Direct N-body simulations of rubble pile collisions

There is increasing evidence that many kilometer-sized bodies in the Solar System are piles of rubble bound together by gravity. We present results fr om a project to map the parameter space of collisions between kilometer-siz ed spherical rubble piles. The results will assist in parameterization of c ollision outcomes for Solar System formation models and give insight into d isruption scaling laws. We use a direct numerical method to evolve the posi tions and velocities of the rubble pile particles under the constraints of gravity and physical collisions. We test the dependence of the collision ou tcomes on impact parameter and speed, impactor spin, mass ratio, and coeffi cient of restitution. Speeds are kept low (<10 m s(-1), appropriate for dyn amically cool systems such as the primordial disk during early planet forma tion) so that the maximum strain on the component material does not exceed the crushing strength, assuming sufficient granularity. We compare our resu lts with analytic estimates and hydrocode simulations. We find that net acc retion dominates the outcome in slow head-on collisions while net erosion d ominates for fast off-axis collisions. The dependence on impact parameter i s almost equally as important as the dependence on impact speed. Off-axis c ollisions can result in fast-spinning elongated remnants or contact binarie s while fast collisions result in smaller fragments overall. Clumping of de bris escaping from the remnant can occur, leading to the formation of small er rubble piles. In the cases we tested, less than 2% of the system mass en ds up orbiting the remnant. Initial spin can reduce or enhance collision ou tcomes, depending on the relative orientation of the spin and orbital angul ar momenta. We derive a relationship between impact speed and angle for cri tical dispersal of mass in the system. We find that our rubble piles are re latively easy to disperse, even at low impact speed. This may provide a way of constraining the energy dissipation parameter and related properties of the initial planetesimal population. (C) 2000 Academic Press.