Fast and slow rotation of asteroids

We present an analysis of the distribution of asteroid spin rates vs. size. The existence of significant populations of both slow and fast rotators among asteroids smaller than D = 40 km, and especially below 10 km (where our sample is mostly near-Earth asteroids), is shown. We have found that the excess of slow rotators is present at spin rates below approximate to0.8 rev/day, and the group of fast rotators occupies the range of spin rates >7 rev/day. The fast rotators show interesting characteristics: The lack of objects rotating faster than 2.2 h period among asteroids with absolute magnitude H < 22, as well as the tendency to spheroidal shapes of fast rotators, is evidence that asteroids larger than a few hundred meters are mostly loosely bound, gravity-dominated aggregates with negligible tensile strength (rubble piles), while monoliths may be abundant among smaller objects. A large fraction (about half) of near-Earth fast-rotating asteroids appear to be binary systems, probably created by tidal disruptions during close encounters with the terrestrial planets. (C) 2000 Academic Press.