| 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. |
Comet
West (1975)
This
photograph was taken by amateur astronomer John Loborde on March 9, 1976.
This picture shows two distinct tails. The thin blue plasma tail is made
up of gases and the broad white tail is made up of microscopic dust particles.
Hale-Bopp
over Zimbabwe 1997, taken by Gunter in Harare.
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