A Search for Aten Asteroids

D. J. Tholen, R. J. Whiteley (Univ. Hawaii)

Aten asteroids are a subset of the near-Earth asteroid population that have semimajor axes of less than 1 AU. Only 24 such objects are currently known, compared with 218 Apollo asteroids, and all 24 of the known Atens have aphelion distances in excess of 1 AU. This situation is due to the fact that these objects are discovered by near-Earth asteroid search efforts that tend to concentrate on the opposition region, so these objects can be seen only if their orbits carry them outside the Earth's orbit. Clearly there is an observational bias against Aten asteroids with smaller aphelion distances. The extent of this bias is unknown, however. Could Aten asteroids with aphelion distances very close to the Earth's orbit represent an impact hazard that is not being addressed by current search efforts?

To tackle this question, we have engaged in a search effort that concentrates on ecliptic regions far from the opposition point. The typical solar elongation angle for our observations has been around 75 degrees. This strategy allows Aten asteroids with heliocentric distances of less than 1 AU to be discovered, though it limits the amount of observing time available on any one night. Another difficulty involves brightness losses caused by observing at high phase angles. To compensate for phase losses, we are using a larger aperture telescope than either the Spacewatch or NEAT search programs (the University of Hawaii 2.24-m telescope), and to compensate for the smaller field of view provided by a longer focal length instrument, we are using the 8192 by 8192 pixel CCD mosaic, which provides a field nearly 19 arcmin on a side.

To date, we have imaged approximately 9 square degrees of sky. No clear-cut Aten asteroids have been discovered so far, though one object was found moving at a rate of about 80 arcsec per hour, which is right at the threshold between the rate of motion we expect for main-belt asteroids and Atens, based on a hypothetical population of Atens. Bad weather prevented followup on that one candidate object.