  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 the theory. Planetesimals likewise may have finished their
accretion in a sequence of 'junior giant impacts', scaled down in size and
velocity. This chapter considers the complicated physics of pairwise accretion,
as planetesimals grow to planetary scales, and considers how the inefficiency
of that process influences the origin of planetesimals and the diversity of
meteorites and primary asteroids.
