asteroid

Any of many thousands of small bodies, made of rock and minerals, that orbit the Sun. Most lie in a region called the asteroid belt between the orbits of Mars and Jupiter, and are thought to be fragments left over from the formation of the Solar System. About 100,000 asteroids may exist, but their total mass is only a few hundredths of the mass of the Moon. These rocky fragments range in size from 1 km/0.6 mi to 900 km/560 mi in diameter.

The largest asteroids are sometimes called minor planets; these include Ceres (the largest asteroid, 940 km/584 mi in diameter) and Vesta (which has a light-coloured surface and is the brightest as seen from Earth). Some asteroids are in orbits that bring them close to Earth and some, such as the Apollo asteroids, which include Eros and Icarus, even cross Earth's orbit. They may be remnants of former comets.

The first asteroid to be identified was Ceres, discovered by the Italian astronomer Giuseppe Piazzi at the Palermo Observatory, Sicily, on 1 January 1801. The first asteroid moon was observed by the space probe Galileo in 1993, orbiting asteroid Ida.

Recent research NASA's Near-Earth Asteroid Rendezvous (NEAR) was launched in 1996, and from February 2000 orbited the asteroid Eros to collect data on asteroid composition. In 2001 it survived a deliberate crash landing to become the first spacecraft to land on an asteroid.

The NASA-funded Near-Earth Asteroid Tracking (NEAT) system at the Maui Space Surveillance Site in Hawaii detected more than 18,000 asteroids between 1995 and 2001. In 2003 astronomers of the Lowell Observatory Near-Earth Object Search Program in Arizona recorded the closest approach of a natural object to Earth - an asteroid, designated SQ222, roughly 10 m/33 ft across, passed within 88,000 km/54,680 mi of Earth, and was only observed as it was moving away from the planet. In 2004 the program discovered an asteroid with the smallest known solar orbit - asteroid 2004-JG6 was observed to orbit the Sun at a closest distance of 50 million km/31,070,000 mi. The asteroid, which is 500-1,000 m/1,640-3,208 ft in size, orbits the Sun every six months. Only the planet Mercury has an orbit closer to the Sun.

There are an estimated 1,000-2,000 near-earth asteroids (NEAs) with a diameter greater than 1 km/0.62 mi, according to research published by US astronomers in 2000.

Asteroids are frequently binary (consisting of two components revolving around each other) or bifurcated (consisting of two chunks of rock that touch each other). It may be that at least 10% of asteroids approaching the Earth are binary or bifurcated.