COMET

COMET, (Lat. 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 in 1705 he successfully predicted the return of the comet in 1758. 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 probes and by another instrumented package launched by the European Space Agency. Two Japanese craft observed the comet at a great distance as it passed.

The comet Hyakutake, which was discovered less than two months before it passed close to earth in March 1996, was found to emit X rays, confirming similar observations made during an all-sky survey in 1990. Comet Hale-Bopp, which may be the brightest comet in 400 years, was discovered in 1995 and passed closest to earth in March 1997.

For many years astronomers have supposed comets to consist of a small nucleus embedded in a nebulous disk called a coma. American astronomer Fred L. Whipple (1906–    ) proposed in 1949 that the nucleus, containing practically all the comet’s mass, is a “dirty snowball” consisting of ices and dust. This view was confirmed by the 1986 Halley studies.

The gases and particles ejected from comets providing their comae and tails consist mostly of molecules or radicals of elements common in space: hydrogen, carbon, nitrogen, and oxygen. The radicals of CH, NH, and OH that are observed may be broken away from the stable molecules CH₄ (methane), NH₃ (ammonia), and H₂O (water), which exist as ices in the nucleus. The escaping gases produce jet actions that propel the nucleus slightly away from its otherwise predictable path. Short-period comets (see below) observed over many revolutions, tend to fade slowly with time, as would be expected of such objects. Even Halley’s comet, although not a short-period comet, is losing about 100 million metric tons with each passage around the sun. Thus, it will also fade after several hundred more passes by the earth—a matter of several hundred more centuries.

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 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 the many comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous.

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; Halley’s comet has a period of 75–76 years. The orbits of most comets are vast and are probably ellipses of great eccentricity, with periods as long as 40,000 years or possibly much longer.

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 (1937–    ) 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.

Although no detailed theory of origin is generally accepted, many astronomers now believe that comets originate in the outer, colder edges of the solar system, where matter remains as it was at the time when the solar system formed. A Dutch astronomer, Jan Hendrik Oort, proposed in 1950 the existence of a “storage cloud” of such material surrounding the solar system at a great distance (as much as 50,000 to 150,000 times more than the distance from the sun to earth), while the Dutch-American astronomer Gerard Peter Kuiper proposed in 1951 a denser, nearer cloud, just beyond Neptune’s orbit but lying more in the plane of the system. Passing galactic visitors, such as molecular clouds or brown dwarfs, may then disturb these clouds and send comets in toward the realm of the planets. The first such comets were discovered in the Kuiper cloud in 1992; by 1996, 31 had been found. The Oort cloud accounts for long-period comets, such as Halley’s and the Swift-Tuttle (with a period of 130 years), and the Kuiper cloud for short-period comets (on the order of 10 years).

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. Some scientists suggest, however, that collisions have taken place in the astronomical past. A somewhat related theory that became controversial during the 1980s is that the earth is constantly being bombarded by tiny, hundred-ton ice comets that were the source of the earth’s oceans over geological time. Most scientists, however, have discounted this notion.