Comet (Latin stella cometa, "hairy star"), nebulous celestial body revolving around the sun. A comet is characterized by a long, luminous tail, but only in the segment of the comet's orbit when it passes closest to the sun.
Appearances of large comets were regarded as atmospheric phenomena until 1577, when the Danish astronomer Tycho Brahe proved that they were celestial bodies. In the 17th century the British scientist Sir Isaac Newton demonstrated that the movements of comets are subject to the same laws that control the planets in their orbits. By comparing the orbital elements of a number of earlier comets, the 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 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. As it once more headed outward, it was visited in March of that year by two Soviet-constructed probes, Vega 1 and 2, and by another instrumented package called Giotto, launched by the European Space Agency. Two Japanese craft observed it at a great distance as it passed.
A comet is generally considered to consist of a small, sharp nucleus embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple proposed in 1949 that the nucleus, containing practically all the mass of the comet, is a "dirty snowball" conglomerate of ices and dust.
Major proofs of the snowball theory rest on various data. For one, of the observed gases and meteoric particles that are ejected to provide the coma and tails of comets, most of the gases are fragmentary molecules, or radicals, of the most common elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for example, of CH, NH, and OH may be broken away from the stable molecules CH4 (methane), NH3 (ammonia), and H2O (water), which may exist as ices or more complex, very cold compounds in the nucleus. Another fact in support of the snowball theory is that the best-observed comets move in orbits that deviate significantly from Newtonian gravitational motion. This provides clear evidence that the escaping gases produce a jet action, propelling the nucleus of a comet slightly away from its otherwise predictable path. In addition, short-period comets, observed over many revolutions, tend to fade very slowly with time, as would be expected of the kind of structure proposed by Whipple. Finally, the existence of comet groups shows that cometary nuclei are fairly solid units.
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 15 by 4 km (about 9 by 2.5 mi) in size.
As a comet approaches the sun, the solar heat evaporates, or sublimates, 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, including carbon monoxide and dioxide. The molecules are blown away from the comet by the action of the solar wind, a thin stream of hot gases continuously ejected from the solar corona, the outermost atmosphere of the sun, at a speed of 400 km (250 mi) per sec. Comets frequently also display smaller, curved tails composed of fine dust blown from the coma by the pressure of solar radiation.
As a comet recedes 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 (200 million mi) 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. Of some 1400 comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous.
Periods and Orbits
Comets have elliptical orbits, and the periods of about 200 comets-the time they take to orbit the sun once-have been calculated. They range from 3.3 years for Encke's comet to 2000 years for Donati's comet of 1858. The orbits of most comets are so vast that they are indistinguishable from parabolas-open curves that would take the comets out of the solar system-but from technical analyses astronomers assume that they also are ellipses, of great eccentricity, with periods as long as 40,000 years or possibly much longer. The bright comet Hyakutake, which was visible from earth in 1996, has an estimated period of 10,000 years.
No comets have been known to approach the earth on a hyperbolic orbit; this would have meant an origin in outer space. Some comets, however, may never return to the solar system because of extreme alteration of their original orbits by the gravitational action of the planets. Such action has been observed on a smaller scale. About 60 short-period comets have orbits that have been influenced by the planet Jupiter, and are said to belong to the family of Jupiter. Their periods range from 3.3 to 9 years.
When several comets with different periods travel in nearly the same orbit, they are said to be members of a comet group. The most famous group includes the spectacular sun-grazing comet, Ikeya-Seki, of 1965, and seven others having periods of nearly a thousand years. The American astronomer Brian G. Marsden has concluded that the 1965 comet and the even brighter comet of 1882 split from a parent comet, possibly the one of 1106. This comet and others of the group probably split away from a truly giant comet thousands of years ago.
A close relationship also exists between the orbits of comets and the orbits of meteor showers. The Italian astronomer Giovanni Virginio Schiaparelli proved that the Perseid meteors, which appear in August, move in the same orbit as Comet 1862 III. Similarly, the Leonid meteors, which appear in November, were found to follow the same orbit as Comet 1866 I. Several other showers have been related with known cometary orbits, and are explained by the earth-type debris scattered by a comet along its orbit.
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. The Dutch astronomer Jan Hendrik Oort has proposed that a "storage cloud" of comet material has accumulated far beyond the orbit of Pluto, and that the gravitational effects of passing stars may send some of the material sunward, where it becomes visible as comets.
Comets have long been regarded by the superstitious as portents of calamity or important events. 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 occasional comets without measurable effect. 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. Some scientists suggest, however, that collisions have taken place in the astronomical past and may even, for example, have had a climatic role in the extinction of the dinosaurs.
In 1992 Comet Shoemaker-Levy 9 broke apart into 21 large fragments as it ventured into the strong gravitational field of the planet Jupiter. During a week-long bombardment in July 1994, the fragments crashed into Jupiter's dense atmosphere at speeds of about 210,000 km/hr (130,000 mph). Upon impact, the tremendous kinetic energy of the comets was converted into heat through massive explosions, some resulting in fireballs larger than the earth.
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