In astronomy, small solid body entering a planet's atmosphere from outer space and raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteoric showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteoric showers generally coincide with those of certain comets. Most meteors are dissipated in flight and fall to the earth as dust; a meteor that reaches the surface of the earth or another planet is called a meteorite.
METEORITES
Meteor that reaches the surface of the earth or of another planet before it is entirely consumed. Meteorites found on earth are classified into types, depending on their composition: irons, those composed chiefly of iron, a small percentage of nickel, and traces of other metals such as cobalt; stones, stony meteors consisting of silicates; and stony irons, containing varying proportions of both iron and stone. Although most meteorites are now believed to be fragments of either asteroids or comets, recent geochemical studies have shown that a few Antarctic stones came from the moon and Mars, from which they presumably were ejected by the explosive impact of asteroids. Asteroids themselves are fragments of planetesimals, formed some 4.6 billion years ago, while the earth was forming. Irons are thought to represent the cores of planetesimals and stones (other than the aforementioned Antarctic ones) the crust. Meteorites generally have a pitted surface and fused charred crust. The larger ones strike the earth with tremendous impact, creating huge craters.
The largest known meteorite, estimated to weigh about 55 metric tons, is situated at Hoba West near Grootfontein, Namibia. The next largest, weighing more than 31 metric tons, is the Ahnighito (the Tent); it was discovered, along with two smaller meteorites, in 1894 near Cape York, Greenland, by the American explorer Robert Edwin Peary. Composed chiefly of iron, the three masses had long been used by the Inuit (Eskimo) as a source of metal for the manufacture of knives and other weapons. The Ahnighito was brought to the United States by Peary and is on display at the Hayden Planetarium in New York City. The largest known crater believed to have been produced by a meteorite was discovered in 1950 in northwestern Québec, Canada. It consists of a circular pit 4 km (2.5 mi) in diameter, containing a lake and surrounded by concentric piles of shattered granite.
