Comets are defined in the World Book Dictionary as, "a bright heavenly body with a starlike center and often with a cloudy tail of light which always points away from the sun. Comets move around the sun like planets, but in an oval course. We can see comets only when they come close to the earth. The diameters of the heads of comets average 80,000miles, approximately the size of Jupiter"(Zeleny)
A typical comet is a small, oblong chunk of ice, about five to ten miles across. This chunk is called the cometís "nucleus". The nucleus may contain frozen carbon dioxide(dry ice), carbon monoxide, and methane, in addition to water ice. Interspersed with all this ice are tiny grains of dust; together, this ice and dust causes the nucleus to be a "dirty snowball". This idea was first put forth by astronomer Fred Whipple in 1950, and firmly verified by the spacecraft Giottoís flyby of Halleyís Comet in 1986.
Far out in space, the comet is nothing more than this "dirty snowball", but as it nears the sun and starts to experience the sunís heat, the gases start to change directly from solid to gas. This sublimation is caused by the near-vacuum of space, and are blown off the nucleus. As this occurs, the imbedded dust grains are expelled with the gas as well, and eventually this dust and gas creates a large diffuse cloud around the nucleus. This cloud is referred to as the "coma" and is usually some tens of thousands of miles across. The coma is what gives a comet its typical fuzzy appearance (Hale).
As the comet comes still closer to the sun, the solar wind, an energetic stream of particles continuously blowing off the sunís surface, encounters the material in the cometís coma and blows it back behind the nucleus. This creates the cometís "tail", which usually extends behind the comet in the opposite direction from the sun. One can think of a comet as a large windsock, with the tail extending in the direction of the solar windís motion. Quite often, two tails will form: one made up primarily of the sublimated gases and the other composed of dust grains, which "shine by reflecting sunlight (Roemer).
While most of the planetsí orbits are near circles, the orbits of most comets are extremely elongated ellipses. That point on the cometís orbit that is closest to the sun is called "perihelion". A comet experiences its strongest solar heating at its perihelion point, that is usually when it is brightest. The distance a comet is away from the Earth also makes a difference to how bright it appears to us. Occasionally, comets have been known to experience "outbursts" when their brightnesses increase dramatically within a short period of time. This can be due to a fresh eruption of new material from a cometís surface, or sometimes this occurs when the nucleus spits into two or more pieces, exposing previously hidden sections of its material to the sunís heat for the first time.
On the average, about a dozen of Jupiterís family of comets will pass perihelion during any given year. Also, as many as a dozen previously unknown comets are discovered each year. Some of the comets discovered may turn out to be additional members of Jupiterís family, and others may be in much larger orbits which will not bring them back to the sun for many centuries or millennia, if ever. On any given clear dark night, two or three dozen comets may be accessible to professional astronomers with suitable telescopes. About once a year, on the average, there will appear a comet bright enough to be visible with the naked eye, and the so-called "Great Comets", which are those that are conspicuous to the naked eyes of even non-astronomers, appear about once every decade or so(Hale).
A list of some of the most Famous comets is listed below (Roemer):
Name First recorded sighting Period of orbit
(years)
Halleyís Comet About 240 B. C. 76-79
Tycho Braheís Comet 1577 Unknown
Bielaís Comet 1772 6.6-6.8
Enckeís Comet 1786 3.3
Comet Flaugergues 1811 3,000
Comet Pons-Winnecke 1819 5.6-6.3
Great Comet of 1843 1843 513
Donatiís Comet 1858 2,000
Great Comet of 1882 1882 760
Comet Morehouse 1908 Unknown
Comet Schwassmann-
Wachmann 1 1927 16.1-16.4
Comet Humason 1961 2,900
Comet Ikeya-Seki 1965 880
Comet Tago-Sato-Kosaka 1969 420,000
Comet Bennett 1969 1,680
Comet Kohoutek 1973 75,000
Comet West 1976 500,000

Works Cited

Hale, Alan. "What Are Comets?" HB Magazine (http://www.halebopp.com/whatare2.htm) 4 Dec. 97.

"Roemer, Elizabeth." The World Book Encyclopedia. 1994 ed. World Book, Inc.

"Zeleny, Robert O.". The World Book Dictionary. 1988 ed. World Book,