Page 316 - NS-2 Textbook

Asteroids, Comets, and Meteors

Between the orbits of Mars and Jupiter there are a multi- As has been previously mentioned, while the GaWeo
tude of small bodies referred to as asteroids. The area in spacecraft was on its way to Jupiter, it passed close by the
which they orbit the Slm is called the asteroid belt. They asteroids Gaspra in 1991 and Ida in 1993, photographing
have irregular orbits, but all revolve around the Still in them and in the process discovering a small satellite
the same direction as the larger planets. Approximately named Dactyl orbiting the latter. In June 1997 a Near
2,000 of these asteroids have been discovered by as- Earth Asteroid Rendezvous (NEAR) spacecraft launched
tronomers so far, and many have been given names. in 1996 came within 750 miles of the carbon-rich asteroid
Some scientists believe there may be 25,000 or more such Mathilde while on its way to a year-long encounter with
bodies. the asteroid Eros, which it reached in February 2000. In
Astronomers long wondered why the large distance Februaly 2001 it landed on Eros's surface, after having
between Mars and Jupiter seemed to be without a planet. trallSmitted some 200,000 detailed pictures of the aster-
After several hundred years of looking, in 1801 the first oid back to Earth. The pictures reveal a surface strewn
and largest asteroid, Ceres, was found. Orbiting at a with boulders, craters, and mysterious bright spots. Eros
mean distance of 257 million miles from the Sun, Ceres is is considered a geologic relic from fhe formation of the
only 480 miles in diameter. Other asteroids are consider- solar system about 4.5 billion years ago.
ably smaller. Many have diameters of less than 1 mile.
Some astronomers think the asteroids are material COMETS
that was left over when the solar system was formed
about 4.6 billion years ago. Others believe that they are Comets appear as bright streaks of light, sometinles visi-
leftovers from a collision of two relatively small planets ble without the aid of a telescope. They are the most
at some tinle in the distant past. Still others believe that plentiful bodies in our solar system, perhaps numbering
they are the remnants of a small planet that exploded for in the millions. TIley are the travelers of the solar system.
some unknown reason. Comets wander in huge elliptical orbits, out of the plane
None of the asteroids have any kind of atmosphere. of the ecliptic, far beyond the planet Pluto, but still re-
They appear to be little more than irregular chunks of volving around our Stm.
rock and metallic substances. Most comets are thought to have little mass. They are
believed to be composed of a nucleus of water ice, frozen
gases, and dust-like particles of such elements as carbon
and sodium, altogether rather like a dirty snowball.
When a comet approaches the interior of the solar sys-
tem, radiation from the Sun beginS to heat it, causing
particles of material alld vapor to begin to be released.
These form a halo around the nucleus called a cOllla. The
nucleus and coma together form the head of the comet.
As the comet comes closer, pressure from sunlight
causes the vapor and dust particles in the coma to fan out
from the head in a direction opposite the Sun, forming
the comet's tail. Certain gases in the coma, stimulated by
the sunlight, begin to glow, much like a fluorescent bulb.
When sunlight is reflected from the dust particles in the
tail, the effects combine to make the comet visible from
A dose-up photo of the asteroid Gaspra, taken by the Galileo space- Earth. Luminous tails more than 200 million miles long
craft while on its way to Jupiter in 1991. NASA have been obsen'ed. As the comet swings around the

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