All other known asteroids are less than 340 km across. T here is some debate as to the classification of asteroids, comets and moons. 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 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 .