Asteroids are rocky objects in our solar system, but they’re too small to be considered planets. They range in size from 30 feet across (10 meters) up to about 600 miles (970 kilometers), and most reside in the space between Mars and Jupiter, known as the main asteroid belt. Although this belt contains hundreds of thousands of these objects, its total mass is only about 4 percent that of our Moon.
Tour the solar system: Asteroids
These small, rocky objects are leftovers from the formation of our planetary system and have changed little since that time.