METEOR

also called Shooting Star, or Falling Star, streak of light in the sky that results when a particle or small chunk of stony or metallic matter enters the Earth's atmosphere and vaporizes. The term is sometimes applied to the falling object itself, but the latter is properly called a meteoroid (q.v.). The vast majority of meteoroids burn up in the upper atmosphere, but occasionally one of relatively large mass survives its fiery plunge and reaches the surface as a solid body. Such an object is known as a meteorite (q.v.). The speed at which a meteoroid enters the upper atmosphere is dependent primarily on two motionsthat of the meteoroid in its elongated orbit and that of the Earth in its path around the Sun. Also, the Earth's gravitational pull on the meteoroid increases its velocity by 3 km (1.9 miles) per second or more. Observations have shown that meteoroids penetrate the atmosphere at velocities ranging from 11 to 72 km (7 to 45 miles) per second. When a meteoroid plunges toward the Earth at high speed, its surface collides with the atoms and molecules of the atmosphere. These collisions dislodge material from the meteoroid's surface and break up atoms and molecules of both the meteoroid and atmosphere into charged particles. The ionized atoms are excited to produce visible light. Much of this light is generated near the moving solid body, but in the case of very fast, bright meteors a residual luminosity called the train may be left behind the meteor head. As the meteoroid plunges closer to the ground, an extremely dense, hot gas cap develops in front of it. A shock wave also may be produced in the air ahead of the meteoroid. Such atmospheric shock waves may propagate to the Earth's surface because meteoroids travel through the air at five times the speed of sound or higher. A meteoroid weighing about 1 kg (2.2 pounds) penetrating to an altitude of roughly 40 km (25 miles) can produce sounds on the ground resembling thunder or sonic booms. Most meteoroids crumble and disintegrate completely before they reach an altitude of 80 km (50 miles). It has been speculated that meteoroids that readily fragment originated with comets, whereas the denser objects that penetrate to the surface are remnants of asteroids that broke apart hundreds of millions of years ago. When the Earth encounters swarms of meteoroids moving together, meteor showers become visible. Photographic data indicate that many such phenomena are associated with the orbits of observed comets. For example, the meteor shower known as Eta Aquarid, which can be seen each year in early May, has been identified with Halley's Comet. The meteoroids in the swarm producing this shower are thought to be debris ejected from the comet along its orbit. Much accurate observational data on meteors has been obtained in recent years by means of special high-speed, wide-angle cameras and electronic image intensifiers. Radar also has been widely used in studying meteors, since radio waves can be bounced off the ionized gases yielded by the vaporizing constituent matter. It is very useful in determining the speed, direction, and height of meteors, particularly in daylight and under weather conditions unfavourable for visual observations. also called shooting star, or falling star, streak of light in the sky that results when a particle or small chunk of stony or metallic matter enters the Earth's atmosphere and vaporizes. The term is sometimes applied to the falling object itself, but the latter is properly called a meteoroid. The vast majority of meteoroids burn up in the upper atmosphere, but occasionally one of relatively large mass survives its fiery plunge and reaches the surface as a solid body. Such an object is known as 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