ASTEROID

ASTEROID

ASTEROID, one of the many small or minor planets that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.

Sizes and Orbits

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The largest representatives are Ceres, with a diameter of about 930 km (about 578 mi); Pallas, with a diameter of about 552 km (about 343 mi); and Vesta, with a diameter of about 521 km (about 324 mi). About 200 asteroids have diameters of more than 97 km (more than 60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (less than 100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hr. Certain asteroids may be binary, or have satellites of their own.

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.

The so-called Trojan asteroids lie in two clouds, one moving 60° ahead of Jupiter in its orbit and the other 60° behind. In 1977 the asteroid Chiron was discovered in an orbit between that of Jupiter and Saturn. By the late 1980s, about 75 asteroids, the Amor asteroids, were known to intersect the orbit of Mars, about 50 Apollo asteroids to intersect the orbit of the earth, and less than 10 Aten asteroids to have orbits smaller than the earth’s orbit. One of the largest inner asteroids is Eros, an elongated body measuring 36 by 12 km (22 by 7 mi). The peculiar Apollo asteroid Phaethon, about 5 km (about 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.

Several earth-approaching asteroids are relatively easy targets for space missions. In 1991, the National Aeronautic and Space Administratio’s Galileo space probe, on its way to Jupiter, took the first close-up pictures of an asteroid. They showed that the small, lopsided body, 951 Gaspra, is pock-marked with craters, and revealed evidence of a blanket of loose, fragmental material, or regolith, covering the asteroid’s surface.

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. They reveal that asteroids, like meteorites, can be classified into a few distinct types.

Three-quarters of the asteroids visible from earth, including Ceres, belong to the C type, which appear to be related to a class of stony meteorites known as carbonaceous chondrites. These are considered to be the oldest materials in the solar system, with a composition reflecting that of the primitive solar nebula. Extremely dark in color, probably because of their hydrocarbon content, they show evidence of having adsorbed water of hydration. Thus, unlike the earth and the moon, they have never either melted or been reheated since they first formed.

Asteroids of the S type, related to the stony iron meteorites, make up about 15 percent of the total population. Much rarer are the 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.

Very few asteroids, notably Vesta, are probably related to the rarest meteorite class of all: the achondrites. These asteroids appear to have an igneous surface composition like that of many lunar and terrestrial lava flows. Thus, astronomers are reasonably certain that Vesta was, at some time in its history, at least partly melted. Scientists are puzzled that some of the asteroids have been melted but others, such as Ceres, have not. One possible explanation is that the early solar system contained certain concentrated, highly radioactive isotopes that might have generated enough heat to melt the asteroids.