A comet is a small  icy celestial body which rotate around the sun. It contains a nucleus solid  frozen ice and a coma which is made of gas (water vapor, CO₂ and other gases) and dust and also a long tail (possessing dust and ionized gases) which is developed when comet is nearby the sun. Due to force of solar wind, its long ion tail always point away from the sun. this long tail can increase up to 250 million km and because of this long tail we can see it when comets are in their highly eccentric orbits near the sun.

                         PARTS of A COMET                        

Nucleus:

 The frozen center of a comet's head is nucleus. It is made of ice, gas and dust which is main mass of comet, but it is so small of about 1 to 10 km across or may be more.

Coma:

 Nucleus is surrounded by roughly spherical bolb of gas about a million Km across and it is made of water vapor, carbon dioxid, ammonia, dust and extra gases which sublimed from nucleus. Head of a comet is formed by nucleus and coma.

Ion tail:

 Due to solar wind, a tail of charged gases(ion) always face away from the sun at high velocities and this is called ion tail or plasma tail. When comet reaches to the sun’s ion tail trails and when it leaves the sun, this tail disappears.

 Dust tail:

 It is a long  wide tail made of microscopic dust particles which is emitting from the sun during comet’s  motion and  due to photons that are buffeted.  It slightly curves and disappears when comet moves far from the sun.

Hydrogen Envelope:

The coma of comet is surrounded by hydrogen gas which trails for million of miles and often located between ion tail and dust tail. It's longitude is about 100 million km and it is about 10 million km across at nucleus of comet. By closing to sun,  hydrogen envelope will be bigger.

                          A COMET'S ORBIT                       

Comets rotate around the sun in highly elliptical orbits. The velocity of them increases when they are close to sun and decreases when they are far from it. When comets are near to the sun they are bright because the solar wind pushes the tail from the sun and therefore but it is virtually invisible through most of their orbit.

Some of comets that come close to the sun, they burn up or crash in to the sun. they are called sun grazers.

                         Comet Exploration                       

NASA'S stardust mission will take sample of comet wild 2 and bring them to Earth in 2004. One small spacecraft about 770 pounds or 350 kg flew in feb. 7, 1999 and  met comet wild 2 in Jan 2004 and will return to Earth on Jan 15, 2006 in western Utah USA. A short period comet called comet wild 2 (aka comet 81 p).It was discovered by Pauel Wild a astronomer from Swiss on Jan 6, 1978.The nucleus of comet is about 3 miles across (5km).It's elliptical orbit changes in rang of Mars to Jupiter's orbit and rotates the sun every 6.39 years.

                       COSMIC SNOWBALLS                    

The scientists discussing about a new theory that says comets made of frozen water, bombarding the Earth constantly and are called cosmic snow bulls may be seen by the visible imaging system of polar satellite.

According to this theory they vaporize in atmosphere and adding this vapor to environment.