ORIGIN AND EVOLUTION OF THE VESTA ASTEROID FAMILY

We model the formation and subsequent collisional evolution of the Ves ta asteroid family. The outcomes of the cratering event(s) which gener ated the family are predicted from current cratering physics, whereas the subsequent erosion of the family members due to collisions with ba ckground asteroids is simulated according to the model of Marzari et a l. (1995). Comparing the size and orbital distribution of the model Ve sta families with the observed family, we estimate the number and size of the projectile(s) which have impacted Vesta. The observed morpholo gy of the family suggests two possible scenarios: (1) The family is th e outcome of a major cratering event, resulting from the impact of an asteroid approximate to 40 km in diameter on the surface of Vesta abou t 1 Gyr ago, and followed by another more recent lower-energy impact ( by a projectile approximate to 20 km in diameter), producing the famil y's subgrouping close to the 3:1 mean motion Jovian resonance. (2) A s ingle impact occurred approximate to 1 Gyr ago and formed the whole fa mily at the same time. In this case we have to assume that the fragmen ts were ejected isotropically over a hemispheric region of Vesta, inst ead of being concentrated near the surface of a 90 degrees aperture co ne, as suggested by laboratory impact experiments with planar targets. This different ejection geometry yields a more scattered distribution of the orbital elements, resulting into a better agreement with the o bserved family. In both scenarios the cratering event(s) which formed the family is/are likely to have injected a significant number of km-s ized and smaller fragments into the 3:1 resonance, thus generating V-t ype near-Earth asteroids and HED meteorites. However, it appears likel y that the current influx of HED meteorites cannot be directly traced back to the family-forming event(s), but results from more recent, s s maller impacts on Vesta (or other family members).