one of the many small or minor planets that are members of the solar system and that move in elliptical orbits primarily in the “main asteroid belt” between the orbits of Mars and Jupiter. Unlike the myriad partly icy bodies found in the Kuiper Belt, main-belt asteroids are believed to be primarily rocky or metallic in composition.

Sizes and Orbits.

The largest bodies traditionally classified as asteroids are Ceres, with an average diameter of about 950 km (590 mi); the irregularly shaped Pallas, with dimensions ranging from 570 km (354 mi) to 482 km (300 mi); and Vesta, with a diameter of about 530 km (330 mi). In 2006 the International Astronomical Union voted to place Ceres in the new category of “dwarf planet,” along with the demoted planet Pluto and Eris, a body larger than Pluto, that was found in the outer solar system.

An estimated 250 asteroids have diameters of more than 100 km (60 mi), and researchers estimate that the number of bodies with diameters of at least 1 km (0.6 mi) in the main asteroid belt exceeds 1 million. The total mass of all asteroids in the solar system still is much less than the mass of the moon. Except for a very few, even the larger bodies are only roughly spheroidal; and elongated and irregular shapes are common for those with diameters of less than 160 km (100 mi). Most asteroids, regardless of size, rotate on their axis every 3 to 20 hr. Dozens of asteroids are “binary,” or have a satellite. The first asteroid found to have a moon was the main-belt asteroid Ida, whose satellite, given the name Dactyl, was discovered by the U.S. Jupiter-bound space probe Galileo in 1993. In 2005 the main-belt asteroid Sylvia, already known to have one satellite, was found to have another; the two moons were named Romulus and Remus.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the solar system where a sizable planet could have formed, but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present.

A small proportion of asteroids are located outside the main asteroid belt. The so-called Trojan asteroids lie in two clouds along Jupiter's orbit, one moving 60° ahead of Jupiter and the other 60° behind. Similarly positioned bodies have been found in the orbits of Neptune and Mars as well. Several small objects known as Centaurs have orbits between Jupiter and Neptune. The Centaurs have sometimes been regarded as asteroids, but at least some of them, such as Chiron, show comet like characteristics; some scientists think the Centaurs may originate in the Kuiper Belt.

Asteroids that cross the orbit of Mars or whose orbit lies entirely within that of Mars are called near-earth asteroids. More than 1000 belong to the group of near-earth asteroids known as the Amors, which do not cross earth's orbit. Another group, the Apollo asteroids, do cross earth's orbit, usually in the portion of their orbit nearest the sun (perihelion); this group also has more than 1000 known members. The Aten asteroids, of which a couple of hundred are known, also may cross earth's orbit, but they do so near their furthest point from the sun (aphelion); the few whose orbit lies entirely within earth's are called Apohele asteroids. One of the largest near-earth asteroids is Eros, an elongated body measuring 33 x 13 x 13 km (21 x 8 x 8 mi) that belongs to the Amors. The peculiar Apollo asteroid Phaethon, about 5 km (3 mi) wide, approaches the sun more closely, at 20.9 million km (13.9 million mi), than any other known asteroid. It is also associated with the yearly return of the Geminid stream of meteors.

Surface Composition.

With the exception of a few that have been traced to the moon and Mars, most of the meteorites recovered on earth are thought to be asteroid fragments. Remote observations of asteroids by telescopic spectroscopy and radar support this hypothesis. Researchers have accordingly tried to classify asteroids, like meteorites, into distinct types. A variety of schemes have been proposed involving numerous types, typically based on asteroids' spectra (indicative of chemical composition) and albedo, or reflective capability. A common traditional approach assigns most asteroids to types C, S, or M.

The majority of the asteroids visible from earth belong to the C type, thought to be related to a class of stony meteorites known as carbonaceous chondrites. These are considered among the oldest materials in the solar system, with a composition reflecting that of the primitive solar nebula. They are extremely dark in color, probably because of their hydrocarbon content. Many of them show evidence of the presence of minerals containing so-called water of hydration, or water of crystallization, in their crystal structure. This suggest that such asteroids, unlike the earth and the moon, have never either melted or been reheated since they first formed.

Asteroids of the comparatively bright S type, possibly similar to the stony iron meteorites, make up perhaps 15 percent of the total population. Much rarer are the bright M-type objects, corresponding in composition to the meteorites known as “irons.” Consisting of an iron-nickel alloy, they may represent the cores of melted, differentiated planetary bodies whose outer layers were removed by impact cratering.

A very few asteroids, among them Vesta, appear to have an igneous surface composition like that of many lunar and terrestrial lava flows. This suggests that the asteroid, at some time in its history, was at least partly melted. Scientists are puzzled that some of the asteroids have been melted but others, such as Ceres, have not.

Exploration.

Prior to its flyby past Ida and Dactyl, the U.S. probe Galileo in 1991 took the first close-up pictures of an asteroid: Gaspra, in the main belt. The images showed that the small, lopsided body is pock-marked with craters, and they revealed evidence of a blanket of loose, fragmental material, or regolith, covering the asteroid's surface. The first probe specifically devoted to asteroid study was the U.S. spacecraft Near Earth Asteroid Rendezvous–Shoemaker (NEAR Shoemaker), which was launched toward Eros in 1996. En route, it photographed the main-belt asteroid Mathilde in 1997. The craft went into orbit around Eros on Valentine's Day 2000 and landed on its surface about a year later. The U.S. probe Deep Space 1 flew by the near-earth asteroid Braille in July 1999 but obtained only limited data, owing to malfunctions. The U.S. probe Stardust, en route to Comet Wild 2, performed a close flyby of the main-belt asteroid Annefrank in November 2002. In November 2005 the Japanese probe Hayabusa landed on the near-earth asteroid Itokawa and then took off again, on a mission to return samples back to earth. This achievement marked the first controlled landing on an asteroid and the first ascent from a solar system body other than the earth and the moon.

The U.S. was scheduled to launch the spacecraft Dawn in 2007 on a mission to the asteroid belt, where it would orbit Vesta for several months beginning in late 2011, then proceed to Ceres, which it would study from orbit beginning in early 2015.

For further information on this topic, see the Bibliography, sections 376. Astronomy, 382. Solar system, 385. Comets and meteorites.