• What are their thermal and collisional histories, and their relationships to meteorites and other bodies in the solar system?

The scientific goals of an NEO research program can be stated succinctly: To understand the orbital distribution, physical characteristics, composition, origin, and history of near-Earth objects. These goals are responsive to scientific objectives for the exploration of small bodies in the solar system previously articulated by the Space Studies Board and its committees.^4,5

Asteroids in near-Earth space are categorized as Amor, Apollo, or Aten objects, depending on whether their orbits lie outside that of Earth, overlap that of Earth with periods greater than 1 year, or overlap that of Earth with periods less than 1 year, respectively (Box 1.1). Comets are classified as short period or long period, depending on whether their orbital periods are less or greater than 200 years. This report focuses specifically on Amor, Apollo, and Aten objects (collectively referred to as NEOs), some of which may be currently inactive short-period comets. Most NEOs probably originate when collisions in the main asteroid belt eject fragments into resonances with Jupiter and Saturn. They may also derive from the Oort Cloud or the Kuiper Belt. A systematic inventory of NEOs will permit a better understanding of their orbital distribution, as well as the relationships among asteroids, comets, meteorites, and interplanetary dust.

An assessment of the physical characteristics of these objects includes determining their shapes, sizes, albedos, spin characteristics, and masses. Shapes, sizes, and spin characteristics are central to understanding collisional histories; albedos (as functions of wavelength), reflectance spectra, and calculated densities provide information on asteroid and comet compositions and internal structures. Their magnetic and thermal properties relate to composition and thermal history. Studies of surface morphology and materials, including craters, fractures and other structural features, regoliths, and bedrock outcrops, allow the geologic evolution of these objects to be reconstructed.

Determining the chemical and mineralogical compositions of NEOs provides critical constraints on their formation and evolution, as previously emphasized by the Space Studies Board.⁶ Their bulk chemistries relate to condensation and other processes thought to have occurred within the solar nebula, and their mineralogies are functions of temperature, pressure, and geologic history (or orbital history, in the case of comets). Quantification of mineralogy provides a bridge between asteroid spectroscopy and studies of meteorites. Returned samples would also allow determination of their times of formation and fragmentation based on their radiogenic and cosmogenic isotopic compositions, as well as studies of processes resulting from interactions with the space environment (solar wind implantation, space weathering, and so on). The petrology of returned samples would reveal details of accretional, thermal, and regolith-forming processes.

The origins of NEOs must be inferred from their physical characteristics, compositions, and orbital properties. All of these objects are thought to be relics from the early solar system. Meteorite studies tell us that many bodies retain primordial characteristics and thus provide unique opportunities to constrain presolar and solar nebula events. Others may be geologically processed and differentiated, and the relative importance of subsequent

Front Matter (R1-R12)

Executive Summary (1-2)

1 Introduction to Near-Earth Objects (3-7)

2 Status of Current Research Programs (8-15)

3 Future Research Activities (16-21)

4 Technological Aspects of Studies of Near-Earth Objects (22-26)

5 Conclusions and Recommendations (27-29)

Glossary (30-32)