More than 5,000 such asteroids are now known, and many more have been sighted at least once.
Asteroids range from many kilometers to a few meters or less in diameter.
Probes had earlier photographed the two tiny moons of Mars, however, and it is likely these also are asteroids, captured by that planet's gravitational field. Asteroids are thought to be remnants of the early solar system that never grew to planetary size. Collisions between such objects must have been numerous in those early times. Most of the asteroids seen at present are probably fragments of once-larger ones, as indicated by their irregular shape. While some of those found in the inner solar system may be burned-out comets, most probably have compositions in the general range found in meteorites.