Asteroids are space rocks that have existed since the formation of the planets over 4.6 billion years ago. Scientists believe that asteroids are pieces of rock that were never swept up into planets. Like the planets, asteroids are kept in orbit by the gravity of the Sun. Most of the asteroids in the solar system lie in the Asteroid Belt between the orbits of Mars and Jupiter.

Image credits: main image, Asteroid Gaspra's Best Face, courtesy of NASA, The Galileo Project; Mordecai-Mark MacLow: AMNH.