They are the only asteroids which have been studied closely so far. T he largest asteroid by far is 1 Ceres . There are many planetary satellites that are probably better thought of as captured asteroids. Mars 's tiny moons Deimos and Phobos , Jupiter 's outer eight moons , Saturn 's outermost moon, Phoebe , and perhaps some of the newly discovered moons of Saturn , Uranus and Neptune are all more similar to asteroids than to the larger moons. There are also a dozen or so other rare types.
T here is little data about the densities of asteroids.
Surprisingly, its density turns out to be not much greater than that of water, suggesting that it is not a solid object but rather a compacted pile of debris. Between the main concentrations of asteroids in the Main Belt are relatively empty regions known as the Kirkwood gaps . These are regions where an object's orbital period would be a simple fraction of that of Jupiter . An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit. 5145 Pholus orbits from Saturn to past Neptune .