COMETS
Comets are small, fragile, irregularly shaped bodies composed of a mixture
of non-volatile grains and frozen gases. They have highly elliptical orbits
that bring them very close to the Sun and swing them deeply into space,
often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop
a surrounding cloud of diffuse material, called a coma, that usually grows
in size and brightness as the comet approaches the Sun. Usually a small,
bright nucleus (less than 10 km in diameter) is visible in the middle of
the coma. The coma and the nucleus together constitute the head of the comet.
As comets approach the Sun they develop enormous tails of luminous material
that extend for millions of kilometers from the head, away from the Sun.
When far from the Sun, the nucleus is very cold and its material is frozen
solid within the nucleus. In this state comets are sometimes referred to
as a "dirty iceberg" or "dirty snowball," since over
half of their material is ice. When a comet approaches within a few AU of
the Sun, the surface of the nucleus begins to warm, and volatiles evaporate.
The evaporated molecules boil off and carry small solid particles with them,
forming the comet's coma of gas and dust.
THE ONLY PROBLEM WITH SEEING COMETS IS THAT THEY TAKE SO MANY YEARS
TO VISIT EARTH'S VIEWING RANGE. HALE-BOPP WILL NOT BE VISIBLE FOR AT LEAST
75 MORE YEARS!
On July 23, 1995, an unusually bright comet outside of Jupiter's orbit
was discovered independently by Alan Hale, New Mexico and Thomas Bopp, Arizona.
The new comet, designated C/1995 O1, is the farthest comet ever discovered
by amateurs and appeared 1000 times brighter than Comet Halley did at the
same distance.
Comet Hyakutake was found on January 30, 1996 by a man in Japan named
Yuji Hyakutake. He spotted it using strong binoculars. This comet was visible
from late March until late April, 1996. On March 25, the comet reached its
closest approach to Earth of 9.3 million miles away. Comet Hyakutake has
probably not approached Earth for thousands of years.