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Meteoroid

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"Meteor" redirects here. For other uses, seeMeteor (disambiguation).
Ameteoroidshown entering the atmosphere, causing a visiblemeteorand hitting the Earth's surface, becoming ameteorite

Ameteoroid(/ˈmiːtiərɔɪd/MEE-tee-ə-royd)[1]is a small rocky or metallic body inouter space. Meteoroids are distinguished as objects significantly smaller thanasteroids,ranging in size from grains to objects up to a meter wide.[2]Objects smaller than meteoroids are classified asmicrometeoroidsorspace dust.[2][3][4]Many are fragments fromcometsor asteroids, whereas others arecollision impactdebrisejected from bodies such as theMoonorMars.[5][6][7]

Ameteororshooting star[8]is the visible passage of a meteoroid, comet, or asteroidentering Earth's atmosphere.At a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph),aerodynamic heatingof that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. Meteors typically become visible when they are about 100 km (62 mi) above sea level. 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 debrisenter Earth's atmosphere each day,[9]which results in an estimated 15,000 tonnes of that material entering the atmosphere each year.[10] Ameteoriteis the remains of a meteoroid that has survived theablationof its surface material during its passage through the atmosphere as a meteor and has impacted the ground.

Meteoroids

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See also:Micrometeoroid
Meteoroid embedded inaerogel;the meteoroid is 10μmin diameter and its track is 1.5 mm long
2008 TC3meteoritefragments found on February 28, 2009, in theNubian Desert,Sudan

In 1961, theInternational Astronomical Union(IAU) defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than anasteroidand considerably larger than an atom ".[11][12]In 1995, Beech and Steel, writing in theQuarterly Journal of the Royal Astronomical Society,proposed a new definition where a meteoroid would be between 100μmand 10 m (33 ft) across.[13]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 TS26withH= 33.2[14]and2011 CQ1withH= 32.1[15]both with an estimated size of one m (3 ft 3 in).[16]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.[17]

Objects smaller than meteoroids are classified asmicrometeoroidsandinterplanetary dust.TheMinor Planet Centerdoes not use the term "meteoroid".

Composition

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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 aschondrulesand 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.[18]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,[19]to nickel-iron rich dense rocks. The study ofmeteoritesalso gives insights into the composition of non-ephemeral meteoroids.

In the Solar System

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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 velocityfrom 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 orbitsuch 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),[20]but due to Earth's gravity meteors such as thePhoenicidscan 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 cloudentered Earth atmosphere overCaliforniaandNevada.[21]The object had a retrograde orbit with perihelion at 0.98 ± 0.03AU.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)[21]vaporising more than 100 km (330,000 ft) above ground over a period of several seconds.

Collision with Earth's atmosphere

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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.[11]NASAhas 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 (see below).

Meteors

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Several terms redirect here. For other uses, seeMeteor (disambiguation),Falling star (disambiguation),andShooting star (disambiguation).
Meteor seen from the site of theAtacama Large Millimeter Array(ALMA)[22]
World map of large meteoric events(also seeFireballbelow)[23]

Ameteor,known colloquially as ashooting starorfalling star,is the visible passage of a glowingmeteoroid,micrometeoroid,cometorasteroidthrough Earth's atmosphere, after being heated to incandescence by collisions with air molecules in the upper atmosphere,[11][24][25]creating a streak of light via its rapid motion and sometimes also by shedding glowing material in its wake. Although a meteor may seem to be a few thousand feet from the Earth,[26]meteors typically occur in themesosphereat altitudes from 76 to 100 km (250,000 to 330,000 ft).[27][28]The root wordmeteorcomes from theGreekmeteōros,meaning "high in the air".[24]

Millions of meteors occur in Earth's atmosphere daily. Most meteoroids that cause meteors are about the size of a grain of sand, i.e. they are usually millimeter-sized or smaller. Meteoroid sizes can be calculated from their mass and density which, in turn, can be estimated from the observed meteor trajectory in the upper atmosphere. [29] Meteors may occur inshowers,which arise when Earth passes through a stream of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific stream ofspace debris.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. The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (67,000 mph),[30]the orbital speeds of meteoroids, and thegravity wellof Earth.

Meteors become visible between about 75 to 120 km (250,000 to 390,000 ft) above Earth. They usually disintegrate at altitudes of 50 to 95 km (160,000 to 310,000 ft).[31]Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized. For bodies with a size scale larger than 10 cm (3.9 in) to several meters meteor visibility is due to the atmosphericram 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 vaporised meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second.

History

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Meteors were not known to be an astronomical phenomenon until early in the nineteenth century. Before 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. In 1807,Yale Universitychemistry professorBenjamin Sillimaninvestigated a meteorite that fell inWeston, Connecticut.[32]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.[33]People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Careful observers noticed thatthe radiant,as the point is called, moved with the stars, staying in the constellation Leo.[34]

The astronomerDenison Olmstedextensively studied this storm, concluding that it had a cosmic origin. After reviewing historical records,Heinrich Wilhelm Matthias Olberspredicted the storm's return in 1867, drawing other astronomers' attention to the phenomenon.Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved correct.[33]WithGiovanni Schiaparelli's success in connecting theLeonids(as they are called) with cometTempel-Tuttle,the cosmic origin of meteors was firmly established. Still, they remain an atmospheric phenomenon and retain their name "meteor" from the Greek word for "atmospheric".[35]

Fireball

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Main article:List of bolides
Footage of asuperbolide,a very bright fireball thatexploded over ChelyabinskOblast, Russia in 2013

Afireballis a brighter-than-usual meteor that also becomes visible when about 100 km from sea level. TheInternational Astronomical Union(IAU) defines a fireball as "a meteor brighter than any of the planets" (apparent magnitude−4 or greater).[36]TheInternational 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 atzenith.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.[37]

Fireballs reaching apparent magnitude −14 or brighter are calledbolides.[38]The IAU has no official definition of "bolide", and generally considers the term synonymous with "fireball". Astronomers often use "bolide" to identify an exceptionally bright fireball, particularly one that explodes in ameteor air burst.[39]They are sometimes called detonating fireballs. It may also be used to mean a fireball which creates audible sounds. In the late twentieth century, bolide has also come to mean any object that hits Earth and explodes, with no regard to its composition (asteroid or comet).[40]The wordbolidecomes from theGreekβολίς (bolis)[41]which can meana missileorto flash.If the magnitude of a bolide reaches −17 or brighter it is known as asuperbolide.[38][42]A relatively small percentage of fireballs hit Earth's atmosphere and then pass out again: these are termedEarth-grazing fireballs.Such an event happened inbroad daylight over North America in 1972.Another rare phenomenon is ameteor procession,where the meteor breaks up into several fireballs traveling nearly parallel to the surface of Earth.

A steadily growing number of fireballs are recorded at theAmerican Meteor Societyevery year.[43]There are probably more than 500,000 fireballs a year,[44]but most go unnoticed because most occur over the ocean and half occur during daytime. AEuropean Fireball Networkand a NASA All-sky Fireball Network detect and track many fireballs.[45]

Fireball Sightings reported to the American Meteor Society [43]
Year 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021
Number 734 676 953 1,660 2,183 3,599 3,789 4,250 5,391 5,510 5,993 6,978 8,259 9,629

Effect on atmosphere

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"Ionization trail" and "Dark flight (astronomy)" redirect here. For the movie, seeDark Flight.
A meteoroid of thePerseidswith a size of about ten millimetres entering the earth's atmosphere in real time. The meteorid is at the bright head of the trail, and the ionisation of themesosphereis still visible in the tail.

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 passage. During the entry of a meteoroid or asteroid into theupper atmosphere,anionization trailis created, where the air molecules areionizedby the passage of the meteor. Such ionization trails can last up to 45 minutes at a time.

Small,sand-grainsized 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 calledmeteor burst communications.Meteor radars can measure atmospheric density and winds by measuring thedecay rateandDoppler shiftof a meteor trail. Most meteoroids burn up when they enter the atmosphere. The left-over debris is calledmeteoric dustor 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.[46]Meteoroids or their fragments achievedark flightafter deceleration toterminal velocity.[47]Dark flight starts when they decelerate to about 2–4 km/s (4,500–8,900 mph).[48]Larger fragments fall further down thestrewn field.

Colours

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A meteor of theLeonid meteor shower;the photograph shows the meteor, afterglow, and wake as distinct components

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 colour of the light emitted may change according to the layering of minerals. Colours of meteors depend on the relative influence of the metallic content of the meteoroid versus the superheated air plasma, which its passage engenders:[49]

Acoustic manifestations

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The sound generated by a meteor in the upper atmosphere, such as asonic boom,typically arrives many seconds after the visual light from a meteor disappears. Occasionally, as with theLeonid meteor showerof 2001, "crackling", "swishing", or "hissing" sounds have been reported,[50]occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth'sauroras.[51][52][53][54]

Theories on the generation of these sounds may partially explain them. For example, scientists at NASA suggested that the turbulent ionized wake of a meteor interacts withEarth's magnetic field,generating pulses ofradio waves.As the trail dissipates,megawattsof electromagnetic power could be released, with a peak in thepower spectrumataudio frequencies.Physicalvibrationsinduced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames, the hearer's own body (seemicrowave auditory effect), and other conductive materials vibrate.[55][56][57][58]This proposed mechanism, although proven plausible by laboratory work, remains unsupported by corresponding measurements in the field. Sound recordings made under controlled conditions in Mongolia in 1998 support the contention that the sounds are real.[59](Also seeBolide.)

Meteor shower

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Main articles:Meteor showerandList of meteor showers
Multiple meteors photographed over an extendedexposure timeduring ameteor shower
Meteor showeron chart

Ameteor showeris the result of an interaction between a planet, such as Earth, and streams of debris from acometor other source. The passage of Earth through cosmic debris from comets and other sources is arecurring eventin many cases. Comets can produce debris by water vapor drag, as demonstrated byFred Whipplein 1951,[60]and by breakup. Each time a comet swings by the Sun in itsorbit,some of its ice vaporizes and a certain amount of meteoroids are shed. The meteoroids spread out along the entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure).

The frequency offireballsightings increases by about 10–30% during the weeks ofvernal equinox.[61]Evenmeteoritefalls 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. Others have pointed out that during this period the ecliptic is (in the northern hemisphere) high in the sky in the late afternoon and early evening. This means that fireball radiants with an asteroidal source are high in the sky (facilitating relatively high rates) at the moment the meteoroids "catch up" with Earth, coming from behind going in the same direction as Earth. This causes relatively low relative speeds and from this low entry speeds, which facilitates survival of meteorites.[62]It also generates high fireball rates in the early evening, increasing chances of eyewitness reports. This explains a part, but perhaps not all of the seasonal variation. Research is in progress for mapping the orbits of the meteors to gain a better understanding of the phenomenon.[63]

Notable meteors

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See also:Near-Earth object § Notable objects
Comparison of approximate sizes of notable impactors with the Hoba meteorite, a Boeing 747 and a New Routemaster bus
1992 – Peekskill, New York
ThePeekskill Meteoritewas recorded on October 9, 1992 by at least 16 independent videographers.[64]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 aground pathof some 430 to 500 mi (700 to 800 km).[65]One meteorite recovered atPeekskill, New York,for which the event and object gained their name, had a mass of 27 lb (12.4 kg) and was subsequently identified as an H6 monomict breccia meteorite.[66]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.
2009 – Bone, Indonesia
Alarge fireballwas observed in the skies nearBone,Sulawesi,Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 m (33 ft) in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice theNagasaki atomic bomb.No injuries were reported.[67]
2009 – Southwestern US
A large bolide was reported on 18 November 2009 over southeastern California, northern Arizona, Utah, Wyoming, Idaho and Colorado. At 00:07 local time a security camera at the high altitudeW. L. Eccles Observatory(9,610 ft (2,930 m) above sea level) recorded a movie of the passage of the object to the north.[68][69]It had a spherical "ghost" image slightly trailing the main object (likely a lens reflection of the intense fireball), and a bright fireball explosion associated with the breakup of a substantial fraction of the object. An object trail continued northward after the fireball. The shock from the final breakup triggered seven seismological stations in northern Utah. The seismic data yielded a terminal location of the object at 40.286 N, −113.191 W, altitude 90,000 ft (27 km).[citation needed]This is above the Dugway Proving Grounds, a closed Army testing base.
2013 – Chelyabinsk Oblast, Russia
TheChelyabinsk meteorwas an extremely bright, exploding fireball, orsuperbolide,measuring about 17 to 20 m (56 to 66 ft) across, with an estimated mass of 11,000 tonnes as the relatively smallasteroidentered Earth's atmosphere.[70][71]It was the largest natural object known to have entered Earth's atmosphere since theTunguska eventin 1908. Over 1,500 people were injured, mostly by glass from shattered windows caused by theair burstapproximately 25 to 30 km (80,000 to 100,000 ft) above theenvirons of Chelyabinsk,Russia on 15 February 2013. An increasingly bright streak was observed during morning daylight with a large contrail lingering behind. At no less than one minute and up to at least three minutes after the object peaked in intensity (depending on distance from trail), a large concussive blast was heard that shattered windows and set-off car alarms, which was followed by a number of smaller explosions.[72]
2019 – Midwestern United States
On November 11, 2019, a meteor was spotted streaking across the skies of the Midwestern United States. In theSt. LouisArea, security cameras, dashcams, webcams, and video doorbells captured the object as it burned up in the earth's atmosphere. Thesuperbolidemeteor was part of the South Taurids meteor shower.[73]It traveled east to west ending its visible flight path somewhere over the US state of South Carolina becoming visible once again as it entered the earth's atmosphere creating a large fireball. The fireball was brighter than the planetVenusin the night sky.[74]
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Meteorites

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Main article:Meteorite
Murnpeowiemeteorite,aniron meteoritewithregmaglyptsresembling thumbprints (Australia, 1910)

A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and hits the ground without being destroyed.[75]Meteorites are sometimes, but not always, found in association with hypervelocityimpact craters;during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.Geologistsuse the term, "bolide", in a different sense fromastronomersto indicate a very largeimpactor.For example, theUSGSuses 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".[76]

Meteoroids also hit other bodies in the Solar System. On such stony bodies as theMoonorMarsthat have little or no atmosphere, they leave enduring craters.

Impact craters

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Main article:Impact crater

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 bytectonicsover time. In early literature, before the significance of impact cratering was widely recognised, the termscryptoexplosionor cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.[77]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.

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See also

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Relating to meteoroids

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Relating to meteors

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Relating to meteorites

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References

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