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Meteor

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

   Photo of a burst of meteors with extended exposure time
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   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.

Definitions

   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.

   Retrieved from " http://en.wikipedia.org/wiki/Meteor"
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