The visible passage of a meteoroid, comet, or asteroidentering Earth's atmosphere is called ameteor, and a series of many meteors appearing seconds or minutes apart and appearing to originate from the same fixed point in the sky is called ameteor shower.
An estimated 25 million meteoroids, micrometeoroids and otherspace debris enter Earth's atmosphere each day,[8] which results in an estimated 15,000 tonnes of that material entering the atmosphere each year.[9]Ameteorite is the remains of a meteoroid that has survived theablation of its surface material during its passage through the atmosphere as a meteor and has impacted the ground.
In 1961, theInternational Astronomical Union (IAU) defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than anasteroid and considerably larger than an atom".[10][11] In 1995, Beech and Steel, writing in theQuarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid would be between 100μm and 10 m (33 ft) across.[12] In 2010, following the discovery of asteroids below 10 m in size, Rubin and Grossman proposed a revision of the previous definition of meteoroid to objects between 10 μm (0.00039 in) and one meter (3 ft 3 in) in diameter in order to maintain the distinction.[2] According to Rubin and Grossman, the minimum size of an asteroid is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy. Some of the smallest asteroids discovered (based onabsolute magnitudeH) are2008 TS26 withH = 33.2[13] and2011 CQ1 withH = 32.1[14] both with an estimated size of one m (3 ft 3 in).[15] In April 2017, the IAU adopted an official revision of its definition, limiting size to between 30 μm (0.0012 in) and one meter in diameter, but allowing for a deviation for any object causing a meteor.[16]
Almost all meteoroids contain extraterrestrial nickel and iron. They have three main classifications: iron, stone, and stony-iron. Some stone meteoroids contain grain-like inclusions known aschondrules and are calledchondrites. Stony meteoroids without these features are called "achondrites", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron.[17] The composition of meteoroids can be inferred 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(seemeteor showers) often associated with a parentcomet, 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,[18] to nickel-iron rich dense rocks. The study ofmeteorites also gives insights into the composition of non-ephemeral meteoroids.
Most meteoroids come from theasteroid belt, having been perturbed by the gravitational influences of planets, but others are particles fromcomets, giving rise tometeor showers. Some meteoroids are fragments from bodies such as Mars or theMoon, that have been thrown into space by an impact.
Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest move at about 42 km/s (94,000 mph) through space in the vicinity of Earth's orbit. This isescape velocity from the Sun, equal to the square root of two times Earth's speed, and is the upper speed limit of objects in the vicinity of Earth, unless they come from interstellar space. Earth travels at about 29.6 km/s (66,000 mph), so when meteoroids meet the atmosphere head-on (which only occurs when meteors are in aretrograde orbit such as theLeonids, which are associated with the retrograde comet55P/Tempel–Tuttle) the combined speed may reach about 71 km/s (160,000 mph) (seeSpecific energy#Astrodynamics). Meteoroids moving through Earth's orbital space average about 20 km/s (45,000 mph),[19] but due to Earth's gravity meteors such as thePhoenicids can make atmospheric entry at as slow as about 11 km/s.
On January 17, 2013, at 05:21 PST, a one-meter-sized comet from theOort cloud entered Earth atmosphere overCalifornia andNevada.[20] The object had a retrograde orbit with perihelion at 0.98 ± 0.03 AU. It approached from the direction of the constellationVirgo (which was in the south about 50° above the horizon at the time), and collided head-on with Earth's atmosphere at 72 ± 6 km/s (161,000 ± 13,000 mph)[20] vaporising more than 100 km (330,000 ft) above ground over a period of several seconds.
Animated illustration of different phases as a meteoroid enters the Earth's atmosphere to become visible as ameteor and land as ameteorite
When meteoroids intersect with Earth's atmosphere at night, they are likely to become visible asmeteors. If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are calledmeteorites. Meteorites are transformed in structure and chemistry by the heat of entry and force of impact. A noted 4-metre (13 ft)asteroid,2008 TC3, was observed in space on a collision course with Earth on 6 October 2008 and entered Earth's atmosphere the next day, striking a remote area of northern Sudan. It was the first time that a meteoroid had been observed in space and tracked prior to impacting Earth.[10]NASA has produced a map showing the most notable asteroid collisions with Earth and its atmosphere from 1994 to 2013 from data gathered by U.S. government sensors.[21]
A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and hits the ground without being destroyed.[22] Meteorites are sometimes, but not always, found in association with hypervelocityimpact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.Geologists use the term, "bolide", in a different sense fromastronomers to indicate a very largeimpactor. For example, theUSGS uses the term to mean a generic large crater-forming projectile in a manner "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".[23]
Meteoroids also hit other bodies in the Solar System. On such stony bodies as theMoon orMars that have little or no atmosphere, they leave enduring craters.
Meteoroid collisions with solid Solar System objects, including the Moon,Mercury,Callisto,Ganymede, and most small moons andasteroids, create impact craters, which are the dominant geographic features of many of those objects. On other planets and moons with active surface geological processes, such as Earth,Venus,Mars,Europa,Io, andTitan, visible impact craters may becomeeroded, buried, or transformed bytectonics over time. In early literature, before the significance of impact cratering was widely recognised, the termscryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.[24] Molten terrestrial material ejected from a meteorite impact crater can cool and solidify into an object known as atektite. These are often mistaken for meteorites. Terrestrial rock, sometimes with pieces of the original meteorite, created or modified by an impact of a meteorite is calledimpactite.
^Millman, Peter M. (1961). "A report on meteor terminology".Journal of the Royal Astronomical Society of Canada.55:265–267.Bibcode:1961JRASC..55..265M.
^Beech, Martin; Steel, Duncan (September 1995). "On the Definition of the Term Meteoroid".Quarterly Journal of the Royal Astronomical Society.36 (3):281–284.Bibcode:1995QJRAS..36..281B.)
^Yeomans, Donald K.; Chodas, Paul; Chesley, Steve (November 9, 2009)."Small Asteroid 2009 VA Whizzes By the Earth". NASA's Near Earth Object Program Office.Archived from the original on November 12, 2009. Retrieved2013-01-28.
![Children posing behind a replica of the Tucson Meteorite at the Arizona Museum of Natural History[25]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMesa-Arizona_Museum_of_Natural_History-Tucson_Meteorite.JPG)
![Meteorite with brecciation and carbon inclusions from Tindouf, Algeria[26]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMeteorite_Tindouf_NWA_869.jpg)
