Exploration of the outermost regions of the solar system is a demanding task, especially in the current environment of tight financial limitations. Advances in technology are needed to improve telescopic observations and to enable spacecraft exploration of the trans-neptunian region.
The technical challenge posed by surveys to detect KBOs is to efficiently search the sky for faint objects with parallactic motion corresponding to the trans-neptunian region. Currently, less than 0.1% of the ecliptic has been surveyed to <24th magnitude for KBOs. Faint objects require an efficient detector on a modest (2-meter) telescope, as well as dark skies. To illustrate the scale of the task, examining the entire area within ±10° of the ecliptic (7,200 square degrees) with a field of view typical of current searches (~0.02 square degrees) at approximately 12 sets of images per night1 would take ~82 years of observing every night (ignoring such matters as the full moon and bad weather). The efficiency of searches could be improved by using detectors with larger arrays (currently 2,048 × 2,048 pixels) and shortening their readout time to allow a greater area of sky to be covered per night. Additional improvements in array technology are, however, unlikely to help future ground-based studies because current detectors are already background limited.
Telescopic observations of the trans-neptunian region make demands on infrared astronomy. When integrated with large-aperture telescopes, existing near-infrared (~1-micron) instruments can barely detect the brightest objects, allowing the determination of the chemical composition of their surfaces (requiring resolving powers of a few hundred) and atmospheres (requiring resolving powers of several thousand). Access to these instruments and the equivalent of their next generation(s) on large-aperture telescopes (existing and planned), allows the sampling of smaller or more distant objects, providing a more statistically significant sampling of the KBO population (Figure 4.1).
Access to 8-meter-class facilities (e.g., the Gemini telescope now under construction on Mauna Kea) would enable some limited studies of the compositional variations of the brightest KBOs. Indeed, the 10-meter Keck telescope has already been used to obtain near-infrared (1.42- to 2.40-micron) spectra of 1993SC.2 Even using the largest telescope in the world to observe this KBO, one of the brightest known, proved extremely challenging, and the resulting spectrum was not of high quality. Moreover, observing time on the Keck telescope and other such facilities is heavily oversubscribed, and it is far from clear that a long-term program focused on KBOs would
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--> 4 Technology Issues Exploration of the outermost regions of the solar system is a demanding task, especially in the current environment of tight financial limitations. Advances in technology are needed to improve telescopic observations and to enable spacecraft exploration of the trans-neptunian region. Telescopic Observations The technical challenge posed by surveys to detect KBOs is to efficiently search the sky for faint objects with parallactic motion corresponding to the trans-neptunian region. Currently, less than 0.1% of the ecliptic has been surveyed to <24th magnitude for KBOs. Faint objects require an efficient detector on a modest (2-meter) telescope, as well as dark skies. To illustrate the scale of the task, examining the entire area within ±10° of the ecliptic (7,200 square degrees) with a field of view typical of current searches (~0.02 square degrees) at approximately 12 sets of images per night1 would take ~82 years of observing every night (ignoring such matters as the full moon and bad weather). The efficiency of searches could be improved by using detectors with larger arrays (currently 2,048 × 2,048 pixels) and shortening their readout time to allow a greater area of sky to be covered per night. Additional improvements in array technology are, however, unlikely to help future ground-based studies because current detectors are already background limited. Telescopic observations of the trans-neptunian region make demands on infrared astronomy. When integrated with large-aperture telescopes, existing near-infrared (~1-micron) instruments can barely detect the brightest objects, allowing the determination of the chemical composition of their surfaces (requiring resolving powers of a few hundred) and atmospheres (requiring resolving powers of several thousand). Access to these instruments and the equivalent of their next generation(s) on large-aperture telescopes (existing and planned), allows the sampling of smaller or more distant objects, providing a more statistically significant sampling of the KBO population (Figure 4.1). Access to 8-meter-class facilities (e.g., the Gemini telescope now under construction on Mauna Kea) would enable some limited studies of the compositional variations of the brightest KBOs. Indeed, the 10-meter Keck telescope has already been used to obtain near-infrared (1.42- to 2.40-micron) spectra of 1993SC.2 Even using the largest telescope in the world to observe this KBO, one of the brightest known, proved extremely challenging, and the resulting spectrum was not of high quality. Moreover, observing time on the Keck telescope and other such facilities is heavily oversubscribed, and it is far from clear that a long-term program focused on KBOs would
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--> Figure 4.1 The estimated sensitivities of various ground-and space-based facilities likely to play roles in studies of trans-neptunian objects in the next 10 to 15 years. The current capabilities of the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) are plotted relative to those of the 8-meter Gemini telescope currently under construction in Hawaii, and the Space Infrared Telescope Facility scheduled for launch in 2001. Also shown are two different concepts for NASA's proposed Next Generation Space Telescope (NGST). The 6-meter NGST is in a heliocentric orbit at 3 AU, i.e., in the outer portion of the asteroid belt where the zodiacal emission is 30 to 100 times lower than it is at 1 AU. The 8-meter NGST is in a heliocentric orbit at 1 AU. Gemini (outfitted with low-order adaptive optics), NICMOS, SIRTF, and NGST will readily detect the reflected sunlight from Pluto (off scale) and a typical KBO out to wavelengths of ~5 microns. Pluto's thermal emission should be detectable by either of the two NGST concepts at wavelengths greater than ~15 microns. The thermal emission from the KBO, however, appears to be beyond the capability of any of the facilities in the wavelength range illustrated. SIRTF will, however, be able to detect such a KBO at wavelengths greater than 35 microns (not illustrated). The capabilities indicated assume a 10,000-sec integration, a 10-σ signal-to-noise ratio, and a wide bandpass of λ/Dl = 3. The Kuiper Belt object is taken to have a radius of 100 km and is located at 35 AU. Its temperature is 35 Kelvin; it has a visual magnitude of 22.0 and a V-K color index of 2.0 (i.e., the red color characteristic of some, but not all, trans-neptunian objects). Pluto is assumed to have a diameter of 1,200 km and a temperature of 40 Kelvin, and to be located at its current distance from the Sun. Adapted from The Next Generation Space Telescope: Visiting a Time When Galaxies Were Young, H.S. Stockman, ed., Space Telescope Science Institue, Baltimore, Maryland, 1997, with information on Pluto and the KBO courtesy of D.P. Cruikshank.
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--> receive support. The cost of such support is, however, small compared with the cost of a spacecraft, and funding agencies will have to make the necessary decisions regarding trade-offs. The Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), recently installed on HST, was designed to obtain near-infrared images and spectra. NICMOS could provide compositional information for some of the brighter KBOs. However, due to technical difficulties, spectra cannot currently be obtained, and so compositional inferences based on NICMOS observations are limited to broadband photometric measurements (see Figure 4.1). If these technological difficulties can be overcome, then the spectra of a limited number of KBOs could be obtained. The low temperatures (<40 Kelvin) of objects in the trans-neptunian region mean that their thermal emissions are in the far-infrared (~10- to 100-micron) region of the spectrum. The detectors on the Space Infrared Telescope Facility (SIRTF) are very close to providing background-limited performance and will be capable of radiometrically detecting KBOs (with diameters of >100 km) at wavelengths greater than approximately 35 microns.3 However, if telescopes larger than SIRTF were available, then the diffraction-limited performance would improve as a result of the lower background. In this case, improved detector performance would be important, although in some cases the current detectors are near the theoretical limit of performance.4 Much work is needed to construct larger detector arrays, as well as the cooling systems that would be required for long-term operation in space. Advancing our knowledge of the physical and chemical properties of the KBOs using both Earth-and space-based telescopes to determine accurate photometry and radiometry is directly related to having large, efficient detector arrays, especially in the far infrared. Procuring appropriate arrays may present some problems. Although those operating in the 1- to 40-micron bands are available commercially, ones operating at longer wavelengths are only being made by a few university research groups for use in, for example, SIRTF.5 For observations of comet-size (<10-km) objects in the Kuiper Belt we look to the next generation of space telescopes. The report of the HST & Beyond (Dressler) Committee recommended the construction of a 4-meter space-based observatory optimized for imaging and spectroscopy in the 1- to 5-micron region.6 In response to this recommendation NASA initiated the planning for the Next Generation Space Telescope (NGST).7 This 6- to 8-meter facility would be suitable for composition studies of KBOs if it had the capability to observe moving targets (see Figure 4.1). This capability could, potentially, place severe demands on the systems that control the spacecraft's attitude and point the optical system. The ability to track will probably need to be incorporated from the earliest phases of the spacecraft design, possibly by provision of an internal steering mirror capable of tracking objects within a limited angular range without moving the entire telescope. Spacecraft Missions The demands of exploring the outer solar system under tight fiscal constraints have led to a major change in approach to mission design, development, and operation.8, 9 The high costs of sending large and heavy, spacecraft with a dozen separate scientific instruments to the giant planets (e.g., Voyager, Galileo, Cassini) have resulted in the interval between missions increasing to nearly 20 years. The past few years have seen the emphasis change to small, integrated spacecraft with highly focused science objectives that use new technologies to improve efficiency. Reducing the cost of missions by at least a factor of 10 brings the possibility of more frequent missions to the outer solar system. The move away from large missions has had an additional, indirect benefit to studies of the trans-neptunian region. Many of the “smaller, cheaper, faster” missions initiated in the last few years are targeted at asteroids and comets. Given the close connection between these objects and those found in the distant outer solar system, our overall knowledge about primitive bodies is likely to increase greatly over the next few years. Thus, New Millennium missions such as Deep Space 1 and Deep Space 4, together with Discovery missions like Near-Earth Asteroid Rendezvous and Stardust, not only will provide valuable experience on how to design and conduct low-cost science missions but also are likely to greatly expand the overall context within which all studies of primitive bodies are conducted. While considerable progress has been made in developing new-style missions to the outer solar system, particularly Pluto flyby missions (Box 4.1),10–12 the technological obstacles of returning substantial scientific data from >30 AU remain formidable. The issues that drive technological development to enable lower-cost, higher-output missions to the outer solar system include the following:13
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--> High launch energy—To climb out of the Sun's gravitational well and reach >30 AU, a spacecraft must be launched by a powerful launch vehicle (or take lengthy detours via other planets). Alternatively, the size and cost of the launch vehicle can be reduced by lowering the mass of the spacecraft. The spacecraft mass can be reduced by designing the spacecraft functions around a limited number of integrated scientific instruments and by using lightweight components (e.g., integrated microelectronics; lightweight telecommunications; precise, low-impulse attitude thrusters; and advanced, stellar-navigation cameras). To achieve orbit around Triton or to fly past multiple objects, the spacecraft will require an efficient propulsion system. Long mission duration—Spacecraft missions to the trans-neptunian region last on the order of 10 years, a duration that places serious demands on the reliability and longevity of components as well as requiring long-term mission operations. Technologies required to enable long-duration missions include spacecraft autonomy and active fault management. The difficulty in longer missions is to focus the scientific objectives and limit the complexity of operations to lower the cost of operation without reducing the scientific return. Low sunlight—The low levels of sunlight in the outer solar system provide insufficient solar energy to power the spacecraft, and longer exposures are required for taking images of scientific targets (which puts demands on attitude control to prevent smearing). Advances in radioisotope power sources as well as in the development of low-power electronics and instrumentation will be important for outer solar system missions. Long telecommunications links—Sending information efficiently across >30 AU requires major advances in telecommunications (e.g., carbon-composite antennas, efficient amplifiers, optical telecommunications, and on-board data processing and compression). Radiation-hardened electronics—A consequence of the long duration of missions to the outer solar system is that spacecraft might accumulate significant doses of radiation. The development of reliable, radiation-hardened, integrated solid-state electronics capable of surviving the extreme conditions of the trans-neptunian region may thus be important. Microinstrumentation—The budgetary pressure leading to NASA's increasing emphasis on small spacecraft developed on rapid time scales has created a parallel pressure to develop a new generation of highly capable microinstruments to fly on these missions. Many first-generation small missions (e.g., Mars Global Surveyor) are equipped with copies of instruments designed to fly on traditional, “large” missions. This trend cannot continue, and COMPLEX has previously recommended that NASA devote more attention to the development of microinstruments for planetary missions.14 The programmatic constraints likely to be imposed on future missions to the outer planets will have important consequences for the types of instruments that can be flown. Limited spacecraft resources (e.g., power, mass, data rate, and so on) will favor the development of integrated instrumentation. In other words, the functions of several instruments will be ingeniously combined in one package proposed by a single team of investigators. An example of this trend is the Mars Volatiles and Climate Surveyor integrated payload selected for the Mars Surveyor 1998 lander or the Plasma Experiment for Planetary Exploration to be carried by Deep Space 1. Similarly, the combination of rapid advances in instrumentation technology and short mission development schedules is likely to promote the flight of instrumentation incorporating more technically advanced components than has been the case for traditional missions with decades-long development schedules. A full discussion of the likely ramifications of these and other trends with respect to issues such as strategic planning by NASA and groups such as COMPLEX is beyond the scope of this study. Exploration of the outer solar system will probably benefit from the testing of new technologies in spacecraft components and scientific instruments on missions to the inner solar system under the New Millennium and Discovery programs. For example, Deep Space 1, the first flight in NASA's New Millennium technology demonstration program, will test the Miniature Integrated Camera Spectrometer (an integrated camera, ultraviolet imaging spectrometer, and infrared imaging spectrometer), which is a candidate for use on a Pluto mission. Similarly, NASA's recently published Roadmap for the exploration of the solar system highlights specific technologies that are required to enable missions to the trans-neptunian region.15 The Roadmap's discussions of the Pluto/Kuiper Express and Neptune Orbiter with Triton Flybys “portrait” missions, for example, note the importance of solar-electric propulsion, autonomous operations, and lightweight spacecraft systems.
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--> Box 4.1 History of NASA's Pluto Mission Concepts The Space Science Board's 1986 report A Strategy for the Exploration of the Outer Planets: 1986–1996 (National Academy Press, Washington, D.C., 1986) identified the Pluto-Charon system as a likely and logical long-term candidate for a flyby reconnaissance mission. Since then, NASA plans for a Pluto mission have been repeatedly redesigned. Each iteration has been geared toward producing a mission architecture that could, in general, be accomplished either more rapidly or at lower cost than could its predecessors. NASA's 1991 Solar System Exploration Division Strategic Plan called for a Pluto Flyby/Neptune Orbiter program, in which a pair of Mariner Mark II (Cassini-class) spacecraft would be launched in the period 2001 to 2003 by Titan IV/Centaurs on a 15- and 20-year cruise to Pluto and Neptune, respectively. Each 5,000- to 6,000-kg spacecraft would carry a payload of 14 instruments and cost in excess of $2 billion. By 1992, however, it was becoming clear that space science funds through the turn of the century would not support such an ambitious architecture. The project was officially descoped to a single Mariner Mark II Pluto Flyby or Neptune Orbiter mission and then quietly dropped. At the same time, the idea for a Pluto Very Small or Pluto Very Small or Pluto Fast Flyby program was born out of technological feasibility studies at NASA's Jet Propulsion Laboratory. In this scenario, two spacecraft, each carrying four instruments, would be launched toward Pluto in the period 2001 to 2003. Even though each spacecraft would have a mass of approximately 150 kg, launch on a Titan IV/Centaur would be required to reduce the flight time to 7 to 8 years. A $400 million price tag included mission development and operations up until 30 days after launch, but excluded both the cost of the radioisotope thermoelectric generators necessary to power the spacecraft in the outer solar system and the cost of the two Titan IV/Centaurs. Actual mission costs would be around $1.2 billion plus expenditures for lifetime operations. At about the same time the Pluto Fast Flyby concept was developed, a number of alternative missions were investigated. Prime among these was the so-called Pluto Flyby 350 concept developed for NASA's Discovery Program Science Working Group. It envisaged a somewhat larger, fully redundant spacecraft (~300 kg), carrying a greater range of instruments than the Pluto Fast Flyby concept. To limit total mission costs, Pluto Flyby 350 eliminated launch by a Titan IV and, instead, relied on Earth and Jupiter flybys to inject it on course to Pluto. If launched on an Atlas or Delta in 2001, Pluto Flyby 350 would have reached Pluto more than 11 years later. The missions savings on its launch vehicle costs were, however, negated by the need for a more elaborate spacecraft with a significantly longer operational lifetime. At about the same time, consideration was also given to a Pluto orbiter mission. Studies indicated that a 35-kg spacecraft could be placed into orbit about Pluto by following a trajectory similar to that of Pluto Flyby 350. This concept was highly unattractive because the total flight time was more than 16 years and, References. 1. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey,” Astronomical Journal 112:1225, 1996. 2. R.H. Brown et al., “Surface Composition of Kuiper Belt Object 1993SC,” Science 276: 937, 1997. 3. D.P. Cruikshank and M.W. Werner, “The Study of Planetary Systems with the Space Infrared Telescope Facility (SIRTF),” Planets Beyond the Solar System and the Next Generation of Space Missions, D.R. Soderblom, ed., ASP Conference Series 119:223, 1997. 4. For a review of the current status of infrared detector technology, see, for example, G.H. Rieke, Detection of Light: From the Ultraviolet to the Submillimeter, Cambridge University Press, Cambridge, U.K., 1994. 5. G.H. Rieke, Detection of Light: From the Ultraviolet to the Submillimeter, Cambridge University Press, Cambridge, U.K., 1994, p. 170. 6. A. Dressler, ed., Exploration and the Search for Origins: A Vision for Ultraviolet-Optical-Infrared Space Astronomy, report of the HST & Beyond Committee, Association of Universities for Research in Astronomy, Washington, D.C., 1996. 7. H.S. Stockman, ed., Next Generation Space Telescope: Visiting a Time When Galaxies Were Young, STScI M-9701, Space Telescope Science Institute, Baltimore, Maryland, 1997. 8. Science Applications International Corporation, Low-Cost Outer Planet Mission Definitions: Report to NASA Headquarters, Washington, D.C., 1995.
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--> more importantly, the orbiter could carry no useful scientific payload. Increasing the orbiter's mass to 100 kg to allow for a useful payload would have increased the flight duration to more than 20 years. Faced with these insuperable difficulties, work on an orbiter was dropped and all efforts were focused on the Pluto Fast Flyby concept. Two years later, in 1994, the out-year funding profile for the space sciences had deteriorated even further. Even with plans for Russian participation (providing launch vehicles) in the Pluto Fast Flyby, the costs to the United States still hovered around $600 million. New start plans for a Pluto mission were put on hold as scientists and engineers worked on a way to reduce costs even further. In parallel with efforts to develop a realistic Pluto mission, another concept, the Kuiper Express “sciencecraft,” was devised to investigate the feasibility of a mission to the Kuiper Belt. By removing the usual compartmentalization between spacecraft and instrument design to such a degree that the dividing line between spacecraft systems and scientific instruments becomes blurred, the sciencecraft reaps significant mass and, therefore, cost savings. Since a mission to a Kuiper Belt object would have many similarities to a Pluto mission, the sciencecraft concept was borrowed to form the basis of the Pluto Express mission detailed in Pluto Express: Report of the Science Definition Team (NASA, Washington, D.C., September 1995). At first blush, the Pluto Express mission looks very similar to that proposed for the Pluto Fast Flyby. The Pluto (or Pluto-Kuiper) Express mission design does, however, include the option of an extended mission into the Kuiper Belt after the Pluto-Charon encounter, provided that no mission requirements are driven by this option. The Pluto-Kuiper Express concept envisages the launch in 2002, 2003, or 2004 of two spacecraft with four instruments apiece on Delta II launch vehicles. Use of the smaller Delta launch vehicle affects both the cost and the duration of the cruise to Pluto. For the ~$77 million cost of launch on a Delta II with an upper stage—as opposed to the Titan IV's cost of around $350 million—cruise time is lengthened from 7 to between 10 and 13 years depending on the exact launch date. An option to utilize solar-electric propulsion in place of a chemical upper stage could yield flight times under 10 years. In addition, the innovative engineering of the sciencecraft concept reduces development expenditures to $145 million to $200 million, depending on whether the mission flies one or two spacecraft. The mass of each spacecraft currently stands at ~100 kg. While NASA includes discussion of the Pluto-Kuiper Express in its recent report Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap (NASA, Washington, D.C., 1996), where it is given a nominal launch date of 2001 to 2003, the only representation the mission currently has in NASA's budget is implicit; an “Outer Planets/Solar Probe” line item, slated for a new start in FY 2000, is included as an element of the Origins Initiative approved as a part of NASA's FY 1998 budget. 9. Science Applications International Corporation, Measure-Jupiter Mission Design Book: Report to NASA's Outer Planet Science Working Group, Washington, D.C., 1994. 10. R.L. Staehle et al., “Exploration of Pluto: Search for Applicable Satellite Technology,” presentation at 6th Annual American Institute of Aeronautics and Astronautics/Utah State University Conference on Small Satellites, Logan, Utah, 1992. 11. R.L. Staehle et al., “Exploration of Pluto,” IAF-92-0558, presentation at 43rd Congress of the International Astronautical Federation, Washington, D.C. , 1992. 12. H.W. Price et al., “Pluto Express Sciencecraft System Design,” IAA-L-0603, presented at the Second International Academy of Astronautics International Conference on Low-Cost Planetary Missions, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, April, 1996. 13. Pluto Express Science Definition Team, Pluto Express: Report of the Science Definition Team, NASA, Washington, D.C., 1995. 14. Space Studies Board, National Research Council, Review of NASA's Planned Mars Program, National Academy Press, Washington, D.C., 1996, p. 25. 15. Solar System Roadmap Development Team, Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap (Version B), Jet Propulsion Laboratory, Pasadena, California, September 20, 1996.