Meteorite, a meteor that reaches the earth's surface. Meteors
are naturally occurring objects that enter the earth's
atmosphere from space, traveling at high speed. Those larger
than about four inches (10 cm) in diameter pass through the
atmosphere without being burned up by the intense heat produced
by air friction. (Those microscopic in size also survive the
passage; they drift to the earth's surface, becoming dustlike
micro-meteorites.)  Thousands of meteorites have been
discovered, some of them by searching in areas where meteors
were seen to fall. Most have been found in Antarctica, lying on
ice fields where they have been concentrated over long periods
of time by glaciers from surrounding areas.  There are three
major types of meteorites.  Stony Meteorites are composed
largely of silicon-containing minerals and other rock-forming
minerals.  Iron Meteorites are primarily iron mixed with lesser
amounts of nickel.  Stony-iron Meteorites are composed of
roughly equal amounts of stony materials and metal.  S