Comets are small, fragile bodies made up of frozen gases and ice mixed with
dust . Most travel around the Sun in elliptical orbits which bring them close to
the Sun and then swing them out deep into space, often outside the orbit of
Pluto.
A comet has a distinct center called a nucleus, which usually measures less
than 10 miles in diameter and is normally irregularly shaped. As a comet
approaches the Sun, the heat causes the outer layers of the icy nucleus to
evaporate. The evaporation releases gases and dust which form the coma (a
hazy cloud which surrounds the nucleus). It's diameter may be as large as 1
million miles. The coma and nucleus make up the comet's head. The pressure
of the sun's radiation and solar wind push materials away from the comet's
head at differing speeds according to the size and mass of the materials. Thus,
relatively massive dust tails are accelerated slowly and tend to be curved. The
ion tail is much smaller and the acceleration is so great that it usually appears
as a straight line extending from the comet opposite the Sun. The tail of a comet
may stream across space as far as 100 million miles. All of a comet's light
comes from the Sun. The brightness of a comet results partly from sunlight
reflected by it's nucleus and coma. In addition, when a comet is closest to the
Sun, gas molecules in the coma release energy absorbed from the Sun's rays.
Most comet's cannot be seen without a telescope. Some are visible to the
unaided eye, but only for several weeks or months when they pass closest to
the Sun.
History of Comets
The Greek philosophers decided that comets were merely lights in the
atmosphere, like prolonged lightning. This theory persisted until the sixteenth
century, when the great Danish astronomer Tycho Brahe proved from his
observations that a comet may be at least six times as far away from the Earth
as the moon. Even then, astronomers believed that comets were not subject to
the laws of celestial motion. Kepler himself believed that comets moved in
straight lines, rather than the ellipses required by his three laws of planetary
motion.
Since ancient times there have been fanciful superstitions connected with
comets. They have been regarded as forerunners of wars, plagues, crop
failures, earthquakes, and all manner of evil. Halley's Comet revived these fears
in 1910, but the Earth passed through the edge of the comets tail with no harm.
Halley's Comet is probably the best-known comet. It was named for the English
astronomer Edmond Halley, who recognized that it could be seen an average of
every 77 years as it orbited the Sun. Astronomers predicted that the comet
would complete two orbits during the 1900's - in 1910 and in 1986.
A number of extremely bright comets have been seen since the Daylight Comet
of 1910 (not Halley's). For example, Comet Arend-Roland and Comet Mrkos
appeared in 1957 and Comet Ikeya-Seki in 1965. In 1973, Comet Kohoutek
became the first comet to be studied by men in space. Astronauts in the Skylab
space station photographed it and so provided much new information about
comets.
In recent times, two comets have surpassed naked eye visibility. Comet
Hyakutake around March 1996 and Comet Hale-Bopp around January 1997.
Comet Hyakutake put on the more spectacular display of the two, due to it's
apparent size, almost the diameter of the full moon.
