Asteroid rotations

The rotations of asteroids larger than ~40 km in diameter have a distribution close to Max-wellian that suggests that they are either original bodies of the asteroid main belt or its largest, collisionally evolved remnants. Small asteroids (0.15 < D < 10 km) show significant excesses of both slow and fast rotations, a “barrier ” against spins faster than ~12 rotations per day, and some of them are binary systems on inner-planet-crossing orbits with a characteristic fast rotation of their primaries. These small asteroids are collisionally derived fragments, mostly with neg-ligible tensile strength (rubble-pile or shattered interior structure). They mostly gained angular momentum through collisions, but noncollisional factors have also affected their spins and perhaps shapes. In the intermediate size range (10 < D < 40 km), the populations of large and small asteroids overlap. Most tiny asteroids smaller than D ≈ 0.15 km are rotating so fast that they cannot be held together by self-gravitation and therefore must be coherent bodies. They likely are single fragments of the rubble that make up larger asteroids from which the smaller ones are derived. 1