METEOROID

any interplanetary body of relatively small size that enters the Earth's atmosphere. In colliding with atmospheric atoms and molecules at high velocity, the object begins to burn up and heats the air around it. The resultant luminous phenomenon is called a meteor (q.v.). If the object survives its plunge through the atmosphere and lands on the ground, it is termed a meteorite (q.v.). Meteoroids range in size from dust particles measuring less than 1 micrometre across to bodies several kilometres in diameter. Most meteoroids consist either of rocky material (largely silicate minerals) or of nickel-iron alloy, though some are made up of silicates intermixed with nickeliron alloy. Observational and theoretical studies of the orbits of meteoroidal bodies, as well as laboratory investigations of recovered meteorites, strongly suggest that the majority of meteoroids are fragments of asteroids produced by collisions between such objects in the asteroid belt lying between the orbits of Mars and Jupiter. There also is evidence that some meteoroids, particularly the smaller ones, are of cometary origin. The nucleus of a comet (q.v.) is composed of a mixture of ice and less-volatile material, chiefly in the form of grains of rock dust. When a comet approaches the Sun, the ice begins to evaporate and the dust particles embedded within it are swept away from the nucleus. Particles of millimetre-to-centimetre size may enter the Earth's atmosphere to become meteoroids. any interplanetary body of relatively small size that enters the Earth's atmosphere. In colliding with atmospheric atoms and molecules at high velocity, the object begins to burn up and heats the air around it. The resultant luminous phenomenon is called a meteor. If the object survives its plunge through the atmosphere and lands on the ground, it is termed a meteorite. George W. Wetherill Additional reading Introductory information can be found in Harry Y. McSween, Jr., Meteorites and Their Parent Planets (1987); Robert T. Dodd, Thunderstones and Shooting Stars: The Meaning of Meteorites (1986); John G. Burke, Cosmic Debris: Meteorites in History (1986); Robert Hutchison, The Search for Our Beginning: An Enquiry, Based on Meteorite Research, into the Origin of Our Planet and of Life (1983); and John A. Wood, Meteorites and the Origin of Planets (1968). More advanced treatments are John T. Wasson, Meteorites: Their Record of Early Solar-System History (1985), and Meteorites: Classification and Properties (1974); V.A. Bronshten, Physics of Meteoric Phenomena (1983; originally published in Russian, 1981); and Robert T. Dodd, Meteorites: A Petrologic-Chemical Synthesis (1981). A descriptive and historical treatment of iron meteorites, including beautiful photographs, is Vagn F. Buchwald, Handbook of Iron Meteorites, Their Distribution, Composition, and Structure, 3 vol. (1975). H.H. Nininger, Out of the Sky: An Introduction to Meteorites (1952, reprinted 1959), provides firsthand experiences of fall phenomena on a nontechnical level. See also D.E. Brownlee, Cosmic Dust: Collection and Research, Annual Reviews of Earth and Planetary Sciences, 13:147173 (1985). A catalog of known meteorites, including data regarding their fall, is A.L. Graham, A.W.R. Bevan, and R. Hutchison (eds.), Catalogue of Meteorites, 4th ed. rev. and enlarged (1985). There are two journals devoted to papers on meteorites and related bodies: Meteoritika (annual), published in Russia; and Meteoritics (quarterly). Many papers on meteorites are published in Geochimica et Cosmochimica Acta (monthly). George W. Wetherill