Origin of the Spacewatch Small Earth-Approaching Asteroids

sions which reach Earth-crossing or near Earth-crossing orbits via a chaotic resonance zone (e.g., the 3 : 1 meanRecent discoveries of small Earth-approaching asteroids by the 0.9 m Spacewatch telescope (referred to here as S-SEAs) motion resonance with Jupiter, the n6 secular resonance). reveal 16 objects which have diameters p50 m or smaller. However, many factors complicate this scenario: (a) shortApproximately half of these objects lie in a region where few period comets frequent some of the same regions as NEAs, large near-Earth asteroids are found, with perihelia (q) and making it possible that ‘‘extinct’’ comets provide a partial aphelia (Q) near 1 AU, e , 0.35, and i from 08 to p308. Possible source for NEAs (Wetherill 1985, 1988). (b) Perturbations origins for these objects are examined by tracking the orbital caused by the terrestrial planets and resonances caused by evolution of test bodies from several possible source regions the jovian planets both scatter NEAs, making it difficult using an Öpik-type Monte Carlo dynamical evolution code, modified to include (a) impact disruption, based on a map in to determine their point of origin. (c) NEAs are products orbital (a, e, i) space of collision probabilities and mean impact of comminution due to collisions, so a single disruption velocities determined using actual main-belt and near-Earth event could create a population of smaller objects in a asteroid orbits, (b) fragmentation, and (c) observational selecrelatively empty region for an extended amount of time. tion effects. (d) Not all asteroids are from the main belt, e.g., some Amor asteroid fragments evolving from low eccentricity Antarctic meteorites and SNC meteorites probably came Mars-crossing orbits beyond the q 5 1 AU line provide a reasonfrom the Moon and Mars, respectively. Distinct sub-popuable fit to S-SEA orbital data. Planetary ejecta from Mars is only consistent with low and moderately inclined S-SEA orbits. lations of NEAs may have their own source regions. Such Asteroidal fragments from the main-belt via the 3 : 1 or n6 populations may be characterized by being dynamically or chaotic resonance zones rarely achieve low-e orbits before planspectrally distinctive. Proposed models of their origins etary impacts, comminution, or ejection remove them from the must also be consistent with the size–frequency distribusystem. This source could produce the observed moderate-totion of the sub-population and must take into account high eccentricity S-SEAs. Plantary ejecta from the Earth–Moon selection effects that affect their distribution and how long system and Venus are only consistent with low-inclination Sthey can remain distinctive before they are scattered. SEA orbits. Moreover, constraints set by the planetary cratering record and the meteorite record suggest that the Earth, Moon, Here we investigate possible sources of a putative suband Venus are unlikely to provide many S-SEAs. All of these population of NEAs identified by Rabinowitz et al. (1993): results are predicated on the observational bias computations a set of bodies discovered by the Spacewatch project, com(Rabinowitz, D.L. 1994. Icarus 111, 364–377) that provide the prising 16 of the smallest NEAs (diameter D # 50 m) current definition of the S-SEA population.  1996 Academic Press, Inc. ever observed. Orbits of these ‘‘Spacewatch small-Earth approachers’’ (S-SEAs) are shown in Fig. 1 and Table I. Half of these bodies lie in a region where few larger NEAs