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Scientific Uses of the Space Shuttle (1974)

Chapter: OPTICAL AND ULTRAVIOLET ASTRONOMY

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Suggested Citation:"OPTICAL AND ULTRAVIOLET ASTRONOMY." National Research Council. 1974. Scientific Uses of the Space Shuttle. Washington, DC: The National Academies Press. doi: 10.17226/12385.
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6 Optical and Ultraviolet Astronomy I. SCIENTIFIC OBJECTIVES A. Solar System Optical telescopes aboard the Shuttle, or carried into orbit by the Shuttle, can increase our knowledge of solar system bodies in several important ways. They can particularly provide for high-resolution imaging, for spectroscopy without the intervening atmosphere of the earth, and for photometry and polarimetry with high spatial resolu- tion. Broadly, the objectives are to investigate the dynamics, com- position, and structure of planetary atmospheres and surfaces, of satellites and asteroids, and of comets. The general goal is to improve our knowledge of the origin and evolution of the solar system. Much of the imaging, spectroscopy, and photometry of solar- system bodies has to be distributed over a period of time. Planets are dynamic bodies with continuously changing observable character- istics. Clouds, dust storms, polar hoods, and polar caps form and move on Mars. Unidentified dynamic modes in the atmosphere of Jupiter produce continuous changes in the belts and cloud patterns. Faint atmospheric markings and spectroscopically variable regions circulate around Venus. Changes and motions in the atmosphere of Saturn are at the very margin of earth-based detection, while those of Uranus are out of reach. Cometary spectra, which also change, are submerged in the earth's airglow. Studies of these phenomena are intensively pursued with earth- based telescopes to the limits possible. Planetary flybys and orbiters permit short-term glimpses of local detail but do not provide planet- wide synoptic coverage. Thus, Shuttle and Shuttle-launched optical telescopes can fill a major need in solar-system science, greatly ex- ceeding the limits of present earth-based observation in synoptic pro- grams of imaging and spectroscopy. The observations have to be made at regular intervals during apparitions, utilizing either the Large 102

Scientific Uses of the Space Shuttle 103 Space Telescope (LST) or a long-focal-length diffraction-limited tele- scope aboard the Shuttle. The same instruments can reveal detail in comets' heads and can provide superior spectra of both real and simulated comets. We do not yet know whether the universe abounds in planets; it is possible that there are billions per galaxy and perhaps many that are closely earthlike. This is not only an exciting scientific question for astronomers and biologists, but it is also one of great potential phil- osophical impact. Although direct-image detection of planets around other stars remains unlikely with foreseeable methods, the l-m//30 diffraction-limited Shuttle telescope and the LST provide, in princi- ple, an opportunity to push the precision of stellar radial velocity variations and stellar proper-motion fluctuations into the range needed to explore this question more seriously than heretofore possi- ble. B. Stars and Stellar Systems Three satellites, OAO-A2, OAO-C, and ESRO-TD1, have now contrib- uted to the investigation of stellar ultraviolet radiation, and a fourth one, IUE, is planned for operation in the 1977- 1980 period. The wealth of information provided by these satellites will allow astronomers to complete the first major step into the field of ultra- violet astronomy. Classification criteria will be better known, discrep- ancies between observations and model atmosphere predictions will be better identified, and many intrinsic properties of stars (rotation, luminosity, abundances, mass ejection) will have been analyzed for the brightest objects. Although unpredictable developments are likely to come in this decade, several major objectives can readily be spelled out for the 1980's. In some cases, these require the LST and a 1-m diffraction- limited Shuttle telescope; but a 0.5-m photometric telescope will sometimes suffice, and ultraviolet survey cameras will play important roles. Emphasis will be on the study of larger samples (thus on fainter objects), on higher photometric and spectrophotometric accu- racies, and on high time-resolution studies of light curves. A refer- ence sample of a few hundred stars will have to be observed as accurately as possible at low, medium, and high spectral resolution in order to establish fully a set of standard stars and to investigate more thoroughly the dependence of ultraviolet classification criteria on physical parameters by comparisons with model atmosphere pre- dictions.

104 OPTICAL AND ULTRAVIOLET ASTRONOMY Photometric observations will have to be extended to larger sam- ples, such as globular and open clusters, where the sensitivity of ultraviolet colors to stellar properties is likely to provide important clues to the understanding of stellar evolution. Such data will also improve the galactic distance scale through better photometric paral- laxes. Among other developments, one can identify systematic studies at medium and high spectroscopic resolution for classifica- tion, for a mass-loss search in order to study driving mechanisms, for a search of stellar chromospheres in late-type stars, and for the abun- dance analysis of normal and abnormal stars in order to improve the understanding of the evolution of the galaxy. The good seeing and the absence of scintillation that characterize space observations can be taken advantage of to investigate variable stars with high time resolution and high photometric accuracy. In addition to resolvable binaries and spectroscopic binaries, these in- clude intrinsic variables, such as RR Lyrae stars, flare stars, and Cepheids. C. Interstellar Matter Though it has long been known that the space between stars contains gaseous and particulate matter, the variety and complexity of this material is only now becoming apparent. Some dozens of molecules are now known to exist, many of them polyatomic, and there is no reason to think that the present inventory is complete. Ultraviolet extinction measurements indicate interstellar dust characteristics quite different from those suggested by observations at visual wave- lengths. Furthermore, several different constituents are probably present. This dust is not only of interest in itself but also in its interaction with the gaseous component of the medium, since it appears that the production of at least some molecules requires the presence of dust either as a shield from the interstellar ultra- violet radiation field or as the site of formation. The mecha- nisms involved cannot be established until the grain composition is better known, and this in turn will probably require an understand- ing of how the grains are formed. The LST and the 1-m diffraction- limited Shuttle telescope will be the most valuable for pursuing these studies in the 1980's. Data now available indicate the general features of interstellar extinction in the ultraviolet—its variability from place to place and the prominent X2200 feature. However, these data are as yet avail- able only at low spectral resolution from a small number of nearby

Scien tific Uses of the Space Shuttle 105 stars. Furthermore, few, if any, reliable ultraviolet polarization obser- vations exist, which would be of special interest at and near the X2200 feature. Extinction and polarization measurements for large numbers and types of stars in a variety of geometries with respect to the dust grains, as well as ultraviolet observations of reflection nebu- lae, would make available important evidence concerning the com- position and characteristics of the grains. With high spectral resolu- tion, faint extinction features might be found that would be of great use in testing various grain models. Because of the large far- ultraviolet extinction, photographs with a Shuttle survey camera in this spectral region might delineate dust clouds better than is now possible. Ultraviolet observations of Lyman-a absorption have an important advantage over 21-cm-wavelength radio observations in mapping the local distribution of hydrogen, in that the geometry of the former is much better known than that of the latter. Several astrophysically important elements have resonance lines in the ultraviolet for which observations provide better determinations of their abundances than heretofore possible. Studies of the relation between gas and dust grains can be made by detailed elemental abundance determinations and grain distributions in neutral hydrogen regions. D. Emission Nebulosities The existence of emission nebulosities formed from condensations in the interstellar medium near hot stars provides unusual opportunity for study of several important areas of astrophysics. These nebulosi- ties are formed from residual gas after the formation of short-lived, massive stars that are extremely bright in the ultraviolet spectral region. The residual gas is partially photoionized by this ultraviolet radiation. The imbalance of gas pressures leads to a general expansion and evolution of the nebulosity, which provides a laboratory for study of plasmas under well-defined conditions but on a scale not possible on the earth. Since the phenomenon only occurs near re- cently formed stars, we expect this study to yield information about the actual processes of star formation itself. Moreover, the ionized gas is a highly efficient converter of stellar continuum energy to atomic line emission, which allows the study of not only the condi- tions of temperature and density within the gas but also of its ele- mental abundances. Not only is this method of abundance deter- mination easier and more reliable than others, but it can be applied to other galaxies and to regions within them. These energetic sources

106 OPTICAL AND ULTRAVIOLET ASTRONOMY can best be studied at ultraviolet wavelengths accessible from above the atmosphere and at higher spatial resolutions than possible with ground-based telescopes. E. Galactic Nuclei and Quasars Violent processes are taking place in the nuclei of many giant gal- axies, especially radio and Seyfert galaxies. Although such a galaxy may be a hundred thousand light-years in diameter, a substantial fraction of its entire luminosity comes from a tiny core, in some cases perhaps only a few tens of light years in size. The output of energy may be billions of times that of the sun. Ordinary large galaxies, such as our own or M31, display what may be much the same kind of process, but at only a thousandth or even a millionth of the rate. If the red shifts of quasars arise from velocities of recession in an expanding universe, they are the most distant objects that we know and their intrinsic luminosities range up to a hundred times those of the most luminous Seyfert or radio galaxies. But the energy output of every quasar appears to vary, in some cases as rapidly as days or even hours, requiring them to be as small as the solar system. The problem of extracting 104 s ergs/sec from so small a volume of space is so severe that some astrophysicists have proposed a nonrecession- velocity origin for much of the apparent red shift, allowing quasars to be relatively nearby and perhaps a thousandfold less luminous. Space observations are essential to solving the riddles posed by galactic nuclei and quasars. The LST, a 1-m diffraction-limited Shut- tle telescope, an ultraviolet survey camera on the Shuttle, and a very large light collector can each contribute in important ways to extend- ing wavelength coverage, measuring the red shifts of fainter objects, resolving diameters and structures, and discovering fainter quasars. Only by learning the physical processes in such objects can we be comfortable with our understanding of basic physical laws. Only by becoming confident about their true remoteness in the universe can we use them as the ultimate probes mentioned in Section I.H. F. Intergalactic Matter The detection and measurement of the intergalactic medium may be one of the frontiers of astronomical research in the 1980's. Current measurements suggest that the mean density of intergalactic matter is very small. However, there is a possibility that the intergalactic me-

Scien tific Uses of the Space Shu ttle 107 dium may be nonuniform and that there may be intergalactic clouds. Detection of such clouds, if they exist, could be accomplished by ultraviolet spectroscopy of quasars, provided they are as distant as their red shifts imply. Both the 1-m diffraction-limited Shuttle tele- scope and the LST can contribute significant data. Alternatively, ultraviolet spectroscopy of the brightest blue stars in external galaxies may give definitive results. For these stars, a very-large-aperture telescope (like the Very Large Light Collector described later in this report) with fairly good spatial resolution (~0.3 sec of arc) will be required. G. Extragalactic Research An important scientific objective of the 1980's will be high spatial resolution studies of distant galaxies. We are interested in whether distant galaxies (red shift > 10 percent) are similar to the nearby galaxies. At a red shift of 10 percent (distance of 6 x 108 pc if the Hubble constant equals 50 km/sec Mpc), the characteristic size of a large spiral galaxy like M31 (3 x 104 pc) subtends an angular size of about 10 sec. An angular resolution of 0.1 sec of arc or less is required to make detailed comparison of the structure and photo- metric profiles of these distant galaxies with the nearby galaxies. The distant ones require the LST, while the nearby ones require the field coverage of the deep-sky survey camera on the Shuttle. In addition, it will be very interesting to compare the stellar con- tent of nearby galaxies with our galaxy. At the present time, very little is known about stellar content beyond the Magellenic Clouds. The color-magnitude diagrams of star clusters in external galaxies will be important in establishing how similar the stars in different galaxies are. Besides its intrinsic interest, this knowledge will help to extend our stellar distance indicators to sufficiently large distances that local deviations from a smooth cosmic expansion can be investigated and that a better mean value for ultraviolet energy distributions of gal- axies, including ellipticals, generally show a minimum at about X2400, followed by a rapid rise toward shorter wavelengths, which is too steep to be accounted for by any combination of early-type stars. This rise may be due to light scattered by dust and may there- fore be useful in studying interstellar dust in other galaxies, along with its implication for infrared measurements, for computing the "K terms" of red-shifted galaxies, and for the interpretation of sky-brightness observations.

108 OPTICAL AND ULTRAVIOLET ASTRONOMY H. Cosmology Optical observations that relate to the origin and large-scale evolution of the universe have required the largest earth-based telescopes work- ing to their very limits. By providing the possibility of putting major telescopes in orbit, the Shuttle opens the exciting opportunity of reaching deeper into the realm of galaxies, galaxy clusters, and qua- sars to seek better answers to the scientifically and philosophically challenging questions of cosmology. In particular, one must search for distant clusters of galaxies with a 1-m deep-sky survey telescope reaching the 25th magnitude in selected sky samples, measure red shifts and magnitudes of cluster members by spectrophotometry with a large light collector, and mea- sure luminosity profiles of cluster members with a diffraction-limited LST. All of these can be expected to result in significant advances, and the last is certain to provide a major breakthrough in the deter- mination of the deceleration parameter, based on plotting profile diameters against red shifts. The deceleration parameter distinguishes an infinite universe from a "closed" one, and that, together with the Hubble constant, specifies the age of the universe and the mean density of matter within it. The red shift-diameter relation is not so sensitive as the red shift- magnitude relation to corrections for evolu- tionary changes associated with the "look-back time." One-meter Shuttleborne telescopes, as well as the LST can also aid the optical identification of radio galaxies and quasars, which may prove to be our ultimate probes of the geometry and evolution of the universe. II. CANDIDATE SHUTTLE-LAUNCHED INSTRUMENTS A. The Large Space Telescope The Large Space Telescope (LST) is identified as by far the most important project for optical and ultraviolet astronomy in the first decade of operation of the Space Shuttle. It is uniquely capable of realizing most of the potential for imaging space astronomy and would represent an enormous step beyond the capabilities of possible ground-based telescopes. The next levels of astronomical and astro- physical research require this instrument. To understand the potential of the LST, one must recognize the basic limitations imposed by the earth's atmosphere on ground-based

Scientific Uses of the Space Shuttle 109 observations. The three principal limitations arise from atmospheric opacity, atmospheric irregularities, and emission or scattering of light from the night sky. The earth's atmosphere is partially obscuring at all wavelengths, but at some wavelengths it is so opaque as to block any penetration to the earth's surface. In the region of optical and ultraviolet astron- omy, only a small fraction of the full wavelength range penetrates the atmosphere. Full wavelength coverage can be obtained by obser- vation from satellite and rocket altitudes; this potential has begun to be exploited with small telescopes during the first decade of space astronomy but must now become a resource normally and contin- uously available to astronomy. Small-scale inhomogeneities in the index of refraction of the earth's atmosphere lead to random deviations of the apparent direc- tion of a star. To the eye, this phenomenon appears as the twinkling of stars, while a small-telescope observer sees an image that moves as the atmosphere appears to "boil." A large telescope gathers light passing through many regions of inhomogeneity and averages them out to form a stationary but large image. These phenomena are all familiar to the optical observer and are collectively known as "see- ing," conventionally measured by the apparent diameter of a stellar image (the seeing disk). Seeing varies with time at any one observa- tory site, and its average value is different at different sites. The most frequent seeing disk with the 5-m telescope at Palomar Mountain is about 2.5 sec of arc in diameter, while that reported with smaller telescopes in the Andes may be as small as 1 sec of arc. The inherent imaging capabilities of a precisely made large tele- scope are remarkably much better than this limit set by atmospheric seeing. The diameter (d) of a visible-light image, limited only by dif- fraction of light as it passes the primary aperture of diameter (D) in meters, is given approximately by d = OA5/D sec of arc; d is also proportional to the wavelength of observation. This means that a diffraction-limited 3-m telescope operating in space, free from atmo- spheric seeing, at 5000 A can give images of 1/20 sec of arc, while at 2500 A the images are only 1/40 sec of arc in diameter. The advantages of improved spatial resolution from a large dif- fraction-limited space telescope are twofold. The first is, of course, the ability to resolve structures in astronomical sources that are some 20- 50 times smaller than can be seen from the ground. Since the gain is made in two dimensions, the effect is to give some 400 to 2500 times more information elements per area of the sky. Some

1 10 OPTICAL AND ULTRAVIOLET ASTRONOMY feeling for so enormous an improvement can be realized by compar- ing the naked-eye view of the moon with that obtained by looking through a good 30-cm telescope. Most astronomical sources will show new structure down to the diffraction limit, while many star fields are so crowded that this improved resolution is required if we are to be able to study any but their brightest stars. When one tries to detect faint starlike objects, the most important source of noise in the signal comes from the background night-sky brightness superimposed on the star image and its immediate vicinity. A darker sky permits the detection of fainter objects. Likewise, in- herently smaller images mean less background light per image area and therefore again a fainter limiting threshold signal. Assuming a four times fainter sky from orbit and 50 times better images, a 3-m LST could detect objects nearly 100 times fainter than could the largest operational telescopes on earth. In turn, this means seeing nearly all kinds of stars out to ten times the present limiting distance, hence to bringing 1000 times the volume of space within view. Al- though these improvements may not be quite so great when very large telescopes are completed at superb sites, the gains will always remain extraordinarily great and attainable only by putting major telescopes in space. The advantages of a large, high-spatial-resolution LST are many. A whole new regime of astronomical problems can be attacked at very low light levels, and much current threshold work can be carried out with improved accuracy. In addition, it will allow the spatial resolu- tion of known sources and the discovery of new sources. It will be a powerful complement to all other disciplines of space astronomy, from infrared through gamma rays. To realize these remarkable potentials of the LST, certain condi- tions are required: 1. It must have as large an aperture as possible, compatible with constraints of cost of manufacture and launch by the Shuttle. 2. It must be of as high optical performance as possible, compat- ible with total system costs. 3. It must be a true space observatory; that is, it must be a long-lived facility capable of refurbishment and repair over a span of many years. 4. The auxiliary instrumentation should reflect the needs and wishes of a diverse user community, compatible with efficient instru- ment design and operation. The program should allow the periodic

Scientific Uses of the Space Shuttle 111 re-evaluation of the instruments then being flown and possible selec- tion of new instruments for flight. 5. The ultraviolet wavelength range should extend far down, if possible, to the Lyman continuum cutoff around 900 A. B. Diffraction-Limited Telescope As prime complement to the LST program, we recommend a me- dium-sized Shuttle telescope for testing experimental instruments and detectors, for performing tasks requiring manned access to instru- ments, for conducting varied observational programs of short dura- tion, for photographic and electronographic image recording, and for covering sky areas up to 30 times larger than can be imaged by the LST. These functions would be served by a diffraction-limited //30 Cassegrain telescope of about 1-m aperture. The opportunity for utilizing photography and electronography arises because such materials will tolerate a space environment for the duration of a Shuttle flight (say, one month) but not for the much longer periods that would be required by the LST. A Shuttle telescope is therefore not limited in function and field coverage by the necessity for re- mote data readout. A sky field of up to 30 min of arc can be made optically available. Both wide-field electronography and smaller-field digital detectors can be expected to reach a threshold of about 27th magnitude and thus for many purposes to exceed the performance of the best ground-based telescopes. The proposed 1-m //30 Cassegrain design is suitable for mounting at the far end of the pallet, where it helps to put the center of gravity of the Shuttle within required limits. It can be equipped to perform two to three types of observation with command and monitoring either from the ground or from the Spacelab. If, for example, wide- field imaging is provided by a film camera (or by the electrono- graphic equivalent) mounted at the normal Cassegrain focus, it would still be easy to do spectroscopy or photometry of individual objects by inserting small diagonal mirrors ahead of the focal plane to feed a spectrograph and a photometer mounted at the side of the tube. In the same way, synoptic planetary image recording can be included on every mission with little or no conflict with other programs, partic- ularly since such work could utilize bright daytime portions of the orbit. If a coude configuration is adopted in place of a conventional Cassegrain, the optical focal plane can be made directly available to manned access inside the Spacelab. In general, this mode of opera-

112 OPTICAL AND ULTRAVIOLET ASTRONOMY tion currently appears to offer more disadvantages than advantages, but it is certainly an option that could be developed if sufficient need arises. C. Small General-Purpose Telescope A useful astronomical instrument for the Shuttle sortie mode appears to be a modest, general-purpose uv telescope. This instrument could be an //15 Cassegrain of about 50-cm aperture able to accept a variety of auxiliary instruments permitting photometric, spectro- photometric, and polarization measurements. Just as ground-based photometry has continued to be an important observational tech- nique even after many decades, the same situation will certainly prevail in the uv. Furthermore, no other uv photometric instrument with these capabilities is currently envisaged. The Orbiting Astronomical Observatory, OAO-2, barely began the study of a large number of interesting objects, e.g., magnetic vari- ables, Cepheids, and close binaries, for which uv light curves are proving to be exceedingly interesting; surface photometry of globu- lar clusters and galaxies; and color-magnitude diagrams of stars in open clusters. It should also be possible to utilize the superior guid- ance of the proposed instrument to observe nuclei of Seyfert galaxies and the brightest QSO's. A particularly interesting application of this instrument might be to observe, simultaneously with high-energy de- tectors, various x-ray objects or suspected sources. OAO showed the usefulness of having even small telescopes in orbit able to study transient objects like comets, novae, and supernovae. The 50-cm tele- scope, although not always in orbit, would be available for launch rather quickly when such opportunities arose (this is an important feature of the sortie mode but one that probably diminishes with increasing payload complexity). In addition, a modest uv photometer is needed to utilize fully the capabilities of the LST. For example, it will enable absolute uv calibrations, typically made on very bright objects, to be extended to stars sufficiently faint for the LST to ob- serve without overloading its detectors. A typical instrument for this telescope might consist of a simple low-resolution uv spectrograph feeding an intensifier coupled to a relatively low-spatial-resolution area detector, e.g., of the silicon- diode-array type. Thus simultaneous multiband photometric observa- tions of the program object, as well as of the sky background, could be made with considerable advantages in observing efficiency and accuracy. With such a system intermediate-band photometry of un-

Scientific Uses of the Space Shuttle 113 reddened early-type stars as faint as 15th magnitude could be done in a few minutes during the dark portion of the orbit. Bright stars could probably be observed during the sunlit portion. In its simplest mode of operation, the telescope mount could be clamped on a small section of pallet, using Shuttle attitude and or- bital data as the reference from which to point the telescope to within 0.5°; a simple bore-sighted star tracker could provide 1 min of arc pointing. The pointing stability would be whatever the Shuttle provided, preferably 1 or 2 min of arc. Alternatively, a simple open- loop gyro system in the mount could provide ~ 1 min of arc stability over some minutes of time. A simple TV system would give an ob- server on the ground occasional access to the image plane and would enable him to check pointing and star acquisition. Observing programs could be transmitted to the Shuttle a few times each day for storage and later execution from a small on-board computer; data could be stored for transmission to a ground station at a convenient time. Data rates would be modest, only occasionally exceeding a few kilobits per second. To observe the faintest objects or to work in crowded fields, the instrument could be mounted on a stabilized platform giving point- ing accuracy and stability of a few seconds of arc. In either case, a total system is planned to weigh no more than 150-200 kg, to use no more than about 50 W of Shuttle power, and to have only loose thermal control requirements for maintaining image quality. It would need no Spacelab facilities (but would need a few feet of electronic rack space somewhere in the Shuttle cabin) and would require astro- naut attention only on special occasions. The Shuttle-payload in- terface would be simple, much like that on an Aerobee rocket, since the Shuttle would provide only power, telemetry, attitude data, and mechanical reference planes. Since this should be a relatively in- expensive instrument, two should be available for quick turnaround and to take advantage of every possible flight opportunity. Such a simple but versatile telescope, occupying so little space and payload might well become the most widely used astronomical in- strument of the Shuttle sortie mode, flying as often as a dozen times a year as a noninterfering accompaniment of a wide variety of pri- mary missions. D. Very-Wide-Field Survey Camera Nearly all observations with rockets and satellites have been devoted to studies of individual stars and have given little information about

1 14 OPTICAL AND ULTRAVIOLET ASTRONOMY the large-scale distribution of the stellar clouds of the Milky Way. Detection of interstellar matter (diffuse and emission nebulae) under the most favorable condition of almost no atmospheric sky back- ground has not yet been undertaken. A general optical study of the Milky Way needs a very wide field, partly to guard against the photometric difficulties of the conven- tional mosaic overlapping and partly to reach the extragalactic sky background on both sides of the Milky Way. Also, any large-scale evaluation of the distribution of galactic light is disturbed by inter- planetary scattering from zodiacal light and gegenschein, which have angular sizes of the same order; it is thus essential to record galactic light and interplanetary scattering on the same field under the same conditions. The instrument required by these constraints must have a very wide field (—60°), be free of vignetting, and operate at a high focal ratio (//I to//2). Pointing accuracy of a few degrees is sufficient; guidance of 3 min of arc is adequate, but 1 min of arc would be better. Exposure times would range from a few seconds up to 30 min when working through narrow interference filters. This instrument would preferably be operated by astronauts in a semiautomatic airlock mode, making it possible to get the camera back inside to recover and change film magazines, to change filters, and to adjust optical parts, for example. Alternatively, this very- wide-field camera would easily fit a hitchhiker mode. In either case, it would be relatively small (0.4 x 0.4 x 1.2m3) and lightweight (about 50 kg). E. Very Large Light Collector Astrophysical study of any class of object usually follows a sequen- tial pattern involving an increasing sophistication of analyzers and use of the flux at progressively higher resolutions. The present pro- gram of NASA for construction of an LST meets the needs for detec- tion and initial analysis of the faintest cosmological sources but will not enable the researcher to study them in great detail. These first steps are essential, but we should not forgo plans for the next generation of telescopes. In particular, there will always exist a class of sources that we can detect with our most powerful telescopes such as the LST but that have flux levels so low and consequent photon arrival rates so slow that it is impossible to perform accurate photometric or high- spectral-resolution analysis. For such sources, we require a telescope

Scientific Uses of the Space Shuttle 115 of dramatically larger aperture but relatively modest optical quality. This need could be well met by the construction in space of a mul- tiple-mirror telescope of some 10-m aperture or larger. Since the costs associated with a telescope of this type would increase only slowly with size, the total cost should not be inordinate. Although this is not the place for a detailed design of such a telescope, some appropriate general properties can be inferred from its necessary constraints and goals. The individual optical elements should be large enough to permit individual diffraction-limited per- formance better than 0.1 sec of arc but small enough to be easily fabricated and readily handled in space. If the array were operated as a single large segmented prime-focus mirror, each mirror element would depart only slightly from a flat surface and would accordingly be relatively quick and economical for an optician to produce. Al- most continuous high-precision alignment of each mirror would probably prove necessary, requiring techniques of active optics. The frame of the array would presumably require prefabricated snap fit- tings permitting assembly in orbit. Guidance, control, and instrumentation might be copied with as few modifications as possible from those developed for the LST, at considerable savings in development cost. Two Shuttle flights should suffice to carry all parts of the system into orbit and to assemble it. Alternatively, the system could be carried up piecemeal over several years on other dedicated flights, as extra payload to be left in orbit; it could even be partly assembled and used with only a few elements while awaiting gradual comple- tion. This more leisurely mode of construction might lead to appre- ciable savings, while permitting important astronomy research to be done from the very beginning. Finally, the lifetime of the array should be indefinite. It is un- likely that the mirrors would deteriorate significantly, but, if so, the removal of a few at a time for resurfacing on the ground and later replacing would have little effect on the efficiency of the system. The potential advantages inherent in this approach are so great as to warrant serious detailed studies to begin in the relatively near future. F. Other Instruments 1. ALL-SKY ULTRAVIOLET SURVEYS All-sky surveys have four major functions: to provide finding and reference data on the many objects that become of interest as astron-

116 OPTICAL AND ULTRAVIOLET ASTRONOMY omers pursue their varied researches; to provide source data for sta- tistical studies and synoptic data for features such as absorption clouds seen clearly only when examined over a wide field; to provide a historical record for objects that prove variable or transient; to permit the discovery of unusual objects and unexpected phenomena. Good surveys retain utility over many decades; they also serve a much larger number of users than is the case with most research. Their cost per data bit is very low, yet the value of the discoveries and studies made with their aid may be equal to that of the most highly specific projects. Ultraviolet surveys using a meter-class all-reflecting Schmidt, reaching m = 19 or 20 photographically (fainter, if electronic cam- eras are available), should be centered at wavelengths on the peak of the interstellar reddening curve (2200 A) and at its far-uv minimum around 1500 A. A low-resolution (about 100 A) objective prism sur- vey with the same instrument would reach about 15th magnitude, bringing out objects with anomalous uv spectra, including bright radio-quiet QSO's. A higher-resolution survey (about 1 A) would reach m = 10, with spectra permitting detailed astrophysical studies of tens of thousands of stars. The various catalogs would augment other sources of objects for more detailed study, particularly with the LST. They would permit improved first-order selection of candidate identifications for optical counterparts of radio, x-ray, and gamma-ray sources. They would be particularly valuable for working out problems related to galactic structure and for correction of extragalactic observations as distorted by extinction in our galaxy. Intergalactic absorption and emissions may be detectable by such surveys. While there is as yet no reason to expect the discovery of new classes of objects with enormously en- hanced uv radiation, nevertheless we will not be sure until an ex- haustive search has been made; even a very few could be important. These typical applications indicate the properties of the instru- ment needed for such work. To cover the necessary wide spectral range, it would have to be an all-reflecting Schmidt. If possible the field should be external and flat to permit use of large rolls of film or of electronic cameras. The aperture must be sufficient (0.5 to 1 m) to reach faint objects in a reasonable time (the uv sky is several magnitudes darker than the visual sky, permitting deeper penetration for a given //ratio; but the typical source brightness and number of photons are lower in the visual, requiring longer exposures to pick up faint objects). A resolution of 1 sec of arc is desirable. The field must

Scientific Uses of the Space Shuttle 117 be large enough to permit rapid coverage of the sky, thus at least 5° in diameter. These all-sky surveys are particularly well suited to extended 28-day Shuttle flights (about thirty-two 15-min dark-sky exposures per day; 900 per flight; 1800 required to cover the entire sky). Even using film, two such missions with the sun in opposite hemispheres of the sky would provide complete coverage for each program, thus requiring a total of eight flight opportunities during the 1980's. If suitable electronographic cameras become available, each survey would need less than half of the number of days, while reaching more than a magnitude fainter; the programs could thus be accom- plished on only four Shuttle flights with substantial cost savings even after including the expense of developing the electronographic cam- era (which would also have important uses in other parts of the space program). A folded Schmidt with 0.75-m aperture, able to accom- plish these programs, would require about 2 x 3 x 4 m3 of space and weigh about 2500 Ib. 2. DEEP-SKY-SURVEY 1-METER TELESCOPE Sky fields intermediate in size between those provided by a general- purpose //30 Cassegrain telescope and by a wide-field Schmidt survey camera would be very useful for some important search and survey problems. Such a system would be particularly powerful for observa- tions of details of diffuse nebulae and H II regions in our galaxy, of the structure and stellar populations of the nearest galaxies, of inter- galactic matter and bridges between systems of galaxies, of very dis- tant clusters of galaxies (red shifts beyond z = 0.5) for cosmological studies, and of selected sky areas to search for optical counterparts of radio and x-ray sources beyond the threshold of the optical and uv all-sky surveys. These goals would be served well by an existing Ritchey- Chretien-Bowen design that provides excellent imaging at f/1 over a 3° field. With a 1-m aperture, it can be expected to have a threshold about 3 magnitudes fainter than the all-sky survey camera described above. This optical system, as a package, can be substituted for the //30 Cassegrain system into the same mounting structure, and it has simpler pointing and stabilization requirements. Sky-limited exposures can be approached in some cases with unaided photography and can be easily reached at all optical wavelengths with electronography. For photographic work on very faint diffuse sources or for the widest-field electronographic studies, a reducing camera option may be desirable.

118 OPTICAL AND ULTRAVIOLET ASTRONOMY 3. SOLAR VARIATION The total emission of energy by the sun is an important parameter in meteorology and climatology. The solar luminosity is normally assumed not to vary significantly—hence the term "solar constant" for the flux received at the standard distance of 1 AU. However, observational evidence for constancy is only at about the 1 percent level, largely because of the difficulty in removing atmospheric ef- fects from the observations, especially in the uv part of the spectrum. The Shuttle offers the possibility of very frequent observations over all the significant wavelength regions from 0.1 to 10 nm. To be most useful the photometric instrument should accept uniformly the entire disk of the sun, be very simple and light-weight, rugged, easy to calibrate, accurate to at least 0.01 percent, and inherently free of significant instrumental change or drift over a decade or more of use. The entire instrument should be only a fraction of a meter in size, weighing less than 20 kg, and requiring only a few watts of power. It could be operated by astronauts through an airlock or could be fastened somewhere inside the Shuttle bay for use once or twice for a few minutes during a mission. 4. MICHELSON INTERFEROMETER Angular resolution beyond that of the LST (— 0.035 sec of arc in the visible) will probably require interferometric techniques. As an ex- ample, a 30-m Michelson interferometer is capable of measuring spa- tial frequencies as small as 0.0035 sec of arc at visible wavelengths and 0.001 sec of arc in the far uv. Such ultra-high resolution would provide powerful constraints on theories of QSO's and galactic nu- clei. Very close binaries and many stellar diameters could be re- solved. Even surface details of certain stars could be studied, particu- larly red giants, where large-scale convective cells are suspected on theoretical grounds and where there is evidence, in at least one case, for nonspherical shape. However, severe technical requirements must be met before such an interferometer could be used for aperture synthesis. An rms guid- ance jitter of the order of 0.1 times the resolution (0.0001 sec of arc in the most severe case) is required to measure the phase and ampli- tude of the spatial frequencies. Although the sortie mode makes it more difficult to achieve the guidance requirements, it does allow manned participation in erect- ing the interferometer in space and in learning how to operate it. Further discussion of this topic will require a feasibility study.

Scientific Uses of the Space Shuttle 119 . | 1 ! (are :heck-out se Optional J j 1^1 •"Urn/Area O *ew Mbits Iptional 5060 min/ ii || \ KC of arc ). 1 deg any time; o S Jt ^ x o V • 2 £ t; * f-l C J2 g,- 111 1 II in —5 1 I sfl.il III 0 ZZZ 6 v do 5 i •s - | g i 2 ll 521 3 O . I 1 - i j «1 j g s •o *fc •s *a 1 2 fe «* k J £ .5 .8 a c O « S 0- a 5 1 f o i | * 5 ¥ TJ :heck-out 4one 103 min of j i ±45 deg "c 'O020'C )* Desirable ( 000800 km 2 E 1 i » 5 Q III any time; 5060 min/ Hire iff < IM-^t! ill Access at o i s •* a ^ S. g sea Q E "c 0 Rare Check-out None 103 min os <. ±45 deg o I* - I Film/Area Few Mbils Optional'' J"i= S 1 < 2 O 0 o o 3 < E S § £ (j "« 1 Q M _£ 5 o < * g g ail * 11 . - 1 1-a- ° A » f Ifa S| ZZ XX s •= § 2 o 0 Z ZZZ • V ZZZ Q 1 „ e 1 |E! s 8 • ej •a • g. -E * ll II 0020% Occasion; Optional • I 0 "' < K •Si 5E*%^I Film/Are Few Mbil Optional' *?"* 2 f " 5 "^ o t ° 5- Q — O m V V O & — VI — 0 g u •= Jt i 0.5-m General- Purpose Telescope For malfunctio only Rare Xone 0.1- 1 min of ai O.I deg a more detailed st I 1 i E a at any time, >it 5030 min/or 11 "0020°C 0° Desirable 250-800 km K I"Ti 7S < gJ fix, 11 III 1 u. u. O | a •c ii-S. g S - c "i? •s 5 J ^•o 11 « S Jj g- £ u £" 1 1 1 ill JF S* if ! j jlj.sjf Ifl II Q J X O j! •s J i is ciSS* £ vi ?- 2 2 . 1 i 1 8 s % i 6 1 fit 1 2 IE • 1 8 ^11 B < •s 1 • • J! JC 1 "C JJ££ v u o ZZ H (2 « b So z S2ii ZZZ 1 M E j M«n /I [tendance Pay load Specials Astronomer /. Pointing Instrument Orbiter 2. Stabilization Contamination Thermal Require Temperature Orfrif Inclination Orbit Altitude S ji! Detectors Data Storage Processing * Image Orbiter S-8 * t Eg e ! rill iii 1 < m U d w u. u i - M! li

120 OPTICAL AND ULTRAVIOLET ASTRONOMY III. TECHNICAL IMPACT ON SHUTTLE A. The Role of Man in Space-Shuttle Astronomy There can be four levels of manned interaction with optical-uv astronomical instruments carried to space by the Shuttle: active ob- servation by the astronomer, operation of observing programs includ- ing film change, repair or maintenance of equipment including detec- tor change, and assembly of instruments too large or too heavy for single-Shuttle flights. Each of these is considered in more detail, not only in the remainder of this section but also where relevant in the description of individual instruments. (See also Table 15.) 1. ACTIVE OBSERVATIONS While a case can be made for active human monitoring of certain transient phonomena such as solar flares, we believe that manned interaction with the observations is unlikely to prove an important mode for nonsolar optical and uv astronomy. An exception to this statement would arise if a coud6 telescope mounting were adopted, bringing the image into the Spacelab for shirtsleeve access. Possible advantages with this scheme include ease of construction and change of instruments at the focus and slightly reduced need for full automatic pointing and guidance. However, the drawbacks seem much more severe, including primarily the weight of the Spacelab, which would preclude having both a substantial tele- scope and an extended mission (only an expected 5000-lb real pay- load for a 28-day mission including a minimal Spacelab); for nearly all contemplated uses the advantage of extended observing time and of flying several major instruments overwhelm any simplifications that might be possible from continuous human access. The telescope mounting and guiding in any event must be of the quality required by the final imaging; TV monitoring of setting and focus are already routine—even at ground-based observatories; and the limited observ- ing time available even with extended Shuttle missions strongly sug- gests that each flight be optimized for use of typically one or at most two or three instruments at the focus. 2. OPERATION OF OBSERVING PROGRAMS FOR PALLET- MOUNTED OR FREE-FLYING INSTRUMENTS Some of the astronomy missions would be likely to profit from planned extravehicular activity (EVA ) at the start of operations, to check out the telescope and its auxiliary instruments, and in photo-

Scien tific Uses of the Space Shuttle 121 graphic programs to recover a test exposure for immediate on-board development. Assuming the likely event that EVA activity will become rela- tively straightforward and routine, the planned changing of film canisters or reconfiguring of telescopes during missions (e.g., from photometric to spectroscopic operation or the change to a different kind of detector) may well be more easily, reliably, and cheaply done by man than by remotely operating devices. 3. MAINTENANCE AND REPAIR Although the LSI will be a free-flying observatory, operated from the ground, its success will depend on revisits for service, mainte- nance, and refurbishment by the Space Shuttle. The present NASA program for the LST has adopted as guidelines that the major repairs and changes will be made following earth return by the Shuttle, with only limited on-orbit service. Because of the unavoidable delays expected between ground return and re- launch, we hope that the on-orbit work can be identified as the most desirable method of normal operation. Since this would mean both servicing flights and return flights, the Shuttle must have docking facilities that permit pressure-suited manned entry of the LST and suitable storage structures for launch and recovery. Servicing visits should be made possible on a shared mission basis, allowing relatively small costs to be incurred. Likewise, the launch and recovery struc- tures should be such as to permit flight sharing. It would seem wise to design other free-flyers and sortie-mode instruments for some degree of manned access so that simple mal- functions could be cleared up by EVA. In particular, for some experi- ments it may be important to carry one or more complete spare detector packages and microcircuit electronic cards covering all essential functions. 4. ASSEMBLY Only one telescope in the optical-uv area is likely to profit from any degree of manned assembly in space, and it will require this mode. A very large optical array or segmented-mirror telescope might need the equivalent of two full Shuttle payloads to bring its elements up to orbit, with several man-weeks of EVA for assembly. B. Pointing and Stabilization The pointing and stabilization requirements of a 1-m diffraction-

122 OPTICAL AND ULTRAVIOLET ASTRONOMY limited telescope are as stringent as those of any astronomical instru- ment that is likely to be flown in the sortie mode. If the higher optical spatial frequencies are not to be severely attenuated, the reduction in amplitude at the limiting frequency due to guidance errors must be less than 50 percent. For a 1-m telescope operating at 5000 A, the resulting guidance error is less than 0.02 sec of arc rms. Imagery at shorter wavelengths would decrease the value to 0.01 sec of arc rms. A stabilization of 0.01 sec of arc rms in the free-flying mode has been shown feasible in several studies, notably the phase A report of the LST for which the requirements are even more stringent. Further- more, the OAO-C (Copernicus) telescope is achieving a guidance of the order of 0.02 sec of arc rms, as did the Stratoscope II balloon- borne telescope. An intermediate level of guidance required by some telescopes is about 1 sec of arc rms. Experience with telescopes mounted aboard aircraft indicates that this level of guidance can readily be obtained using a gyro as a sensor. At the lower end of the requirements, the ±0.1 deg of arc basic Shuttle stabilization is adequate for many important purposes, partic- ularly the general-purpose telescope described above. The current pointing specifications of the Shuttle are ±0.5 deg of arc, which is adequate for wide-field cameras that rely on the basic ±0.1 deg of arc stabilization of the Shuttle. More accurate pointing specifications can readily be accomplished with the aid of bore- sighted star trackers. Search modes, either with the telescope or with the Shuttle as a whole, may be helpful. C. Contamination In the present circumstances, it is impossible to discuss contamina- tion problems in any but the most superficial terms. The contam- inants likely from the Shuttle and their outgassing rates are un- known; detector types and surfaces that will be used in the 1980's are uncertain; and possible shielding, packaging, and other design techniques are not yet determined. However, it is clear that photo- cathodes and optical surfaces must not be exposed to contaminants such as H2O, O2, or organic volatiles. Any change of sensitivity, either by degradation in cathode sensitivity, by optical reflectivity, or by absorption by a cloud of material surrounding the spacecraft, should not be greater than a few percent in any 1 or 2 A interval over

Scientific Uses of the Space Shuttle 123 a 30-day mission. Larger changes might require an inordinate amount of recalibration or compromise the photometric accuracy of the ob- serving program. The effects of light scattered by particles in the line of sight must be held to such a level that the artificial background induced is small compared with the natural level. D. Thermal Requirements Because of the very long thermal time constant of the primary mirror (typically days), most recent thermal studies of diffraction-limited telescopes have assumed that the primary and secondary mirrors are kept at a fixed temperature (~20°C) by means of active thermal control. Most other portions of the optical telescope assembly have passive thermal control. It would seem that the same basic thermal design would be satisfactory for the sortie mode. In thermal designs for the free-flying mode, the scientific instru- ment package generally has a thermostated portion containing the relay optics and other devices that generate little heat. In addition, heat-generating cameras and other instruments are coupled to the outer portions to allow the heat to be radiated to space. In the sortie mode, the scientific instrument package must radiate its heat to the inside of the Shuttle bay rather than to space. Pro- vided that the temperature of the Shuttle bay is kept sufficiently cool, it seems plausible that the basic free-flyer design could be rea- sonably adapted to the sortie mode. Before such a conclusion is made, however, a study should be carried out to establish in detail the problems of adapting a free-flyer thermal design to the sortie mode. In particular, the study should determine whether the 6-kW (—21,500 Btu/h) cooling available to the pallet is adequate. Generally the thermal problems of other op- tical and ultraviolet telescopes are less severe than for a diffraction- limited telescope. E. Orbits The orbit for 1-m telescope operation must be in the 3 00-800 km range, to optimize minimum reasonable sky brightness and radiation environment. The most desirable orbit is equatorial (in order to avoid the South Atlantic Anomaly), although if it is not available the standard 28.5° inclination is nearly as satisfactory as any interme- diate value.

124 OPTICAL AND ULTRAVIOLET ASTRONOMY F. Payload Weight The total weight of a 1-m class sortie-mode telescope with its mount- ing will be about 7000 Ib plus about 2000 Ib of control equipment in the Shuttle cabin or a Spacelab. These numbers are based on the recently constructed 36-in. airborne telescope and are about one half of those of a recent industrial study. If the entire Shuttle is used for coarse pointing, thereby limiting the necessary pitch angles of the telescope, then a bay section of full width and 2-m length would be required. A 4-m length would be required if Shuttle coarse pointing is not available. Control rack space of about 100 ft would be neces- sary. Power requirements during operation will be approximately 3 kW. The maximum weight of a free-flying satellite presently con- templated is that of the LST, which is almost 22,000 Ib. G. Detectors and Telemetry Requirements All the major proposed optical and uv-astronomy missions are being planned to operate primarily by ground command. In some cases, a backup Shuttle control option may be useful. The data rates needed for the ground-based command and control functions are conven- tional and generally low; availability of an on-board computer is desirable and would be essential if the tracking and data-relay satel- lite should not be continuously available. Likewise telemetry require- ments for reporting of housekeeping data would be negligible. Except for photographic missions, the requirements for trans- mission of scientific data to the earth will be relatively high. Photo- metric field studies might use area detectors containing at least 104 elements, to be read out normally at intervals of about a minute, leading to maximum bit rates of perhaps 104/sec. However, occa- sional high-speed photometric problems would require target readout at millisecond rates, or 107 bits/sec. Digital television readout tubes and electronographic cameras are two of the prime candidates foreseen today for optical image detec- tors aboard the Shuttle in the 1980's. Whether they completely re- place unaided photography will depend on the availability of fields up to 20 or 25 cm in diameter. Both have the potential of high photometric accuracy. Some images may have as many as 2 x 109 pixels per picture or about 20 x 109 bits of data per picture, assuming a pixel size of about 5 MOI. Even if data transmission rates can be increased to the 10 Mbits/sec range, more than 30 min would be required to transmit

Scientific Uses of the Space Shuttle 125 a picture to earth. The total production of image bits during a Shuttle mission could exceed 1012. Electronography, if expanded to the field size desired, comes close to meeting the desired storage goal. A modest reduction of storage requirements would be possible by onboard processing of images if the data are in digital form. In many images, for example, an overwhelming majority of the pixels may all have the same brightness value, namely that of the sky background, and the full information contained in the image can be put into a more compressed format than a listing of brightness values separately for every pixel. There may consequently be a tradeoff between stor- age capacity and processing capability. At least some digital ca- pability will be required to handle nonimage data, but the amount will doubtless be small in comparison with image data. IV. MISSION MODEL An observatory in space, just as one on the ground, is extremely versatile and capable of carrying out a wide variety of measurements of an exceedingly large number of objects. Such a facility, be it a small 0.5-m uv telescope or the LST, is in no way an "experiment" in the generally accepted sense of that term. Thus the scientific value of a uv telescope increases with time in operation, since the information gained increases with observing time; there are few, if any, "satura- tion effects." Indeed, many types of investigating, e.g., in planetary science, are greatly increased in value if continuums or repeated ob- servations can be made. It is for these reasons that we place primary emphasis on free-flyers, not only the LST but also smaller payloads. Even where the sortie mode is advantageous, to maximize the sci- entific return of the relatively small number of astronomy sortie- mode flights, presented in the mission model, these flights should be extended beyond 7 days as soon as possible. (See Table 16.) Any such extension immediately increases the mission efficiency in that the times required to prepare for operations after launch as well as to prepare for re-entry and landing will be about the same regardless of mission length. Of prime importance, however, is to increase the astronomy mission lengths to 30 days or longer as soon as possible. Much interesting and valuable work in the uv and visual wave- length regions can be carried out with small instruments, requiring very simple payload-Shuttle interfaces, e.g., the 0.5-m general- purpose telescope, the very-wide-angle camera, the solar-constant in-

126 OPTICAL AND ULTRAVIOLET ASTRONOMY TABLE 16 Optical and Ultraviolet Sortie-Mode Missions, 1981 -199 \a Major Optical and uv Instruments Total number of stellar sortie pay loads 61 or ~ 6/yr Likely number of optical and uv payloads ~ 20 or ~ 2/yr If during 1981-1985 mission length is only 7 days and 5 days/mission are available for observing, then get ~ 10 observing days/yr or ~ 75 h of dark time/yr Small Optical and uv Instruments Total number of stellar sortie flights ~ 30 or 3/yr Assuming one small instrument on half of these flights would have ~ 15 or 1.5/yr which corresponds to ~ 8 observing days/yr or ~ 60 h of dark time/yr This table was extracted from "Potential NASA Scientific Missions: Reference Model Only-Not a NASA Plan-1973." struments. We hope that every effort will be made to reserve 100-200 kg of weight, a few cubic feet of rack space, and very modest power allotments on every astronomy and physics sortie mis- sion so that these and other small instruments may have as many flight opportunities as possible. Such a policy could have only a minor effect on the primary payload but a major effect on the sci- ence done. If this mission model represents the maximum number of flights likely to be available for uv and optical astronomy, we believe that the payload distribution is reasonable. V. SUMMARY AND RECOMMENDATIONS A. Utilizing the Shuttle for Optical and Ultraviolet Space Astronomy Very-high-resolution images in visible and ultraviolet wavelengths will be required to answer many of the most vital questions concerning the nature of our solar system, the evolution of stars and galaxies, and the structure of the universe. Spectroscopy, photometry, and po- larimetry, particularly in the ultraviolet, will also be needed. Unfor- tunately, the earth's atmosphere severely limits the sharpness of astronomical images and is completely opaque to most ultraviolet wavelengths. Consequently, astronomical instruments in space are

Scien tiflc Uses of the Space Shuttle 127 absolutely vital to the discipline. Among proposed space telescopes, the Large Space Telescope (LST) is outstanding in its ability to ob- tain observations of unprecedented quality and importance. It ranks as the single most important program in optical and ultraviolet space astronomy. Planned as a permanent 3-m astronomical observatory, the LST will utilize the capability of the Shuttle to provide servicing in orbit or return to earth. Most optical and uv space-astronomy telescopes are technically more suited to the free-flying mode than to any other Shuttle mode. Generally, tight tolerances on the allowable guidance errors, thermal excursions, and contamination limits are more easily met in the free- flying mode. Most importantly, the free-fly ing mode offers far more observing time than does the sortie mode. However, several impor- tant smaller instruments are well suited to the sortie mode. Besides serving as a booster with a large payload capacity, the Shuttle offers the extremely valuable advantage of manned assistance in deploying a free-flying spacecraft. In addition, the Shuttle pro- vides the ability to periodically update and maintain the telescope in orbit. It also has the capability of returning an astronomical tele- scope to the earth for major refurbishment. However, we are con- cerned that the latter option will result in free-flying spacecraft spending a large fraction of their time on the ground undergoing modifications and awaiting launch. We, therefore, believe that the former option should be used for instrument replacement and that free-flying satellites be returned to earth only when absolutely nec- essary. We recommend that the Shuttle be designed to facilitate instru- ment replacement in orbit. In this connection, we also recommend that all free-flying astronomical telescopes be designed to make it as easy as possible for a space-suited astronaut to replace instruments and to make minor adjustments and repairs. There are situations for which the sortie mode is highly advan- tageous. For example, imaging over large fields of view using photo- graphic film (directly or via electronography) seems preferable in the sortie mode, where frequent manned access is possible. Although it would be possible to conduct a photographic survey with a free- flying telescope that is visited about once per year, the difficulties of protecting against film fogging due to charged particles, against sys- tematically overexposing or underexposing, against misfocusing, and against film jamming are sufficiently large to warrant use of alternate approaches if possible.

128 OPTICAL AND ULTRAVIOLET ASTRONOMY We believe the best approach to optimize the scientific returns is to lengthen the 7-day baseline sortie mission as much as possible. Since at the moment we cannot identify an overriding need for the Spacelab for astronomy experiments, we suggest that the weight penalty in expendables for longer missions be compensated by elim- inating the Spacelab, utilizing only the Shuttle bay. In this approach, manned attendance would require wearing spacesuits. In the absence of a malfunction, we currently estimate that about one extra- vehicular activity per week should be sufficient. We recommend that the Shuttle be designed to facilitate the longest possible sortie missions. Relatively simple telescopes requiring only the basic Shuttle guidance (<0.1 degree of arc) or moderately improved guidance (~ 1 sec of arc) provided by a compact general-purpose stabilized plat- form provide another situation where the sortie mode can be advan- tageous. They could be controlled from ground stations, and data could be telemetered in the same way as for free-flyers. Contained in coarse gimbals of adequate angular range, they could be launched whenever additional space is available. Alternatively, there are several small telescopes with low guidance requirements that could be oper- ated effectively through an airlock in the Shuttle cabin. For the most part, we have not been able to identify any technical reasons that would preclude operation of astronomical telescopes in the sortie mode. However, there are some problems that we wish to emphasize. One of these problems is the increased complexity that will result if continuous command and telemetry are not available. We, therefore, recommend that a Tracking and Data Relay Satellite be available for astronomical space telescopes, particularly those uti- lizing the sortie mode. Optical and uv payloads are sensitive to contamination by the Shuttle. Inadequate control could prevent execution of the projects investigated in this study. We, therefore, recommend that contamina- tion be kept paramount among Shuttle design and operation consid- erations and that quantitatively acceptable contamination levels be established for the instruments proposed. Many astronomical sensors would benefit from an equatorial orbit that avoids the South Atlantic Anomaly, thereby reducing the back- ground of energetic particles. These charged particles will cause a noise background against which it is very difficult to shield or com- pensate. We recognize that the achievement of an equatorial orbit is difficult, unless an equatorial launch site can be developed. We rec-

Scien tific Uses of the Space Shuttle 129 ommend that additional consideration be given to the problem of obtaining equatorial orbits. One of the most exciting long-range prospects for astronomic re- search is the possibility of using the Shuttle to assemble very large telescopes and other instruments in orbit. We recommend that the Space Shuttle be designed to facilitate major assembly operations in space. B. Supporting Research and Technology The potential of the Shuttle for performing a large amount of space astronomy will not be borne out unless the proper telescopes and instruments are available when the Shuttle is ready to fly. At mini- mum, a vigorous supporting research and technology program is now required. A promising imaging technique is electronography, for which fields of only a few centimeters diameter are currently available. For some applications, the required camera is one in which the image is read out electronically, thereby avoiding the use of film in orbit. We recommend that imaging cameras of large surface area (20 cm x 20 cm) and high quantum efficiency be developed. Many of the constraints on the Shuttle cannot be accurately spec- ified until more detailed studies of astronomical payloads have been performed. This is particularly true of the thermal requirements. We, therefore, recommend that a thermal design study be made of a suitable sortie-mode telescope.

7 Solar Physics I. SOLAR-PHYSICS OBJECTIVES AND OVERALL PLAN The outstanding scientific problems in solar physics derive their sig- nificance as much from their intrinsic interest as plasma phenomena of extreme complexity as they do from their importance for the study and elucidation of a range of basic questions arising in our efforts to understand the physical universe.* In summary, these problems center around (a) the origin of solar activity and the mechanisms underlying its various manifestations (especially flares), (b) the nature and origin of the mass and mechan- ical energy flux from the sun, and (c) physical problems of broad significance that can only be studied in the sun. We discuss these broad areas below, giving particular emphasis to the progress to be anticipated from solar observations during the Shuttle era. A. Solar Activity The study of the formation, heating, and long-term development and decay of active regions requires spatial correlation of observations made over a broad spectral range and over consecutive periods of a few days. For example, in order to study the interaction of rising magnetic fields with the plasma of the solar photospheric layers, long-term time-lapse observations with high spatial resolution in the visible portion of the spectrum are needed of velocity fields, small- scale magnetic fields, and features reflecting different temperature and density conditions. These observations must be correlated with *In considering the scientific motivation, we have drawn heavily on the reports of the NASA Payload Planning Working Group (Blue Book) on Solar Physics and of the ESRO-PASOL Group and particularly on the discussions of those prob- lems that they believe should consume a major fraction of the best efforts in solar physics through the first decade of the Shuttle era. 130

Scientific Uses of the Space Shuttle 131 the uv and x-ray observations of the same areas to yield parallel data on the higher levels in the sun's atmosphere—the chromosphere, tran- sition region, and inner corona. Data show the spatial structure of active regions to be extremely complex and to change completely in the higher layers, where the magnetic field dominates; however, the limited spatial resolution currently available severely restricts our ability to interpret such data fully. The evolution of activity and the details of magnetic-field development will almost certainly depend on magnetic-field measurements made with high spatial resolution and extending over periods of a week or more. Little is known about the impulsive nonthermal phase of flare development during which energy is released and charged particles accelerated to very high energies. We would like to know the location of the primary acceleration, the magnetic- and electric-field configu- rations, and the time sequence of the energy release processes. X-ray and radio-wave observations provide essential information on the energetic electron population of a flare, while the white-light, gamma-ray, and neutron emission give clues to the acceleration of protons. Direct measurwnent in space of the isotopic content of energetic flare particles promises to add still another insight into the acceleration, containment, and release of charged particles. Because theory suggests that the energy release and subsequent thermaliza- tion must take place in an extremely small-volume, high-temporal and -spatial resolution is essential. Another area of current interest is the state of an active region prior to the occurrence of a flare. There are periods of rapid mag- netic change in an active region during several hours or days prior to a large flare, during which time the x-ray, xuv, and radio emission tend to increase in intensity. Accelerated particles of comparatively low energy are observed to escape from the buildup area into inter- planetary space. This, with many other aspects of the buildup, is not understood, and further observations of particle densities, fluxes, temperatures, and magnetic fields—and the associated time vari- ations—are needed. B. Energy and Mass Flow in the Solar Atmosphere The mechanisms that produce the large departures from radiative equilibrium that characterizes the chromosphere and corona are not understood. Compelling theoretical and observational evidence sug- gests that these levels are heated by mechanical disturbances such as acoustic, magnetoacoustic, and possibly gravity waves originating in

132 SOLAR PHYSICS the subphotospheric convection zone. The principal mechanism has not been identified in spite of the fact that recent years have pro- duced a wealth of data on the temperature structure of the chromo- sphere- corona transition region as well as microscopic motions in the lower atmosphere. Future work must provide a complete specification of the temper- ature, density, velocity structure, and magnetic field over the entire atmosphere from the photosphere out into the lower corona. Be- cause these layers contain an intricate fine-scale horizontal structure, closely associated with the concentration of the magnetic field into small columns, high spatial resolution at all wavelengths is essential. Without such resolution the critical effects of the channeling of the mechanical energy flux by the magnetic field cannot be determined. The flow of mass and energy in the solar atmosphere continues into interplanetary space in the form of the corona and solar wind. The magnetic field plays a crucial, if incompletely understood, role in modulating the flow of the material and imprinting an intricate density, temperature, and velocity structure on the plasma as it rushes out from the sun. Space probes have measured these at 1 AU; however, the connection between these observations and structures in the inner corona is just beginning to be established. Surprises, such as the recent realization that most of the solar wind originates in quite undistinguished regions of the corona, where the magnetic field is weak and open and the density is low, can be expected to be frequent and to lead to exciting revisions of our ideas on the struc- ture of the outermost atmospheres of the sun and stars. Understanding these processes requires a complete specification of the density, temperature, and magnetic field in the corona and solar wind, with good temporal and spatial resolution, so that a full three- dimensional model can be established. Since the medium is contin- ually evolving, synoptic observations are necessary to describe the influence of activity in the lower atmosphere on the upper levels. Moreover, high time-resolution measures are required to investigate the response of the corona-solar wind plasma to solar flares. A va- riety of tools will be required. Spaceborne coronagraphs have demon- strated their power on the OSO and ATM; however, these data must be supplemented by x-ray, euv, ground-based radioheliograph and coronagraph, spaceborne radiospectrographs, and in situ solar-wind measures if a complete picture is to be obtained. We would particu- larly stress the need for coordination of ground and space observa- tions for incisive attacks on particular scientific objectives.

Scien tific Uses of the Space Shuttle 133 C. Physical Problems of Broader Significance Solar activity originates below the visible levels of the solar atmo- sphere; our knowledge of the structure and dynamics of the interior is, at best, provisional. Models provide a basis for understanding the most obvious properties of the sun—its mass, radius, and luminosity— and show that the presence of a chromosphere and corona depends on the existence of a convection zone, some of whose characteristics are reflected in the photosphere. Similar models applied to other stars provide insight into their evolution, variability, and the pro- cesses of element synthesis. In all these investigations, comparison with the sun furnishes a critical test; several tests lead to only a qualified confidence. For example, the currently accepted solar models predict a neutrino flux well in excess of the measured upper limit. Also, models incorporat- ing convection in a rotating sun are not yet sufficiently advanced to explain the observed differential rotation of the photosphere and the characteristics of the solar magnetic cycle. With these more obvious features of the sun unexplained, it is small wonder that more subtle questions such as the nature of supergranulation cells, solar oblate- ness, and the role of the solar wind in the angular momentum history of the sun remain subjects of speculation. Likewise, broader ques- tions regarding the presence of similar phenomena on other stars remain uncertain. The constancy of the solar "constant"—a funda- mental parameter in all studies involving terrestrial climate—appears to be an article of faith. It is clear that little progress can be made until our ideas concern- ing the role of turbulent convection in determining the structure of the sun and its interaction with solar rotation are clarified. Here, a fundamental advance in the theory of turbulent convection beyond the currently used mixing length models is essential. The application of modern computational tools to these problems will be essential but may be misleading without this fundamental knowledge. A directly related problem is the operation of the solar dynamo and the production of the solar magnetic cycle. If the investigations mentioned earlier are successful, there should be no lack of funda- mental knowledge that would impede progress in the study of the solar cycle. Advancing our knowledge of the stability of the sun, and the consequent implications on the solar constant, and the neutrino deficit must proceed in concert with these studies. Although progress can be made using the current models, the stability of the sun is most

134 SOLAR PHYSICS certainly dependent on the coupling between the energy generating core, the radiative envelope, and the convection zone. Since the char- acteristics of these zones are not fully known, the presence of a solar variability independent of the magnetic cycle remains uncertain. D. Relation of Solar Physics to Other Disciplines The outstanding problems discussed above have an importance far beyond solar physics. Thus, once the processes of mechanical energy production, transport, and dissipation are understood, observations of stellar chromospheres and coronae could be used for further studies of stellar structure and evolution, since the extent of the subphotospheric convection surely varies with spectral type and class. Since it seems clear that small-scale photospheric features are associated with production of the mechanical energy that heat the chromosphere and corona, such motions and fine structures should exist also in the atmospheres of stars exhibiting chromospheric fea- tures; the interpretation of the spectra of such stars must rest heavily on the solution of the mass and energy-flux problem of the solar atmosphere. Continuing studies of the solar wind will find application in understanding stellar winds and mass-loss mechanisms. The process whereby the solar wind removes angular momentum from the sun, thus slowing down solar rotation, is basic to an understanding of the origin and evolution of the solar system and of other stars and planetary systems. This mass loss is important in determining the composition of the interstellar medium and interplanetary plasma. Solar flares exhibit a broad range of high-energy processes, including the generation of hard cosmic rays and associated radiation, extending over the spectrum from gamma rays to radio wavelengths. The sun provides an opportunity for detailed study of the interaction of high-energy particles and magnetic fields, since both of these characteristics can be measured directly. Such studies have clear and direct relevance to the study of other energetic objects in the universe. Similarly, the study of solar-active regions and the long-term interaction of the solar plasma and magnetic fields should increase our understanding of the coupling between solar convection, differential rotation, and the loss of angular momentum, as well as cycles of stellar activity. We can look to a continuing stimulation of many other areas in astrophysics coming from attempts to understand the complex questions posed by solar physics. As a single example, important

Scien tific Uses of the Space Shuttle 135 studies of atomic processes in low-density plasmas have followed efforts to account for the physical state of the solar atmosphere. Finally, as man's technical achievements mount, the importance of a detailed understanding of solar-terrestrial effects will grow. The influences of solar activity on the upper terrestrial atmosphere are well documented, if insufficiently understood. The solar wind stands out as the principal modulator of the magnetosphere. Significant progress has been made in our ability to predict the occurrence of major flares, and a capability for accurate prediction would have economic benefits and may determine the extent to which man can work in space above the atmosphere. Finally, a possible link between solar activity and large-scale terrestrial weather patterns suggests potential significance of solar space studies to all mankind. II. PROFILE FOR A BALANCED PROGRAM IN SOLAR ASTRONOMY With the above objectives as guidelines, we have developed a set of goals that we believe would provide a well-balanced program in solar astronomy through the 1980's. These are outlined briefly below; more detailed descriptions are set out in Section III. A. Spaceflight Aspects A solar maximum satellite for the 1978- 1979 period would allow, in conjunction with ground-based studies, an incisive approach to the study of solar activity in its various manifestations. Furthermore, the basic spacecraft, through Shuttle recovery, relaunch, and revisit, would provide a free-flying payload for long-duration solar experiments in the 1980's. Basic instrumentation for a Shuttle Sortie Solar Observatory (SSO) falls into two categories. First we envisage a set of major telescopes optimized for different wavelength regions and feeding inter- changeable specialized instruments (spectrographs, direct cameras, magnetometers). Second would be a versatile, fine-pointed platform for mounting special-purpose instruments that may be incompatible with the larger feed telescopes or not require their power—examples are coronagraphs and polarimeters. The larger system, at least, should be started soon to provide the opportunity of studying problems of solar activity with more powerful instruments (even if narrower in scope) than those on the free-flying satellite.

136 SOLAR PHYSICS While the smaller fine-pointed platform should be developed on a single pallet as a module for the sortie solar observatory, we also see an attractive possibility in its use to carry payloads on a standby basis—an opportunity whereby an available payload could be carried on an otherwise unfilled sortie mission. This concept needs study to determine its feasibility. Because the ultimate observational needs of solar astronomy may eventually require a free-flying Large Solar Observatory, we recommend that the National Academy of Sciences convene a panel of scientists to investigate all aspects of the need and specifications for, and use of, such a facility. B. Other Necessary Components of a Balanced Program 1. OTHER SATELLITE OBSERVATIONS A coordinated approach to a variety of solar-physics problems requires that numerous observations be made at the same time. Many of these must be made from spacecraft flying outside the magnetosphere. Particularly relevant are very-low-frequency radio measurements, in situ observations of solar-wind plasma and magnetic field, and high-energy particle measurements. Specific attention should be given to the scheduling of launches of such payloads to optimize the scientific returns coordinated with the solar Shuttle missions. 2. DATA ANALYSIS AND THEORETICAL STUDIES Adequate and sustained support for the analysis of experimental data, as for parallel theoretical studies, is imperative if the data are to be used for increasing our understanding of the sun. This support must be provided for at the earliest planning stages. 3. GROUND-BASED OBSERVATORIES In the Shuttle era, solar astronomy will make increasingly heavy demands on the ground-based observatory capabilities at optical and radio wavelengths. The multiparameter observational detail required in order to develop an understanding of solar phenomena necessarily results in the integration of data from a broad variety of sources. Furthermore, as the understanding of basic solar processes unfolds, it is necessary to maintain the ground-based as well as the space-based solar capabilities at the forefront of technological sophistication.

Scientific Uses of the Space Shuttle 137 4. ROCKETS AND BALLOONS The return from the use of rockets and balloons for solar studies has far more than justified the cost. With the augmented payload capability and excellent pointing controls now available, these experiment platforms continue to provide an important part of a balanced solar-astronomy effort. As solar astronomy enters the Shuttle era, it is important that the rocket and balloon programs be continued, both for original solar studies and for the development of Shuttle-compatible instrumentation. Only after we are well into the operational Shuttle era will experience be available to permit a reassessment of the role of the rocket and balloon capability for solar studies. 5. SUPPORTING RESEARCH AND TECHNOLOGY In the past, the SR&T program in NASA has been pivotal in develop- ing and maintaining the solar-astronomy program and has underlain the excellent progress in understanding the sun and its influences. Sadly, the decrease in this type of funding in recent years has not only had an impact on established research efforts but has curtailed the investigation and development of new ideas that represent investment in the future. We most urgently recommend that SR&T support be maintained and augmented as a balanced part of the total NASA program. III. MISSION MODEL Table 17 is the mission model that we recommend to achieve the goals outlined. It is designed to meet the anticipated needs of U.S., European, and other scientific groups. It envisages a launch of the Solar Maximum Mission (SMM) in 1977/78, a schedule of sortie missions starting in 1980 with a buildup to four missions a year from 1983, and, starting in 1980, an annual schedule of new flights, revisits, and refurbishments of the free-flyer spacecraft originally designed for the SMM. A certain fraction of these would carry new payloads; some would simply replace consumables on the spacecraft. The initiation schedule (SMM in 1978, first sortie at the end of 1979) is set by the coming solar maximum and is more fully documented elsewhere. Over a 10-year period, the total number of dedicated missions will be 34 including the following: Solar Telescope Cluster (STC), 17 flights; Large Fine-Pointed Platform (LFPP), 17 flights; High-Energy

138 SOLAR PHYSICS TABLE 17 Mission Model Years Item 77 80 81 82 83 84 85 86 87 Missions on which SSO is prime pay loada 2 2 2 Missions flying the 1 1 1 3 3 3 3 3 SFPP only which are not included above^ Sortie flights of 13222222 opportunity for the SFPPC Solar Maximum Mission 1 (Large Solar Observatory) 1 aThese are dedicated missions for solar physics only. They might carry into space one of the four following packages: Pay load Average Annual Rate (full level of activity) STC + SFPP 2 LFPP + SFPP + FF 1 LFPP + SFPP + HESP 1 These are missions for which solar-physics payloads carried by the SFPP will fly with pay- loads belonging to other disciplines. eThese numbers assume that the number of rocket payloads launched per year in the Shuttle area will be maintained at the present level of activity in the United States, Europe, and Japan. The Solar Maximum Mission satellite will be launched in 1978. It will be recovered by the Shuttle in 1980, refurbished, equipped with updated instruments, and launched by one of the dedicated missions once every year. Solar Package (HESP), 7 flights; Free-Flyer Satellite (FF), 10 flights (or revisits); Small Fine-Pointed Platform (SFPP), 34 flights. The total number of missions flying the SFPP only over 10 years is 24; the total number of flights of opportunity for the SFPP for the same pe- riod is 21. The sounding-rocket program goal of 25 flights per year would continue through 1982 at least; its continuation beyond that must be a subject for study over the coming few years as the Shuttle sortie capability becomes more defined. Also envisaged is a Large Solar Observatory program with annual revisits, although the need for closer definition of this program is reflected in our parenthetical entry of this item in Table 17. A. The Pre-Shuttle Solar Maximum Mission Because of the timing requirement imposed by the 11-year solar

Scientific Uses of the Space Shuttle 139 cycle, we regard a free-flying satellite, with a carefully coordinated complement of instruments for the study of the next solar maximum, as the highest immediate priority item for solar physics. Solar activity may be expected to return in 1977 and reach a peak approximately in 1979, with the likelihood of observing major flares in a seven-day mission decreasing rapidly after 1981. We believe that an immediate start on this project is required in order to use this opportunity, which will not be repeated until 1990. The study of solar activity, especially flares, requires a wide range of instruments to cover the electromagnetic spectrum from visible wavelengths to several MeV, where solar nuclear gamma-ray lines have been observed. In particular, the study of the effects of nonthermal particles at high x-ray and gamma-ray energies requires specialized instrumentation that was not available during the last maximum in 1968 but that is now within the state of the art. Further, the high resolution that will become available simul- taneously in spatial and spectral properties of the thermal flare plasma with the generation of x-ray and euv spectroheliographs, which we believe can be developed in ample time for the Solar Maximum Mission (SMM), will allow studies that can be achieved in no other way. 1. DESIGN OF THE SPACECRAFT The SMM presents an opportunity to develop a standard solar free-flying observatory for the Shuttle era. The SMM satellite concept developed by the Goddard Space Flight Center seems to provide an excellent basic capability that can support the pre-Shuttle SMM and that has the growth capability to accommodate instruments of the class of the Shuttle Sortie Observatory in a free-flying mode. The SMM concept envisages a Delta-launched satellite with the capability of fine pointing of some 500 kg of instruments at the sun. The SMM concept will, by 1977, provide a pointed payload four times greater than OSO-I, with over twice the power, more than 10 times the viewing area for pointed instruments, enhanced pointing accuracy, and comparable telemetry and command capability. Such ca- pabilities, combined in a single spacecraft, will make it possible to achieve the scientific objects with a low-cost approach. We strongly endorse the SMM concept, not only for the pre-Shuttle SMM, which is of paramount importance, but as the basis for a flexible future series of the Solar Free-Flying Observatories.

140 SOLAR PHYSICS Alternative concepts for the spacecraft are not necessarily ruled out; however, it would be essential that the following requirements be met to provide a viable system: 1. The spacecraft should be available for use at the next solar maximum in 1977-1979. 2. The pointing stability should be better than 1 sec of arc over a period of 5 min. 3. The spacecraft should be designed as a revisitable and reusable free-flyer throughout the Shuttle era. 2. SELECTION OF PAYLOAD FOR SMM While we recognize that there may be strong constraints on funding experiments for this mission, it is obvious that the best science, which must be the principal objective of the mission, will not be accomplished by simply reflying experiments that have already successfully returned data, simply in the name of economy. The design of the spacecraft and the ample size and weight provision should allow new experimental approaches. We, therefore, urge that NASA ensure that experiment selection follow the proven method of open competition and impartial review. B. Use of the Space Shuttle for Solar Research The following sections summarize our recommendations for use of the Shuttle as a base for solar experiments and as a transportation system for free-flying satellites. 1. SORTIE MODE We have identified four basic solar-physics sortie payloads, which can provide the flexibility to accommodate the broad range of instrumentation required to implement the observational program outlined. Two payloads have been identified as basic multiuse facilities: the Solar Telescope Cluster, which provides a basic set of optical feeds for a variety of imaging, spectroscopic, and polarization studies between 8 A and 10000 A; and a High-Energy Solar-Physics Package, which can carry out similar studies between 1 keV and 100 MeV and with the high time resolution necessary to study nonthermal events. We have also defined two different size fine-pointed platforms that can accommodate a variety of specialized instruments. These four basic experiment packages are described in this section, and representative instrumentation is presented in Appendix A.

Scientific Uses of the Space Shuttle 141 TABLE 18 Characteristics of the Solar Telescope Cluster Wavelength Range Spatial Resolution (sec of arc) Type Aperture Length° Collect- ing Area 1200 A 0.1 at Gregorian 100-cm, 6 in 7500cm2 5 000 A //5 primary, ;/35 overall 300- 1 600 A 0.5 Normal-incidence 40 cm, 5 m 1250 cm2 mirror /yio 140-600 A 0.5 on axis Wolter type II 80 cm 5 m 1500cm2 8-300 A 1 on axis Wolter type 1 80 cm 5m 450 cm2 (X>20A) 1-40 keV 4 Oda collimator 50 cm 6 m 1000 cm2 Includes anticipated focal-plane instrumentation. (a) SOLAR TELESCOPE CLUSTER Since radiation emitted over the en- tire wavelength range from below 1 A into the millimetric range arises in different height and temperature regimes in the solar atmosphere, a battery of telescopes is required to carry out the needed research. Our recommendation for such a battery is summarized in Table 18, while a brief description of each component appears below. (Alignment of the entire battery on a given solar feature to within 1 sec of arc, as well as independent pointing of individual telescopes to any part of the solar disk, is required.) These specifications are presented as our desired goals; we well recognize that funding or technical constraints may delay the deployment of some elements of the ultimate cluster. . (i) OPTICAL TELESCOPE A 1-m-diameter, //35, diffraction- limited telescope yielding angular resolution of about 0.1 sec of arc at 5000 A is desired. A design goal should be to extend the technology of surface finishing so that the system can operate with similar angular resolution down to Lyman-a. Such a system has the advantage that it builds upon the technology of intermediate-size systems of 65-cm aperture, which are planned for flight in the next several years in stratospheric balloons, while representing a reasonable advance of performance. As a general-use system, such a heliograph should be equipped with a variety of final image magnifications as well as auxiliary devices such as spectrographs, filters, polarimeters, cameras, and magnetographs for investigations in the wavelength range from 1200 A to 1 mm. Such devices should be designed for modular installation of various combinations for

142 SOLAR PHYSICS differing scientific objectives. The use of such a telescope for nonsolar observations should be considered in its design. (ii) EUV TELESCOPE This instrument should be designed for maximum collecting area and greatest possible efficiency (i.e., minimum number of reflections), consistent with use of high- efficiency stigmatic spectrographs as subsidiary instrumentation. It should cover the range 300 to 1500 A with normal-incidence optics, designed to produce image quality better than 0.5 sec of arc within 1 min of arc of the optic axis. Such a system could then feed, for example, a stigmatic spectrograph of ~ 1-m focal length, which also, by rocking the objective by ±15 sec, would produce high-resolution spectroheliograms in a variety of lines. Other possible instruments that could be placed at the focal plane include (1) narrow-band filters (e.g., for Ly-a); (2) special-purpose spectrometers for measuring velocities, particular line ratios, or line profiles; and (3) polarimeters. Although the efficiency of normal-incidence optics drops seriously in the far uv, it is important that every attempt be made to extend the spectral ranges to include the strong He II 304 A line. Consideration should also be given, however, to extending the long-wavelength limit of the grazing-incidence Wolter type II telescope described below to overlap the 300- 1600 A range. (iii) X-RAY TELESCOPES Adequate coverage of the shorter wavelengths will require three individual telescopes. Two grazing- incidence imaging telescopes will operate longward of about 8 A; the shortest wavelengths are probably best covered by a nonimaging mechanical Oda collimator, although this possibility needs further study. The characteristics of the individual Wolter-type reflectors would be tailored to provide the maximum available effective aperture for each range. The short-wavelength limit of this system is strictly set by the brightness of the source; for solar flare studies, this telescope should be usable down to approximately 2 A. For imaging studies, the entire collecting area is available, and a spatial resolution of 1 sec of arc (or better) should be attainable over a 1-2 min of arc field. For spectroscopic studies, the different optics allow separate spectrometers to work in the wavelength ranges from 8 A to ~ 50 A and from 40 A to ~ 300 A. Limitations of collecting area may require spectroscopic or polarization observations to be carried out at lower resolution.

Scientific Uses of the Space Shuttle 143 The Oda collimator covers wavelengths too short for effective imaging, even at grazing incidence. It will require devices to raster or scan its field of view over the regions of interest; it normally will be used to feed spectrometers or polarimeters. (b) COARSE-POINTED HIGH-ENERGY MEASUREMENTS Comprehen- sive measurements of the characteristics of x-ray, gamma-ray, and neutron emission from the flaring and nonflaring sun would give insight into the triggering mechanism and total energy content of a flare (in conjunction with other measurements) and into the acceleration, containment, and release of charged particles. The recent OSO-7 discovery of flare-excited nuclear gamma-ray lines is indicative of the expected new results from future high-energy studies. Use of the sortie mode for these high-energy experiments permits observations to be made simultaneously with longer- wavelength experiments and accommodation of high weight and data rates. These ends could also be accomplished by an appropriate scheduling of free-flyers. The cost-effectiveness of both modes should be investigated. The intensity distribution and its variation with time and position should be measured for photons in the spectral range of 0.001 to above lOMeV. The flux, spectrum, and time history of neutrons should be measured; a representative set of instruments is specified in Appendix A. The measurements taken during flares will be of great significance when compared with simultaneous radio spectral and spatial measurements and with solar-particle measurements obtained by other spacecraft. (c) GENERAL-PURPOSE, FINE-POINTED PLATFORMS Several scientific disciplines will require oriented platforms for the Shuttle sortie mode. To carry the full range of possible solar experiments, we recommend that two pointed platforms be developed; the stability requirements for both platforms are 1 sec of arc, but they differ in size. The smaller fine-pointed platform should accommodate instruments up to 2 m long, and might, for example, be based on a half-pallet section. This unit could be flown on sortie launches with only a limited amount of unused space or load capacity. It represents an important component of the proposed facilities; it will be ideal for carrying the type of experiment now flown on rockets. It will accommodate larger and heavier payloads than do present rockets and will permit the evolution of current rocketborne experiments. Its

144 SOLAR PHYSICS early deployment would allow smaller scientific groups to participate in early sortie flights. The larger fine-pointed platform should accommodate instru- ments up to 2m in diameter by 4 m long and weighing up to 3000 kg. As part of the Sortie Solar Observatory it would, for example, carry large special-purpose instruments not included in the Solar Telescope Cluster or a problem-oriented package of several experiments of a size intermediate between current rocket or OSO- type experiments. The design of these platforms must be such as to allow payload development with minimal interaction with the Shuttle itself. A clean interface for power, thermal-control, data-transfer, and experiment control functions is important. (d) DEPLOYABLE RECOVERABLE FREE-FLYER A semiautomated free-flying pointing platform based on an evolution of the OSO series or the proposed SMM satellite is needed for some observational programs with duration well in excess of that of a single sortie flight. Additionally, some experiments require a higher freedom from contamination than is available on the Shuttle. A deployment recovery mode is likely to be the most efficient way of serving this class of platform since (i) there is no need to build a new spacecraft for every mission; (ii) the instrumentation can be returned, updated, recalibrated (with a high degree of confidence never reached up to now), and flown again; and (iii) spacecraft consumables and components can be replenished, repaired, or replaced within a short lapse of time. The SMM satellite should be designed with these needs closely in mind. (e) OTHER ASPECTS (i) SOLAR FLIGHTS OF OPPORTUNITY The payload carrying capability of the Space Shuttle may be used to permit observations from space in a piggyback mode at modest cost and with great flexibility. In this mode, experiments of an exploratory or develop- mental nature may be carried out on a space-available basis. For solar studies, this mode would make use of the general- purpose fine-pointed instrument platform described above. It is desirable that this platform be built as a modular independent facility to permit mounting into the Space Shuttle with minimum interference to the prime Shuttle mission. For effective and low-cost

Scientific Uses of the Space Shuttle 145 utilization of this mode, it is essential that this facility have clean and standardized interfaces with the Shuttle orbiter. We strongly recommend that the experiment accommodation management of this facility be as direct and informal as possible in order to promote maximum utilization at minimum cost and lead time. (ii) CALIBRATION Accurate instrument calibration is especially critical for solar observations whose analysis demands high photo- metric accuracy. For example, a powerful method of determining density or temperature of the solar plasma makes use of the accurate measurement of ratios of spectral line intensities—often at widely separated wavelengths—and for this the absolute values of these in- tensities are essential. The Shuttle sortie mode is well suited for achieving accurate cali- bration since, in principle, instrument calibration can be monitored during operation and a thorough recalibration made immediately after flight. (iii) THE ROLE OF MAN IN SOLAR SORTIE OBSERVATIONS The re- cent successful operation of manned space solar-astronomy experi- ments of ATM during the Skylab SL/2 mission has given needed per- spective on the role of man in future sortie solar observations. The man-instrument interaction on ATM takes three forms: as an ob- server, as an operator, and as a technician. As observers the SL/2 crew have shown themselves capable of educated and thoughtful choice of pointing coordinates within the solar features chosen for study and have made important real-time decisions such as when and how to observe transient phenomena such as flares. It is fair to say that the presence of educated observers at the telescopes has greatly enhanced the resulting data. As operators the crew have skillfully initiated complicated observ- ing sequences, many of which occurred out of reach of ground sta- tions and therefore could not have been initiated from the ground. It is fair to note, however, that if the Skylab had been in continuous telemetry contact, these operations could have been accomplished from the ground. As a technician man has been essential in Skylab; the crew erected a thermal shield, deployed a faulty solar power panel, overhauled an inoperative stellar uv experiment, repaired faulty voltage regulators, cleared the optics of the ATM coronagraph, repaired faulty experi- ment doors, replaced two jammed film cameras, and returned ex- posed film to earth. It seems probable that the usefulness of man as a

146 SOLAR PHYSICS technician will continue to be paramount in the Shuttle sortie mode. There seem to be very strong reasons, however, for carrying out observational and operational activities from the ground. These include the following: 1. Ground support of scientific operations can continue 24 h per day by rotation of ground personnel, thus substantially increasing the total observing time. 2. Consultations among a number of solar scientists on the ground before and during the observational sequences will improve the quality of the observations. Reasonably high data rates would be required to operate the ex- periments from the ground. However, this capability would also per- mit returning all or a sampling of the data to earth in real-time or near real-time. This leads to a third advantage of ground-based opera- tion. 3. Quick-look evaluation of the data within hours or at most a day of the observation will permit updating and improving observa- tions planned for later in the same mission. Experience on OSO's and ATM have proven the worth of quick-look data evaluation for mission planning. Therefore, we believe that it is important to provide the capability for ground-based evaluation, through use of a Tracking and Data Relay Satellite or other continuous high-data-rate system. One crew member should be thoroughly competent to make technical adjust- ments to the solar instrumentation; if he is also a competent ob- server, he might carry out observations directly as time permits, in close collaboration with ground-based colleagues. IV. REQUIREMENTS IMPOSED ON SHUTTLE AND SPACELAB BY THE SOLAR PROGRAM A. Contamination of the Optical Environment The Panel is concerned that the Shuttle may contaminate the local environment and the optical surfaces of many of the experiments because of the extensive use of volatile materials and the uncon- trolled dumping of wastes.

Scientiflc Uses of the Space Shuttle 147 The Panel recommends that NASA establish a Shuttle Contamina- tion Control Board to examine all materials, engineering approaches, and inflight procedures that may have implications for the contami- nation problem. Such a group could recommend modifications to assure that tolerable limits of contaminating gases and particulates are maintained. A similar Board operated for Skylab, and a consider- able body of observational data on this problem will be available from Skylab. B. Scheduling of Solar Missions Because solar studies typically make use of many coordinated obser- vations, it will be advisable to schedule solar sortie missions to co- incide with supporting ground-based observations. This will in gen- eral be during May-September, as most major solar facilities are in the northern hemisphere and are located at sites where the skies are clearer during the summer than the winter months. C. Orbital Considerations Solar sortie missions will in general make use of orbits requiring minimum fuel consumption in order to maximize available payload weight. Solar free-flyers should be put into orbits that will maximize re- covery and revisit opportunities. Sun-synchronus missions for studies requiring continuous coverage are also possible. D. Tracking and Data-Relay Satellite System (TORS) We believe that a data-relay system permitting nearly continuous contact with the Shuttle sortie is essential for maximum scientific productivity. Further, we believe that the Orbiter/TDRS wideband data link, which in the present mission model is regarded as optional, is indispensable to the effective use of the sortie mode and should be a part of the Shuttle program from the beginning. E. Payload Capacity It appears that the solar sortie will be limited by return payload weight for most missions, which will limit the ability to conduct coordinated experiments. We strongly urge that the weight landing

148 SOLAR PHYSICS capacity of the Shuttle be increased to as near the original goal as possible. F. Mission Duration Some solar missions will benefit greatly from longer missions, up to the full 30-day capability. The Shuttle should be designed to mini- mize the payload impact of such longer missions. G. Use of the Payload Specialist Station Flights of the Solar Telescope Cluster and other major solar payloads will utilize, on occasion, the payload specialist in an interactive role in the experiment. However, if the required console displays and controls are housed in a Spacelab pressurized module (as presently defined), it appears that the weight of that unit will seriously limit the size of the scientific payload and may, indeed, prevent flying the full Solar Telescope Cluster. Assuming that it is impractical to in- crease the permissible landing weight of the Shuttle, then the best solution seems to be to design the payload specialist console to allow adequate servicing of the scientific payload. H. Data and Control Interfacing It is recognized that the time available for payload integration with the Shuttle may be extremely limited. We suggest that these require- ments may be met if the pallet itself includes a general-purpose com- puter of substantial capacity (e.g., 128 kbits of direct-access memory plus mass storage capability of at least 1010 bits) that is used for experiment control and data management and the payload specialist console serves primarily as a terminal for this computer. The console should also include video and CRT displays, the latter for display of information from the computer. Although most control functions would be derived from the pallet computer, it is advisable to have several analog servo-control circuits included in the console for in- strument manipulation and limited analog readouts for critical ex- periment monitors. With such a design, all experiment functions and computer soft- ware could be integrated and checked out using a Payload Specialist Console Simulator prior to mounting the pallet in the Shuttle. Also, this approach will minimize mission peculiar modifications of the Payload Specialist Station, requiring only that the terminal- computer interface be standardized.

Scien tific Uses of the Space Shu ttle 149 V. GENERAL CONSIDERATIONS A. The Impact of Quality Assurance on Costs The Space Shuttle could substantially reduce the cost of transporting payloads to orbit, as well as increasing the number of flight oppor- tunities. To take advantage of these opportunities, the cost per pound of payload must be substantially reduced. Part of this saving may be achieved by substantially streamlining the documentation and verification requirements of present-day quality assurance pro- cedures. We recommend that a panel of experienced Principal Investigators and satellite and experiment program managers from the various NASA Centers and from NASA Headquarters be established to exam- ine the problem of quality assurance in the Shuttle era and to make specific recommendations on procedures for sortie instruments and for instrumentation on free-flyers. We believe that the basic quality assurance approach recommended in the Shuttle sortie model pre- sented by NASA is an excellent one and recommend that Principal Investigators work closely with NASA to implement this approach. We also recommend asking the proposed panel on quality assurance to consider the Solar Maximum Mission proposed for 1978, since this mission will be a prototype of the free-flyer of the Shuttle era. B. Convening of a Shuttle Experimentation Planning Committee Because the Shuttle and sortie laboratory are still in the planning stage, it is important to establish a continuing channel for exchange of information—e.g., payload accommodations, contamination con- trol, and pointing requirements—between the scientific community and the Shuttle and sortie laboratory planners. Accordingly, we rec- ommend that a committee of representative experimenters be set up for this purpose; this committee could be drawn, for example, from the existing U.S. and European working groups. We also recommend that these working groups be continued. C. Selection and Responsibilities of Scientists We consider that the successful construction and operation of indi- vidual instruments of any size is best accomplished under the super- vision of a single responsible scientist. The process of selecting ex- periments must avoid conflict of interest, be open at all program

150 SOLAR PHYSICS phases, and must reflect the requirement that observing time and data are to be made available to guest investigators. A promising start in defining management responsibilities in this area is set out in detail in the report of the NASA Payload Planning Working Group. D. The Crucial Role of SR&T Support Supporting research and technology provides, at modest cost, the basis from which flight programs grow. The Shuttle promises to pro- vide a splendid opportunity for deployment of new and exciting instruments. To produce these in time for solar maximum, a start must be made now on instrument development. Because of the large payloads carried by the Shuttle, a substantial effort is needed, requir- ing a corresponding increase in SR&T funding or special allocation of funds for Shuttle instrument development. VI. RECOMMENDATIONS 1. The occurrence of solar activity presents a unique opportunity to investigate a broad variety of energetic astrophysical processes and, in particular, to study the role played by magnetic fields in such phenomena. For this reason, it is crucial to exploit the forthcoming maximum in solar activity (anticipated for early 1979); an equivalent opportunity will not be repeated until at least 1990. We, therefore, recommend that the highest priority be given to the implementation of a Solar Maximum Mission (SMM ) satellite to be launched in late 1977 to observe the upsurge of solar activity and designed to permit uprating as a free-flyer in the Shuttle era. 2. The early data from the ATM have clearly demonstrated that instrumentation covering a wide range of the electromagnetic spec- trum is essential for a broad attack on the fundamental problems of solar physics. This concept can be used to great advantage on the Shuttle because of its high-payload and data-return capabilities. Such a wide variety of problems can be approached in this way that a series of missions is required, each having different specialized detec- tors at the focal planes of a cluster of generalized light collectors. We, therefore, recommend that a flight program be initiated with the aim of development of a Shuttle Sortie Observatory consisting of (a) a solar telescope cluster of large collectors covering a wide range of the electromagnetic spectrum and designed to feed different focal- plane instruments on different flights; (b) a small, fine-pointed plat- form for experiments of the rocket class; (c) a coarse-pointed pack- age for high-energy solar measurements.

Scientific Uses of the Space Shuttle 151 3. Certain important needs of solar physics are not met by the Shuttle sortie. Among these are (a) long-term, synoptic observations of such long-lived phenomena as active regions and coronal structures where moderate data rates suffice; (b) rare events, such as major flares, which can be studied only by long-duration observations; (c) contamination-free observations; and (d) observations for correlative purposes with observations from other spacecraft or from the ground. All these needs can be met by a free-flying spacecraft. The concept of periodic recovery, refurbishment, and instrument inter- change on the SMM spacecraft offers an attractive, flexible, and in- expensive solution to this need. We, therefore, recommend the creation of a solar free-flyer pro- gram based on Shuttle recovery and upgrading of the SMM spacecraft. 4. Certain important solar instruments such as coronagraphs and some special xuv devices require a large fine-pointed platform but are not adaptable to the general-purpose Solar Telescope Cluster. We, therefore, recommend the development of a large fine- pointed platform to accommodate these larger instruments. 5. Considerable cost savings may be realized by developing instru- ments usable by different disciplines and programming observations so that some of the powerful hardware developed in one area of astronomy can be used for observations in others. We, therefore, recommend that close attention be given at all planning stages to the possibility of development of modular instru- ment packages or interdisciplinary use. 6. The ultimate observational goals of solar studies make the eventual deployment of large instruments on a free-flying platform a most attractive possibility, particularly in view of recent spectacular Skylab, OSO, and ground observations. With the availability of the Shuttle to carry such large loads, the time is ripe to begin planning for such a program. We, therefore, recommend that a panel be convened under the auspices of the National Academy of Sciences to study all aspects of a Large Solar Observatory (LSO). 7. Because of the novelty and complexity of the Shuttle opera- tion, we recommend the establishment of a representative Shuttle experimentation planning board drawn from the disciplines to work closely with the Shuttle and sortie laboratory planners in defining experiment accommodations to be required. 8. Because of the severe weight penalty presently imposed by the use of the sortie laboratory module, we recommend that, as a prior- ity matter, adequate payload specialist console space be provided

152 SOLAR PHYSICS along with sufficient and data storage in the orbiter/sortie pallet mode. 9. Because of the planned operational mode, detailed specifically in the above text, we recommend that the fundamental importance to solar-physics missions of a wideband Shuttle/TDRS relay satellite capability be kept closely in mind in all planning stages. 10. The extent to which the potential of the Shuttle is realized in advancing space science depends intimately on the degree of con- tinued input of the scientific community, especially during the plan- ning stages. The discipline working groups constituted by NASA have set a sound direction for such communication. We, therefore, rec- ommend that a continued and close interaction between scientists and planners be recognized as an essential component in Shuttle development and that appropriate mechanisms (e.g., discipline work- ing groups) be established to ensure this interaction. APPENDIX A 1. Representative Focal Plane Instrumentation for Use with the Solar Telescope Cluster0 Weight (kg) Size Data Rate (bps) Average Power (W) Telescope Instrument (m) a Gregorian (>I200 A) Universal filter camera 20 0.2 x 0.2 x 0.6 10M 100 (or film) Video magnetograph 20 1 x 0.25 X 0.2 3 x 105 (or film) 100 High- resolution spectrograph 500 0.5 X 0.5 X 6 12M (or film) 50 b. Normal incidence telescope (1600-300 A) Spectrograph/ spectroheliometer 270 0.7 x 0.6 x 4 100K 100 c. Grazing incidence telescope (140-600 A) Spectrometer/ spectro heliograph 150 0.2 x 0.4 x 2 IK 15 (or film) d. Grazing incidence telescope (8-300 A) High-resolution spectrograph 250 3 X 0.5 X 0.5 20K SO Filter camera 50 0.5 x 0.5 X 1 IK 50 (or film) flNumbers do not include auxiliary equipment such as electronics racks, computers, and monitors. The weight and power needed for these would be ~200 kg and 500 W continuous, respectively.

Scientific Uses of the Space Shuttle 153 2. Representative Special-Purpose Instruments for a Large Fine- Pointed Platform Instrument Size (m) Weight (kg) Data Rate (bps) Average Power (W) LI. Externally 2x 0.5 x 0.5" 200 10M or film 50 occulted coronagraph b. Concave O.S X 0.5 x 4 200 20K SO grating spectroheliograph c. Polarimeter O.S x 0.5 x 4 250. 1000 so Also requires occulting disk at 10-20 m distance. 3. Representative Instruments for a Small Fine-Pointed Platform The following instruments will be of within a factor of about 2 of current rocket payloads in their weight, volume, power, and data requirements: uv high-resolu- tion echelle spectrograph, matched double (normal and grazing incidence) spec- trograph, monitor instrumentation in support of Solar Telescope Cluster instru- ments (e.g., H-a heliograph x-ray spectrum monitor), developmental instruments (e.g., xuv magnetograph). 4. Representative Coarse-Pointed, High-Energy Instrumentation Instrument Type MeV Energy Range Required Pointing Primary Measure- ment Objective Weight, Power, Volume, Telemetry Proportional counters and/or cooled solid- state detectors 0.001-0.050 -y Spectral (atomic line and continuum) 90kg, 10W, 0.13 m3, 3 kbps Actively shielded scin- tillators Csl and Nal 0.030-0.600 "y Spectral (con- tinuum) 450kg, 20 W, 1 .0 m3 , 3 kbps Actively shielded scin- tillators Csl and Nal 0.300-10 "s° Spectral (con- tinuum) 900 kg, 20 W, 1.0m3, 1 kbps Actively shielded cooled solid-state device 0.100-10 -5• Spectral (nuclear line) 700kg, 20 W, 0.64m3, 4 kbps Li and Be scattering block polarimeters 0.001-0.005 0.005-0.030 0.030-0.200 1* Continuum polarization 100kg, 15W, 0.34m3, 3 kbps Bragg reflection crystal polarimeter/ spectrometer 0.001-0.010 I* Line and continuum polarization 40kg, 35W, 0.13 m3, 1 kbps Neutron-sensitive scintillators 1-100 Coarse Solar neutrons 230kg, 25 W, 0.8 m3, 1.5 kbps 5. Representative Instruments for a Large Fine-Pointed Platform The LFPP is required for flights of instruments of the following representative types-ATM instruments: balloon payloads, which are too large for the SFPP; large instruments under development for eventual use on the SRC or the free- flyer; arrays of medium-sized problem-oriented experiments. On sorties when the STC is not available, the LFPP may be used for flying large instruments such as the coronagraph and polarimeter.

8 Life Sciences I. INTRODUCTION The Shuttle era will provide the first opportunity to carry out a thorough experimental program in the life sciences in space under conditions approximating those of ground-based laboratories. The Skylab experience has already demonstrated the feasibility of performing many kinds of general experimental manipulations under weightless conditions and has provided considerable support for the concept of a manned space laboratory in which sophisticated biological and medical experiments can be done. The ability to manipulate experimental material directly rather than by automated remote control alone is essential in the life sciences, and the capability for immediate follow-up of new experimental findings is an important facet of any effective biological experimentation. Having these attributes the pressurized laboratory concept con- stitutes a major advance in capability for definitive life-sciences investigations in space. In addition to biomedical investigations relevant to man's well-being in space, basic principles of biology and medicine can be examined using the 0-g environment as a research tool. The laboratory will provide the operational conditions neces- sary for the evaluation of components of advanced life-support systems and of man-machine integration technology. In the following sections are outlined examples of investigations in life sciences that are necessary to assure man's safety and to define further the conditions in which he can take part in spaceflight or that utilize the space environment to learn more of basic biological processes on earth. Shuttle capabilities and requirements in these studies are discussed. Studies relative to life elsewhere in the universe (exobiology) are not discussed because Shuttle capabilities here, other than as a launching or collecting platform, are minimal. 154

Scientific Uses of the Space Shuttle 155 Observations of the terrestrial environment (e.g., ecology, agricul- ture) are omitted as earth applications were excluded from the Study's charge. It is recognized that the scientific questions asked are based on 1973 concepts and technology and that maximum flexibility must be maintained for restructuring the questions and protocols with the development of new knowledge. II. CELLULAR AND MOLECULAR BIOLOGY Theoretical and experimental grounds for predicting detectable effects of 0 g at the cellular, subcellular, and molecular levels are still fragmentary. The experiments of Biosatellite 2 and other early flights have given some clues to potentially significant effects of weightless- ness at the cellular level, and early Shuttle flights should include well-designed and well-controlled experiments to confirm and con- solidate potential problems identified by observations in these early experiments. The weakness of the gravitational force relative to other physical and chemical forces operating over molecular dimensions suggests that 0 g is not likely to affect directly kinetic properties of bio- chemical reactions in vitro in a detectable or physiologically significant way. Similarly, the small size of procaryotic microor- ganisms would be expected to minimize potential gravitational effects. In any event, procaryotes cannot be used as models for eucaryotic systems, because of major differences between bacteria and higher forms in size, cellular organization, and mechanisms of fundamental processes such as cell division. It seems more likely that gravity may significantly influence complex intracellular or intercellular processes involving oriented supramolecular structures such as the mitotic apparatus. Indeed, both U.S. and Soviet studies have suggested a slight increase in random chromosome aberrations and mitotic abnormalities in response to spaceflight conditions. Significant perturbation of the process of cellular replication in the absence of gravity would be of clear importance to the ultimate understanding of fundamental mechanisms; it could also have serious consequences as a source of potential disturbance in normal processes of cellular proliferation and turnover during long-duration spaceflight. Studies of the kinetics of cell growth and cell division in plant and animal tissue culture and in rapidly proliferating tissues in vivo (e.g., bone marrow, skin, intestinal epithelium) will be required to assess quantitative and qualitative effects of the space environment and 0 g on cell

156 LIFE SCIENCES replication and turnover time, including parameters such as chro- mosome replication, function of the mitotic apparatus, and cy- tokinesis. It would be of interest to test responses to mitogens such as phytohemagglutinin as a probe for potential alterations in control of the division cycle. Wound repair involves another sort of proliferative system appropriate for study in 0 g: the rate of repair of a cored skin wound has been very carefully analyzed under conditions of normal gravity as has the repair of mechanical injury to bone marrow, and these systems could be used to study the kinetics of cell growth and division in 0 g. Investigations of embryonic development under 0 g (see Section III on Organismic Biology) may also give information pertinent to the cell-division question. An on-board centrifuge providing variable g in the range of 0 to 1.5 g and capable of handling both tissue cultures and small animals of the size of mice or rats will be essential for these studies. The Spacelab environment would not seem to offer any special opportunity for genetic studies other than those concerned with cell division and chromosome replication. The suggestion of synergism between radiation mutagenesis and weightlessness is probably neither a genetic nor a radiation problem but a problem in molecular or cellular reactions or both to 0 g. It is not certain at this time that a better understanding of such 0-g effects would have any importance for molecular or cellular genetics per se, but the possibility should be kept in mind for long-range planning. It is not clear to what extent gravity may influence processes involving cellular and intracellular movement such as cytoplasmic streaming, rapid axonal flow, and amoeboid locomotion. Phase- contrast microscopy and photomicrography provide simple experi- mental means for investigating these questions. The intriguing possibility of gravitational effects on membrane- mediated processes deserves careful consideration. At the present time there seems little reason to anticipate significant gravitational effects on carrier-mediated solute transport across biological mem- branes. Current information suggests that convectional forces are not important to the mechanism of biological membrane transport systems and that gravitational effects on the molecular organization of biological membranes should be negligible. For these reasons, investigation of possible effects of 0 g on the kinetics of carrier- mediated transport does not appear to offer a particularly fruitful area of study. However, study of electrolyte and water transport in model systems such as the toad bladder may be appropriate in

Scientific Uses of the Space Shuttle 157 relation to the problem of redistribution of fluid volume and electrolyte balance in man. It is of interest to examine effects of weightlessness on phenomena involving specific cell- cell interactions, such as cell sorting in sponge and embryonic tissue and differentiation in the cellular slime mold. Density-dependent (contact) inhibition of growth in tissue culture would also be of interest. III. ORGANISMIC BIOLOGY In considering the potential effects of the space environment on the organism as a whole, a central scientific question is the long-term influence of weightlessness on growth, development, maturation, and reproduction. This question is conceptually the same for both plants and animals but for convenience is discussed sequentially below. Other major questions—effects on individual body systems of the mature organism and effects of radiation—are treated in Section IV on Biomedicine. A. Rant Biology The classic picture of the influence of gravity on plants is that of the positive and negative geotropism exhibited by stem and root tissue. Obvious questions have arisen as to what is the receptor mechanism by which plants perceive gravity and what are the mechanism(s) that mediate the tropic response. Although numerous ground-based experiments have been performed, no single hypothesis has been formulated that explains or clarifies the phenomenon to the satisfaction of a majority of scientists. Studies carried out on Biosatellite 2, although providing rather uncertain results, indicated that ground-based control experiments employing the clinostat gave results comparable with those carried out at 0 g. We agree in general with the consensus of earlier study groups that future experimen- tation should be largely limited to precise ground-based experiments aided by clinostat studies to mimic weightlessness. However, it should be emphasized that the validity of the clinostat as an adequate model of 0-g conditions for plant studies must be unequivocally demonstrated. A variable-g centrifuge in the pres- surized laboratory would strongly aid in this evaluation. Geotropic experiments should be performed only as part of experiments designed to examine other aspects of plant and cell

158 LIFE SCIENCES development that are thought to be influenced by gravity. Only those experiments that have had the most thorough exploration under l-g conditions should be considered for analysis at 0g. As part of this evaluation, it is important that thorough analysis and review of any potential flight experiment by a good cross section of the concerned section of the scientific community be obtained. Equal attention should be directed toward growth and develop- ment of plant cells in Og. For example, does the absence of gravity influence the morphogenesis of plant cells and affect chromosome replication to alter the mechanism of cell division? Previous flight studies have indicated that chromosomal aberrations do occur, and clinostat data suggest alteration of certain enzyme activity (gluta- mine synthetase) and of tracheid formation. Additional information as to the cause of such effects might contribute to a more tenable explanation for the ability of plants to respond to a gravitational field. In preparation for Shuttle experiments, a thorough study should be made of the types of plants that would provide the most useful information concerning the wide range of phenomena expressed by plants as a function of gravitational field. Many of the experiments proposed for 0-g conditions encountered in spaceflight are directed toward short-term responses of organisms because of the limited duration of available flights. Although results from short-term 0-g experiments will augment thinking about possible effects produced by longer exposure, the obvious question, derived from extrapolation of the possible effects of weightlessness on chromosome replications, is whether plants can grow and develop normally in long-duration spaceflight. For example, can a microalgae population continue in a normal fashion after several generations at 0 g? Such information would be essential if one were to propose plant systems as secondary life-support systems in long-duration spaceflights. If malfunction of cell division is induced by extended 0 g, populations of microalgae and higher plants would diminish, photosynthetic capacity decrease, and the support systems ulti- mately fail. Long-duration growth and development experiments might also provide important evidence for the possible evolutionary mechanisms of terrestrial plants. Plants that are able to complete one or more life cycles within 30 days, such as Aribidopsis, or that are easily grown in tissue culture (carrot, tobacco) or that produce extensive xylem proliferation (sunflower) would be typical objects for experimentation.

Scien tific Uses of the Space Shuttle 159 B. Animal Biology Thirty-day Shuttle flights and recoverable free-flier flights of six months or more provide an excellent opportunity for long-term Q-g experiments with animal systems. Results of complete life-cycle studies on small mammals such as mice and rats should provide important information, transferable to man in many cases, on physiological effects of weightlessness. A unique feature of the 0-g conditions in the pressurized laboratory will be the opportunity to determine in aqueous medium the morphogenic characteristics of some simple metazoans (hydra or perhaps copepods) and to make further studies of protozoa. The purpose of this type of experiment would be to evaluate whether the earth's aquatic and marine environments are truly 0 g in their morphogenic effects. Similar studies on the fine structure of various diatoms would represent an alternative way to examine this interesting aspect of possible gravitational effects on the develop- ment of aquatic organisms. The placental mammal in utero, like aquatic animals, might be considered as developing in a generally omnidirectional force field. If so, then gestation in a weightless environment should have little or no effect on the developmental processes of organogenesis and fetal growth. However, the suggestion is speculative and should be tested. For example, do blastocyst formation and implantation have any dependence on external force fields? To what extent might critical delineations of organ systems be thus dependent? Is parturition at all gravity-dependent? Postnatal growth and development in 0 g should then identify the adaptive sequence to which we are committed by our terrestrial confinement. For example, if cardiac "decon- ditioning" is solely a function of 0 g, then a mammal brought from birth to young adulthood (~ 25- 30 days for the mouse) in the absence of gravity might be irreparably "deconditioned." Aspects of mineral metabolism and concurrent musculoskeletal development might be clarified by studies of early postnatal growth in 0 g. To carry out these studies, animal-holding facilities are required for small mammals and selected avian species. An on-board cen- trifuge with a range of 0.1 to 1.5 g should be available for the smaller species to define effects of fractional g. Dissection and preliminary fixation of tissue samples would be required. Mission duration should be no less in duration than the period from fertilization through parturition of the animals in question; and a polar orbit, with concomitantly higher levels of radiation, should be avoided.

160 LIFE SCIENCES Study of the behavioral patterns of lower vertebrates, perhaps small fish such as goldfish or guppies when exposed to 0 g in an aquatic environment, would provide additional information on the ability of such organisms to maintain themselves in the appropriate pressure/buoyancy gradient. This type of study would give insight into the mechanism of function of the swim bladder and possible other position-orienting sensors in aquatic vertebrates. A preliminary demonstration on Skylab 3 using minnows indicated that fish brought into space adapted fairly quickly and that those born in space were adapted to 0 g from birth. Whether they can subsequently survive under 1 g is not yet clear. Rhythmicity in biological phenomena has fundamental im- portance in all life-sciences disciplines. Processes that are cyclic in their time response (circadian rhythm) are found over a broad range of responses, i.e., from nucleic acid synthesis to complicated stress reactions in plants, animals, and birds, and have had extensive study in biological systems on earth. This topic was covered in depth by the Santa Cruz study in 1969,* and some of the experiments proposed there were flown on Skylab but failed because of electrical malfunction. While they should be studied under the unique circumstances provided by spaceflight, several circadian rhythms have been shown in ground-based experiments to be genetically determined and genetically modifiable rather than governed by external processes.! This renders experiments on circadian rhythms in actual spaceflight less critical. Barring observations on the ground or in space that override the work of Konopka and Benzer, a lower priority for this type of biological experimentation is recommended. IV. BIOMEDICINE The space environment offers new experimental approaches to analysis of mechanisms of a variety of fundamental physiological control systems. In addition, certain human (and mammalian) functions have become deranged in space, and corrective or preventive measures must be sought in order that man may perform adequately. Other functions offer special advantages for investigation of the 0-g condition. *Space Science Board, Space Biology (NAS, Washington, D.C., 1970), Chap. 2. tR. J. Konopka and S. Benzer, Proc. Nat. Acad. Sci. (U.S.), 68, 2112 (1971).

Scientific Uses of the Space Shuttle 161 A. Cardiovascular System Prolonged spaceflight is accompanied by complex physiological changes, and evidence to date suggests that these changes may be manifest most in the cardiovascular system. Disturbances that can be related to the cardiovascular system have consistently occurred in astronauts and cosmonauts and have been primarily evident as a decreased tolerance to the burden of gravity after re-entry to earth. The regulatory pathways that normally maintain arterial blood pressure and peripheral blood flow on earth necessarily are still present at 0 g, and some alteration has apparently taken place, rendering compensation for 1 g less effective. The essential responses of heart rate, strength of contraction of the heart muscle, and peripheral vascular resistance are the result of complex reflex interactions among (a) the mechanoreceptors in the heart, lungs, aortic arch, and carotid sinus; (b) the central nervous system and autonomic nerve outflow; and (c) the smooth muscle of the blood vessels and the heart, all reacting to maintain blood flow to the vital organs. Experiments in both animals and humans will be needed to determine which parameters must be monitored inflight to detect functional changes in the cardiovascular control system and what procedures are effective in combating or preventing these changes. Examples of pertinent experiments that could be carried out in the pressurized laboratory are measurement of peripheral blood flow by venous occlusion plethysmograph during lower-body negative pressure (LBNP) or during a deep breath and determination if exercise, isometric exercise, or LBNP prevent the abnormal cardi- ovascular regulatory responses that are seen after exposure to weightlessness. It is presumed (see Section IV. C on Kidney and Metabolism) that substantial loss of sodium and water occurs on entry into 0 g. If so, it may be initiated by the movement of extravascular fluid into the capillaries of the lower body under osmotic forces. Experiments are needed to determine the validity of this hypothesis, particularly in relation to the long-term changes that can be expected. There are reasons to expect that the pressure and volume workload on the right heart might be reduced in space, leading to some decreased efficiency. This could be critical on return to the 1-g environment after prolonged residence at Og. Experiments should be designed to study possible atrophy of cardiac and other muscles.

162 LIFE SCIENCES B. Respiration There are no apparent reasons to expect any significant alterations in cellular respiration, gas diffusion exchange, or control of respiration as a result of 0 g except for changes secondary to any different total body energy requirements. As the atmosphere of the orbiter cabin and pressurized laboratory will be air at sea-level pressure (20% oxygen, 80% nitrogen at 760 mm Hg), with the partial pressure of carbon dioxide maintained at less than 7 mm Hg, the basic composition of the gas that the crew and passengers will inspire presents no difficulty. However, as discussed in Section VII on Life Support, the atmosphere must be monitored assiduously and excess gases and particulate matter removed. The absence of gravity will alter the mechanical function of the lungs. Lung function should be studied after a protracted stay in space to determine if there is significant deterioration of muscles and supporting structures. The lack of hydrostatic pressure in the pulmonary circulation may alter the distribution of capillary blood flow/alveolar ventilation through the lungs and decrease the effective pulmonary vascular resistance. Spaceflight provides a unique oppor- tunity to study these effects. Ciliary transport, lymphatic drainage, and phagocytic functions may become abnormal after exposure to 0 g. These mechanisms are of great importance in protecting the lung from infection and are deserving of study. C. Kidney and Metabolism It is our present hypothesis that, with the entry of man into a weightless environment, there occurs a rapid translocation of interstitial fluid into the intravascular space with an attendant activation of volume control mechanisms leading to an increased rate of renal excretion of sodium, chloride, and water. With the net contraction of extracellular fluid volume, there may follow an increase in the rate of aldosterone secretion leading to an increase in the renal excretion of potassium. If the excretion of potassium exceeds the concurrent rate of intake, total body potassium depletion will ensue. The latter could have widespread physiological effects on the cardiovascular system (as may have occurred in Apollo 15) and on the rate of secretion and effects of certain hormones such as insulin and growth hormone. The perturbation of volume regulation imposed by the weightless state offers the opportunity to

Sctentific Uses of the Space Shuttle 163 examine the component parts of this complex control system in a manner that is not possible on earth. The astronauts have shown a negative calcium balance in space, presumably analogous to that seen in prolonged bedrest at 1 g. Although the calcium loss has not been accompanied by signs of general demineralization of the bones, it is important to determine if it will continue for as long as man is at 0 g or whether it levels off or can be made to do so. It may be possible to obtain information about calcium metabolism as well as about any perturbations in the rate of bone formation in 0 g by monitoring the repair of minute mechanical injuries made to the bone marrow of small animals. The rate of repair on earth has been very carefully timed. It provides a useful model because it involves the deposition and resorption of cancellus bone. The absence of gravitational force may reduce the total basal energy requirements of the body, which would produce a lower metabolic rate, or qualitatively altered metabolism, detectable for example as a decreased food intake or lower body temperature. D. Hematology There has been a consistent decrease in red-blood-cell mass in the astronauts produced by increased destruction of red blood cells as well as a suppression of the bone marrow's normal compensatory response. It has been generally assumed that the basic cause was the hyperoxic atmosphere breathed by the astronauts, because ground studies in similar atmospheres were able to duplicate the effect. How- ever, an anemia appeared in the first two Skylab missions despite a normal inspired O2 tension, suggesting the influence of other factors. It is important to uncover the cause of this decrease in red-cell mass and to determine whether it plateaus or progresses with the stay in space. Studies should be done to learn if the rate of turnover is changed and also to learn the rate of granulocytic response to in- flammatory stimuli. E. Neurology It was anticipated that 0 g would affect the vestibular apparatus. Astronauts have indeed had some difficulty with motion sickness, but except in Skylab 3 it has been relatively transient; susceptibility is not predictable from ground-based tests. Recent results from Skylab 2 and 3 indicate that once adaptation has taken place the

164 LIFE SCIENCES otolith organ is functionally denervated in 0 g because moving the head forward and sideways while rotating in a chair (a maneuver that leads to symptoms of motion illness at 1 g) produced no effect. Advantage should be taken of the 0-g environment to study vestibular functions in man and animals. Other neurophysiological dysfunctions may appear, secondary to the changed relationships between the antigravity and other muscle groups or to the altered afferent input to the cardiovascular system. Animals raised from conception in 0-g environment might show important neurophysiological defects at 1 g, because development of the nervous system is critically dependent on the presence of the relevant stimuli at certain stages. F. Microbiology Microbiological problems in spaceflight divide themselves into two major categories: those concerned with interactions of microbial flora within an enclosed system, including possible alterations in host resistance, and problems arising from the possibility of mutation during flight leading to the emergence of additional pathogenic microorganisms. The possibilities of mutation leading to new pathogens have been explored, and, although this must always be considered, the risks do not seem much greater than they would be on earth. It seems likely that alterations in host resistance attendant upon prolonged space- flight and the spread of microorganisms within an enclosed space will be more important considerations. Although there is a tendency to equate host resistance with antibody formation, function of phagocytes, and activity of im- munocytes, it is often forgotten that the first lines of defense consist of relatively nonspecific reactions, such as the intact integument, rinsing mechanisms such as tears and other secretions, the motility of the gut, the extremely active capacity of the alveolar macrophage system to kill inhaled bacteria in aerosols, and the antibacterial mechanisms in mucous membranes. It is still not clear whether 0 g will have a deleterious effect on some of these defenses, such as rinsing mechanisms or the capacity of the lungs to clear inhaled bacteria because of altered deposition of aerosols in the lungs. A major potential problem is gastrointestinal disturbance. It is known that the motility of the gastrointestinal tract is a major factor in keeping the duodenum and jejunum relatively free of bacteria. The degree to which such motility and the movement of bacteria will be

Scientific Uses of the Space Shuttle 165 influenced by 0 g remains unexplored. There is increasing evidence that prevention of and recovery from disease are dependent on cell- cell interactions and cellular immunity. Study of the effect of 0 g on these cellular functions relative to host resistance is thus very important. Specific immune responses to spaceflight have been monitored in previous flights and do not seem to have been greatly disturbed. However, we must remain alert to this possibility. Preflight isolation of astronauts and immunization procedures have reduced almost to zero the incidence of inflight illnesses, and it is mandatory that precautions of this nature be continued in Shuttle flights. They will require continuing review and adjustment in accordance with changing knowledge and increasing experience. Epidemiological study of the environments of the flight personnel and their families during the preflight period would also appear to be essential. Conditions favoring the exchange of microorganisms among individuals are not yet well understood on earth let alone in space, and further work is needed so that control measures can be fashioned if such exchange proves harmful. The life-support systems should remove viable microorganisms from the Shuttle atmosphere or kill them as the most practical way of reducing this potential hazard. Finally, the possibility of replacing the usual earth flora in the microbial environment with relatively innocuous organisms should be given much more attention than it has received so far. V. BEHAVIOR Although much indirect evidence has been accumulated about man's ability to perform in spaceflight, there is a need to quantify man's performance in order to plan properly for his effective utilization in all modes of spaceflight activity. Most of the planned Shuttle missions have tasks that might be used to investigate and quantify the human operator's visual, mental, and psychomotor performance and levels of stress, boredom, or fatigue. The development of precise tests that are able to discern subtle decrements in performance, and perhaps changes in behavior, will be necessary in preparation for longer flights when the ability to predict decrements will be important. Study of existing tapes of communications during flight should yield detailed observations on the interchanges among personnel and permit the study of interpersonal interactions and relationships. The data should identify reaction patterns between

166 LIFE SCIENCES colleagues under these unique conditions and perhaps suggest improved personnel screening techniques. Preliminary findings indicate that quality of sleep is not signifi- cantly altered in spaceflgiht, whereas there may be an alteration in sleep quantity. Regardless of what may be learned concerning sleep during Skylab missions, the question of mental state, i.e., alertness, and performance in relation to sleep, will not have been answered. The Shuttle flights provide an excellent opportunity to investigate this relationship. The participation of a scientist-passenger population with profes- sional astronauts in the Shuttle flights will require modification of selection criteria and screening procedures and, finally, assessment of the techniques for evaluating the performance of those individuals ultimately selected. The results of such investigations would serve the dual purpose of validating the selection criteria and providing insight for improving subsequent screening methods for future participants in spaceflight missions. In addition, studies should be made to determine how much and what kinds of training are necessary to enable the scientist-passenger to tolerate and perform well under the unusual environmental conditions in space. VI. RADIOBIOLOGY The radiations encountered in spaceflight can present a hazard to man during long-duration flights where flight trajectories forbid quick return. There are no unique radiations in space, with the exception of the heavy-ion or HZE-particle component. Thus, pres- ent understanding of radiation effects is quite adequate to permit accurate prediction of the consequences of space-radiation exposure and to prepare countermeasures as needed or appropriate. There are two areas of possible exception: the biological effects of HZE-particle irradiation and the possibility of synergisms between radiation and other elements of the space environment. The first is a matter of ascertaining if randomly located microlesions, induced by HZE particles being stopped in deep tissue, might cause sufficient cumulative damage to nonproliferating cells that pathological conse- quences might ultimately result. Could HZE-particle hits, for exam- ple, induce a detectable loss of retinal function? For both scientific and practical reasons HZE-particle effects are best studied initially in ground-based laboratories. The studies required and the methods of study are described in detail in a

Scientific Uses of the Space Shuttle 167 recently issued Space Science Board report.* One strong recom- mendation has been made in that and other related reports: basic studies on radiation effects should not be attempted in flight laboratories. Ground-based studies with accelerator-produced heavy ions are thus the first requirement. NASA has the operational problem of assessing the potential hazard of HZE particles to man during long- duration space missions and should therefore take full advantage of the ground-based facilities that are becoming available. Flight studies can then be designed for proof-of-principle. If flight experiments on HZE-particle effects do develop sufficient priority as a result of the earth-based work, then a polar orbit should be sought. With minimal shielding, about 3 iron nuclei might be stopped per day in each gram of tissue. At lower orbital inclinations, the geomagnetic cutoff would reduce the expected number of hits by a factor of 10 or more. The exposure of biological specimens to the ambient flux of heavy particles should not be encouraged on the assumption that the HZE- particle microlesions will be readily observable in deep tissue and their pathologic consequences then identified. In fact, it is forbid- dingly difficult to detect such lesions because of their minute size and random location. All significant HZE events (thindowns or stopped particles) impinging on the biological target must be recorded and geometrically correlated with the target so that the precise location of the microlesion can be anticipated. The Biostack and Biocore experiments of the Apollo program are examples of two methods for correlating physical and biological events. Additional techniques should be sought, especially for lesions in deep tissue. The question of synergism between radiation injury and weight- lessness, or other flight-related factors, largely raised by some of the uncertain results of the Biosatellite 2 experiments, cannot logically be resolved by immediately embarking upon a series of flight experiments. There seems to be evidence accruing from both U.S. and Soviet studies that 0 g has an impact on cell division. The evidence is seen in the form of small increases in certain chromosome aberrations, spindle misorientation, and cell lethality (of chromo- somal origin?), but the mechanism that might produce these findings is not clear. The molecular basis of 0-g perturbations of cell division *Radiobiological Advisory Panel, SSB Committee on Space Biology and Medicine, HZE-Particle Effects in Manned Space/light (National Academy of Sciences, Washington, D.C., 1973).

168 LIFE SCIENCES should first be thoroughly studied (see Section II on Cellular and Molecular Biology) before studies of synergism are undertaken. Some critical studies might be possible on molecular aspects of radiation target theory and the many associated concepts of genetic and cellular radiation injury. These, too, must be preceded by more complete evaluation of strictly 0-g effects on cell metabolism and reproduction. Ultimately, sophisticated multivariate radiobiological experiments for flight could be conceived. These would require on-board radiation sources, both low- and high-LET, the capacity for in vitro and in vivo cell labeling with tritiated and carbon-14 labeled compounds, and the use of both cell cultures and small mammals. While such studies would best be done in orbits of about 30° inclination to avoid the cosmic radiations as much as possible, studies on heavy-ion effects, as noted above, will require a polar orbit. The latter, however, may also best be designed not to require any complex inflight manipulation but to allow a maximum buildup of particle hits for later evaluation on the ground. All Shuttle flights should contain dosimetric devices and materials positioned to accumulate data on the flux and energy of the several radiations in space and the shielding effect of spacecraft components on their intensity and scattering. While the influence of these components on the actual radiation levels in different parts of the spacecraft can be predicted to some degree, it is necessary to test these expectations against observations. VII. LIFE-SUPPORT TECHNOLOGY Requirements for atmospheric control in spacecraft are (a) to supply oxygen in the amounts needed to sustain life; (b) to maintain appropriate temperature, pressure, and relative humidity; (c) to remove carbon dioxide and water produced by metabolism; and (d) to monitor and remove trace contaminants and particulate matter. With the present state of the art, for the short-duration missions presently planned for the Shuttle, open-loop systems, in which oxygen is supplied and wastes are removed and stored, weigh less and use less energy than closed-loop systems in which wastes are recycled. From the standpoint of weight and power requirements, the present breakeven point between open-loop and closed-loop systems is about 30 days.

Scientific Uses of the Space Shuttle 169 If development of recylcing systems proceeds, there is a high probability that in a very few years such systems will be able to compete with open-loop systems for missions of two weeks or even less. A wide variety of systems have been proposed for the recovery of oxygen from both carbon dioxide and water under the conditions prevailing in space. Examples of such systems include chemical conversion (e.g., Bosch, Sabatier), electrochemical (e.g., fused car- bonate, solid electrolyte), electrolysis cells, and bioregenerative systems. None of these systems is yet at the point of acceptability in space from the standpoint of weight, energy requirement, or reliability, although substantial progress has been made. It has been suggested that the lack of gravitational force would affect transport through artificial membranes, an important process in many proposed life-support systems. The rate of transport of a molecular species across a membrane system can be dependent on gravity only if gravitational forces affect concentration gradients in the system. This might occur, for example, if gravity-dependent convection is required to prevent the development of concentration gradients in the boundary layer adjacent to the membrane. However, it is presently possible, within the current state of the art, to design out any adverse effects caused by 0 g. Forced convection and the use of capillary action in porous structures are presently utilized to minimize adverse 0-g effects on ionic and nonionic solute rejection, concentration gradients at the membrane surface, absorption phenomena, etc., in such devices as batteries and fuel cells designed for operation at 0 g. The orbiter cabin and pressurized laboratory should provide facilities for the testing of components of closed-loop life-support systems developed on the ground on a competing basis, so that efficient and reliable systems can be developed. In particular, appropriate space, power, and access to the main atmospheric circulation loop should be provided for at least two prototype component units to be tested simultaneously. The closed environment of the Shuttle will undoubtedly produce a wide variety of trace contaminants, some of which will be dangerous or distressful. Many of these contaminants have synergistic effects on body function. For example, trace amounts of ozone in the atmosphere produce lung tissue damage and also have a profound ef- fect on the ability of the lungs to clear microorganisms. Man and other animals also produce a variety of chemicals that are eliminated into the ambient in trace amounts. The rates of their buildup in a closed

170 LIFE SCIENCES system have not been studied adequately, nor have their possible deleterious effects on man, animals, or plants. Plants, for example, are exquisitely sensitive to ethylene. Apparently the only con- taminant-monitoring systems currently being contemplated for the Shuttle are a rudimentary carbon monoxide sensor and, possibly, a hydrogen leak detector. The only contaminant-scavenging systems now being planned include the use of LiOH for carbon dioxide removal, filters for particulate matter, and activated carbon for removal of material for which it has an affinity, such as high- molecular-weight matter. Clearly, a better system to remove trace contaminants will be required, such as a catalytic burner or a rechargeable absorbing system. It should be pointed out that rather sophisticated systems for sensing and scavenging contaminants are currently available and in commercial use. Both for crew health and safety and for the success of scientific experiments, a substantial improvement in the contaminant control system over what is presently contemplated will be necessary. For safety reasons it is mandatory that the air in the orbiter cabin be monitored continuously for oxygen and hydrogen, the first to guard against a significant reduction in oxygen for breathing and the second, as an index of a fuel leak, to prevent fire. In addition, it is most important that the atmosphere be monitored periodically (several times a day) for trace contaminants such as carbon monoxide, oxone, amines, sulfides, mercaptans, and hydrocarbons. The particulate content of the cabin air must also be sampled and measured periodically and the number of viable microorganisms determined. There will need to be a mechanism for removing this particulate matter, as by filters, and possibly for sterilizing the air stream, as by ultraviolet radiation. Trace contaminants and par- ticulate matter, including microorganisms, must be removed to levels consistent with standards recommended in reports of the NRC Com- mittee on Toxicology* and the Space Science Board.f Recent developments in the computer sciences indicate that, by the 1980's, Shuttle users can expect that their needs for inflight *Panel on Air Quality in Manned Spacecraft, NRC Committee on Toxicology, Atmospheric Contaminants in Spacecraft (available from NASA Director of Life Sciences, Washington, D.C.). fSSB Panel on Air Standards for Manned Space Flight, Atmospheric Contami- nants in Spacecraft (NAS-NRC, Washington, D.C., 1968); Physiology in the Space Environment, Vol. II, Respiration (NAS-NRC, Washington, D.C., 1967); Infectious Disease in Manned Spaceflight: Probabilities and Countermeasures (NAS, Washington, D.C., 1970).

Scien tific Uses of the Space Shu ttle 171 computation will not be limited by size and weight considerations. One important potential use for this improved on-board computer capability will be in the teleoperator, or remote manipulator, system. Projected developments in teleoperators, particularly improved hand geometry and use of tactile- and force-feedback, make it probable that many of the presently contemplated needs for extravehicular activity in the Shuttle-Spacelab can be handled remotely. Ac- cordingly, new and improved designs of teleoperators should be field-tested in the Shuttle-Spacelab to determine their suitability for general use in space, and facilities for such field-testing should be provided. VIII. LABORATORY OPERATIONS The wide range of experimental approaches employed by the various biological and medical disciplines will require a pressurized labora- tory module of the maximum size possible and with maximum flexibility as a major design principle. Requirements for specific experiments will vary greatly among different investigators and from flight to flight. However, certain common requirements can be identified; these include light, water, electricity, thermal control within ±2 °F, suction, laboratory refuse disposal (solid, liquid, gaseous), and such generalized equipment as a small multipurpose centrifuge, refrigerator, and freezer. It is impossible to identify or enumerate all types of specialized instrumentation required for individual missions, but it is likely that major classes of equipment used in many types of investigation would include, for example, radioisotope-handling apparatus, constant-temperature incubators, plant-growth and tissue-culture facilities, aquaria, photographic instrumentation, microscopes, and animal-holding and -handling facilities. We do not anticipate special requirements or problems with respect to data handling and storage. However, need for an on-board centrifuge to provide a control for the gravity component and to test responses to fractional g forces has been emphasized by several disciplines. The centrifuge should be a variable-speed device capable of generating accelerations up to 1.5 g and handling tissue cultures, plants, and small animals up to 0.5 kg. Because animals in the centrifuge would be exposed to variations in acceleration and to Coriolis forces, which could produce physiological effects invali- dating their use as \-g controls, the centrifuge should have as large a radius as possible to reduce the necessary speed of rotation and

172 LIFE SCIENCES therewith the acceleration gradients. Rodents (white mice) have a normal metabolic rate during chronic exposure to between 1 and 2 g in a 6-ft-radius centrifuge, suggesting relatively normal behavior.* However, there is little similar information available on the behavior of higher vertebrates, especially primates, and such data should be obtained. The increased resistance of Skylab astronauts to Coriolis sickness may indicate that under 0-g conditions centrifuged animals will be less susceptible to vestibular-type dysfunctions in a centri- fuge. The speed of rotation of the centrifuge should be regulated so that variations in g force are minimal. Data are lacking on how large a variation is acceptable; intuitively we estimate that the variation should be less than ±2%, but this should be tested. Certainly the radius should not be less than 5 ft. The addition and removal of animals or biological materials to and from the centrifuge should be possible without stopping or starting the centrifuge. The capability of 0.5 kg will permit study of a small primate—a spider monkey— which is important for man-related experiments. The centrifuge will not be required for all experiments and should therefore be removable from the laboratory module in order to conserve weight and space. The above, very incomplete enumeration implies two important points. The first is that there can be no such thing as the ideal space laboratory for all biomedical work: there are too many different sets of equipments that will be needed at one time and not another. The design of the pressurized laboratory should therefore be based on modular concepts and interchangeable components. This will further- more reduce lead times necessary for experimental work as the investigator will be responsible for furnishing his specialized equip- ment that can be plugged into the basic laboratory facility. The second point is the need to assure, insofar as possible, that the basic laboratory facilities (power and atmosphere control, for example) will in fact accommodate the requirements of the majority of potential users. This will require further contract study. A prime requirement of the life-sciences laboratory is the ability to carry out investigations on the intact animal. The exact species that might be employed cannot be set out other than to identify the familiar vertebrates used in experimental biology and medicine. Among the mammals this would include mice, rats, Chinese *W. Fethke, K. M. Cook, S. M. Porter, and C. C. Wunder, "Oxygen Metabolism Measurements during Chronic Centrifugation of Mice," J. Appl. Physiol. 35:572, 1973.

Scientific Uses of the Space Shuttle 173 hamsters, rabbits, and possibly dogs and small primates. Among the nonmammalian species, needs might arise for avian species up to the size of chickens and fish, frogs, and diverse amphibia and reptiles. Adequate atmospheric and thermal control of the animal-holding facilities is essential. Ambient temperature should be held to about 72 ±2 °F and about 40 to 60% relative humidity for most mammals and birds. Lower or higher temperatures may be required for some reptiles, amphibia, and fish. We would prefer that a separate air input system be provided for the animal facility in order to avoid mixing of the two atmospheres except when animal handling is required, but there is no reason why the two atmospheres should not vent into a common exhaust and revitalization system. Total isolation defeats the purpose of the facility: full access to the animals must be available throughout flight. In addition to simple access, provision should also be made for some rather detailed manipulations including dissection and removal of selected organs and tissues. An especially difficult problem might arise when the gastrointestinal tract will have to be sampled, as may well be required in certain studies on cell proliferation. As microorganisms comprise the major component of intestinal contents, these contents as well as other body fluids, hair, and dander will have to be safely contained without inhibiting the investigator's manual or instrumental access. Additional presently foreseeable requirements involving animals are the capability for radiologically safe use of radioisotope-labeled compounds for injection and on-board tissue or fluid sampling, counting, and disposal; artificial insemination of small mammals; small mammal nesting, parturition, and litter-rearing; avian egg incubation; and sampling of tissues and fluids and their storage for postflight chemical analysis. Small, live-in compartments may also be needed on the centrifuge for those species upon which the critical tests of weightlessness are made. While certain of the projected experiments in the life sciences can be accomplished within the span of 7-day missions, others, such as those concerned with embryonic and fetal development, wound healing, and other aspects of cellular proliferation (e.g., marrow, skin, and gut), will require the full capability of the 30-day mission. In general, we anticipate that the longer missions will be of particular value to the life sciences. In some cases, free-flying unmanned satellites, recoverable after 6 months or so, will be necessary. For most purposes, orbital attitude and inclination will not be critical. Polar orbits may be required occasionally for radiobiological ex- periments.

174 LIFE SCIENCES IX. EXPERIMENTAL AND ADMINISTRATIVE APPROACHES If the potential of the Shuttle for life-sciences research is to be realized to any meaningful extent, certain experimental and adminis- trative approaches are essential. All persons on all missions should be available for routine biomedical tests and monitoring. The critical need for information about the responses of humans to 0 g, necessary to assure the safety of manned flight, means it would be wasteful not to obtain physiological data on all crew members and other personnel in addition to any life-sciences experimenters. Many of the Shuttle flights will not have the pressurized laboratory or life scientists on board, so the types of measurements made would necessarily be relatively unsophisticated. For example, at this time there is an identifiable need for metabolic balance studies, requiring collection and preservation of urine and fecal samples. This demands that the feeding and waste management systems of the orbiter be designed to permit such tests and that space be allotted for the samples obtained. Measurement protocols should be designed not to interfere with mission tasks. The degree to which nonastronaut scientists of all disciplines are able to participate in Shuttle flights will have a strong impact on the amount of research that can be done in the Shuttle. Medical and psychological screening should therefore be as lenient as possible, within safety limitations. The initial period of conservatism in selection should be as short as possible, its end hastened by continuing tests to warrant lowering of criteria. As a general principle, biomedical experiments should be con- ducted in humans first, where feasible, and supplemented by animal studies. Experiments should not overload the test subjects by trying to obtain many different kinds of data in one protocol: early experiments at least are likely to be more successful if they are quite simple in design and execution. A cardinal rule is that all flight experiments should be preceded by adequate and thorough ground-based preparation. The responses of the test materials must be completely familiar under 1 g and the other experimental conditions. In no other way can the quality of the experiment and the legitimacy of any flight results be assured. The above implies that financial support for ground-based prepara- tory work will be necessary in some cases. Such support is justified, but it must not be abused or allocated without careful review.

Scientific Uses of the Space Shuttle 175 Peer review of the entire structure of space life-sciences programs, both supporting research and technology (SR&T) and flight experi- ments, both proposed and on-going, is essential. The reviews should be made at regular intervals, systematically, by formal panels appointed for fixed terms and consisting of members of the national and international scientific community. This procedure should be initiated promptly, because out of the current SR&T programs will doubtless come many of the Shuttle flight experiments. It is imperative that the base of life scientists participating in space research be broadened. Similarly, communication between life scientists and the space program must be improved. Broad dissemi- nation of information on flight opportunities should be helpful, as should appointment of a wider circle of scientists to review panels. Nevertheless, the most important influence to bring about a lasting, satisfactory participation of the scientific community is the con- tinuing execution of high-quality work. At present there is no formal representation for the life sciences within the administrative structure of ESRO. If the European com- munity proceeds with the space laboratory, this lack will almost inevitably have an adverse impact on the quality of the biomedical laboratory. Appointment of staff and panels, as appropriate, would seem essential to this work and to increasing the participation of European life scientists in the Shuttle program. X. RECOMMENDATIONS 1. For life-sciences work in space, within the Shuttle concept, a pressurized biomedical laboratory that is as large as feasible is necessary. The laboratory will be used for a broad array of biological and medical experiments and must have flexibility as an intrinsic characteristic of design. We can identify some general requirements that will have to be built in, for example, that about 3 to 4 kW, or about 3000 kWh, of power will be needed for a 30-day mission. In addition, many special requirements will be dictated by specific experiments that will differ from flight to flight. Thus modularity of equipment and facilities is important to permit interchange within common spatial dimensions and consequent saving of weight and space. 2. We foresee a requirement for recoverable unmanned free-flying satellites, or bioresearch modules, for experiments requiring long periods (~ 6 months) in orbit. A typical example would be the

176 LIFE SCIENCES long-term effects of weightlessness on small mammals over several generations, including hematological effects. 3. On all missions all Shuttle personnel should be available as possible subjects for routine medical and performance monitoring inflight. Facilities must therefore be available on the orbiter to conduct metabolic balance studies and to measure, sample, and preserve medical specimens, including urine and feces. This will place requirements on the feeding and waste management systems of the orbiter cabin and on space in the orbiter cabin. 4. The composition of the atmosphere of the orbiter cabin and pressurized laboratory must be monitored continuously for oxygen and hydrogen and periodically for trace contaminants (e.g., CO, hydrocarbons, particularly ethylene, Freons, O3, sulfides, mer- captans, and amines) and viable microorganisms. Trace contaminants and particulate matter, including microorganisms, must be removed to levels consistent with standards recommended in reports of the NRC Committee on Toxicology* and the Space Science Board.f 5. It will be necessary to conduct onboard control experiments for the gravity component and to generate gravity-level response curves for many biological studies. This will require a variable-speed centrifuge with a radius not less than 5 ft, capable of producing accelerations up to 1.5 g and handling plants, tissue cultures, and animals up to 0.5 kg. Since the centrifuge will not be required for every life-science experiment, it should be detachable from the laboratory. 6. In order to capitalize on the laboratory's potential for animal experimentation, easy access to the animals, for direct manipulation such as for surgery, must be provided. Although an entirely isolated (sterile) environment is unnecessary and impractical, the gas outflow from the animal-holding area should not be discharged directly into the atmosphere inspired by the crew. The animals can be maintained *Panel on Air Quality in Manned Spacecraft, NRC Committee on Toxicology, Atmospheric Contaminants in Spacecraft (available from NASA Director of Life Sciences, Washington, D.C.). fSSB Panel on Air Standards for Manned Space Flight, Atmospheric Con- taminants in Spacecraft (NAS-NRC, Washington, D.C., \968); Physiology in the Space Environment, Vol. II, Respiration (NAS-NRC, Washington, D.C., 1967); Infectious Disease in Manned Space flight: Probabilities and Countermeasures (NAS, Washington, D.C., 1970).

Scientific Uses of the Space Shuttle 177 in a closed compartment, with intrinsic environmental control, that could be opened as required. 7. To permit certain biological studies that are time-dependent, late access to the payload is essential. We have been informed that access up to 2 h prelaunch is feasible, and at the present time this would seem to be sufficient. 8. Dosimetric devices and materials should be positioned in the orbiter cabin on all flights to accumulate data on radiation fluxes and energies and on the effect of diverse spacecraft components on intensities and scattering. While some predictions can be made about the influence of spacecraft materials on the actual radiation exposure parameters, it is necessary to test these expectations against the observed radiation environment. 9. In order to test new or redesigned components of life-support systems at 0 g in the course of their development, there should be access to the gas-flow loops in the pressurized laboratory. 10. Recent developments in remote manipulation using force- reflecting master-slave servomechanisms and stand-alone force- and tactile-sensitive manipulators have great potential for Shuttle op- erations and should be applied and encouraged. If this instrumenta- tion is properly developed, remote manipulation can be at least partly substituted for extravehicular activity, with concomitant increases in payload efficiency. 11. In order to permit maximal participation by the research community, criteria for selection of scientific participants should be no more stringent than is necessary to protect the health and safety of the passengers themselves and the crew. 12. There is need for a standardized procedure to inform and to attract potential scientific users of the Shuttle. In addition to the Announcement of Flight Opportunities, there should be announce- ments in major relevant scientific publications. All proposals for flight experiments and SR&T should be subject to peer review by panels drawn from the international scientific community. Similarly, peer review of ongoing flight and SR&T projects should be made at regular intervals. 13. All flight experiments should be preceded by adequate and thorough ground-based preparation. The responses of the test materials must be completely familiar under 1 g and the other experimental conditions, for in no other way can the quality of the experiment and the legitimacy of any flight results be assured. Thus,

178 LIFE SCIENCES after an experiment proposal has been accepted, ground-based research and control experiments specific to the flight experiment will normally be required and should be supported. 14. If ESRO proceeds with the Space Laboratory, we strongly urge that life sciences be formally represented in the ESRO manage- ment structure.

9 Planetary Exploration I. OBJECTIVES The late 1960's and early 1970's were marked by the unique achievements of manned exploration of the moon. In its later stages, the Apollo program provided scientific data that add a new dimension to our knowledge of the earth- moon system. During the same period, unmanned lunar and planetary explora- tion, by both the Soviet Union and the United States, provided technological successes comparable to Apollo and scientific returns of similar importance because of the many different objects in the solar system accessible to deep-space probes. Planetary exploration has, however, only begun, and as long as there is a space program, planetary exploration should continue to be a major objective. Given television, remote operation, and sample return, unmanned operations have a significant capability to extend man's knowledge and experience throughout the solar system. From the scientific point of view, the aim of planetary research has extended beyond the stimulating but diffuse ambition to understand the origin of the solar system. This remains a major concern, as does the search for life, but of equal importance is the understanding of planetary processes, including those of earth, in a more generalized framework. There is a growing influence through- out the earth sciences of knowledge gained from planetary explora- tion. New ideas have been introduced, and, more importantly, existing rationales have been challenged. The rapidity with which our knowledge of the earth advances may now be significantly influenced by the rate at which our knowledge of the other planets increases. The technical achievements of planetary exploration are well illustrated by the spectacular quality of the soundings and images from Mariner 9, by the Soviet landings on the inhospitable surface of Venus, by the Pioneer probes now approaching Jupiter, and by the 179

180 PLANETARY EXPLORATION Lunakhod rovers and sample return from the moon. Viking '75 to Mars has yet to achieve its objectives; it is probably the most complex instrument package yet designed by man. A variety of space systems have been developed during this period of technical advance: in the United States, Mariner was designed for high-quality remote sensing, Pioneer for direct measurements and low-cost exploration, the Viking soft lander for surface studies. The Soviet Union developed entry probes and a sample return capability. Pioneer Venus and subsequent missions to the outer planets should establish probe systems also for the U.S. program. Sample return is the next technical challenge for American scientists. The above space systems must be preserved for use with the Shuttle if we are to avoid continuous redevelopment. In addition, capabilities for sample return and remote operation should be developed in parallel with the Shuttle in order to ensure that an effective program of planetary exploration will be available for use with this new capability. Some additions to the Shuttle capability will be necessary to ensure adequate escape energies for deep-space missions. These will be discussed in a subsequent section. In the quest for new information obtainable only from planetary and lunar spacecraft missions, one should not forget the important role played by observations and measurements from earth and from instruments in earth orbit, either on the Shuttle itself or on free-flyers. The essential feature of these observations is that they can be made almost continuously over very long time periods, and thus they can detect subtle changes that could easily escape spacecraft instruments. Thus, synoptic imagery, spectrophotometry, photometry, and polarimetry at high resolution in the optical and ultraviolet ranges are of enormous importance for the study of planetary atmospheric dynamics and chemistry. Infrared observa- tions are particularly important for studies of molecular com- positions and reactions. Since these problems and the instrumental requirements are treated in the infrared, optical, and ultraviolet chapters of this report, they will not be discussed here in detail. Another feature of terrestrial observations that may have signifi- cant scientific and philosophical consequences is the search for other planetary systems. Such searches involve the detection of small systematic changes in stellar velocities and proper motions. It is an arduous task that might lead to significant discoveries. We see four major thrusts in planetary science: comparative planetary science of the inner planets, the nature of a major planet,

Scientific Uses of the Space Shuttle 181 exobiology, and the nature and origin of small objects. Some activity should continue in each of these major areas in order to support an active science community and a stimulating flow of ideas. The specific objectives in each area should be developed by continuing review processes that will identify the next promising target to advance the general field. This assessment was judged inappropriate to the present study, but we believe that NASA and its centers should provide the mechanisms for such review. The nature of the Space Shuttle creates a greater difference between deep-space exploration and orbital science than has existed up to now, and an effort is required to ensure that NASA's traditional and highly successful commitment to planetary exploration con- tinues. II. THE INNER PLANETS The last two or three years have brought a new sense of perspective to our knowledge of the inner planets. There is or soon will be sufficient information about the state of differentiation and chemical composition of Mars, earth, and Venus to give hopes for a unified theory of planetary formation. High-resolution images of Mars show features similar to, but essentially different from, those on the earth and the moon—features that tectonic theories must explain. The state of water and carbon dioxide on the inner planets suggests how the atmospheres were formed. The comparative meteorology of Mars, earth, and Venus has become a major concern involving fundamental climatology, tidal theory, nonrotating circulations, clouds, violent storms, boundary-layer phenomena, and other topics. Study of the photochemistry of Mars and Venus has led to a better understanding of the role of interactions between lower and upper atmospheres. For the inner planets, substantial advances in fundamental knowledge are likely to be achieved from data from all three planets. Despite their similarities, they differ significantly, and measurements on each planet are necessary. Little is known about Mercury. The first probe to fly by that planet will do so next year. If the history of tectonic processes does not appear to be substantially different from the moon and Mars, the difficulties of making measurements so close to the sun may defer future efforts on this planet in favor of other targets. For the United States, the major lacuna in past programs has been the absence of a program to explain the surface and lower

182 PLANETARY EXPLORATION atmosphere of Venus. Entry probes and synthetic-aperture radar mapping using Pioneer and Mariner spacecraft can change this situation. The key to advance, and to the development of probe systems for planetary exploration in general, is the Pioneer Venus multiple-probe mission, followed by orbiters and simple landers. To preserve this program, funding in the fiscal year 1975 budget is essential. For Mars, the current task is to assimilate the large amount of information available from Mariner 9 with that which will become available from Viking. The most important complement to the detailed images that are or will be available will probably be surface-sample analysis, preferably at many locations. The tools will be Pioneer hard landers for widespread sample analyses, Viking landers with rover capability, and sample return. For meteorological research, the Mariner 9 instrumentation was well designed, but the lifetime was short and nightside data are scarce. For aeronomy. a Pioneer probe is the ideal entry science mission. The Viking 1975- 1976 soft lander for Mars is the mission chosen for the initial investigation of life in the solar system. Titan, a Saturn satellite, has been identified as another possible habitat for primitive life. Viking/Mars is an expensive and complex mission. It uses an elaborate approach, which, if it acquires positive signals, could give rise to the most spectacular discovery of the space program to date. The prognosis is not optimisitc, however. There is no responsible alternative, therefore, but to wait for the results of Viking before planning further biological exploration. If Viking gives tentative signals, there will be strong reasons to continue with the Viking approach, modified in the light of the results achieved. If the signals are negative or confused, it may be preferable to change the approach and emphasize the slow accretion of relevant knowledge, culminating later in an elaborate mission to confirm ideas formed from indirect knowledge. It might also prove desirable to wait for the opportunity to analyze a returned sample. Sample return capability must eventually be available to the U.S. planetary exploration program, and Martian exobiology may be a good first target. Finally, we may wish to turn attention to Titan when we are more familiar with the exploration of the outer planets. III. THE MOON The Apollo data tell us that the moon formed about 4.5 billion years ago, a time accepted as the age of the solar system. A large

Scientific Uses of the Space Shuttle 183 proportion of the moon consists of compounds that condensed at high temperatures in the solar nebula. For the first few hundred million years of its existence, the moon consisted of an aluminum- rich highland crust and iron-rich mantle, both depleted in volatile compounds such as water and methane. During this early period, the moon was heavily cratered by asteroid and meteoroid impacts, of which the largest formed the great circular mare basins. About 0.5 billion to 1.5 billion years later the frontside mare basins, located in a thinner crust than farside basins, filled with basaltic lava, which originated from melting in the deep iron-rich interior. Since that period, the moon has been volcanically quiet; today its seismic energy release is about 109 times less than that of the earth. The lunar quiescence has been to our benefit because it has provided an accessible surface on which is recorded the history of meteoroid, cometary, solar-wind, and cosmic-ray activity for over 3 billion years. This lunar story is fascinating in its own right, but more so because of its applicability to understanding how the earth and other planets formed and evolved. For example, if a planet as small as the moon formed a crust soon after formation, so probably did all planets of similar or larger size such as Mars and earth. In fact, some 2.5-billion- to 3.75-billion-year-old aluminum-rich rocks found on earth are now suspected to be remnants of an early crust, one that possibly formed in the same manner as the lunar highlands. It further appears that other old terrestrial volcanic rocks may be equivalent to lunar mare basalts, perhaps representing volcanism triggered by the giant impacts that are suspected to have occurred on the earth 4 billion years ago. The analysis of lunar soil samples leads us to believe that solar activity has been relatively constant over the past million years; this severely constrains the hypothesis that the earth's glacial cycles, or ice ages, are related to changes in the sun's activity. Embedded in the lunar soil samples were found solar-wind atoms over 0.5 billion years old, atoms that appear to correspond to today's solar wind. Such atoms are also found in some 4-billion-year-old rocks from the lunar highlands. We thus expect our analyses over the coming few years to provide us with a reliable history of much of the sun's activity since it formed, a record of particular value to our understanding of the evolution of our sun. The observations of the Apollo astronauts, the lunar samples, and the information continuing to be provided by the instruments left on the lunar surface (such as seismeters, mass spectrometers, and solar-wind detectors), as well as the Explorer orbiters, have answered many of our questions about the moon. But as we learn more about

184 PLANETARY EXPLORATION the moon, we find new questions to be asked—questions that form the basis for a second generation of lunar exploration.* Answers to these and other questions about the moon will help us to understand the evolution and nature of our earth, an under- standing that contains both scientific and practical significance. Continued analysis and correlation of data from previous studies with new data as it comes in should put us in a position to resume fruitful exploration of the moon by the early to mid-1980's. IV. THE OUTER PLANETS The exploration of the outer planets—Jupiter, Saturn, Uranus, and Neptune—is of great interest for several reasons; the most funda- mental is the importance of their overall composition and past history for the reconstruction of the radial, chemical, and thermal structures of the primeval solar nebula. This, in turn, is closely related to the mechanism and chronology for the accretion of the planets. The outer planets contain over 99 percent of the mass and angular momentum of the total planetary system, and thus the structure and composition of their interiors is of great importance. Fortunately, in contrast to the terrestrial planets, the outer planets are large and have low densities so that the possible choices for their chemical composition are very limited. In fact, helium and hydrogen must be their major constituents. As a result, self-consistent theoretical models of the interiors of these planets may eventually be obtained. Uranus and Neptune were once thought to be rather dense, so that their composition was difficult to establish. Recent more accurate optical and occultation measurements indicate that the radii are larger and the densities appreciably lower. In fact, the density of Uranus is lower than that of Jupiter, and that of Neptune somewhat higher. Thus there is hope for obtaining meaningful theoretical models of their internal structure. Pluto is excluded from these considerations because it is small, dense, and in a strongly inclined orbit, which suggests that it is a captured body not characteristic of that part of the solar system. Among the outer planets, Jupiter is of particular interest because its huge escape velocity and relatively low surface temperature make it a uniquely well-preserved sample of the early solar composition. However, its atmospheric composition, which is susceptible to *See Post Apollo Lunar Science, Report of a Study by the Lunar Science Institute, July 1972, "Statement of Major Unsolved Problems," pp. 27-30.

Scientific Uses of the Space Shuttle 185 remote observation or to spacecraft sampling, does not provide a unique answer about the interior composition, even with respect to the basic problem of the hydrogen-helium ratio. The second major interest in the outer planets stems from the apparent broad similarity of the structure and dynamics of their atmospheres. Here Jupiter and Saturn stand out not only because they are the biggest planets but also because of the striking analogies and differences between them. The problem of the vertical structure of the deep adiabatic supercritical atmospheres of these planets and of their complicated dynamics has been the subject of innumerable investigations. The wavelength dependence of the limb darkening or brightening appears to be understood. Recently, some insight has been obtained into the mechanism of their atmospheric circulation including the number, structure, and appearance of the cloud bands, and many of these phenomena appear to be related to the exceedingly high rotational velocity of these planets. Recently improved observations have led to improved models of the atmo- sphere of Uranus; but observations and theoretical understanding of Neptune are still in a rudimentary stage. Interest in the outer planets also focuses on several unique features of Jupiter and Saturn. Among these are the enormous magnetic field of Jupiter and its direct relation to the nature and dynamics of the deep interior of the planet. This magnetic field is closely related to decimetric radiation, which originates in the external radiation belt, and presumably also to the rotating systems of decametric radiation. The mysterious Great Red Spot, known for over 300 years, is another fascinating Jovian phenomenon for which many models have been proposed, although none is completely satisfactory. There is, as yet, no evidence for the presence of a strong magnetic field on Saturn, which could indicate that either the deep interior of that planet is appreciably different from that of Jupiter or that a radiation belt, which would be the best evidence for a magnetic field, is suppressed by the Saturnian rings. The rings themselves, their origin, nature, and dynamics present a fascinating topic for planetary research. The discovery of a significant magnetic field on Saturn would have a profound influence on the admissible models of its interior. The list of the unusual phenomena on Jupiter and Saturn would not be complete without mentioning the enormous source of internal energy that is much higher than the energy received from the sun. There are many proposals that attempt to explain the origin of this energy, from a gradual loss of primordial heat to a continuous

186 PLANETARY EXPLORATION phase change and gravitational segregation. The source of internal energy is closely related to the still unsolved problem of the radial temperature profile and central temperature of Jupiter. The presence of the internal energy source is well established for Jupiter but not for Saturn, which presents an interesting dilemma in view of the usually assumed similarity of the two planets. The great variety of questions that need to be answered before the outer planets are reasonably well understood is reflected in the variety of quantitative and qualitative data that are required. For atmospheric studies, one requires information about, among other things, chemical composition, thermal gradients, optical properties, dynamic parameters, and heat fluxes. These should be measured not only as a function of depth but also latitude and perhaps even longitude; probes and radio occultation experiments may be most useful for these purposes. In order to refine our knowledge of the interiors, one needs better gravitational multipole coefficients; oblateness measurements; heat-flux data; information about the magnetic field (electronic and protonic belts); and any indication of local gravitational, thermal, or magnetic anomalies (such as the one recently reported at 220° longitude). In particular, any information about a north-south asymmetry, either gravitational or thermal, would be significant because of its bearing on the question of the absence or presence of a solid mantle. None of the required data are obtainable with sufficient accuracy from terrestrial observations and must be deduced from planetary missions. Among these missions, flybys provide the first, rather limited, information; then come the probes, which it is hoped will lead to valuable atmospheric data; and, finally, the orbiters, which alone can give sufficiently complete data to construct satisfactory models of the interior and exterior of each planet. We suggest that the primary focus of the exploration of the outer planets in the period 1977-1990 be placed on Jupiter and Saturn. Our con- siderably advanced knowledge of these planets permits us to pose sophisticated questions whose answers can have far-reaching conse- quences. Some preliminary investigation of the more distant outer planets should also be pursued, but it is clear that technical difficulties and cost will make progress slow. During the 1980's, the propulsion and the design of lightweight instrumentation may advance so that in the 1990's the other outer planets will become more accessible. In view of these remarks, the proposed sequence of missions to outer planets (see Table 19) mission 5 and missions 15 to 21 seems

Scientific Uses of the Space Shuttle 187 reasonable. The flyby missions 5, 15, and 21 will give initial information about Jupiter, Saturn, Uranus, and Neptune, to be followed by probes on missions 16, 17, and 19 to Saturn, Uranus, and Jupiter, and finally orbiter missions 18 and 20 to circle Jupiter and Saturn. It is important that each planet be explored by one orbiter close to the equatorial plane and the other at high inclination. The Saturn probe precedes the Jupiter probe because Jupiter has more than twice the gravitational acceleration of Saturn, leading to many times higher heating of the probe on entry. The Saturn probe is in a sense a test for the new problem of entering the atmospheres of the outer planets. V. PLANETARY SPACECRAFT The exploration of the planets was initiated by NASA in 1962 with the launch of Mariner 2 to Venus. Since 1962, a plan for planetary exploration has been formulated with the assistance of many science advisory groups. The sequence of missions begins with an explora- tory survey by flyby missions, which yield the necessary data to plan more intensive observations by orbiter and entry-probe missions. The next step would be to conduct long-term observations on the surface of a planet such as will be done by the Viking '75 mission to Mars. Finally, sample return missions would be initiated for detailed geological and biological analysis of material. The missions described increase in complexity, and the more complex missions demand greater propulsive capability as well as increased spacecraft capability. A mission to swing by Venus and fly by the planet Mercury was launched in November 1973. Pioneer 11 is currently en route to Jupiter, and Pioneer 10 flew by the planet in December 1973. The Viking '75 mission will place two landers on the surface of Mars to initiate the search for life outside of earth. The Mariner/Jupiter-Saturn program will initiate NASA's high-resolution observations of the outer solar system. The flyby will also permit observations of the Jovian Galilean satellites and the Saturn satellite Titan, one of only three satellites in the solar system known to have an atmosphere. Currently, three spacecraft systems exist: Pioneer, designed to explore the environment of a planet and to accomplish this with maximum economy; Mariner, either as a flyby or orbiter, designed to obtain high-resolution imagery of the planets and their satellites; and Viking, designed to soft land on Mars and to conduct surface analysis of the planet over an extended period of time. The next step in the

188 PLANETARY EXPLORATION exploration of the solar system requires development of an atmo- spheric entry probe. The first such mission planned is a Pioneer spacecraft to deliver probes to Venus. This mission would be followed by measurements in the atmospheres of Saturn, Uranus, and in the satellite Titan. These missions require very modest launch energy capability and can easily be accomplished with a launch vehicle having the equivalent performance of a Shuttle/Centaur system. The next step in the exploration of Mars might be a roving vehicle on the surface of the planet or a sample return mission. It has been estimated that such missions might weigh some 2000 kg; since the energy requirements for Mars missions are fairly low, they also could be accomplished by a launch vehicle having the capability of a Shuttle/Centaur. The great success of the U.S. man-driven rover and of the semiautomatic earth-controlled Russian Lunakhod makes it clear that an automatic exploration of the Martian surface should be considered. The essential difference between a mission to Mars and to the moon is the much greater distance to Mars, which makes communication and earth control very slow. The round-trip time in the most favorable case is 18 min, but it could be as long as 40 min. Thus, it is essential to develop an automatic obstacle-avoidance system that would permit the rover to travel to a predetermined destination following a path chosen in detail by the rover's sensors. Such sensors should be able to direct the rover around such things as boulders and steep slopes. An earth-controlled override contingency should also be available. The rover should be able to move on hard and on soft surfaces and should provide protection of the instru- ments from diurnal thermal variations. The scientific tasks of the rover could be manifold: TV imagery, thermal and spectroscopic observations, solid and gaseous sample collection and analysis, drilling for deep samples, microscope and x-ray examination, seismological experiments, radioactivity, mag- netic observations, and others. Biological studies may require a sophisticated array of instru- ments, which, if they are too heavy, too big, or too delicate to be located on the rover, could be left near or in the lander. The possibility of establishing a fairly permanent fully automated station on Mars should also be considered once the technology of remote experimentation is developed. This may not be realizable until the 1990's. In any case, an automatic adaptive control of the operation of the rover and of the fixed station should be developed.

Scientific Uses of the Space Shuttle 189 Sample return offers many types of information that cannot reasonably be obtained in any other way. The most sophisticated array of instruments that can be landed on the surface of another planet is a poor comparison to the analytical powers of numerous established terrestrial laboratories. The types of sample information that can be obtained from Martian samples can be broadly categorized in three major discipline areas: exobiology, organic geochemistry, and geosciences. The importance of the investigation of the surface of Mars, and returned Mars samples, for the possible presence of extraterrestrial life forms has been elaborated in previous NAS documents (e.g., Biology and the Exploration of Mars*) and is of paramount scientific interest; the probability of a positive result is a speculative matter. Similarly, the analysis of the Martian surface and Martian samples for organic compounds is of extreme interest; the possibility that Martian samples will contain organic compounds of biologic or prebiologic interest is difficult to evaluate. The geochemical, mineralogical, petrological, and geophysical value of the Martian samples is assured. Regardless of their composition, miner- alogy, and structure, the samples will be exceedingly informative about the history, surface and internal processes, age, and degree of differentiation of the planet. Analogy with the lunar exploration program demonstrates the tremendous improvement in our under- standing of the moon with the return of the first samples from the lunar surface (e.g., see "Proceedings of the Lunar Science Con- ferences," Geochim. Cosmochim. Acta, Suppls. 1-4). Recently completed studies by the NASA Langley Research Cen- ter and the Jet Propulsion Laboratory indicate that 10 to 1000 g of Martian sample can be returned to the earth by a Viking-class mission. Although we would like to return the largest possible sample, it should be noted that many of the major conclusions about the moon based on the samples returned by Apollo 11 also derived from the very much smaller Luna 16 and Luna 20 samples. During the past 10 years, NASA has supported a number of lunar sample investigators to improve their abilities to work with very small amounts of returned samples, and many sample investigators now have the ability to derive significant information from individual grains (1 to 100 Aim in diameter). Thus, the information that can be derived from a returned sample of the size contemplated is *Space Science Board, Biology and the Exploration of Mars, C. S. Pittendrigh, W. Vishniac, and J. P. T. Pearman, eds., NAS-NRC Publ. 1296 (National Academy of Sciences-National Research Council, Washington, D.C., 1966).

190 PLANETARY EXPLORATION significant. The question of quarantining a Martian sample has been raised. In view of past experience and the new knowledge about Mars that has accumulated in recent years, it is our judgment that a decision on this question be deferred until the problem can be considered in detail. VI. PROPULSION REQUIREMENTS Each planetary mission has propulsion requirements that depend not only on the distance to the planet and weight of the spacecraft but also on the inclination of the orbit of the Shuttle, the launch period, the path and velocity of the spacecraft in proximity of the planet, and the lifetime of power sources such as the radioisotope thermoelectric generators (RTG's). If the Shuttle provides the first propulsion stage, then the choice concerns only the escape, or upper, stage. The presently considered escape stages include the Centaur and the not yet developed low- and high-performance Tug. Whether the Tug is recoverable or expendable is a distinction that affects the feasibility and cost of the mission. Thrust can be augmented by a kick stage, which is either the existing Burner II or an as yet not developed solid-fuel device. The latter has the advantage of a rapid burn, but it may present structural strength problems during launch. It is expected that at the time the Shuttle becomes operative a 20-kW solar-electric-propulsion (SEP) system will become available. The approximate weight, size, and cost of the various upper stages are as follows: Centaur, 3-m diameter, 10 m long, 14.000 kg. $8 million. Tug, 5-m diameter, 12m long, 27,000kg (2700kg empty), $8 million low technology, $17 million high technology; if reusable, $1 million per mission. Kick Stage (Burner II), 1-m diameter, 2m long, 1400kg, $0.2 million. 20-kW SEP, 4.5-m diameter, 6 m long, 2000 kg, cost as yet unknown. There are several technical questions to be answered before the upper stages listed above could be accommodated in the cargo bay of the Shuttle orbiter. Outstanding is the compatibility of the weight and size of the spacecraft, escape stage, and kick or SEP stages, par-

Scientific Uses of the Space Shuttle 191 ticularly the location of the center of gravity of the total load. Of special significance are potential problems associated with the inability to vent the Centaur or the Tug in the cargo bay and the problems of thermal control of the RTG's during the flight and the protection of the crew from radiation. Table 19 lists the planetary missions, their date of launch, their weight and propulsion requirements (A/0 and C3) in the ballistic mode and, in certain cases, when using the additional SEP stage. The table indicates also the weight of the delivered payload as well as of the scientific equipment, listing separately the weights of the science on the probes, on the lander, and so on. Figure 5 illustrates the performance of the Centaur (with a kick stage for higher C3) of the high-technology Tug, either expendable or recoverable, whether with or without a kick stage. In addition, the performance of an expendable low-technology Tug with a kick stage is indicated. The huge differences between the performance of a Tug with or without kick should be noted, as should the relatively small difference between the high- and low-technology Tugs with a kick stage. Figure 5 also illustrates the propulsion requirements of the various missions that are proposed for the period after 1978. Some of them are indicated without and with a SEP stage, and mission 24 is shown in two variants that differ in the flight time, 800 versus 1100 days. It is clear that Centaur with a kick stage permits the launch of all but two missions to the outer planets, if the SEP is used. Without the SEP, two additional missions would not be possible. In this respect, the comparable performance of the reusable high-technology Tug with a kick stage should be noted. The extremely important Mariner Jupiter orbiter mission 18 is feasible with a Centaur, but barely so, which results in a rather short launch period of 20 days. Actually this mission is scheduled for an unusually favorable planetary configuration in 1981 and would be impossible with a Centaur otherwise. Mission 21, which is a Mariner Uranus-Neptune flyby, requires the use of an expendable high- technology Tug. Mission 22, which is an orbiter and lander on the Jovian satellite Ganymede, appears out of the reach of the single-escape stages, and it may require two Tugs that would have to dock in an earth orbit before launch. It should be noted, however, that other Galilean satellites of Jupiter are easier to reach than Ganymede. In fact, they may be reachable without a double upper stage. On the whole, the prospects for a successful launch of missions to the outer planets are very encouraging provided the Centaur

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194 PLANETARY EXPLORATION 2 0 1 0.8 0.6 0.4 21 1 10 12 14 16 18 20 22 24 26 28 30 32 AV ABOVE 160-naut. mi CIRCULAR ORBIT, 1000 ft/sec | | | | | | | | | | | 0 20 40 60 80 100 120 140 160 180 200 C3, km2/sec2 FIGURE 5 Planetary missions propulsion requirements. •, Ballistic; X, 20-kW SEP; C, Centaur; AT, advanced Tug; IT, intermediate Tug; E, expended; R, reused;"K, kick stage; NK, no-kick stage. Numbers refer to the missions as listed in Table 19. capability is maintained, the necessary development and construction of the Tugs and SEP completed, and the problems of transporting them in the cargo bay of the Shuttle orbiter solved. It should be stressed that the list of planetary missions given in Table 19 is the minimum viable program of planetary exploration in the period 1979- 1990. It requires a minimum of 20 to 24 Shuttles fully dedicated to spacecraft launch. The continuation of the excellent lunar explorations will require additional Shuttle launches. It is suggested that in order to assure the observation of suddenly appearing comets, such as the 1973 Kohoutek comet, one payload dedicated to cometary studies should be kept in readiness at all times. The sample return from Mars in 1985- 1986 may require the use of a fully equipped laboratory in the sortie mode of the Shuttle. This is a major task that should be investigated at an early date. With additional funding, a series of planetary and lunar missions of comparable or even greater scientific interest could be prepared.

Scientific Uses of the Space Shuttle 195 VII. RECOMMENDATIONS 1. We recommend the development of the solar electric propul- sion (SEP) auxiliary stage for the minimum planetary program; we further recommend a gradual development of Tugs and of other more advanced propulsion methods for a full program of planetary explorations. 2. We recommend that NASA maintain and improve its present spacecraft (Pioneer, Mariner, Viking), develop associated science- oriented atmospheric probes and automatic rovers, and solve the problem of automatic planetary rendezvous in connection with return of samples from Mars. 3. We recommend that the need for a sortie laboratory for the analysis of a Martian sample be investigated and the laboratory be built if necessary. 4. We recommend that a Shuttle payload be prepared to stand in readiness for cometary studies. 5. We recommend that NASA plan to undertake as a developmen- tal program a block of missions rather than considering two single missions for each opportunity. Such an approach can lead to significant cost reductions.

Appendix: The Space Shuttle System The Space Shuttle is a reuseable transportation system designed to routinely carry scientific and applications payloads to and from low earth orbit in the period after 1980. This system consists of three basic elements: solid rocket boosters, an external tank, and an orbiter. The two solid rocket boosters, after they are expended, are parachute-dropped into the ocean, recovered, and reloaded for subsequent reuse. The external tank, which contains the cryogenic fuels used by the orbiter main engines during launch, is dropped from the orbiter just prior to orbital insertion and is destroyed during its re-entry into the atmosphere. The orbiter, about the size of a DC-9 airplane (122 ft long), has a pressurized cabin able to accommodate up to ten persons for short missions and a large bay in which to carry payloads. The payload bay has an available volume 15 ft in diameter by 60 ft in length. Within this payload bay the orbiter can carry up to 65,000 Ib into a due east orbit and up to 40,000 Ib into a polar orbit. In orbit, which is typically 100-200nm, the payload bay doors open to expose the payload to the space environment. Upon completion of the mission, the orbiter bay doors are closed and the spacecraft is deorbited with the orbiter performing cross-range maneuvers and aerodynamic flight resulting in a horizontal landing at a landing strip. As a transporta- tion system, the Shuttle can 1. Deliver various free-flying spacecraft to orbit and act as a first stage for geosynchronous missions and planetary injection (in addition to deploying spacecraft, it permits functional checkout of spacecraft prior to injection); 2. Revisit previously deployed spacecraft either to service them or to retrieve them and return them to earth for refurbishment; 3. Carry into low earth orbit a laboratory from which scientific and technical investigations are directly performed. This provides an 196

Scientific Uses of the Space Shuttle 197 opportunity to carry scientists as passengers to operate their own experiments and also makes possible the repeated use of equipment in the pressurized laboratory or on the pallet. The Space Shuttle has limited flexibility as to the orbit achieved. From the Eastern Test Range (ETR) 65,000 Ib could be delivered to a 210-naut mi circular orbit at 28.5-deg inclination using the basic Shuttle propulsion. A 30,000-lb payload could be placed in a 240-naut mi circular orbit system. By adding one Orbital Maneuver- ing System (QMS) kit, this 30,000-lb payload could be placed in 350-naut mi circular orbit. Up to three QMS kits can be added to the basic Shuttle system. With three kits, a 30,000-lb payload could be placed in a 510-naut mi circular orbit. From the ETR, orbits with in- clinations up to 56 deg can be achieved with some reduction in payload capability over that for a 28.5-deg orbit. It is planned that the Space Shuttle will be operated from the Western Test Range as well as the ETR to make possible high-inclina- tion orbits. These high-inclination orbits require greater energy than do low-inclination orbits. The maximum payload that can be delivered into a 90-deg orbit is 40,000 Ib, with a maximum altitude without an QMS kit of 145 naut mi circular. By adding a single kit, the 40,000-lb payload can be lifted to 300 naut mi circular. To ensure safe supersonic and subsonic flight and landing, certain constraints are placed on the weight and location of the returned payload. The maximum returned payload is limited to 32,000 Ib. A 32,000-lb payload would have to have its center of gravity located between about 32 and 44 ft behind the forward payload bay bulkhead. Heavier payloads may be landed in an abort situation, with some degradation in safety, providing the payload center of gravity is contained within the so-called 2 percent envelope. This means that a 65,000-lb payload could be landed in abort if its center of gravity is located between 38 and 44 ft behind the payload bay forward bulkhead.

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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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