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Priorities in Space Science Enabled by Nuclear Power and Propulsion 8 Applications of Nuclear Power and Propulsion in Astronomy and Astrophysics: Missions NUCLEAR POWER AND PROPULSION FOR SPECIFIC ASTROPHYSICAL APPLICATIONS To gauge the possibility that radioisotope power systems (RPSs) and/or fission reactors might enhance or enable the achievement of important astrophysical goals, this chapter examines several types of astrophysical missions for which nuclear systems might potentially be considered. The discussion focuses on several of the important categories of applications outlined in the section “Implementation and Techniques” in Chapter 7. As the discussion below makes clear, nuclear power and propulsion technologies are not enabling nor enhancing for any of the current high-priority scientific goals of astronomy and astrophysics. Generating Very Long Baselines Nuclear power systems would be truly enabling for astronomical observations that require multiple telescopes separated by distances of >5 AU and involve operation of (at least) one observatory far enough from the Sun that solar power would be more expensive than nuclear power. One such new opportunity is created by the ability to conduct radio interferometry using baselines extending over many AU. Figure 8.1 shows the angular resolution obtained from interferometry as a function of observing wavelength (λ) and baseline. It is notable that an interferometer with 1-AU baseline—for which nuclear power systems offer no advantage—is capable of achieving an angular resolution of a nanosecond of arc at all wavelengths less than 1 mm. Such a resolution is sufficient to resolve neutron stars or active galactic nuclei (AGN) black holes, or to measure parallaxes at 100 megaparsecs. There is limited return from obtaining yet finer angular resolution, and in any case a telescope much larger than any currently imagined as remotely feasible would be required to obtain observations with a sufficient signal-to-noise ratio for such small features. Three other possibilities for the use of nuclear systems include the following: Geometrical Parallax Mapper. A 1980s study of the so-called Thousand Astronomical Units (TAU) mission emphasized the possibility of microarcsecond parallax measurement, which would enable determination of geometric distances to Local Group galaxies and hence a more accurate extragalactic distance scale.1 It is now believed to be far more feasible to obtain these measurements using interferometry with baselines well below 1 AU.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 8.1 Diagram illustrating that there is little justification for astronomical interferometry on baselines longer than the Earth-Sun distance (1 AU). Sloping black lines: angular resolution as a function of baseline (ordinate) and photon wavelength λ, frequency υ, or energy E (abscissas). One arcsecond is 1/3600 degree. Right ordinate: bottom to top, baselines of the Keck Telescope, Palomar Testbed Interferometer (PTI), Very Large Array (VLA), Earth’s radius , the distance to the Earth-Sun L2 point , the Earth-Sun separation (1 AU), and the semi-major axis of Pluto’s orbit. Lightface labels—angular diameters of a Sun-like star at 100 pc, of the horizon of the 3 billion solar-mass black hole in the nearby galaxy M87 (the black hole in the center of the Milky Way appears 3 times larger), of the 100 million solar-mass black holes in typical quasars at redshift of 2, and of a nearby neutron star at 1-kpc distance. Boldface labels: resolution needed to obtain a 1 percent parallax (π) or resolve a 1-year period solar-mass binary at 10 kpc and to measure a 1 percent parallax at a distance of 100 Mpc. Scattering in the interstellar medium causes the images formed by combinations of wavelength and baseline falling in the hatched region (upper left) to be blurred to the size of the region’s lower boundary line. Thin horizontal lines: strong multipath scattering. Dotted vertical lines: weak scattering, with images appearing as scattered speckles; some information about source size may remain. Gamma-Ray Burst Locator. Time-of-arrival triangulation on ≈50-AU baselines can localize the sources of gamma-ray bursts (GRBs) down to the arcsecond scales required for unambiguous identification of the host galaxy. The Swift mission may demonstrate that all GRBs have afterglow emissions that can be used to locate the host galaxy. But if there are classes of GRBs with no localizable afterglow, a long-baseline GRB network would be scientifically compelling. GRB detector sensitivities would be enormously degraded by the gamma-ray background from a nearby fission reactor, but GRB detectors have successfully coexisted with the radioisotope thermoelectric generator (RTG) on the Ulysses spacecraft. For a small payload like a GRB detector, solar sails
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Priorities in Space Science Enabled by Nuclear Power and Propulsion may offer a faster transit to 50 AU than would nuclear-electric propulsion (NEP). Even if a solar sail is used, an RPS would still be required to supply the spacecraft’s electrical-power requirements. Long-Baseline Radio Interferometer. Radio (λ >1 mm) interferometry on interplanetary baselines appears, at first glance, to be an ideal application for the use of nuclear power, given that it is only at that wavelength that an attempt has been made, successfully, to correlate wavefronts using a space-based antenna. However, the scattering of radio waves by the interstellar medium blurs radio images to angular sizes much greater than can be resolved by a radio interferometer with >1-AU baselines (see Appendix D for additional details). Some AGN are relatively bright sources at high radio frequencies where scattering is minimized. Nevertheless, sources with the brightness temperatures required to produce correlated signals on such large baselines may not exist. Moreover, without an armada of observatories, the time to fill in the so-called UV plane would likely exceed the time over which source structures on the smallest observable scales could vary. The only sources with sufficiently high intrinsic brightness temperature and stable morphologies are radio pulsars, but these are intrinsically low-frequency objects that thus suffer too much scattering. There thus appears to be no compelling justification for radio interferometry on baselines greatly exceeding the radius of Earth’s orbit. Permitting More Favorable Observing Locations: The Moon and Moons of Mars The question of observatories on the Moon and elsewhere in the solar system has received new attention within the context of NASA’s new exploration initiative. Four possibilities are considered below: Lunar Astronomical Observatory. The use of the Moon as a site for an observatory was carefully considered by the astronomical community 20 years ago and found to offer science potential at that time.2,3 The Moon offers a solid surface on which to anchor telescopes, no absorbing atmosphere, and, in lunar polar craters that are permanently shadowed, a thermally convenient place to locate sensitive infrared telescopes. In these respects, the surface of the Moon is vastly better for astronomy than the surface of Earth. These facilities would need power for operations and communication, and in some cases would need heat sources for survival. Providing such power through the lunar night would likely require nuclear systems.a Within the astronomical community, however, enthusiasm for lunar surface observatories has waned considerably over the last two decades.4 During this time, enormous progress has been made in free-space telescope operations—e.g., guiding, tracking, and stabilization—and there is no longer the need for a solid surface to anchor telescopes. Furthermore, the lunar surface has distinct disadvantages over free-space sites, including the presence of gravity—which imposes serious structural requirements on precision optical systems that are pointed around the sky—and dust. Free space offers the same vacuum as does the lunar surface, and although the lunar polar craters are naturally very cold, similarly low temperatures have been achieved in free space with deployable sun shields. Although human-aided deployment and maintenance of astronomical instruments on the Moon could be advantageous, it is by no means clear that, in the context of the new exploration agenda, free-space sites will be less accessible than lunar surface sites to human service crews. Thus, for achieving most astronomical goals (see the discussion below for two possible exceptions), the use of the lunar surface as an observatory site does not appear to offer any enabling advantages over the use of free space. Farside Radio Observatory. The Moon’s farside is a generally radio-quiet location, because it is effectively shielded from both natural and human-caused electromagnetic emissions from Earth. The effect of terrestrial radio noise on passive scientific research such as radio astronomy is enormous. A variety of astronomical objects are expected to emit in the spectral regions affected by terrestrial interference, and study of these objects is likely to be scientifically interesting (see Appendix D for details). Is a lunar-based radio telescope essential to the continued existence of radio astronomy as a science? The answer to this question is not yet clear. Some locations on Earth are still relatively radio quiet, although whether a Although by careful selection of the observing site, operations in permanently shadowed craters on the Moon’s polar regions could be enabled by power transmitted from sites, such as crater rims, located in permanent sunlight.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion they will remain so is a primary issue. However, quantifying the deleterious effects of interference, and projecting them into the future, is extremely difficult. Techniques for mitigating radio-frequency interference are currently under very active development in preparation for the next generation of Earth-based radio telescopes. Certainly more will be known regarding the growth in the sources of interference, as well as the ability to mitigate it, once these telescopes are operating, over the next two decades. The use of a free-flying fleet of dipole receivers reliant on solar power could reduce the terrestrial radio-frequency interference by r−2 and might possibly suffice to achieve high-priority science goals, obviating the need to leverage the shielding effect of the Moon and the concomitant need to use nuclear power systems. A farside dipole array could possibly be operated with an RPS rather than a reactor, with telecommunications and/or computing requiring most of the power. These alternatives suggest that a nuclear reactor-powered farside observatory is not a uniquely enabling solution to the problem of radio interference. Lunar Gravitational Wave Array. The Moon could be instrumented as a sensitive gravitational-wave detector. By combining signals from several superconducting (2 K) displacement sensors distributed over the surface of the Moon, a sensitive detector (hmin ≤ 10−22 Hz−1/2) with full-sky coverage could be constructed. Such displacement sensors could be tuned to the lowest quadrupole mode of the Moon (1 mHz) or operated as a wideband detector below the fundamental frequency (<1 mHz). Phobos/Deimos Gravitational Wave Array. Another interesting possibility is instrumenting the martian moons, Phobos or Deimos, as gravitational-wave detectors. With a radius of only several kilometers, one of the moons of Mars will cover a medium frequency band (0.1 to 1 Hz), which will be missed by both the ground detectors and the Laser Interferometer Space Antenna (LISA). RPSs would be needed to cool and operate these displacement sensors continuously. The cryocooler is expected to dominate the power budget (requiring ~100 to 200 watts of electrical power per site). However, the sensitivity of such gravitational-wave detectors would be substantially less than that of currently envisioned free-flying (solar-powered) laser interferometer gravitational wave detectors in the same 0.1- to 1-Hz band, such as the proposed Big Bang Observer. Detailed studies of cost trade-offs would be necessary to determine if there is a niche for such detectors. Permitting More Favorable Observing Locations: Beyond 3 AU Both the zodiacal-light background (Figure 8.2) and the solar thermal load on a telescope decrease with increasing distance from the Sun. Beyond ~2 to 3 AU from the Sun, nuclear power might become more cost-effective than solar power for propelling or powering an observatory. FIGURE 8.2 Brightness of the infrared background as a function of wavelength at high ecliptic latitudes from the ground, in Earth-trailing solar orbit (1 AU), and at 3 AU.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion The maximization of scientific benefit per dollar invested involves a complex trade-off between the advantages offered by the superior ~3- to 5-AU environment versus the larger mass/aperture and earlier operations obtainable for the same cost with a telescope located at the Sun-Earth L2 or the Spitzer-like drift-away orbits. Two major possibilities and one mission of opportunity are considered below. Note that there is no incentive for placing radio or high-energy observatories at 5 AU. 5-AU Optical/Near-Infrared Observatory. For observations at λ ≈ 0.2 to 3 μm, sunlight scattered by zodiacal dust grains is the dominant source of diffuse background emissions and can, thus, be the dominant source of noise for observations of faint sources. An observatory at 5 AU could have ≈ 50× lower zodiacal background than current or planned ultraviolet/optical/infrared observatories in Earth-trailing or L2 orbits. For nuclear power and propulsion systems to be enabling for a 5-AU observatory mission, placing and operating a 2-meter telescope at 5 AU from the Sun would have to cost much less than placing and operating a 15-meter telescope (or 50 2-meter telescopes) at the Sun-Earth L2 point. Otherwise, the clear choice would be the L2 option, which would offer a superior signal-to-noise ratio (see Appendix D for details) for other observations, as well as better resolution. Certainly, the priorities of the ultraviolet/optical/infrared community will emphasize first building a larger collecting area near 1 AU. 5-AU Far-Infrared Observatory. At thermal infrared wavelengths (λ > 1 μm), space observatories with ambient temperature at 1 AU (300 K) are strongly background limited by emissions from the telescope itself. Therefore, cooling the observatory’s optical system greatly increases observing sensitivity. For smaller telescopes, such as the 0.8-m Spitzer Space Telescope, onboard expendable cryogens can be used for this purpose, resulting in telescope temperatures on the order of between 4 and 8 K. Such a cooling strategy is more difficult for larger astronomical telescopes. Active cooling with onboard refrigerators is probably the best way to ensure a long observatory lifetime (see Appendix D for details). These systems do, however, have substantial power requirements, which can be met by solar arrays at 1 AU. Telescopes at larger heliocentric distances, however, may be enabled by RPSs. Of some interest is the possibility that an infrared observatory at a large heliocentric distance could be powered by a separate spacecraft, with reactor power beamed across a distance that would obviate problems associated with waste heat and radiation.5,6 With enormous generation capability, the power-plant spacecraft could be outfitted for propulsion as well, and could serve as a tug to bring the observatory to the science venue before standing off Sun-ward of the observatory. Such a tug could be outfitted for a powerful communication link back to Earth and could act as a relay station for observatory communications. In this case, the communication power budget for the observatory itself could be quite modest, because it would only need to link to the relay station. Infrared Background/Zodiacal Light Mapper. It could be argued that missions such as the Interstellar Observatory (see Box 4.1 and Appendix B) or, possibly, the Titan Express/Interstellar Pioneer (see Box 6.4) present opportunities for the conduct of astronomical observations. These missions are designed to travel out to 200 AU, where zodiacal light is no longer a concern. A small (~10-cm-aperture) mid/far-infrared telescope on such a mission could measure the cosmic background at wavelengths from 3 to 100 μm, believed to be a result of the superposition of emissions from dusty galaxies at high redshifts. Knowledge gained from such measurements would help identify how much had been “missed” at low surface brightness or low flux in imaging missions such as the Spitzer Space Telescope and the James Webb Space Telescope. This is an interesting, but not a high-priority opportunity, because the relevant observations can be made at distances of between 3 and 5 AU from the Sun and do not require travel to 200 AU. And similar measurements could also be performed by a large infrared telescope at 5 AU that would have much wider applicability, such as described above. Permitting More Favorable Observing Locations: Accessing Special Alignments In gravitational lensing events, the alignment between the photon source, the lensing mass, and the observer can grossly affect the received image. Adjustment of the observer’s position can therefore be exploited, opening a window for nuclear propulsion systems to enable new scientific opportunities. Three types of missions have been proposed:
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Microlensing Parallax Mapper. The amplification of background stars in the Local Group galaxies through gravitational lensing by foreground stars or dark masses is now routinely detected. The proper motions of the source and lens cause the amplification pattern to sweep across Earth. The time-series information gives the characteristic Einstein-ring size divided by the (projected) transverse velocity of the source/lens.7 With additional telescope(s) separated by ~1 AU, the characteristic size can be determined independent of the velocity. When applied to Local Group microlensing events, this approach results in improved knowledge of the masses and dynamics of the lensing and source populations. Because the sizes of the projected Einstein rings are typically a few AU, there is no gain from baselines of 5 AU or greater. Similarly, distant AGN can be microlensed by the individual stars in intervening galaxies, and observations from a network of three telescopes on ~10-AU baselines can provide sufficient information to disentangle the effects of transverse velocity and size imprinted on the lensing caustic sweeping across the solar system. In this way, improved constraints on the sizes of AGN can be determined. Binary-Star Gravitational Telescope. Gould and Gaudi propose placing a telescope in a location where the gravitational lensing caustic of a nearby binary star aligns with a distant AGN.8 Under these circumstances, it may be possible to map the AGN with AU-scale resolution, relying on the extreme (one-dimensional) magnification that the lensing caustic provides. To find the nearest point that lies along the line connecting an AGN with a known binary star would require traveling tens of AU and having substantial maneuverability once there. It is likely, however, that equivalent resolution in the mapping of AGN could be obtained using interferometers with baselines much smaller than 1 AU. Solar Gravitational Telescope. Several studies have investigated the possibility of placing an observatory 550 AU or more beyond the Sun, where the gravitationally lensed rays near the Sun’s limb come to a focus.9,10 Proper placement of the observatory would use the Sun as a gravitational telescope to provide enormous flux and angular magnification (in one dimension) for a chosen target. This could be an attractive method for resolving a terrestrial planet (or several) around another star. However, in addition to the great distance involved, there are severe practical difficulties in, for instance, blocking the Sun’s light or observing through the surface brightness of the solar corona. None of these applications offer a compelling, unique scientific value that would justify the development of nuclear propulsion systems. The Solar Gravitational Telescope could be interesting, but it is either completely infeasible or must be left to the consideration of astronomers later in this century. TECHNOLOGY ENHANCEMENTS AND ISSUES The report of the Aldridge Commission identified 17 technologies as enabling for NASA’s exploration initiative.11 One of these—advanced power and propulsion, primarily nuclear thermal and nuclear electric—is the focus of this report. As is demonstrated above, however, nuclear power and propulsion technologies are far less likely to promote progress in space astronomy and astrophysics than are several of the other 16 enabling technologies identified in the commission’s report. As is discussed below, some of these other technical capabilities—e.g., affordable heavy lift, advanced structures, large-aperture systems, formation flying, cryogenic fluid management, high-bandwidth communications, and scientific data collection/analysis—are much more relevant to addressing the scientific goals of astronomy and astrophysics. Enabling technologies for astrophysical missions over the next decades are expected to be those that make possible larger collecting areas; better high-precision optics; on-orbit assembly; precision formation flying; active cooling of telescopes and detectors; and high-communication bandwidth. These are not technologies that benefit obviously from nuclear power or propulsion. Nuclear power and propulsion are neither uniquely enabling of nor even clearly enhancing for missions that address the high-priority science goals of astronomy and astrophysics. The missions require power in the range of 100 We to 10 kWe per spacecraft. Cost and mass per watt are important considerations, because they affect allowable science capabilities under established mission budget and launcher constraints. Only if nuclear power were to become sufficiently cheap, reliable, and low risk might it be of interest for astrophysics missions. For
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Priorities in Space Science Enabled by Nuclear Power and Propulsion operations at nonshadowed 1-AU locations, solar-electric power generation is a low-risk, low-cost benchmark against which any nuclear system application must be measured. Current Cassini-class radioisotope thermoelectric generators weigh 56 kg and provide 285 We or 190 kg/kWe, about 20 times the mass/power ratio of solar-electric power at 1 AU, and the proposed Stirling radioisotope generators have even lower mass/power ratios.b Thus, such power systems do not become cost-effective until well beyond 3 AU except in locations that are shadowed for long periods (e.g., the lunar surface). Important technical challenges for space astrophysics that may prove more relevant than power and propulsion technologies include the following: High-bandwidth communications. As sensor format size becomes larger, data rate rises proportionally. Opportunities for system control and robotic assembly and servicing are also greatly enhanced by improvements in data rate. Higher power transmitter output is an option for which nuclear systems can be considered. For a fleet of co-located spacecraft in which the power budget is dominated by communications, a high-power communications relay station that is nearby could offer significant advantages. However, there are alternatives that appear to be cheaper, including: Provision of high-bandwidth optical communications links; and Improvement and upgrades of the bandwidth and collecting area of the Deep Space Network (DSN). Such an upgrade, whose cost can be amortized over all future missions, saves the expense of installing high-power transmitters on every subsequent spacecraft. Onboard processing. The processing power of onboard computer systems is increasingly important, and such power relates to the electric power available. Missions now being proposed must satisfy the substantial (~10 kWe) power demands (see Table 7.1) of, for example, the correlators supporting coherent detection, the processors required for efficient compression of large data sets (relating to communication bandwidth), and the hardware necessary for autonomous operations. Developing highly capable processing hardware that is radiation hardened would be of particular importance for operations near nuclear power systems and would also be of value for the harsh particle and radiation environment outside low Earth orbit. Faster transit times. Although it is anticipated that most astrophysics missions will operate near 1 AU, minimizing transit times for both deployment and servicing can maximize mission efficiency and reduce risk. With the Sun-Earth L2 point as an important destination for astrophysics missions, it is particularly important to minimize transit times between low Earth orbit and that site. Low-energy travel to L2 with lunar-gravity assist, used by the Wilkinson Microwave Anisotropy Probe and baselined for JWST, takes several months to complete. To the extent that a nuclear propulsion system, whether in the form of a ferry/tug or a spacecraft component, could reduce these transit times substantially and cost-effectively, science might benefit. Multiple spacecraft systems. There is a general trend in astrophysics toward multiple-spacecraft systems, and certain destinations such as L2 will become heavily populated with observatories. Some missions require substantially similar craft operating independently in physical proximity to each other to enhance capability and are often launched and delivered to orbit on a single craft. As the number of craft in a cluster grows, the total demand for power and communications also grows, increasing the attractiveness of a centralized source of power for operations and communications. Although not directly relevant to power and propulsion considerations, there are technology needs for space astrophysics that could be advanced as an indirect consequence of Project Prometheus. These technologies include the following: Heavy-lift launch. Astrophysics, with its continuous need for greater sensitivity, will move inexorably toward larger, more massive instruments. Improved heavy-lift capability would enable new classes of instrumentation with greater sensitivity to be affordably built and launched. b The Stirling radioisotope generator has a nominal power of 112 We at the beginning of a mission and a mass of 34 kg.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Large launcher fairings. Many missions feature large, lightweight structures that are launch-limited not by weight but by size. Larger fairing sizes are needed. Contamination mitigation. Astronomical instrumentation has always been sensitive to environmental factors, including sources of instrument contamination and background. As the instrument requirements for new, cutting-edge science grow more challenging, the environment must be better controlled. Any new support technology must meet these constraints. Specifically, sensors must be shielded, instrument radiation-hardness requirements met, and gaseous contaminants kept from sensitive areas. In-space assembly. Between the current instruments sized to fit inside a launch shroud and the large, formation-flying arrays lies a class of instruments that is better supported by large, deployable structures. Significant advances are being made in this area in the context of the development of the JWST and the Space Interferometry Mission. Nevertheless, additional attention needs to be paid to the technologies for deployment and assembly of large, precision structures in space. Space interferometry. A new generation of observatories (both optical and gravitational wave) is coming that uses the techniques of physical optics to create long-baseline interferometers for vastly improved imaging of the sky and detection of gravitational waves. These missions require long-lived, space-qualified lasers, and also formation flying, often over great distances with the associated large, tidally disruptive forces. Many will need continuous precision thrusting to hold position, precise metrology, drag-free sensors, and in some cases high-speed inter-craft communications. Sensors. Development of next-generation sensors offers enormous advantages to space astrophysics. Recent technological progress suggests order-of-magnitude improvements in sensitivity per sensor element at many wavelengths. New array technology multiplies this progress by offering large numbers of detectors at telescope focal planes, providing huge increases in the figure of merit for astronomical instruments. Materials research. High-performance Sun shields, solar sails, tethers, and low mass/area mirrors are all empowering for space astrophysics. CONCLUSIONS Nuclear power and propulsion are neither uniquely enabling nor unambiguously enhancing for any of the current high-priority goals of astronomy and astrophysics. Most envisaged missions (radio, infrared spectroscopy, optical, ultraviolet, x-ray, gamma ray, and gravitational radiation) work as well at 1 AU as anywhere and have power requirements of <10 kWe, and so the clearly preferred cheap and reliable choices are solar power and chemical propulsion (aided in some cases by solar-electric propulsion). Nuclear power is more costly, heavier (hence more costly to launch), and has dauntingly malign effects on most astronomical detectors. The lifetime of, and the risks posed by, nuclear systems are also significant concerns. The one major exception is infrared imaging, for which there is a cost trade-off to consider between a larger telescope in the high-zodiacal-light background at 1 AU versus a smaller one in the lower background at ≥3 AU. Nuclear power (e.g., RPSs or a small reactor) might be attractive for such a mission, but there are serious issues—e.g., the effect of radiation on sensitive detectors—that would have to be addressed. Some of the more exotic missions considered in this chapter (but not previously evaluated in any recent NRC decadal survey or NASA strategic planning exercise), including lunar observatories, gravitational lensing telescopes, and the use of moons as gravitational wave detectors, might be enabled or enhanced by the use of nuclear technologies. They do not, however, uniquely address high-priority goals of the astronomy and astrophysics community. There are interesting small instruments of opportunity that might be considered in the context of missions to the outer solar system and interstellar space (whether nuclear powered or not). These opportunities include a small telescope to measure infrared background radiation and small gamma-ray burst detectors. Although these opportunities also do not address major, high-priority questions in astronomy and astrophysics, they may be considered as cost-effective add-ons. Finally, the direct measurement of the properties of the local interstellar medium beyond the heliosphere is of astronomical interest, although again it is not a high-priority goal enunciated in either Astronomy and Astrophysics in the New Millennium or Connecting Quarks with the Cosmos.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion REFERENCES 1. M.I. Etchegaray, ed., Preliminary Scientific Rationale for a Voyage to a Thousand Astronomical Units, JPL Publication 87-17, Jet Propulsion Laboratory, Pasadena, Calif., 1987. 2. See, for example, National Research Council, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991, pp. 100–109. 3. See, for example, J.O. Burns and W.W. Mendell, eds., Future Astronomical Observatories on the Moon, NASA Conference Publication 2489, National Aeronautics and Space Administration, Washington, D.C., 1988. 4. D.F. Lester, H.W. Yorke, and J.C. Mather, “Does the Lunar Surface Still Offer Value as a Site for Astronomical Observatories?” Space Policy 20: 99–107, 2004. 5. M.W Henley, J.C. Fikes, J. Howell, and J.C. Mankins, “Space Solar Power Technology Demonstration for Lunar Polar Applications,” 34th COSPAR Scientific Assembly, The Second World Space Congress, 2002, p. R-4-04. 6. M.W. Henley, S.D. Potter, J. Howell, and J.C. Mankins, “Wireless Power Transmission Options for Space Solar Power,” 34th COSPAR Scientific Assembly, The Second World Space Congress, 2002, p. R-4-08. 7. C. Alcock, R.A. Allsman, D. Alves, T.S. Axelrod, D.P. Bennett, K.H. Cook, K.C. Freeman, K. Griest, J. Guern, M.J. Lehner, S.L. Marshall, B.A. Peterson, M.R. Pratt, P.J. Quinn, A.W. Rodgers, C.W. Stubbs, and W.C. Sutherland, “First Observation of Parallax in a Gravitational Microlensing Event,” Astrophysical Journal Letters 454: 125, 1995. 8. A. Gould and B.S. Gaudi, “Femtolens Imaging of a Quasar Central Engine Using a Dwarf Star Telescope,” Astrophysical Journal 486: 687, 1997. 9. C. Maccone and G.L. Matloff, “SETIsail: A Space Mission to 550 AU to Exploit the Gravitational Lens of the Sun for SETI and Astrophysics,” Journal of the British Interplanetary Society 47: 3–4, 1994. 10. S.G. Turyshev and B.-G. Andersson, “The 550-AU Mission: A Critical Discussion,” Monthly Notice of the Royal Astronomical Society 341: 577–582, 2003. 11. President’s Commission on Implementation of United States Space Exploration Policy, A Journey to Inspire, Innovate, and Discover, U.S. Government Printing Office, Washington, D.C., 2004, p. 28.
Representative terms from entire chapter: