meteor

meteor A brief streak of light in the Earth's upper atmosphere between altitudes of 85 and 115 km, produced by the high-speed entry of a small fragment of interplanetary debris (a meteoroid). An estimated 100 million meteors are visible to the naked eye over the whole Earth in an average 24-hour period. Meteoroids enter the atmosphere at velocities of 11–72 km/s. A typical naked-eye meteor of magnitude +2 is produced by a meteoroid about 8 mm in diameter. Over 0.1–0.2 s, its kinetic energy is converted principally to heat and ionization; only a small proportion is converted to visible light. The surface of a meteoroid is rapidly vaporized by the process of ablation. Material eroded from the meteoroid's surface goes on to collide further with atmospheric particles, producing excitation and ionization along a column perhaps 20–30 km long. The excess energy imparted to the atmospheric particles is re-emitted in a fraction of a second as visible light.

Meteors may be produced by particles sharing an orbit around the Sun (a meteor stream), or by solitary, random particles (sporadic meteors). During a meteor shower, more meteors will be seen. Most meteors are faint. The naked eye can detect events down to about magnitude +5, while binoculars show meteors as faint as magnitude +8. Smaller material gives rise to radio meteors or radar meteors, perhaps equivalent to visual magnitudes around magnitude +12. Occasionally, the arrival of a more substantial piece of debris produces an extremely bright fireball.