Editing Meteoroid (section)

== Meteoroids ==
{{See also|Micrometeoroid}}
[[Image:Meteoroid track through aerogel from EURECA mission.jpg|thumb|Meteoroid embedded in [[aerogel]]; the meteoroid is 10&nbsp;[[μm]] in diameter and its track is 1.5&nbsp;mm long]]
[[File:323213main Petersmeteorites 946-710.jpg|thumb|{{mpl|2008 TC|3}} [[#Meteoroids|meteorite]] fragments found on February 28, 2009, in the [[Nubian Desert]], [[Sudan]]]]

In 1961, the [[International Astronomical Union]] (IAU) defined 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 name="IMO-meteoroid"/><ref name="Millman1961"/> In 1995, Beech and Steel, writing in the ''[[Quarterly Journal of the Royal Astronomical Society]]'', proposed a new definition where a meteoroid would be between 100 [[Micrometre|μm]] and {{cvt|10|m|ft|}} across.<ref name="Steel1995"/> 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 {{cvt|10|μm|in}} and {{convert|1|m|spell=in|sp=us}} in diameter in order to maintain the distinction.<ref name="Rubin2010"/> 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 on [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] ''H'') are {{mp|2008 TS|26}} with ''H'' = 33.2<ref name="smallest-jpl"/> and {{mpl|2011 CQ|1}} with ''H'' = 32.1<ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2011CQ1;cad=1#cad |title=JPL Small-Body Database Browser: (2011 CQ1) |type=2011-02-04 last obs}}</ref> both with an estimated size of {{cvt|1|m|spell=in|sp=us}}.<ref name="NASA-166"/> In April 2017, the IAU adopted an official revision of its definition, limiting size to between {{cvt|30|μm|in}} and one meter in diameter, but allowing for a deviation for any object causing a meteor.<ref>{{cite news |author=Vincent Perlerin |title=Definitions of terms in meteor astronomy (IAU) |date=September 26, 2017 |work=News |publisher=[[International Meteor Organization]] |url=https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau/ |access-date=2018-01-22}}</ref>

Objects smaller than meteoroids are classified as [[micrometeoroid]]s and [[interplanetary dust cloud|interplanetary dust]]. The [[Minor Planet Center]] does not use the term "meteoroid".

=== Composition ===

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 as [[chondrule]]s and are called [[chondrite]]s. Stony meteoroids without these features are called "[[achondrite]]s", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron.<ref>{{cite web | last = Notkin | first = Geoffrey | title = Meteorite types and classification | work = Meteorwritings | publisher = Geology.com | url = http://geology.com/meteorites/meteorite-types-and-classification.shtml | access-date = 2014-03-02}}</ref> 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 ''(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>{{cite journal |last=Povenmire |first=Harold |url=http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1183.pdf |title=Physical Dynamics of the Upsilon Pegasid Fireball – European Network 190882A |journal=Lunar and Planetary Science Conference |page=1183 |bibcode=2000LPI....31.1183P |year=2000}}</ref> to nickel-iron rich dense rocks. The study of [[Meteorite#Meteorite types|meteorites]] also gives insights into the composition of non-ephemeral meteoroids.

=== In the Solar System ===
Most meteoroids come from the [[asteroid belt]], having been perturbed by the gravitational influences of planets, but others are particles from [[comet]]s, giving rise to [[meteor shower]]s. Some meteoroids are fragments from bodies such as Mars or the [[Moon]], 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 {{cvt|42|km/s|mph|}} through space in the vicinity of Earth's orbit. This is [[escape 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 {{cvt|29.6|km/s|mph|}}, so when meteoroids meet the atmosphere head-on (which only occurs when meteors are in a [[retrograde orbit]] such as the [[Leonids]], which are associated with the retrograde comet [[55P/Tempel–Tuttle]]) the combined speed may reach about {{cvt|71|km/s|mph|}} (see [[Specific energy#Astrodynamics]]). Meteoroids moving through Earth's orbital space average about {{cvt|20|km/s|mph|}},<ref>{{cite journal |author=Interagency Group (Space) Working Group on Orbital Debris |title=Report on Orbital Debris |date=February 1989 |website=NASA Technical Reports Server |page=1 |url=https://ntrs.nasa.gov/citations/19900003319 |hdl=2060/19900003319 |access-date=2023-05-31 |archive-date=2023-05-31 |archive-url=https://web.archive.org/web/20230531171611/https://ntrs.nasa.gov/citations/19900003319 |url-status=dead}}</ref> but due to Earth's gravity meteors such as the [[Phoenicids]] can make atmospheric entry at as slow as about 11&nbsp;km/s.

On January 17, 2013, at 05:21 PST, a one-meter-sized comet from the [[Oort cloud]] entered Earth atmosphere over [[California]] and [[Nevada]].<ref name="CAMS"/> The object had a retrograde orbit with perihelion at 0.98&nbsp;±&nbsp;0.03&nbsp;[[Astronomical unit|AU]]. It approached from the direction of the constellation [[Virgo (constellation)|Virgo]] (which was in the south about 50° above the horizon at the time), and collided head-on with Earth's atmosphere at {{cvt|72|±|6|km/s|mph}}<ref name="CAMS">{{cite web |title=2013 January 17 Sierra Nevada fireball |publisher=[[SETI Institute]] |first=Peter |last=Jenniskens |author-link=Peter Jenniskens |url=http://cams.seti.org/index-archive3.html |access-date=2014-11-16}} | {{cite web |url=http://www.seti.org/seti-institute/news/earth-collides-with-comet |title=Earth Collides Head-On with Small Comet |publisher=[[SETI Institute]] |access-date=2013-01-25 |archive-date=2013-01-28 |archive-url=https://web.archive.org/web/20130128052127/http://www.seti.org/seti-institute/news/earth-collides-with-comet}}</ref> vaporising more than {{cvt|100|km|ft|-4}} above ground over a period of several seconds.

=== Collision with Earth's atmosphere ===
{{main|Meteor}}
[[File:Meteoroid meteor meteorite.gif|Animated illustration of different phases as a meteoroid enters the Earth's atmosphere to become visible as a [[meteor]] and land as a [[meteorite]]|thumb|right]]

When meteoroids intersect with Earth's atmosphere at night, they are likely to become visible as [[#Meteors|meteors]]. If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are called [[#Meteorites|meteorites]]. Meteorites are transformed in structure and chemistry by the heat of entry and force of impact. A noted {{convert|4|m|adj=on|}} [[asteroid]], {{mpl|2008 TC|3}}, was observed in space on a collision course with Earth on 6&nbsp;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.<ref name="IMO-meteoroid"/> [[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.<ref>{{cite web |title=New Map Shows Frequency of Small Asteroid Impacts, Provides Clues on Larger Asteroid Population |url=https://www.jpl.nasa.gov/news/new-map-shows-frequency-of-small-asteroid-impacts-provides-clues-on-larger-asteroid-population/ |website=NASA Jet Propulsion Laboratory (JPL) |access-date=17 February 2025}}</ref>