Title: | On the shapes and spins of "rubble pile" asteroids |
Author: | Alan W. Harris, Space Science Institute, Eugene G. Fahnestock, Univ. of Michigan, Petr Pravec, Ondrejov Observatory |
We examine the shape of a "rubble pile" asteroid as it slowly gains angular momentum by YORP torque, to the point where "landsliding" occurs. We find that it evolves to a "top" shape with constant angle of repose from the equator up to mid-latitude. A similar calculation for a non-spinning, extremely elongate rubble pile body suggests that it should collapse into the rough shape of a prolate ellipsoid of about 2.5:1 axis ratio. We also investigate the tidal effects of a binary system with a "top shape" primary spinning at near the critical limit for stability. We find that very close to the stability limit, the tide from the secondary can actually levitate loose debris from the surface and re-deposit it, in a process we call "tidal saltation", such that angular momentum is transferred from the primary spin to the satellite orbit, thus maintaining the equilibrium of near-critical spin as YORP continues to add angular momentum to the system.