Comets are nebulous celestial bodies orbiting the sun. A comet is characterised by a long, luminous tail, but only in the segment of the comet's orbit when it passes closest to the sun. Comets are commonly distinguished from other components of the solar system by their rather nebulous appearance and extremely elongated orbits.
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 kilometres. The dust-blackened nucleus of Halley's comet, for example, is about 15 by 4 km (about 9 by 2.5 miles) in size with an estimated mass of 1017 grams.
As the cometary nucleus approaches the Sun, its dust surface becomes hotter, more heat is transferred through the crust, and the subsurface ice begins to sublimate. The resultant gas leaves the comet and carries with it some of the loosely bound dust particles. Sublimation starts when the comets are closer than about three astronomical units from the Sun (one astronomical unit [AU] equals about 150,000,000 km, or 93,000,000 miles). The chemical composition of the vaporising gases is dominated by water (about 80 percent), followed by carbon monoxide, carbon dioxide, methane, ammonia, and carbon disulphide, quantitatively in that order. Parent molecules leaving the nucleus quickly break up into daughter molecules, radicals, and ions. These absorb solar radiation and subsequently reradiate it at another wavelength. The dust also scatters sunlight. When within 1 AU of the Sun, a typical cometary nucleus is surrounded by a spherical envelope, or coma, of gas and dust, which can be up to 100,000 km (62,000 miles) across. The coma gases travel outward at a speed of roughly 0.6 km per second, dragging dust particles away from the nucleus.
On nearing the Sun, a comet may develop two tails. The solar wind of high-speed protons and electrons sweeps cometary ions in a direction away from the Sun, producing a straight plasma tail. A second tail consisting of dust particles about a micrometer in size may appear. This dust tail has a greater curvature than the plasma tail and is usually shorter. It also points away from the Sun because of the repulsive force exerted by solar radiation pressure on the minute particles. Larger particles released from the nucleus take up orbits that have nearly the same parameters as the parent comet. Some gain on the comet and others fall behind until eventually an annulus of dust is formed around the comet's orbit. This is called a meteoroid stream. A meteor shower is produced in the Earth's upper atmosphere when it passes through such a stream. 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 miles) 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.
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.
Some accretion models of the solar system suggest that an early cometary bombardment of the Earth may have played a major role in the formation of the atmosphere and the oceans. In addition, comets may have supplied the organic molecules needed for life to develop on the planet. Such theories remain unproved.
Comets are generally divided into short-period comets (those with periods of less than 200 years) and long-period comets (those with periods of more than 200 years). Among those comets that can be seen easily with the naked eye, Comet Halley, with an average period of 76 years, is the only one that returns in a single lifetime.
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.
Cometary bodies were investigated for the first time with spacecraft during the mid-1980s. In 1985 the U.S. probe known as the International Cometary Explorer (ICE) passed through the dust tail of Comet Giacobini-Zinner. The following year, spacecraft launched by Japan and the Soviet Union, along with the aforementioned Giotto probe of the European Space Agency (ESA), flew by Comet Halley, transmitting many useful data about its composition and sending photographs of its nucleus and coma.
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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