Tucked away in the far fringes of the Solar System is a great void space. This is a rather quite eventless place of the Universe. From here, a bright yellow star, the sun, can easily be seen. The Sun never appears to the brightest star as seen from this strange place. Rather stars like Sirius and Canopus appear brighter. Attached to the weak gravity of the Sun are a band of space rocks. They have been existing there for several million years and going around the Sun at nearly at an airplane’s speed.
Due to the irregular motion of the stars around the centre of our galaxy, occasionally some stars come closer to these rocks and push them a little. This force is sufficient for them to slip away from their orbits. Out of these, some head into interstellar space never to be seen again. But paths of a few are very interesting! They bend towards the Sun. They are the comets. At this point time, the comet is simply a chunk of ice which is called its ‘nucleus’.
As the comet approaches the Sun, the solar heat vaporizes the materials from its surface. Tiny ice crystals that make most of the surface become unstable and explode into a jet of gas. As the comet nears further and its darkened surface is flashed by bright sunlight, hundreds of jets shoot up into the comet’s sky. The nucleus is now surrounded by a bright envelope of gas and dust. This is the second structure of the comet known as the ‘coma’. A typical coma may stretch upto tens of thousands kilometres in diameter. Now the comet has moved very close to the Sun. The dust particles that form the comet are blown away by the strong radiation pressure of the sun. This gives rise to a curved ‘dust’ tail. This is the third and most spectacular structure of the comet. It may extend upto several million kilometers in space. Now, it appears that there is not just one tail. A straight hazy tail is also visible. This is the ‘gas’ tail consisting of ions which absorb ultraviolet light from the Sun to radiate visible light.
Now, the comet has become a spectacular sight in the sky. Its bizarre shape attracts everybody’s attention. The comet has come a long way from its remote home to become visible. It also grips people with fear and panic and is normally associated with doom, disaster and death of kings. Eventually, the comet turns away from the Sun and the materials that formed its tail are swept away into space. Its violent processes quickly subside in the frigid interstellar temperature and the comet sheds most of its materials. Finally, it returns to its quite home in the far fringes of the solar system.
Periodic comets used to come to our sky in a particular interval of time to be seen again and again. Halley’s Comet, a short period comet having a period of 76 years, belong to this category. Long period comets can have periods ranging upto several hundred years. Amongst the spectacular comets seen in recent past are the comet West, Bennet, Ikeya-Seki, Hyakutake and Hale-Bopp, to name a few. Collision of a comet with the earth may result in a major catastrophe that can eventually end life on our planet. The mass extinction of dinosaurs some 65 millions years ago is believed to have been caused due to one of such impacts. Comets also prove to be boon as well. Life on the earth is believed to have been transferred from a comet during one of such collisions long ago. Also due to very low temperature, the comets preserve materials that formed our early solar system, and hence, serve as messengers from space for the astronomers to study the evolutionary process of our solar system.