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The brief luminous trail observed as a particle of dust or piece of rock from space enters the Earth's upper atmosphere. The popular name for a meteor is shooting star or falling star. The Earth is constantly bombarded with material from space. The individual objects range in size from rocks of several kilograms down to microscopic particles weighing less than one millionth of a gram. It is estimated that more than 200 million kilograms of meteoric material are swept up by the Earth in the course of a year. One-tenth of this mass reaches the ground, in the form of meteorites and micrometeorites. The remainder burns up in the atmosphere, becoming visible as meteor trails. The meteoric material enters the atmosphere at typical speeds of around 15 km/s. Frictional heating causes medium-sized particles to vaporize, creating visible light and leaving a temporary trail of ionized gas. This trail is capable of reflecting radar signals, and radar has been used to detect meteors too faint to be seen by eye and meteors occurring during the daytime. Much of the meteoric material in the solar system orbits the Sun in distinct streams. The orbital characteristics can be calculated from observations of the meteor trails. In this way it has been shown that many such meteor streams have the same orbits as known comets. The particles may be strung out all along the orbit or concentrated in a particular swarm. When the Earth's orbital motion causes it to cut through a stream, a meteor shower is observed. The effect of perspective makes the meteors, which are travelling along parallel paths, appear to radiate from a single point in the sky, the radiant. In addition to the dozens of regular meteor showers, a background of sporadic meteors is observed throughout the year. They may come from any direction. See also: fireball. |