Meteor. As the term is commonly used, a meteor is a small celestial body that enters the atmosphere of the earth. To astronomers, however, a meteor is the streak of light such an object produces as it passes through the earth's atmosphere and the object itselfwhether in the atmosphere or in outer spaceis a meteoroid. This article uses the common definition.

Meteors that produce extremely bright streaks of light are called fireballs, and those that explode in the air, bolides. In popular usage, meteors are often referred to as falling stars, or shooting stars. Meteorites are meteors that reach the earth's surface.

Meteors travel at speeds ranging from 7 to 45 miles (11 to 72 km) per second relative to the earth and are visible only briefly as they travel through the upper atmosphere. Most meteors first become visible at heights of 50 to 75 miles (80 to 120 km). The friction between meteors and the air at these heights produces enough heat to vaporize the meteors' outer material and to cause the meteors and the air they pass through to glow. At heights of 35 to 50 miles (56 to 80 km) the meteors cease to be visible. By then, the larger ones have been slowed so they no longer glow, and the smaller ones have been completely vaporized. Some, called micrometeoroids, are so small that they are slowed by the atmosphere before they heat up enough to vaporize and glow. Micrometeoroids drift through the atmosphere like dust.

It is estimated that every day there are billions of meteors, although most of them cannot be seen. On a dark, clear night an observer can usually see five or more meteors an hour. More meteors can be seen after midnight than before because the earth, as it moves in its orbit, sweeps up meteoroids before it; after midnight an observer is on the side of the earth that faces the direction of the earth's orbital motion. Meteors are only rarely visible during daylight hours.

Studies of the paths taken by meteors through the atmosphere have shown that almost all of them were in orbit around the sun and therefore belonged to the solar system. (A few remain of questionable origin.) Meteors are thought to be the remains of comets and asteroids that have broken up. Most meteors are no larger than a grain of sand. A few, however, are extremely large, weighing many tons.

Meteor Showers

At times, large numbers of meteors fall in a short period. These occurrences are called meteor showers. Observations of as many as 60,000 meteors an hour have been made. All the meteors that form part of a shower travel in the same direction in parallel paths. Because of the effect of perspective, they seem to radiate from a single point in the sky. This point is called the radiant. Most meteor showers are named for the constellation in which their radiant is located. The radiant of the Leonids, for example, is in the constellation Leo.

Meteor showers are produced by swarms of meteoroids that travel around the sun in elliptical orbits that cross the earth's orbit. A meteor shower occurs when the earth passes through a swarm; the earth passes through a given swarm at the same time every year. A meteor shower may have more meteors some years than others, because meteoroid swarms are not spread out evenly in their orbits.

Meteoroid swarms are thought to be the debris from comets. Several swarms travel in the same orbits as known comets. In one case, a comet (Biela's Comet) failed to return at the expected time in 1872, and a meteor shower occurred instead.

Important meteor showers
ShowerDate of peakPeriod of activity Parent body
Quadrantids Jan. 4Jan. 1 to Jan. 6 2003 EH1
Lyrids April 22April 16 to April 25 Comet Thatcher
Eta Aquariids May 4April 19 to May 28Comet Halley
Perseids Aug. 12July 17 to Aug. 24 Comet Swift-Tuttle
Orionids Oct. 21Oct. 2 to Nov. 7Comet Halley
Leonids Nov. 17Oct. 31 to Nov. 23Comet Tempel-Tuttle
Geminids Dec. 13Nov. 27 to Dec. 183200 Phaethon
Ursids Dec. 22Dec. 17 to Dec. 26Comet Tuttle