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over an extended Exposure Time . The meteors may have actually occurred several seconds to several minutes apart.]] A meteoroid is a large Sand to Boulder -sized particle of debris in the Solar System . The visible path of a meteoroid that enters the Earth's (or another body's) Atmosphere is a '''meteor''', commonly called a '''shooting star''' or '''falling star'''. Many meteors are part of a Meteor Shower . DEFINITIONS The root word meteor comes from the Greek ''meteōros'', meaning ''high in the air''. Meteoroid ]] The NEO definition includes larger objects, up to 50 m in diameter, to this category. Meteor ] A meteor is the visible event that occurs when a meteoroid or asteroid enters the earth's atmosphere and becomes brightly visible. For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) 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. The meteor is just what we see. Fireball A fireball is a very bright meteor. The (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of -3 or brighter if seen at Zenith . This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude -1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude -6. International Meteor Organization - Fireball Observations Bolide The word ''bolide'' comes from the Greek βολις, (''bolis'') which can mean ''a missile'' or ''to flash''. The IAU has no official definition of bolide and generally considers the term synonymous with fireball. The term is more often used among Geologist s than Astronomer s where it means a very large impactor. For example, the USGS uses the term to mean a generic large crater forming projectile ''"to imply that we do not know the precise nature of the impacting body . . . whether it is a rocky or metallic asteroid, or an icy comet, for example"''. usgs.com - What is a Bolide Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball). Meteorite A Meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed. Meteorites are sometimes, but not always, found in association with hypervelocity Impact Crater s; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites. Tektite Molten terrestrial material "splashed" from a crater can cool and solidify into an object known as a Tektite . These are often mistaken for meteorites. Meteoric dust Most meteoroids are destroyed when they enter the atmosphere. The left-over debris is called meteoric dust or just meteor dust. Meteor dust particles 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 or asteroid 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 Burst Communications . Meteor Radar s can measure atmospheric density and winds by measuring the Decay Rate and Doppler Shift of a meteor trail. SOUND? Numerous people have over the years reported sounds being heard while bright meteors flared overhead. This would seem impossible, given the relatively slow speed of sound. Any sound generated by a meteor in the upper atmosphere, such as a sonic boom, should not be heard until many seconds after the meteor disappeared. However, in certain instances, for example during the . Many invesigators believe the sounds to be imaginary... essentially sound effects added by the mind to go along with a light show. However, the persistence and consistency of the reports have caused others to wonder. And sound recordings made under controlled conditions in Mongolia in 1998 by a team lead by Slaven Garaj , a physicist at the Swiss Federal Institute Of Technology In Lausanne, Switzerland , supports the contention that the sounds are real. How these sounds, assuming they are in fact real, could be generated remains something of a mystery. It has been hypothesized that the turbulent ionized wake of a meteor interacts with the magnetic field of the Earth generating pulses of radio waves. As the trail dissipates, Megawatt s of electromagnetic energy could be released, with a peak in the Power Spectrum at Audio Frequencies . Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames, and other conductive materials vibrate http://science.nasa.gov/headlines/y2001/ast26nov_1.htm {Link without Title} {Link without Title} {Link without Title} . This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field. FORMATION Many meteoroids are formed by impacts between asteroids though many are also left in trails behind Comets that form Meteor Showers and many members of those trails are eventually scattered into other orbits forming random meteors too. Other sources of meteors are known to have come from impacts on the Moon , or Mars as some meteorites from them have been identified. See Lunar Meteorite s and Mars Meteorite s. ORBIT Meteoroids and asteroids orbit around the Sun. Not all of the orbits are the same. Some of these objects orbit together (stream component); these are probably comet remnants that would form a Meteor Shower . Other meteors are not associated with any clustering of meteoroids to form a shower (though there must also be meteoroids clustered in orbits which do not intercept the Earth's or any other planet.). The fastest objects travel at roughly 26 miles per second (42 km per second) through space in the vicinity of the Earth's orbit. Together with the Earth's orbital motion of 18 miles per second (29 km per second) speeds can reach 44 miles per second (71 kilometers per second) in head-on collisions. This means the meteor would also be in a Retrograde Orbit and have roughly a fifty percent chance of a daylight (or near daylight) event as the Earth orbits in the direction of roughly west at noon. Most meteors are observed at night as light conditions allow fainter observations but meteors are still seen from 40 to 75 miles away high in the sky above observers.[http://www.nasa.gov/worldbook/meteor_worldbook.html NASA Home > World Book @ NASA, Meteors] A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the incoming meteors or meteorites have been calculated. All of them came from orbits from the vicinity of the Asteroid Belt . Diagram 2: the orbit of the Peekskill meteorite along with the orbits derived for several other meteorite falls Perhaps the best-known meteor/meteorite fall is the , 1992 Videos] Eyewitness accounts indicate that the fireball entry of the Peekskill meteorite started over West Virginia at 23:48 UT (+/- 1 min.). The fireball, which traveled in a northeasterly direction had a pronounced greenish colour, and attained an estimated peak visual magnitude of -13 (comparable to the Full Moon ). During a luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 700 to 800 km. One meteorite recovered at Peekskill, N.Y., for which the event and object gained its name, (at 41.28 deg. N, 81.92 deg. W) had a mass of 12.4 kg and was subsequently identified as an H6 monomict breccia meteorite."Meteoritical Bull", by Wlotzka, F. published in "Meteoritics", # 75, 28, (5), 692, 1994. The video record suggests that the Peekskill meteorite probably had several companions over a wide area especially in the harsh terrain in the vicinity of Peekskill. SPACECRAFT DAMAGE Even very small meteoroids can damage spacecraft. The Hubble Space Telescope for example, has about 572 tiny craters and chipped areas.http://www.space.com/scienceastronomy/astronomy/hubble_impact_020226.html EXTERNAL LINKS
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