asteroid

asteroid   planetoid, or minor planet, small body orbiting the sun. More than 10,000 asteroids have orbits sufficiently well known to have been cataloged and named; thousands more exist. Most asteroids are irregularly shaped, unlike the spherically shaped major planets. The largest asteroids, Quaoar (diameter c.800 mi/1,300 km) and Ixion and Varuna (both c.750 mi/1,200 km), reside in the Kuiper belt (see comet ) and are about half the size of Pluto. The largest main-belt asteroid, Ceres , has a diameter of c.630 mi (1,000 km); the three next largest are Pallas , Vesta , and Juno . Only Vesta can be seen with the naked eye. Many asteroids are no larger than a few kilometers; in 1991, an asteroid only 33 ft (10 m) in diameter was found. Many asteroids are so small that their sizes cannot be measured directly; in many cases, their sizes have been estimated from their brightness and distances. The average orbital distance of the asteroids from the sun is about 2.9 astronomical units (AU).

The orbits of most asteroids lie partially between the orbits of Mars and Jupiter. These so-called main-belt, or cisjovian, asteroids are divided into subgroups named for the main asteroid in the grouping: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles, and Hildas. The near-earth asteroids, which closely approach the earth, are classed as Atens (with orbits between the earth and the sun), Apollos (with orbits similar to that of the earth), and Amors (with orbits between the earth and Mars). Two groups of asteroids share Jupiter's orbit; they are known as Trojan asteroids . In 1990, a similar asteroid was found in the orbit of Mars. Centaurs are asteroids with orbits in the outer solar system.

Asteroids are also classified by composition and albedo , most being one of three types. The majority (C-type) are similar to carbon-chrondite meteorites with approximately the same composition as the sun (excluding hydrogen) and are relatively dark. Those with a composition of nickel iron mixed with silicates of iron and magnesium (S-type) are relatively bright. The M-type are composed of nickel iron and are bright. Some of the Trojan asteroids appear to be captured comets, composed of ice and dirt, rather than rocky asteroids.

Toward the end of the 18th cent. astronomers were searching for a planet whose orbit should, according to Bode's law , have an average distance from the sun of 2.8 AU On Jan. 1, 1801, G. Piazzi discovered Ceres while studying the sky in the constellation Taurus; Ceres was later found to have an orbit very near that predicted by Bode's law. Ceres and the asteroids Juno, Pallas, and Vesta, which were discovered soon (1802-7) after Ceres, were initially regarded as planets by many astronomers, a view that was not overturned until additional asteroids were identified in the 1840s and 50s. By 1890 more than 300 asteroids had been discovered by visual means. In 1891, Max Wolf introduced the method of identifying an asteroid by the record of its path on an exposed photographic plate; it appears as a short line in a time exposure, rather than as the sharp point of a star. Brucia was the first asteroid discovered by this method. A more modern approach uses two photographs taken less than an hour apart and examined through a stereomicroscope that allows the asteroid to appear suspended above the background of stars. Still more modern techniques were employed in the discoveries of Ixion, found in 2001 using virtual telescope techniques, and of Quaoar, found in 2002 using photographs taken with the Hubble Space Telescope .

More than 200 asteroids have been identified that regularly intersect the orbit of the earth, and over geologic time asteroids in similar orbits have struck the earth. Hermes, discovered in 1937 and subsequently lost until 2003 when it was identified as a pair of asteroids, comes within 378,000 mi (608,000 km), and Eros comes within 14 million mi (22 million km). More recently, a small asteroid provisionally designated 2002 MN, 150-360 ft (45-110 m) in diameter, passed within 75,000 mi (121,000 km) of the earth—about a third of the distance to the moon—in 2002. Astronomers have observed about several hundred small asteroids, most measuring less than 55 yd (50 m) across, in near-earth orbits that are spread thinly between the earth and Mars. Many of these small asteroids have orbits that intersect the earth's.

Asteroids have been implicated in several mass extinctions of large numbers of animal and plant species in the past. From evidence found in sediments, Luis Walter Alvarez and others hypothesized that the great mass extinction of the dinosaurs and other species 65 million years ago, at the end of the Cretaceous period , was caused by the atmospheric and climatic effects of an asteroid impact; a possible crater exists in the Yucatán region of Mexico. In 1992, scientists reported that the appearance of patterns of shattered quartz crystals imbedded in Triassic shale and other fossil evidence suggest that another major mass extinction, about 200 million years ago, was caused by three closely spaced asteroid impacts.

The origin of asteroids is unclear; one theory claims that they were formed from material that could not condense into a single planet because of perturbation effects involving Jupiter. Some asteroids are actually nuclei of comets that are no longer active.

The space probe Galileo, which passed near and photographed Gaspra (1991) and Ida (1993), provided the first close images of an asteroid. The pictures revealed that Ida has a natural satellite, Dactyl. Ida, in the main asteroid belt between Mars and Jupiter, is about 35 mi (56 km) long and 15 mi (24 km) in diameter. Its tiny moon is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida. Since then several other asteroids have been found to have companions, leading astronomers to believe that it may not be uncommon. The probe NEAR-Shoemaker examined Mathilde (1997) on its way to rendezvous (1999) and orbit (2000) Eros. After providing the most information ever obtained about an asteroid (measurements of size, shape, mass, and gravitational field; elemental and mineral composition of the surface; topographic mapping; and measurement of the magnetic field and its interaction with the solar wind), NEAR-Shoemaker made an originally unplanned landing on Eros in 2001, returning close-up images as it descended and data about surface composition. In 1999 the probe Deep Space 1 accomplished the then closest-ever flyby of an asteroid, coming within 16 mi (26 km) of the surface of Braille ; spectroscopic data suggests that Braille broke off from Vesta millions of years ago.

Bibliography: See T. Gehrels and M. S. Matthews, ed., Hazards Due to Comets and Asteroids (1995); J. S. Lewis, Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment (1997); C. T. Russell, ed., The Near Earth Asteroid Rendezvous Mission (1998); N. F. Michelson, The Asteroid Ephemeris 1900-2050 (1999).

asteroid Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in size from almost 1000 km for Ceres (the first asteroid discovered, in 1801) down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 10²¹ kg, about one-twentieth the mass of the Moon.

When an asteroid is discovered it is given a temporary designation, consisting of the year of discovery followed by two letters; the first indicates the half-month during which the asteroid was discovered, and the second the order of discovery within that half-month. Only when an accurate orbit has been determined is it assigned a permanent number, and the discoverer then has the right to name it. Increasing numbers are being discovered in dedicated searches such as the Catalina Sky Survey, Lincoln Near-Earth Asteroid Research, Lowell Observatory Near-Earth Object Search, Near Earth Asteroid Tracking, Pan-STARRS, and Spacewatch. There are thought to be between 1–2 million larger than 1 km, mostly in the main belt, although only around 10% of these are currently known.

The orbits of most asteroids have higher eccentricities and inclinations than those of the major planets. Within the main asteroid belt, orbital eccentricities average about 0.15, and inclinations about 10°; occasionally they exceed 0.5 and 30°, respectively, more typical of the orbits of short-period comets. Indeed, some objects classified as asteroids may be defunct cometary nuclei. Rotation periods of asteroids range from a few hours to several weeks, but are typically 6–24 hours. The larger asteroids are roughly spherical, but those smaller than 150 km are commonly elongated or irregular. Radar studies of a few asteroids have revealed that some may be dumbbell-shaped or possibly double; these include Castalia and Toutatis. A few asteroids have small moons, the first of which was photographed in orbit around Ida by the Galileo space probe.

Some main-belt asteroids form groups with similar orbital characteristics (semimajor axis, orbital eccentricity, and inclination), for example the Cybele, Hilda, Hungaria, and Phocaea groups. Where the group seems to have originated from the break-up of a single parent body, it is called a Hirayama family. A small percentage of asteroids orbit outside the main asteroid belt. Members of the Amor group cross the orbit of Mars, while Apollo and Aten group asteroids cross that of Earth; these three groups are collectively termed near-Earth asteroids. Farther out, the Trojan asteroids orbit at Jupiter's distance, while beyond Neptune is the Kuiper Belt.

Asteroids are divided into various classes according to their reflectance spectra, which reveal differences in composition. The proportion of different asteroid classes changes markedly with increasing distance from the Sun. S-class (silicaceous) asteroids predominate in the inner main belt (at less than 2.4 AU). C-class (carbonaceous) asteroids are more prevalent in the middle and outer regions of the belt, with a peak near 3 AU. The dark asteroids near the outer edge of the main belt have a reddish tinge, and may be richer in organic components; these are the P-class asteroids. Still farther out, many of the Trojan asteroids are even redder; they are termed D-class asteroids. There is an apparent concentration of M-class (metallic) asteroids in the middle of the belt, at 2.5–3.0 AU.

Asteroids are thought to have formed through the accretion of metre-sized bodies, but were prevented from aggregating into a planet by the gravitational effect of Jupiter, which had already formed. In addition, some planetesimals left over from the formation of Jupiter may have been scattered into the asteroid belt. The largest asteroids were heated by the decay of radioactive isotopes within them. They melted and became differentiated, acquiring a metallic core, overlain by a mantle and crust. Subsequent collisions led to fragmentation, and almost every asteroid is probably a fragment of a once-larger body. In addition, most meteorites are believed to be pieces of asteroids.