A term used to describe the internal structure of certain asteroids and comet nuclei. The low densities of these objects imply that they are composed of a loosely bound aggregation of smaller objects with large voids between them. Objects with such weak structures cannot spin too quickly or they would fly apart. Neither do they support large craters, for if they are struck they will rearrange themselves internally to smooth out the depression. Rubble piles are formed by the coalescence of pristine smaller objects, rather than from the fragments of larger objects.