comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months. They were long regarded with awe and even terror and were often taken as omens of unfavorable events. In 1987 a comet orbiting the star Beta Pictoris was identified, and since then an increasing number of exocomets have been found.
The Orbits of Comets
Although the occurrence of many comets had been recorded, it was not until 1577 that the Danish astronomer Tycho Brahe suggested that they traveled in elongated rather than circular orbits. A century later Giovanni Borelli concluded that the orbits were parabolic and that comets passed through the solar system but once, never to return. In 1705, however, Edmond Halley concluded that the comet observed in 1682 was the same one that had been described in 1531 and 1607, and he predicted that it would return again in late 1758 or early 1759. The comet was sighted on Christmas Day in 1758, and it returned again in 1835, 1910, and 1986 (see Halley's comet). While some comets appear to have parabolic orbits (see parabola), others return to the inner solar system in highly elongated orbits with periods ranging from a hundred to thousands of years. Still others return at shorter intervals of less than 10 years and reach aphelion (the orbital point farthest from the sun) near the planet Jupiter; these have been captured into their smaller orbits by Jupiter's gravitational attraction.
Structure of Comets
A comet far from the sun consists of a dense solid body or conglomerate of bodies a few miles in diameter called the nucleus. As it approaches the sun the nucleus becomes enveloped by a luminous "cloud" of dust and gases called the coma; this luminosity is caused by the molecules absorbing and reflecting the radiation of the sun. According to the icy-conglomerate theory proposed by F. L. Whipple in 1949, the nucleus consists of frozen water and gases with particles of heavier substances interspersed throughout, thus being in effect a large, dirty snowball, although more recent research has suggested that comets may contain a higher proportion of dust and rock than previously proposed. The Stardust probe—passed near Comet Wild 2 in 2004, collected particles from the coma, and returned the samples to earth in 2006—found evidence that many of the dust particles were formed at high temperatures not found in the Oort cloud and Kuiper belt (see below), where comets are believed to have formed. Data from the Deep Impact mission, which sent a projectile crashing into Comet Tempel 1 in 2005, suggests that suggests that the interior structure of comets may consists of layers of accreted material. As the comet approaches the sun, the solar wind drives particles and gases from the near the surface of the nucleus and coma to form a tail which can extend as much as 100 million mi (160 million km) in length. Thus the tail always streams out in the direction opposite the sun; i.e., it follows the head as the comet approaches the sun and precedes it as the comet passes perihelion (its closest point to the sun) and moves away.
Near the sun a comet can change drastically in size and shape; it may even split into two or more pieces, as Comet Biela did in 1846, and Comet West did in 1976. The comas of comets vary widely in size, some being the size of the earth or larger. However, the nucleus, which makes up virtually all a comet's mass, is small; in 1986 the Giotto and Vega spacecrafts observed Comet Halley's nucleus to be only about 6 mi (10 km) in diameter. In 2014 Rosetta became the first space probe to orbit a comet's nucleus (that of Comet 67P); it also deployed a lander on the comet. Comets lose material and thus brightness with successive passages near the sun. Some of this material moves around the comet's orbit as a stream of meteoroids (see meteor); when the earth passes through this path, a meteor shower is observed.
In 1992 the periodic comet Shoemaker Levy 9 made an extremely close passage of Jupiter. The tidal stresses induced by the giant planet's gravity shattered the comet's nucleus, estimated to have been 5–9 km (3–5 mi) in diameter, into more than 20 major fragments, the largest of which was about 4 km (2.5 mi) in diameter. Two years later, the returning fragmented comet crashed into Jupiter; observations from both terrestrial observatories and artificial satellites such as the Hubble Space Telescope yielded vast amounts of information about the structure of comets and about Jupiter's atmosphere.
In 1996 the Polar satellite discovered a constant rain of small comets impacting the earth. Unlike large comets, whose cores are estimated to be as much as 25 mi (40 km) in diameter, these are only up to 40 ft (12 m) wide. It is estimated that as many as 43,000 reach the earth each day and break up at altitudes of 600–15,000 mi (950–24,000 km). Also in 1996 the ROSAT satellite (see X-ray astronomy) detected X-rays emanating from the Comet Hyakutake. This was completely unexpected, and can be explained by no known mechanism. Observation of more large comets passing through the solar system by orbiting X-ray observatories will be necessary to corroborate this finding.
The Origin of Comets in the Solar System
The Oort Cloud
The origin of the solar system's comets is still uncertain. They were once thought to have originated outside the solar system, but more recent theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis proposed by J. H. Oort and since modified, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 20,000 AU to 50,000 AU or greater (1 AU, or astronomical unit, being the mean distance from the earth to the sun); some astronomers have suggested an inner Oort cloud exists beginning at 2,000 to 5,000 AU and extending to 20,000 AU. While the comets move very slowly in this huge storage cloud, a passing star may change the orbits of bodies in the outer reaches of the Oort cloud enough to force some of them into the inner part of the solar system. The mechanism for the Oort cloud's creation, however, is unclear; it has been suggested that the Oort cloud may include a significant amount of material that originated outside the solar system and was gravitationally captured by the sun.
The Kuiper Belt
In 1951, G. P. Kuiper, noting that Oort's cloud of comets did not adequately account for the population of short-period comets (those making complete orbits around the sun in less than 200 years), proposed the existence of a disk-shaped region of minor planets outside the orbit of Neptune, now called the Kuiper belt, as a source for such comets. The Kuiper belt acts as a reservoir for these in the same way that the Oort cloud acts as a reservoir for the long-period comets. This theory was validated in 1992 with the discovery of the first of the more than 70,000 so-called transneptunian objects, bodies more than 60 mi (100 km) in diameter in an orbit 30–50 AU from the sun. Astronomers regard Pluto not as a planet but rather as dwarf planet that is a member of the Kuiper belt. The discoveries of several Kuiper belt objects led to this view. Eris, an object discovered in 2003 (and originally nicknamed Xena), has an elongated orbit that extends to roughly three times the distance of Pluto's, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto, and has a moon; Quaoar is more than half the size of Pluto; and Ixion and Varuna are almost half the size of Pluto. 2003 VS2 (roughly a fourth the size of Pluto) and a number of other Kuiper belt objects, called plutinos, have an orbital synchrony with Neptune like that of Pluto (Neptune completes three orbits around the sun in the same time that Pluto and the plutinos complete two orbits).
See D. Yeomans, Comets (1991), C. Sagan and A. Druyan, Comet (1997), D. H. Levy, Comets (1998), G. Kronk, Cometography (1999), and D. J. Eicher, Comets! (2013).