Comets

Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records of Comet Halley going back to at least 240 BC. The famous Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley.

As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 are periodic comets; some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure.

Comets are sometimes called dirty snowballs or "icy mudballs". They are a mixture of ices and dust that for some reason didn't get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

When they are near the Sun and active, comets have several distinct parts:

nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;
coma: dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus;
hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;
dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the naked eye;
ion tail: up to 100 million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets, stay within the orbit of Pluto for a significant fraction of their orbits.

After 500 or so passes near the Sun off most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter.

By far the most famous comet is Comet Halley but SL 9 was a "big hit" for a week in the summer of 1994.

Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October.