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An asteroid is any one of a large number of small solid objects 
in the SOLAR SYSTEM, sometimes called minor planets because
they orbit the Sun directly.  The vast majority are found in a
swarm called the asteroid belt, between the orbits of Mars and
Jupiter, at average distances of 2.1 to 3.3 ASTRONOMICAL UNITS
(AU) from the Sun.  They are given a number (and sometimes then 
named) when their orbits are well established.  More than 5,000 
such asteroids are now known, and many more have been sighted
at least once.  Numbered ones include about 38 to 45 Amor
asteroids, whose orbits intersect the orbit of Mars;  about 40
Apollo asteroids, whose orbits intersect the Earth's orbit;
about 6 Aten asteroids, with orbits smaller than the Earth's
orbit;  and about 96 Trojan asteroids, which precede or follow
Jupiter in its orbit.  One Apollo asteroid, now numbered 4581
and called Asclepius, passed within 800,000 km (500,000 mi) of
Earth in 1989--the closest approach of a large asteroid since
that of Hermes in 1937.
Asteroids range from many kilometers to a few meters or less in 
diameter.  The three largest, CERES, Pallas, and Vesta, are
about 785, 610, and 540 km (488, 379, and 336 mi) wide,
respectively.  Gaspra, an irregular asteroid about 20 km (12
mi) long on its largest axis, was the first to be photographed
at close range--by the space probe GALILEO in 1991, at a
distance of 1,600 km (1,000 mi).  Probes had earlier
photographed the two tiny moons of Mars, however, and it is
likely these also are asteroids, captured by that planet's
gravitational field.
Asteroids are thought to be remnants of the early solar system
that never grew to planetary size.  Collisions between such
objects must have been numerous in those early times.  Most of
the asteroids seen at present are probably fragments of
once-larger ones, as indicated by their irregular shape.  While 
some of those found in the inner solar system may be burned-out 
comets, most probably have compositions in the general range
found in meteorites.
Collisions of larger asteroids with the Earth have been
implicated in mass EXTINCTIONS over geological time, as well as 
with the onset of an ice age about 2.3 million years ago.  Some 
interest has been expressed in planning ways to avert future
cataclysmic encounters with the Earth.

Lawrence Grossman
Bibliography:  Binzel, R. P., et al., Asteroids II (1990);
Cunningham, C., Introduction to Asteroids (1987).
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