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Scientific Uses of the Space Shuttle (1974)

Chapter: HIGH-ENERGY ASTROPHYSICS

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Suggested Citation:"HIGH-ENERGY ASTROPHYSICS." National Research Council. 1974. Scientific Uses of the Space Shuttle. Washington, DC: The National Academies Press. doi: 10.17226/12385.
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Suggested Citation:"HIGH-ENERGY ASTROPHYSICS." National Research Council. 1974. Scientific Uses of the Space Shuttle. Washington, DC: The National Academies Press. doi: 10.17226/12385.
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Suggested Citation:"HIGH-ENERGY ASTROPHYSICS." National Research Council. 1974. Scientific Uses of the Space Shuttle. Washington, DC: The National Academies Press. doi: 10.17226/12385.
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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

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,

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.

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.

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.

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

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,

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

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

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

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

Scientific Uses of the Space Shuttle 51 diffusion mean free path of X~ 20 pc will show an anisotropy of 5 = X/Z, of about 10 percent). Possible nearby sources, such as pulsars, may be identified by this technique. The specific details of the low-energy spectra, particularly of heavy nuclei, are profoundly important with regard to the role of cosmic rays in the dynamics of the galaxy, in particular the heating of the interstellar medium by ionization loss. 4. To measure the energy spectra and elemental and isotopic composition of solar energetic particles and solar x rays and gamma rays. Solar-particle events provide a microcosm of what is happening on a galactic scale. The charge composition of solar particles is a function of both the acceleration process and the source region. Measurements of isotopes such as 2 H, 3 H, and 3 He give information on the dynamics of the source region. Coupled with particle accelera- tion are radio, x-ray, and gamma-ray bursts. The time history of particle intensity provides information on the travel of energetic particles through the magnetic irregularities. Observing these features and the development of the magnetic configuration of the solar-flare region offers probably the most favorable opportunity to understand one of nature's acceleration processes. III. INSTRUMENTS A. Achievement of Objectives High-energy astrophysics involves the detection of an extreme range of information carriers: electromagnetic radiation from soft x rays of 100 eV to gamma rays of 100 GeV and particles ranging in character from electrons to transuranic nuclei and in energy from 1 MeV to 108 MeV. Precision is needed in determining such different param- eters as mass, charge, and velocity of the particles and energy, direction, and time of the photons. Clearly, a wide diversity of instruments is needed. With balloons, rockets, small satellites, or as piggyback riders on OSO's, these diverse instruments could be de- ployed one by one. With more advanced instruments having greater requirements of mass, volume, pointing, and other supporting subsystems, the costs tend to rise; but in the high-energy field great savings are possible by developing the concept of a multiple-experi- ment pallet and free-flyer in which the subsystems do not need to be duplicated for each experiment.

52 HIGH-ENERGY ASTROPHYSICS Three modes of deployment will be needed to achieve the objectives of these subdisciplines. One is the free-flying observatory, which may contain only a large focusing x-ray telescope, a very-large-area collimated x-ray receiver, or a large-area high-energy gamma-ray telescope; or, alternatively, there may be a big platform containing many different experiments on both particles and radiation (including, for instance, a cryogenic magnetic spectrometer for particles, a cooled high-resolution detector for low-energy gamma-ray lines, and several other particle and photon detectors). A second type of mission of great importance is the kind that uses supplementary propulsion to go out of the solar magnetosphere to measure the undisturbed low-energy galactic particle fluxes, to see the directional asymmetry of the galactic particles, and to apply long-baseline timing to x-ray sources and to the mysterious short- term gamma-ray bursts to determine the directions. The third mode is the performance of attached experiments on sortie pallets. Many of the proposed experiments in high-energy astrophysics are adapt- able to this mode. It will be a great step forward from rocket, balloon, and small satellite experiments in terms of observing time and instrument capability while still accommodating experiments in a developmental .state, when moderately rapid redirection and readjustment, in response to the experimental results, are vital. This will be the place for exploratory measurements, where instruments are proved out before their commitment on long-duration free-flying observatories and where many important results will be obtained that will guide and enhance the free-flying programs. Of the above modes, it must be emphasized that by far the most important is the free-flying, long-life observatory. Some of the sub- disciplines of high-energy astrophysics have become highly developed areas of precise measurements and sophisticated technology, in which the most significant objectives require the continuity of observations and long observing times, which are properties of the free-flying mode but not of the brief sortie missions. To carry out the long-range programs of x-ray, gamma-ray, and cosmic-ray astronomy with efficiency, reasonable economy, and maximum scientific return, the most profitable use of the Shuttle will be in establishing these free- flying observation platforms and periodically refurbishing them and supplying them with needed expendables to prolong their lifetimes of automated measurements under control from earth.

Scientific Uses of the Space Shuttle 53 B. X-Ray Instruments for the Shuttle Era X-ray astronomy has reached a state of development where its scope and methods closely resemble those of other branches of astronomy. Since x-ray observations can only be carried out in space, it is imperative that permanent orbiting observatories, operated as na- tional facilities, be provided—a requirement that can be satisfied economically only with free-flyers. In particular cases, the capability for recovery and refurbishment will decrease operating costs. Most instrument development and specialized observation can be carried out unmanned and are adaptable to the pallet mode on the Shuttle. The ability to recover and refurbish the large permanent facilities is quite important to allow for continuity of observations at moderate cost. An important concept to reduce the cost of the free-flyer is the commonality of the support system for all subdisciplines of high-energy astronomy. Thus we propose to use a standard spacecraft support system such as the presently conceived mini-HEAO (High Energy Astronomical Observatory) system. The support system includes the telemetry system, power distribution; and attitude- control systems. The experiment support structure should be modular to permit instruments or groups of instruments of various sizes. Standard modules would provide for free-flyers from 3-m diameter x 3-m length up to the full Shuttle size of 3-m diameter x 18-m length. Two types of mission are envisaged: those in which the instruments' view direction is along the long axis of the spacecraft and those with view directions along the short axis of the spacecraft. Both require ~ 1 min of arc pointing, although ~ 3 min of arc may be sufficient for the second type of mission. A mission of each type should be launched in the early 1980's about 4 years after the mini-HEAO launches. These would be of size compatible with the original HEAD. Follow-on missions in the late 1980's would be full Shuttle-sized payloads of each type. The smallest free-flyer module will continue to be used to carry out more specialized tasks, in the same sense that small-aperture telescopes are still used in optical astronomy. X-ray astronomy has developed to the point where a large national facility, multiuser, high-resolution telescope is clearly justified by our knowledge of the x-ray sky. Such an instrument is described below. It should receive major financial support commensurate with a

54 HIGH-ENERGY ASTROPHYSICS launch early in the 1980's. The succeeding sections describe more specialized instruments for which priorities have not been assigned. Priorities will depend strongly on our knowledge at the time of selection. We would presently give high priority to a large-area array for the study of the known variability of x-ray sources, including black hole candidates with microsecond timing resolution. All of these instruments should be multiuser devices, several of them being part of an x-ray facility. The instrument descriptions and scientific justifications follow. Their gross parameters are summarized in Table 4. Instruments for the sortie pallet and for the free-flyer are designated S and FF, respectively. 1. High Resolution Imaging Telescopes (0.1-4 keV; free-flyer; focal plane instruments for imaging, spectroscopy, and polarimetry): SX-5, SX-5', FFX-l.FFX-2. The fact that practical x-ray telescopes can be built that will reflect and focus x-ray photons to at least 4 keV allows for observational capability comparable with that available to optical astronomers. In terms of sensitivity and resolution, and in terms of the great diversity of measurements that can be made, there is no other single instrument or set of instruments available in x-ray astronomy that can approach the capability of a large x-ray telescope with a complement of focal-plane instruments. Besides the imaging instruments, the devices that can be used in the focal plane include Bragg crystal spectrometers, grating spectrometers, polarimeters, and solid-state detectors. In addition, the telescope can be used as an element of an objective spectrometer with either a grating or a Bragg crystal. For each of these instruments, with the possible exception of the Bragg crystal spectrometers, the telescope allows for a qualitative improvement over the equivalent instrument built without a tele- scope. The facility can be justified purely on the basis of its imaging capability. Because of problems of source confusion, high angular resolution is required to study any sources in the range fainter than 10t4 Sco X-l or sources present in crowded regions such as single galaxies, globular clusters, or clusters of galaxies. The limiting sensitivity should allow reaching sources as faint as 10t8 Sco X-l (10t1s erg cmt2 sect1), which is a range in brightness comparable with that achieved in optical astronomy and greater by about an order of magnitude than what is now achieved in radio astronomy. This allows the observation of solarlike coronas to about 50 pc,

Scientific Uses of the Space Shuttle 55 galactic x-ray sources to about 30 Mpc, and weak extragalactic sources such as NGC 4151 or NGC 5128 to about 103 Mpc. Other classes of extragalactic sources such as rich clusters of galaxies, which are much more luminous, are observable to the very edge of the universe. Furthermore, this sensitivity is achieved without any sacrifice in angular resolution, which should be better than 1 sec of arc. This facility has the power to attack virtually every classical problem in astronomy—evolution of stellar systems, the structure of the galaxy, the nature of galactic nuclei, the origin and distribution of cosmic rays, the structure and evolution of clusters of galaxies, the evolution of active galaxies, or the large-scale structure of the universe. There are four general sizes of x-ray telescope of interest: (a) Several monitor x-ray telescopes (S X-5*) of 30-cm aperture or less to be used in connection with other experiments (for instance, imaging at higher x-ray energies) for the study of selected intense objects (for example, the Crab nebula, the Vela supernova remnant). Such devices could be used on the Shuttle or on free-flyers. (b) A research instrument (S X-5) of the general dimensions of HEAO-B with 60-cm aperture to study selected sources from fre- quent sortie pallet missions. The angular resolution is of order of 2 sec of arc. Such an instrument, in addition to performing specialized observations for which time at the larger observatories could not be made available, could be used as a test bed for improved focal-plane instruments and for development of new technology. (c) A 1.2-m aperture x-ray telescope (FF X-l) with better than 0.6 sec of arc resolution and about 9-m focal length. This instrument would have a factor of 10 greater sensitivity than the HEAO-B experi- ment and would constitute a frontier line research instrument for the early 1980's. (d) A 2.2-m aperture x-ray telescope (FF X-2) with 0.1 sec of arc resolution and a factor of 10 greater sensitivity than FF X-1. This facility would make full use of the Shuttle launch capabilities with dimensions of 3-m diameter x 17-m length. This is conceived as a national facility of permanent value with very long (10-year) utilization through use of refurbishment and revisit or return. 2. Large-Area Arrays with Concentrators (0.1-4 keV, 5-50 m2, pallet and free-flyer, resolution 1 min of arc within a field of view of 1°): SX-l,FFX-4.

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58 HIGH-ENERGY ASTROPHYSICS High sensitivity surveys for very faint sources cannot be efficiently conducted with large-area proportional counters with conventional mechanical collimators, because of source confusion. The criterion of 0.03 expected source per resolution element used normally in radio astronomy is already violated in the HEAO-A large-area survey, where extrapolating the results of Uhuru for extragalactic sources one expects 0.3 source per resolution element at the ultimate sensitivity of the survey (~ 10t6 Crab). In order to search for even fainter sources, it is necessary to combine large area with moderate angular resolution. While focusing x-ray telescopes have extremely fine angular resolution (better than 1 sec of arc), they cannot easily achieve large areas. Thus the measurement of the spectrum of the faintest sources, the measurement of time variations of 1036 erg/sec sources in external galaxies, and, finally, the survey of the sky for fainter sources can more conveniently be accommodated by this instrument. Some specific objectives could be as follows: (a) Ln N- Ln S and spectra of very distant galaxies. (b) Survey of the sky at 45 A. (c) X-ray structure of the galaxy. (d) Properties of x-ray background. (e) Estimation of absorbing matter in galactic media. (f) Low-resolution imaging of superno\ia remnants and similar large-scale features. (g) Study of x-ray pulses and other variable sources in the galaxy and in M 31. The large-area arrays are conveniently constructed in modular sections of 1 to 2 sq m area and about 3-m length. The achievement of some specific objectives outlined above can be accomplished by Shuttle sortie modes (SX-1) and of others by free-flyers (FF X-4). Items S X-l and FF X-4 in Table 4 give the gross features of the experiments. 3. Large-Area Proportional Counter Array (1-20 keV; 5-50 m2; pallet and/or free-flyer; collimation variable, e.g., modulation col- limators for high angular resolution): S X-2, FF X-3. A large proportional counter array is the only means presently known to obtain a large effective area for x-ray detection in the 3- 15 keV region, where many sources emit most of their energy. The detection of large numbers of photons in short time intervals is essential for the detection and measurement of transient or

Scientific Uses of the Space Shuttle 59 nonperiodic intensity variations. Variations from matter falling into a black hole are expected to have time constants on the order of 10-100 Msec. The present black hole candidate, Cyg X-l, is known to exhibit aperiodic intensity variations down to an -50 msec time scale. An effective area of 10 m2 will yield a counting rate of ~ 10s photons/sec for Cyg X-l or ~ 1 photon/10 ^sec. An area of 50 m2 yields 5 photons/10 Msec. Almost every galactic x-ray source is vari- able, and transient irregular phenomena are clearly very common. Angular structure of x-ray sources or the background could be studied with modulation collimators mounted upon the detector array. Such studies might be indicated by unusual or energy- dependent angular structure detected at lower energies by the telescope experiments. There are a number of different time domains that need to be investigated. Short Periodic Variability: Periodic components of the emission in the range 1 msec to several seconds are expected based on our present knowledge of pulsars and pulsating x-ray sources. Short Aperiodic Variability: Certain sources exhibit erratic fluc- tuations on a short time scale that is important to study. It may be necessary to make measurements in the microsecond range, which is the typical rotation period around collapsed stars. Eclipse Phenomena: Binary periods are now seen between 5 h and 7 days. More realistically, one must be prepared to search for periodicities down to the shortest known binary periods, which are about 15 min. The time scale within which the actual eclipse may be is in the minute time range. Long-Term Variability: Certain objects, such as supernovae, may undergo secular changes in intensity that should be studied in x rays. Active galaxies also need to be examined for x-ray variability down to the level at which optical variations are now reported. It is quite possible that in these cases the x-ray variability is substantially greater than the optical or radio variability. Consequently there are requirements both for very-high-time- resolution studies of short (days) duration on specific sources and much longer duration (years) studies of a large number of objects. The first requirement can conveniently be met in a Shuttle sortie mode (S X-2), the other requires a free-flyer (FF X-3). The construction of such large arrays can usually be made modular with typical modules of 1 to 2 sq m. The items S X-2 and FF X-3 in Table 4 describe the gross features of each experiment mode.

60 HIGH-ENERGY ASTROPHYSICS 4. High-Energy X-Ray Source Study (scintillation counters, 10-300 keV range): S X-3, FF X-6. This will permit detection of sources to an intensity of 5 x 10ts of the Crab nebula and will therefore measure the hard x-ray continuum of many x-ray sources. Such data, which contrast the spectrum and time variations in such diverse emitters as the binary x-ray objects, supernova remnants, nonperiodic but varying galactic sources, and extended and compact extragalactic objects, cannot fail to distinguish emission mechanisms and source-region properties. Scintillation counters with active or semipassive shielding configured in modules will be used. An array weighing ~ 1000 kg will have an area of ~ 104 cm2. This instrument is contemplated for use both in Shuttle sorties and in the free-flyer mode, and the two modes are described under S X-3 and FF X-6. 5. Hard X-Ray Imaging (scintillation counters, modulation col- limators, 10-200 keV): S X-6, FF X-7. Studies of the angular size of extended sources such as the Crab nebula, Perseus cluster, and galactic disk at various energies provide unique information on electron distributions and distinguish between regions of thermal and nonthermal emission in the same object. In the Crab, for example, the higher-energy electrons injected by the central pulsar will diffuse further before being reduced in energy by lifetime limitations. Studies of the Crab require angular resolutions of 10 sec of arc, while nearby clusters of galaxies may have structures in the 0.1° regime. At present it appears that a modulation collimator on a high-sensitivity scintillation counter device is the best technique for achieving these objectives. The basic sensitivity should be of the order of 10"4 Crab in order to obtain a large number of sources. This implies an area of some 104 cm2 in a low-background configuration. The device can be used in the sortie mode in conjunction with S X-5* to study high-intensity sources and on a free-flyer in conjunction with FF X-l or FF X-2 for study of fainter sources. 6. Low-Energy Telescope (30-300 A; pallet/free-flyer, 5° x 5° field of view, focal-plane imaging, spectroscopy, and polarimetry): S X-4, FF X-9. The opacity of the interstellar medium would appear to severely limit observations between 912 and 20 A. However, the interstellar medium is inhomogeneous, with small dense clouds contained in a

Scientific Uses of the Space Shuttle 61 low-density intercloud medium that can be as low as TV// ~ 0.1 atom/cm3. Thus, we can expect to see out of our own galaxy for wavelengths < 100 A when we look normal to the galactic plane; and even at 304 A, local sources should be observable. The K and L absorption edges of the most abundant light elements in the interstellar medium are in the xuv. Xuv absorption and scattering can allow us to study the chemical composition and size of the interstellar gains. Attempts can be made to detect the intergalactic medium by means of comparative studies of the detailed shape of the long-wavelength cutoff and the observation of absorption edges in the spectra of the same class of x-ray sources from nearby galaxies (such as the Clouds of Magellan) and from more distant galaxies. Discrete xuv emitters that can be studied are chromospheres and coronas of late-type stars, supernova remnants, soft extragalactic sources, early-type stars, and peculiar stars such as flare stars. More hypothetical candidates are defunct pulsars, uv stars, and galactic halos. Focal-plane instruments would include imaging devices, a spectrometer, filters, and a polarimeter. This instrument is optimized for the study of a presently little explored wavelength region. Thus final definition of a free-flyer instrument should probably await the results of exploratory sounding rocket and/or sortie missions. 7. Bragg Crystal Spectrometers and Polarimeters (1-30 A, 1.5 m x 2mx 3m, effective area 2000-4000 cm2): S X-7, FF X-5. A separate crystal spectrometer facility, not in the focal plane of a large x-ray telescope, is necessary because the hydrogenic and heliumlike emission lines and the K absorption edge of iron, as well as of calcium and argon, are at high energies (E > 3 keV) not accessible to a focusing telescope; and a specialized crystal spec- troscopy instrument will have a much greater collecting area for a given weight and volume. The observation of emission lines or absorption edges will play an extremely important role in the development of our knowledge of discrete galactic x-ray sources, the interstellar medium, and extragalactic sources. The wavelength range from 1 to 25 A will most probably be covered by three crystals. For X < ~6 A, the focusing is accomplished by the configuration of the crystal array itself. At longer wavelengths, the focusing is accom- plished by the use of grazing-incidence optics placed behind (or in front of) the crystal array. The extension of the instrument to 100 A could be carried out with a grazing-incidence grating spec- trometer.

62 HIGH-ENERGY ASTROPHYSICS Large arrays of crystals can be used to measure the polarization of the flux from x-ray sources. The detection of polarization in the Crab nebula is a clear indication that the synchrotron process is giving rise to the x rays. A 1 5 percent polarization was detected with a ~ 0.3 m2 crystal during a rocket flight of ~ 300-sec duration. The detection of faint spectral features or precise determination of polarization requires extremely long observation times. Typically then, the instruments can be tested in Shuttle flights and then should be flown on free-flyers. 8. Broadband X-Ray Spectrometer (2-50 keV): S X-8. This instrument will be used to study spectra in an effort to identify specific features such as spectral breaks, nuclear gamma-ray lines, or heavy-element K-line emission. The data necessary for an unambiguous determination of such effects involve both the line profile and the continuum, since, in most instances, physical conditions within the emitting regions will broaden any features. Thus, the need arises for a broadband spectrometer that will give best possible spectral resolution commensurate with available exposure. An array of cooled Si(Li) detectors will give spectral resolution at least one order of magnitude better than other nondispersive techniques and afford the best means for identification of the broad spectral features predicted for x-ray sources. The above instrument should be developed and tested on Shuttle sorties. Its characteristics are described in S X-8 in Table 4. 9. Long-Baseline X-Ray Probe (1-10 AU, proportional counters, 1 m2, crystal scintillator 100 cm2, 1-^sec timing): FF X-10. The existence of time variations on the time scale of 10t2 to 10t3 sec in several of the galactic x-ray sources makes it possible to determine their position by simultaneous measurement from two detectors placed at planetary distances with a precision greatly superior to any yet achieved in x-ray astronomy (1-0.01 sec of arc). If even faster time variations should occur, the technique could yield positional information to the order of 0.001 to 0.0001 sec of arc. Such a very large increase in angular resolution makes it possible to perform a variety of experiments involving measurement of angular positions as a function of time. Such experiments might lead to precise determination of distance of stellar systems, to the determi- nation of their binary nature, and to refined comparison between structure observed in the x-ray and radio regions.

Scientific Uses of the Space Shuttle 63 The sources of the recently discovered gamma-rays bursts could be pinpointed by this technique also; they may be the direct signal from stellar collapses in other galaxies. The importance of this extension of x-ray and gamma-ray techniques cannot be overemphasized. The gross features of the instrument are described under FFX-10 in Table 4. 10. All-Sky Monitor (1-10 keV): FF X-8. The need to monitor continuously the gross intensity variations of the 100 galactic sources presently known has been clearly demon- strated by the Uhuru study of eclipsing binaries of several days' period, by the observation of novalike explosive events, and by the recent indication of new classes of transient phenomena such as the recently observed gamma-ray bursts. Also, stellar flares of detectable x-ray intensity are expected to occur at a frequency of 10 events/year. In all of these measurements it is necessary to keep the entire sky under observation, with moderate sensitivity, for very long periods of time. The instrumentation could consist of pinhole devices of several different types in conjunction with large imaging detectors. As an example, a set of six simple pinholes each with an imaging proportional counter of about 1000 cm2 could monitor all presently known galactic x-ray sources with a time resolution of ~ 1000 sec. Larger arrays or more efficient imaging schemes would be necessary to monitor sources with resolutions of 0.1 sec as necessary to observe the pulsating characteristics of x-ray pulsars and study the variations of their orbital periods. Obviously such an instrument should be considered for small free-flyers. Its characteristics are described in Table 4 under FF X-8. C. Gamma-Ray Experimental Program The low flux levels of high-energy quanta, coupled with increasing demand for detailed resolution of source features, drive most gamma-ray experiment designs to include long exposures (a week or more) on individual sources or regions, even with instruments of great area. In order to study many sources or to survey the whole sky, the only possible mode of deployment is then a free-flyer. There are, however, objectives for which an exposure of a few days to a week or two would not only be adequate but in some cases superior. These include specific missions such as a concerted attack on a

64 HIGH-ENERGY ASTROPHYSICS particular source with instruments covering different parts of the spectrum mounted on the same pallet or the attempt to apply gamma-ray polarimetry to a few of the strongest sources or the exploratory application of a new instrument, where a short period of use could profitably be followed by one of instrument modification before commitment to a free-flyer. A major subdivision among the gamma-ray experiments occurs between those detecting low- and medium-energy gamma rays, 0.1 to 20 MeV, and those detecting high-energy gamma rays, 20 to at least 10s MeV. The techniques of detection as well as the observational objectives differ markedly for these two ranges. For instance, only in the low-energy range are sharp spectral lines expected or techniques available for fine spectral resolution; while only in the very high-energy range can presently conceived techniques yield angular resolution of a small fraction of a degree. The investigations characterized below and in Table 5 include some that are recommended for the sortie mode, especially in the early part of the Shuttle era. Sortie experiment designations begin with the letter S in the table, and free-flyers with FF. 1. High-Energy Gamma-Ray Survey (FF G-l) This is a full-sky survey at high sensitivity, carried out with a wide-field instrument incorporating moderately good resolution in both direction and energy. This combination of requirements can probably best be met by a pictorial, multiplane wire readout system such as a spark chamber, with a transparent absorbing crystal 6 to 10 radiation-lengths thick for energy determination. With an area of about 8 m2, this instrument will have a sensitivity ten times better than that of the high-energy telescope planned for HEAO-C*. Its an- gular accuracy will be about 1° at 100 MeV and 1/4° above 500 MeV, and the energy resolution will be 10- 20 percent depending on energy and on thickness of the absorbing crystal. Timing accuracy to at least 10t4 sec should be incorporated, since high-energy gamma- ray sources are known that exhibit pulse structure on that time scale. Aspect information to a precision of 2 min of arc will be useful. This instrument will detect point sources down to levels more than 100 times lower than the strongest gamma-ray sources now known, and therefore low enough to include hundreds of examples. It will give spectra accurate enough to determine the main emission mechanism from each source and will locate positions of the stronger sources to 0.1°. For those sources that have angular dimensions on the order of a degree or more, it will map out the source contours.

Scientific Uses of the Space Shuttle 65 For pulsing and eclipsing sources, it will accurately determine pulse profiles and features of the eclipse cycle. The intense radiation from the galactic plane will be mapped out in detail. With the high sensitivity available, exceptionally high-energy gamma rays can be used in this scan of the galactic plane to obtain fine angular resolution, avoid source confusion, and thus distinguish between diffuse origins and an extensive collection of point emitters. The instrument will also obtain a thorough survey and spectral analysis of the radiation from high galactic latitudes and find out whether much of this radiation is in fact isotropic and hence of cosmological origin. It is urgent that this survey instrument be launched as early as possible because the great wealth of information it will uncover will be of particular value in guiding the rest of the high-energy gamma-ray program. 2. High-Energy Gamma-Ray Pointing Instrument (FF G-2) In addition to the all-sky survey instrument, a high-energy telescope is needed that will concentrate for longer periods of time on particular sources and areas of the sky of special interest. This is necessary to accumulate the statistical volume of data required to resolve spatial and spectral features of the sources in satisfactory detail and to study temporal variations continuously over periods adequate to decipher complex dynamic behavior. The field of view of this telescope does not need to be as wide as that of the survey instrument (10° diameter is enough), but large area is just as essential as in the survey and should be on the order of 5 to 10 m2. This instrument will study single sources or small regions long enough to define some spectra up to 1011 eV or more; and significant spectral features in the 101 °- 101' eV range are predicted for the diffuse radiation and known to occur in at least one source—the Crab pulsar. Therefore, energy resolution to about 20 percent should be provided up. to at least 1011 eV. Angular resolution should be optimized to the extent possible without loss of sensitivity—at least 0.2° at energies above 500 MeV. Aspect information to 2 min of arc is desired, although pointing of the vehicle to better than 0.5° is not necessary. Timing accuracy is needed to at least 10t4 sec. This telescope will provide the same types of information as the gamma-ray survey instrument, but in greater detail, extending up to higher energies, and with finer statistical precision, which adds to the accuracy of spectral and angular determinations. Although the

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68 HIGH-ENERGY ASTROPHYSICS programming of observations with FF G-2 will profit from the results of FF G-l, it need not wait for the all-sky survey before deployment, since many exciting targets of observation can be defined even now, and more will be uncovered by the telescope planned for HEAO-C'. 3. High-Sensitivity, Low- and Medium-Energy Gamma-Ray Survey (FF G-3) Measuring the spectrum of galactic sources over the 0.3-10 MeV range will determine the relative role of nonthermal electrons and nuclear processes in such objects as pulsating binary x-ray stars, supernova remnants, and the galactic plane. Gamma-ray fluxes at 1 MeV detect 20-GeV electrons scattering on the 3 K radiation, which permeates halos and clusters. Detection of perhaps 50 galactic and extragalactic sources requires a cluster of scintillation counters with active anticoincidence shielding, a sensitivity of at least 5 x 10t6 photon cmt 2 sect' MeVt' at 1 MeV, and a total area of 5000 cm2. An aperture of 5° FWHM represents a compromise between source confusion and sensitivity. These objectives can only be obtained with masssive, large-area detectors and long-term observations. 4. Medium-Energy Gamma-Ray Monitor (FF G-4) The recent unexpected discovery of gamma-ray bursts of less than a minute duration, occurring a small number of times per year, shows the need of maintaining a monitor designed to sense rapid intensity changes in any direction and to record the properties of such events (including direction) that are essential to their interpretation. The types of celestial cataclysms that cause abrupt changes in the gamma radiation are not established, but they include supernovae, the early phase of which ought to be detected in medium-energy gamma rays (the first radiation to emerge) if such an event should happen in the local cluster. 5. Nuclear Gamma-Ray High-Resolution Spectrometer (S G-5, FF G-5) Determination of the intensities of gamma-ray lines from such processes as radioactivity in supernova remnants, positron annihila- tion in the galactic disk or in extragalactic interactions, lower-energy cosmic rays passing through dense matter, and nucleosynthesis in violent events in distant galaxies will put nuclear astrophysics on an observational basis. Detection of such lines at a sensitivity of 2 x 10t6 photon cmt2 sect1 (1/10 that of the HEAO-C'instrument)

Scientific Uses of the Space Shuttle 69 will require a detector of 250 cm2 with active anticoincidence shielding and collimators, and extreme care must be taken to reduce background effects. Present technology indicates that a cooled Ge(Li) system provides the best energy resolution, although other devices are now under development. The flight of such an instrument on a sortie pallet early in the Shuttle era will permit a detailed investigation of a few important selected sources and give thorough knowledge of the response of the instrument in the space environment. Later deployment on a free-flyer will permit study of many more sources and improvement of sensitivity by use of long exposures. 6. Developmental Pallet for Gamma-Ray Experiments (S G-6) Many experiments in a developing scientific field, such as low-energy gamma-ray astronomy, require a close and timely interaction between technical developments and scientific dis- coveries. A system that has basic capabilities for mounting fairly large and massive instruments with modest pointing requirements is needed during the early Shuttle era. This system will serve the same function as rockets for soft x-ray astronomy and balloons for hard x-ray astronomy. Observations that are specific and one-time, such as exploratory polarimetry on strong sources, and observations of new objects found in other wavelengths for phenomena predicted in the gamma-ray regime are examples of possible investigations. Technical developments, such as liquid xenon proportional counters and new shielding or collimation techniques, can have their initial application on such a device. 7. Double Compton Telescope (S G-7) At medium gamma energies (1-20 MeV), both energy and angular resolution are particularly difficult to achieve with conventional single-element detectors. A double Compton telescope is capable of providing good energy resolution in this interval, along with modest angular resolution. The spectrum of selected individual sources will be measured, and also that of the diffuse radiation. 8. Study of Individual High-Energy Gamma-Ray Sources (S G-8) An instrument similar to FF G-l or FF G-2, but a slightly smaller area, can be flown profitably on a sortie mission, to perform a detailed study of spatial, spectral, and temporal features of selected areas of the sky. This could be of particular value for (a) the earliest

70 HIGH-ENERGY ASTROPHYSICS application of the instrument, when the results might dictate desirable modifications prior to commitment on a free-flyer; (b) comparatively rapid response to discoveries made in another part of the spectrum, which should be complemented by gamma-ray studies of the same sources; (c) investigation of unique objects like the Crab nebula; and (d) a combined attack on a particular source with multispectral instruments mounted on the same pallet, along with coordinated observations from ground observatories. 9. Precise Energy Spectra of Known Gamma-Ray Sources at High Energies (S G-9) With the aid of a transparent crystal of sufficient thickness to be totally absorbing, gamma-ray spectra from 20 to 10s MeV can be measured with an energy resolution of 3 percent. This capability permits looking for sharp features in the high-energy spectra of selected gamma-ray emitters. Such features, when observed, are highly specific indicators of the physical interactions responsible for the emission and are also capable of identifying previously un- discovered particles and processes. 10. High-Sensitivity Measurement of Low- and Medium-Energy Gamma Rays from Selected Areas of the Sky (S G-10) An actively collimated scintillator telescope like that of FF G-3 can profitably be deployed in a sortie mission in which the objective is not a complete sky survey but a detailed study of individual sources or areas of the sky. This could be of special value (a) in the earliest application of the instrument, when the results might dictate desirable modifications for the free-flyer; (b) in response to dis- coveries elsewhere in the spectrum, which should be complemented by gamma-ray observations; (c) for investigation of unique objects such as a young supernova; and (d) as part of a concerted attack on a particular source, using multispectral instruments mounted on the same pallet. D. Cosmic Rays The anticipated large payload capacities of the Shuttle sortie pallets and of Shuttle-launched free-flyers, with correspondingly large detector systems and observation times, promise major scientific advances from detailed studies of the charge, mass, and energy spectra of cosmic rays.

Scientific Uses of the Space Shuttle 71 The technology to measure charge and energy spectra up to energies of ~ 10's eV now exists. In particular, the steady evolution of detector resolution means that individual elements from hydrogen through uranium and beyond will be unambiguously resolved. Because of the relatively small fluxes of cosmic rays at higher energies and charges (the intensity scales with energy roughly as 1:10 :104 GeV/nucleon ~ 1:1(T5:10t1 °, and with charge, H:C:Fe: U~ 1:10t 2 :10t 3 :10t8), these high-energy cosmic-ray instruments must be generally massive, of large surface area, and require large observation times. They are dependent on a Shuttle-like capability to carry them above the earth's atmosphere. In view of the complexity of the instruments, most of the high-energy systems should be test-flown on short sortie missions before being committed to a free-flyer. Such a program allows verification of crucial detector performance parameters. Most of these sortie missions will return scientifically significant results and will meet part of the larger experiment objectives. 1. Experiments on Low-Altitude Spacecraft Table 6 represents a listing of presently identified major high- energy cosmic-ray investigations for the Shuttle era. Estimates are given of what can be achieved on typical 4- 7 day sortie missions and what are the extended capabilities of 0.5- 1 year duration free-flyer missions. All listed investigations have no specific pointing require- ments beyond a viewing direction away from the earth. The discus- sion attempts to outline some of the possible experimental tech- niques and their present state of development and tries to indicate where further improvements are required. N 2. High-Energy Spectra and Charge Composition (S C-l, FF C-l) A class of detector systems has been developed that measures both the energy spectra of cosmic rays over the range of 10 to ~ 106 GeV and their elemental abundances from hydrogen through iron, with typical resolution of 50.2 charge unit. These detectors include possible combinations of ionization calorimeters, superconducting magnets, and multithreshold Cerenkov counters. It is possible that transition radiation techniques or other new techniques will permit significant extensions of this range to higher energies within permis- sible weights. While typical geometric factors of ~ 1 m2 sr limit the effective energy range for protons during short sortie missions to ~ 5 x 104

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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

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.

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.

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

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

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.

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.

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.

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

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.

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Scientific Uses of the Space Shuttle focuses on those aspects of the Shuttle most different from conventional launch-vehicle capabilities. It especially considers the sortie mode, in which the Shuttle carries into orbit a payload that remains attached to the Shuttle and then returns to earth with the payload after one to four weeks. Interest in the sortie mode is particularly great because of the contemporary decision by several European countries to develop a space laboratory (Spacelab). The report also considers the use of the Shuttle for launching, servicing, and recovering satellites and for launching lunar, planetary, and interplanetary missions.

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