COMETS ORIGIN:Most comets are believed to come from the Oort Cloud, which lies beyond the planets, about one light year away from the sun, and also from the Jupiter belt which is situated just beyond the orbits of Neptune and Pluto.

COMPOSITION

Comets resemble hazy stars and travel in definite paths through the solar system. As some of the brightest comets approach the sun they form a long shining tail. The tail of a comet may stretch as far as 160km. The nucleus is the distinct centre be as large as 16km in diameter.  A coma (a hazy cloud ) surrounds the nucleus. A coma's diameter may reach up to 116km.The coma together with the nucleus make up the head of a comet.

The nucleus is believed to have formed in the cold outer regions of the solar system and probably consists of frozen gases (like ammonia, carbon dioxide, carbon monoxide and methane), together with ice mixed with dust (70-80%). The other 20-30% may be similar to meteoric particles. Meteoric particles are made up of fine grains and large pieces of metal and rocky material.

The icy layers on the nucleus begin to evaporate as they approach the sun.During this evaporation period, gases and dust is set free so forming the coma around the nucleus. Due to pressure of the sun's light , the tiny dust particles and gas molecules of the coma are pushed away so forming the tail of the comet. The brightness of a comet is partly achieved by the nucleus and coma reflecting the sun's light. Also, when a comet is closest to the sun, gas molecules in the coma release energy absorbed from the sun's rays.

COMET"S PATHS

Most comets travel in elliptical (oval-shaped) paths around the sun. The time taken to complete a full orbit is called its period. A few comet's have short periods of less than seven years.Others move in such huge orbits that they only pass near the sun once in thousands, even millions of years. Comets only become visible for a few weeks/months when they pass very close to the sun.eg. Halleys Comet is one of the best examples.  

Name	1st seen	PERIOD OF ORBIT	NAME	1ST SEEN	PERIOD OF ORBIT
Halley's Comet	Before 240 BC	76-79 years	Great Comet of 1882	1882	760 years
Tycho Brahe's Comet	1577	Unknown	Comet Morehouse	1908	16,1-16,4 years
Biela's Comet	1772	6,6-6,8 years	Comet Humasan	1961	2900 years
Eneke's Comet	1786	3,3 years	Comet Ikeya-Seki	1965	880 years
Comet Flaugergues	1811	3000 years	Comet Tago-Sato-Kosaka	1969	420000 years
Comet Pons Winneke	1819	5,6- 6,3 years	Comet Benett	1969	1680 years
Great Comet of 1843	1843	513 years	Comet Kohoutek	1973	75000 years
Donati's Comet	1858	2000 years	Comet West	1976	500000 years

IMPACT EFFECTS

A comet would do just as much damage as an asteroid of the same size, if it hit the earth at the same speed. To site an example astrophysicists at the Los Alamos National Laboratory in the Us anticipate that if a 5km wide asteroid landed in th Mid- Atlantic it would swamp the eastern seaboard of the US to the Appalachian Mountains. Comets, however, usually travel much faster than an asteroid (as fast as 2km/sec),therefore the resulting damage would be much more.

Comet Shoemaker-Levy 9 is the comet that crashed into Jupiter in July 1994 and left a distinct mark on this planet. If fragment G had struck earth instead jupiter, the resulting 6 million megaton explosion would have created a crater 60km across.This devastation would have caused a biological catastrophe similar to that which wiped out the dinosaurs and may other species 65 million years ago.