Basic nature of sub-km main-belt asteroids: their size and spatial distributions Original Research Article

This paper is a summary from the results of the first systematic survey of small Main-Belt Asteroids (MBAs) using 8.2 m Subaru telescope. We searched about 3 square-degree sky area near opposition and near the ecliptic. About 1100 moving objects down to R∼26 mag (including very slow Kuiper Belt Objects) were detected. In this survey, the observational arc for each moving object was only 2 hours, so that only their semi-major axis (a) and inclination (I) were estimated statistically based on their apparent motions. We found that (1) the sky number density of MBAs is about 290 per square-degree down to R∼24.4 mag (for MBAs), (2) the slope of the cumulative size distribution for detected MBAs ranging from 0.5 to 1 km in diameter is considerably shallower (about 1.2) than that for large MBAs (about 1.8) and (3) the shallowness of the size distribution slope for sub-km MBAs is more conspicuous in the outer part of the main-belt than in the inner part. These findings suggest that the number of small MBAs is much more depleted than an estimate extrapolated from the existing statistics of large asteroids. We also propose a new form of the power-law distribution with a size-dependent slope to express the observed sub-km MBA distribution.