For years astronomers have speculated that small moons and asteroids are not coherent, solid pieces of rock, but loosely bound piles of rubble, cleverly called “rubble piles”. The reason for this speculation is that while space is fairly empty, time is also fairly long, and given enough time the debris in the solar system has a tendency to crash together. Small objects are more easily broken apart than larger ones, but the fragments of a shattered asteroid will frequently run back into each other as they orbit the Sun on nearly the same path, and their feeble gravity can be enough to reassemble them into a rubble pile. The Japanese spacecraft Hayabusa visited the small asteroid Itokawa and returned the most visually compelling images of a rubble pile I have seen.

This tiny asteroid is a little over 500 meters long, and only 200 meters across its narrow waist. To escape its gravity you only need to move about a half a mile per hour. Check out also this picture where the shadow of Hayabusa can be seen on the asteroid’s strange smooth plain, nicknamed the Muses Sea.