Many have regular orbits and appear at predictable time intervals. In 1705, Edmund Halley hypothesized that four previously assumed to be individual entities were actually one comet with a regular orbit, based on his awareness that all items in space were affected by Newton’s law of gravity. Halley successfully predicted that the comet would appear again in 1758.

Comets are normally equally distributed between those that have direct orbits and revolve around the sun in the same manner as planets, and those that have retrograde orbits and revolve in a manner opposite to the planets. Orbits are mea­sured in Earth years.

Comets are categorized by their orbits and fall into three categories. More than 1,000 comets have fairly well defined orbits. The most widely studied comets are short-period, or periodic, com­ets that orbit around the sun every 200 years or less. Popular short-period comets include Encke: 3.3 years; Halley: 76.1 years; Kopf: 6.5 years; Temple: 5.3 years; and Wolf: 8.4 years. Long- period comets are those whose orbits are longer than 200 years but are still influenced by the gravitational forces of the sun. The Hale-Bopp comet has an orbital period of 2,400 years, and the orbital period of Hyakutake is 31,000 years. Some comets appear once and do not find an orbit but pass around the sun and get lost in space. These are referred to as single-apparition comets.

Although generally regular, orbits of comets are not always consistent. While there are more than 100 short-period comets whose orbits are mapped, astronomers occasionally lose track of one of them. Likewise, although infrequently, a comet that is thought to be “new” turns out to be a for­merly “lost” comet. An example of this is Comet 11P/Tempel-Swift-LINEAR, which was discov­ered in 1869, lost after 1908 due to disturbances caused by Jupiter, and rediscovered in 2001.

Comets do not last forever. Each time they pass around the sun, comets lose some of their matter. Eventually, the comet will be reduced to a meteor. Comets can also collide with the sun or a planet and be destroyed. Some comets also develop para­bolic or hyperbolic orbits that shoot them out of the .

Comets are sometimes referred to as “dirty snowballs” due to their makeup of 50% gas and dust and 50% that is primarily frozen gas. They are believed to be ejected from either the or the . Most originate from the , an outlying area of the solar system hypoth­esized by the Dutch astronomer Jan Hendrik Oort. Some of the short-period comets develop from the Kuiper belt, also known as the transneptunian region, a theoretical collection area of celestial debris beyond the orbit of Neptune described by K. E. Gerard Kuiper in 1951. Following ejection, the comet becomes caught in the sun’s gravitation field and develops an orbit around the sun.

As comets get closer to the sun, the ice begins to melt, forming a gaseous cloud tail called a coma. There is also a second tail of dust and gas, though the two are generally perceived as one tail. Sometimes the tail of the comet will be seen in front of the comet since the dust and gas tails are lighter than rest of comet, and the solar winds can push the tail out in front of the comet when the comet is close to the sun. When the earth passes through the orbit of a comet, meteor showers are created. For example, in May and October every year, the earth encoun­ters the meteor stream from Halley’s comet.

Beth Thomsett-Scott

See also Extinction and Evolution; Life, Origin of; Meteors and Meteorites; Nuclear Winter; Omens; Star of Bethlehem; Time, Galactic; Time, Sidereal

Further Readings

Levy, D. H. (1998). Comets: Creators and destroyers. New York: Touchstone.

Schechner Genuth, S. (1997). Comets, popular culture, and the birth of modern cosmology. Princeton, NJ: Princeton University Press.

Whipple, F. L. (1985). The mystery of comets. Washington DC: Smithsonian Institution Press.

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Christopher Columbus

Christopher Columbus

Auguste Comte

Auguste Comte