ORIGIN OF ASTEROID ROTATION RATES IN CATASTROPHIC IMPACTS

The rotation rates of asteroids, which are deduced from periodic fluct uations in their brightnesses(1), are controlled by mutual collisions( 2-8). The link between asteroid spin and collision history is usually made with reference to impact experiments on centimetre-scale targets, where material strength governs the impact response(2,3,9-11). Recent work, however, indicates that for objects of the size of most observe d asteroids (greater than or equal to 1 km in diameter), gravity rathe r than intrinsic strength controls the dynamic response to collisions( 12-14). Here we explore this idea by modelling the effect of impacts o n large gravitating bodies. We find that the fraction of a projectile' s angular momentum that is retained by a target asteroid is both lower and more variable than expected from laboratory experiments, with spi n evolution being dominated by 'catastrophic' collisions that eject si milar to 50 per cent of the target's mass. The remnant of an initially non-rotating silicate asteroid that suffers such a collision rotates at a rate of similar to 2.9 per day, which is close to the observed me an asteroid rotation rate of similar to 2.5 d(-1) Moreover, our calcul ations suggest that the observed trend in the mean spin frequency for different classes of asteroids(4) (2.2 d(-1) for C-type asteroids, 2.5 d(-1) for S-type, and 4.0 d(-1) for M-type) is due to increasing mean density, rather than increasing material strength.