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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

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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.

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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

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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

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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-

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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.

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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

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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

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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

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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-

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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-

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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).

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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-

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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.

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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.

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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

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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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