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Astronomy and Astrophysics in the New Millennium: Panel Reports 7 Report of the Panel on Ultraviolet, Optical, and Infrared Astronomy from Space
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Astronomy and Astrophysics in the New Millennium: Panel Reports SUMMARY The following missions are the priority recommendations of the Astronomy and Astrophysics Survey Committee’s Panel on Ultraviolet, Optical, and Infrared Astronomy from Space. All recommendations are a consensus of the panel. MAJOR MISSIONS When it prioritized major missions, the panel assumed that the Space Interferometry Mission (SIM), one of the initiatives recommended in the 1991 survey committee report,1 will be flown and that the Hubble Space Telescope (HST) will operate until 2010. NEXT GENERATION SPACE TELESCOPE NGST, ranked by the panel as the top-priority major mission for the decade, will reveal the onset of star and galaxy formation in the early universe. Its combination of scientific breadth and depth make it a compelling successor to the Hubble Space Telescope. It is the first of two logical paths to improved image resolution and sensitivity in space: increase overall aperture size. It should be technologically ready to be launched before 2010. The panel considered extensions of the core mission, currently 1 to 5 µm, and favors an extension to longer wavelengths, beyond 20 µm, for example, as scientifically more useful than extension to shorter wavelengths. TERRESTRIAL PLANET FINDER TPF was ranked as the second-priority major mission for the decade. Designed to observe directly Earth-sized planets near other stars, it is potentially the most scientifically exciting of all the major missions, depending on the breadth of its mission goals. It is the second logical path to improved image resolution and sensitivity in space: distributed aperture interferometry. Because TPF will depend on the successful 1 Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics (Washington, D.C.: National Academy Press).
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Astronomy and Astrophysics in the New Millennium: Panel Reports technology developed for both SIM and NGST, the panel saw it as being less technologically ready for the coming decade and therefore gave it lower priority. MODERATE MISSIONS SINGLE-APERTURE FAR INFRARED OBSERVATORY SAFIR, the panel’s top-priority moderate-size mission, is a large, filled-aperture telescope sensitive in the far infrared using NGST technology that will enable a distributed array in the decade 2010 to 2020. Such a telescope will be able to determine the total energy output from the first galaxies and will unambiguously determine the separate contributions of stars and accretion onto black holes to the total radiant energy. It will also be able to see through the opaque cores of molecular clouds that are creating new stars, thus providing a window on the first stages of star formation. An 8-m-class telescope will improve observational speed by factors of 104 to 105 over SIRTF, using the definition of “astronomical capability” in the 1991 survey committee report. The far infrared presently has enormous discovery potential. The single most important requirement is improved angular resolution. The logical build path is to develop a large, single-element (8-m-class) telescope leveraging NGST technology on time scales set by NGST’s pace of development. A later generation of interferometric arrays of far-infrared telescopes could then be leveraged on SIM or TPF technologies on correspondingly longer time scales. The panel considered SAFIR to be a moderate project and rated it as such. The survey committee placed it in the major category because its fairly uncertain costs fall close to the boundary separating moderate from major missions. The panel recognized that this mission could appropriately be included in the class of major missions in the survey committee report, and the committee decided to do so. SPACE ULTRAVIOLET OBSERVATORY SUVO, the panel’s second-priority moderate-class mission, is a development effort leading to an 8-m-class ultraviolet/optical telescope in the decade starting with 2010. The main science goal is to map the distribution of matter between the galaxies by observing its absorption of light against distant quasars and to search for dark matter by obtaining
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Astronomy and Astrophysics in the New Millennium: Panel Reports gravitationally lensed images of galaxy clusters. It is thought that most of the baryons in the universe may reside in this intergalactic matter. Other core science includes studies of protostellar disks in Orion-like environments and starburst galaxies at z<1. An 8-m-class UV/optical telescope will achieve gains of 100 to 1000 over the current capabilities of the Hubble Space Telescope, especially when combined with the next generation of energy-sensitive detectors for the UV/optical bands. Because the costs of a large-aperture UV/optical telescope are presently unknown, the recommendation is for technology development leading up to a new start at the end of this decade or later. SMALL MISSIONS ULTRALONG-DURATION BALLOON FLIGHTS The panel’s top priority for small missions is to include ULDB flights in the Explorer line so that they can compete for funding at least at the SMEX level—$75 million. Recent developments in ballooning have resulted in flights lasting on the order of 100 days. Atmospheric models show that the atmosphere is exceptionally transparent and stable at altitudes of 30 km and above. A whole class of missions, ranging from hard x rays to the submillimeter regime, could benefit from missions at cost levels of mid-Explorer-class projects and lower. The panel recommends that NASA allow long-duration ballooning projects to compete within the Explorer program with space missions for funding. With funding similar to that for a SMEX, it should be possible to improve the technology of balloons to make them attractive alternatives to spacecraft for some applications. Such suborbital flights also have great potential for training students who will one day become the technical force in space science. LABORATORY ASTROPHYSICS The panel recommends an expanded National Aeronautics and Space Administration (NASA) investment in laboratory research in several areas of astrophysics in support of space missions. The areas requiring fundamental studies and measurements include (1) spectroscopic properties of irradiation-processed ices, (2) the properties of refractory components of interstellar grains, (3) astrophysical and space plasmas, (4) the organic chemistry of the interstellar medium, (5) radia-
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Astronomy and Astrophysics in the New Millennium: Panel Reports tive and dielectronic recombination, and (6) collisional and photoionization studies, including measurements of iron cross sections for both processes. New space missions will explore wavelength regions and temperature regimes not previously observed and study interstellar dust and organic molecules with unprecedented sensitivity, requiring a broader suite of supporting laboratory investigations than have ever before been conducted. TECHNOLOGY DEVELOPMENT ENERGY-RESOLVING DETECTORS The highest priority for technology development is detectors sensitive to ultraviolet/optical wavelengths. Energy-resolving detectors, both superconducting tunnel junctions (STJ) and transition-edge sensors (TES), will completely revolutionize observations at ultraviolet and optical wavelengths when fully developed. The potential is so enormous that it is important to invest heavily in making arrays of flight-qualified detectors with either of the two new technologies. Improving detectors will expand observing capability in the UV-optical range by large factors and is a much less expensive way to achieve gains than building larger telescopes with today’s detectors. FAR-INFRARED DETECTORS State-of-the-art far-infrared detectors, as represented by the arrays to be flown on SIRTF, have background-limited sensitivity but are of modest format (32×32). If SAFIR and subsequent far-infrared missions are to achieve their full potential, NASA should support continued development of these photoconductor and bolometer array technologies to formats of at least 128×128. Because there are no commercial or defense-related efforts in this area, as there are at shorter wavelengths, it is essential that NASA provide the support. An investment of $10 million in the next decade would lead to array sizes better matched to the capabilities of future telescopes. REFRIGERATORS At the same time, it is important to develop a new class of refrigerators for these new detectors, allowing routine operation at millikelvin
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Astronomy and Astrophysics in the New Millennium: Panel Reports temperatures. The techniques for cooling are well known but require investment to produce reliable, long-lived devices for spacecraft. SPACECRAFT COMMUNICATIONS It is essential for NASA to improve the bandwidth of space telemetry systems, so that expensive missions will not be underutilized as a result of low data rates. The limit to science capability for many of the future missions will be the rate at which data can be transmitted to the ground. Modern detectors can produce data at rates far exceeding those at which data are transmitted through the Telemetry Data Relay Satellite System (TDRSS) or the Deep Space Network. ULTRALIGHTWEIGHT OPTICS Three of the major recommendations from this panel involve large-aperture space telescopes, for which very lightweight optics are highly desirable. The panel supports NASA’s initiative to develop gossamer optics to enable the next generation of large telescopes for ultraviolet, optical, and infrared applications. SCIENCE OPPORTUNITIES Ultraviolet, optical, and infrared (UVOIR) astronomy, the region between about 0.1 and 1000 µm, is the largest source of information about the universe. Stars and planets emit most of their radiation in this energy range. The electronic transitions of molecules, atoms, and ions as well as the vibration-rotation transitions of molecules fall within the UVOIR; the range includes all chemistry important to life. Approximately 50 percent of the photon energy density in the Galaxy is starlight in the UVOIR; the rest is primarily in the cosmic background radiation. For these reasons, observations at these wavelengths dominate astronomical study and are likely to continue to do so. UVOIR astronomy flourished in the decade 1990 to 1999. The Hubble Space Telescope was launched, repaired, and enhanced to produce images of unparalleled resolution and depth and spectra that are only now being rivaled by the new class of giant telescopes on the ground. Its public impact has been profound. Hubble alone has put astronomy much more in the minds of nonscientists than any other
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Astronomy and Astrophysics in the New Millennium: Panel Reports facility as the number of Hubble discoveries continues to capture the attention of the public. The Hubble Deep Field opened the way to study galaxies when the universe was a small fraction of its current age and focused the world’s attention on astronomy. The Hubble ultraviolet spectrographs provided a new way to observe stars, planets, and galaxies. The first of the giant ground-based telescopes, the 10-m Keck and the VLT, went into operation and began to revolutionize ground-based astronomy. The Keck telescopes produced a number of exciting new discoveries, especially about the early universe, for which smaller telescopes cannot collect enough light for spectral analysis of the most distant objects. Interferometers were developed and used for astrometry and imaging. Instruments were improved. Advances in the precision with which spectroscopic lines could be measured resulted in the first discoveries of planets outside the solar system. Hipparcos, the European astrometric survey satellite flown early in the decade, provided the most accurate map of stellar positions in history. The European Infrared Space Observatory (ISO), flown in mid-decade, returned a wealth of new data on the mid- and far-infrared bands. The Far Ultraviolet Spectroscopic Explorer (FUSE) was launched in 1999. The Space Infrared Telescope Facility (SIRTF) got a new start and will be launched early in this decade. Research with these new facilities produced two scientific themes. One was to observe ever more distant objects in an attempt to discover the first stars and galaxies as they came into being after the Big Bang. This theme continues to dominate extragalactic astronomy. The primary motivation for constructing 8-m-class telescopes was to collect enough light to make it practical to take spectra of the faint galaxies and quasars at redshifts greater than 1. The second motivation was the growing recognition that planetary systems may be common in the Galaxy, culminating in the first, albeit indirect, discoveries of planets outside the solar system. The discoveries rekindled the age-old interest in the uniqueness of the solar system, an understanding of our origins, and the possibility that the detection of extraterrestrial life is within our reach. These two themes dominate any discussion of scientific priorities for the next decade. They embody some of the oldest philosophical questions: How did the universe come into being? How did life arise? Are we alone? We are, indeed, fortunate to live at a time when our technology makes possible the answers to these questions, where we see a clear path to addressing these old questions directly with the scientific method. The priorities of the panel reflect the deep urge to understand our roots
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Astronomy and Astrophysics in the New Millennium: Panel Reports through measurement and observation, testing of theory, and discovery of those properties of the universe now beyond our imagination. The most distant objects in the universe are typically less than an arcsecond in size. We need subarcsecond resolution simply to discern their shapes and structures. Even with adaptive optics and artificial guide stars, only a small fraction of the youngest galaxies can be observed with adequate resolution from ground-based telescopes now, and the challenge to provide good sky coverage with theoretical systems of the future is daunting. At redshifts exceeding 2, most of the observable energy of these objects emerges in the near-infrared part of the spectrum, at wavelengths beyond 1 µm. They are so faint that the nighttime sky is usually more than 10,000 times brighter than the objects, even at the best sites on Earth. And the most interesting parts of the spectra are often blocked by the Earth’s atmosphere, as the important spectral lines shift from the nearly transparent visual window to wavelengths between 1.5 and 4 µm. Extrasolar planetary systems pose an even greater challenge to observation. The light from planets either is emitted in the far-infrared parts of the spectrum, where detection from ambient-temperature telescopes on Earth is essentially impossible, or is reflected starlight, in which case the stars are more than a billion times brighter than the planets. Distortion by Earth’s atmosphere smears the starlight, causing it to overwhelm the light from a planet in any ground-based observation. Even from space, the optics of a telescope create insurmountable limits to the observation of a faint planet next to a bright star unless the telescope is enormous—100 m in diameter, say—or the starlight is reduced by interference effects. The study of planet building through observations of the circumstellar disks from which planets are born is easier but nevertheless demands much higher angular resolution at infrared wavelengths than anyone has attempted. Two scientific themes—discovering the formation of stars and galaxies after the Big Bang and searching for planets around other stars— challenge our technical prowess to develop the next generation of astronomical observatories. Many of the wavelengths covered by this panel are inaccessible from ground-based sites and can only be observed from space. Even at optical and near-infrared wavelengths, where the Earth’s atmosphere is largely transparent, the enormous gains in resolution and sensitivity made possible by freedom from the distortion and background created by Earth’s atmosphere mean that spacecraft are poised to dominate many observations. The large impact of the Hubble
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Astronomy and Astrophysics in the New Millennium: Panel Reports Space Telescope with its aperture of modest size is a good example of how the advantages of space make up for disadvantages in size. These missions are just beginning to tap the potential of space-based observations of the cosmos in the UVOIR bands. Scientifically, there are important areas not touched by the current generation of space satellites. Technology has now progressed to where gains of several orders of magnitude are possible in each of the areas covered by the current generation of missions. The Hubble Space Telescope sees deeply into the universe but not yet deeply enough to see the “edge of light,” where the first stars and galaxies came into existence after the Big Bang. Both HST and FUSE are still inadequate to detect faint quasars needed to map out the intergalactic medium using observations of their absorption lines. For these tasks, more collecting area—a larger aperture—will be needed. Hipparcos measured positions of stars to about 1 milliarcsec, an impressive precision but still more than an order of magnitude shy of the level needed to detect planetary companions to nearby stars by measuring the stellar wobble. Improving on the precision of Hipparcos requires interferometers with baselines on the order of 10 m, many times larger than that of Hipparcos. ISO and SIRTF use telescopes less than 1 m in diameter, meaning that source confusion sets the limit to the sensitivity throughout the far-infrared band. The high density of sources in the far infrared means that the greatest advances will come from increases in angular resolution or image sharpness. Gains in resolution imply larger structures, since all far-infrared telescopes have been limited entirely by the diffraction of the primary optics. Therefore, larger telescopes and eventually interferometers with telescopes spread across several hundred meters or more are needed to exploit the potential of this rich wavelength regime. The recommendations of this panel are made so that known scientific problems of great significance can be addressed by the new facilities. But the history of astronomy suggests that the most important and interesting results will be new discoveries unanticipated by those who advance our observational capability. New discoveries often result when capability is increased by a factor of 10 in some dimension. They are common for improvements of 100-fold. Increases by a factor of 1000 or more virtually guarantee discoveries. The panel was especially enthusiastic about initiatives such as NGST that promise 1000-fold (or more) gains, since it believes that such gains are likely to revolutionize our knowledge of the universe. There are, as a consequence, many opportunities for new UVOIR
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Astronomy and Astrophysics in the New Millennium: Panel Reports spacecraft. Some of these fall within NASA’s Explorer (<$140 million) and Discovery (<$300 million) programs. These programs are peer-reviewed in regular calls for proposals, so the panel did not attempt to rank any projects that could be proposed as Explorer or Discovery missions, although there are some broad directions that it highlights for special attention. NASA selects its Grand Challenge missions, whose total costs typically exceed $1 billion, by means of a strategic planning process involving much of the astronomical community. The panel independently reviewed the missions in the current strategic plan and produced its own ranking. It also reviewed the complementary character, both scientifically and technologically, of the Grand Challenge missions—SIM, NGST, and TPF—and found that they constitute a compelling package. It discussed several possibilities for moderate missions that are highly desirable and are ranked by priority. Several small missions and technology investments received strong endorsement from the panel. In fact, modest investments in technology are likely to increase astronomical capability in several areas where alternative approaches are very expensive, so these relatively inexpensive recommendations are among the most important. ASSUMED FACILITIES THE HUBBLE SPACE TELESCOPE The panel fully supports the recommendations relating to the Hubble Space Telescope contained in the 1996 report of the Dressler committee, HST and Beyond,2 which emphasized the importance of an extended life for HST.3 The report’s rationale remains as apt today as when it was written. The first 10 years of HST have been a remarkable period of advance in the science and practice of astronomy. The qualitative superiority of 2 HST and Beyond Committee. 1996. HST and Beyond (Washington, D.C.: Association of Universities for Research in Astronomy, Inc.); also known as the Dressler report for the committee’s chair, Alan Dressler. 3 Information about HST can be found online at the Space Telescope Science Institute’s Web site at <http://www.stsci.edu>.
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Astronomy and Astrophysics in the New Millennium: Panel Reports Hubble’s imaging and spectroscopy, combined with its sophisticated data management, has enabled astronomers worldwide to make observations that further reveal the remarkable complexity of the universe. The panel assumes that the Hubble operations will extend until 2010, currently NASA’s plan for the facility. It will be the only facility covering the ultraviolet portion of the spectrum, and its imaging power at all wavelengths will remain a necessary adjunct to the suite of facilities planned for the coming decade. THE SPACE INTERFEROMETRY MISSION The 1991 survey committee report, The Decade of Discovery in Astronomy and Astrophysics,4 recommended an astrometric interferometry mission as a high priority. In line with that recommendation, a major initiative of the last decade was the development of the Space Interferometry Mission (SIM).5 While the advanced state of SIM’s development and an anticipated launch in 2005 precludes its inclusion among the new missions being prioritized here, its successful completion and execution will require an ongoing major investment by both NASA and the astronomical community throughout this decade. The panel believes that the present high level of commitment to the SIM mission is well merited, and it fully supports the launch of SIM. The primary scientific objective of the SIM mission is ultrahigh-accuracy astrometry. The spacecraft is a Michelson interferometer with a 10-m baseline, operating in visible light. The performance goals are 4-µarcsec absolute accuracy anywhere on the sky and 1-µarcsec relative precision over a 1-deg field to a limiting visual magnitude of 20. These astrometric goals are approximately 200 times more accurate than the measurements of the Hipparcos mission, and the faint limit is nearly 1000 times more sensitive. It is important to combine high astrometric accuracy with deep sensitivity, for both are required to study the entire Galaxy and local group. The additional mission goals—to demonstrate interferometric nulling 4 Astronomy and Astrophysics Survey Committee, National Research Council. 1991. The Decade of Discovery in Astronomy and Astrophysics (Washington, D.C.: National Academy Press). 5 Information on SIM can be found online at <http://sim.jpl.nasa.gov>.
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Astronomy and Astrophysics in the New Millennium: Panel Reports FIGURE 7.9 This simulation shows the expected distribution of intergalactic gas resulting from structure formation in the early universe. Most of the matter is cold and dark but can be detected via its absorption of light from distant quasars. SUVO could map this intergalactic structure that may contain most of the normal matter in the universe through systematic observations of many quasars. Courtesy of R.-Y.Cen and J.P.Ostriker, Princeton University.
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Astronomy and Astrophysics in the New Millennium: Panel Reports Studying the Feedback from Star Formation An important challenge for UV astronomy in 2010 will be to follow the emergence of the modern universe over the last 10 billion years (z<1.65). As described earlier, the gaseous components of the universe during this epoch can best be studied in the ultraviolet, where one has access to the resonance lines of H (Lyman-alpha) and other key heavy elements. At z>1.65, some of these lines are redshifted into the optical. However, this leaves some 70 to 80 percent of cosmic time inaccessible without UV instruments. The lack of high-throughput UV spectroscopy is a major hindrance to understanding the cosmic chemical evolution of the modern universe at z< 1.65. With SUVO, astronomers will be able to make precision studies of the modern (z<1.65) counterparts of star-forming galaxies in the high-redshift universe studied with NGST. The fact that stars at redshifts less than 1.65 produced over 90 percent of the heavy elements seen today further justifies the need for access to the UV, which will give astronomers the key resonance lines of most elements. For the most recent 70 percent of cosmic time, the UV band provides the best means for them to quantify the rate at which galaxies accrete mass from the IGM, form new heavy elements in stars, and expel mass and radiation as a result of this star formation. UV spectra of both emission and absorption lines provide quantitative measurements of the effects of “feedback” from star formation on the surrounding IGM and on the galaxies themselves. This feedback takes the form of newly synthesized heavy elements (C, O, and so on), of powerful ionizing radiation from massive stars, and of hot gas produced by stellar winds and exploding stars. This feedback of radiation and energy modulates the rate at which galaxies continue to accrete gas and determines how heavy elements are spread throughout the universe. Studying Galactic Star Formation With Hubble, spectral imaging in the optical and ultraviolet proved its value in measuring the products of star formation in local regions of the Milky Way. The usefulness of these images is dictated by the physics of line emission and the peak of the radiative cooling function at 105 K, arising from strong UV emission lines of C, N, and O. The UV and visible bands also allow astronomers to probe the effects of stellar radiation on exposed molecular clouds and protoplanetary disks. Understanding the effects of Galactic ionizing radiation and stellar outflows is critical to understanding large-scale star formation and the survivability of disks in the Galactic environment.
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Astronomy and Astrophysics in the New Millennium: Panel Reports The narrow-field UV/optical detectors aboard HST provided detailed images of nearby star-forming regions, Orion’s protoplanetary disks, planetary nebulae, and shock waves. With its wide-field imagers and filters, SUVO will be able to map, in a single pointing, entire star-forming regions in diagnostic emission lines and broadband colors. Within the Milky Way, but more importantly in external star-forming galaxies, SUVO will be able to study global star formation and the interactions between young star clusters and the ISM. Diffraction-limited imaging will allow astronomers to measure the interactions of stars and the ISM at scale lengths necessary to probe important physical processes (winds, shock waves, thermal conduction, and wind ablation of disks). The dynamic range and wide field of these images will enable astronomers to study the stellar initial mass function, after the molecular cloud is blown away, from the most massive stars down to brown dwarfs. In this respect, UV/optical imaging is complementary to infrared studies of the initial stages of star formation. Some of these science goals are complementary to those in the IR and x-ray regimes. However, some can only be achieved using the wide-field images and sensitive spectral diagnostics of the UV and optical bands. In particular, images of weak gravitational lensing and large-scale surveys of Lyman-alpha absorption may be the only way to find the dominant components of the invisible universe. SUVO will allow astronomers to map out the large-scale distribution of dark matter and the nearly invisible warm components of the cosmic web of gaseous matter that may dominate the missing baryons. Technology Development No high-throughput UV/optical mission will be possible without significant NASA investments in technology, including UV detectors, gratings, mirrors, spectrographs, and imagers. The panel assumes the telescope will use the technology developed for NGST: lightweight, deployable optics. With an 8-m aperture, SUVO will achieve a 10-fold increase in collecting area over HST for imaging and spectroscopy. With improved efficiencies in detectors and gratings and with increases in camera sizes and in spectral multiplex efficiency by means of simultaneous wavelength coverage, SUVO should achieve, overall, a 100-fold increase in power. The most critical needs for SUVO are the following:
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Astronomy and Astrophysics in the New Millennium: Panel Reports Develop more sensitive UV detectors with low background noise, high quantum efficiency, large dynamic range, and large formats. Develop three-dimensional, energy-resolving detectors such as photon-counting superconducting tunnel junction (STJ) or transition-edge sensor (TES) devices. These cryogenic detectors have the potential to revolutionize UV/optical astrophysics, and the cryogenic technology would be shared with other missions (far-IR and submillimeter). Space-qualify large mosaics of low-noise, high-quantum efficiency charge-coupled device (CCD) detectors (at least 16K×16K) for wide-field imaging cameras in the optical and near UV. Develop micromirror arrays for use in multiobject and integral-field spectrographs in the UV and visible. Develop large, lightweight precision mirrors for use in the UV/ optical. Although SUVO could be done with a 4-m monolith, the extension to an 8-m aperture will probably require segmented deployable optics. SMALL MISSIONS ULTRALONG-DURATION BALLOONS For many small payloads, the top of the stratosphere accrues essentially all the benefits of space, and these payloads could be flown on balloons at a fraction of the cost of satellites or of using a space shuttle. The principal disadvantages of balloons—short-duration flights and lack of control over the flight path—can be alleviated with new technologies that would enable flights of 1-ton payloads at altitudes above 35 km for 100 days or longer with enough control over the flight path to ensure a safe landing at the point of launch. As such, balloons would be more attractive for many small payloads than traditional spacecraft and would give NASA an inexpensive way to carry out novel science experiments as well as to prove new payload technology and train future scientists in instrumentation. Examples of the kind of science that can be carried out with balloons include searches for planets with a coronagraph on a diffraction-limited telescope a few meters in diameter; the imaging of convective flows and magnetic fields in the Sun’s photosphere with a large solar telescope; extragalactic observations with a moderate-size far infrared telescope; and all-sky surveys at hard x-ray wavelengths. The top of the stratosphere is far superior to terrestrial sites and enables a wide range of
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Astronomy and Astrophysics in the New Millennium: Panel Reports small-mission science experiments at wavelengths not transmitted to Earth. The typical balloon science payloads would be similar to those proposed under the Explorer program. The panel recommends that ULDBs be permitted in the competition for Explorer missions. Furthermore, it finds that it would be appropriate to invest money in balloon technology to enable flight path control and very long flights. The investment could come from either the technology program or the Explorer program as part of the cost of a SMEX mission. LABORATORY ASTROPHYSICS Much of the data provided by new space missions will have spectral signatures that cannot be interpreted given our current knowledge of atomic and molecular lines. Resonance transitions from small particles are commonly observed that lack counterparts in the limited number of laboratory studies of solid-state lines. It is already the case that space missions reveal spectral lines that are unidentified owing to a lack of laboratory data. This problem spans the spectrum from the x ray to the far infrared and means that the space missions do not realize their full potential for basic research. Laboratory research can lead to important discoveries. For example, spectra observed in the disks around young stars showed the presence of the mineral forsterite, whose spectrum had been measured in the laboratory. Forsterite is a magnesium-rich silicate that is seen in the spectra of some comets and is also known on Earth. The close match between spectra of the circumstellar disks, thought to be the birthsites of new planetary systems, and those of comets, consisting of the same material in the disk surrounding the proto-Sun, greatly strengthened the belief that the young stars are akin to the early phases of our solar system and that the early solar system consisted of cometlike bodies orbiting the young Sun. The material from these comets is believed to have aggregated in the solar system planets known today. Similar conditions dominate the optical and ultraviolet spectra of many ionized molecular and atomic species and of radicals that are unstable under normal terrestrial conditions. Such substances, although short-lived, can often be produced in the laboratory and studied in sufficient depth to permit unambiguous identification when astronomically observed. A particular instance of this is the identification of the diffuse interstellar bands. More than 150 of these are now known,
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Astronomy and Astrophysics in the New Millennium: Panel Reports though none has to date been unambiguously identified. Preliminary laboratory studies now indicate that some of these bands may be associated with ionized buckyballs, Another recent laboratory study indicates that the anion of the linear carbon chain may be associated with the narrow bands at 496.4, 561.0, 574.7, 606.5, and 627.0 nm. This study led to the suggestion that other bands may be due to carbon chain anions that are six, eight, and nine atoms long. Suggestions such as these require laboratory follow-up, since there are no theoretical means for calculating the energy levels of such molecular ions or the transition probabilities between these levels. The cost of laboratory studies tends to be low in comparison with overall mission costs but can make a decisive difference in obtaining the astrophysical insights that these missions were designed to yield. The panel recommends an increased investment in laboratory astrophysics to take advantage of the wealth of new data already being returned by space science missions. TECHNOLOGY FOR THE FUTURE ENERGY-SENSITIVE UV/OPTICAL DETECTORS A revolution is occurring in the technology for ultraviolet detectors. Two technologies, superconducting tunnel junctions (STJ) (Figure 7.10) and transition-edge sensors (TES), make it possible to measure the energy of individual photons as they are detected. Unlike all previous techniques, these detectors simultaneously give the photon rate (brightness) and spectral energy distribution. For many applications, all images will immediately yield modest-resolution spectra of each pixel. The increase in information capacity relative to today’s images is huge. These detectors will increase the efficiency of spectral observations by factors on the order of 100. When fully developed, these detectors will increase the astronomical capabilities of even small telescopes by an equivalent factor. The increased capability will permit many scientific problems to be solved with current telescopes, without the expense of building larger telescopes. This potential is so great that the panel recommends NASA invest aggressively in these technologies, perhaps selecting the most promising after an initial investment, to ensure that the new potential is fully exploited. The panel believes that investments of about $25 million would
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Astronomy and Astrophysics in the New Millennium: Panel Reports FIGURE 7.10 A schematic diagram of a superconducting tunnel junction detector shows the basic elements for the first ofthe ultraviolet-energy-resolving detectors (left). An array of 6×6 detectors (pixels) has been fabricated and is shown on the right in a microphotograph. Courtesy of ESTEC (<http://www.estec.esa.nl>). save several times that amount by enabling new science without further increasing the size of telescopes in the next two decades. Investment in energy-resolving detectors is the top priority of the panel for new technology development. REFRIGERATORS Most detectors require cooling to achieve their full sensitivity. The STJ and TES detectors operate at millikelvin temperatures. Far-infrared detectors typically must be cooled to a few kelvin. Far-infrared telescopes require optics below about 10 K to achieve their full potential. For these applications, missions will need refrigerators capable of operating for long periods with minimal power to keep the detectors and optics cold. NASA should invest in technology to develop space-qualified refrigerators needed for the suite of missions enabled by the new detector technologies. It should be straightforward to develop devices for far-infrared applications. It will be more challenging to make refrigerators capable of millikelvin temperatures, but because the new UV detectors have tremendous potential, it is vitally important to invest in the needed
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Astronomy and Astrophysics in the New Millennium: Panel Reports technology. The panel believes an investment comparable to that for detectors, about $25 million, will be needed to develop technologies for next-generation cryocoolers. SPACECRAFT COMMUNICATIONS Missions currently under consideration envisage gathering data at rates many orders of magnitude higher than were traditionally considered possible. Missions such as NGST are expected to have focal plane arrays with more than 3×108 pixels, and even this number will not capture all the information delivered by the telescope—more than 10 times as many pixels would be needed to critically sample the entire diffraction-limited field of view. At 16 bits per pixel, the characterization of a single frame then would require almost 1010 bits of information. Exposure times lasting no more than a minute will be routine to ensure that none of the pixels become saturated, meaning that bandwidths on the order of 1 billion bits per second (GHz) would be needed to transmit the data to Earth. Current telemetry systems for NASA’s science missions are limited to data transmission rates of a few million bits per second (MHz). Moreover, the telemetry bands need to be shared among all the different missions simultaneously operating in space. The rate has not been upgraded significantly in the last few decades and is rapidly becoming the bottleneck that will limit the utility of the more ambitious missions being planned. Onboard processing and data compression can help but cannot completely alleviate this bottleneck. Most missions encounter unanticipated sources of noise that can be removed only with processing that involves interactive analysis. Interactive processing requires the transfer of all the data to Earth, where sophisticated algorithms can be developed on computers much more powerful than those on board the mission. Commercial systems are now becoming available that permit transmission at bandwidths of 100 GHz. These normally work at infrared wavelengths and would require an approach to ground stations different from that used by the radio telescopes now in place. However, investment in new ground stations permitting vastly increased telemetry bandwidths is likely to be needed if we are to realize the enormous gains made possible by the advanced detector arrays that will be used with the next generation of space science missions.
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Astronomy and Astrophysics in the New Millennium: Panel Reports A dedicated optical communications satellite at L2 would cost about $70 million. Operational costs for using a dedicated 3-m telescope as a receiver would need to be added, although it is almost certain that one or more telescopes already exist that could be used for this purpose. The panel urges NASA to make an investment of this magnitude to capitalize on the great potential for science from L2. ULTRALIGHTWEIGHT (“GOSSAMER”) OPTICS Filled-aperture telescopes have been the principal sources of information for astronomers for almost 400 years, since Galileo turned the first telescope to the heavens in the early 17th century. Very large telescopes in space have an enormous advantage over those on the ground because they avoid Earth’s atmosphere and its concomitant distortion of the light. If it were possible to develop very large optical surfaces for space telescopes—more than 100 m, say—such telescopes would enable astronomical studies of great interest, in particular, the study of Earthlike planets around nearby stars, without having to use special interferometric techniques, where the collection of light is important for high-resolution spectroscopy of the planetary atmospheres. Very large ultraviolet telescopes would be extremely useful for high-throughput spectroscopy of the intergalactic medium, galactic halos, and heavy-element evolution in the ISM and IGM, where the background targets are faint. The panel supports NASA’s initiative to develop gossamer optical surfaces that would be the basis for a new generation of giant telescopes in space. Although how much money would be needed to achieve this breakthrough technology is unknown at present, the panel believes it would be prudent to invest several million dollars per year to explore the means of manufacturing very large, accurate optical surfaces for future space telescopes. ACRONYMS AND ABBREVIATIONS AGN —active galactic nuclei ALMA —Atacama Large Millimeter Array AU —astronomical unit, a basic unit of distance equal to the separation between Earth and the Sun, about 150 million km
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Astronomy and Astrophysics in the New Millennium: Panel Reports CCD —charge-coupled device, an electronic detector used for low-light-level imaging and astronomical observations; CCDs were developed by NASA for use in the Hubble Space Telescope and the Galileo Probe to Jupiter and are widely used on ground-based telescopes COBE —Cosmic Background Explorer, a NASA mission launched in 1989 to study the cosmic background radiation from the Big Bang CSA —Canadian Space Agency DIRBE —Diffuse Infrared Background Explorer, an instrument aboard COBE DOD —Department of Defense EELV —Evolved Expendable Launch Vehicle ESA —European Space Agency, the European equivalent of NASA ESTEC —European Space Research and Technology Centre FIRST —Far-Infrared Space Telescope (ESA) FOV —field of view FUSE —Far-Ultraviolet Spectroscopic Explorer GALEX —Galaxy Evolution Explorer, a space ultraviolet emission and spectroscopic Small Explorer mission Hipparcos —European Space Agency mission for measuring the distances, motions, and colors of stars HST —Hubble Space Telescope, a 2.4-m-diameter space telescope designed to study visible, ultraviolet, and infrared radiation; the first of NASA’s Great Observatories IFS —integral field spectrograph IGM —intergalactic medium ISM —interstellar medium ISO —(European) Infrared Space Observatory L2 orbit —Lissajous orbit about the L2 Sun-Earth Lagrange point 1.5 million km from Earth LBT —Large Binocular Telescope, an American, Italian, and German collaboration MAP —Microwave Anisotropy Probe MCP —microchannel plate MIPS —Multiband Imaging Photometer for SIRTF, a far-infrared instrument capable of imaging photometry and high-resolution imaging MOS —multiobject spectrograph NASA —National Aeronautics and Space Administration NGST —Next Generation Space Telescope, an 8-m infrared space telescope
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Astronomy and Astrophysics in the New Millennium: Panel Reports PTI —Parkes-Tidbinbilla Interferometer (it has a baseline of 275 km) QE —quantum efficiency QSOs —quasi-stellar objects; together with active galactic nuclei, they form the group of objects known as active galaxies SAFIR —Single-Aperture Far-Infrared Observatory, an 8-m space-based telescope SCUBA —Submillimeter Common-User Bolometer Array, a British-French-Canadian ground-based telescope in Hawaii operating at wavelengths between 350 and 2000 µm SIM —Space Interferometry Mission SIRTF —Space Infrared Telescope Facility, NASA’s fourth Great Observatory, will study infrared radiation Sloan survey, also known as the Sloan Digital Sky Survey (SDSS) —a mission to produce high-resolution pictures of one quarter of the sky and to measure the redshift of distant galaxies SMEX —Small Explorer, a NASA program to fly small, inexpensive satellites on a rapid timescale ST-3 —NASA’s Space Technology 3, two spacecraft launched together and put into an orbit around the Sun to demonstrate interferometry STJ —superconducting tunnel junctions SUVO —Space Ultraviolet Observatory, a proposed 8-m-class telescope SWAS —Submillimeter Wave Satellite, one of NASA’s Small Explorer missions; it studies interstellar clouds TDRSS —Telemetry Data Relay Satellite System TES —transition-edge sensors TPF —Terrestrial Planet Finder, a free-flying infrared interferometer designed to study terrestrial planets around nearby stars ULDB —ultralong-duration balloon flights UV —ultraviolet UVOIR —ultraviolet, optical, and infrared VLT —Very Large Telescope, the European Southern Observatory’s four 8-m telescopes
Representative terms from entire chapter: