Abstract
The 8.2m Subaru Telescope equipped with the Suprime-Cam can detect ~100 Main-Belt Asteroids (MBAs) with an apparent magnitude of 20-24 mag in the single field of view. The apparent magnitudes correspond to the asteroid size from several 100 m to a few km in diameter. Since MBAs move only 5-6 arcmin during the whole night, almost MBAs remain in the single field of view of the Suprime-Cam.This means that ~100 lightcurves of MBAs can be obtained in a single night. The observations were performed in 2002 September, two different fields were observed and ~200 moving objects were detected. MBAs were extracted from the moving objects, and their spin period distribution was examined. We got a surprising result that about 25 % of our MBAs are so-called fast rotators whose spin period is less than 2 hours.Fast rotators were discovered till now only in a group of near earth asteroids, their origin was enigmatic so far. Our result suggests that there is fast rotators at high ratio in the population of small MBAs which has not been examined until now.
