"Origin of the Spacewatch Small Earth-Approaching Asteroids"

W. F. Bottke, Jr., M. C. Nolan, H. J. Melosh,

A. M. Vickery, and R. Greenberg

(1996) Icarus 122, 406-427

Recent discoveries of small Earth-approaching asteroids by the 0.9 m Spacewatch telescope (referred to here as S-SEAs) reveal 16 objects which have diameters ~50 m or smaller. Approximately half of these objects lie in a region where few large near Earth asteroids are found, with perihelia (q) and aphelia (Q) near 1 AU, e < 0.35, and i from 0 to ~30 deg. Possible origins for these objects are examined by tracking the orbital evolution of test bodies from several possible source regions using an Opik-type Monte-Carlo dynamical evolution code, modified to includes (a) impact disruption, based on a map in orbital (a, e, i) space of collision probabilities and mean impact velocities determined using actual main-belt and near-Earth asteroid orbits, (b) fragmentation, and (c) observational selection effects.

Amor asteroid fragments evolving from low eccentricity Mars-crossing orbits beyond the q = 1 AU line provide a reasonable fit to S-SEA orbital data. Planetary ejecta from Mars is only consistent with low and moderately inclined S-SEA orbits. Asteroidal fragments from the main-belt via the 3:1 or nu_6 chaotic resonance zones rarely achieve low-e orbits before planetary impacts, comminution, or ejection remove them from the system. This source could produce the observed moderate-to-high eccentricity S-SEAs. Planetary ejecta from the Earth-Moon system and Venus, are only consistent with low-inclination S-SEA orbits. Moreover, constraints set by the planetary cratering record and the meteorite record suggest that the Earth, Moon, and Venus are unlikely to provide many S-SEAs. All of these results are predicated on the observational bias computations (Rabinowitz 1994) that provide the current definition of the S-SEA population.

For more information, contact Bill Bottke:bottke@astrosun.tn.cornell.edu