An asteroid, also called a minor planet or planetoid, is a member of a group of small, planet-like bodies that are part of our solar system. They are believed to be remnants of the interstellar clouds, nebula, that were not incorporated into planets during the formation of the solar system.

The largest asteroid in the inner solar system is Ceres with a diameter of 1003 km. It also was the first to be discovered, by Giuseppe Piazzi on January 1, 1801. Nowadays, over 9000 asteroids are known, some less than 1 km across. Two other large asteroids are Pallas and Vesta. The first nearby pictures of an asteroid were taken by the Galileo spacecraft of Gaspra and Ida in 1991, while NEAR Shoemaker landed on Eros in 2001.

One large group of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the "main belt." These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence also results in Kirkwood gaps in the asteroid belt, orbits cleared by orbital resonance.

Another important group is called Trojans; they are in the orbit of Jupiter, on two Lagrangian points. Similar sets of asteriods appear to lie in the trojan points in almost all planetary orbits.

There is increasing interest in identifying asteroids whose orbit crosses the Earth's, and that could, given enough time, collide with the earth. The two most important groups of near-Earth asteroids are the Amors, and the Atens. Various asteroid deflection strategies have been proposed.

Another group of objects that didn't accrete to form planets (but are more icy, and so are not really asteroids) are the Kuiper Belt objects. The Kuiper belt is the source of about half of the comets that come to the inner solar system. Some of these are not much smaller than Pluto and Charon -- the largest found so far is Quaoar, thought to be around 1250 km in diameter, the same size as (or even slightly larger than) Charon - and some astronomers expect that we shall one day find some Trans-Neptunian objects bigger than Pluto.

There are also a few objects that orbit the Sun between the orbits of the giant planets, called Centaurs. The first of these to be discovered was 2060 Chiron in 1977. These are generally supposed to be asteroids or comets that were ejected from their proper orbits.

When the orbit of an asteroid is confirmed, it is given a number, and later it may also be given a name (e.g. 1 Ceres). The first few are named after figures from Graeco-Roman mythology, but as such names started to run out, others were also used - famous people, the names of the discover's wives, even television characters. A few groups have names with a common theme - for instance Centaurs are all named after legendary Centaurs, and Trojans after heroes from the Trojan War. The Centaurs are of special interest; many of them are massive comets, such as Chiron.