comet

An icy body, orbiting in the solar system, which partially vaporizes when it nears the Sun, developing a diffuse envelope of dust and gas and, normally, one or more tails. Ground-based observations of the behaviour of many comets, together with results from the investigation in 1986 of Halley's Comet from space probes, support the view first proposed by F. Whipple in about 1949 that the nuclei of comets are essentially "dirty snowballs" a few kilometres across. They appear to be composed of frozen water, carbon dioxide, methane and ammonia, in which dust and rocky material is embedded. As a comet approaches the Sun, solar heating starts to vaporize the ices, releasing gas that forms a diffuse luminous sphere, called the coma, around the nucleus. The coma may be up to a million kilometres across. The nucleus itself is too small to be observed directly. Observations in the ultraviolet region of the spectrum made from spacecraft have shown comets to be surrounded by huge clouds of hydrogen, many millions of kilometres in size. The hydrogen comes from the breakdown of water molecules by solar radiation. In 1996, X-ray emission from Comet Hyakutake was discovered, and it was subsequently found that other comets are also X-ray sources. Dust and gas leave the comet nucleus from jets on the side facing the Sun, then stream away under the Sun's influence. Electrically charged ionized atoms are swept away directly by the magnetic field of the solar wind, forming straight ion tails (alternatively called Type I, plasma or gas tails). Variations in the solar wind cause the ion tail to take on structure, or even break off in a disconnection event. Small neutral dust particles are not carried along by the solar wind but get "blown" gently away from the Sun by radiation pressure. Dust tails (also called Type II tails) are often broad and flat. Comet Hale-Bopp was found to have a third distinct tail, made of neutral sodium atoms. The tails grow as a comet approaches the Sun and are always directed away from the Sun: they can be as much as a hundred million kilometres long. Large dust particles become strewn along the comet's orbit and form meteor streams. Despite their often dramatic appearance, comets contain very little material, perhaps only a billionth the mass of the Earth. The tails are so tenuous that only one five-hundredth of the mass of the nucleus may be lost in a passage round the Sun. A dozen or so new comets are discovered every year. Some are short-period comets, in elliptical orbits that take between about 6 and 200 years to complete. Most are Long-period comets, in orbits so elongated that the period is many thousands of years. Short-period comets orbit close to the ecliptic plane, but long-period comet orbits are not confined to the main plane of the solar system. It is now generally accepted that many comets originate in a spherical cloud that surrounds the solar system at a distance of perhaps 50,000 AU. This "reservoir" of comet nuclei is called the Oort Cloud. Others appear to come from the Kuiper Belt, located beyond the orbit of Neptune. The short-period comets have been captured within the planetary system by the gravitational perturbation of their orbits that can arise from a close encounter with Jupiter. When a new comet is discovered, or a periodic comet recovered, it is given a designation consisting of the year followed by an upper-case letter indicating the half-month of discovery in that year, e.g. A = 1-15 January, B = 16-31 January ... Y = 16-31 December. The prefix P/ is added for short-period comets and C/ for long-period comets. Periodic comets that have disappeared or been destroyed are prefixed by D/. New comets are named after their discoverers, no more than three names being permitted when there are several independent discoverers. A few comets have been named after individuals who calculated their orbits (e.g. Halley and Encke) or after observatories or satellites, where discovery was essentially through the efforts of a team. When a short-period comet has been fully established as such, it is also allocated a number (e.g. 1P/Halley). This system for designating and naming comets was introduced in 1995. Prior to 1995, comets were given designations consisting of the year of discovery, temporarily followed by a lower-case letter indicating the order of discovery in that year. The letters were subsequently replaced with "permanent" designations in the form of Roman numerals determined by the order of perihelion passage of comets in the particular year. Authority for naming rests with the International Astronomical Union. Its designated centre coordinates discovery reports and observations, and distributes information to subscribers.