The asteroids, also called the minor planets, are smaller than any of the eight major planets in the solar system and are not satellites of any major planet. The term asteroid means “starlike” because asteroids appear to be points of light when seen through a telescope.

Most asteroids are located between Mars and Jupiter, between 2.1 and 3.3 AUs (astronomical units) from the sun. Ceres, the largest and first to be discovered, was found by Giuseppe Piazzi on January 1, 1801, and has a diameter of 582 miles (936 kilometers). A second asteroid, Pallas, was discovered in 1802. Since then, astronomers have identified more than 18,000 asteroids and have established orbits for about 5,000 of them. Some of these have diameters of only 0.62 mile (1 kilometer). Originally, astronomers thought the asteroids were remnants of a planet that had been destroyed; now they believe asteroids to be material that never became a planet, possibly because it was affected by Jupiter’s strong gravity.

Not all asteroids are in this main asteroid belt. Three groups of asteroids, the Near-Earth asteroids (NEAs), reside in the inner solar system. The Aten asteroids have orbits that lie primarily inside Earth’s orbit. However, at their farthest point from the sun, these asteroids may cross Earth’s orbit.

The Apollo asteroids cross Earth’s orbit; some come even closer than the moon. The Amor asteroids cross the orbit of Mars, and some come close to Earth’s orbit. The Trojan asteroids move in virtually the same orbit as Jupiter but at points 60° ahead or 60° behind the planet. In 1977 Charles Kowal discovered an object now known as Chiron orbiting between Saturn and Uranus. Originally cataloged as an asteroid, Chiron was later observed to have a coma (a gaseous halo), and it may be reclassified as a comet.