From Wikipedia
Jump to: navigation, search
File:Leonid Meteor.jpg
A Leonid meteor, or a shooting star seen in the 2009 Leonid Meteor Shower.

A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite. Many meteors appearing seconds or minutes apart are called a meteor shower. The root word meteor comes from the Greek meteōros, meaning "high in the air".

Around 15,000 tonnes of meteoroids, space dust, and debris of different types enters Earth's atmosphere each year.<ref>Stuart Gary - Survey finds not all meteors the same - ABC Science</ref>




File:Esquel pallasite partial slice.jpg
A slice of a pallasite meteorite fragment discovered in Argentina; on display at the Museum of Nature, Ottawa, Canada.

As of 2011 the International Astronomical Union officially defines a meteoroid as a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".<ref>Template:Cite journal</ref><ref>Template:Cite web</ref> Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 µm and 10 m across.<ref>Template:Cite journal)</ref> Following the discovery and naming of asteroids below 10 m in size (e.g., 2008 TC3), Rubin and Grossman refined the Beech and Steel definition of meteoroid to objects between 10 µm and 1 m in diameter.<ref>Template:Cite journal)</ref> The NEO definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

The Minor Planet Center does not use the term "meteoroid".

The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give 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. Debris from meteoroid streams may eventually be scattered into other orbits. 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. PHYSICAL DYNAMICS OF THE UPSILON PEGASID FIREBLL – EUROPEAN NETWORK 190882A. Florida Institute of Technology</ref> to nickel-iron rich dense rocks.

Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit.Template:Fact The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second). Meteoroids moving through the earth's orbital space average about 20 km/s.<ref>Template:Cite web</ref>


"Meteor" and "Meteors" redirect here. For other uses, see Meteor (disambiguation).
See also Hydrometeor.
File:Meteor burst.jpg
Photo of a part of the sky during a meteor shower over an extended exposure time. The meteors have actually occurred several seconds to several minutes apart.

A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 76 km to 100 km. (46-62 miles) <ref>Template:Cite web</ref> Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble.

The velocities of meteors result from the movement of the Earth around the Sun at about 30 km/s (18.5 miles per second),<ref name="earth_fact_sheet">Template:Cite web</ref> the orbital speeds of meteoroids, and the gravitational attraction of the Earth.


Meteors become visible between about 75 to 120 kilometers (34 - 70 miles) above the Earth. They disintegrate at altitudes of 50 to 95 kilometers (31-51 miles).Template:Cn Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized.

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres)Template:Clarify the visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball).

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific single cause. 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 meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt.<ref>Template:Cite web</ref>


A fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude −4 or greater).<ref>Template:Cite web</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 name=IMO>Template:Cite web</ref> For 2011 there are 4589 fireballs records at the American Meteor Society.<ref name="Fireballs2011">Template:Cite web</ref>



A bolide (an especially bright meteor)


The word bolide comes from the Greek βολίς (bolis) <ref></ref> 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 generally applies to fireballs reaching magnitude −14 or brighter.<ref name=Belton>Template:Cite book:156</ref> Astronomers tend to use "bolide" to identify an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball). It may also be used to mean a fireball which creates audible sounds.


Geologists use the term "bolide" more often than astronomers do: in geology it indicates 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".<ref>Template:Cite web</ref>


If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide.<ref name=Belton/><ref>Template:Cite book:133 </ref>



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 craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.



Molten terrestrial material "splashed" from a meteorite impact crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.

Meteoric dust

Most meteoroids burn up 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.<ref>Template:Cite web</ref>

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 any given region of the atmosphere, 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 radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail.


The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the color of the light emitted may change according to the layering of minerals. Possible colors (and elements producing them) include:

  • orange/yellow (sodium)
  • yellow (iron)
  • blue/green (copper)
  • purple (potassium)
  • red (silicate)


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 disappears. However, in certain instances, for example during the Leonid meteor shower of 2001, several people reported sounds described as "crackling", "swishing", or "hissing"<ref>Template:Cite news</ref> occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Sound recordings made under controlled conditions in Mongolia in 1998 by a team led by Slaven Garaj, a physicist at the Swiss Federal Institute of Technology at Lausanne, support the contention that the sounds are real.<ref>Template:Cite web</ref>

How these sounds could be generated, assuming they are in fact real, remains something of a mystery. It has been hypothesized by some scientists at NASA 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, megawatts 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref> This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field.

Frequency of large meteors

Template:See also

The biggest asteroid to hit Earth on any given day is likely to be about 40 centimeters, in a given year about 4 meters, and in a given century about 20 meters. These statistics are obtained by the following:

Over at least the range from 5 centimeters (2 inches) to roughly 300 meters (1,000 feet), the rate at which Earth receives meteors obeys a power-law distribution as follows:

<math>N(>D) = 37 D^{-2.7}\ </math>

where N(>D) is the expected number of objects larger than a diameter of D meters to hit Earth in a year.<ref>Template:Cite web</ref> This is based on observations of bright meteors seen from the ground and space, combined with surveys of near Earth asteroids. Above 300 meters in diameter, the predicted rate is somewhat higher, with a two-kilometer asteroid (one million-megaton TNT equivalent) every couple of million years — about 10 times as often as the power-law extrapolation would predict.

Seasonal variation in frequency of fireball sightings

The frequency of fireball sightings increases by about 10-30% during the weeks of vernal equinox.<ref name="fireball season NASA">Template:Cite web</ref> Even meteorite falls are more common during the northern hemisphere's spring season. Although this phenomenon has been known for quite some time, the reason behind the anomaly is not fully understood by scientists. Some researchers attribute this to an intrinsic variation in the meteoroid population along Earth's orbit, with a peak in big fireball-producing debris around spring and early summer. Research is in progress for mapping the orbits of the meteors in order to gain a better understanding of the phenomenon.<ref name="camera network NASA">Template:Cite web</ref>

Notable meteors

Template:See also

Perhaps the best-known meteor/meteorite fall is the Peekskill Meteorite, filmed on October 9, 1992 by at least 16 independent videographers.<ref>The Peekskill Meteorite October 9, 1992 Videos</ref>

Eyewitness accounts indicate 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.<ref>Brown, P. et al., 1994. Nature, 367, 6524 - 626</ref>

One meteorite recovered at Peekskill, New York, for which the event and object gained their name, 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 had several companions over a wide area. The companions are unlikely to be recovered in the hilly, wooded terrain in the vicinity of Peekskill.

2008 TC3 was discovered on 6 October 2008 and entered the Earth's atmosphere the next day, striking a remote area of northern Sudan. It was the first time a meteoroid had been observed in space and tracked prior to impacting Earth.

A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the Nagasaki atomic bomb. No injuries were reported.<ref name="NEO165">Template:Cite web</ref>

A large bolide was reported on 18 November 2009 over southeastern California, northern Arizona, Utah, Wyoming, Idaho and Colorado. At 12:07 a.m., a security camera at the high altitude W. L. Eccles Observatory (9600 ft above sea level) recorded a movie of the passage of the object to the north.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Of particular note in this video is the spherical "ghost" image slightly trailing the main object (this is likely a lens reflection of the intense fireball), and the bright fireball explosion associated with the breakup of a substantial fraction of the object. An object trail can be seen to continue northward after the bright fireball event. The shock from the final breakup triggered seven seismological stations in northern Utah; a timing fit to the seismic data yielded a terminal location of the object at 40.286 N, -113.191 W, altitude 27 km.<ref>Patrick Wiggins, private communication</ref> This is above the Dugway Proving Grounds, a closed Army testing base.


Although meteors have been known since ancient times, they were not known to be an astronomical phenomenon until early in the 19th century. Prior to that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky. Thomas Jefferson wrote "I would more easily believe that (a) Yankee professor would lie than that stones would fall from heaven."<ref name=jefferson> The Early Years of Meteor Observations in the USA</ref> He was referring to Yale chemistry professor Benjamin Silliman's investigation of an 1807 meteorite that fell in Weston, Connecticut.<ref name=jefferson/> Silliman believed the meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833.<ref name=1833leonids> The Leonids and the Birth of Meteor Astronomy</ref> People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.<ref>Template:Cite journal</ref>

The astronomer Denison Olmsted made an extensive study of this storm, and concluded it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted the storm's return in 1867, which drew the attention of other astronomers to the phenomenon. Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct.<ref name=1833leonids/> With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle, the cosmic origin of meteors was now firmly established. Still, they remain an atmospheric phenomenon, and retain their name "meteor" from the Greek word for "atmospheric".<ref> October's Orionid Meteors</ref>


See also



External links

Template:Commons Template:Wikisource index Template:Wiktionary Template:Wiktionary

Template:Impact cratering on Earth Template:Small Solar System bodies Template:Solar SystemTemplate:Link FA

af:Meteoroïde ar:نيزك ast:Meteoroide bn:উল্কাপাত be:Метэароід be-x-old:Мэтэароід bg:Метеорно тяло bar:Meteoroid bs:Meteoroidi ca:Meteoroide cs:Meteoroid da:Meteoroid de:Meteoroid et:Meteoorkeha el:Διάττοντας αστέρας es:Meteoroide eo:Meteoroido fa:شهاب‌وار fr:Météoroïde ko:유성체 hy:Աստղաքար hi:उल्का hr:Meteoroid id:Meteoroid it:Meteoroide he:מטאורואיד kk:Метеор sw:Kimondo lv:Meteoroīds lb:Meteoroid lt:Meteoroidas li:Meteoroïde hu:Meteoroid mk:Метеороид ml:ഉൽക്ക mr:उल्का ms:Meteoroid nl:Meteoroïde ja:流星物質 no:Meteoroide nn:Meteoroide pnb:شہاب ثاقب nds:Meteoroid pl:Meteoroid pt:Meteoroide ro:Meteoroid ru:Метеороид sk:Meteoroid sl:Meteoroid sr:Meteoroid sh:Meteoroid fi:Meteoroidi sv:Meteoroid tl:Meteoroid ta:எரிவெள்ளி th:สะเก็ดดาว chy:Hotohke tséana'ôhtse uk:Метеороїд vi:Thiên thạch zh:流星体

Personal tools