A comet is a small body in space. Its study was a matter of great interest to astronomers like Sir Edmond Halley. Many decades later, comets still remain a challenging mystery in the field of astronomy. Space research institutes around the world have reported the existence of 3,628 comets, that are known and studied. Annually, an average of one comet is sighted by the naked eye from the surface of the Earth.
According to the definitions given by various space research organizations, a comet is a space body that travels in an elliptical orbit around the sun. When the comet comes in the vicinity of the star it is orbiting around, its comma-like tail gets illuminated. Majority of the comets are visible only through telescopes and other similar aids, however, some are also visible to the naked eye.
A comet is made up mainly of ice, dust, and different forms of gases. The nature of the components differ from comet to comet. The central part of a comet that looks like a ball, is termed as the nucleus. The nucleus is principally made up of ice and frozen gases. The frozen components also include other compounds, like ammonia, methane, and carbon dioxide. Due to its composition, it often resembles a huge ball of dirt and snow.
It is also believed by some astronomers, that the nucleus is rather fragile, because several comets have split into parts without any explicable reason, except immense velocity.
The huge elliptical orbit takes the comet in the range of the heat of the star around which it is orbiting. When it enters the inner parts of the solar system, the frozen content of the nucleus starts melting. The gases and water within the nucleus vaporize as a result of radiation from the star and solar wind. The immense velocity of the comet and the solar wind push the vaporized matter away from the nucleus. This gives the comet its distinct tail and its comma-like appearance. The high amount of heat and ions (charged particles that are formed due to the heat of the sun) cause its tail to glow.
According to their formation or birth, comets can be classified into two types. The first type is short-term comets, which originate in the Kuiper belt as space bodies beyond the orbit of Pluto. These enter the solar system and take the form of active and glowing comets. Long-term comets originate as traveling bodies in the Oort cloud (located around 50,000 light years away from the sun). The gravitational attraction of the planets in the solar system, and other gravitational forces, cause these to form elliptical orbits.
The study of comets has revealed that they have a fragile structure. Sometimes, they split into parts due to sheer velocity. In course of its orbit in the solar system, the amount of matter (ice and frozen gases) in it reduces. Due to this reduction in matter, they break up and disperse into clouds. A dead comet is nothing but an asteroid or a similar space body. Sometimes, comets also crash into other planets when their orbits cross each other. The most famous example being the Shoemaker Levy 9, that crashed into Jupiter.
For astronomers who have spent their lives studying comets, it is a very heart-breaking moment to gaze at a dying one. The Halley's Comet is said to be one of the most beautiful one around. Another very beautiful comet is the Hale-Bopp, that was discovered in the year 1995, on 23rd July.
It was predicted by astronomers that Halley's Comet would be sighted in the November of 1835. Mark Twain, the famous writer, was born just two weeks after the sighting. He developed a very special liking towards Halley's Comet. In his biography, he comments,"I came in with the Halley's Comet, and expect to go with it. The Almighty has said no doubt, now here are two unaccountable freaks; they came in together; they must go out together.", Halley's Comet was next sighted on 20th April, 1910. Twain ended the journey of his life the very next day.
The journey of any comet is indeed like that of Mark Twain; remarkably long, fast, and adventurous, as it streaks across the dark sky.