Size Distributions of Bluish and Reddish Small Main-belt Asteroids Obtained by Subaru/Hyper Suprime-Cam

We performed a wide-field survey observation of small asteroids using the Hyper Suprime-Cam installed on the 8.2 m Subaru Telescope. We detected more than 3000 main-belt asteroids with a detection limit of 24.2 mag in the r -band, which were classified into two groups (bluish C-like and reddish S-like) by the g – r color of each asteroid and obtained size distributions of each group. We found that the shapes of the size distributions of asteroids with C-like and S-like colors agree with each other in the size range of 0.4–5 km in diameter. Assuming the asteroid population in this size range is under collision equilibrium, our results indicate that compositional difference hardly affects the size dependence of impact strength, at least for the size range between several hundred meters and several kilometers. This size range corresponds to the size range of “spin barrier,” an upper limit observed in the rotation rate distribution. Our results are consistent with the view that most asteroids in this size range have a rubble-pile structure.