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.
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 Administration’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.
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.