoff energies somewhat lower. Red shift due to the universal expansion has moved all features such as these to lower energies in contemporary spectra. Spatial as well as temporal features are to be sought. For instance, electrons that have escaped from radio galaxies should produce a gamma-ray halo due to scattering of the 2.7° background during their limited lifetime at high energy. If supernovae are the principal source of cosmic rays in our galaxy, the heavy particle yield per supernova must be large enough that if these particles had remained trapped in the nebula the filaments would have been dragged out more rapidly. Therefore, the fast nuclei must have escaped in the early history and still (because of interstellar fields) inhabit a surrounding region 1 or 2 deg in diameter, where they interact with ambient gas to produce TT°-decay gammas, Massive dark clouds in the galaxy, too, serve as sources of w0-decay gammas, with which the columnar mass density of the clouds can be mapped. Information already available from SAS-2 shoVs the presence of spatial structure in the intensity distribution along the galactic plane and also of variations in the spectrum from different directions. The discovery of unexpectedly high-intensity gamma radiation along the galactic plane in the neighborhood of the galactic center, and of possible discrete sources in this region, is one of the most remarkable outcomes of balloon flights and the few small satellite observations conducted of gamma rays thus far. In order to unfold the structure of the galactic center region, as well as to resolve the other phenomena mentioned above, the energies of high-energy gamma rays must be measured well enough to distinguish differences in the broad spectral features of the different sources; it is vital to measure angles to the smallest fraction of a degree permitted by the fundamental requirement of being able to apply these measurements to extremely small fluxes (<10t7 photon cmt2 sect1 in many important cases). Large detector area, at least a few square meters, is therefore a necessity. In the regime of low-energy gamma radiation, the most distinctive clues are sharp spectral lines, which require fine spectral resolution not only to identify the lines but to discern them in the presence of a strong background continuum. Here, too, adequate sensitivity requires large area. Possibly the most significant of the gamma-ray discoveries is that of a diffuse high-energy radiation apparently uniformly bright over the entire sky. The existence of this radiation has profound implications for cosmology. It is therefore urgent to measure the

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48 HIGH-ENERGY ASTROPHYSICS high-galactic-latitude flux in many directions with enough precision to set fine limits on its anisotropy and to follow its spectrum up to the high energies where opacity in early epochs may have left significant marks. Again, these purposes require very large detector area, fair energy resolution up to high energies, and good angular resolution. The discrete source studied most extensively is the Crab nebula, from which the pulsed flux has been detected up to more than 109 eV. Processes impossible to duplicate in the laboratory, such as gamma-ray absorption via pair production in extremely strong magnetic fields, may be observable in the study of pulsed gammas from neutron stars. The high-energy pulses from the Crab show features on a time scale considerably finer than 10t3 sec, pointing up the necessity of including the time dimension in gamma-ray detection, to a precision of at least 1(T4 sec. A startling, newly observed phenomenon in need of investigation is the bursts, lasting tens of seconds, of hard x rays and gamma rays discovered with Project Vela low-energy gamma-ray monitors. What these remarkable events signify is not yet known. They show a need to monitor the whole sky for abrupt changes on as broad a temporal bandwidth and energy bandwidth as possible. Even more clearly, this recent discovery emphasizes that for gamma rays, as happened before for x rays and radio waves, the discovery of unexpected phenomena may well outweigh the importance of systematic investigation of known processes. This always happens when one looks at the external universe with new eyes—in a new part of the spectrum or with instru- ments that have new dimensions of resolution and sensitivity, as do those designed for gamma-ray measurements in the period of Shuttle availability. C. Cosmic-Ray Astronomy The study of high-energy cosmic-ray particles plays a unique role in modern high-energy astrophysics and has a direct bearing on a variety of basic astrophysical problems. X rays and gamma rays are produced frequently in conjunction with, or by, these high-energy particles. The energy density of cosmic rays in the galaxy, ~1 eV/cm3, is comparable with that of the containing magnetic fields, of starlight, and of the kinetic motion of interstellar matter. The cosmic rays themselves therefore are a major, and perhaps controlling, element of galactic structure. Cosmic rays provide the only direct material samples from outside

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Scientific Uses of the Space Shuttle 49 the solar system, as well as data on the origin and nature of the most interesting stellar sources, where cosmic rays are believed to originate. These nuclei will have been synthesized in stellar furnaces, then accelerated, ejected, stored, and propagated in the interstellar medium. The physical environments under which nucleosynthesis takes place impart definite signatures to both the charge and isotopic abundances of the manufactured elements. These abundances are altered in a known way by spallation processes in the interstellar medium. After corrections for the propagation effects, the resulting abundances of cosmic-ray sources may be directly compared with predictions from nucleosynthesis theory for different types of astrophysical sources. In addition, the measurement of unique radioisotopes, which represent "nuclear clocks," can give direct evidence for presently ongoing nucleosynthesis in the galaxy. Once the particles escape from the vicinity of the sources, they are contained in the microgauss galactic magnetic fields. One sees then a superposition of many sources in which the galactic cosmic rays are largely isotropic upon reaching earth. Beacuse of the containment process, cosmic-ray particles are major elements in the structure and dynamics of the galaxy. The thermal and dynamical state of the interstellar gas, the formation of clouds and stars from the interstellar gas, the structure of the gaseous disk, and the galactic halo are dominated by cosmic rays and can be understood only on the basis of quantitative observations of cosmic-ray charge, energy, and mass spectra. Electrons and positrons have the unique feature that in their passage through the intergalactic medium, they interact with the microwave background radiation, and their observed spectral behavior places constraints on the universality of this radiation. Also these particles are a source of radio waves through synchrotron radiation and of x rays through the inverse Compton process. The interpretation of interstellar processes must take account of all these aspects. The major observational objectives are 1. To determine accurately, from direct measurements, the energy, mass, and charge spectra of the cosmic-ray nuclei (e.g., H-U and beyond). The physics of the cosmic accelerators producing the immense energies of cosmic rays is not understood, although a number of ideas involving supernovae, neutron stars, pulsars, and other energetic objects have been proposed. An accurate determination of the energy

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50 HIGH-ENERGY ASTROPHYSICS spectra of different nuclei will provide clues to the nature of the acceleration process. Changes in the shape of the energy spectra and the charge and mass distribution at high energies have important astrophysical consequences, often uniquely related to the sources of the nuclei, their storage in the gravitational/magnetic fields of the galaxy, and their extragalactic history. The discovery of even one complex antinucleus such as anticarbon would imply the existence of antimatter stars and element building and would have profound significance regarding the nature of the galaxy and the universe. 2. To determine accurately the energy spectra of cosmic-ray electrons and positrons. The shape of the high-energy spectrum will provide important clues on the age of the electrons, their source spectrum, the galactic storage mechanism, and their distribution in the galaxy. The galactic electron spectrum below several hundred MeV is unknown and must be derived from in situ observations in interstellar space. Their flux is importantly related to the production of the diffuse x-ray and gamma-ray background in the galactic disk, to the dynamics of the galactic disk-halo configuration, and to the galactic nonthermal radio emission and derived data on interstellar matter and temperature distributions. 3. To measure the energy spectra and elemental and isotopic composition of low-energy nuclei (^ 109 eV) and electrons in situ in the interstellar medium. It is not possible to measure the characteristics of the galactic flux of low-energy particles (< several hundred MeV) near the earth, since the effects of the solar wind prevent their penetration to a heliocentric radius of 1 AU. However, in order to reach a complete understanding of the source characteristics and dynamics of galactic cosmic rays, including their identification with unique sources, it is essential to extend the spectral coverage to low energies. At low energies, because of the ionization range requirements, we are sampling very local distributions of galactic cosmic rays (e.g., the range of a 1-MeV proton in the typical galactic magnetic fields is ~ 200 pc). A comparison of elemental abundances at low and high energies will be crucial to the separation of features related to the cosmic-ray production and subsequent propagation in the galaxy. The bulk streaming patterns of low-energy nuclei are expected to show large anisotropies, allowing the probing of interstellar space over different scale lengths as functions of energy (e.g., 1-MeV protons with a density gradient of L ~ 200 pc and a typical galactic

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72 HIGH-ENERGY ASTROPHYSICS c .ts.S o in tn ^ in •si oo i 1 oo i OS * 00 " 00 r* n s Js B » « ft (N is 0 I § o o y o m « ^ s n 8 *^ 3 1 ^j E (^* e 3 CM « o x So ii8 8 §§ o o o o O *^ O oo Vt fl no ^H eg o r, i s |1 C || «£ n e > n n 1 GeV/nucle* .^ i 250 1 GeV/nucle« ^Z^92 e M O •5 -Flyer (FF) aperheavies 0 i 2-S a. fc) V? ~ 2 C/3 u *g •O V u o T! A>O V? * 3 £ ^ - uj01 •f o-S 3» • - ^ 1 3 •£- t "> o c .1 c3« o o " iy Investigate isurement Objec tie Pallet (S) -" X X VI 5 GeV/nucleon £A £ 60 1 GeV/nucleon £Z<; 50 u o "o o ^J \$ V! 2 ty U V! S i II i£ t« ^ 1«S S1 V3 O c o T3 •^ „ £ o, u e u rt 01 C u c a -a 1 la 1 a o e 1 8 I on 0 3 JD Is •< (3 s> s * "e « a a .i Ejp 11 "a o ii I ig| .SP i S u « u c CO X wC u ^0 c TABLE t| On. oo bu fS U B. OCR for page 40
Scientific Uses of the Space Shuttle 73 GeV, the longer observation times of the free-flyer allow an ex- tension to £ 106 GeV. This extended coverage is particularly signifi- cant since it allows, for the first time, a significant energy overlap with results from extensive air-shower work, thereby establishing a connection with the extremely energetic phenomena (5 x 1014 to 5 x 102 ° eV) explored by these ground-based instruments. 3. Isotopic Abundances (S C-2, FF C-2) The study of the isotopic abundances of cosmic rays at low and at high energies is a vital but relatively unexplored new area of cosmic- ray astrophysics. To date only the low-energy isotopes of hydrogen and helium have been measured. However, recent advances in experi- mental techniques have produced a number of promising new sys- tems, which make it possible to begin serious study of isotopic com- position. At lower energies these include high-precision multiple -dE/dx versus range measurements. At moderate energies, the energy range can be extended with Cerenkov counters. At energies of several GeV/nucleon the use of multiple-threshold Cerenkov counters and the geomagnetic field allows the measurement of mean elemental masses. At these intermediate energies the superconducting magnet also promises to be a very useful experimental tool. The extension of isotope determination to even higher energies represents a formidable experimental task. It involves, most likely, greatly improved Cerenkov counters at low indices of refraction, significant advances in the momentum resolution of magnet spectrometers, and totally new experimental techniques. These developments will benefit partic- ularly from the heavy-ion beams that have recently become avail- able at high-energy accelerators. 4. High-Z Elemental Abundances (S C-3, FF C-3) The study of the abundances of high-Z elements (Z £ 26) is with- in the state of the art of present technology. Because of the prevail- ing extremely low intensities at high energies, these investigations rely on very large-area detectors and long observation times. Proven detection techniques include passive devices, e.g., large plastic sheets, which require recovery for analysis, and large ionization chamber/ Cerenkov counter hodoscopes, which are linked to the observer via telemetry. It is possible to achieve resolution better than a charge unit for individual elements up to and beyond uranium, including the hypothesized "stable islands" of nuclei near charge 115. 5. Electron and Positron Energy Spectra (S C-4, FF C-4) The intensity of cosmic-ray electrons at a given energy is generally

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74 HIGH-ENERGY ASTROPHYSICS on the order of 1 percent of the corresponding nuclear fluxes. Elec- tron detectors therefore have to operate under severe background conditions. Precise spectral information over a large energy range is necessary to understand the interaction of electrons with the cosmic microwave background (inverse Compton effect) and the galactic magnetic field (synchrotron emission). To understand the origin of these particles, it is necessary to measure the sign of their charge. The most straightforward experimental techniques for electron investiga- tions utilize magnetic spectrometers and electromagnetic calorim- eters singly or in combination. If the electron spectrum at higher energies continues with a roughly Et2-5 power-law shape, present technology allows extension of the measurements up to -10s GeV. At lower energies (;£ 5 GeV) proven electron/positron spectrometers exist and await appropriate flight opportunities. 6. Experiments on Eccentric Orbit and Deep-Space Spacecraft In addition to these requirements of high-energy cosmic-ray astro- physics, the Shuttle has an important function as launch platform for both highly eccentric satellites as well as Tug-assisted deep-space mis- sions, which serve the studies of solar energetic particle phenomena and in situ investigations of low-energy interstellar cosmic rays. Highly developed and sophisticated instruments for measurements of low-energy solar and interstellar particles (with isotope resolution of the order of one tenth amu) exist at present. In the past, these instruments generally were lightweight (~5 kg), small [<(0.3 m)3], with low power consumption (<10W). Considerable scientific ad- vances in the study of solar composition and dynamical phenomena can be expected from somewhat larger instruments (~ 15-20 kg) flown by the Shuttle. Interstellar in situ observations of low-energy cosmic rays are presently in their infancy. They can be expected to add totally new dimensions to the near-earth studies of high-energy galactic cosmic rays, including the very exciting possibility of identi- fying spatial features and unique cosmic-ray sources. E. Shuttle Sortie Mode Requirements In order to give input to the design of the Shuttle sortie mode, a number of possible experiment combinations were established and investigated to determine constraints on the sortie mode pallets. The baseline requirements for these are (a) no man required; (b) 3-m pallet element length, 4.5 m width; (c) power and telemetry provided by the pallet; (d) 10-15 pallet element flights/year for this 'iscipline.

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Scientific Uses of the Space Shuttle 75 As a result of this investigation, the following recommendations on the capability of the pallet and sortie mode are made: 1. Pointing Requirements Four modes of pointing requirement have been identified: (a) Experiments are mounted on pallet, Shuttle used for orien- tation, accuracy to 1°, readout to 0.1°. (b) Two-axis stabilized platform, full pallet element area. Accuracy a= 0.1°, jitter b= l*/sec. This should carry much larger weights than the 2000 kg presently envisaged, e.g., 5000 kg. (c) Three-axis stabilized, large-diameter platform. 2000 kg, 2.5-m diameter, 4.5-m length, a = l',b= \"/sec. (d) Three-axis stabilized, small-diameter platform, 1000 kg, 1-m diameter, 2.5-m length, a = 1 ,b = 1 /sec. For all oriented platforms, the slewing rate should be 0.5- l°/sec. 2. Orbit Orbit requirements are for both low-inclination (< 30°) and high- inclination (30-55°) latitude. Altitude should be minimum yet be consistent with a 1-year life- time if free-flyers are ejected. 3. Thermal Control The system should provide sufficient flexibility to provide ther- mal control in each individual case. 4. Contamination We consider the ATM standards as sufficient for our requirements. There should be no radiation sources on board, and no large changes of background-producing masses or release of large quantities of material should occur. External magnetic fields should be small (roughly a few gauss). 5. Weight, Power, Telemetry Most pallet elements are in the £ 6000-kg payload class. Those that exceed this limit are modularized so that they can be made to fit available weight capabilities. Occasionally weight/pallet element re- quirements run up to ~ 10,000 kg. Power requirements are of the order of ~ 300 W/pallet element, exclusive of thermal control. Data rates are usually 10-100 kbits/sec but may, for short periods up to 1000 sec, run up to 10 Mbits/sec. A pallet data storage system of ~ 101 ° bits and/or periods of telemetry rates exceeding those now planned would be required.

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76 HIGH-ENERGY ASTROPHYSICS IV. PROGRAM IMPLEMENTATION The Space Shuttle provides the potential to conduct new and excit- ing scientific investigations at costs and on a time scale not previous- ly possible from space. The realization of this potential requires the modification of many accepted management practices and the im- plementation of new practices. The large weight-lifting capability, recoverability, and short turn-around time to reflight are the charac- teristics that will contribute most to the realization of the high sci- entific potential at relatively low cost of the Shuttle. The balloon and sounding-rocket programs have proven highly effective in carrying low-cost, but scientifically valuable, payloads into space. The Working Group recommends extending the balloon and rocket experiment philosophy to the Space Shuttle. Adopting this philosophy requires acceptance of relaxed quality assurance and reliability standards, greater reliability on the performance of the principal investigator (PI), and standardization of most systems in- terfaces. Large national facilities require a new management/ investigator approach. A. Single-Investigator Experiments Achievement of the goal of performing valuable scientific research at reasonable costs infers placing greater responsibility for development and testing on the PI. That is, the burden of providing a tested, func- tioning instrument to NASA for integration must be with the PI. At present, NASA monitors the activities of the PI through a large in-house organization. A more practical and economical approach appears to be to provide the PI with an experiment handbook in which standardized electrical and mechanical interfaces, safety re- quirements, and launch environment are provided. The burden of meeting these requirements would be on the PI not on the NASA con- tract monitor. NASA would, however, assure that the intent of the requirements is being met through one or more design reviews early in the development of an instrument. The frequency of review would be determined by the complexity of the instrument. At delivery of the instrument, the integration center would perform functional tests: inspection for adherence to safety requirements and simple, flight-level vibration and thermal tests. If the instrument fails, the flight opportunity would be lost to the PI. When it passes, the in- strument would be integrated and launched. The Working Group

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Scientific Uses of the Space Shuttle 77 recognizes the necessity for NASA to maintain accountability over public resources. We believe, however, that this can be accomplished with a lower level of monitoring than presently exercised by NASA on the larger projects. Experiment selection and development and the long lead time from selection to flight are major factors in increasing experiment costs. Under the present system, an announcement for flight oppor- tunity is issued. Interested investigators submit proposals, which are evaluated by a NASA committee, and a number are selected for flight. Selection is often tentative and is made final only after a 6- month or longer definition study. Following formal selection, the PI is funded for major hardware fabrication. The hardware is delivered to the integration center for integration, test, and finally launch. This entire procedure from proposal to launch can often extend over a period of four or more years. During this period, the Pi's staff is be- ing funded; the spacecraft contractor has a major design, fabrication, and testing effort in progress; and the NASA management center has a staff monitoring the activities of all participants. The Working Group proposes a modification of this system into an evolutionary instrument program from Supporting Research and Technology (SR&T ) funding through pallet flight into flight on a free-flying spacecraft. Under the proposed system, a potential PI would be given an agreed upon sustaining level of SR&T funding. The level would probably be higher than present-day SR&T levels and would be supplemented on occasion when new construction or modi- fication is required. Within this fixed budget, the PI would develop a new instrument concept and construct the instrument to the relaxed quality assurance standards, much as he does today for a balloon flight. If the instrument is flown successfully (scientifically as well as technically) on the pallet, the same instrument would be upgraded and the interfaces modified as necessary for flight on a free-flyer. It is recognized that an impartial selection committee must be inter- posed at various stages to advise NASA on specific selection actions. The ability of the Shuttle to check out spacecraft in space, recover malfunctioning spacecraft, refurbish standardized spacecraft, and re- fly instrumentation enables an investigator to carry through from development to spacecraft flight with a single basic instrument. The PI would, under this system, carry out a complete research program over the period of a decade without the major perturbations of spe- cific flight instrumentation construction or the long and expensive

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78 HIGH-ENERGY ASTROPHYSICS lead times from proposal submission to spacecraft flight. The requirement for a PI to operate on a fixed budget over an extended period of time will also have the beneficial effect of requir- ing long-term planning of a research program, sharing common com- ponents with other Pi's, and integrating design and development into a standardized form that does not require major expenditures for relatively small modifications necessitated by spacecraft interfaces or man rating. Particle physicists using accelerators, for example, often borrow detectors or other major pieces of equipment rather than build an entirely new module for an experiment. The practice of sharing equipment and data output at an accelerator has often been necessitated by funding limitations under which an experimenter must operate. The same philosophy should be applicable to investiga- tions in space. B. National Facilities Large national facilities require major commitments in national resources. This situation requires the establishment of special pro- cedures that assure that the broadest possible segment of the scien- tific community are able to participate in the benefits of the invest- ment. Where possible, we recommend applying the above procedures to national facilities. In addition we recommend the following imple- mentation procedures for national facilities: 1. A permanent staff should be developed to assume full opera- tional management of the facility when it is ready for flight. This staff may overlap or even coincide with the group that guided the payload through its construction stage or with an enlarged group constituting the Scientific Steering Committee for the program. The staff would have responsibility for the operation of the facility and the execution of its research program. 2. Selection of research programs and of the users should be the responsibility of a research committee of representatives from the high-energy community, NASA, and the permanent staff. This com- mittee would review research proposals to determine their com- patibilities and would then judge their scientific worth. When ap- proved, proposals would be given to the permanent staff, who would determine requirements for observations and related support. In order to ensure impartial consideration, the research committee would be composed of representatives from most of the major re- search centers, and its membership would be changed periodically.

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Scientific Uses of the Space Shuttle 79 All users are responsible to NASA for effective use of their as- signed observing time, prompt analysis and interpretation of their data, and publication of their findings. V. MISSION MODEL The flight program in high-energy astrophysics can utilize the Space Shuttle as a launch and recovery vehicle for unattended automated spacecraft and in the attached sortie mode analogous to the present balloon and sounding-rocket program. The Summer Study group identified approximately 60 high-energy astrophysics groups that are presently pursuing active experimental research programs and that are capable of mounting valuable scientific investigations during the period from 1980 to 1991. The mission model developed here assumes the continuation of the present NASA automated, balloon, and rocket programs as outlined in the NASA mission model through the 1970's. The Space Shuttle is assumed to become available on a limited operational basis in 1980, and fully operational in 1983-1984. A. Automated Program The Shuttle-launched and -recovered automated spacecraft program is centered on standardized spacecraft of the HEAO class (see Table 7). These spacecraft are visualized as being launched and recovered on a six-month basis: that is, two launch/recovery missions per year. In addition, smaller Explorer-class spacecraft are required on a sched- ule of about one per year. The exact frequency will depend on the .74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 UK-5 1a ANS 1° SAS-C 1° IME 1° la Explorers 1 1 1 1 1 1 1 1 1 1 1 1 1 HEAD class Aa Ba C° D E 2» 2 2 2 2 2 2 2 2 2 Recover D E 1 2 2 2 2 2 2 2 2 2-m telescope 1 R 1 Inner-solar-system and deep-space probes 1 1 1 TABLE 7 High-Energy Astrophysics Mission Model—Automated Spacecraft aApproved. "One of these missions is a 1.2-m x-ray telescope.

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80 HIGH-ENERGY ASTROPHYSICS extent to which moderate-sized standardized spacecraft can be mass produced in an inexpensive manner. The decision on whether to recover and refurbish the Explorer spacecraft will be made on a case-by-case basis. Special requirements exist in the discipline for grazing-incidence x-ray telescopes and for deep-space probes. Assum- ing that HEAO-B is launched in 1978, the study group recommends replacing HEAO-B with a 1.2-m telescope in 1982, using the standard spacecraft. This telescope would be recovered in 1984, refurbished, and relaunched in 1985. The 1.2-m telescope would be replaced by a 2-m telescope in 1987, which would be serviced and recovered as required by the telescope technology in the late 1980's. Both telescopes would be operated as national x-ray observatories. Long-baseline observations are required for x-ray, gamma-ray, and cosmic-ray observations. Deep-space and inner-solar-system probes are programmed for launch in 1981, 1985, and 1989 to meet these require- ments. It is estimated that the above program can be carried out at an average cost of $120 million/yr to $150 million/yr in 1972 dollars. B. Sortie Mode The sortie mode can serve as a research platform to conduct short- term observations and to test and check out new instruments prior to flight on free-flying spacecraft (see Table 8). It has been assumed that the balloon and sounding-rocket programs will continue into the TABLE 8 High-Energy Astrophysics Mission Model—Sortie Model 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 Number of pallet elementsa a. Direct Shuttle mounting 123245334344 b. Two-axis stabilized platform 123244434334 c. Three-axis stabilized platform (2.5 m X 4.5 m) 12222121122 d. Three-axis stabilized platform (1-m diam) 112211221222 e. High-inclination direct Shuttle mounting 222211122 TOTAL 3 6 10 10 13 14 12 11 11 10 13 14 aA pallet element is assumed to be a structure 3 m X 4.5 m.

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Scientific Uses of the Space Shuttle • 81 Shuttle era. Shuttle pallet flights are planned to begin at a modest level in 1980 and will grow to a level of 10-15 pallet element flights per year. The standard unit in the mission model is the 3 m x 4.5 m pallet element. The working group found it possible to describe nu- merous 3-m pallet size payloads, which are adequate to satisfy the needs of the discipline. Four types of stabilized platform are re- quired. Shuttle flights at 28° inclination and at high inclinations are required. It is estimated that a research program of 10-15 pallet elements launched per year can be carried out at a cost of approxi- mately $30 million/yr in 1972 dollars. VI. SUMMARY AND RECOMMENDATIONS The broad research area now called high-energy astrophysics is a most rapidly expanding field of modern astronomy, which is having a profound influence on astrophysics and fundamental physics and which requires instruments located above the earth's atmosphere. The discovery potential of this area is unique, as testified by the NAS Astronomy and Physics Survey Committees. Therefore, with respect to the program in high-energy astrophysics 1. We recommend that increased resources be devoted to this new and exciting area in order that the potential for discovery during the era of Shuttle operation be realized. 2. We recommend that the major allocation of resources be given to free-flyers, i.e., large automated spacecraft for x-ray, gamma-ray, and cosmic-ray studies, since it is only on these missions that the long observing times and the continuity of observations that will be required for this discipline can be obtained. 3. We recommend that the HEAO program, which was recently considerably reduced, be continued and expanded, since it will extend naturally into the Shuttle era and forms the basis of our free-flyer concepts; that the developed and ready-for-construction instruments left over from the earlier HEAO program be implemented on either unmanned or Shuttle-launched missions in the late 1970's or very early 1980's. 4. We recommend that an intermediate x-ray telescope of at least 1.2-m aperture be launched in the early 1980's to be followed by the launch of a large x-ray telescope of at least 2-m aperture, both facili- ties to be operated as national observatories. 5. We recommend that space be allocated on inner-solar-system

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82 HIGH-ENERGY ASTROPHYSICS missions for experiments to measure solar cosmic rays and to provide long baselines for x-ray and gamma-ray burst studies and that deep- space probes be implemented to study cosmic-ray phenomena out- side the modulation of the solar magnetosphere. We have devoted considerable effort to investigate methods by which the Shuttle opportunity can be used to obtain maximum sci- ence at minimum cost in this discipline area. We recognize that in addition to the large free-flyers there must be a range of oppor- tunities from rockets and balloons to inexpensive single-experiment spacecraft. We regard the Shuttle sortie mode pallet as at least equiv- alent to a one-week or longer rocket or balloon flight with consider- able enhanced capabilities. Therefore, with respect to high-energy astrophysics, 6. We recommend that the sortie be used to provide frequent inexpensive and rapid turnaround flight opportunities for a broad segment of the discipline. We believe that the best method of achiev- ing this objective is to fly single 3-m pallet-sized elements often, rather than total missions dedicated to our discipline. This requires a new and simplified approach to management philosophy analogous to that now employed in the balloon and sounding-rocket programs. 7. We recommend that a standard support system for free-flyers similar to that available in the mini- HEAO program be defined. This support system could accommodate most of the high-energy astron- omy experiments either as single instruments or in a multiexperiment bus. 8. We recommend that retrieval and return to earth of free-flyers is a possible valuable concept, which can materially reduce cost and increase flexibility, particularly with respect to national facilities.