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 am ong 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 e xcess 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 re v/day. The fast rotators show interesting characteristics: The lack of obje cts rotating faster than 2.2 h period among asteroids with absolute magnitu de H < 22, as well as the tendency to spheroidal shapes of fast rotators, i s evidence that asteroids larger than a few hundred meters are mostly loose ly bound, gravity-dominated aggregates with negligible tensile strength ("r ubble piles"), while monoliths may be abundant among smaller objects. A lar ge fraction (about half) of near-Earth fast-rotating asteroids appear to be binary systems, probably created by tidal disruptions during close encount ers with the terrestrial planets. (C) 2000 Academic Press.