Observations of NEOs are governed by the short time available to study them. Because they traverse near-Earth space very quickly and are intrinsically very faint, the window of opportunity for physical studies of most objects is measured in days to about a week at most. However, for some objects the observations can be planned. New technology need not necessarily be developed, but access to existing facilities (optical and infrared telescopes, radar facilities) or development of search and observational telescopes for more or less continuous use is desirable.
Routine or priority access to optical and infrared telescopes and radar facilities requires strong and focused coordination. However, because observing time on large-aperture telescopes generally is scheduled long in advance, there is little flexibility for observing most newly discovered objects during the short period that they are sufficiently bright. Moreover, the appropriate instruments may not be available on short notice. Data that can be expected for most objects will be of uneven consistency and quality except for NASA-related facilities, such as the Infrared Telescope Facility (IRTF) and Goldstone.
A dedicated 2-m-class telescope situated at a good site could provide a vast amount of highly consistent data on the physical characteristics and mineralogical compositions of NEOs. Such a telescope should have a visible-wavelength charge-coupled device (CCD) and a suite of instruments for physical observations. A minimum set of instruments would include the following:
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--> 4 Technological Aspects of Studies of Near-Earth Objects Support and Development Required for Ground-Based Observations Telescope Technology and Observations Needed Observations of NEOs are governed by the short time available to study them. Because they traverse near-Earth space very quickly and are intrinsically very faint, the window of opportunity for physical studies of most objects is measured in days to about a week at most. However, for some objects the observations can be planned. New technology need not necessarily be developed, but access to existing facilities (optical and infrared telescopes, radar facilities) or development of search and observational telescopes for more or less continuous use is desirable. Routine or priority access to optical and infrared telescopes and radar facilities requires strong and focused coordination. However, because observing time on large-aperture telescopes generally is scheduled long in advance, there is little flexibility for observing most newly discovered objects during the short period that they are sufficiently bright. Moreover, the appropriate instruments may not be available on short notice. Data that can be expected for most objects will be of uneven consistency and quality except for NASA-related facilities, such as the Infrared Telescope Facility (IRTF) and Goldstone. A dedicated 2-m-class telescope situated at a good site could provide a vast amount of highly consistent data on the physical characteristics and mineralogical compositions of NEOs. Such a telescope should have a visible-wavelength charge-coupled device (CCD) and a suite of instruments for physical observations. A minimum set of instruments would include the following: Visible-and near-infrared-wavelength CCDs with broad-band filters operating in the range of 0.3 to 2.5 microns that would provide the basic set of observations, including light-curve studies, on most discovered objects. This instrument could be a dual optical-infrared photometer to obtain multicolor photometry of the faintest objects; Spectrographs covering 0.3 to 2.5 microns that would obtain the surface mineralogical composition of some of the brighter objects; and A 10-micron radiometer that would provide thermal infrared observations to establish the size and albedo of the brighter objects.
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--> Laboratory Studies and Technology Needed. A number of laboratory investigations will aid in understanding NEO spectra and processes even in advance of sample-return missions. Such investigations are also important for defining the scientific objectives and sampling strategies for future sample-return missions. Asteroid spectra, for example, depend critically on the nature of the outermost surface layers. Regolith breccias contain materials that once resided on the surfaces of meteorite parent bodies, and some interplanetary dust particles may also be surficial materials. Further study of regolith breccias and their relation to asteroidal soils is valuable, but emphasis should be on understanding the processes involved in soil formation and space weathering and on identifying those that produce optical and compositional effects. A direct comparison of the spectral properties of regolith breccias and soils has been done in the case of the Moon, revealing that breccias have flatter spectra and stronger absorption than do soils. The question of weathering by long-term exposure to space is important for the determination of asteroid-meteorite connections. Systematic searches for space weathering products in chondrite regolith breccias, as well as experimental studies of possible space weathering processes, should be undertaken. Quantitative information on the mineralogical effects of shock blackening is also needed to interpret some NEO spectra. Although the identities and compositions of the minerals composing different meteorite groups are well known, quantitative data on relative mineral proportions commonly are not available. These data are critical for interpreting asteroid spectra. Also, more rigorous methods for deconvolving spectra to obtain information on mineral proportions and composition, as well as physical properties, have to be pursued. The petrogenetic connections between meteorite types must be explored more fully, so that the coexistence of different types on asteroids can be predicted and sought in spectral data. Thermal models for asteroids provide a powerful way to relate meteorites with different metamorphic grades or aqueous alteration histories; petrologic and geochemical studies allow the relationships among igneous meteorites to be understood. These studies are also critical for determining whether complex NEOs have inherited accretional structure or have acquired heterogeneity by internal geologic processing or by chance collisions that resulted in rubble pile objects. Finally, the development of new microanalytical instruments will benefit the chemical and mineralogical characterization of returned samples from NEOs. NASA's Cosmochemistry program, especially those parts devoted to the study of interplanetary dust particles collected in the stratosphere and interstellar grains separated from meteorites, has greatly expanded the ability to handle and analyze very small samples. However, some analytical techniques are not currently applicable to the characterization of very small samples. Continued development of such instrumentation, and acquisition of existing instruments for use in providing access and training, are necessary steps that should precede sample return. Technology Status and Development for Robotic Missions to Near-Earth Objects The missions currently planned, along with ground-based spectroscopy, radar, and meteorite studies, should greatly increase our understanding of NEOs by the year 2000. It is virtually certain that important questions will be suggested by the new data. However, only a small subset of these bodies will be explored, and the diversity of NEOs will require, for further progress, either a spacecraft cruising among them or multiple missions targeted on individual objects. Learning how to conduct more, and more effective, missions for less money seems particularly urgent in this particular area of planetary exploration. The three main types of small-body missions (flyby, rendezvous, and sample return) are discussed briefly above. Because no one mission of any type can characterize the variety of NEOs, the goal of technology development must be to reduce costs and increase capabilities. Experience so far is very limited, but the paths available for progress seem to be many. One possible path is the use of one of the various types of nonchemical propulsion systems that can allow multiple encounters with a large number of objects, thereby lowering unit costs. In an earlier report, the Space Studies Board concluded that the value of electric propulsion systems for missions to comets and asteroids would be immense.1 Among the many nonchemical propulsion methods discussed to date, the most fully developed and most
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--> likely to be used soon for multiple targets is solar electric propulsion (SEP),2 that is, ion engines powered by solar arrays. These have been designed, built, and tested in several forms since the 1960s. NASA's Space Electric Rocket Test (SERT) program, for example, launched an ion engine on a sounding rocket (SERT 1) in July 1964 and on an orbital flight (SERT 2) in February 1970. To date, however, SEP has not yet been employed to accomplish an actual deep-space mission. Ions can be accelerated across an electric potential, which in most concepts is provided by solar power, to a much higher velocity than chemical fuel systems can reach. Thus, massive quantities of fuel are not necessary. The disadvantage—that these systems provide low thrust (typically a few millinewtons or less) for any reasonable power level—is balanced by the fact that such engines can be run continuously, not merely for a few minutes but for periods comparable to the flight duration. Later legs of such a mission can even be retargeted, within broad limits, based on new information derived from the mission itself or otherwise. Deep Space 1 will test the utility of SEP as a propulsion system for small spacecraft. Another path is the use of multiple penetrators or small landers on one spacecraft, which could provide knowledge of surface and subsurface properties. Additionally, miniaturization, which could allow reduction of the required mass of spacecraft and science payload, may permit the launch of multiple spacecraft at one time. A more detailed discussion of attractive lines of progress follows for each of the three main mission types. Flyby Missions The simplest mission type is the flyby, without provision for matching velocities to achieve rendezvous. A variety of scientific goals can be achieved, as already illustrated by Galileo's flyby imaging of Gaspra and Ida and NEAR's flyby observations of Mathilde. In addition to imaging and broad spectral characterization, other instruments such as magnetometers and spectrometers could yield important information about the magnetic properties, composition (particularly the presence and proportion of metallic iron), and thermal evolution of NEOs. Flyby missions to individual objects probably will not be as attractive as other mission types for scientific exploration of NEOs, except for special high-priority objects or unless they can be done very inexpensively. With the anticipated massive increase in the number of NEOs known, multiple flyby missions3 may be attractive as a cost-effective means for increasing our understanding of the variety of taxonomic groups represented among NEOs and perhaps making new, secure asteroid-meteorite connections. Rendezvous Missions Rendezvous requires matching the velocity as well as the position of the spacecraft with the NEO and hence is a more difficult task with respect to both energy and precision performance. Attractive mission opportunities, such as those for multiple flybys, are relatively rare but will increase with the number of known NEOs. When the number of known interesting objects reaches the thousands (and probably before), this should not be a problem. The NEAR mission should demonstrate the special opportunities created by the weak gravitational field of small objects. When the local gravitational acceleration (g) is on the order of a few cm/sec2, attitude jets are sufficient to permit maneuvering close to the asteroid. Automated “landing” at multiple targeted sites should be achievable, and the distinction between an orbiter and a rover becomes blurred. Among other advantages, this will widen the range of instruments available to satisfy a given objective such as chemical analysis. Sample-Return Missions Creating a system capable of robotically collecting and returning material from a small body would be a major advance in capability for asteroid and comet research. The only robotic space missions to date that have accomplished such a task were three Soviet missions (Lunas 16, 20, and 24), which returned core samples of lunar regolith. Stratigraphic detail was not preserved as well as in core samples collected by the Apollo astronauts. A recent study of a mission involving two spacecraft on a single launch vehicle gives an example of the opportunities and problems associated with the use of solar sails for sample-return missions. The only use of chemical rockets in this mission, after launch, is for a small lander/penetrator to take the sample and transfer it
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--> robotically to the spacecraft. The two asteroids chosen for the study were both large main-belt asteroids, with escape velocities of hundreds of meters per second, so that conventional rocketry is still needed for landing and takeoff from the asteroid. Similar missions to smaller objects, particularly NEOs, would be technically much easier. Solar electric propulsion, proposed years ago for the Halley-Temple 2 mission, would be an alternative option. Although the use of a single spacecraft might be preferable for multiple flyby or rendezvous missions, use of multiple spacecraft is preferred for sample-return missions because it would increase the likelihood of successful sample acquisition. Human Exploration of Near-Earth Objects It would be difficult to justify the human exploration of NEOs on the basis of any cost-benefit analysis of the strictly scientific results obtained.4 As stated in the National Academy of Sciences' 1988 space policy report, “the ultimate decision to undertake further voyages of human exploration and to begin the process of expanding human activities into the solar system must be based on non-technical factors.”5 If, for other reasons, the United States should choose to utilize human exploration beyond low Earth orbit, a strong case can be made for starting with NEOs. Missions to these bodies could serve effectively as stepping stones or “waypoints,” in the language of the Synthesis Group's report.6 A few percent of NEOs are the most accessible bodies beyond Earth for both robotic and human exploration. In the case of human expeditions, a primary concern would be to keep the total mission times as short as possible in order to minimize the hazards and attendant risks to which astronauts are exposed. These include, but are not limited to, weightlessness, a high-radiation environment, meteoroid impact, and equipment failure. NEO missions can reduce the requirements for life support and total mission costs. Short-duration, low delta-V missions to NEOs require either six-month or one-year round trips, since return to Earth occurs at either the ascending or the descending node of the transfer trajectory. Still shorter “sprint” missions (i.e., of a few months' duration) are conceivable with a large commitment of additional propulsion. Among the currently known NEOs, at least two and perhaps more can be reached in six-month or one-year round-trip missions with realistically achievable delta-V. An example of a six-month mission profile to asteroid 1991 JW is shown in Figure 4.1.7 With the anticipated accelerated discovery of NEOs, the number of potential targets for human expeditions can be expected to increase by an order of magnitude in the next 10 to 15 years. Hence, there most likely will be opportunities for launch to one or more accessible asteroids in any given year, and there could conceivably be a diversity of physical types from which to choose. The range of geologic problems to be solved by astronauts on NEOs would depend on the type of body. In nearly all cases, the distribution, depth, and physical and petrologic characteristics of the regolith, the original locations of fragments in the regolith, and the structure of the underlying body of the asteroid represent basic problems to be solved. Studies of bedrock or regolith will provide clues to its origin and evolution.8 The advantage that human explorers bring to the study of NEOs is the ability to conduct on-site observations at a great range of scales, from the hand specimen to the entire body, coupled with the ability to manipulate materials and take diverse samples in the context of complex field relationships. Although some of these tasks can be done robotically with difficulty, the human observer has a great advantage in dexterity and in integrating diverse observations.9 Humans can also adapt to new situations quickly and effectively, enabling real-time decisions to be made. The field aspects of scientific investigation of the surface of an asteroid are, in many ways, analogous to field geology. The surface of a 1-km asteroid is a geologic field area well suited for study during the stay times of two weeks to a month that are possible on a six-month human expedition. Expeditions to NEOs represent the easiest and least expensive next step in human exploration of space beyond Earth. Scientific exploration of these bodies could provide the experience and technology needed for fruitful human exploration of Mars and even deeper space.
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--> Figure 4.1 Six-month round-trip mission profile to NEO 1991 JW. This mission requires a lower delta-V than does a one-way trip to the lunar surface. (SOURCE: T.D. Jones et al., “Human exploration of near-Earth asteroids,” Hazards Due to Comets and Asteroids, University of Arizona Press, Tucson, Ariz., 1994, p. 683.) References 1. Space Science Board, National Research Council, Strategy for Exploration of the Outer Planets: 1986–1996, National Academy Press, Washington, D.C., 1986, p. 82. 2. M.D. Rayman and D.H. Lehman, “NASA's first New Millennium deep-space technology validation flight,” 2nd International Conference on Low-Cost Planetary Missions, IAA-L-0502, International Academy of Astronautics, Paris, France, 1996. 3. J. Veverka, Y. Langevin, R. Farquhar, and M. Fulchigoni, “Spacecraft exploration of asteroids: The 1988 perspective,” pp. 970–996 in Asteroids II, R.P. Binzel, T. Gehrels, and M.S. Matthews, eds., University of Arizona Press, Tucson, Ariz., 1989. 4. Space Studies Board, National Research Council, Scientific Prerequisites for the Human Exploration of Space, National Academy Press, Washington, D.C., 1993, p. 2. 5. Committee on Space Policy, National Academy of Sciences, Toward a New Era in Space: Realigning Policies to New Realities, National Academy Press, Washington, D.C., 1988, p. 14. 6. Synthesis Group, America at the Threshold: Report of the Synthesis Group on America's Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991, p. A-37. 7. T.D. Jones, D.B. Eppler, D.R. Davis, A.L. Friedlander, J. McAdams, and S. Krikalev, “Human exploration of near-Earth asteroids,” pp. 683–708 in Hazards Due to Comets and Asteroids, T. Gehrels, ed., University of Arizona Press, Tucson, Ariz., 1994. 8. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 96. 9. Space Studies Board, National Research Council, Scientific Prerequisites for the Human Exploration of Space, National Academy Press, Washington, D.C., 1993, p. 8.