Comets are the most exciting heavenly bodies, which used to create more awe than curiosity in the early days of human history. They often exist at the edge of, or even beyond our Solar System. Most of them are believed to inhabit in an area known as the ‘Oort Cloud’, far beyond the orbit of Pluto.
Comets are often called as dirty snowballs, though some scientists prefer to call them snowy dirtballs. It is commonly believed that the Comets are leftovers from the material that initially formed the solar system about 4.6 billion years ago. They are comprised of frozen gases, rock and dust that orbits the Sun in regular orbits, though their orbits are very much longer and in elongated eclipses. In fact, orbits of some of the Comets are so much elongated that it takes them hundreds and even thousands of years to complete one revolution around the Sun.
Some Comets, known as the ‘Periodical Comets’, visit the Earth regularly at fixed intervals. ‘Halley’s Comet’ is one of such important Comets, which can be seen from the Earth once in every 76 years. The ‘Non-Periodic Comets’, on the other hand, have passed through the Solar System only once. They do not orbit around the Sun, or they may take longer than 200 years to orbit.
The difference between a Comet and an Asteroid lies in the presence of the coma and tail in a Comet. A big Comet is comprised of a nucleus, a coma, and a tail. The solid core is called the nucleus, which contains icy chunks, frozen gases with bits of embedded dust. As it nears the sun, the comet gets hot and develops a surrounding cloud of diffuse material, called a coma, along with one or more tails, when a comet sweeps close to the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun. The Sun's heat causes the comet's ices change to gases and consequently, the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles, known as Solar Wind, is strong enough to blow the coma dust and gas away from the sun, sometimes forming a long, luminous bright tail extended for millions of kilometers from the head, away from the Sun.
Note:Comets actually have two tails - a dust tail and an ion or gas tail.