In 1801, the Italian astronomer Giuseppe Piazzi discovered a new object in the Solar System orbiting the Sun out beyond the planet Mars. At first, people thought that it was a new planet between Mars and Jupiter. But over the next few years several more such objects were sighted, all of them much smaller than a planet. Astronomers now refer to them as asteroids, minor planets, or planetoids.
Ceres, the asteroid that Piazzi discovered in 1801, is the largest known of these objects. It has a diameter of about 1,020 kilometers (640 miles). But most asteroids are very small. Pallas, the next in size to Ceres, is only 585 kilometers (365 miles) across. Of the 2,000 asteroids so far known, there are probably only 250 with a diameter of 100 kilometers (60 miles) or more.
Astronomers think that there are millions of asteroids in the Solar System, ranging in size from a small planetary moon to a tiny object the size of a boulder. Experts who have analyzed light reflected from asteroids believe that most of them are made of stony material mixed with metals such as iron and nickel. Asteroids found farther out from the Sun may be made of rock and ice.
The great majority of asteroids travel around the Sun between the orbits of Mars and Jupiter. Astronomers used to think that they were the remains of a planet that blew up, but it is now thought that no planet ever formed between Mars and Jupiter. Because the very strong gravitational influence of Jupiter, the largest planet in the Solar System, would have pulled apart any planetary material that might have begun to collect there.
Although most asteroids orbit between Mars and Jupiter, there is a small group whose orbits bring them closer to the Sun. A group known as the Apollo Group comes close to the Earth. They include Apollo (from which the group takes its name), Icarus, and Eros. Two other groups are trapped in Jupiter's orbit around the Sun. One group precedes the huge planet and the other follows it by the same distance. These asteroids are called the Trojans, because many of them have names taken from characters in the Trojan War, such as Achilles, Hector, Agamemnon, and Patroclus. About 40 Trojans asteroids are now known.
© 1988 Encyclopaedia Britannica, Inc.