4:00 PM - 5:00 PM
Terrestrial planets grew in a series of similar-sized collisions that swept up most of the next-largest bodies. Theia was accreted by the Earth to form the Moon according to this theory. The stochastic nature of this process can explain why Mercury and Mars are so completely different (Asphaug and Reufer 2013, 2014). Planetesimals likewise might have finished their accretion in a sequence of ‘junior giant impacts’, scaled down in size and velocity, with important differences. I will review the inefficiency and complexity of pairwise accretion, and show how it influences the origin of planetesimals and the diversity of meteorites and primary asteroids. I will argue that Vesta and Ceres are two of the largest planetesimals that accreted in the region, the last of ~100 comparable oligarchs that were scattered by giant planet migration. The scenario offers an explanation for the remarkable diversity of ~100-300 km asteroids and associated meteorites, and furthermore predicts that most asteroids are 'orphans' of vanished planetesimals. The Psyche mission, currently in Phase A study, propose to visit one of the largest of the mantle-stripped remnants.