Asteroids are small, rocky, irregular bodies orbiting the sun. The first asteroid was discovered by Giuseppe Piazzi in 1801. He originally thought he had found a comet, but found that its orbit more closely matched those of planets rather than comets. He named the object Ceres. Ceres is the largest of all known asteroids. It is 933 km in diameter and contains about 25% of the mass of all the asteroids. The asteroids together have a mass less than that of the Moon.