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--> 5 Conclusions and Recommendations COMPLEX has identified alternatives for the continued discovery and characterization of NEOs. These are summarized below, and some advantages and disadvantages of each are discussed. Under each heading, the baseline program describes the current or necessary level of activity, and augmentations describe more ambitious levels of activity that could be combined with the baseline program. Availability of funding and programmatic priorities will determine whether augmentations can be implemented. These recommendations are consistent with priorities articulated previously by the Space Studies Board and its committees for the scientific study of asteroids and comets.1,2 Ground-Based Telescopic Observations and Instrumentation Baseline Recommendation: Support research programs for interpreting the spectra of near-Earth objects (NEOs), continue and coordinate currently supported surveys to discover and determine the orbits of NEOs, and develop policies for the public disclosure of results relating to potential hazards. Interpreting the spectra of NEOs already cataloged in NASA's Planetary Data System should continue. The NEAT, LONEOS, and Spacewatch systems, when used in a coordinated program of detection and orbit determination, could discover and characterize perhaps 1000 new Earth-crossing objects larger than 1 km in diameter, as well as thousands of smaller NEOs over a 10- to 15-year period. The baseline program, however, will not provide information on physical properties, compositions, and origins of newly discovered NEOs. NASA should undertake a study of how to communicate these findings to the public, especially the discoveries of potentially threatening NEOs, and appropriate protocols should be established. Augmentation 1: Provide routine or priority access to existing ground-based optical and infrared telescopes and radar facilities for characterization of NEOs during favorable encounters. Frequent access to existing facilities would allow most objects discovered in any given month to be observed. Alternatively, access could be granted contingent upon discovery of an object using a system perhaps analogous to the Hubble Space Telescope's (HST's) target of opportunity program This augmentation would allow a subset of NEOs to be classified, and their sizes, shapes, rotation, and surface compositions would be characterized. The current level of effort in spectral classification of NEOs is perhaps 20 objects per year, although access to appropriate instruments may decrease. Prioritized access to telescopes could increase this level by an order of
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--> magnitude. The present rate of NEO characterizations by radar, approximately 1 per year, might be increased by a factor of 10. Infrared measurements of size and albedo require large-aperture instruments and could double or triple from the present rate of fewer than 10 per year. Priority access would affect ongoing observation programs (depending on the number of participating facilities). The quality and consistency of data might be uneven, depending on the instrumentation available, and the cooperation and flexibility of observers would be required. Augmentation 2: Provide expanded, dedicated telescope access for characterization of NEOs. With the construction and planned commitment of national operational resources to large (4- to 10-m) telescopes, consideration is being given to decommissioning several national facility telescopes in the 2-m class.3 If funds for NEO research were provided for maintenance of an existing telescope of at least 2-m aperture (including the cost of necessary support personnel) in order to ensure more or less full-time access, use of such a facility would allow physical and mineralogical characterization of as many as half of the NEOs discovered each year. A minimum set of instruments would include CCDs with broad-band filters, a 10-micron photometer, and a near-infrared array spectrometer. Funding would be required for instrument construction. Laboratory Studies and Instrumentation Baseline Recommendation: Support continued research on extraterrestrial materials to understand the controls on spectra of NEOs and the physical processes that alter asteroid and comet surface materials. Meteorite regolith breccias and some interplanetary dust particles potentially provide samples of asteroidal and cometary surface materials. Mineralogical, chemical, textural, and spectral studies of such materials can constrain the interpretation of NEO spectra and help quantify the proportions of phases that dominate their spectra. The discovery and characterization of altered surficial layers in regolith breccias would greatly assist in solving the question of space weathering. Laboratory studies would also be useful in addressing this problem. The occurrence of different meteorite types on the same asteroids can be understood from petrologic studies, as well as theoretical models of the accretional, thermal, and collisional histories of asteroids and comets. An advantage of continuing such work is that it tends to be an inexpensive aspect of NEO research. However, it is often difficult or impossible to make connections between specific NEOs and meteorites or interplanetary dust particles. Augmentation 1: Support the acquisition and development of new analytical instruments needed for further studies of extraterrestrial materials and for characterization of returned NEO samples. Materials research provides important information used in defining NEO exploration targets and the instrumentation required for spacecraft missions.4 State-of-the-art instruments in laboratories on Earth are necessary to develop and maintain scientific expertise in readiness for asteroid sample-return missions. Even adapting existing instrumental technologies for the analysis of extraterrestrial materials requires a great deal of time and effort, and so laboratory facilities and protocols must be in place long before they are needed to analyze returned NEO samples. The development of new and improved microanalytical techniques and instruments will also be necessary for characterization of organic compounds, isotopes, and other constituents of tiny meteoritic and interplanetary dust samples, as well as the modest-sized samples to be collected on NEO sample-return missions. An advantage of supporting instrument development is that laboratory equipment is versatile and can be used in many research programs (e.g., the search for evidence of life in martian meteorites and returned martian samples); additionally, facility instruments can be shared among many investigators. Spacecraft Technology, Instrumentation, and Missions. Baseline Recommendation: Support NEO flyby and rendezvous missions. Spacecraft such as NEAR and Deep Space 1 are already capable of NEO flyby and rendezvous missions. The high spatial resolution afforded by such missions provides important information on NEO physical characteristics, composition, formation, and geologic history that is otherwise unobtainable. Some flight instruments for remote sensing are already in a reasonably advanced state of development but require miniaturization. An advantage of
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--> supporting further reconnaissance missions is that they can draw on the technological heritage of previous spacecraft and thus have lower costs. Augmentation 1: Develop technological advances in spacecraft capabilities, including nonchemical propulsion and autonomous navigation systems, low-power and low-mass analytical instrumentation for remote and in situ studies, and multiple penetrators and other sampling and sample-handling systems to allow low-cost rendezvous and sample-return missions. The definition and development of new analytical instruments to be flown on flyby, orbital, and landed spacecraft must be a high priority. In situ imaging and measurements of physical and chemical properties are the justification for NEO spacecraft missions that do not return samples, and the severe mass and power limitations imposed by smaller spacecraft will demand a new generation of instruments. With increased emphasis on low-cost, rapid-pace, and highly competitive missions, new instrument development is a difficult challenge, and programs such as NASA's Planetary Instrument Definition and Development Program (PIDDP) have to be supported. However, in the case of some NEOs, it may be easier to collect and return samples than to do adequate in situ analyses. Flight instrument development focused on sampling devices and on autonomous navigation and control systems would enable NEO sample-return missions. Spacecraft and instrument miniaturization and multiple penetrators or landers are among other potential mission-enhancing developments. Nonchemical propulsion concepts hold particular promise for NEO missions. Sampling missions would be most effective if focused on collecting samples from well-characterized geologic units or the subsurface of NEOs and from an object exhibiting cometary behavior. Sample return involves many complex manipulations that pose engineering challenges. Augmentation 2: Study technical requirements for human expeditions to NEOs. Human exploration of NEOs would provide considerable improvement in understanding because of the ability to make intensive geologic observations and take carefully chosen samples. The technical requirements for human expeditions to NEOs, although undefined, are intermediate between those for lunar and martian missions. A particularly attractive aspect is that human spaceflight beyond the Moon will probably require incremental steps, and an expedition to a NEO would be of considerably shorter duration and risk and lower in cost than one to Mars. References 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 3. 2. Space Science Board, National Research Council, Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, National Academy Press, Washington, D.C., 1980, p. 52. 3. Space Studies Board and Board on Physics and Astronomy, National Research Council, A Strategy for Ground-Based Optical and Infrared Astronomy, National Academy Press, Washington, D.C., 1995, p. 2. 4. H.Y. McSween, “The role of meteorites in spacecraft missions, and vice versa,” Meteoritics and Planetary Science, 31:272–738, 1997.
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