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| − | {{Update Science}}
| + | [[File:Leonid Meteor.jpg|thumb|right]] |
| − | [[Image:Meteor burst.jpg|thumb|right|200px|Photo of a part of the sky during a meteor shower over an extended exposure time. The meteors may have actually occurred several seconds to several minutes apart.]] | + | [[File:Meteor crater.gif|thumb|right]] |
| − | A '''meteoroid''' is a small sand- to boulder-sized particle of debris in the [[Solar system]]. The visible path of a meteoroid that enters [[Earth|Earth's]] (or another body's) atmosphere is a meteor, commonly called a "shooting star" or "falling star". On reaching the ground, a meteor is then called a meteorite. Many meteors are part of a meteor shower. The root word '''meteor''' comes from the Ancient Greek ''meteōros'', meaning ''high in the air''.
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| − | ===Meteoroid===
| + | A '''meteoroid''' is a small sand- to boulder-sized particle of astronomical debris in the [[Solar system]], smaller than an asteroid. <br>The visible path of a meteoroid that enters a planet's atmosphere is a meteor, commonly called a "shooting star" or "falling star". Upon reaching the ground, a "meteor" is then called a "meteorite." |
| − | [[Image:IMG 8505n3.JPG|thumb|175px|A meteor (possibly 2) and Milky way.]]
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| − | Larger than a meteoroid, the object is an asteroid; smaller than that, it is interplanetary dust. The current official definition of a meteoroid from the ''International Astronomical Union'' is "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom."<ref>http://www.imo.net/glossary Glossary International Meteor Association</ref> The ''Royal Astronomical Society'' has proposed a new definition where a meteoroid is between 100 Micrometre (µm) and 10 m across.<ref>{{cite journal
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| − | |author=Beech, M.
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| − | |authorlink=Martin Beech
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| − | |coauthors=[[Duncan I. Steel|Steel, D. I.]]
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| − | |year=1995
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| − | |month=September
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| − | |title=On the Definition of the Term Meteoroid
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| − | |journal=Quarterly Journal of the Royal Astronomical Society
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| − | |volume=36
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| − | |issue=3
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| − | |pages=281–284
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| − | |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&db_key=AST&data_type=HTML&format=&high=44b52c369007834
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| − | |accessdate=2006-08-31
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| − | }})</ref>
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| − | The [[near-earth object|NEO]] definition includes larger objects, up to 50 m in diameter, in this category.
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| − | The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectory and the light spectra of the resulting meteor. Their effects on radio signals also yield information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see [[Meteor showers]]) often associated with a parent comet, others apparently sporadic. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice,<ref>Povenmire, H. [http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1183.pdf PHYSICAL DYNAMICS OF THE UPSILON PEGASID FIREBALL – EUROPEAN NETWORK 190882A]. Florida Institute of Technology</ref> to nickel-iron rich dense rocks. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed [[The Great Daylight 1972 Fireball#All known Earth-grazing fireballs|Earth-grazing fireballs]].
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| − | ===Meteor===
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| − | [[Image:Comet holmes and Geminid121307.jpg|Comet [[17P/Holmes]] and [[Geminid]].|thumb|left|200px]]
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| − | A '''meteor''' is the visible event that occurs when a meteoroid or [[asteroid]] enters Earth's atmosphere and becomes brightly visible. This typically occurs in the [[mesosphere]], and most visible meteors range in altitude from 75km to 100km.<ref>{{cite web|url=http://www.haystack.mit.edu/~pje/meteors/| title = Millstone Hill UHF Meteor Observations: Preliminary Results| author = Philip J. Erickson}}</ref>
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| − | 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 simply the visible event rather than an object itself.
| + | ===Size=== |
| | + | Ranging in size from small grains to one-meter-wide objects, objects smaller than this are classified as "micrometeoroids," or simply, space dust. Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as moons or other planets. |
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| − | ===Fireball===
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| − | A ''fireball'' is brighter than a usual meteor. The [[International Astronomical Union]] defines a fireball as "a meteor brighter than any of the planets" ([[Apparent magnitude|magnitude]] -4 or greater).<ref>[http://www.meteorobs.org/maillist/msg13871.html MeteorObs Explanations and Definitions (states IAU definition of a fireball)]</ref> The [[International Meteor Organization]] (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.<ref>[http://www.imo.net/fireball International Meteor Organization - Fireball Observations]</ref>
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| − | ===Bolide===<!-- This section is linked from [[Eocene]] --> | + | ===Atmosphere=== |
| − | The word ''bolide'' comes from the [[Greek language|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 [[United States Geological Survey|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"''.<ref>[http://woodshole.er.usgs.gov/epubs/bolide/introduction.html usgs.gov - What is a Bolide?]</ref> Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball). | + | The entry of meteoroids into [[Earth]]'s atmosphere produces three main effects: ionization of atmospheric molecules, dust that the meteoroid sheds, and the sound of its passage. During the entry of a meteoroid or asteroid into the upper atmosphere, an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere. |
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| − | ===Meteorite===
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| − | 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.<ref name="oxford">The Oxford Illustrated Dictionary. 1976. Second Edition. Oxford University Press. page 533</ref> 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
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| − | ===Tektite===
| + | Sound generated by a meteor in the upper atmosphere of a planet, such as a sonic boom, typically arrives many seconds after the visual light from a meteor disappears. Occasionally, "crackling," "swishing," or "hissing" sounds have been reported, occurring at the same instant as a meteor flare. |
| − | [[Image:Two tektites.JPG|thumb|right|Two tektites.]]
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| − | Molten terrestrial material "splashed" from a crater can cool and solidify into an object known as a [[tektite]]. These are often mistaken for meteorites.
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| − | ===Meteoric dust===
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| − | 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.
| + | ===Impact=== |
| | + | Meteoroid collisions with solid Solar System objects create impact craters, which are the dominant geographic features of many objects. On planets and moons with active surface geological processes, visible impact craters may become eroded, buried, or transformed by tectonics over time. |
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| − | ==Ionization trails==
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| − | 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 [[ionization|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]].
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| − | [[Meteor radar]]s can measure atmospheric density and winds by measuring the [[decay rate]] and [[Doppler shift]] of a meteor trail.
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| − | ==Sound==
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| − | 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 [[Leonid meteor shower]] of 2001, several people reported sounds described as "crackling", "swishing", or "hissing"<ref>[http://findarticles.com/p/articles/mi_m1134/is_6_111/ai_87854873/pg_1 Psst! Sounds like a meteor: in the debate about whether or not meteors make noise, skeptics have had the upper hand until now - Now Hear This | Natural History | Find Articles at BNET.com<!-- Bot generated title -->]</ref> occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's [[Aurora (astronomy)|auroras]].
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| − | Many investigators 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. Sound recordings made under controlled conditions in Mongolia in 1998 by a team led by [[Slaven Garaj]], a physicist at the [[École Polytechnique Fédérale de Lausanne|Swiss Federal Institute of Technology]] at [[Lausanne]], support the contention that the sounds are real.
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| − | How these sounds could be generated, assuming they are in fact real, 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 frequency|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.<ref>[http://science.nasa.gov/headlines/y2001/ast26nov_1.htm Listening to Leonids]</ref><ref>[http://homepages.tesco.net/~John.Dawes2/extract.htm Hearing Sensations in Electric Fields]</ref><ref>[http://homepages.tesco.net/~John.Dawes2/frey.htm Human auditory system response to Modulated electromagnetic energy. ]</ref><ref>[http://homepages.tesco.net/~John.Dawes2/frey2.htm Human Perception of Illumination with Pulsed Ultrahigh-Frequency Electromagnetic Energy]</ref> This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field.
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| − | ==Formation==
| + | [[category:science]] |
| − | 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.
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| − | ==Orbit==
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| − | Meteoroids and asteroids orbit around the Sun, in greatly differing orbits. Some of these objects orbit together in streams; these are probably comet remnants that would form a [[meteor shower]]. Other meteoroids are not associated with any stream clustering (although there must also be meteoroids clustered in orbits which do not intercept Earth's or any other planet). The fastest objects travel at roughly 42 kilometers per second (26 miles per second) through space in the vicinity of Earth's orbit. Together with the Earth's orbital motion of 29 km/s (18 miles per second), collision speeds can reach 71 km/s (44 miles per second) during head-on collisions. This would only occur if the meteor were in a [[retrograde orbit]]. Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth as the Earth orbits in the direction of roughly west at noon. Most meteors are however, observed at night as low light conditions allow fainter meteors to be observed. Meteors are usually seen when they are 60 to 120 km (40 to 75 miles) above the ground.<ref>[http://www.nasa.gov/worldbook/meteor_worldbook.html NASA Home > World Book @ NASA, Meteors]</ref>
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| − | 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]].<ref>[http://uregina.ca/~astro/mb_5.html Diagram 2: the orbit of the Peekskill meteorite along with the orbits derived for several other meteorite falls]</ref>
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| − | Perhaps the best-known meteor/meteorite fall is the [[Peekskill Meteorite]] which was filmed on October 9, 1992 by at least 16 independent videographers.<ref>[http://aquarid.physics.uwo.ca/~pbrown/Videos/peekskill.htm The Peekskill Meteorite [[October 9]], 1992 Videos]</ref>
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| − | 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. During a luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 700 to 800 km.
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| − | 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 (27 lb) and was subsequently identified as an H6 monomict breccia meteorite.<ref>"Meteoritical Bull", by Wlotzka, F. published in "Meteoritics", # 75, 28, (5), 692, 1994.</ref> 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.
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| − | ==Spacecraft damage==
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| − | Even very small meteoroids can damage spacecrafts. The [[Hubble Space Telescope]] has about 572 tiny craters and chipped areas.<ref>[http://www.space.com/scienceastronomy/astronomy/hubble_impact_020226.html SPACE.com - How Hubble Has Survived a Decade of Impacts<!-- Bot generated title -->]</ref>
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| − | ==Gallery==
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| − | <Gallery>
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| − | Image:Orionid, pedia.org/wiki/Milky_way Milky Way] and to the right of [[Venus]]. [[Zodiacal light]] is also seen at the image.
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| − | Image:Orionid meteor.jpg|[[Orionids|Orionid]]
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| − | Image:Orionid meteor1.jpg|[[Orionids|Orionid]]
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| − | Image:Two orionids and milky way.jpg|Two Orionids and [[Milky Way]]
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| − | Image:Multi colored Orionid.jpg|Multi-colored Orionid
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| − | Image:Orionids and Orion.jpg|Orionid
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| − | Image:Meteor trail.jpg|The brightest meteor, a fireball, leaves a smokey persistent trail drifting in high-altitude winds, which is seen at the right-hand side of the image left by [[Orionids|Orionid]].
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| − | </gallery>
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| − | == See also ==
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| − | *[[North American Meteor Network]]
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| − | *[[International Meteor Organization]]
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| − | *[[American Meteor Society]] (AMS)
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| − | *[[Baetylus]]
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| − | *[[Impact crater]]
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| − | *[[Impact event]]
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| − | *[[Meteor shower]]
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| − | *[[Meteorite]]
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| − | *[[Perseids]]
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| − | *[[Tektite]]
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| − | *[[Tollmann's hypothetical bolide]]
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| − | *[[Green fireballs]]
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| − | *[[Interplanetary_dust_cloud#Collecting_interplanetary_dust_on_earth|Stratospheric micrometeorites]]
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| − | ==References==
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| − | == External links==
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| − | * [http://www.cbc.ca/cp/science/080307/g030708A.html Astronomers spot meteor streaking across central Ontario sky] - [[CBC News]] March 7, 2008
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| − | * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Meteors Meteoroids Page] at [http://solarsystem.nasa.gov NASA's Solar System Exploration]
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| − | * [http://www.imo.net/fireball International Meteor Organization fireball page]
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| − | * [http://britastro.org/baa/content/view/90/121/ British Astronomical Society fireball page]
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| − | * [http://www.gsfc.nasa.gov/scienceques2005/20060406.htm A Goddard Space Flight Center Science Question of the Week where the answer mentions that a fireball will cast a shadow.]
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| − | * [http://www.omcea.be/article-19,19563,Meteor,showers.html Meteor showers] - view tips
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| − | * [http://astroclub.tau.ac.il/ephem/Meteors/ Meteor shower predictions]
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| − | * [http://www.popastro.com/sections/meteor.htm Society for Popular Astronomy - Meteor Section]
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A meteoroid is a small sand- to boulder-sized particle of astronomical debris in the Solar system, smaller than an asteroid.
The visible path of a meteoroid that enters a planet's atmosphere is a meteor, commonly called a "shooting star" or "falling star". Upon reaching the ground, a "meteor" is then called a "meteorite."
Size
Ranging in size from small grains to one-meter-wide objects, objects smaller than this are classified as "micrometeoroids," or simply, space dust. Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as moons or other planets.
Atmosphere
The entry of meteoroids into Earth's atmosphere produces three main effects: ionization of atmospheric molecules, dust that the meteoroid sheds, and the sound of its passage. During the entry of a meteoroid or asteroid into the upper atmosphere, an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere.
Sound generated by a meteor in the upper atmosphere of a planet, such as a sonic boom, typically arrives many seconds after the visual light from a meteor disappears. Occasionally, "crackling," "swishing," or "hissing" sounds have been reported, occurring at the same instant as a meteor flare.
Impact
Meteoroid collisions with solid Solar System objects create impact craters, which are the dominant geographic features of many objects. On planets and moons with active surface geological processes, visible impact craters may become eroded, buried, or transformed by tectonics over time.
