Size and spatial distributions of sub-km Main-Belt Asteroids

This paper presents the result of the first systematic investigation of very small Main-Belt Asteroids (sub-km MBAs) using the Subaru Prime-Focus Camera (Suprime-Cam) with an 8 K × 10K mosaic CCD array attached to the 8.2 m Subaru Telescope atop Mauna Kea, Hawaii. We call this survey SMBAS (Sub-km Main-Belt Asteroid Survey). Observations were carried out on 2001 February 22 and 25 (HST) and a ∼ 3.0deg² sky area near the opposition and near the ecliptic was searched, We detected 1111 moving objects down to R ∼ 26 mag (including very slow Trans-Neptunian Objects). In this survey, we could not determine the exact orbits of the moving objects, because of their short observational arc of only 2 hours. Instead, we statistically estimated the semi-major axis (a) and inclination (I) of each moving object from its apparent sky-motion vector, and then obtained the size and spatial distributions of sub-km MBAs. The main results of SMBAS are: (1) The sky number density of MBAs is found to be ∼ 290deg^-2 down to R ∼ 24. 4mag (for MBAs) near the opposition and near the ecliptic. (2) The slope of the cumulative size distribution for sub-km MBAs ranging from 0.5 km to 1 km in diameter is fairly shallower (∼ 1.2) than that for large MBAs of more than ∼ 5 km in diameter (∼ 1.8), which was obtained from past asteroid surveys. This means that the number of sub-km MBAs is much more depleted than a result extrapolated from the size distributions for large asteroids. (3) The depletion of sub-km MBAs is clearer in the outer main-belt than in the inner main-belt. (4) It seems that SMBAS asteroids distribute more widely in the I-direction in the outer zone (a = 2.8-3.1 AU) of the main-belt than known large asteroids do. We also discuss the possible causes for the characteristics of the distributions of SMBAS-observed small asteroids.