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.
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