The search for the most ancient asteroid collisions reveals the original planetesimals of our solar system

Hundred years of asteroid family research and new asteroid family identification methods have shed new light on the collisions among asteroids that shaped the main belt. In particular, in the inner portion of the belt - bracketed by the nu6 secular resonance at 2.1 au and by the J:3/1 mean motion resonance at 2.5 au - it is now possible to distinguish those asteroids that formed as collisional fragments from the break up of larger and older parents, and those other that are not fragments. This second population, which constitutes those asteroids that formed as planetsiamals by the accretion of dust in the protoplanetary disk, has a size distribution skewed towards bodies with diameters larger than 50 km. This indicates that planetesimals formed big, as previously suggested.