COMETS Comets are celestial bodies of small mass that travel around the Sun (see SOLAR SYSTEM), usually in elongated orbits. They become visible as they near the Sun, and sometimes they form a tail. For centuries, comets have been considered harbingers of catastrophe. Their appearances, and sometimes their motions, were accurately chronicled. Babylonian and Chinese astronomers believed that comets were celestial bodies that moved through space just like the planets. The Greeks considered comets a phenomenon in the Earth's atmosphere--a kind of vapor or exhalation from the Earth. This view was later generally accepted. Only in the 16th century did Tycho BRAHE establish that comets are not atmospheric phenomena but are considerably farther away from the Earth than the Moon is. A century later Sir Isaac NEWTON discovered a method of deriving the true orbit of a comet from its observed trajectory in the sky, and he determined that the comet of December 1680 followed a very elongated, parabolic orbit. Edmond HALLEY, a contemporary of Newton's, found that the orbits of the comets of 1531, 1607, and 1682 were almost identical, a discovery that led him to conclude that they were in reality a single comet, whose return he predicted in 1758. The return actually occurred, and since then the comet has been known as HALLEY'S COMET. It has been observed 20 times since 239 BC, its most recent appearance occurring in 1985-86. A newly discovered comet is provisionally designated by the year of discovery and a letter indicating its order in comet sightings that year. Once the date at which the comet reaches perihelion (closest approach to the Sun) is determined, the comet is officially designated by the calendar year followed by a Roman numeral specifying the chronological order of the perihelion passages in that year (for example, 1882 II). Some comets are named for their discoverers. Thus the comet IRAS-Araki-Alcock, which was discovered in 1983 and came within 4.7 million km (2.9 million mi) of Earth--the closest known comet approach in more than 200 years--was named for the Infrared Astronomy Satellite and the two amateur astronomers who first detected the comet. At present comets are sought with the help of large reflecting telescopes, which can photograph extensive parts of the sky simultaneously. This technique is used by many amateur astronomers. There are about ten comet discoveries a year. On the average, one comet every three years is visible without a telescope. In 1985 the U. S. Interplanetary Sun Earth Explorer (ISEE) 3 was maneuvered into an encounter with comet P/Giacobini-Zinner, and in 1986 the European Space Agency, Japan, and the Soviet Union all obtained closer views of Halley's comet with their separate spacecraft. COMET ORBITS All comet orbits that have been established are elliptical. Short-period comets have periods of less than 200 years, and their orbits are mostly inclined at a small angle to the orbital plane of the Earth (the ecliptic). The comet with the shortest known period is Encke's comet (3.3 years). Long-period comets have periods of more than 200 years--some may reach several thousand years--and their orbital planes may lie at various angles to the ecliptic. About 100 of each group of periodic comets are known, and 800 nonperiodic comets have been observed. Some comets observed only once appear to have parabolic or hyperbolic orbits that would bring them near the Sun only once, suggesting a possible origin beyond the solar system, but lack of data may account for such seeming orbits. Almost all known comets approach to between 0.005 and 2.5 astronomical units of the Sun at perihelion (1 AU = mean Earth-Sun distance). If a comet's perihelion is farther from the Sun than 2.5 AU, it is usually not observable. Many comets have their aphelia (points of greatest distance from the Sun) in the region of the outer planets. A group of about 75 comets known as the Jupiter family have their aphelia near the orbit of Jupiter. There are other comet groups consisting of comets that move in strikingly similar orbits around the Sun. The members of such a group are the remnants of a larger comet that broke up because of tidal forces exerted by the Sun or a planet. PHYSICAL NATURE OF COMETS Nucleus and Coma Almost the entire mass of a comet is concentrated in its nucleus. The diameter of the nucleus is on the order of a few kilometers. The density, between 0.1 and 1 g/cu cm (6 and 60 lb/cu ft), indicates that the nucleus is very tenuous. According to Fred L. WHIPPLE's "dirty snowball" model--confirmed by recent comet observations--the nucleus consists of a conglomerate of such compounds as water, carbon dioxide, ammonia, and methane, all frozen and mixed with grit and dust. When the comet approaches the Sun, this frozen matter sublimes and forms a cloud of gas and grit--called the coma--around the nucleus. Closer to the Sun, the production of gases increases. The gas and dust particles are repelled from the nucleus by the solar RADIATION PRESSURE and the SOLAR WIND (a stream of charged particles), forming the tail. The average diameter of the coma is about 100,000 km (62,000 mi), but its mass and density are small. Some molecules are decomposed and ionized by ultraviolet light from the Sun on their way from the nucleus to the tail. The chemical composition of the nucleus can be discovered indirectly by spectral analysis of this released gas. The main products observed are hydrogen and oxygen atoms, water, and hydroxyl (OH) radicals. The coma of a comet generally becomes smaller as the distance from the Sun decreases. Near the Sun, the molecules of the coma are decomposed more rapidly by the solar wind and pulled into the tail. The apparent brightness of a comet depends on its distance from the Sun and from the Earth: the brightness depends on about the fourth power of the distance from the Sun, which indicates that a comet not only reflects light, but also absorbs and then itself emits light. Consequently the brightness may increase quite rapidly upon approach to the Sun. Some comets show abrupt, striking increases in brightness in the vicinity of the Sun, the result of temporary increases in solar activity. Other comets fade away upon approaching the Sun, probably because the nucleus disintegrates (for example, Comet Ensor, 1926 III). In some short-period comets the brightness decreases slightly with each revolution, probably owing to loss of matter. Tail When a bright comet becomes visible, the most noticeable feature is the tail. At the appearance of Halley's comet in 1910, the tail stretched for more than 90 deg over the celestial sphere. During the comet's most recent appearance, however, this elongation of the tail took place while the comet was on the far side of the Sun from the Earth, so that the show was far less dramatic. In the majority of faint comets, no tail is ever observed. The length of the tail ranges from 1 to 100 million km (0.62 to 62.14 million mi). Usually it first appears at a distance of about 1.5 AU from the Sun. Despite the enormous size of a comet's tail, it usually contains less material in 1 cu km than in 1 cu mm of ordinary air. The tail is formed of gas from the coma and always points away from the Sun. It was initially thought that the solar radiation pressure alone was responsible for driving the tail away; it is now clear that the solar wind has a far greater effect. The solar wind consists of charged particles ejected from the Sun. The force exerted by these particles on the gas molecules of the coma is about 100 times stronger than the gravitational force of the Sun, so that the molecules are pulled along by the wind. The solar wind is not constant, and its variations are responsible for the threadlike structure of the tail. Solar flares or other perturbations on the Sun can sometimes be seen to affect the tail: it becomes turbulent and is sometimes bent. A comet may have one of two types of tail, and many comets have both types--a double tail. One is elongated and almost straight, has a fibrous structure, and consists of ionized gases. This type of tail is called a Type I tail, a gas tail, or a plasma tail. Type II tails, or dust tails, are more strongly curved and hazier; they consist of dust repelled by sunlight. A comet may possess several dust tails in addition to a gas tail. Some comets have an anomalous tail, or antitail, which points toward the Sun (for example, the Arend-Roland comet, 1957 III). The antitail is actually a very thin layer of dust lying along the orbit of the comet, having been expelled at an earlier stage. DISINTEGRATION OF COMETS Many comets, especially short-period ones, slowly disintegrate, mainly under the influence of the Sun's gravitational force. Recently, several collisions of comets with the Sun have been observed with coronographs aboard satellites. A regular decrease in the brightness of short-period comets is often observed. Comets also leave waste products behind in their orbits, in the form of millions of meteoroids. When the Earth crosses such an orbit, METEOR showers are frequently observed EARTH COLLISIONS Scientists speculate that collisions of comets--or cometary fragments--with the Earth may occasionally occur, with devastating results. In one hypothetical scenario, past cometary collisions are thought to have thrown enough dust into the Earth's atmosphere to have caused the extinctions of some species of plants and animals (see EXTINCTION). Also, the impact of a comet, or piece of a comet, remains one of the more plausible explanations for a tremendous blast that occurred in the Tunguska region of Russia in June 1908. ORIGIN OF COMETS Various theories have been developed in recent centuries to account for comets, but the one most widely accepted at present is that comets were formed at the same time as the rest of the solar system. In 1950 the Dutch astronomer Jan OORT proposed that the Sun is surrounded by an enormous "cloud" of comet material at a distance about 1,000 times that of the radius of the known solar system. This theory was followed in 1951 by Dutch-American astronomer Gerard KUIPER's proposal that a ring of cometary material lies in the plane of the solar system, several hundred times as far from the Sun as the Earth is. Both proposals have been widely accepted, with some astronomers suggesting that the Oort and Kuiper clouds actually merge along the Oort cloud's much denser inner boundary. Perturbations by interstellar clouds or passing stars would cause some of the comet materials to dislodge and enter the inner solar system in the form of comets, with short-period comets more likely arising from the Kuiper belt. Steven J. Dick Bibliography: Brandt, John C., and Chapman, Robert D., Introduction to Comets (1981); Brandt, John C., and Niedner, Malcolm B., Jr., "The Structure of Comet Tails," Scientific American (January 1986); Delsemme, A. H., "Whence Come Comets?" Sky & Telescope, March 1989; Sagan, Carl, and Druyan, Ann, Comet (1985); Whipple, F. L., The Mystery of Comets (1985); Yeomans, Donald K., Comets: A Chronological History of Observation, Science, Myth, and Folklore (1991).