3
The New Initiatives: Building on the Current Program



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Astronomy and Astrophysics in the New Millennium 3 The New Initiatives: Building on the Current Program

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Astronomy and Astrophysics in the New Millennium INTRODUCTION Astronomers seek to understand phenomena as diverse as the formation of planets, the origin of solar activity, the evolution of black holes, and the large-scale structure of the universe. Electromagnetic radiation—radio, infrared, optical, ultraviolet, x-ray, and gamma-ray—provides most of the information we have on these distant phenomena, but other messengers can contribute as well: Energetic charged particles (the cosmic rays) carry information about where they were accelerated, neutrinos tell us about the deep interior of stars, and gravitational waves may reveal how some black holes form. This chapter describes how the recommended new initiatives grew out of the existing program, how they complement each other, and how they will address the major scientific questions in astronomy. The characteristics of the major and moderate new initiatives are listed in Tables 3.1 and 3.2. More detailed descriptions of the facilities and the science they will accomplish can be found in Astronomy and Astrophysics in the New Millennium: Panel Reports (NRC, 2001). The committee emphasizes that telescopes alone do not lead to a greater understanding of the universe. So that maximum benefit can be obtained from the current and proposed new facilities, the committee recommends a vigorous and balanced program of astrophysical theory, data archiving and mining, and laboratory astrophysics. One of the key recommendations is the establishment of theory challenges to be associated with most new major and moderate programs. These challenges should describe theoretical problems that are ripe for progress, relevant to the planning and design of the program, and essential to the interpretation and understanding of its results in the broadest context. The specific theory challenges tied to each mission and project should be determined by the informed astronomical community—probably through ad hoc panels, drawn from the theory community and convened for this purpose. However, to illustrate the concept the committee gives examples of possible theory challenges below in the discussion of the new initiatives. THE ULTRAVIOLET, OPTICAL, AND INFRARED WINDOWS ONTO THE UNIVERSE Ultraviolet, optical, and infrared (UVOIR) observations provide extremely important sources of information about the universe. Stars,

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Astronomy and Astrophysics in the New Millennium TABLE 3.1 Characteristics of Prioritized Major Initiatives Project Wavelength or Energy Coverage Increase in Sensitivitya Spatial Resolution (arcsec)b Spectral Resolving Powerc NGST 0.6 to 27 µm (goal) 100 to 600 5 to 5,000 GSMT 0.3 to 25 µm 10 3 to 105 Con-X 0.25 to 40 keV 100 15 300 to 5,000 EVLA 3 mm to 100 cm 10 10 to 107 LSST 0.3 to 1 µm 0.3 to 2.4 µm (goal) 20 to 60 0.6 3 to 100 TPF 3 to 30 µm n.a.d 3 to 300 SAFIR 30 to 300 µm 100 to 300 5 to 103 aIncrease in sensitivity compared with existing or other planned facilities. bFor entries with a (λ/a) term included, the spatial resolution is for wavelength λ > a. The number in front of the parenthesis therefore corresponds to the best spatial resolution. cSpectral resolving power is defined as λ/Δλ. dn.a., not applicable, is assigned to TPF because there is no existing facility with which to compare the sensitivity. This unique facility will work at spatial resolutions not currently accessible in this wavelength range. and the interstellar gas and dust that make stars, emit most of their radiation at these wavelengths. The atoms and ions in the interstellar medium and in stellar atmospheres have rich UVOIR spectra that can be used to probe the density, dynamics, magnetic field structure, temperature, and elemental abundances of astronomical objects. Infrared radiation penetrates the obscuring dust that surrounds regions of star and

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Astronomy and Astrophysics in the New Millennium TABLE 3.2 Characteristics of Prioritized Moderate Initiatives Project Wavelength or Energy Coverage Increase in Sensitivitya Spatial Resolution (arcsec)b Spectral Resolving Powerc GLAST 10 MeV to 300 GeV >40 600 50 LISA 10−4 to 10−1 Hzd n.a.e 1,800 3 × 104 AST 0.3 to 35 µm 300 103 to 106 SDO 0.02 to 1 µm 25 1 100 to 6 × 104 CARMA 860 µm to 1 cm 4 to 12 3 to 107 EXIST 5 to 600 keV 1,000 300 100 VERITAS 100 to 104 GeV 10 180 10 ARISE 3 mm to 4 cm n.a.e 20 to 107 FASR 1 to 100 cm n.a.e 30 to 100 SPST 200 µm to 1 mm 10 10 to 107 aIncrease in sensitivity compared with existing or other planned facilities. For EXIST the comparison is with a previous survey by HEAO-1. bFor entries with a (λ/a) term included, the spatial resolution is for wavelength λ > a. The number in front of the parenthesis therefore corresponds to the best spatial resolution. cSpectral resolving power is defined as λ/Δλ. dFrequency of gravitational waves. All other recommended projects detect electromagnetic waves. en.a., not applicable, is assigned to LISA, ARISE, and FASR because there are no existing facilities with which to compare the sensitivity. LISA will be a unique facility operating at frequencies (see footnote above) not accessible to the LIGO experiment. ARISE will operate at unprecedented spatial resolutions. FASR will observe the Sun over an unprecedented frequency range and at spatial and temporal resolutions beyond current capabilities.

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Astronomy and Astrophysics in the New Millennium planet formation and many galactic nuclei. Although stars generally radiate mostly at optical and ultraviolet wavelengths, the expansion of the universe transforms such radiation from very distant galaxies to infrared wavelengths. Consequently, a number of the new initiatives cover the infrared portion of the spectrum. Advances in UVOIR technology offer the promise of enabling great leaps in our understanding of the universe. Typically, these advances bring greater sensitivity—the ability to detect faint signals—and greater angular resolution, the ability to see fine detail in distant objects. Increased sensitivity requires more collecting area, which is most easily achieved with large filled-aperture telescopes. The angular resolution also improves with the diameter of the telescope mirror. However, to achieve the extremely high angular resolution needed for some of the long-range scientific goals described in Chapter 2 would require excessively large mirrors, up to thousands of kilometers in size. For these angular resolutions filled-aperture telescopes are not practical, and interferometers are used instead. Interferometers combine the photons from two or more telescopes to produce the equivalent angular resolution of a single telescope whose diameter equals the maximum separation of the interferometer telescopes. The cost of using interferometers is reduced sensitivity and ambiguities in image reconstruction. The latter can be corrected by reobserving with many different baseline separations and orientations. Interferometers also can cancel the light from the central star in a planetary system so that astronomers can see the relatively dim planets nearby. Both space- and ground-based telescopes are needed to open the UVOIR window onto the universe. Ground-based telescopes have the advantage that they are generally less expensive and much easier to maintain and upgrade. Space-based telescopes have the advantage that they are free from the distortion, absorption, and background emission of the atmosphere and allow observations in many UVOIR wavelength bands not accessible from the ground: for example, the far ultraviolet (0.1 to 0.3 µm), many regions in the infrared (1 to 30 µm), and the entire far infrared (30 to 300 µm). Even at wavelengths that do reach the ground, the turbulence in the atmosphere blurs the angular resolution of ground-based telescopes. Recent advances in adaptive optics have substantially reduced this problem in the infrared, but it persists in the optical band, particularly for wide-field imaging. Together, ground- and space-based telescopes enable a comprehensive attack on many of the fundamental questions in astronomy.

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Astronomy and Astrophysics in the New Millennium LARGE FILLED-APERTURE OPTICAL AND INFRARED TELESCOPES: NGST AND GSMT At the present time, the premier UVOIR telescopes are the Hubble Space Telescope (HST) in space and the Keck 10-m telescopes on the ground. When they are completed, the 8-m Gemini telescopes and the European Very Large Telescope (VLT) will provide powerful additions to the available ground-based telescopes (see Table 3.3). The committee’s top recommendations for this decade are to dramatically increase the capability of UVOIR observations with the Next Generation Space Telescope (NGST) in space and the Giant Segmented Mirror Telescope (GSMT) on the ground. Both of these telescopes have filled apertures and are very substantial improvements on HST and Keck, respectively. Their large mirrors must be segmented, and fast-responding actuators must bring the segments into nearly perfect alignment. The 1990s saw revolutionary advances in the development of lightweight mirrors and segmented structures that will be put to use in constructing these new telescopes. NGST (Figure 3.1) consists of a passively cooled, segmented telescope that will deploy to its full diameter of about 8 m once it is in space. It will orbit the Sun roughly a million miles from Earth. At present, its planned wavelength range is 0.6 to 27 µm. NGST will be far more capable than its space predecessors HST and SIRTF and its airborne predecessor SOFIA. Figure 3.2 compares the sensitivity of NGST with that of other space facilities at low spectral resolving power, which is appropriate in searches for distant galaxies and faint stellar objects. Much of its increase in sensitivity compared with previous space telescopes comes from its large aperture, which not only gathers more photons from each source but also reduces the number of photons from the background by virtue of its greater angular resolution. Astronomical capability is defined in the 1991 survey, The Decade of Discovery in Astronomy and Astrophysics (NRC, 1991), in terms of the speed of an observation. Improvements in sensitivity and angular resolution make NGST roughly 1,000 times more capable than HST and SIRTF; its low temperature makes it up to a million times more capable than similar-size ground-based telescopes. The discovery potential of NGST is enormous. Having NGST’s sensitivity extend to 27 µm would substantially improve its ability to study Kuiper Belt objects (KBOs) in the solar system, star formation and planet formation in our galaxy, and dust emission in galaxies out to a redshift of 3. Not only would this extension take full

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Astronomy and Astrophysics in the New Millennium TABLE 3.3 Existing and Approved Large Ground-Based Optical and Infrared Telescopes Project Commencement of Operations Partners Involved Percent U.S. Aperturesa Gemini   United States, United Kingdom,     Northern 1999 Canada, Chile, Australia, 52 2 × 8 m Southern 2001 Argentina, Brazil 42   Gran Telescopio Canarias 2003 Spain 0 1 × 10.4 m HET 1998 United States,b Germany 90 1 × 9.2 m Keck 1993, 1996 United Statesb 100 2 × 10 m LBT 2002 United States,b Italy, Germany 50 2 × 8.4 m Magellan 2000, 2002 United States,b Chile 90 2 × 6.5 m MMT 1999 United Statesb 100 1 × 6.5 m SALT 2002 South Africa, United States,b Poland, New Zealand, Germany, United Kingdom 20+ 1 × 10 m Subaru 1999 Japan 0 1 × 8 m VLT 1999+ Europe 0 4 × 8 m aNotation: 2 × 8 m denotes two telescopes with 8-m-diameter apertures. bU.S. private or university telescopes. SOURCE: Includes data from NRC (2000), Table 5.13. advantage of the effort to cool the instrument, but NGST would also gain its greatest advantage over any ground-based telescope at the longer infrared wavelengths (see Figure 3.3). Considerable progress has been made in developing the challenging technology required by NGST, including sensitive detectors, lightweight deployable primary mirrors, and control and image analysis systems. To enable NGST to reach its full potential, the committee recommends technology development to increase telemetry rates in spacecraft communication and for cryocoolers that enable detectors to operate at wavelengths longer than 5 µm. GSMT complements NGST, both in technical capabilities and in its ability to probe distant galaxies and nearby star-forming regions. GSMT is a 30-m-class, ground-based, filled-aperture, segmented-mirror, optical and infrared telescope that will operate in the atmospheric windows over the wavelength range from 0.3 to 25 µm. Adaptive optics will give it diffraction-limited performance down to wavelengths as short as 1 µm. GSMT will complement NGST much as the Keck telescope has comple-

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Astronomy and Astrophysics in the New Millennium FIGURE 3.1 An artist’s conception of the NGST mission. NGST, the highest priority for the new decade, will be a segmented, filled-aperture, 8-m-class telescope that will utilize a large sunshield and an orbit at least 1 million km from Earth to achieve very cold operating temperature. Its sensitive measurements, primarily in the infrared region of the spectrum, will revolutionize the fields of galaxy formation and evolution and star formation. Courtesy of J. Lawrence (NASA Goddard Space Flight Center). mented HST, by making studies with high spatial and spectral resolution of the sources seen by the smaller space telescope. Figure 3.3 shows a comparison of the sensitivity of GSMT and NGST at various spectral resolving powers, demonstrating the power of NGST at low spectral resolution and longer wavelengths and the power of GSMT at high spectral resolution and shorter wavelengths. Furthermore, with the ability to add new instrumentation, a step that is not possible with NGST, GSMT can evolve its capabilities and become increasingly powerful. In agreement with the Panel on Optical and Infrared Astronomy from the Ground (see Chapter 2 of the Panel Reports; NRC, 2001), the committee believes that the 30-m scale of GSMT is the appropriate next step in the construction of large ground-based OIR telescopes. The more ambitious 100-m “OWL” telescope under development by the European

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Astronomy and Astrophysics in the New Millennium Southern Observatory represents an excellent opportunity for shared technology development and possibly for eventual U.S. collaboration. International participation in GSMT would offer strong benefits as well. Experience with the Keck, Gemini, and Hobby-Eberly telescopes shows that the cost of large telescopes need not increase with a high power of the mirror diameter. Nevertheless, the large size of GSMT means that substantial advances in telescope design and adaptive optics will be required if it is to be built for a reasonable cost. The committee recommends that this work commence soon so that construction of the telescope can begin in this decade. Figure 3.4 shows the enormous gains in spatial resolution that are made possible by the use of adaptive FIGURE 3.2 Relative performance of planned and recommended space initiatives, as well as the performance of the ground-based ALMA in the submillimeter wavelength band. The vertical axis denotes the 5σ flux detectable in 3 hours of integration; therefore better performance is lower on the figure. Estimated performance is based on the combination of photon and confusion noise appropriate to the high-galactic-latitude sky. Courtesy of G. Rieke (University of Arizona) and S. Beckwith (Space Telescope Science Institute).

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Astronomy and Astrophysics in the New Millennium FIGURE 3.3 A comparison of GSMT to NGST for spectral resolving powers of R = 10 through 100,000 over a range of wavelengths. Plotted is the signal-to-noise ratio achieved in an observation of given duration on a given object (much fainter than the sky) for GSMT relative to that achieved with NGST. The comparison shows that GSMT is substantially more effective in obtaining high-resolution spectra of faint objects for short-wavelength radiation that penetrates the atmosphere. NGST is substantially more effective at longer wavelengths, at wavelengths blocked by the atmosphere, and for observations done at low spectral resolution. Courtesy of L. Ramsey (Pennsylvania State University). optics on ground-based telescopes such as GSMT. GSMT requires a large investment of resources and offers an opportunity for partnership between national and university/independent observatories in producing and operating a world-class facility within the coordinated system of these two essential components of U.S. ground-based astronomy. Together, NGST and GSMT will trace the formation and evolution of galaxies from the end of the “dark ages,” when the first stars formed, until the present. While NGST’s infrared capability will enable it to study that early epoch of the universe when clouds of hydrogen gas collapsed to form the first galaxies and stars, GSMT will be especially powerful in studying galaxies and intergalactic gas at a somewhat later period of cosmic history when most of today’s stars and chemical elements were formed. NGST will observe the development of the clustering and

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Astronomy and Astrophysics in the New Millennium FIGURE 3.4 The power of adaptive optics (AO) as shown by comparison of a high-quality 2.2-µm ground-based image of the galactic center without AO and with AO. Adaptive optics corrects for the distortions caused by the turbulence in the atmosphere and results in an image of much higher resolution (diffraction-limited). Courtesy of the W.M. Keck Observatory Adaptive Optics Team. (This figure originally appeared in Publications of the Astronomical Society of the Pacific [Wizinowich, P., et al., 2000, vol. 112, pp. 315-319], copyright 2000, Astronomical Society of the Pacific; reproduced with permission of the Editors.)

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Astronomy and Astrophysics in the New Millennium radar remote sensing of solar system objects as well as passive radio astronomy. For compact sources, however, it is necessary to use interferometry, a technique pioneered by radio astronomers in which radio signals detected at antennas separated by distances of up to thousands of miles are combined to form an image of the source. The highest resolutions are achievable with the Very Long Baseline Array (VLBA), used to image radio sources associated with quasars, active galactic nuclei (AGNs), molecular masers, and other compact sources. For larger radio sources, however, the Very Large Array (VLA) is the instrument of choice. It provides spatial resolutions of 0.1 to 1 arcsec, comparable to those of the large ground-based optical telescopes. The VLA is the most powerful and productive centimeter-wave telescope in the world, despite the fact that its instrumentation is 25 to 30 years old. The Expanded VLA (EVLA) is the second priority among major, ground-based projects. The EVLA will have 10 times the sensitivity and angular resolution and 1,000 times the spectroscopic capability of the VLA. The first stage of the expansion will replace instruments, computers, and software and install wideband fiber-optics data links. In the second stage, up to eight new antennas will be sited within 250 km of the VLA and connected via fiber-optics links. The resulting angular resolution of 0.01 to 0.1 arcsec will be comparable to that of ALMA and of NGST, facilitating multi-wavelength studies. These new antennas will also enhance the field of view and sensitivity of the VLBA when the two systems are used together. The committee also notes that a complementary small project, the Low Frequency Array (LOFAR), would extend wavelength coverage to 20 m and provide improvement by a factor of 100 to 1,000 in sensitivity and resolution over existing instruments at these wavelengths. The overall dimension of LOFAR would be several hundred kilometers, possibly using the VLA site as the primary location. The high angular resolution and sensitivity of the EVLA, combined with the penetrating power of centimeter-wave radio waves, will enable the detailed study of nearby protostars and protoplanetary disks as well as the active nuclei of distant galaxies. The EVLA will produce images of protogalaxies with sufficient detail to determine whether AGN activity associated with a supermassive black hole precedes, is contemporaneous with, or follows bursts of star formation in galactic nuclei. A possible theory challenge for the EVLA is To understand the roles of star formation and supermassive black holes in powering luminous active galactic nuclei.

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Astronomy and Astrophysics in the New Millennium The Square Kilometer Array Technology Development. The EVLA, VLBA, LOFAR, and One Hectare Telescope (1HT), a privately funded array to be used in part for the search for extraterrestrial intelligence (SETI) project, form the foundation of ground-based interferometric centimeter-wave astronomy for this decade. To study how the first galaxies condensed out of vast clouds of atomic hydrogen, a substantially larger radio telescope is needed: the Square Kilometer Array (SKA), with 1 million square meters of collecting area. (By comparison, the EVLA has some 13,000 square meters of collecting area.) The extraordinary sensitivity of SKA will also revolutionize fields of study newly accessible to centimeter-wave astronomy, including the study of jets and the disks of protostars, the measurement of magnetic fields in collapsing clouds, and the study of the distribution of dark matter on the largest scales by means of weak gravitational lensing. The SKA is a major international project that may start in the decade 2010 to 2020. The committee recommends a coherent development program over the current decade to develop the technology that will enable the science objectives to be met at a reasonable cost. A possible theory challenge for the SKA development is To understand the formation of the first generation of stars and their effect on reionizing the universe. The Advanced Radio Interferometry between Space and Earth (ARISE) mission is a 25-m-class space antenna to be linked with the ground-based VLBA. It is recommended as a means of achieving the highest spatial resolution for bright sources such as jets emanating from near supermassive black holes in galactic nuclei. ARISE will operate at wavelengths as short as about 3 mm. Its elliptical orbit will reach up to 50,000 km from Earth, giving an angular resolution six times better than that obtained with the VLBA. It will be an order of magnitude more sensitive than the Japanese HALCA space interferometric system that is currently in operation. A theory challenge for ARISE might be To understand maser emission and other physical processes in nonrelativistic accretion disks in our own and other galaxies.

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Astronomy and Astrophysics in the New Millennium MILLIMETER- AND SUBMILLIMETER-WAVE ASTRONOMY: CARMA AND SPST Table 3.7 lists the major ground-based millimeter- and submillimeter-wave telescopes in the world. The Atacama Large Millimeter Array (ALMA), an international collaboration among the United States, Canada, Europe, and possibly Japan, will be by far the most powerful of these telescopes when it becomes operational. Situated at a high, dry site in Chile, ALMA will dominate millimeter-wave observations of the southern sky. Under another name (the Millimeter Array), ALMA was the first-ranked radio project a decade ago (NRC, 1991); the committee reaffirms support for this project. The high spatial resolution (as fine as 0.01 arcsec) and sensitivity of ALMA will allow unprecedented views of diverse astronomical phenomena ranging from comets and Kuiper Belt objects in the solar system, to planet-forming disks in nearby regions of star formation, to the structure of the interstellar medium in distant galaxies. The Combined Array for Research in Millimeter-wave Astronomy (CARMA). The committee recommends support for the construction of a Northern Hemisphere array, somewhat different in design, that will complement ALMA. CARMA would combine nine of the current 6-m Berkeley-Illinois-Maryland Association (BIMA) antennas, the six 10.4-m Owens Valley Radio Observatory (OVRO) dishes, and ten new 2.5-m antennas at a higher and better site in California. The resultant hybrid array would offer unique imaging capabilities to study structure on all scales with particular sensitivity to low-surface-brightness, extended emission. CARMA will be a powerful tool for studying the chemistry, dynamics, and structure in star-forming regions as well as for mapping the deviations in the cosmic microwave background caused by the hot gas in clusters of galaxies. As a project, CARMA is run by a university consortium and is largely funded by nonfederal sources but will provide significant access to the entire astronomical community. It could be undertaken immediately, fostering the training of students and the U.S. capability in millimeter-wave interferometry at the start of the ALMA era and beyond. The South Pole Submillimeter-wave Telescope (SPST). For its combination of low opacity and stable seeing, the South Pole is the best site in the world for ground-based observations at submillimeter wavelengths. To take advantage of the opportunities offered by this site, the committee recommends the construction there of a 7- to 10-m-class

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Astronomy and Astrophysics in the New Millennium filled-aperture submillimeter-wave telescope. Such a telescope should be equipped to survey the sky so that it can identify sources such as primordial galaxies, study the distortion of the cosmic microwave background caused by clusters of galaxies, and survey the dusty universe. Its survey capability will make the SPST an important complement to ALMA. A possible theory challenge for CARMA and SPST is To understand the dynamical and chemical evolution of molecular clouds in galaxies. THE COSMIC MICROWAVE BACKGROUND RADIATION The cosmic microwave background radiation was emitted early in the history of the universe, before stars and galaxies formed. The radiation was emitted with a spectrum dominated by optical and near-infrared radiation, but the expansion of the universe has increased its wavelengths by a factor of about 1,000, so that it is now concentrated in the millimeter and submillimeter parts of the spectrum. When the radiation was emitted, the universe was almost, but not quite, perfectly homogeneous. The small inhomogeneities present at that time were the seeds of the formation of galaxies, clusters of galaxies, and larger structures in the universe. The Cosmic Background Explorer (COBE) was flown in the past decade and made the most precise measurements at that time of the background radiation and the tiny variations in its intensity over the sky (about 1 part in 100,000). Since then, ground- and balloon-based experiments have probed the microwave background on smaller angular scales than did COBE. Recent balloon observations have shown that the total density of matter and energy is just what is needed to make the geometry of the universe flat (see Chapter 2). Measurements of the microwave background are the primary means available for probing the large-scale structure of the early universe, and as such they are essential for addressing one of the primary science goals for this decade. Scientists’ knowledge of the microwave background will be transformed by ongoing ground-based experiments and by the upcoming Microwave Anisotropy Probe (MAP) MIDEX (mid-size Explorer) mission. The Planck Surveyor, a European-led mission with significant U.S. involvement that is planned for launch in 2007, will study smaller-scale fluctuations in the background. Future microwave background experiments, such as measuring the polarization, are of great importance, but the committee recommends that prioritization of such

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Astronomy and Astrophysics in the New Millennium TABLE 3.7 Ground-Based Millimeter- and Submillimeter-wave Telescopes Project Nations Involved Wavelength Band Aperturea Angular Resolutionb (arcsec) Existing and Approved Interferometers ALMA United States, Europe, Japan Millimeter 64 × 12 m BIMA United Statesc Millimeter 10 × 6 m IRAM Europe Millimeter 5 × 15 m Nobeyama Japan Millimeter 6 × 10 m OVRO United Statesc Millimeter 6 × 10.4 m SMA United States, Taiwan Submillimeter 8 × 6 m New Initiative CARMA United Statesc Millimeter 25 × (2.5 to 10 m) Existing and Approved Single Dish CSO United Statesc Submillimeter 10.4 m FCRAO United Statesc Millimeter 14 m Hertz (SMT) Germany, United Statesc Submillimeter 10 m IRAM Europe Millimeter 30 m

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Astronomy and Astrophysics in the New Millennium Project Nations Involved Wavelength Band Aperturea Angular Resolutionb (arcsec) Existing and Approved Single Dish (continued) JCMT United Kingdom, Submillimeter 15 m LMT United States,c Mexico Millimeter 50 m Nobeyama Japan Millimeter 45 m New Initiative SPST United States Submillimeter 7 to 10 m aNotation: 64 × 12 m denotes 64 antennas with 12-m diameters. bThe wavelength is scaled to the shortest operating wavelength; the number in front of the parenthesis is the angular resolution at that wavelength in arcseconds. cUniversity or private facility in the United States. experiments await the results from MAP (assuming it is successful) and the ongoing suite of ground-based and balloon projects. THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE Are we alone in the universe? Finding evidence for intelligence elsewhere would have a profound effect on humanity. Searching for evidence for extraterrestrial life of any form is technically very demanding, but, as indicated in the discussion of TPF above, there is a clear approach for doing so. The search for extraterrestrial intelligence is far more speculative because researchers do not know what to search for. Radio astronomers have taken the lead in addressing this challenging Netherlands, Canada

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Astronomy and Astrophysics in the New Millennium problem, and SETI programs are under way at many radio telescopes around the world. This committee, like previous survey committees, believes that the speculative nature of SETI research demands continued development of innovative technology and approaches, which need not be restricted to radio wavelengths. The privately funded 1HT, which will be the first radio telescope built specifically for SETI research, is a good example of such an innovative approach, and it will pioneer new radio techniques that could be used in the SKA. THE NATIONAL VIRTUAL OBSERVATORY AND OTHER HIGH-LEVERAGE, SMALL INITIATIVES The National Virtual Observatory (NVO). As the new millennium begins, astronomy faces a revolution in data collection, storage, analysis, and interpretation of large data sets. Data are already streaming in from surveys such as the Two Micron All Sky Survey and the Sloan Digital Sky Survey, which are providing maps of the sky at infrared and optical wavelengths, respectively. The LSST, which will survey the sky every 3 days, will add a third dimension, time, to the data. What is needed is to make the data from all these surveys available to astronomers, educators, and the public so that they can view images of the evolving sky at any wavelength (or color) surveyed by astronomical telescopes. Each day, trillions of bits of information from telescopes will have to be rapidly archived and made available for viewing and analysis. The NVO is designed to enable this, and it is the committee’s highest priority for small projects. The NVO will link the major astronomical data assets into an integrated, but virtual, system to allow automated multiwavelength search and discovery among all cataloged astronomical objects. The computers used in the NVO would be distributed across the country, but high-speed networks would link them into a unified system. The NVO not only would archive the data but also would provide advanced analysis services for the astronomical community, create data standards and tools for mining data, and provide a link between the exciting astronomical data and the educational system in the United States. The opportunities for communicating the most recent discoveries in the dynamic sky into

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Astronomy and Astrophysics in the New Millennium classrooms and homes worldwide give the NVO the potential of becoming a powerful tool for increasing the general public’s science literacy. Theoretical Astrophysics. All the major and moderate programs recommended in this report are aimed at making substantial advances in astronomers’ ability to observe the universe. However, these data would have little significance in the absence of a theory to interpret them. Theorists play a major role in defining the intellectual frontier of astronomy and astrophysics, in developing the models that quantitatively relate observational data to the underlying physics and chemistry, and in synthesizing a world view that is accessible to the general public. In view of the explosion in the rate of astronomical discoveries, the committee believes that the resources recently devoted to theory are not adequate for an optimized program, and it therefore recommends three small initiatives to redress this imbalance: (1) Theory challenges tied to major and moderate projects. These challenges should be administered as a competitive grants program that is budgeted and programmed as an integral part of its associated project. Examples of possible theory challenges are given above. Each challenge should identify a theoretical problem that is ripe for progress, relevant to the planning and design of the mission, and essential to the interpretation and understanding of its results in the broadest context. Funding for this program might typically amount to 2 to 3 percent of the cost of the project, although this amount could vary substantially depending on the nature of the project. (2) The National Astrophysical Theory Postdoctoral Program modeled on the highly successful Hubble postdoctoral program. Currently, grants to individual theorists rarely have the funding or the longevity to support postdoctoral fellows. A national postdoctoral program in theoretical astrophysics will support innovative research by the new generation, foster their intellectual development, and encourage ethnic and gender diversity. This program should be supported jointly by NASA and NSF to provide 10 new 3-year postdoctoral positions each year. (3) Augmentation of the Astrophysics Theory Program at NASA. This program has been highly successful, but the report Federal Funding of Astronomical Research (NRC, 2000) documents the difficulties facing it—the 3-fold oversubscription in 1987 increased to oversubscription by a factor of 4.8 in 1997, and the overall funding for theory declined by about 20 percent from 1990 to 1999. The committee recommends that the Astrophysics Theory Program be augmented by $3 million per year to bring it into better balance with the vigorous experimental and observational pro-

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Astronomy and Astrophysics in the New Millennium grams at NASA. More details on these recommendations can be found in Chapter 1 of the Panel Reports (NRC, 2001). These three programs should not divert funds from existing grants programs for broadly based theory. Laboratory Astrophysics. Existing missions and facilities are returning spectroscopic data of unprecedented breadth and detail, yet in many cases these data cannot be interpreted because the underlying atomic and molecular properties are unknown. Even in the Sun, almost 40 percent of the coronal lines observed by SOHO in the range from 50 to 160 nm are unidentified; for those lines that are identified, the wavelengths and excitation cross sections may be so poorly known that quantitative interpretation of the data is impossible. Existing missions such as Chandra and XMM-Newton and proposed missions such as NGST, Constellation-X, and SAFIR will obtain spectra of much more exotic objects where the uncertainties are much greater. In addition to atomic and molecular physics, the properties of irradiated ices, refractory grains, and fluids at high energy densities also require study. At present, support from both NASA and NSF for these types of investigations is extremely limited. The committee recommends a significant increase in support for these areas, primarily from NASA, since the largest residual uncertainties in the atomic and molecular databases pertain to transitions that lie primarily in wavelength bands accessible only from space. Specifically, the committee recommends a dedicated NASA grants program for laboratory astrophysics funded at the level of $4 million per year ($40 million for the decade), which should enable roughly 8 to 10 significant experimental programs that collectively cover most of the spectrum. The committee also recommends a smaller level of support from NSF ($500,000 per year or $5 million for the decade) that would be directed at computational atomic and molecular physics and database development. In addition to the above recommended initiative for laboratory astrophysics, the committee notes several proposed programs that will provide significant benefit to astrophysics. Direct laboratory simulation of magnetic reconnection in plasmas and of radiative-hydrodynamical instabilities in supernovae and supernova remnants have the potential to be of great benefit in interpreting astronomical observations, particularly when coupled with computational modeling of the same phenomena. Better knowledge of key nuclear reaction rates is essential for advancing scientists’ understanding of the late stages of stellar evolution, of supernovae, and of Big Bang nucleosynthesis. Finally, experiments at the Relativ-

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Astronomy and Astrophysics in the New Millennium istic Heavy Ion Collider and the Large Hadron Collider will determine the properties of the quark-gluon plasmas that existed a few microseconds after the Big Bang. Ultralong-Duration Balloons (ULDBs). NASA’s Explorer program has been extremely successful because it provides frequent access to space for innovative projects across the entire field of astrophysics. In many cases, most of the advantage gained by going into a low Earth orbit could be achieved at far less cost with balloons capable of remaining aloft for periods of 100 days or more. 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, imaging convective flows and magnetic fields in the Sun’s photosphere with a large solar telescope, extragalactic observations with a moderate-sized far-infrared telescope, or 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 small-mission science at wavelengths not transmitted to Earth. The committee recommends that NASA invest the necessary resources (estimated to be about $35 million) to develop steerable ULDBs, and that use of ULDBs be allowed as an alternative to spacecraft (where warranted) in all Explorer programs.

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