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.