Comets have been known since ancient times, unlike the other small bodies of the Solar System. Chinese observers recorded Comet Halley as early as 240 BC. In Europe, the famous Bayeux Tapestry commemorates the Norman Conquest of England in 1066 AD and depicts an appearance of Comet Halley.
The orbits of more than 878 comets have been cataloged. Of these, 184 are referred to as periodic comets because they swing into view on schedules of less than 200 years. Some of the others may be periodic, but their orbits are not understood clearly.
Muddy snowballs. Comets have been described as dirty snowballs or icy mudballs. They are mixtures of water ice, frozen gases and dust that for whatever reason didn't get picked up and incorporated into the planets when the Solar System formed. Astronomers think of comets as samples of the earliest history of the Solar System.
Comets are invisible to people on Earth, except when they are near the Sun. Most fly along highly eccentric orbits, which take them far out beyond the orbit of the most-distant planet, Pluto. They are seen once and then disappear for thosuands of years. Only the so-called short-period and intermediate-period comets -- like Comet Halley -- stay inside the orbit of Pluto for a significant portion of their orbits.
What's in a comet? After 500 or so trips around the Sun, most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. It could be that half of the near-Earth asteroids may be dead comets. Also, a comet whose orbit brings it near the Sun is likely to either hit one of the planets or the Sun, or be thrown out of the Solar System by a close encounter with a major body like Jupiter.
However, the active comets we see near the Sun are composed of several elements:
Nucleus mostly ice and gas with some dust and other matter
relatively solid and stable
Coma carbon dioxide and other gases from the nucleus
in a dense cloud of water
Dust Tail the most-visible part of a comet
smoke-sized dust particles up to 10 million km long
driven off the nucleus by escaping gases
Ion Tail plasma laced with rays and streamers up to 100 million km long
caused by interactions with the solar wind
Hydrogen Cloud huge sparse envelope of neutral hydrogen
millions of km in diameter
Best known is Comet Halley, but Shoemaker-Levy 9 was a big media hit for a week in the summer of 1994 as it broke apart and pieces of it slammed into Jupiter.
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