Asteroids

Asteroids are a class of small Solar System Bodies in orbit around the Sun. Asteroids are increasingly referred specifically to the small rocky–icy and metallic bodies of the inner Solar System out to the orbit of Jupiter.

There are millions of asteroids, many thought to be the often shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets. A large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter.

Naming

A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name.

Distribution within the Solar System

1. Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e., not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km in diameter, and millions of smaller ones. These asteroids may be remnants of a failed planet in the protoplanetary disk. In in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

2. Trojans

Trojan asteroids are a population that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.

The most significant population of Trojan asteroids are the Jupiter Trojans. A couple trojans have also been found orbiting with Mars.

3. Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross the Earth's orbital path are known as Earth-crossers. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500–1,000.

Quasi-satellites

Some asteroids have unusual horseshoe orbits that are co-orbital with the Earth or some other planet.

Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.