An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation due to excessive gravitational perturbations by Jupiter. Some asteroids have moons. The vast majority of the asteroids are within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter.

Hundreds of thousands of asteroids have been discovered within the solar system, and the present rate of discovery is about 5000 per month. As of April 14, 2006, from a total of 330,795 registered minor planets, 129,436 have orbits known well enough to be given permanent official numbers. Of these, 13,040 have official names (trivia: at least 610 of these names require diacritics). The lowest-numbered but unnamed minor planet is (3360) 1981 VA; the highest-numbered named minor planet is 118172 Vorgebirge [2].

Current estimates put the total number of asteroids above 1 km in diameter in the solar system to be between 1.1 and 1.9 million[3]. The largest asteroid in the inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large inner solar system belt asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. Vesta is the only main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions, a near-Earth asteroid may be visible without technical aid; see 99942 Apophis).

The mass of all the asteroids of the Main Belt is estimated to be about 3.0-3.6x1021 kg[4][5], or about 4% of the mass of our moon. Of this, 1 Ceres comprises 950x1018 kg, some 32% of the total. Adding in the next three most massive asteroids, 4 Vesta (9%), 2 Pallas (7%), and 10 Hygiea (3%), bring this figure up to 51%; while the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 3 Juno (0.9%), only add another 3% to the total mass. The number of asteroids then increases exponentially as their individual masses decrease.