Editing Asteroid belt (section)

==Characteristics==
[[File:Main belt asteroid size distribution.svg|thumb|upright=1.6|Size distribution of asteroids in the main belt<ref>{{cite book | chapter=Collisional Evolution of Small-Body Populations | last1=Davis | first1=D. R. | last2=Durda | first2=D. D. | last3=Marzari | first3=F. | last4=Campo Bagatin | first4=A. | last5=Gil-Hutton | first5=R. | title=Asteroids III | editor1-first=W. F. | editor1-last=Bottke Jr. | editor2-first=A. | editor2-last=Cellino | editor3-first=P. | editor3-last=Paolicchi | editor4-first=R. P. | editor4-last=Binzel | publisher=University of Arizona Press | location=Tucson | pages=545–558 | date=March 2002 | doi=10.2307/j.ctv1v7zdn4.41 | bibcode=2002aste.book..545D | isbn=978-0-8165-2281-1 | chapter-url=https://www.lpi.usra.edu/books/AsteroidsIII/pdf/3029.pdf | access-date=2023-02-11 }}</ref>]]
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100&nbsp;km,<ref>{{cite web
| last = Yeomans
| first = Donald K.
| date = April 26, 2007
| url = http://ssd.jpl.nasa.gov/sbdb_query.cgi
| title = JPL Small-Body Database Search Engine
| publisher = NASA JPL
| access-date = 2007-04-26
}}&nbsp;– search for asteroids in the main belt regions with a diameter&nbsp;>100.</ref> and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7&nbsp;million asteroids with a diameter of 1&nbsp;km or more.<ref>{{cite journal
| last1=Tedesco | first1=E. F. | last2=Desert | first2=F.-X. | title=The Infrared Space Observatory Deep Asteroid Search
| journal=The Astronomical Journal
| year=2002
| volume=123
| issue=4
| pages=2070–2082
| bibcode=2002AJ....123.2070T| doi = 10.1086/339482
| doi-access=free
}}</ref>

The number of asteroids in the main belt steadily increases with decreasing size. Although the size distribution generally follows a [[power law]], there are 'bumps' in the curve at about {{val|5|u=km}} and {{val|100|u=km}}, where more asteroids than expected from such a curve are found. Most asteroids larger than approximately {{val|120|u=km}} in diameter are primordial, having survived from the accretion epoch, whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.<ref>{{Cite journal | last1=Bottkejr | first1=W. | last2=Durda | first2=D. | last3=Nesvorny | first3=D. | last4=Jedicke | first4=R. | last5=Morbidelli | first5=A. | last6=Vokrouhlicky | first6=D. | last7=Levison | first7=H. | date=May 2005 | title=The fossilized size distribution of the main asteroid belt | url=https://linkinghub.elsevier.com/retrieve/pii/S0019103504003811 | journal=Icarus | language=en | volume=175 | issue=1 | pages=111–140 | doi=10.1016/j.icarus.2004.10.026 | bibcode=2005Icar..175..111B }}</ref><ref>{{Cite journal | last1=O'Brien | first1=David P. | last2=Sykes | first2=Mark V. | date=December 2011 | title=The Origin and Evolution of the Asteroid Belt—Implications for Vesta and Ceres | url=http://link.springer.com/10.1007/s11214-011-9808-6 | journal=Space Science Reviews | language=en | volume=163 | issue=1–4 | pages=41–61 | doi=10.1007/s11214-011-9808-6 | bibcode=2011SSRv..163...41O | s2cid=121856071 | issn=0038-6308 }}</ref>

On average the distance between the asteroids is about {{convert|965600|km|miles|abbr=on}},<ref name="EarthSky Updates on your cosmos and world 2021">{{cite web | title=The asteroid belt contains solar system remnants | website=EarthSky {{!}} Updates on your cosmos and world | date=2021-11-03 | url=https://earthsky.org/space/what-is-the-asteroid-belt/ | access-date=2023-01-20}}</ref><ref name="Williams 2016">{{cite web | last=Williams | first=Matt | title=How Far is the asteroid Belt from Earth? | website=Universe Today | date=2016-08-10 | url=https://www.universetoday.com/130136/far-asteroid-belt-earth/ | access-date=2023-01-20}}</ref> although this varies among asteroid families and smaller undetected asteroids might be even closer. The total mass of the asteroid belt is estimated to be {{val|2.39e21}} kg, which is 3% of the mass of the Moon.<ref name="Pitjeva2018"/> The four largest objects, Ceres, Vesta, Pallas, and Hygiea, contain an estimated 62% of the belt's total mass, with 39% accounted for by Ceres alone.<ref>{{Cite web|title=In Depth &#124; Ceres|url=https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/in-depth|access-date=2023-02-11|website=NASA Solar System Exploration|date=November 9, 2017 }}</ref><ref name="halfmass">For recent estimates of the masses of Ceres, Vesta, Pallas and Hygiea, see the references in the infoboxes of their respective articles.</ref>

===Composition===
{{main|Asteroid spectral types}}
[[File:Asteroid populations by orbital distance.svg|upright=1.6|right|thumb|Distribution of asteroid spectral types by distance from the Sun<ref>{{cite journal | title=Compositional Structure of the asteroid Belt | last1=Gradie | first1=J. | last2=Tedesco | first2=E. | journal=Science | date=June 1982 | volume=216 | issue=4553 | pages=1405–1407 | doi=10.1126/science.216.4553.1405 | pmid=17798362 | bibcode=1982Sci...216.1405G | s2cid=32447726 }}</ref>]]
The present day belt consists primarily of three categories of asteroids: C-type carbonaceous asteroids, [[S-type asteroid|S-type]] silicate asteroids, and a hybrid group of X-type asteroids. The hybrid group have featureless spectra, but they can be divided into three groups based on reflectivity, yielding the [[M-type asteroid|M-type]] metallic, P-type primitive, and E-type [[enstatite]] asteroids. Additional types have been found that do not fit within these primary classes. There is a compositional trend of asteroid types by increasing distance from the Sun, in the order of S, C, P, and the spectrally-featureless [[D-type asteroid|D-types]].<ref name=DeMeo_et_al_2015>{{cite book
 | chapter=The Compositional Structure of the Asteroid Belt
 | last1=DeMeo | first1=F. E. | last2=Alexander | first2=C. M. O'D.
 | last3=Walsh | first3=K. J. | last4=Chapman | first4=C. R.
 | last5=Binzel | first5=R. P.
 | title=Asteroids IV
 | editor1-first=Patrick | editor1-last=Michel
 | editor2-first=Francesca E. | editor2-last=DeMeo
 | editor3-first=William F. | editor3-last=Bottke
 | publisher=University of Arizona Press
 | location=Tucson | isbn=978-0-816-53213-1 | pages=13–41
 | year=2015 | doi=10.2458/azu_uapress_9780816532131-ch002 
 | arxiv=1506.04805 | bibcode=2015aste.book...13D | s2cid=4648806 }}</ref>

[[File:AllendeMeteorite.jpg|right|thumb|Fragment of the [[Allende meteorite]], a carbonaceous chondrite that fell to Earth in Mexico in 1969]]
[[C-type asteroid|Carbonaceous asteroids]], as their name suggests, are carbon-rich. They dominate the asteroid belt's outer regions,<ref name="ApJ133">{{cite journal
| last1=Wiegert | first1=P. | last2=Balam | first2=D. | last3=Moss | first3=A. | last4=Veillet | first4=C. | last5=Connors | first5=M. | last6=Shelton | first6=I. | title=Evidence for a Color Dependence in the Size Distribution of Main-Belt Asteroids
| journal=The Astronomical Journal
| year=2007
| volume=133
| issue=4
| pages=1609–1614
| url=http://astro.uwo.ca/~wiegert/papers/2007AJ.133.1609.pdf |archive-url=https://web.archive.org/web/20110706212244/http://astro.uwo.ca/~wiegert/papers/2007AJ.133.1609.pdf |archive-date=2011-07-06 |url-status=live
| access-date=2008-09-06| doi = 10.1086/512128
| bibcode=2007AJ....133.1609W
|arxiv = astro-ph/0611310 |s2cid=54937918}}</ref> and are rare in the inner belt.<ref name=DeMeo_et_al_2015/> Together they comprise over 75% of the visible asteroids. They are redder in hue than the other asteroids and have a low [[albedo]]. Their surface compositions are similar to [[carbonaceous chondrite]] [[meteorite]]s. Chemically, their spectra match the primordial composition of the early Solar System, with hydrogen, helium, and [[Volatile (astrogeology)|volatiles]] removed.<ref name=Blair_2002>{{cite book
 | title=Asteroids: Overview, Abstracts, and Bibliography
 | page=2 | year=2002 | isbn=978-1-59033-482-9
 | publisher=Nova Science Publishers
 | editor-first=Edward C. | editor-last=Blair
 | url=https://books.google.com/books?id=oa289IxCvAAC&pg=PA2 }}</ref>

S-type ([[silicate]]-rich) asteroids are more common toward the inner region of the belt, within 2.5&nbsp;AU of the Sun.<ref name="ApJ133" /><ref>{{cite journal
| last=Clark
| first=B. E.
| title=New News and the Competing Views of Asteroid Belt Geology
| journal=Lunar and Planetary Science
| year=1996
| volume=27
| pages=225–226
| bibcode=1996LPI....27..225C }}</ref> The spectra of their surfaces reveal the presence of silicates and some metal, but no significant carbonaceous compounds. This indicates that their materials have been significantly modified from their primordial composition, probably through melting and reformation. They have a relatively high albedo and form about 17% of the total asteroid population.<ref name=Blair_2002/>

M-type (metal-rich) asteroids are typically found in the middle of the main belt, and they make up much of the remainder of the total population.<ref name=Blair_2002/> Their spectra resemble that of iron-nickel. Some are believed to have formed from the metallic cores of differentiated progenitor bodies that were [[Disrupted planet|disrupted]] through collision. However, some silicate compounds also can produce a similar appearance. For example, the large M-type asteroid [[22&nbsp;Kalliope]] does not appear to be primarily composed of metal.<ref>{{cite journal
| last1=Margot | first1=J. L. | last2=Brown | first2=M. E. | title=A Low-Density M-type Asteroid in the Main Belt
| journal=Science
| year=2003
| volume=300
| issue=5627
| pages=1939–1942
| bibcode=2003Sci...300.1939M| doi = 10.1126/science.1085844
| pmid=12817147
|s2cid=5479442|url=https://pdfs.semanticscholar.org/d014/0c3feee1d41f01ff67e1637b4d367da520ef.pdf|archive-url=https://web.archive.org/web/20200226121156/https://pdfs.semanticscholar.org/d014/0c3feee1d41f01ff67e1637b4d367da520ef.pdf|url-status=dead|archive-date=2020-02-26}}</ref> Within the asteroid belt, the number distribution of M-type asteroids peaks at a semimajor axis of about 2.7&nbsp;AU.<ref name="lang2003">{{cite web
| last = Lang
| first = Kenneth R.
| year = 2003
| url = http://ase.tufts.edu/cosmos/print_images.asp?id=15
| title = Asteroids and meteorites
| publisher = NASA's Cosmos
| access-date = 2007-04-02
| archive-date = 2012-03-24
| archive-url = https://web.archive.org/web/20120324083246/http://ase.tufts.edu/cosmos/print_images.asp?id=15
| url-status = dead
}}</ref> Whether all M-types are compositionally similar, or whether it is a label for several varieties which do not fit neatly into the main C and S classes is not yet clear.<ref>{{cite journal
| last1=Mueller | first1=M. | last2=Harris | first2=A. W. | last3=Delbo | first3=M. | others=the MIRSI Team
| title=21 Lutetia and other M-types: Their sizes, albedos, and thermal properties from new IRTF measurements
| journal=Bulletin of the American Astronomical Society
| year=2005| volume=37| page=627
| bibcode=2005DPS....37.0702M }}</ref>

One mystery is the relative rarity of [[V-type asteroid|V-type]] (Vestoid) or [[basaltic]] asteroids in the asteroid belt.<ref name="Duffard">{{cite conference
| last1=Duffard | first1=R. D. | last2=Roig | first2=F. | title=Two New Basaltic Asteroids in the Main Belt?
| book-title=Asteroids, Comets, Meteors 2008
| date=July 14–18, 2008| location=Baltimore, Maryland
| bibcode=2008LPICo1405.8154D| arxiv=0704.0230 }}</ref> Theories of asteroid formation predict that objects the size of Vesta or larger should form crusts and mantles, which would be composed mainly of basaltic rock, resulting in more than half of all asteroids being composed either of basalt or of [[olivine]]. However, observations suggest that 99% of the predicted basaltic material is missing.<ref name="olivine">{{cite web|title=Strange Asteroids Baffle Scientists| last=Than | first=Ker | year=2007|work=space.com|url=http://www.space.com/scienceastronomy/070821_basalt_asteroid.html|access-date=2007-10-14}}</ref> Until 2001, most basaltic bodies discovered in the asteroid belt were believed to originate from the asteroid Vesta (hence their name V-type), but the discovery of the asteroid [[1459&nbsp;Magnya]] revealed a slightly different chemical composition from the other basaltic asteroids discovered until then, suggesting a different origin.<ref name="olivine" /> This hypothesis was reinforced by the further discovery in 2007 of two asteroids in the outer belt, [[7472&nbsp;Kumakiri]] and {{mpl|(10537) 1991 RY|16}}, with a differing basaltic composition that could not have originated from Vesta. These two are the only V-type asteroids discovered in the outer belt to date.<ref name="Duffard" />

[[File:Hubble views extraordinary multi-tailed asteroid P2013 P5.jpg|thumb|[[Hubble Space Telescope|Hubble]] views the  multi-tailed cometary asteroid [[P/2013&nbsp;P5]].<ref>{{cite news|title=When is a comet not a comet?|url=http://www.spacetelescope.org/news/heic1320/|access-date=12 November 2013|newspaper=ESA/Hubble Press Release}}</ref> ]]

The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200&nbsp;K (−73&nbsp;°C) at 2.2&nbsp;AU down to 165&nbsp;K (−108&nbsp;°C) at 3.2&nbsp;AU.<ref>{{cite journal
| last1=Low | first1=F. J. | last2=Beintema | first2=D. A.
| last3=Gautier | first3=T. N. | last4=Gillett | first4=F. C.
| last5=Beichman | first5=C. A. | last6=Neugebauer | first6=G.
| last7=Young | first7=E. | last8=Aumann | first8=H. H.
| last9=Boggess | first9=N. | author-link9=Nancy Boggess|last10=Emerson | first10=J. P.
| last11=Habing | first11=H. J. | last12=Hauser | first12=M. G.
| last13=Houck | first13=J. R. | last14=Rowan-Robinson | first14=M.
| last15=Soifer | first15=B. T. | last16=Walker | first16=R. G.
| last17=Wesselius | first17=P. R.
| title=Infrared cirrus&nbsp;– New components of the extended infrared emission
| journal=Astrophysical Journal Letters
| year=1984
| volume=278
| pages=L19–L22
| bibcode=1984ApJ...278L..19L
| doi=10.1086/184213 }}</ref> However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation then to the stellar background.

===Main-belt comets===
{{Main|Main-belt comet}}
Several otherwise unremarkable bodies in the outer belt show [[comet]]ary activity. Because their orbits cannot be explained through the capture of classical comets,  many of the outer asteroids are thought to be icy, with the ice occasionally exposed to sublimation through small impacts. Main-belt comets may have been a major source of the Earth's oceans because the deuterium-hydrogen ratio is too low for classical comets to have been the principal source.<ref>{{cite web|url=https://www.youtube.com/watch?v=B1W4NTmI5Bk| archive-url=https://ghostarchive.org/varchive/youtube/20211030/B1W4NTmI5Bk| archive-date=2021-10-30|title=Interview with David Jewitt|publisher=YouTube.com|date=2007-01-05|access-date=2011-05-21}}{{cbignore}}</ref>

===Orbits===
[[Image:Main belt e vs a.png|thumb|300px|right|The asteroid belt (showing eccentricities), with the asteroid belt in red and blue ("core" region in red)]]
Most asteroids within the asteroid belt have orbital eccentricities of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids reaches a maximum at an eccentricity around 0.07 and an inclination below 4°.<ref name="mpc">{{cite web
| last = Williams
| first = Gareth
|date=September 25, 2010
| url = http://www.minorplanetcenter.org/iau/lists/MPDistribution.html
| title = Distribution of the Minor Planets
| publisher = Minor Planet Center
| access-date = 2010-10-27
}}</ref> Thus, although a typical asteroid has a relatively circular orbit and lies near the plane of the [[ecliptic]], some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.

Sometimes, the term "main belt" is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 [[Kirkwood gap]]s at 2.06 and 3.27&nbsp;AU, and at [[eccentricity (orbit)|orbital eccentricities]] less than roughly 0.33, along with orbital [[inclination]]s below about 20°. {{as of|2006}}, this "core" region contained 93% of all discovered and numbered minor planets within the Solar System.<ref name="basedon1">This value was obtained by a simple count of all bodies in that region using data for 120,437 numbered minor planets from the [http://www.minorplanetcenter.org/iau/MPCORB.html Minor Planet Center orbit database], dated February 8, 2006.</ref> The [[JPL Small-Body Database]] lists over 1 million known main-belt asteroids.<ref name="JPL-MBA">{{cite web
|title = JPL Small-Body Database Search Engine: orbital class (MBA)
|publisher = JPL Solar System Dynamics
|url = http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=MBA;OBJ_field=0;ORB_field=0;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsCkCqAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AcA
|access-date=2018-02-26}}</ref>

====Kirkwood gaps====
{{Main|Kirkwood gap}}
[[File:Kirkwood Gaps.svg|320px|thumb|Number of asteroids in the main belt as a function of their [[Semi-major and semi-minor axes|semimajor axis]] (a). The dashed lines indicate [[Kirkwood gaps]], while colors designate the following zones:<br />{{legend2|#005aff|border=1px solid #333|I: inner main-belt ({{nowrap|[[Semi-major and semi-minor axes|''a'']] < 2.5 [[Astronomical unit|AU]]}})}}<br />{{legend2|#ffa500|border=1px solid #222|II: middle main-belt ({{nowrap|2.5 AU < ''a'' < 2.82 AU}})}}<br />{{legend2|#55d400|border=1px solid #333|III: outer main-belt ({{nowrap|''a'' > 2.82 AU}})}}]]

The [[Semi-major and semi-minor axes|semimajor axis]] of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's [[orbital period]]. In 1866, [[Daniel Kirkwood]] announced the discovery of gaps in the distances of these bodies' orbits from the Sun. They were located in positions where their period of revolution about the Sun was an integer fraction of Jupiter's orbital period. Kirkwood proposed that the gravitational perturbations of the planet led to the removal of asteroids from these orbits.<ref>{{cite journal
| last=Fernie
| first=J. Donald
| title=The American Kepler
| journal=American Scientist
| year=1999
| volume=87
| issue=5
| page=398
| url=http://www.americanscientist.org/issues/pub/1999/9/the-american-kepler/2
| access-date=2007-02-04
| doi=10.1511/1999.5.398
| doi-broken-date=February 27, 2025
| archive-date=June 21, 2017
| archive-url=https://web.archive.org/web/20170621121158/http://www.americanscientist.org/issues/pub/1999/9/the-american-kepler/2
| url-status=dead
}}</ref>

When the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter, a [[mean-motion resonance]] with the gas giant is created that is sufficient to perturb an asteroid to new [[orbital element]]s. Primordial asteroids entered these gaps because of the migration of Jupiter's orbit.<ref>{{cite journal
| last1=Liou | first1=Jer-Chyi | last2=Malhotra | first2=Renu | title=Depletion of the Outer Asteroid Belt
| journal=Science| year=1997
| volume=275| issue=5298| pages=375–377
| doi = 10.1126/science.275.5298.375
| pmid=8994031
|bibcode = 1997Sci...275..375L|hdl=2060/19970022113|s2cid=33032137| hdl-access=free}}</ref> Subsequently, asteroids primarily migrate into these gap orbits due to the [[Yarkovsky effect]],<ref name=DeMeo_et_al_2015/> but may also enter because of perturbations or collisions. After entering, an asteroid is gradually nudged into a different, random orbit with a larger or smaller semimajor axis.