Collisional history and internal structure of small asteroids

Recent spacecraft visits to asteroids have led to new views of asteroid collisional evolution. Despite predictions that most asteroids larger than about km size are strengthless gravitational aggregates (rubble piles), NEAR found Eros not to be a rubble pile, but a shattered collisional fragment. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? A new synthesis of asteroid collisional evolution is proposed, in which the well-known Dohnanyi result, that collisional evolution produes a self-similar power law size distribution, is overturned because of scale-dependent collision physics. The new picture reveals the possibility of a transition size around 5 km, below which asteroids would be primarily collisional breakup fragments, whereas much larger asteroids are mostly eroded or shattered survivors of collisions. If so, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover.