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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS 4 New Initiatives INTRODUCTION Progress in understanding the universe comes from new or improved observing techniques and from theoretical insights. Over the next decade astronomers will use new technologies to increase dramatically both the sensitivity and the spatial and spectral resolution of their observations. The program of new initiatives recommended in Chapter 1 (see Table 1.1) calls for telescopes that improve these key areas of performance by factors of tens to millions. Chapter 1 contains thumbnail sketches of the major initiatives. This chapter gives more complete technical descriptions of the large and moderate programs and their anticipated scientific returns. Most of the instrumental initiatives result from advances in infrared technology, in spatial resolution, in the construction of large telescopes, and in the linking of electronic detectors to powerful computers. Four areas of research are currently of particular significance, and these will be used to illustrate the importance of the recommended instruments. They are the following: The birth of stars and planets. Observations and theory suggest that the formation of a rotating disk of gas and dust, containing enough material to make planets, is a natural stage in the birth of stars like the sun. Astronomers now strongly suspect that planets form in such protoplanetary disks of gas and dust around young stars. New telescopes with high infrared sensitivity or hundredth-of-an-arcsecond spatial resolution will be used to investigate how
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 4.1 Characteristics of Recommended Large and Moderate Programs Project Wavelength Coverage Increase in Sensitivitya Typical Spatial Resolution (arcsec) Typical Spectral Resolution SIRTF 2.5 to 700 µm 30 to 103 7.5 (λ/30)b 3 to 2000 Infrared-optimized 8-m telescope 1 to 30 µm 3 to 10 0.7 (λ/30)b 3 to 105 MMA 900 to 7000 µm 30 0.07 (λ/1000)b 100 to 106 Southern 8-m telescope 0.3 to 2 µm 1 0.1 at 0.5 µm 3 to 105 Adaptive optics 0.3 to 10 µm 50 0.1c 3 to 104 SOFIA 1 to 1000 µm 10 2.5 (λ/30)b 3 to 106 Optical and infrared interferometers 0.3 to 10 µm 103 2.4 × 10−4λb , d 3 to 100 AIM 0.1 to 1 µm < 30 × 10−6 3 to 100 LEST 0.3 to 1 µm 0.1 at 0.5 µm 10 to 105 VLA extension 1 to 100 cm 0.1 at 6 cm 50 to 106 a Relative to other existing or planned facilities of comparable nature. b Wavelength in microns. c 2 µm with 8-m telescope. d 1-km baseline. stars form out of interstellar gas and how the disks that surround young stars might evolve into planets. Active galaxies and quasars. Are active galaxies powered by black holes? What is the link between infrared-luminous galaxies and quasars? These questions will be explored using ground- and space-based telescopes with high spatial and spectral resolution operating at wavelengths from radio to gamma rays. Large-scale structure of the universe. Observations indicate that clusters of galaxies are strewn in sheets and filaments surrounding large voids and that as much as 90 percent of the mass in the universe may have escaped detection. New ground-based instruments operating at infrared, optical, and radio wavelengths will map the three-dimensional distribution of matter in the universe out to distances of a billion light-years and may reveal the physical processes that create such unexpected patterns. The birth of galaxies. The greatest single burst of star formation in the history of the universe attended the birth of galaxies between 10 billion and 15 billion years ago. New instruments will be used to investigate how galaxies and quasars come into being and how their existence can be reconciled with models of the infant universe. Searches for protogalaxies will require observations with sensitive, large-format arrays of infrared detectors on telescopes in space and on the ground. Table 4.1 summarizes the performance of the recommended large and
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 4.2 Contribution of Major Recommended Programs to Scientific Themes Program Formation of Planets and Stars Origin of Energetic Galaxies and Quasars Distribution of Matter and Galaxies Formation and Evolution of Galaxies SIRTF Comets, primordial solar system Planetary debris disks Protostellar winds Brown dwarf surveys Spectra of luminous galaxies to z = 5 Deep 10- to 100-µm surveys Galactic halos and missing mass Protogalaxies Galaxy evolution Infrared-optimized 8-m telescope Imaging and spectra of protostellar disks and jets Planet searches Planetary structure, atmospheres High-resolution imaging and spectra of energetic galaxies Deep 2-µm surveys Galaxy evolution High-z protogalaxies High-z supernovae MMA Motions, structure, chemistry of molecular clouds and protostellar disks Comets Planetary atmospheres Motions, structure, chemistry of quasars, luminous galaxies CO redshift surveys Dusty galaxies at z ≤ 10 Sunyaev-Zeldovich effect Southern 8-m telescope Doppler searches for planets Spectra of active galaxies and quasars Deep imaging of galaxies Quasar abosrption lines z = 1 redshift surveys Galaxy evolution Quasar distribution Distant globular clusters
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS moderate projects in key areas. Table 4.2 compares the contributions of the major recommended projects to the scientific themes. More detailed descriptions of the instruments and the science that they might accomplish can be found in the Working Papers (NRC, 1991). THE DECADE OF THE INFRARED The infrared and submillimeter portion of the spectrum, from 1 µm to 1000 µm, is poorly explored but is of fundamental importance for almost all aspects of astronomy, from solar system studies to cosmology. Four factors make these wavelengths critical: (1) the expansion of the universe shifts the radiation from primeval objects out of the ultraviolet and visible bands into the infrared; (2) most of the known mass in galaxies is in the form of cool stars that are brightest at wavelengths in the range 1 to 10 µm, depending on the distance to the galaxy; (3) the dust associated with cold gas and star-forming regions obscures objects at wavelengths shorter than 1 µm, but glows brightly at longer wavelengths due to the absorbed energy; and (4) atoms and molecules have rich infrared spectra that can be used to probe the density, temperature, and elemental abundances of astronomical objects. The technology for detecting infrared and submillimeter radiation has been revolutionized in the last 10 years. A decade ago, astronomers used single detectors on ground-based telescopes to observe in a few spectral windows between 1 and 30 µm. Radio astronomers struggled to detect radiation at wavelengths as short as 1,000 µm. Pioneering astronomers used balloons or airborne telescopes to work between 30 and 1000 µm. In 1983 the Infrared Astronomical Satellite (IRAS) demonstrated that a telescope cooled with liquid helium could approach the theoretical sensitivity limit set by the faint light emitted by interplanetary dust grains. The 1,000-fold increase in sensitivity compared with that of earth-bound telescopes permitted IRAS to survey the entire sky at wavelengths from 12 to 100 µm and to discover important new phenomena, including trails of solid material behind comets, disks of solid material orbiting nearby stars —possibly the remnants of planet formation (Plate 2.1 and Plate 4.1)—and luminous galaxies emitting more than 90 percent of their energy in the infrared. At wavelengths between 100 and 1000 µm, new techniques for detecting radiation using the Kuiper Airborne Observatory (KAO) and ground-based telescopes like the Caltech Submillimeter Observatory (CSO) led to the discovery of spectral lines from atoms and molecules that brought new information about planetary atmospheres, star formation, and interstellar chemistry. Most recently, the development of arrays of detectors operating from 1 to 200 µm has led to the replacement of single-channel photometers by cameras and two-dimensional spectrographs with 50,000 or more individual detectors. The large initiative accorded the highest priority by this committee is
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS the Space Infrared Telescope Facility (SIRTF), consisting of a 0.9-m telescope cooled by a five-year supply of liquid helium and mounted on a free-flying spacecraft. As proposed, SIRTF will be launched by a Titan IV-Centaur into a high earth orbit with an altitude of 100,000 km. It will operate at high efficiency, with tens of hours of uninterrupted coverage possible on a single area of sky. This initiative will unite two proven technologies to make a national observatory of unprecedented power. First, the technology for cooled telescopes has been demonstrated by two Explorer satellites, IRAS and the Cosmic Background Explorer (COBE). The thermal background for SIRTF will be a million times less than that for a terrestrial telescope. Second, SIRTF will take full advantage of the U.S.-led revolution in infrared detector arrays. IRAS had only 62 detectors; SIRTF will have over 100,000. Figure 4.1b shows SIRTF's expected sensitivity compared to the brightness expected for representative extragalactic objects. Figure 4.2 compares SIRTF with the European Infrared Space Observatory (ISO) mission, to be launched in 1994. Because of its larger aperture and its use of larger and more sensitive detector arrays, SIRTF will be thousands of times more capable than ISO. SIRTF will follow up on ISO's discoveries in addition to breaking new ground with its own deep surveys. All key technologies have been successfully demonstrated, either on the ground or with precursor space missions, during a decade of study by NASA. SIRTF could be initiated in 1994 and launched around 2000 to provide valuable overlap with the Hubble Space Telescope (HST) and the Advanced X-ray Astrophysics Facility (AXAF). The committee's highest priority for ground-based astronomy is an 8-m-diameter telescope for the summit of Mauna Kea, Hawaii, optimized for low-background, diffraction-limited operation in the infrared between 2 and 10 µm but also useful in the optical regions of the spectrum. Mauna Kea is recognized as the best terrestrial site for an infrared telescope because of its low level of water vapor, the primary absorber and emitter of infrared radiation in the earth's atmosphere. Further, the remarkable stability of the atmosphere above this mountain results in minimal distortion of astronomical images. From this dry and stable site, an 8-m telescope would achieve diffraction-limited resolution of 0.1 arcsecond at 2 µm using modest “adaptive optics” techniques to correct for residual atmospheric distortion. The infrared-optimized 8-m telescope will gain its marked increases in sensitivity relative to the sensitivity of other large telescopes owing to the 0.1-arcsecond images possible in the infrared with adaptive optics, and to the low emissivity expected for a silver-coated monolithic mirror. The telescope would provide large amounts of data from new infrared detectors on a large telescope at high spatial resolution. Among the many projects the infrared-optimized 8-m telescope would carry out, two for which it would be particularly well suited are deep searches for and spectroscopic studies of primeval galaxies (Plate 4.2). The proposed national infrared-optimized 8-m telescope will differ from
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 4.1a (top) Photometric sensitivity of three proposed facilities, SIRTF, SOFIA, and the infrared-optimized (IRO) 8-m telescope (solid squares) for broadband observations. For reference the figure also gives the sensitivity of the KAO, the 3-m IRTF (open squares), and the IRAS survey (solid triangles). Technical assumptions underlying these curves can be found in the report of the Infrared Astronomy Panel in the Working Papers (NRC, 1991). FIGURE 4.1b (bottom) SIRTF's sensitivity (solid lines) for photometry [resolution (R) = 2] and spectroscopy [resolution (R) = 100] compared with the predicted brightnesses of representative extragalactic objects scaled to a common redshift of 5, when the universe was only one-sixth its present size. SIRTF could detect luminous galaxies, quasars, and protogalaxies in the early universe.
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 4.2 The power of SIRTF compared with existing telescopes and with the European ISO mission. “Astronomical capability” is defined as (facility lifetime) × (efficiency) × (number of detectors)/(sensitivity)2 and is normalized at various wavelengths to the capabilities of NASA 's 3-m telescope on Mauna Kea, IRAS, or the KAO. The figure shows how much more quickly SIRTF would be able to map or survey a region of sky to a particular flux limit than would other telescopes. the private 10-m Keck telescope in three ways: (1) it will be accessible to all U.S. astronomers, whereas the Keck telescope will be available to only about 3 percent of the national astronomical community; (2) it will be the only large telescope in the world optimized for performance in the infrared; and (3) it will use adaptive optics to achieve the maximum possible spatial resolution in the near-infrared. The third telescope needed to cover this factor-of-1,000 range in wave-lengths is the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA), a moderate-sized 2.5-m telescope. Mounted in a Boeing 747 aircraft, SOFIA will fly over 100 8-hour missions per year. At altitudes of 41,000 ft, above 99 percent of the water vapor in the earth's atmosphere, SOFIA will open the wavelength range from 30 to 350 µm to routine observation and make valuable contributions at still longer wavelengths. In particular, spectral observations at these wavelengths hold the key to understanding the physics in regions of high density and moderate temperature that characterize the primitive nebulae around newly formed stars, and the cores of infrared-luminous galaxies and quasars. SOFIA's capability for diffraction-limited imaging and high-resolution spectroscopy at wavelengths inaccessible from the ground will complement SIRTF's great sensitivity at infrared and submillimeter wavelengths. SOFIA is a joint project with Germany, which will supply the telescope system and support about 20 percent of the operations. NASA and the German space agency
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 4.3 Comparison of Different Recommended Infrared Facilities Facility Most Important Attributes SIRTF Unequaled sensitivity for imaging and moderate-resolution spectroscopy Broad wavelength coverage from 2 to 700 µm 7.5 (λ/30 µm) arcsecond imaging of faint sources at λ > 30 µm Infrared-optimized 8-m telescope 0.7 (λ/30 µm) arcsecond imaging for λ < 30 µm High-resolution spectroscopy in atmospheric windows for λ < 30 µm Evolving instrumentation SOFIA High-resolution spectroscopy at λ > 30 µm 2.5 (λ/30 µm) arcsecond imaging at λ > 30 µm Training of instrumentalists have successfully completed preliminary design studies for SOFIA, drawing heavily on the technical heritage of the KAO. SOFIA could begin observations in 1998. Table 4.3 compares the most important attributes of the three recommended infrared facilities, SIRTF, the infrared-optimized 8-m telescope, and SOFIA. One can see from this comparison that the three instruments are mutually complementary. SIRTF has the highest sensitivity for photometry, for imaging, and for low- to moderate-resolution spectroscopy (~100 km s−1). Between 3 and 20 µm, SIRTF will be 10 to 40 times more sensitive than the infrared-optimized 8-m telescope. Despite advances in ground-based telescope design and detector technology, SIRTF will maintain fundamental advantages in sensitivity longward of 3 µm. SIRTF will also have the uninterrupted spectral coverage from 2 to 200 µm needed to detect important molecular and atomic spectral features. The great strength of the infrared-optimized 8-m telescope compared with SIRTF will be its ability to operate at high spectral or spatial resolution, or both. The telescope, capable of subarcsecond resolution owing to its adaptive optics, will make maps with 100 times more spatial information than those made by the 0.9-m SIRTF telescope. Information on this angular scale will be critical for understanding the disks around young stars and the energy source of infrared-luminous galaxies. For spectroscopy shortward of 5 µm, and at resolving powers in excess of 100,000 from 2 to 20 µm, the infrared-optimized 8-m telescope will be more sensitive than a space-based one throughout the wavelength region accessible from the ground. The infrared-optimized 8-m telescope will make seminal contributions to problems requiring both high spatial and spectral resolution, such as probing the centers of dusty galaxies like our own to look for evidence of massive black holes.
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS Instruments on SOFIA will be able to operate with the spectral resolution of 1 km s−1 needed for dynamical studies of galactic sources in wavelengths inaccessible from the ground. SOFIA will make spectroscopic observations in the large variety of molecular and atomic transitions that characterize far-infrared and submillimeter wavelengths. For bright sources, SOFIA 's maps will contain nine times the spatial information of maps made by SIRTF, although SIRTF will be much more sensitive for broadband observations. This information will be extremely useful in probing the physical conditions in protostars, planetary debris disks, and galaxies. Finally, SOFIA will provide excellent training for young experimentalists and a valuable opportunity to develop and test new instruments. Other projects operating in the infrared and submillimeter region are discussed in the Working Papers (NRC, 1991) and include an Explorer mission to make a spectral survey of important classes of objects at submillimeter wavelengths; a ground-based survey of the entire sky at 1 to 2 µm with a threshold 50,000 times fainter than that of the only other survey, which is now 20 years old; allocation of funds to equip ground-based telescopes with revolutionary new infrared arrays; and a radio telescope using arrays of receivers to look for anisotropies in the cosmic background radiation that might provide clues to when and how galaxies formed. HIGH SPATIAL RESOLUTION Basic physical principles limit the smallest angle that a telescope can discern to a value approximately equal to the wavelength of the radiation observed divided by the characteristic size of the telescope. At visible wavelengths this limit is a few hundredths of an arcsecond for a 5-m telescope, although this limit has not been achieved until very recently, and then only under special circumstances, the limiting factor in practice being the degradation by turbulence in the earth 's atmosphere. The highest quality of astronomical images at the best ground-based sites on the rarest, most stable nights is about 0.3 arcseconds. At radio wavelengths, the limiting resolution of the largest single antennas varies with wavelength from about five to a few hundred arcseconds. Radio astronomers have developed the technique of interferometry in which small telescopes spread over large distances are linked together to simulate a single telescope with an aperture equal to the largest separation between the component telescopes. The Very Large Array (VLA) consists of 27 telescopes separated by up to 35 km and is capable of resolving structures about 0.3 arcsecond in size. Very long baseline interferometry uses telescopes distributed over the entire earth to resolve objects smaller than a thousandth of an arcsecond. Six of the programs recommended in Chapter 1 stress 10-fold or greater improvements in spatial resolution compared with that possible with existing facilities. At visible and infrared wavelengths new technologies may reduce the
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 4.3 An artist's conception of the proposed MMA shows the 40 8-m telescopes spread out in a ring 900 m in diameter. Inner and outer rings of 70 m and 3 km, respectively, are also visible. Courtesy of the National Radio Astronomy Observatory/Associated Universities, Inc. effects of atmospheric distortion and lead to interferometers capable of resolving a few thousandths of an arcsecond. At radio wavelengths, interferometric techniques will make possible an array of telescopes capable of subarcsecond imaging at 1 mm (1,000 µm), bringing many classes of phenomena into clear view for the first time. The Millimeter Array The recommended Millimeter Array (MMA) will be a sensitive, high-resolution instrument providing high-fidelity images at wavelengths between 0.9 mm (900 µm) and 9 mm. Comprising 40 individual 8-m telescopes, it will have a resolution at 1 mm that ranges from 0.07 arcsecond for the largest (3-km) configuration of the telescopes, to 3 arcseconds for the compact (70-m) configuration (Figure 4.3). The technology for the MMA is well understood and draws on expertise developed with centimeter interferometers like the VLA, university millimeter interferometers, and commercial systems. Compared with any other millimeter telescope in the world, existing or planned, the MMA will have better angular resolution by more than a factor of 10, better sensitivity by a factor of 30, and better imaging speed by a factor of 100 or more. The MMA will provide observations in a wavelength regime and with spatial and spectral resolution that are highly complementary to the three infrared telescopes discussed above. The MMA will aid the studies of galaxy formation by detecting the
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS dust and gas emission from very young galaxies, early in the history of the universe. Surveys tracing emission from carbon monoxide molecules will lead to three-dimensional maps of the large-scale distribution of spiral galaxies out to cosmologically exciting distances. Images with high spatial and spectral resolution will unveil the kinematics of optically obscured galactic nuclei and quasars on spatial scales smaller than 300 light-years. Images of young stars taken with existing millimeter interferometers have already demonstrated the existence of enough material in orbit around younger versions of the sun to make 10 to 100 Jupiters. The MMA will measure the mass, temperature, and composition of such protoplanetary disks with greatly improved sensitivity and resolution. Adaptive Optics The recommended program in adaptive optics will give existing and future large telescopes the ability to remove atmospheric distortions, thereby increasing the resolution and sensitivity of astronomical measurements. “Adaptive optics” is different from “active optics.” The latter refers to techniques, like those planned for the Keck 10-m optical telescope or the Green Bank 100-m radio telescope, to correct for minute- or hour-long drifts in mirror shape due to gravity, wind, and temperature drifts. “Adaptive optics,” however, attempts to compensate for the rapid, hundredth-of-a-second effects of atmospheric turbulence. Of the two, adaptive optics is the more challenging, but also the more rewarding scientifically. Turbulence in the atmosphere scrambles light waves in patches larger than a characteristic size, r0, about 8 in. at visible wavelengths. Consequently, light reaching a telescope of diameter larger than this is so badly disordered that diffraction-limited imaging is normally impossible. European astronomers, as well as U.S. scientists, have developed techniques to monitor the wave-front errors in each r0-sized patch of a telescope mirror, and to correct them with reference to a nearby standard star by warping the mirror appropriately. Corrections must be made within the “coherence time” of the atmosphere, τ0, roughly every hundredth of a second at visible wavelengths. Complete phase corrections require a reference star brighter than a visual magnitude of about 9 located within an isoplanatic angle θ0 fof the object of interest. This angle is only a few arcseconds at visible wavelengths. Partial corrections can still improve angular resolution and might utilize stars as faint as 15 magnitude separated by larger distances. The size parameter, r0, the coherence time, τ0, and the allowable distance between object and reference star all increase with wavelength. Thus adaptive optics will probably be applied first in the infrared. The number of r0 patches across a telescope is much smaller in the infrared than in the visible, so that the number of correcting actuators is tens instead of hundreds. Corrections are
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS needed less frequently, and fainter and more numerous reference stars can be used, since the coherence time is longer, up to half a second. Finally, more sky is available to search for reference stars because θ0 can be as large as 100 arcseconds. Ultimately, a solution to the problem of finding suitable reference stars at visible wavelengths may be to use lasers to generate “artificial stars” in the sky close to sources of interest. This technique has been partially developed by the Department of Defense but needs to be adapted to astronomy. The committee proposes that adaptive optics techniques be developed and implemented on existing and planned large telescopes, such as NASA's existing Infrared Telescope Facility (IRTF) on Mauna Kea and on the new generation of 8- and 10-m telescopes. It should be possible to produce near-diffraction-limited performance at wavelengths as short as 1 µm. The proposed program would also support development and implementation of adaptive optics on existing solar telescopes, as pioneered at Sacramento Peak, and the development of adaptive optics for the proposed Large Earth-based Solar Telescope (LEST). The scientific gains from applying adaptive optics will have an enormous impact on many branches of astronomy. The desired 0.1-arcsecond imaging resolution at 2 µm is adequate to resolve details on many planets and satellites in our own solar system, to examine nascent solar systems around young stars, to search for signs of black holes in the cores of energetic galaxies, and to look for supernovae in young galaxies at redshifts of 1 or greater. Furthermore, the reduction in background-noise contamination made possible with diffraction-limited optics could improve the limiting sensitivity of ground-based telescopes by up to a factor of 50. Optical and Infrared Interferometers In the 1920s A. Michelson and F. Pease showed that two or more telescopes linked together can achieve a spatial resolving power proportional to the distance between the apertures. Although such interferometers have been built with great success at radio wavelengths, relatively little progress has been made, until very recently, at optical or infrared wavelengths. In the past decade optical interferometers in the United States and Europe have produced useful scientific results, including wide-angle astrometry with thousandth-of-an-arcsecond precision, measurements of stellar diameters with a precision sufficient to constrain stellar atmosphere models, and the resolving of close binary stars. These projects have demonstrated that the key technologies are in hand to make interferometers using 1- to 2-m-diameter telescopes separated by baselines up to a kilometer, with corresponding resolution better than a thousandth of an arcsecond at a wavelength of 1 µm. Figure 4.4 and Plate 4.3 show two recent results from optical interferometry. Continued support for existing small interferometers and the development
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 4.4 Apparent orbit of the binary star Beta Arietis observed with the Mark III interferometer operating on Mt. Wilson. The Mark III system has routinely demonstrated the ability to control the active optical elements to the hundredths-of-a-micron precision necessary for a scientifically productive instrument. Reprinted by permission from Pan et al. (1990) and the Astrophysical Journal. of at least one larger one would yield immediate scientific return and would help solve technical problems. An array of five 2-m telescopes with baselines exceeding 100 m would provide infrared images of important astronomical objects with a resolution of a few thousandths of an arcsecond, and would lead the way for more ambitious programs in the next decade. The facilities operated in the 1990s would have a major impact on the study of individual stars, of newly forming stars, and of the compact cores of luminous galaxies that may harbor black holes. As discussed in Chapter 6, ground-based interferometers will provide tests of key technologies and concepts for future space interferometry and are an important part of a balanced program leading to major space facilities. Late in the 1990s the program would support planning and design of an advanced interferometer to be built in the next decade. Astrometric Interferometry Mission Despite the promise of interferometry carried out from the ground, the ultimate power of this technique will probably be fully realized only with a
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS system operating in space, where no limits to the coherence size, angle, or time are imposed by the terrestrial atmosphere. NASA has been studying a number of possible space missions as a first step in interferometry from space. A concept that appears particularly appealing is to use interferometric techniques to achieve a 1,000-fold improvement in our ability to measure celestial positions. The mission requirement would be to measure positions of widely separated objects to a visual magnitude of 20 with a precision of 30 millionths of an arcsecond; a more challenging goal would be to measure positions with a precision of 3 millionths of an arcsecond. The Astrometric Interferometric Mission (AIM) would permit definitive searches for planets around stars as far away as 500 light-years through the wobbles of the parent star, trigonometric determination of distances throughout the galaxy, and the study of the mass distributions of nearby galaxies from stellar orbits. AIM would demonstrate the technology required for future space interferometry missions. Large Earth-based Solar Telescope The Large Earth-based Solar Telescope is a solar telescope with a 2.4-m aperture that would use adaptive optics to increase the spatial resolution of solar observations. It would be the premier terrestrial telescope for high-resolution solar observations at optical and near-infrared wavelengths. With LEST, it will be possible to investigate in unprecedented detail the interactions of magnetic fields and turbulent motions under way in the solar surface and overlying atmosphere, which are responsible for the hot solar corona, the solar wind, solar flares, and solar-terrestrial phenomena. The program will be a cooperative international venture among nine countries. It will combine U.S. expertise in adaptive optics and instrument design with European contributions of an outstanding site in the Canary Islands and a major share in the costs of construction. The committee endorses a plan in which the United States would pay one-third of the construction and operation costs of LEST in return for a proportionate share of the observing time. VLA Extension The Very Large Array can currently produce images with better-than-arcsecond resolution. This will be complemented by the better-than-a-thousandth-of-an-arcsecond resolution of the Very Long Baseline Array (VLBA) when it is completed in 1992. There will still remain, however, a gap between the capabilities of the VLA and the VLBA that will restrict the performance of the combined instruments at intermediate resolutions between about 0.01 and 0.1 arcsecond. A plan to bridge this gap would be carried out in three phases: (1) supply the VLA with VLBA tape recorders, (2) build four new antennas to provide intermediate spacings between the VLA and VLBA, and (3)
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS add fiber-optic links between the VLA and nearby VLBA antennas. Together these improvements will produce a genuine intercontinental telescope, one that combines the full angular resolution of the VLBA with the enormous sensitivity and dynamic range of the VLA. This will permit a wide range of new astrophysical applications, including, for example, rapid and high-angular-resolution measurements of solar flares, the imaging of both thermal and nonthermal emission from nearby stars, and astrometric observations with a precision better than a thousandth of an arcsecond. The high-resolution images of the internal structure of jets and lobes in radio galaxies and quasars obtained with this intercontinental telescope will provide a new tool for cosmological studies. CONSTRUCTION OF LARGE TELESCOPES Optics technology has progressed to the point that telescope makers are confident of being able to build successfully the first optical and infrared telescopes larger than the 5-m Hale telescope. Recent advances in the technologies for casting and polishing of fast mirrors, in the precise alignment and support of segmented and monolithic mirrors, and in simple altitude-azimuth mounting will enable the construction of 8- and 10-m telescopes and will make the construction of 4-m-class telescopes less expensive. The 8- and 10-m telescopes will bring 10-fold improvements in sensitivity and, when coupled with the adaptive optics techniques described above, will permit a remarkably sharp view of astronomical objects. Table 3.1 and Appendix B list the telescopes proposed or under construction for the 1990s. The Keck instrument nearing completion on Mauna Kea will employ a segmented primary mirror made up of 36 hexagonal mirrors locked together with an advanced servomechanism to operate as a single 10-m telescope (Figure 4.5). The 8-m Spectroscopic Survey Telescope will be even more highly segmented than the Keck, using 85 1-m-diameter spherical mirrors. The other planned 8-m telescopes will use lightweight monolithic mirrors fabricated at the Mirror Laboratory of the Steward Observatory at the University of Arizona (see Chapter 3) with new casting and polishing techniques. Much of the required technology has already been tested with two 3.5-m mirrors, and a 6.5-m telescope is now under construction for the Smithsonian Institution. If appropriate funding is secured, the Mirror Laboratory will make the 8-m mirrors for the public telescopes recommended in this report, as well as for some of the private ventures listed in . A Southern 8-m Telescope The second major ground-based telescope recommended by the committee is an 8-m telescope, optimized for operation at optical wavelengths, to be built in the Southern Hemisphere. The initial set of instruments would include
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 4.5 Two plots show results from the first operation of the Keck telescope in Hawaii. Above, individual images of a single star, separated by about 12 arcseconds, are formed by the nine mirrors that make up the partially completed telescope. Shown below is the result of the telescope's servosystem bringing the nine images together to form a single image; the resultant image is less than an arcsecond across. The two horizontal scales are in tenths of arcseconds; the vertical scale is in arbitrary intensity units. When completed in 1992, the Keck telescope will have 36 mirrors operating together as a single 10-m telescope. Courtesy of the California Association for Research in Astronomy.
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS multiobject and high-resolution optical spectrometers. Such a telescope would complement its Northern Hemisphere counterpart through its ability to carry out essential scientific programs for which an infrared-optimized telescope would be less suitable. The telescope should have a monolithic mirror of optical quality sufficient to take full advantage of developments in adaptive optics techniques. The Southern Hemisphere 8-m telescope would provide U.S. astronomers with a vital window on objects that are uniquely or best observed from the Southern Hemisphere, such as the Magellanic Clouds, the galactic center, and some of the most prominent globular clusters and radio galaxies. Without national access to a Southern Hemisphere 8-m telescope, U.S. astronomers will be at a disadvantage in understanding new objects and phenomena discovered by NASA's orbiting observatories. Construction and Support of 4-m Telescopes At present, only a small fraction of the first-rate investigations proposed for the 4-m telescopes of the National Optical Astronomy Observatories can be granted time, and often the time available is so minimal as to preclude ideas and programs with high risk but potentially great return, or the assembly of databases adequate to ensure proper interpretation. This pressure will increase as discoveries made with 8-m telescopes and the space observatories place insuperable demands on existing facilities for supporting observations. Examples of the range of important scientific programs requiring extensive time on 4-m-class telescopes include characterizing, through imaging and spectroscopy, the physical properties of sources discovered at nonvisible wavelengths; determining the interior structures of stars through long-term programs of spectroscopic monitoring of stellar oscillations; searching for planetary systems and subsolar mass objects by means of long-term radial velocity studies of large samples of stars; carrying out statistically complete spectroscopic and photometric studies of supernovae in galaxies and in active galactic nuclei; determining the mix of stellar populations in galaxies of a wide variety of ages and morphologies; mapping the large-scale structure of the universe out to a distance of 1 billion light-years (corresponding to redshifts of z ~ 0.1) by determining the redshifts of a million galaxies; and mapping the large-scale structure of the universe far beyond a billion light-years (corresponding to redshifts of z ~ 1) by determining redshifts of galaxies in carefully selected areas. Fortunately, the technological advances that enable the construction of 8-m-class telescopes have also greatly reduced the expected size, weight, and cost of 4-m telescopes, while enhancing their image quality and operational efficiency. The superb image quality obtained with the New Technology Telescope of the European Southern Observatory attests to the potential of these new facilities. As a result of these advances, powerful telescopes can be built by individual universities or small consortia of institutions. The committee points out that
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS strong university support of these facilities is essential to reduce costs, to involve students in important scientific programs, and to develop novel instrumentation. THE INFORMATION EXPLOSION Astronomers of the 1990s will collect an enormous volume of data with the widespread use of large-format electronic detectors and arrays of antennas. Many existing or planned telescopes will be equipped with instruments capable of producing tens of gigabytes of data per day in the form of sky surveys, multiobject spectra, three-dimensional spectroscopic images, and multidimensional polarization maps. Chapter 5 discusses these areas in more detail and makes specific recommendations regarding the computers, software, and procedures needed to obtain, process, and interpret these data, and about the necessity of electronic archives to make selected datasets generally available. OTHER INITIATIVES Progress in a field as diverse as astronomy cannot be completely summarized by just a few technological or scientific themes. The remaining recommendations of Chapter 1, not discussed in previous sections, will produce important scientific results in various areas of astronomy. Dedicated Spacecraft for FUSE A decade ago the Field Committee listed as its highest-priority moderate program a far-ultraviolet spectrometer in space. In 1989 NASA selected the Far Ultraviolet Spectroscopy Explorer (FUSE) mission to enter the Explorer queue for launch sometime around 1999. The committee strongly endorses the scientific importance of FUSE and recommends as its first choice in the moderate space category that this timetable for FUSE be ensured, and possibly accelerated, by the purchase of a dedicated spacecraft. As currently conceived, FUSE would be the third payload to use the single, reusable Explorer platform. FUSE would be carried aloft by the Space Shuttle and exchanged by Shuttle astronauts for the X-ray Timing Explorer (XTE) on the orbiting Explorer platform. A few years prior to this, the same procedure would have been used to replace the Extreme Ultraviolet Explorer (EUVE) with XTE. As discussed in Chapter 1 and Chapter 7, the committee believes strongly that coupling Explorer missions to the manned space program will lead to unnecessary delays and expense compared with launching such satellites on Delta rockets. Additional advantages of a dedicated Delta launch would be an increase in the on-orbit lifetime of FUSE and an optimized orbit that would improve its operational efficiency.
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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS Acceleration of the Explorer Program The goal of an accelerated Delta-class Explorer program is to fly a total of six astrophysics Explorers during the 1990s, two or three more than are currently planned. This recommended flight rate is considered essential for a revitalized program of moderate missions, as discussed in . Although specific missions should be selected by the normal peer review process, the committee believes that three areas of space astronomy are particularly primed for Delta-class experiments: gamma-ray spectroscopy of galactic and extragalactic sources, a complete submillimeter line survey of important astronomical objects, and an x-ray telescope capable of making images with 60-arcsecond resolution in the energy range 10 to 250 keV. These and other possible Explorers are discussed in the Working Papers (NRC, 1991). A related recommendation is the acceleration of the Small Explorer (SMEX) program from its currently planned two or three astronomy missions in the 1990s to a total of five missions in the decade. Fly's Eye Telescope Cosmic-ray protons with energies greater than 1019 eV are not confined by the galactic magnetic field, so that their observation can reveal their point of origin, either galactic or extragalactic. The existing Fly's Eye telescope in Utah (see Plate 2.9) has detected some 200 fluorescent trails of highly energetic cosmic rays (1019 to 1020 eV) moving through the atmosphere. The direction, energy, and longitudinal development of the airshower can be measured. The present data suggest an isotropic distribution of particles with a flattening of the spectrum at energies above 1019 and a possible cutoff at energies above 1020. The longitudinal development of the airshower suggests that these particles are protons. These particles may come from outside of the galaxy, but few mechanisms are known that can accelerate particles to these energies, and no mechanisms are known that can fill the universe with such energetic particles. However, the statistics on which these conclusions are based are sparse. A new Fly's Eye telescope would be 10 times more sensitive and would detect many more events than the existing instrument. The statistics of more than 2,000 events in a few years would lead to a better determination of the energy spectrum and the isotropy of these energetic cosmic rays. The improved spectrum would help determine whether the cutoff at energies of 10 20 eV, expected from pion-producing interactions of protons with the 2.7 K cosmic background radiation (the Greisen effect), is real. The improved spatial resolution would be used to make more detailed studies of the longitudinal development of the airshowers and thereby infer the composition of the particles. This modestly priced facility will explore a new domain in cosmic-ray physics and could yield fundamental new insights.
Representative terms from entire chapter: