Editing Asteroid belt (section)

===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.