internal porosity. This may be because they have been large enough to withstand all impacts, and have never been shattered. Alternatively, Ceres and some few other of the largest asteroids may be massive enough that, even if they were shattered but not dispersed, their gravity would collapse most voids upon recoalescing. Vesta, at least, has withstood intact one major impact since its formation and shows signs of internal structure from differentiation in the resultant crater that assures that it is not a rubble pile. This serves as evidence for size as a protection from shattering into rubble. Comets Observational evidence suggest that the cometary nucleus may not be a well-consolidated single body, but may instead be a loosely bound agglomeration of smaller fragments, weakly bonded and subject to occasional or even frequent disruptive events, although the larger cometary fragments are expected to be primordial condensations rather than collisionally derived debris as in the asteroid case. However, in situ observations by the Rosetta mission indicate that it may be more complex than that. Moons The moon Phobos, the larger of the two natural satellites of the planet Mars, is also thought to be a rubble pile bound together by a thin regolith crust about thick. Spectroscopy of Phobos' composition suggests that Phobos may be a captured main-belt asteroid. See also Circumplanetary disk Comet nucleus List of slow rotators (minor planets) References External links Close-up images of Itokawa, a rubble pile asteroid Hyper-Velocity Impacts on Rubble Pile Asteroids pdf online @ kent.ac.uk Astrophysics Bodies of the Solar System