Asteroid belt
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[[Image:InnerSolarSystem-en.png|350px|thumb|The main asteroid belt (shown in white) is located between the orbits of [[Mars]] and [[Jupiter]].]]
The '''asteroid belt''' is the region of the [[Solar System]] located roughly between the orbits of the [[planet]]s [[Mars]] and [[Jupiter]]. It is occupied by numerous irregularly shaped bodies called [[asteroid]]s or [[minor planet]]s. The asteroid belt region is also termed the '''main belt''' to distinguish it from other concentrations of minor planets within the Solar System, such as the [[Kuiper belt]] and [[scattered disk]].
More than half the mass within the main belt is contained in the four largest objects: [[Ceres (dwarf planet)|Ceres]], [[4 Vesta]], [[2 Pallas]], and [[10 Hygiea]]. All of these have mean diameters of more than 400 km, while Ceres, the main belt's only [[dwarf planet]], is about 950 km in diameter.<ref name=Krasinskyetal2002/><ref name=Pitjeva2005/><ref name=halfmass/><ref name="jplsbdb"/> The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that multiple unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form an [[asteroid family]] whose members have similar orbital characteristics and compositions. Collisions also produce a fine dust that forms a major component of the [[zodiacal light]]. Individual asteroids within the main belt are categorized by their [[spectrum|spectra]], with most falling into three basic groups: [[carbon]]aceous ([[C-type asteroid|C-type]]), [[silicate]] ([[S-type asteroid|S-type]]), and [[metal]]-rich ([[M-type asteroid|M-type]]).
The asteroid belt formed from the primordial [[solar nebula]] as a group of [[planetesimal]]s, the smaller precursors of the planets. Between Mars and Jupiter, however, [[gravity|gravitational]] perturbations from the giant planet imbued the planetesimals with too much orbital energy for them to [[accretion (astrophysics)|accrete]] into a planet. Collisions became too violent, and instead of sticking together, the planetesimals shattered. As a result, most of the main belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably [[Perturbation (astronomy)|perturbed]] whenever their period of revolution about the Sun forms an [[orbital resonance]] with Jupiter. At these orbital distances, a [[Kirkwood gap]] occurs as they are swept into other orbits.
==History of observation==
{{seealso|Definition of planet}}
[[Image:Giuseppe Piazzi.jpg|thumb|200 px|[[Giuseppe Piazzi]], discoverer of the first object, Ceres]]
In an anonymous footnote to his 1766 translation of [[Charles Bonnet]]'s ''Contemplation de la Nature'',<ref name=asteroids>{{cite web|title=When Did the Asteroids Become Minor Planets?|author=J. Hilton|wgjhgjuytijork=US Nava Observatory|year=2001|url=http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html|accessdate=2007-10-01}}</ref> the astronomer [[Johann Daniel Titius]] von [[Wittenburg]]<ref name=Dawn>{{cite web|title=Dawn: A Journey to the Beginning of the Solar System|work=Space Physics Center: UCLA|year=2005|url=http://www-ssc.igpp.ucla.edu/dawn/background.html|accessdate=2007-11-03}}</ref><ref name=Hoskin>{{cite web|title=Bode's Law and the Discovery of Ceres|author=Michael Hoskin|work=Churchill College, Cambridge|url=http://www.astropa.unipa.it/versione_inglese/Hystory/Bode's Law.htm|accessdate=2007-10-01}}</ref> noted an apparent pattern in the layout of the planets. If one began a numerical sequence at 0, then included 3, 6, 12, 24, 48, etc., doubling each time, and added four to each number and divided by 10, this produced a remarkably close approximation to the orbits of the known planets as measured in [[astronomical units]] (one astronomical unit, or AU, equals the average distance between the Earth and the Sun). This pattern, now known as the [[Titius-Bode Law]], predicted the [[semi-major axes]] of the six planets of the time (Mercury, Venus, Earth, Mars Jupiter and Saturnffytuyt) provided one allowed for a "gap" between the orbits of Mars and Jupiter. In his footnote Titius declared, "But should the Lord Architect have left that space empty? Not at all".<ref name=Dawn /> In 1768, the astronomer [[Johann Elert Bode]] made note of Titius's relationship in his ''Anleitung zur Kenntniss des gestirnten Himmels'' but did not credit Titius, which led many to refer to it as "Bode's law".<ref name=Hoskin /> When [[William Herschel]] discovered [[Uranus]] in 1781, the planet's position matched the law almost perfectly; leading astronomers to conclude that there had to be a planet between the orbits of Mars and Jupiter.
In 1800, astronomer Baron [[Franz Xaver von Zach]] recruited 24 of his fellows into an informal club he dubbed the "Lilienthal Society". Determined to bring the Solar System to order, the group became known as the "Himmelspolitzei", or Celestial Police. Notable members included Herschel, British astronomer Royal [[Nevil Maskelyne]], [[Charles Messier]], and [[Heinrich Olbers]].<ref name=police>{{cite journal|title=Call the police! The story behind the discovery of the asteroids|journal=[[Astronomy Now]]|issue=June 2007|pages=60–61}}</ref> Each of the 24 astronomers was assigned a 15° region of the [[zodiac]] in which to search for the missing planet.<ref>{{cite web|title= An Introduction to Solar System Astronomy: Lecture 45: Is Pluto a Planet?|author= Prof. Richard Pogge|work= An Introduction to Solar System Astronomy|publisher= [[Ohio State University]]|year=2006|url=http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit6/dwarfs.html
|accessdate=2007-11-11}}</ref>
Only a few months later, a non-member of the Celestial Police confirmed their expectations. On [[January 1]], [[1801]], [[Giuseppe Piazzi]], Chair of Astronomy at the [[University of Palermo]], [[Sicily]], found a tiny moving object in the exact location predicted by the Titius-Bode Law. He dubbed it [[Ceres (dwarf planet)|Ceres]], after the [[Ceres (mythology)|Roman goddess]] of the harvest and patron of Sicily. Piazzi initially believed it a comet, but its lack of a [[Coma (cometary)|coma]] suggested it was a planet.<ref name=police /> Fifteen months later, Olbers discovered a second object in the same region, [[2 Pallas|Pallas]]. Unlike the other known planets, the objects remained points of light even under the highest telescope magnifications, rather than resolving into discs. Apart from their rapid movement, they were indistinguishable from [[star]]s. Accordingly, in 1802 William Herschel suggested they be placed into a separate category, named ''asteroids,'' after the [[Ancient Greek language|Greek]] ''asteroeides'', meaning "star-like".<ref>{{cite web|title=etymonline: asteroid|url=http://www.etymonline.com/index.php?search=asteroid&searchmode=none|accessdate=2007-11-05}}</ref><ref name="aster-root">{{cite web
| author= DeForest, Jessica
| date = 2000
| url = http://www.msu.edu/~defores1/gre/roots/gre_rts_afx2.htm
| title = Greek and Latin Roots
| publisher = Michigan State University
| accessdate = 2007-07-25
}}</ref> Upon completing a series of observations of Ceres and Pallas, he concluded,<ref>{{cite web|title=William Hershel and the First Two Asteroids|author=Clifford Cunningham|work=Dance Hall Observatory, Ontario|year=1984
|url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1984MPBu...11....3C&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES&high=471afd7a7403993
|accessdate=2007-11-05}}</ref>
<blockquote>Neither the appellation of planets, nor that of comets, can with any propriety of language be given to these two stars ... [They] resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself however the liberty of changing that name, if another, more expressive of their nature, should occur.</blockquote>
Despite Herschel's reservations, for several decades it remained common practice to refer to these objects as planets.<ref name=asteroids /> By 1807, further investigation revealed two new objects in the region: [[3 Juno]] and 4 Vesta.<ref name="serendipity">{{cite web
| author=Staff
| year=2002
| url = http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_06.asp
| title = Astronomical Serendipity
| publisher = NASA JPL
| accessdate = 2007-04-20
}}</ref> The [[Napoleonic wars]] brought this first period of discovery to a close,<ref name="serendipity" /> and it was not until 1845 that another object ([[5 Astraea]]) was
discovered. Shortly thereafter new objects were found at an accelerating rate, and counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list and William Herschel's choice of nomenclature, asteroids, at last came into common use.<ref name=asteroids />
The discovery of [[Neptune]] in 1846 led to the discrediting of the Titius-Bode Law in the eyes of scientists, as its orbit was nowhere near the predicted position. To date, there is no scientific explanation for the law, and the consensus among astronomers is that it is a coincidence.<ref>{{cite web|title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries?|work=astronomy.com|url=http://www.astronomy.com/asy/default.aspx?c=a&id=4494|accessdate=2007-10-16}}</ref>
One hundred asteroids had been located by mid-1868, and in 1891 the introduction of [[astrophotography]] by [[Max Wolf]] accelerated the rate of discovery still further.<ref>{{cite web
| first=David W. | last=Hughes | year=2007
| url = http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/history.html
| title = A Brief History of Asteroid Spotting
| publisher = BBC
| accessdate = 2007-04-20
}}</ref> A total of 1,000 asteroids had been found by 1923, 10,000 by 1951, and 100,000 by 1982.<ref name="jplsbdb">{{cite web
| first=Donald K.
| last=Yeomans
| date = [[July 13]], [[2006]]
| url = http://ssd.jpl.nasa.gov/sbdb.cgi
| title = JPL Small-Body Database Browser
| publisher = NASA JPL
| accessdate = 2007-04-25
}} — Asteroids are numbered by order of discovery.</ref> Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.
== Origin ==
[[Image:Main belt i vs a.png|thumb|300px|right|The asteroid belt (showing inclinations), with the main belt in red and blue ("core" region in red)]]
=== Formation ===
In [[1802]], [[Heinrich Olbers]] suggested to [[William Herschel]] that the belt had been formed from a [[Phaeton (hypothetical planet)|planet that somehow shattered]].<ref>{{cite web|title=A Brief History of Asteroid Spotting|work=Open2.net|url=http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/history.html|accessdate=2007-05-15}}</ref> Over time however, this hypothesis has fallen from favor. The large amount of energy that would have been required to achieve this effect and the low combined mass of the current asteroid belt, which is only a small fraction of the mass of the Earth's [[Moon]], do not support the hypothesis. Further, the significant chemical differences between the asteroids are difficult to explain if they come from the same planet.<ref>{{cite web
| author=Masetti, M.; Mukai, K.
| date=[[December 1]], [[2005]]
| url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980810a.html
| title=Origin of the Asteroid Belt
| publisher=NASA Goddard Spaceflight Center
| accessdate=2007-04-25
}}</ref> Today, most scientists accept that, rather than fragmenting from a progenitor planet, the asteroids never formed a planet at all.
In general in the Solar System, [[planetary formation]] is thought to have occurred via a process comparable to the long-standing nebular hypothesis: a cloud of interstellar dust and gas collapsed under the influence of gravity to form a rotating disk of material that then further condensed to form the Sun and planets.<ref>{{cite web
| last = Watanabe
| first = Susan
| date = [[July 20]], [[2001]]
| url =http://www.jpl.nasa.gov/news/features.cfm?feature=520
| title =Mysteries of the Solar Nebula
| publisher = NASA
| accessdate = 2007-04-02
}}</ref> During the first few million years of the Solar System's history, an [[Accretion (astrophysics)|accretion]] process of sticky collisions caused the clumping of small particles, which gradually increased in size. Once the clumps reached sufficient mass, they could draw in other bodies through gravitational attraction and become [[planetesimal]]s. This gravitational accretion led to the formation of the rocky planets and the [[gas giant]]s.
Planetesimals within the region which would become the asteroid belt were too strongly [[Perturbation (astronomy)|perturbed]] by gravity to form a planet. Instead they continued to orbit the Sun as before, while occasionally colliding.<ref name="icarus153">{{cite journal
| author=Petit, J.-M.; Morbidelli, A.; Chambers, J.
| title=The Primordial Excitation and Clearing of the Asteroid Belt
| journal=Icarus
| year=2001
| volume=153
| pages=338-347
| url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
| format=PDF
| accessdate=2007-03-22 | doi = 10.1006/icar.2001.6702 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,<ref>{{cite journal
| author=Edgar, R.; Artymowicz, P.
| title=Pumping of a Planetesimal Disc by a Rapidly Migrating Planet
| journal=Monthly Notices of the Royal Astronomical Society
| year=2004
| volume=354
| issue=3
| pages=769–772
| url=http://www.astro.su.se/~pawel/edgar+artymowicz.pdf
| format=PDF
| accessdate=2007-04-16 | doi = 10.1111/j.1365-2966.2004.08238.x <!--Retrieved from CrossRef by DOI bot-->
}}</ref> preventing the formation of planet-sized bodies. [[Orbital resonance]]s occurred where the orbital period of an object in the belt formed an integer fraction of the orbital period of Jupiter, perturbing the object into a different orbit; the region lying between the orbits of Mars and Jupiter contains many such orbital resonances. As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.<ref>{{cite conference
| first = E. R. D.
| last = Scott
| title=Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids
| booktitle = Proceedings 37th Annual Lunar and Planetary Science Conference
| publisher = Lunar and Planetary Society
| date = March 13–17, 2006
| location = League City, Texas
| url =http://adsabs.harvard.edu/abs/2006LPI....37.2367S
| accessdate = 2007-04-16
}}</ref>
During the early history of the Solar System, the asteroids melted to some degree, allowing elements within them to be partially or completely differentiated by mass. Some of the progenitor bodies may even have undergone periods of explosive [[volcanism]] and formed [[magma]] oceans. However, because of the relatively small size of the bodies, the period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago, in the first tens of millions years of formation.<ref>{{cite journal
| author=Taylor, G. J.; Keil, K.; McCoy, T.; Haack, H.; Scott, E. R. D.
| title=Asteroid differentiation - Pyroclastic volcanism to magma oceans
| journal=Meteoritics
| year=1993
| volume=28
| issue=1
| pages=34-52
| url=http://adsabs.harvard.edu/abs/1993Metic..28...34T
| accessdate=2007-04-19 }}</ref> In August 2007, a study of [[zircon]] crystals in an Antarctic meteorite believed to have originated from [[4 Vesta]] suggested that it, and by extension the rest of the asteroid belt, had formed rather quickly, within ten million years of the Solar System origin.<ref>{{cite web|title=U of T researchers discover clues to early solar system|author= Karen Kelly|year=2007|work=University of Toronto|url=http://www.news.utoronto.ca/bin6/070803-3321.asp|accessdate=2007-10-30}}</ref>
===Evolution===
The asteroids are not samples of the primordial Solar System. They have undergone considerable evolution since their formation, including internal heating (in the first few tens of millions of years), surface melting from impacts, [[space weathering]] from radiation, and bombardment by [[micrometeorites]].<ref>
{{cite web|title=Asteroid Space Weathering and Regolith Evolution|author=Clark, B. E.; Hapke, B.; Pieters, C.; Britt, D.|work=University of Arizona|year=2002|url=http://adsabs.harvard.edu/abs/2002aste.conf..585C|accessdate=2007-11-08}}
{{cite web|title=The Spectral and Physical Properties of Metal in Meteorite Assemblages: Implications for Asteroid Surface Materials|author=Michael J. Gaffey|year=1996|url=http://observatory.space.edu/f3_research/f4_faculty%20research/gaffeyResumePDFs/1986/Gaffey%201986%20Spectra%20of%20Metal%20in%20Meteorites.pdf
|accessdate=2007-11-08}}
{{cite web|title= Thermal alteration of asteroids: evidence from meteorites|author=Keil K.|work=Planetary and Space Science|url=http://www.ingentaconnect.com/content/els/00320633/2000/00000048/00000010/art00054|year=2000
|accessdate=2007-11-08}}
{{cite web|title=Impact of ions and micrometeorites on mineral surfaces: Reflectance changes and production of atmospheric species in airless solar system bodies|author=Baragiola, R. A.; Duke, C. A.; Loeffler, M.; McFadden, L. A.; Sheffield, J.|year=2003|url=http://adsabs.harvard.edu/abs/2003EAEJA.....7709B|accessdate=2007-11-08}}
</ref> While some scientists refer to the asteroids as residual planetesimals,<ref>{{cite web|title=From Dust to Planetesimals: Workshop at Ringberg Castle Germany|year=2006|url=http://www.mpia-hd.mpg.de/homes/fdtp/talks/index.html|accessdate=2007-11-08}}</ref> other scientists consider them distinct.<ref>{{cite web|title=Asteroid 433 Eros and partially differentiated planetesimals: bulk depletion versus surface depletion of sulfur|author=A. Kracher|work=Ames Laboratory|year=2005|url=http://www.cosis.net/abstracts/EGU05/03788/EGU05-J-03788.pdf
|accessdate=2007-11-08}}</ref>
The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about a million years of formation, leaving behind less than 0.1% of the original mass.<ref name="icarus153" /> Since their formation, the size distribution of the asteroid belt has remained relatively stable: there has been no significant increase or decrease in the typical dimensions of the main belt asteroids.<ref>{{cite news
| first=Lori
| last=Stiles
| title=Asteroids Caused the Early Inner Solar System Cataclysm
| publisher=University of Arizona News
|date=[[September 15]], [[2005]]
| url=http://uanews.org/cgi-bin/WebObjects/UANews.woa/7/wa/SRStoryDetails?ArticleID=11692
| accessdate=2007-04-18 }}</ref>
The 4:1 [[orbital resonance]] with Jupiter, at a radius 2.06 [[astronomical unit|AU]], can be considered the inner boundary of the main belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed inside the radius of this gap were swept up by [[Mars]] (which has an [[aphelion]] at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System.<ref>{{cite web
| author=Alfvén, H.; Arrhenius, G.
| year=1976
| url =http://history.nasa.gov/SP-345/ch4.htm
| title =The Small Bodies
| work=SP-345 Evolution of the Solar System
| publisher = NASA
| accessdate = 2007-04-12 }}</ref> The [[Hungaria asteroids]] lie closer to the Sun than the 4:1 resonance, but are protected from disruption by their high inclination.<ref>[http://adsabs.harvard.edu/abs/1990JRASC..84..123S The Hungaria group of minor planets]</ref>
When the main belt was first being formed, the temperatures at a distance of 2.7 AU from the Sun formed a "snow line" below the condensation point of water. Planetismals formed beyond this radius were able to accumulate ice.<ref>{{cite journal
| author=Lecar, M.; Podolak, M.; Sasselov, D.; Chiang, E.
| title=Infrared cirrus - New components of the extended infrared emission
| journal=The Astrophysical Journal
| year=2006
| volume=640
| pages=1115–1118
| url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/500287
| accessdate=2007-04-11 }}</ref><ref>{{cite news
| first=Phil
| last=Berardelli
| title=Main-Belt Comets May Have Been Source Of Earths Water
| publisher=Space Daily
| date=[[March 23]], [[2006]]
| url=http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html| accessdate=2007-10-27 }}</ref>
In 2006 it was announced that a population of [[Main-belt comet|comet]]s had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans. According to some models, there was insufficient [[outgassing]] of water during the Earth's formative period to form the oceans, necessitating an external source such as a cometary bombardment.<ref>{{cite web
| last = Lakdawalla
| first = Emily
| date = [[April 28]], [[2006]]
| url = http://www.planetary.org/blog/article/00000551/
| title = Discovery of a Whole New Type of Comet
| publisher = The Planetary Society
| accessdate = 2007-04-20
}}</ref>
==Characteristics==
[[Image:951 Gaspra.jpg|thumb|250 px|The asteroid [[951 Gaspra]], the first ever imaged by a spacecraft, taken by ''[[Galileo (spacecraft)|Galileo]]'' as it passed by it in 1991]]
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. 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 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
| accessdate = 2007-04-26
}} — search for asteroids in the main belt regions with a diameter >100.</ref> while a survey in the infrared wavelengths shows that the main belt has 700,000 to 1.7 million asteroids with a diameter of 1 km or more.<ref>{{cite journal
| author=Tedesco, E. F.; Desert, F.-X.
| title=The Infrared Space Observatory Deep Asteroid Search
| journal=The Astronomical Journal
| year=2002
| volume=123
| pages=2070–2082
| url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/339482
| accessdate=2007-04-10 | doi = 10.1086/339482 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> The [[apparent magnitude]]s of most of the known asteroids are 11–19, with the median at about 16.<ref name="mpc" />
The total mass of the asteroid belt is estimated to be 3.0×10<sup>21</sup>–3.6×10<sup>21</sup> kilograms, which is just 4% of the Earth's [[Moon]].<ref name=Krasinskyetal2002>{{cite journal| authorlink= Georgij A. Krasinsky | first=G. A. | last= Krasinsky | coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. | url=http://adsabs.harvard.edu/abs/2002Icar..158...98K| title=Hidden Mass in the Asteroid Belt| journal=Icarus| volume=158| issue=1| pages=98–105| month= July| year= 2002| doi=10.1006/icar.2002.6837}}</ref><ref name=Pitjeva2005>{{cite journal | last= Pitjeva | first=E. V. | authorlink= Elena V. Pitjeva | title= High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants | journal= Solar System Research | year= 2005 | volume= 39 | issue= 3 | pages= 176 | url= http://iau-comm4.jpl.nasa.gov/EPM2004.pdf | format= [[PDF]] | doi= 10.1007/s11208-005-0033-2}}</ref> Its four largest objects, 1 Ceres, 4 Vesta, 2 Pallas and 10 Hygiea, account for almost half of the belt's total mass, with one-third accounted for by Ceres alone.<ref name=halfmass>For recent estimates of the masses of [[Ceres (dwarf planet)|Ceres]], [[4 Vesta]], [[2 Pallas]] and [[10 Hygiea]], see the references in the infoboxes of their respective articles.</ref><ref name="jplsbdb"/> Ceres's orbital distance, 2.8 AU, is also the location of the asteroid belt's [[center of mass]].<ref name="mnras244">{{cite journal
| author=McBride, N.; Hughes, D. W.
| title=The spatial density of asteroids and its variation with asteroidal mass
| journal=Monthly Notices of the Royal Astronomical Society
| year=1990
| volume=244
| pages=513-520
| url=http://adsabs.harvard.edu/abs/1990MNRAS.244..513M
| accessdate=2007-04-19 }}</ref>
===Composition===
[[Image:AllendeMeteorite.jpg|right|thumb|[[Allende Meteorite|Allende]] is a carbonaceous chondrite meteorite that fell to Earth in [[Mexico]] in 1969.]]
The current belt consists primarily of three categories of asteroids: C-type or carbonaceous asteroids, S-type or silicate asteroids, and M-type or metallic asteroids.
Carbonaceous asteroids, as their name suggests, are carbon-rich and dominate the belt's outer regions.<ref name="ApJ133">{{cite journal
| author=Wiegert, P.; Balam, D.; Moss, A.; Veillet, C.; Connors, M.; Shelton, I.
| title=Evidence for a Color Dependence in the Size Distribution of Main-Belt Asteroids
| journal=The Astronomical Journal
| year=2007
| volume=133
| pages=1609–1614
| url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/512128
| accessdate=2007-03-27 | doi = 10.1086/512128 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> Together they comprise over 75% of the visible asteroids. They are more red in hue than the other asteroids and have a very low [[albedo]]. Their surface composition is similar to [[carbonaceous chondrite]] [[meteorite]]s. Chemically, their spectra match the primordial composition of the early Solar System, with only the lighter elements and [[volatiles]] removed.
[[S-type asteroid|S-type]] or [[silicate]]-rich asteroids are more common toward the inner region of the belt, within 2.5 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
| url=http://adsabs.harvard.edu/abs/1996LPI....27..225C
| accessdate=2007-03-27 }}</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 via melting and reformation. They have a relatively high albedo, and form about 17% of the total asteroid population.
[[M-type asteroid|M-type]] ([[metal]]-rich) asteroids form about 10% of the total population; their spectra resemble that of iron-nickel. Some are believed to have formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, there are also some silicate compounds that can produce a similar appearance. For example, the large M-type asteroid [[22 Kalliope]] does not appear to be primarily composed of metal.<ref>{{cite journal
| author=Margot, J. L.; Brown, M. E.
| title=A Low-Density M-type Asteroid in the Main Belt
| journal=Science
| year=2003
| volume=300
| issue=5627
| pages=1939–1942
| url=http://adsabs.harvard.edu/abs/2003Sci...300.1939M
| accessdate=2007-04-10 | doi = 10.1126/science.1085844 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 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
| accessdate = 2007-04-02
}}</ref> It is not yet clear 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.<ref>{{cite journal
| author=Mueller, M.; Harris, A. W.; Delbo, M.; 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 | pages=627
| url=http://adsabs.harvard.edu/abs/2005DPS....37.0702M
| accessdate=2007-07-23 }}</ref>
One mystery of the asteroid belt is the relative rarity of [[V-type asteroid|V-type]], or [[basaltic]] asteroids.<ref name=Duffard>{{cite web|title=Two new basaltic asteroids in the Outer Main Belt|author=Duffard, R.; Roig, F.|year=2007|url=http://adsabs.harvard.edu/abs/2007arXiv0704.0230D|accessdate=2007-10-14}}</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 [[olivine]]. Observations, however, suggest that 99 percent of the predicted basaltic material is missing.<ref name=olivine>{{cite web|title=Strange Asteroids Baffle Scientists |author=Ker Than|year=2007|work=space.com|url=http://www.space.com/scienceastronomy/070821_basalt_asteroid.html|accessdate=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). However, the discovery of the asteroid [[1459 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 Kumakiri]] and [[(10537) 1991 RY16]], with differing basaltic composition that could not have originated from Vesta. These latter two are the only V-type asteroids discovered in the outer belt to date.<ref name=Duffard />
The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU<ref>{{cite journal
| author=Low, F. J. ''et al''
| title=Infrared cirrus - New components of the extended infrared emission
| journal=Astrophysical Journal, Part 2 - Letters to the Editor
| year=1984
| volume=278
| pages=L19-L22
| url=http://adsabs.harvard.edu/cgi-bin/bib_query?1984ApJ...278L..19L
| accessdate=2007-04-11 }}</ref> However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.
===Orbits and rotations===
[[Image:Main belt e vs a.png|thumb|300px|right|The asteroid belt (showing eccentricities), with the main belt in red and blue ("core" region in red)]]
Most asteroids within the main 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 of around 0.07 and an inclination below 4°.<ref name="mpc">{{cite web
| last = Williams
| first = Gareth
|date=[[April 3]], [[2007]]
| url = http://cfa-www.harvard.edu/iau/lists/MPDistribution.html
| title = Distribution of the Minor Planets
| publisher = Minor Planets Center
| accessdate = 2007-04-15
}}</ref> Thus while 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 [[astronomical unit|AU]], and at [[eccentricity (orbit)|orbital eccentricities]] less than roughly 0.33, along with orbital [[inclination]]s below about 20°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.<ref name="basedon1">This value was obtained by a simple count up of all bodies in that region using data for 120437 numbered minor planets from the [http://cfa-www.harvard.edu/iau/MPCORB.html Minor Planet Center orbit database], dated [[February 8]], [[2006]].</ref>
Measurements of the rotation periods of large asteroids in the main belt show that there is a lower limit. No asteroid with a diameter larger than 100 metres has a period of rotation of less than 2.2 hours. For asteroids rotating faster than approximately this rate, the [[Centrifugal force (fictitious)|centrifugal force]] at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that the majority of asteroids with a diameter over 100 metres are actually [[rubble pile]]s formed through accumulation of debris after collisions between asteroids.<ref>{{cite web
| last = Rossi
| first = Alessandro
| date = [[May 20]], [[2004]]
| url = http://spaceguard.esa.int/tumblingstone/issues/current/eng/ast-day.htm
| title = The mysteries of the asteroid rotation day
| publisher = The Spaceguard Foundation
| accessdate = 2007-04-09
}}</ref>
====Kirkwood gaps====
{{main|Kirkwood gap}}
[[Image:Kirkwood Gaps.png|300px|thumb|This chart shows the distribution of asteroid [[semi-major axis|semi-major axes]] in the "core" of the main belt. Cyan arrows point to the Kirkwood gaps, where orbital resonances with [[Jupiter]] destabilize orbits.]]
The [[semi-major 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 at 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=The Americal Scientist
| year=1999
| volume=87
| issue=5
| pages=398
| url=http://www.americanscientist.org/template/AssetDetail/assetid/26603
| accessdate=2007-02-04 }}</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. In effect, asteroids that become located in the gap orbits (either primordially because of the migration of Jupiter's orbit,<ref>{{cite journal
| author=Liou, Jer-Chyi; Malhotra, Renu
| title=Depletion of the Outer Asteroid Belt
| journal=Science | year=1997
| volume=275 | issue=5298 | pages=375-377
| url=http://www.sciencemag.org/cgi/content/full/275/5298/375
| accessdate=2007-08-01 | doi = 10.1126/science.275.5298.375 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> or due to prior perturbations or collisions) are gradually nudged into different, random orbits with a larger or smaller semi-major axis.
The gaps are not seen in a simple snapshot of the locations of the asteroids at any one time because asteroid orbits are elliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps does not differ significantly from the neighboring regions.<ref name="mnras244" />
The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit, for instance. Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby. (For example, an 8:3 resonance for asteroids with a semi-major axis of 2.71 AU.)<ref name="iau160">{{cite conference
| first = S.
| last = Ferraz-Mello
| title = Kirkwood Gaps and Resonant Groups
| booktitle = proceedings of the 160th International Astronomical Union
| pages = 175-188
| publisher = Kluwer Academic Publishers
| date = June 14–18, 1993
| location = Belgirate, Italy
| url = http://adsabs.harvard.edu/abs/1994IAUS..160..175F
| accessdate = 2007-03-28 }}</ref>
The main or core population of the asteroid belt is sometimes divided into three zones, based on the most prominent Kirkwood gaps. Zone I lies between the 4:1 resonance (2.06 AU) and 3:1 resonance (2.5 AU) Kirkwood gaps. Zone II continues from the end of Zone I out to the 5:2 resonance gap (2.82 AU). Zone III extends from the outer edge of Zone II to the 2:1 resonance gap (3.28 AU).<ref>{{cite journal | last=Klacka
| first=Jozef
| title=Mass distribution in the asteroid belt
| journal=Earth, Moon, and Planets
| year=1992
| volume=56
| issue=1
| pages=47-52
| url=http://adsabs.harvard.edu/abs/1992EM&P...56...47K
| accessdate=2007-04-12 }}</ref>
The main belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 AU), and the outer belt formed by those asteroids closer to Jupiter's orbit. (Some authors subdivide the inner and outer belts at the 2:1 resonance gap (3.3 AU), while others suggest inner, middle, and outer belts.)
==Collisions==
[[Image:zodiacal.jpg|thumb|200 px|The [[zodiacal light]], created in part by dust from collisions in the asteroid belt]]
The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years.<ref name="backman_report">{{cite web
| last=Backman
| first=D. E.
| date=[[March 6]], [[1998]]
| url=http://astrobiology.arc.nasa.gov/workshops/zodiac/backman/backman_toc.html
| title=Fluctuations in the General Zodiacal Cloud Density
| work=Backman Report
| publisher=NASA Ames Research Center
| accessdate=2007-04-04
}}</ref> A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new [[asteroid family]]). Conversely, collisions that occur at low relative speeds may also join two asteroids together. After more than 4 billion years of such processes, the members of the asteroid belt now bear little resemblance to the original population.
In addition to the asteroid bodies, the main belt also contains bands of dust with particle radii of up to a few hundred [[micrometre]]s. This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Due to [[Poynting-Robertson effect|Poynting-Robertson drag]], the pressure of [[solar radiation]] causes this dust to slowly spiral inward toward the [[Sun]].<ref name="apj392">{{cite journal
| last = Reach
| first = William T.
| title=Zodiacal emission. III - Dust near the asteroid belt
| journal=Astrophysical Journal
| year=1992
| volume=392
| issue=1
| pages=289-299
| url=http://adsabs.harvard.edu/abs/1992ApJ...392..289R
| accessdate=2007-04-04 }}</ref>
The combination of this fine asteroid dust, as well as ejected cometary material, produces the [[zodiacal light]]. This faint auroral glow can be viewed at night extending from the direction of the [[Sun]] along the plane of the [[ecliptic]]. Particles that produce the visible zodiacal light average about 40 μm in radius. The typical lifetimes of such particles are on the order of 700,000 years. Thus, in order to maintain the bands of dust, new particles must be steadily produced within the asteroid belt.<ref name="apj392" />
===Meteorites===
Some of the debris from collisions can form [[meteoroid]]s that enter the Earth's atmosphere.<ref>{{cite web
| last=Kingsley
| first=Danny
| date=[[May 1]], [[2003]]
| url=http://abc.net.au/science/news/stories/s843594.htm
| title=Mysterious meteorite dust mismatch solved
| publisher=ABC Science
| accessdate=2007-04-04
}}</ref> More than 99.8 percent of the 30,000 [[meteorites]] found on Earth to date are believed to have originated in the asteroid belt.<ref>{{cite web|title=Meteors and Meteorites|work=NASA|url=http://64.233.183.104/search?q=cache:KaispJ9RSuIJ:www.nasa.gov/pdf/145945main_Meteors.Meteorites.Lithograph.pdf+%22percent+of+meteorites%22+asteroid&hl=en&ct=clnk&cd=1&gl=uk |accessdate=2007-10-17}}</ref> A September 2007 study by a joint US-Czech team has suggested that a large-body collision undergone by the asteroid [[298 Baptistina]] sent a number of fragments into the [[inner solar system]]. The impacts of these fragments are believed to have created both the [[Tycho crater]] on the Moon and the [[Chicxulub, Yucatán|Chicxulub]] crater in Mexico, the remnant of the massive impact which triggered the [[extinction of the dinosaurs]] 65 million years ago.<ref>{{cite web|title=Breakup event in the main asteroid belt likely caused dinosaur extinction 65 million years ago|work=Southwest Research Institute|year=2007|url=http://www.physorg.com/news108218928.html|accessdate=2007-10-14}}</ref>
==Largest asteroids==
:''See also: [[List of notable asteroids#Largest known asteroids (out to the orbit of Jupiter)|Largest asteroids]]''
[[Image:Ceres optimized.jpg|thumb|150px|The dwarf planet Ceres]]
Although their location in the asteroid belt excludes them from planet status, the four largest objects, [[Ceres (dwarf planet)|Ceres]], [[4 Vesta|Vesta]], [[2 Pallas|Pallas]], and [[10 Hygiea|Hygiea]], hover on the edge of [[hydrostatic equilibrium]], the boundary that separates objects from planethood. They share many characteristics common to planets, but also show qualities more akin to rock-like asteroids.
Ceres is the only object in the belt large enough for its gravity to force it into a roughly round shape, and so, according to the IAU's 2006 resolution on the [[2006 definition of planet|definition of a planet]], it is now considered a [[dwarf planet]].<ref>{{cite web | date = August 24, 2006 | url = http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html | title = The Final IAU Resolution on the Definition of "Planet" Ready for Voting | publisher = IAU | accessdate = 2007-03-02 }}</ref> The other three may also eventually be reclassified as well.<ref>{{cite web|title=IAU draft resolution|year=2006|url=http://www.iau.org/iau0601.424.0.html|accessdate=2007-10-20}}</ref><ref name=dwarf>
{{cite web
| url = http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html
| title = IAU 2006 General Assembly: Result of the IAU Resolution votes
| accessdate = 2007-03-29 }}</ref> Ceres has a much higher absolute magnitude than the other asteroids, of around 3.32,<ref>{{cite journal
| author=Parker, J. W.; Stern, S. A.; Thomas, P. C.; Festou, M. C.; Merline, W. J.; Young, E. F.; Binzel, R. P.; Lebofsky, L. A.
| title=Analysis of the First Disk-resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope
| journal=The Astronomical Journal
| year=2002
| volume=123
| pages=549–557
| url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/338093
| accessdate=2007-04-15 | doi = 10.1086/338093 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> and may possess a surface layer of ice.<ref name=planetary>{{cite web|title=Asteroid 1 Ceres|work=The Planetary Society|url=http://www.planetary.org/explore/topics/asteroids_and_comets/ceres.html
|accessdate=2007-10-20}}</ref> Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.<ref name=planetary /> Vesta, too, has a differentiated interior, though it formed inside the Solar System's "snow line", and so is devoid of water;
<ref>
{{cite web |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |title=Key Stages in the Evolution of the Asteroid Vesta| work=Hubble Space Telescope news release|year=1995|accessdate=2007-10-20}}
{{cite web|title=Dawn mission and operations|author=CT Russel et al.|work=NASA/JPL|url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=414750|year=2007|accessdate=2007-10-20}}</ref> its composition is mainly of basaltic rock such as olivine.<ref name=olivine /> Pallas is unusual in that, like [[Uranus]], it rotates on its side, with one pole facing the Sun and the other facing away.<ref name="Torppa1996">{{cite journal
| author=J. Torppa ''et al''
| title=Shapes and rotational properties of thirty asteroids from photometric data
| journal=Icarus | year=1996
| volume=164 | issue=2 | pages=346-383
| url=http://adsabs.harvard.edu/abs/2003Icar..164..346T
| accessdate=2007-03-15 }}</ref> Its composition is similar to that of Ceres: high in carbon and silicon.<ref>{{cite web|title=The composition of asteroid 2 Pallas and its relation to primitive meteorites|author=Larson, H. P.; Feierberg, M. A.; Lebofsky, L. A.|url=http://adsabs.harvard.edu/abs/1983Icar...56..398L|year=1983|accessdate=2007-10-20}}</ref> Hygiea is a carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the [[ecliptic plane]].
<ref>
{{cite web|title=10 Hygiea: ISO Infrared Observations|author=M. A. Barucci et al.|url=http://www.lesia.obspm.fr/~crovisier/biblio/preprint/bar02_icarus.pdf|year=2002|accessdate=2007-10-21}}
{{cite web|title=Ceres the Planet|work=orbitsimulator.com|url=http://www.orbitsimulator.com/gravity/articles/ceres.html|accessdate=2007-10-20}}</ref>
==Families and groups==
{{main|Asteroid family}}
[[Image:Asteroid proper elements i vs e.png|right|thumb|300px|This plot of orbital inclination (''i<sub>p</sub>'') versus eccentricity (''e<sub>p</sub>'') for the numbered main belt asteroids clearly shows several clumps of asteroid families.]]
In 1918, the Japanese astronomer [[Kiyotsugu Hirayama]] noticed that the orbits of some of the asteroids had similar parameters, forming families or groups.<ref>{{cite web
| first=David W. | last=Hughes | year=2007
| url =http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/finding.html
| title = Finding Asteroids In Space
| publisher = BBC
| accessdate = 2007-04-20
}}</ref>
Approximately one third of the asteroids in the main belt are members of an asteroid family. These share similar orbital elements, such as semi-major axis, eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the main belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families. Additional groupings have been found that are less certain. Asteroid families can be confirmed when the members display common spectral features.<ref>{{cite conference
| first=Anne
| last=Lemaitre
| title=Asteroid family classification from very large catalogues
| booktitle=Proceedings Dynamics of Populations of Planetary Systems
| pages=135-144
| publisher=Cambridge University Press
|date=August 31-September 4, 2004
| location=Belgrade, Serbia and Montenegro
| url=http://adsabs.harvard.edu/abs/2005dpps.conf..135L
| accessdate=2007-04-15 }}</ref> Smaller associations of asteroids are called groups or clusters.
Some of the most prominent families in the main belt (in order of increasing semi-major axes) are the [[Flora family|Flora]], [[Eunomia family|Eunoma]], [[Koronis family|Koronis]], [[Eos family|Eos]], and [[Themis family|Themis]] families.<ref name="lang2003" /> The Flora family, one of the largest with more than 800 known members, may have formed from a collision less than a billion years ago.<ref>{{cite web
| last = Martel
| first = Linda M. V.
| date = [[March 9]], [[2004]]
| url = http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html
| title = Tiny Traces of a Big Asteroid Breakup
| publisher = Planetary Science Research Discoveries
| accessdate = 2007-04-02
}}</ref>
The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the [[Gefion family]]) is 4 Vesta. The [[Vesta family]] is believed to have formed as the result of a crater-forming impact on Vesta. Likewise, the [[HED meteorite]]s may also have originated from Vesta as a result of this collision.<ref>{{cite journal
| last=Drake
| first=Michael J.
| title=The eucrite/Vesta story
| journal=Meteoritics & Planetary Science
| year=2001
| volume=36
| issue=4
| pages=501-513
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001M%26PS...36..501D
| accessdate=2007-02-04 }}</ref>
Three prominent bands of dust have been found within the main belt. These have similar orbital inclinations as the Eos, Koronis, and Themis asteroid families, and so are possibly associated with those groupings.<ref>{{cite journal
| author=Love, S. G.; Brownlee, D. E.
| title=The [[IRAS]] dust band contribution to the interplanetary dust complex - Evidence seen at 60 and 100 microns
| journal=Astronomical Journal
| year=1992
| volume=104
| issue=6
| pages=2236-2242
| url=http://adsabs.harvard.edu/abs/1992AJ....104.2236L
| accessdate=2007-04-11 | doi = 10.1086/116399 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
===Periphery===
Skirting the inner edge of the belt (ranging between 1.78 and 2.0 AU, with a mean semi-major axis of 1.9 AU) is the [[Hungaria family]] of minor planets. They are named after the main member, [[434 Hungaria]]; the group contains at least 52 named asteroids. The Hungaria group is separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars are likely a factor in reducing the total population of this group.<ref>{{cite journal
| last=Spratt
| first=Christopher E.
| title=The Hungaria group of minor planets
| journal=Journal of the Royal Astronomical Society of Canada
| year=1990
| volume=84
| issue=2
| pages=123-131
| url=http://adsabs.harvard.edu/abs/1990JRASC..84..123S
| accessdate=2007-02-04 }}</ref>
Another high-inclination group in the inner part of the main belt is the [[Phocaea family]]. These are composed primarily of S-type asteroids, where as the neighboring Hungaria family includes some [[E-type asteroid|E-types]].<ref>{{cite journal
| author=Carvano, J. M.; Lazzaro, D.; Mothé-Diniz, T.; Angeli, C. A.; Florczak, M.
| title=Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups
| journal=Icarus
| year=2001
| volume=149
| issue=1
| pages=173–189
| url=http://adsabs.harvard.edu/abs/2001Icar..149..173C
| accessdate=2007-02-04 | doi = 10.1006/icar.2000.6512 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> The Phocaea family orbit between 2.25 and 2.5 AU from the Sun.
Skirting the outer edge of the main belt is the [[65 Cybele|Cybele group]], orbiting between 3.3 and 3.5 AU. These have a 7:4 orbital resonance with Jupiter. The [[Hilda family]] orbit between 3.5 and 4.2 AU, and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are few asteroids beyond 4.2 AU, until Jupiter's orbit. Here the two large groups of [[Trojan asteroid]]s can be found; they are not usually considered part of the main asteroid belt.
===New families===
Some asteroid families have formed recently, in astronomical terms. The [[Karin Cluster]] apparently formed about 5.7 million years ago from a collision with a 16 km radius progenitor asteroid.<ref>{{cite news
| title=SwRI researchers identify asteroid breakup event in the main asteroid belt
| publisher=SpaceRef.com
| date=[[June 12]], [[2002]]
| url=http://www.spaceref.com/news/viewpr.html?pid=8627
| accessdate=2007-04-15 }}</ref> The [[490 Veritas|Veritas family]] formed about 8.3 million years ago; evidence includes interplanetary dust recovered from [[ocean]] sediment.<ref>{{cite news
| first=Maggie
| last=McKee
| title=Eon of dust storms traced to asteroid smash
| publisher=New Scientist Space
| date=[[January 18]], [[2006]]
| url=http://space.newscientist.com/channel/solar-system/comets-asteroids/dn8603
| accessdate=2007-04-15 }}</ref>
In the more distant past, the [[1270 Datura|Datura cluster]] appears to have formed about 450 million years ago from a collision with a main belt asteroid. The age estimate is based on the probability of the members having their current orbits, rather than from any physical evidence. However, this cluster may have been a source for some zodiacal dust material.<ref>{{cite journal
| author=Nesvorný, D.; Vokrouhlick, D.; Bottke, W. F.
| title=The Breakup of a Main-Belt Asteroid 450 Thousand Years Ago
| journal=Science
| year=2006
| volume=312
| issue=5779
| pages=1490
| url=http://www.sciencemag.org/cgi/content/full/312/5779/1490
| accessdate=2007-04-15 | doi = 10.1126/science.1126175 <!--Retrieved from CrossRef by DOI bot-->
}}</ref> Other recent cluster formations, such as the [[4652 Iannini|Iannini cluster]]
(''circa'' 1–5 million years ago), may have provided additional sources of this asteroid dust.<ref>{{cite journal
| author=Nesvorný, D.; Bottke, W. F.; Levison, H. F.; Dones, L.
| title=Recent Origin of the Solar System Dust Bands
| journal=The Astrophysical Journal
| year=2003
| volume=591
| pages=486–497
| url=http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/374807
| accessdate=2007-04-15 | doi = 10.1086/374807 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
==Exploration==
[[Image:Dawn Flight Configuration 2.jpg|right|thumb|320px|Artist's concept of the Dawn Mission spacecraft with Vesta (left) & Ceres (right)]]
The first spacecraft to traverse the asteroid belt was [[Pioneer 10]], which entered the region on [[July 16]], [[1972]]. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft, but it has since been safely traversed by 9 Earth-based craft without incident. [[Pioneer 11]], [[Voyager program|Voyagers 1 and 2]] and [[Ulysses probe|Ulysses]] passed through the belt without imaging any asteroids. [[Galileo (spacecraft)|Galileo]] imaged the asteroid [[951 Gaspra]] in 1991 and [[243 Ida]] in 1993, [[NEAR Shoemaker|NEAR]] imaged [[253 Mathilde]] in 1997, [[Cassini–Huygens|Cassini]] imaged [[2685 Masursky]] in 2000, [[Stardust (spacecraft)|Stardust]] imaged [[5535 Annefrank]] in 2002, and [[New Horizons]] imaged [[132524 APL]] in 2006. Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.<ref>{{cite news
| first=Alan
| last=Stern
| title=New Horizons Crosses The Asteroid Belt
| publisher=Space Daily
| date=[[June 2]], [[2006]]
| url=http://www.spacedaily.com/reports/New_Horizons_Crosses_The_Asteroid_Belt.html
| accessdate=2007-04-14 }}</ref>
All spacecraft images of belt asteroids to date have come from brief [[planetary flyby|flyby]] opportunities by probes headed for other targets. Only the NEAR and [[Hayabusa]] missions have studied asteroids for a protracted period in orbit and at the surface and these were [[near-Earth asteroid]]s. However, the [[Dawn Mission]] has been dispatched to explore [[4 Vesta|Vesta]] and [[Ceres (dwarf planet)|Ceres]] in the main belt. If the probe is still operational after examining these two large bodies, an extended mission is possible that could allow additional exploration.<ref>{{cite web
| author=Staff
| date=[[April 10]], [[2007]]
| url = http://dawn.jpl.nasa.gov/
| title = Dawn Mission Home Page
| publisher = NASA JPL
| accessdate = 2007-04-14
}}</ref>
== See also ==
* [[Asteroids in astrology]]
* [[Asteroids in fiction]]
* [[Centaur (planetoid)|Centaur]]
* [[Colonization of the asteroids]]
* [[Debris disk]]
* [[Trojan asteroid]]
==References==
{{Reflist|2}}
==Further reading==
* {{cite book
| first=Linda T.
| last=Elkins-Tanton
| year=2006
| title=Asteroids, Meteorites, and Comets
| edition=First edition
| publisher=Chelsea House
| location=New York
| id=ISBN 0-8160-5195-X }}
==External links==
* {{cite web
| last = Arnett | first = William A.
| date = [[February 26]], [[2006]]
| url = http://www.nineplanets.org/asteroids.html
| title = Asteroids | publisher = The Nine Planets
| accessdate = 2007-04-20}}
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids Asteroids Page] at [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* {{cite web | author=Fraser Cain|url=http://www.astronomycast.com/astronomy/episode-55-the-asteroid-belt/|publisher=Universe Today|title= The Asteroid Belt|accessdate=2008-04-01}}
* {{cite web
| last = Hsieh | first = Henry H.
| date = [[March 1]], [[2006]]
| url = http://www.ifa.hawaii.edu/~hsieh/mbcs.html
| title = Main-Belt Comets | publisher = University of Hawaii
| accessdate = 2007-04-20}}
* {{cite web
| url = http://www.solstation.com/stars/asteroid.htm
| title = Main Asteroid Belt | publisher = Sol Company
| accessdate = 2007-04-20}}
* {{cite web
| last = Munsell | first = Kirk | date = September 16, 2005
| url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids Asteroids Page
| title = Asteroids: Overview
| publisher = [http://solarsystem.nasa.gov NASA's Solar System Exploration]
| accessdate = 2007-05-26 }}
* Plots of [http://hamilton.dm.unipi.it/astdys/propsynth/allae.gif eccentricity vs. semi-major axis] and [http://hamilton.dm.unipi.it/astdys/propsynth/allai.gif inclination vs. semi-major axis] at Asteroid Dynamic Site
* {{cite web
| author=Staff | date=[[October 31]], [[2006]]
| url = http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html
| title = Asteroids | publisher = NASA
| accessdate = 2007-04-20}}
* {{cite web
| author=Staff | year = [[2007]]
| url = http://www.planetary.org/explore/topics/asteroids_and_comets/facts.html
| title = Space Topics: Asteroids and Comets
| publisher = The Planetary Society | accessdate = 2007-04-20}}
{{Small Solar System bodies}}
{{Solar System}}
[[Category:Asteroid groups and families]]
[[als:Asteroidengürtel]]
[[ast:Cinturón d'asteroides]]
[[az:Asteroid qurşağı]]
[[zh-min-nan:Sió-he̍k-chheⁿ-toà]]
[[bg:Астероиден пояс]]
[[ca:Cinturó d'asteroides]]
[[cs:Hlavní pás]]
[[da:Asteroidebælte]]
[[de:Asteroidengürtel]]
[[es:Cinturón de asteroides]]
[[eo:Asteroida zono]]
[[eu:Asteroide gerriko]]
[[fr:Ceinture d'astéroïdes]]
[[ga:Crios astaróideach]]
[[gl:Cinto de asteroides]]
[[ko:소행성대]]
[[hr:Asteroidni pojas]]
[[is:Smástirnabeltið]]
[[it:Fascia principale]]
[[he:חגורת האסטרואידים]]
[[kn:ಕ್ಷುದ್ರಗ್ರಹ ಹೊನಲು]]
[[ka:ასტეროიდთა სარტყელი]]
[[la:Cingulum asteroidum]]
[[lb:Asteroidenceinture]]
[[lt:Asteroidų žiedas]]
[[mk:Астероиден појас]]
[[nl:Planetoïdengordel]]
[[ja:小惑星帯]]
[[no:Asteroidebeltet]]
[[nn:Asteroidebeltet]]
[[pl:Pas planetoid]]
[[pt:Cintura de asteróides]]
[[ksh:Asteoridejüddel]]
[[ro:Centura de asteroizi]]
[[ru:Главный пояс астероидов]]
[[simple:Asteroid belt]]
[[sk:Pásmo planétok]]
[[sl:Asteroidni pas]]
[[sr:Појас астероида]]
[[fi:Asteroidivyöhyke]]
[[sv:Asteroidbältet]]
[[te:ఆస్టెరాయిడ్ పట్టీ]]
[[th:แถบดาวเคราะห์น้อย]]
[[vi:Vành đai tiểu hành tinh]]
[[tr:Asteroit kuşağı]]
[[uk:Пояс астероїдів]]
[[zh:小行星带]]