Asteroids are spread far apart in the asteroid belt, making collisions unlikely. They formed from collisions of early planetary bodies. Most asteroids have a covering of dust called regolith. While planets have large moons, some asteroids also have small moons. Asteroids group together in the main belt, Kuiper belt, as Trojans, or in the scattered disc. Meteoroids range in size from dust grains to baseballs and the largest may be asteroid fragments. Meteor showers occur when Earth passes through debris left by comets. There are about 9 major annual meteor showers.