Asteroid Density, Porosity, and Structure

New data from observations of asteroid mutual perturbation events, observations of asteroid satellites, and spacecraft encounters have revolutionized our understanding of asteroid bulk density. Most asteroids appear to have bulk densities that are well below the grain density of their likely meteorite analogs. This indicates that many asteroids have significant porosity. High porosity attenuates shock propagation, strongly affecting the nature of cratering and greatly lengthening the collisional lifetimes of porous asteroids. Analysis of density trends suggests that asteroids are divided into three general groups: (1) asteroids that are essentially solid objects, (2) asteroids with macroporosities ~20% that are probably heavily fractured, and (3) asteroids with macroporosities >30% that are loosely consolidated "rubble pile" structures.