ETYM Latin cometes, cometa, from Greek, comet, prop. long-haired, from koman to wear long hair, from kome hair.
(Astronomy) A relatively small celestial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit.
Small, icy body orbiting the Sun, usually on a highly elliptical path. A comet consists of a central nucleus a few miles across, and has been likened to a dirty snowball because it consists mostly of ice mixed with dust. As the comet approaches the Sun the nucleus heats up, releasing gas and dust which form a tenuous coma, up to 100,000 km/60,000 mi wide, around the nucleus. Gas and dust stream away from the coma to form one or more tails, which may extend for millions of miles.
Comets are believed to have been formed at the birth of the Solar System. Billions of them may reside in a halo (the Oort cloud) beyond Pluto. The gravitational effect of passing stars pushes some toward the Sun, when they eventually become visible from Earth. Most comets swing around the Sun and return to distant space, never to be seen again for thousands or millions of years, although some, called periodic comets, have their orbits altered by the gravitational pull of the planets so that they reappear every 200 years or less. Of the 800 or so comets whose orbits have been calculated, about 160 are periodic. The brightest is Halley’s comet. The one with the shortest known period is Encke’s comet, which orbits the Sun every 3.3 years. A dozen or more comets are discovered every year, some by amateur astronomers.
A vast amount of data concerning comets and their orbits have been accumulated, much of it consistent with the hypothesis that they have their origin in a reservoir of appropriate material on the confines of the solar system. This material and the lumps into which it accretes move round the Sun in long-period orbits, relatively few of which have perihelion distances of less than 50 astronomical units. Every now and again, however, some of these orbits are perturbed, either by mutual action or by the action of passing stars, so that lumps traveling in them will pass sufficiently close to the Sun to become visible as comets. Comets are divided into two classes according to their orbital period, those with periods less than 200 years being known as short-period, the remainder as long-period comets. The orbits of the long-period comets are inclined at all angles to the ecliptic, and about equal numbers of them are direct and retrograde. The short-period comets, on the other hand, move mainly in direct orbits that.
Lie close to the mean plane of the solar system and have aphelion distances close to the orbit of Jupiter. Such comets are sometimes referred to as belonging to Jupiter’s family, and there are similar, but less numerous, families associated with the other major planets. There is little doubt that they are long-period comets that have been captured. B. G. Marsden’s Catalog of Cometary Orbits compiled in 1972 lists 97 short-period comets and 503 long-period ones. Most comets are named for their discoverers and denoted by letters in order of discovery each year, but are subsequently numbered in order of their perihelion passage; e.g. the Arend-Roland Comet was 1956h but became 1957 III. Some of the orbits of the long-period comets so closely resemble each other that there is little doubt that they are traced out by fragments of a former bigger comet that broke into pieces as it passed round the Sun. One of the best known of these groups is that of the bright Sun-grazing comets which includes the Great Comet of 1.
668, 1843 I, 1880 I, 1882 II, 1887 I, 1945 VII, 1963 V, 1965 VIII, 1970 VI, and possibly one or two others. These comets passed through the solar corona and near perihelion were bright enough to be observed in daylight. Few of the short-period comets show conspicuous tails, presumably because they have lost most of their volatile material. Some disintegrate and are not seen again, though some of the fragments may cause periodic meteor showers.