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5 Infrared Astronomy I. INTRODUCTION During the past decade, substantial contributions to astronomical knowledge have been made possible by rapid advances in infrared and millimeter-wave technology. These contributions have had an impact on the fields of solar-system physics, stellar formation and evolution, physics and chemistry of interstellar matter, galactic struc- ture", and the origin of the universe. Infrared and submillimeter waves (1 urn to 1 mm) are especially suited for the study of cool states of matter in the universe. Thermal radiation covering the temperature range from cool stars to the cosmic background radiation is pre- dominantly in this spectral region. Infrared observations are also im- portant for understanding the emission mechanism of nonthermal sources and for identifying and understanding the molecular constit- uents of astronomical objects. Progress in infrared astronomy has been made in the past pri- marily by ground-based observations. Atmospheric absorption, how- ever, limits these observations to narrow spectral windows; and even in these windows, atmospheric emission is substantial. Promising re- sults have been obtained with small instruments flown on aircraft, balloons, and rockets, but aperture size and observing time have set severe limitations on these observations. The Space Shuttle will per- mit observations throughout the infrared and submillimeter spectral range without atmospheric interference and with considerably greater aperture, observing time, and sensitivity than would currently be possible on existing high-altitude platforms. In particular, orbital operation provides the unique circumstance of both low residual atmo- sphere and very low infrared sky radiance, making a large cryogeni- cally cooled telescope both feasible and advantageous. Such an in- strument is not possible on the ground and is of limited potential in the stratosphere. Operating in space, it can provide substantial gains 83

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84 INFRARED ASTRONOMY in sensitivity relative to large ground-based instruments operating in the 10-jum atmospheric window. II. SCIENTIFIC OBJECTIVES Several previous reports have addressed themselves to the subject of scientific goals and programs for infrared astronomy in considerable detail.* From these studies, it is clear that infrared astronomical observations are as crucial as are optical and radio observations to the understanding of nearly every fundamental question that can be posed in astronomy today. For the purpose of illustrating the potential of spaceborne infrared astronomy, a few examples are discussed briefly. A. Solar-System Formation High-resolution spectroscopy of planets and comets over the broad spectral regions that are available in space can give molecular abundances and hence atomic abundances and isotopic ratios for the solar system. This information provides a clue to the history of the objects within the solar system. B. Stellar Evolution Spectroscopy with the sensitivity available from instruments in space can provide measurements on the composition and structure of the interstellar medium, stellar atmospheres, and circumstellar gas. These measurements, together with those within the solar system mentioned above, are crucial to theories of nucleosynthesis, stellar atmospheres, and evolutionary models of stars. C. Galactic Structure and Evolution Maps of portions of the galaxy at 100 jum with high spatial resolution, which can be made with a large cooled telescope, can define the distribution of gas and dust and its relation to the stellar *A Long-Range Program in Space Astronomy, Position Paper of the Astronomy Missions Board, R. O. Doyle, ed., NASA SP 213 (National Aeronautics and Space Administration, Washington, D.C., 1969). Astronomy Survey Committee, Astronomy and Astrophysics for the 1970's, Vol. 2, Reports of the Panels (National Academy of Sciences, Washington, D.C., 1973).

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Scientific Uses of the Space Shuttle 85 content in the galaxy. Sensitive long-wavelength photometry will be able to test theories of the mechanisms that generate the enormous infrared luminosities of many galaxies and cosmological objects. D. Cosmology It is of fundamental importance to understand fully the cosmic background radiation, as it may well be a remnant of the hot early phase of our universe. Data from ground-based observations and balloon and rocket flights are consistent with this interpretation, but what is now required is a good measurement of the short-wavelength portion of the spectrum, which can only be made from outside the earth's atmosphere with narrow-band instruments. Observation of any spatial or spectral anisotropies in this radiation would have great cosmological significance. III. SHUTTLE TELESCOPES: EVOLUTIONARY APPROACH The primary aim of infrared space astronomy is to set up an observatory in space. Existing technology is adequate to commence immediately with the design and construction of two classes of telescopes capable of making ir observations from space, one to operate at ambient temperature and the other to be cryogenically cooled. Each of these types of instruments is peculiarly suited to specific types of measurement, and both can be used to great advantage for ir observations outside the atmosphere. The type of instrumentation at the focal planes can be changed, depending on the particular scientific goal in mind, just as at a ground-based facility. It is intended that these general-purpose telescopes be flown early in the program. A spatial interferometer for obtaining high angular resolution at long infrared wavelengths is proposed for later in the program. This system could incorporate two or more 1-m ambient- temperature telescopes. These instruments are the first phase in an evolutionary program. With the experience gained, the design of larger instruments with collecting areas an order of magnitude greater can begin. A 2.5-m cryogenically cooled telescope is probably the largest that can be used on the Shuttle, but one can consider the practicality of constructing an even larger ambient-temperature telescope (10 m) in space.

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86 INFRARED ASTRONOMY TABLE 9 Evolutionary Stages in the Development of Observational-Type Infrared Instruments Early 1980's Mid-1980's Late 1980's Evolutionary stage Cryogenically cooled telescope 1 m 2.5 m Ambient-temperature telescope 1 m 3-m (LST) 10m Spatial interferometer 1 0-m base 1-km base A similar development is proposed for the spatial interferometer. Initially it will operate with a 10-m baseline on the Shuttle, but this will be extended to much longer baselines between the Shuttle and an associated free-flyer. The evolutionary approach is shown in Table 9. It is important to note that the early instruments will continue to be useful throughout the Shuttle period for investigations that do not require the higher performance of the later systems. In particular, a 1-m ambient telescope can be used with the spatial interferometer. During the early Shuttle period it will be important to launch free-flyers with specific objectives such as the measurement of the cosmic background and its isotropy as well as an exploratory survey mission. Also, it is anticipated that throughout the Shuttle period smaller instruments of the "hitchhiker" class, which use rocket-type systems, are needed to test new ideas and instrumentation. A. Cryogenically Cooled Telescopes Ground-based telescopes utilizing the best broadband infrared de- tectors available are presently limited by background noise from the thermal emission of the telescope and atmosphere. Beam- switching techniques, which provide discrimination between a faint discrete source and a bright uniform background cannot over- come this fundamental limitation. Full utilization of current and anticipated broadband detector technology requires the use of a Cryogenically cooled telescope operating above the atmosphere. Such an instrument would be optimum for broadband photometry and multiplex spectroscopy of faint galactic and extragalactic sources. In addition, a cryogenic telescope would make possible wide-angle (large-beam) photometry, permitting the mapping of very low sur- face-brightness extended sources and absolute flux measurements using total field chopping.

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Scientific Uses of the Space Shuttle 87 The limiting performance for a Shuttleborne (1.5-m) cryogeni- cally cooled telescope is given in Table 10. We recommend two instruments in this category. The first is a 1- to 1.5-m telescope, which can be constructed largely with existing technology. The second—a long-range goal—is a 2.5-m telescope whose design will be based on experience with the smaller telescope. The characteristics of these two instruments are given in Table 11. The temperature to which the telescope should be cooled for broadband diffraction-limited operation depends on the anticipated detector noise equivalent power (NEP), detector quantum efficiency, detector size, wavelength interval, temperature of the detector cav- ity, and telescope emissivity. Broadband detectors with an NEP of 1 Q- ' 6 yj m- 2 Hz-. are currentiy available for use between 10 and 30 jum. Comparable detectors should be achieved at longer wave- lengths by the 1980's. To take full advantage of such a detector in the 10- to 20-Mm region requires reducing the background radiation e (min of arc) ^ Object \ D w, Dm (pc)c De (pc)c (urn) (pc)c m-2)c a Orionis .10 450 K-9) 13(5) 100 K-12) 10(3) Orion nebula 100 6 450 H-8) 1(6) 10(3) Kleinmann-Low nebula 10 0.5 450 25(-1l) 7(5) 80(3) 100 1 5(-9) 7(5) 15(2) 350 1 4(-1l) 70(3) 15(2) Sagittarius B2 100 : 10(3) 4(-9) 13(6) 50(3) 350 2 6(-1l) 17(5) 15(3) Galactic center 10 0.4 10(3) 15(-11) 1(6) 150(3) 100 5 2(-9) 10(6) 200(3) 100 390 K-7) 70(6) 15(6) M82 10 0.5 32(5) 8(-12) 7(8) 60(6) 100 0.5 6(-1l) 7(8) 6(6) NGC 1068 10 0.02 17(6) 75(-13) 4(9) 3C273 10 12(8) 9(-14) 30(9) TABLE 10 Limiting Performance of a 1.5-m LH2-Cooled Infrared Telescopea- b aFrom Volume I of the Final Report of the NASA Shuttle Payload Working Groups. D is the distance of the objects that have been observed in the infrared. Dm is the distance at which similar objects could be observed with a 1.5-m cooled telescope. Dg is the maximum distance beyond which the extended objects shown, of angular diameter 6, could not be spatially resolved with this telescope. The flux levels actually observed for these objects are denoted by \F\. The calculations of Dm assume a signal-to-noise ratio of 1 and a 1-sec integration time. Detector NEP is taken as 10"16 W irr2 Hzt5'2 at 10 urn and 2 X 10~15 W m"2 Hz"'/2 at 100 /urn with assumed transmission efficiency 0.44. cNumbers in parentheses are powers of 10.

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88 INFRARED ASTRONOMY flux approximately four orders of magnitude below that which is encountered in ground-based observations. The greatest gain comes from cooling the telescope to approximately 20 K (liquid hydrogen temperature). The cryogenic telescope should be constructed with provision for more than one instrument that can be mechanically switched into the focal plane. Such instruments could include a multiband photometer, imaging devices, and a multiplex spectrom- eter. The detectors would be cooled to 2 to 4 K by liquid helium. The 1 -m cryogenic telescope is well suited to a sortie payload by its developmental nature, its cryogenic requirements, and the desire to update frequently the instruments at the focal plane. It should be fully gimbaled. Special attention must be paid to baffling against radiation from the earth, sun, and moon and the Shuttle bay radia- tors. A baffle extending outside the Shuttle bay will be needed to provide rejection of off-axis radiation. A field of view of approxi- mately 15 min of arc is required to permit fine acquisition and position objects in the focal plane and to allow for multiple detectors and imaging devices. The cooled telescope is to be the prime experiment on the partic- ular flight because of pointing demands and weight. Attention must be paid to the problem that contamination pre- sents for the cryogenically cooled telescope. The low operating tem- TABLE 11 Cryogenically Cooled Telescopes A B Mode (1980-1985) Sortie (1985-1990) Sortie or Free-flyer Diameter of primary (m) 1 2.5 Effective area(m') 0.7 4.2 Diffraction image size at 5 um (sec of arc) 2.5 1 (2.5X/0) 10 Mm (sec of arc) 5 2 100 jjm (sec of arc) SO 20 Field of view (min of arc) IS 5 Pointing stability (sec of arc) 0.5 0.2 Operating temperature (K) 20 20 Detector temperature (K) 2-4 2-4 Weight (kg) 5000 -15,000 Power (kW) 2 Operation From ground or cabin Data rate (kHz) <25 Mount Fully gimbaled Gimbaled or free-flying

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Scientific Uses of the Space Shuttle 89 perature is sufficient to freeze out the residual atmospheric gas, as well as outgassing and leakage from the Shuttle. Also, particulate matter, or "dust," can be a serious problem by increasing the back- ground radiation to the point of degrading the instrumental per- formance and making the total-field-chopping mode of data taking unsatisfactory. Further discussion of the contamination problem is in Section V.D. B. Ambient-Temperature Telescopes Cryogenic cooling (< 20 K) is highly advantageous whenever thermal emission from the telescope is the major limitation in reaching ulti- mate sensitivity, as discussed in Section III.A. When other noise sources dominate, emission from the telescope cooling yields negligi- ble improvement. This is true for the following two cases: 1. Spectroscopy in very narrow spectral intervals throughout the infrared and submillimeter region where detector noise generally dominates telescope background noise within the narrow interval. 2. Studies of relatively bright sources, such as the planets and other objects in the solar system, where the source dominates emis- sion from the telescope. In both cases observations can be made more efficiently and at lower cost with an uncooled telescope. Independent of the question of telescope emission, investigations that require a high angular res- olution must, for the foreseeable future, take advantage of the larger apertures (3 m, 10m) feasible with uncooled telescopes. A cooled telescope with 10-m aperture, although desirable from a scientific point of view, seems not to be feasible from an economic point of view in the 1980's. The 3-m telescope of the Large Space Telescope (LST) mission can provide many of the capabilities needed for infrared observa- tions. The potential available time on the LST might, however, not be adequate to satisfy the needs of the infrared community. In addition, the LST will not be optimized for low emissivity in the infrared. We therefore consider, as part of an evolutionary approach, ambient- temperature telescopes optimized specifically for infrared and sub- millimeter-wave operation. If sufficient utilization can be made of the LST capabilities, the easy steps in this approach can be leap- frogged.

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90 INFRARED ASTRONOMY 1. ONE-METER AMBIENT-TEMPERATURE TELESCOPE The first telescope in this category is a 1-m telescope optimized specifically for infrared and submillimeter-wave operation. Optimiza- tion consists of elimination or at least substantial reduction of emis- sion from baffles, support structure of the secondary, and the mirror surfaces themselves. Careful design should achieve a telescope emis- sivity of not more than 0.01 or 0.02. The 1-m telescope is viewed as an early Shuttle sortie payload, possibly beginning operation during the Shuttle test flights. The moderate size and weight of the telescope will allow operation on a nondedicated mission and is well suited to gain operational experi- ence with pointed astronomical instruments in the sortie mode. Fur- thermore, experience with this telescope is expected to be valuable for the design and operation of later, larger ambient temperature instruments. This telescope will yield higher sensitivity than can be obtained from stratospheric balloon and aircraft telescopes of similar size. In the late 1980's this telescope could be used as one element of a spatial interferometer as discussed in Section IV.C. The charac- teristics of the 1-m ambient-temperature telescope are given in Table 12. 2. THREE-METER AMBIENT-TEMPERATURE TELESCOPE The second stage in the evolutionary process of uncooled infrared telescopes is an instrument in the 3-m class, which represents an order of magnitude increase in the collecting area over the 1-m unit. This telescope should again be of a low-emissivity design optimized for operation at infrared and millimeter waves and diffraction limited at 5 jum. The characteristics of this telescope are also given in Table 12. While not optimized for infrared observations, regular use of the LST with cryogenically cooled detectors could provide some of the capability of the 3-m infrared telescope in terms of large aperture for narrow-band spectroscopy and spatial resolution of bright sources throughout the infrared and submillimeter region. It will also be possible to take advantage of the high optical quality of the LST to ob- tain sensitivity and resolution superior to what can be done from the ground by using efficient imaging detectors that should be available in the next decade for the near infrared.

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Scientific Uses of the Space Shuttle 91 TABLE 12 Ambient-Temperature Telescope A B c Parameter (1979) (1980-1985) (1985-1990) Mode Sortie Sortie or Construction of free-flyer free-flyer Diameter of primary (m) 1 3 10 Effective area (m1) 0.64 6 64 Diffraction limited at wavelength (aim) 1 5 20 Image size at 1 pm (sec of arc) 0.5 0.8 1 (2.5X./0 when applicable) 10 um (sec of arc) 5 1.7 1 100|im(secof arc) 50 17 5 Field of view (min of arc) -40 20 Pointing stability (sec of arc) 0.2 0.1 0.1 Operating temperature of primary (K) 200-250 150-200 150-200 Weight (kg) -1000 ~ 10,000 -25,000 Emissivity (telescope and mirror) 0.01 0.01 0.01 Detector temperature (K) 2-10 2-10 2-10 Power (Wav) -100 Operation From ground or cabin Data rate (kHz) -100 Mount Fully gimbaled Gimbaled or Free-flying free-flying 3. TEN-METER-DIAMETER AMBIENT-TEMPERATURE TELESCOPE The long-range goal in the development of ambient-temperature tele- scopes is a very large instrument that would provide the collecting area needed for high-resolution spectral studies and the aperture for good spatial resolution. The characteristics of a 10-m instrument are given in Table 11. This instrument is diffraction limited at 20 Mm with an image of 1 sec of arc. The 10-m telescope represents a new mode of Shuttle use because the telescope must be assembled in orbit. The telescope might be deployed by unfolding an array of rigid segments, the relative orien- tation of the segments to be determined by a set of laser beams. More than one Shuttle flight might be required to assemble and test the system. Thereafter, regular visits will be required for replacing detector cryogenic supplies, changing instrumentation, and main- tenance. A high orbital inclination, synchronous in a 6 a.m.-6 p.m. orbit, seems desirable from the viewpoint of solar power and thermal con- trol. Thermal control and baffling need very careful considerations in order to achieve reasonable low mirror temperatures by passive means, as well as a low emissivity. Many control and orientation subsystems can be adapted from the LST.

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92 INFRARED ASTRONOMY IV. SPECIALIZED INSTRUMENTS For certain measurements the observatory type of telescope is not suitable and specialized instruments are required. Instruments in this class are not envisaged as always being major undertakings but should also include relatively small pieces of equipment that individual groups with modest budgets could undertake to develop and con- struct. This point is particularly important since it significantly en- larges the community of astronomers involved in the Shuttle pro- gram and greatly increases the chance for new and exciting science. Examples are given in this section of the type of specialized in- strumentation for space use that can have an impact on infrared astronomy in the 1980's. A. Cosmic Background Radiation The 3 K cosmic background radiation affords astronomers a unique opportunity to study a fundamental problem. Considerable effort should be devoted to make accurate measurements of this radiation, both spectrally and spatially. A critical test of the blackbody nature of the radiation is to determine the spectrum on the short-wavelength side of the peak. This is probably best done with a free-flying satellite containing a cooled spectrometer of moderate resolution. The same satellite should also be capable of investigating the ex- tent to which the background is isotropic. This is of great cosmologi- cal interest, as it gives information on the structure of our universe and its past history. Measurements of sufficient accuracy would en- able our velocity to be determined relative to the frame in which the radiation is isotropic. Anisotropies can also arise from many other causes such as density fluctuations in the universe caused, for ex- ample, by clustering of OSO's. These measurements require the abil- ity to point the satellite and to be able to detect small relative temperature differences of the order of 0.1 percent. The long integra- tion times necessary will be possible with a free-flyer. B. Discrete-Source Sky Survey, Infrared Monitor There is a requirement for a free-flying cooled telescope to make an unbiased survey of infrared sources on the celestial sphere. This would complement and improve upon the rather selective and limited scans possible from aircraft, rockets, and balloons and would

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Scientific Uses of the Space Shuttle 93 identify objects of interest for further study using the larger tele- scopes on the Shuttle. In addition, and of great importance, the same instrument can be used to monitor changes of intensity of selected sources by repetitive scanning; the variability of sources could thus be followed continuously. C. Spatial Interferometer To obtain spatial resolution in the far infrared that is comparable with that obtained at optical and radio wavelengths, it is necessary to increase the size of the collecting optics in proportion to the wave- length. The largest size contemplated for a conventional telescope in the far infrared is the 10-m Shuttle telescope, which would give an angular resolution (2.5X/.D) of 5 sec of arc at lOOjurn. Better angular resolution than this is required for many astronomical problems, such as the separation of the various components of complex H II re- gions and the determination of infrared sizes of objects. This im- proved resolution can probably best be achieved by the use of a spatial interferometer operating over a relatively long baseline (as in the radio region). Such an instrument will rely heavily on the prog- ress made during the next few years in the development of tunable lasers and coherent detection techniques. In addition, there is the problem of determining the parameters of the baseline to the required accuracy. It is proposed that initially both telescopes (1m) be mounted in the Shuttle, giving an angular resolution similar to a 10-m telescope. This angular resolution would later be improved by operating with one or both of the 1-m tele- scopes mounted in associated free-flyers. The extent to which the resolution can be improved depends on how long a baseline it is practical to use—two 1-m dishes operating 1 km apart would give an angular resolution of the order of hundredths of seconds of arc, i.e., two orders of magnitude improvement over a conventional telescope. V. REQUIREMENTS ON THE SHUTTLE CAPABILITY A. Sortie Mode All instruments considered for the sortie mode will be mounted on the pallet. The Shuttle should provide a mechanical interface, electri- cal power, control links from the Shuttle cabin, data storage and handling facilities, and communication with the ground.

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94 INFRARED ASTRONOMY Operation of the instruments, including guidance, will be done remotely either from the ground or by the payload specialist. Manned access to the infrared detectors on the telescopes is not required and would be extremely difficult, as the detectors are cry- ogenically cooled, even on the ambient-temperature telescope. Although man's presence in the Shuttle can be used to advantage, it does not appear necessary to have a large crew. Missions up to 30 days offer the greatest rewards for ir astronomy, and large crews will decrease payload weight and mission length. Much of the telescope operation, and virtually all of the data analysis and decision making, can be done best by a ground-based team of scientists who have ready access to libraries and computers. Acquisition of predeter- mined objects could be accomplished most efficiently by a computer using continuous readings on Shuttle attitude and position relative to earth and sun. These considerations suggest that only the minimum crew of two pilots, a mission specialist, and a payload specialist be used for ir astronomy sortie missions. There is adequate room in the Shuttle cabin for the payload control console, display units, and some data analysis and storage equipment. This appears to eliminate the need for an additional pressurized cabin with its serious weight penalty. The use of the payload specialist in the Shuttle cabin requires that provisions exist for electrical interfaces between the payload special- ist's console and the pallet-mounted telescope. In addition to two or three display panels, a control panel, and other electronics associated with the scientific instruments, a small computer and tape recorder will be required for operating the telescope and for data handling. These instruments must interface with the Shuttle telemetry system as well as with the telescope. Tentative values for a number of parameters that make demands on Shuttle capability are tabulated below for the case of the 1-m cooled telescope: Orbital Altitude: 400 km. This is a compromise between the need to stay below the Van Allen belts to minimize high-energy radiation noise on the detectors and the need to minimize infrared radiation from the overlying earth atmosphere. Spacecraft Attitude: The Shuttle bay must be pointed away from the earth. This is required both to permit viewing of celestial objects and to reduce the number of constraints on telescope viewing angle produced by thermal radiation into the telescope barrel from the earth and the Shuttle bay doors.

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Scientific Uses of the Space Shuttle 95 Flight Frequency (2 per year): A greater frequency is desirable. If the frequency is less than two per year it may become difficult to justify the investment in money and scientists' time required to de- velop the facilities. Duration in Orbit: 7 to 30 days, as long as possible. Telescope Weight Estimate (5000 kg): This includes the stabiliza- tion and acquisition systems, the Dewars, the control electronics, the optics and their support structure, baffling, and the entire comple- ment of attached instruments. The weight could be decreased con- siderably at increased cost in materials and engineering. Telescope Volume Estimate: 16m3. Shuttle Stabilization Limits (±5°): Note that the telescope gimbal system is assumed to provide stabilization to 0.5 sec of arc and pointing capability over a large fraction of a hemisphere. Unobstructed Viewing Angles: At least 90° in the plane of the long axis of the Shuttle and its tail and at least 120° in the plane perpendicular to the long axis. The later requirement permits 30 min of continuous viewing of objects near the celestial equator without slewing the Shuttle if the Shuttle has its long axis perpendicular to the orbital plane. Electrical Power (2 kW, continuous): This would supply the tele- scope its instrument complement, the stabilization and pointing system, the 32K word computer, tape recorder, and other pay load electronics. Communications: Continuous voice up and down plus a com- mand link up and data link (25 kbits/sec or more) down. Short periods of 200 kbits/sec down are also required. The requirements of the 1-m ambient temperature telescope are similar but less demanding in weight, volume, and power, as in- dicated in Table 11. If the 2.5-m cooled telescope is used in the sortie mode, its weight, power, and volume requirements on the Shuttle obviously would be much greater. It would occupy the full diameter of the Shuttle bay for a length of at least 12 m. Its weight and power would be determined from tradeoffs between Shuttle capacity and tele- scope cost and performance. The 10-m baseline spatial interferometer has the additional re- quirement that the two components will be placed 10 m apart in the Shuttle bay. The 1.5-km baseline spatial interferometer, still in a very early conceptual phase, will require that the Shuttle orbit approxi- mately 1.5 km from a subsatellite that it launches early in the mis- sion. The subsatellite must always be visible from the payload bay.

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96 INFRARED ASTRONOMY These requirements will probably demand more maneuvering of the Shuttle by the astronauts than most other sortie-mode experiments. B. Launch Mode In this mode, the Shuttle is used as a checkout and launch facility. Payloads with instruments operating in the infrared and submilli- meter wavelengths will range in size from small rocket-type free- flyers to large planetary probes requiring Tug or other additional propulsion units. The role of man will be primarily to check out the payloads prior to their separation from the Shuttle. Man will also maneuver the Shuttle and deployment mechanisms during payload separation from the Shuttle. C. Assembly Mode By the late 1980's the need for a telescope with a primary mirror larger than the Shuttle bay diameter will require construction, assem- bly, or deployment of a large structure in orbit. New techniques such as laser alignment of individual elements will be necessary. Although most of the mechanical deployment will have to be automatic, it is expected that a substantial crew, perhaps as many as eight, will be required. Revisits to the telescope may be needed. D. Contamination One of the principal advantages of infrared observations from space is the elimination of the most disturbing effects of the earth's atmo- sphere. The full potential sensitivity of the space environment can be realized only if sufficient care is given to the effects of the residual atmosphere or the contamination by the spacecraft. The sources of contamination that have to be considered are (1) residual atmo- sphere; (2) spacecraft [outgassing, reaction control subsystem (RCS) firings, dumps, and dust emission]; (3) high-energy radiation (Van Allen radiation belts, South Atlantic Anomaly). 1. THE RESIDUAL ATMOSPHERE The large cryogenically cooled telescope imposes the most severe requirements on contaminants. Because the temperature of the tele- scope is expected to be ~ 20 K, major atmospheric constituents can

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Scientific Uses of the Space Shuttle 97 condense on optical surfaces. For low orbits, the atomic and molec- ular gas densities will be approximately 109/cm3. Therefore a maxi- mum rate of condensation, of one monolayer every 10 sec, will occur when the telescope is pointed in the direction of motion. In this case, a sortie mission of approximately 106 sec will deposit 10s mono- layers, which would severely impair the telescope performance. The rates will be substantially less when viewing normal to the direction of motion. However, more detailed studies of this problem will be required in conjunction with the design of a specific instrument. 2. CONTAMINATION INTRODUCED BY THE SPACECRAFT All contaminants introduced by the spacecraft should be kept below the level of the residual atmosphere. The outgassing and leakage from the Shuttle is a potential source of contamination for cryogenic op- tics; there will also be infrared emission from the warm gas. Apollo 15 data indicate that a water leakage and outgassing rate of 3 x 10t4 g sect' produced a return rate of ~1012 cmt2 sect1 srt!, with other gases 100 times more abundant. If the telescope shield reduces the solid angle to 10t 2 sr, the rate of deposition of all leakage gases is ~ 1 monolayer/h, which may be tolerable for the proposed Shuttle missions. The 25-lb RCS used for attitude control will generate gaseous products at a mean rate of 0.3 g/sec, which may be some hundred times higher than leakage from the Shuttle. Therefore these systems must be designed to have a return rate of less than 1 percent to the vicinity of the spacecraft to avoid significant increase in contamina- tion. All waste should be stored until the end of the mission. Particulate emission from the Shuttle is an important considera- tion for infrared missions. For example, a single dust particle of 10-Mrn diameter at ambient temperature is readily detectable up to tens of kilometers with the sensitive systems envisaged (NEP ~W Hzt /2). If the rate of emission of particles larger than 10 nm is ~ 10t 4 cmt2 sect', there will be, on the average, one particle in the beam of the 1-m telescope at any time. Therefore it is important to determine how dust emissions can be reduced. Unwanted infrared radiation scattered into the telescope from sources such as the sun, the earth, the moon, and the Shuttle itself must be reduced by appropriate baffling.

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98 INFRARED ASTRONOMY TABLE 13 Preliminary Acceptable Contamination Levels for Infrared Astronomy Sortie Missions Return rate (due to leakage and outgassing) < 1012 molecules cmt2 sect1 Particulate emission (10 Mm or larger) < 10t4 cm t2 sect1 RCS fuel expended (1% return) < 20 kg daytl Waste dumps None Column density of HjO etc. (molecules with dipole moments) < 1010- 1012 cmt2 Background ionizing radiations < one event per sq cm per sec in region of the detector Any discussion of contamination of Shuttle-based infrared astron- omy depends on the design of the spacecraft and the state of the art in detector and instrument design in the 1980's. Preliminary discus- sions such as this are sufficient to demonstrate that a problem exists. We, therefore, recommend that NASA set up a group of working astronomers and engineers to study this problem in detail and to follow the evolution of Shuttle design and infrared technology so that serious conflicts do not arise. 3. HIGH-ENERGY RADIATION The photoconductors used for detection of infrared radiation are also sensitive to high-energy radiation. Regions such as the Van Allen belts and the South Atlantic Anomaly, where the density of this radiation is high, are to be avoided. Contamination level requirements are summarized in Table 13. VI. POTENTIAL MISSION MODEL Table 14 gives a potential mission model for the infrared and sub- millimeter instruments described in this report. VII. SUPPORTING RESEARCH AND TECHNOLOGY Orbiting cryogenic telescopes will offer an unparalleled opportunity for astronomical observations in the infrared. The large technological effort to construct and orbit an infrared observatory must be bal- anced by a substantial scientific supporting research and technology (SR&T ) effort to ensure maximum effectiveness of an overall infra- red program. A vigorous program of infrared astronomy should be

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100 INFRARED ASTRONOMY pursued in the 1970's to prepare the instrumental and astronomical foundations required for a Shuttleborne infrared program. The re- quired scientific and technological advances can best be achieved by supporting a substantial effort in infrared astronomy by many indi- vidual research groups working on both ground-based and strato- spheric infrared instruments. This program will develop a pool of knowledgeable users for Shuttle-based astronomy as well as produce a variety of instrumenta- tion and ideas that can be incorporated into the planning and con- struction of payloads for Shuttle flights in the 1980's. We consider the present SR&T funding in this area to be totally inadequate for this purpose and deem it essential that the SR&T budget be increased. The level of funding necessary to produce the desired balance between SR&T and hardware represents a small fraction of the cost of a major orbiting payload, yet the SR&T program will increase the scientific payoff of infrared astronomy in space by a very substantial factor. VIII. SUMMARY AND RECOMMENDATIONS Infrared space astronomy will begin as a substantial effort during the Shuttle era. The primary aim in this field will be to set up observa- tories in space, in particular, a large (l-P/^m) cryogenically cooled telescope and a very large (3-10-m) ambient-temperature telescope. These instruments are appropriate for Shuttle transportation because of their large size and weight and because of the rapidly advancing nature of infrared instrumentation, which requires frequent modifi- cation and improvement. Infrared telescopes are especially sensitive to potential gaseous and particulate Shuttle contamination. It appears that the infrared requirements on contamination levels can be met with some special- ized Apollo-type contamination control. Some specialized small in- struments, for which contamination requirements are particularly severe, will have to be launched as free-flyers. Much of the operation of these observatories, and most of the data analysis and decision making, will be carried out by a ground- based team of scientists and by a control computer on the Shuttle. The mission specialist will be utilized during initial setup and instru- ment mode changes and for overcoming unanticipated problems. Space observatories offer a potential for utilizing infrared instru- mentation far more sensitive and advanced than can be used in ground-based observatories. Realizing this potential requires a contin-

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Scien tific Uses of the Space Shuttle 101 uing and substantial research effort with ground-based and strato- sphere techniques to maintain the flow of new technology. Specific recommendations for infrared and submillimeter Shuttle astronomy follow. 1. We recommend initiation of design of a cryogenically cooled 1 -m class telescope for sortie missions. 2. We recommend that the use of the Large Space Telescope for infrared astronomy be pursued. 3. We recommend that a vigorous program of infrared astronomy be pursued in the 1970's in order to prepare the instrumental and astronomical foundation required for the recommended Shuttle pro- gram. 4. We recommend the creation of a Shuttle contamination board composed of engineers and astronomers to establish standards for contamination based on the requirements of astronomy and to par- ticipate in reviews of Shuttle design. 5. We recommend that provision be made for small gimbal- mounted payloads attached to the Shuttle bay. 6. We recommend that the payload specialists, control console be designed for easy inclusion of equipment unique to individual experi- ments.