2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

Photo of a burst of meteors with extended exposure time
Photo of a burst of meteors with extended exposure time

A meteor is the visible path of a meteoroid that enters the Earth's (or another body's) atmosphere, commonly called a shooting star or falling star.


For bodies with a size scale larger than the atmospheric mean free path (10 cm to several meters) the visibility is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry. Since the majority of meteors are from small sand-grain size meteoroid bodies, most visible signatures are caused by electron relaxation following the individual collisions between vaporized meteor atoms and atmospheric constituents.

A very bright meteor may be called a fireball or bolide. The International Meteor Organisation defines fireballs as being meteors of magnitude -3 or brighter. The meteor section of the British Astronomical Association on the other hand has a much stricter definition, requiring the meteor to be magnitude -5 or brighter.

A meteor is a meteoroid that has entered the earth's atmosphere. It will then become brightly visible due to the heat produced by the ram pressure. If a meteor survives its transit of the atmosphere to come to rest on the Earth's surface, the resulting object is called a meteorite. A meteor striking the Earth or other object may produce an impact crater.

Molten terrestrial material "splashed" from such a crater can cool and solidify into an object known as a tektite.

Meteor dust particles left by falling meteoroids can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere.

Ionization trails

During the entry of a meteoroid into the upper atmosphere, an ionization trail is created, where the molecules in the upper atmosphere are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in a given region, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor scatter communication.

Meteor scatter has been used for experimental secure military battlefield communications systems. The basic idea of such a system is that such an ion trail will act as a mirror for radio waves, which can be bounced off the trail. Security arises from the fact that as a mirror, only receivers in the correct position will hear the transmitter, much as with a real mirror, what is seen in reflection depends upon one's position with respect to the mirror. Because the sporadic nature of meteor entry, such systems are limited to low data rates, typically 459600 baud.

Amateur radio operators sometimes use meteor scatter communication on VHF bands. Snowpack information from the Sierra Nevada mountains in California is transmitted from remote sites via meteor scatter. Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail.

Large meteoroids can leave behind very large ionization trails, which then interact with the Earth's magnetic field. As the trail dissipates, megawatts of electromagnetic energy can be released, with a peak in the power spectrum at audio frequencies. Curiously, although the waves are electromagnetic, they can be heard: they are powerful enough to make grasses, plants, eyeglass frames, frizzy hair, the middle ear and other materials vibrate. See for example, Listening to Leonids (NASA, 2001), Hearing Sensations in Electric Fields (1964), Human auditory system response to Modulated electromagnetic energy (J. Appl. Physiol. 17(4):689-692. 1962), Human Perception of Illumination with Pulsed Ultrahigh-Frequency Electromagnetic Energy (Science 27 July 1973 Vol. 181. no. 4097, pp. 356 - 358) for details and references on the electrophonic auditory phenomenon.

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