Rubble pile

In astronomy, a rubble pile is an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make them up. Many asteroids and comets are thought to be composed of coalesced rubble.

Overview

Asteroids

A close analogy to a rubble pile is a partially melted shot and wax slug projectile commonly fired from a shotgun as opposed to a monolithic pure lead slug, with the former having a lower density as it is loosely held together by wax and is composed of numerous individual lead bird shot spheres.

Rubble piles form when an asteroid or moon (which may originally be monolithic) is smashed to pieces by an impact, and the shattered pieces subsequently fall back together, primarily due to self-gravitation. This coalescing usually takes from several hours to weeks.

When a rubble-pile asteroid passes a much more massive object, tidal forces change its shape.