comet

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 60,000 mi/100,000 km 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. US astronomers concluded in 1996 that there are two distinct types of comet: one rich in methanol and one low in methanol. Evidence for this comes in part from observations of the spectrum of the comet Hyakutake.

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 towards 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. Periodic comets are thought to come from the Kuiper belt, a zone just beyond Neptune. 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 has been accumulated, much of it consistent with the hypothesis that they have their origin in a reservoir of appropriate material beyond the orbit of Neptune. This material and the lumps into which it accretes move round the Sun in long-period orbits, relatively few of which have perihelion distances (that is, distances when closest to the Sun) of less than 50 astronomical units (50 times the Earth–Sun distance). 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.

Short- and long-period 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 (that is, greatest distances from the Sun) 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.

Names, visibility, and disintegration Most comets are named after their discoverers and denoted by letters in order of discovery each year, but are subsequently numbered in order of their perihelion passage; for example 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 1668, 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.

Hale-Bopp The brightest periodic comet, Hale-Bopp, flew past the Earth in March 1997. NASA launched rockets to study the comet, which came within 122 million mi/196 million km of the Earth. Its icy core is estimated to be 25 mi/40 km wide. It was discovered by astronomers Alan Hale of Cloudcroft, New Mexico, and Thomas Bopp of Glendale, Arizona, in July 1995 when it was 650 million mi/1,050 million km away.

Recent research In 2004 NASA's Stardust mission at NASA's Jet Propulsion Laboratory in Pasadena, California, collected samples of comet dust for the first time. The stardust probe passed within 150 mi/240 km of the nucleus of Comet Wild 2 and collected samples of comet dust using a lightweight glass foam mesh. The probe also obtained the highest-resolution pictures ever taken of a comet.

In July 2005 scientists successfully launched a probe into the core of comet Tempel 1. The 820 lb/372 kg probe launched by the NASA Deep Impact mission spacecraft recorded high-resolution images of the impact. The 23,000 mph/37,000 kph collision took place 83 million mi/133 million km from Earth. Comets are thought to be unchanged since the formation of the Solar System and obtaining information from a comet's interior should provide new insight on the original composition of the Solar System.