An asteroid is one of the many small planets that are members of the solar system and that move in orbits primarily between the orbits of Mars and Jupiter. The largest asteroid is Ceres, with a diameter of about 640 miles. About 200 asteroids have diameters of more than 60 miles, however, thousands of smaller ones exist. The larger asteroids are roughly rounded, with the smaller ones being irregularly shaped. The asteroid Chiron was discovered in an orbit between Saturn and Uranus. About 75 asteroids are known to intersect the orbit of Mars, and about 50 asteroids intersect the orbit of Earth. The Apollo asteroid Phaethon approaches the sun more closely, at 13.9 million miles, than any other known asteroid. Most asteroids are extremely dark in color, and show evidence of having absorbed water.