A comet is a nebulious celestial body orbiting the sun. They have a head and a long luminous tail which only appears when the comet approaches the sun in its orbit.
The appearance of comets were treated as atmospheric phenomena until 1577, when the Danish astronomer Tycho Brahe proved that they were celestial bodies. Newton too demonstrated that the movement of comets and planets in their orbits are identical. British astronomer Edmund Halley showed the comet of 1682 to be identical with the two that had appeared in 1607 and 1531, and he successfully predicted the return of the comet named after him in 1759.
The earlier appearances of Halley's comet have now been identified from records dating from as early as 240 BC, and it is probable that the bright comet observed in 466 BC was also an apparition of this famous comet. Halley's comet most recently passed around the Sun again early in 1986 and will return in 2066.
American astronomer Fred L. Whipple proposed in 1949 that the nucleus is a dirty snowball conglomerate of ices and dust. Major proofs of the snowball theory rest on various data. Maybe of the observed gases and particles that are the coma and tails of comets, most of the gases are fragmentary radicals of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen.
The head of a comet, including the hazy coma, may exceed the planet Jupiter in size. The solid portion of most comets, however, is equivalent to only a few cubic kilometers. The dust-blackened nucleus of Halley's comet, for example, is about 60km2 in size.
As a comet approaches the Sun, the heat from the sun evaporates the ices so that the comet brightens enormously. It may develop a brilliant tail, sometimes extending many millions of kilometers into space. The tail is generally directed away from the Sun, even as the comet recedes again. The great tails of comets are composed of simple ionized molecules. Comets frequently also have tails which are smaller composed of fine dust.
As a comet moves away from the Sun, the loss of gas and accompanying dust decreases in quantity, and the tails disappear. Some of the comets with small orbits have tails so short that they are practically invisible. On the other hand, the tail of at least one comet has exceeded 320 million km in length. The variation in length of the tail, together with the closeness of approach to the Sun and the Earth, accounts for the variation in the visibility of comets.
Comets have elliptical orbits, and the periods of about 200 comets have been calculated. They range from 3.3 years for Encke's comet to 2000 years for Donati's comet of 1858. Most comet's orbits are indistinguishable from parabolas - open curves that would take the comets out of the solar system. Actually, they are ellipses with orbits of more than 40 thousand years. About 60 short-period comets have orbits that have been influenced by the planet Jupiter. Their periods range from 3.3 to 9 years.
Comets are also divided into groups. A group includes several comets with different periods travelling in nearly the same orbit. The most famous group includes the spectacular Sun-grazing comet, Ikeya-Seki, of 1965, and seven others having periods of nearly a thousand years. Actually, the 1965 comet and the even brighter comet of 1882 split from a parent comet, possibly the one of 1106 which probably split away from a truly giant comet thousands of years ago.
Comets were once believed to come from interstellar space. Although no detailed theory of origin is generally accepted, many astronomers now believe that comets originated in the outer, colder part of the solar system from residual planetary matter in the early days of the solar system. Comets have long been regarded by the superstitious as influential.
The appearance of a comet has also given rise to the fear of collision between the comet and the Earth. The Earth, in fact, has passed through the tails of a few comets without being destroyed. The collision of the nucleus of a comet with a large city would probably destroy the city but the probability of such an event occurring is exceedingly small. Of course, these collusions have probably had a role in the extinction of the dinosaurs.
During March to April 1997, the comet Hale-Bopp which comes only once in a few thousand years visited Earth. It was visible from most parts of the earth. In fact, it generated so much excitement that when a leading local newspaper published an April Fool's Joke on the comet visible from Sabah, many people fell for the trick.
| Did you know...
The nucleus contains nearly all the mass of the comet! |
Wow!
Of some 1400 comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were clearly visible! |
| Pop Quiz
What does a comet group include? |
 Bennet's Comet |
 Halley's Comet in early days |
 Comet classification by period |
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