# Asteroid belt <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *The asteroid belt in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=belt&embed=1" data-title="The asteroid belt in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=belt`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: under show, pick small bodies so that only the sampled belt and the named bodies remain, and look for the thin empty lanes at the 3:1, 5:2, 7:3 and 2:1 resonances; drag to an edge-on view to see how thick the torus is next to the flat orbits of Mars and Jupiter; then set show back to everything and the speed to 10 years/s, and watch the named bodies near the inner edge run ahead of Jupiter, which needs almost 12 years for one circuit.* The **asteroid belt**, often called the **main belt**, is the ring-shaped region of the [[PORTAL_Solar_System|Solar System]] between the orbits of [[Mars]] and [[Jupiter]], roughly 2.1 to 3.3 [[Astronomical_unit|AU]] from the [[Sun]], in which most of the known [[Asteroid|asteroids]] travel.[^williams2015][^mpc-dist] It is the smallest and innermost of the Sun's discs of debris. Its entire mass is estimated at about 2.4 × 10²¹ kg, roughly 3% of the mass of the [[Moon]], and about three-fifths of that sits in four bodies: the [[Dwarf_planet|dwarf planet]] [[Ceres_(dwarf_planet)|Ceres]] and the asteroids [[4_Vesta|Vesta]], [[2_Pallas|Pallas]] and [[10_Hygiea|Hygiea]].[^pitjeva2018][^nasa-ceres] The rest is spread over a volume so large that more than a million catalogued objects sit, on average, about a million kilometres apart.[^jpl-mba][^earthsky2021] The belt is not the wreck of a planet. It is what remains of a population of planetesimals whose growth into a planet was prevented by Jupiter; most of the original material was lost early, and the survivors have been ground down, reshaped and sorted by collisions and by resonances with Jupiter ever since.[^petit2001][^obrien2011] Those resonances leave the [[Kirkwood_gap|Kirkwood gaps]], and the collisions leave asteroid families, dust bands and most of the meteorites that reach [[Earth]]. The explorer at the top of this page draws the belt as a torus from 2.3 to 3.3 AU filled with ILLUSTRATIVE points, a random sample rather than a catalogue, with empty lanes at the 3:1, 5:2, 7:3 and 2:1 resonances; the named large bodies in it are real objects on their catalogued orbits.[^jpl-sbdb] ## History of observation [[Johannes_Kepler|Kepler]], working on the spacing of the planetary orbits in his *Mysterium Cosmographicum* of 1596, placed an unseen planet in the wide interval between the two.[^dawn-kepler][^cunningham2022] In 1766 Johann Daniel Titius added a note to a translation of Charles Bonnet's *Contemplation de la Nature* describing a numerical rule for the planetary distances, later popularised by Johann Bode: take 0, 3, 6, 12, 24, 48 and so on, add 4 and divide by 10, and the results approximate the orbital radii in AU. The rule has an empty slot at 2.8 AU, and when [[Uranus]] was found in 1781 near the distance it predicted, a search for the missing body seemed justified.[^hilton2001][^nieto2014] An organised hunt, the so-called celestial police convened by Franz Xaver von Zach, was being planned when Giuseppe Piazzi at Palermo found a slowly moving star-like object on 1 January 1801. He named it Ceres, and its orbit lay close to the empty slot.[^winterburn2021][^dawn-serendipity] Heinrich Olbers found Pallas in the same region in 1802, and Juno and Vesta followed by 1807.[^dawn-serendipity] None of them showed a disc in the telescopes of the time, and William Herschel proposed calling them "asteroids", from the Greek for star-like.[^cunningham1984][^etymonline] For decades they were nevertheless listed as planets, numbered in order of discovery. Karl Ludwig Hencke's discovery of Astraea in 1845 started a steady flow of finds, and as the list lengthened the objects were moved into a class of their own.[^hilton2001] The Titius–Bode rule lost its standing when [[Neptune]], found in 1846, turned out to lie far from its predicted slot; no physical basis for it has been established.[^astronomy-tb] The phrase "asteroid belt" appears in English by 1850, in Elise Otté's translation of Alexander von Humboldt's *Cosmos*, and in Robert James Mann's *Guide to the Knowledge of the Heavens* of 1852.[^humboldt1850][^mann1852] Discovery accelerated once Max Wolf introduced photographic searches in 1891. About 1,000 asteroids were known by 1921 and 100,000 by 2000, and automated surveys now add them by the thousand.[^hughes2007][^moore2011][^mpc-stats] In January 2014 a team using the [[NASA]]- and ESA-supported Herschel Space Observatory reported water vapour released from localised sources on Ceres, the first clear detection of water vapour in the belt.[^kuppers2014][^nasa2014] ## Origin ### Formation An early explanation, put forward by Olbers in 1802, was that the asteroids were fragments of a single planet destroyed by an explosion or an impact. The idea fails on two counts: the belt holds only a few per cent of the mass of the Moon, far too little to be the remains of a planet, and the asteroids differ chemically in ways that a single parent body cannot explain.[^cunningham2017][^krasinsky2002][^masetti2005] The accepted picture starts from the [[Protoplanetary_disk|protoplanetary disc]] described in [[Formation_and_evolution_of_the_Solar_System|the formation of the Solar System]]. Dust grains stuck together into larger aggregates, and once these reached kilometre sizes their own [[Gravity|gravity]] let them sweep up neighbours and grow into planetesimals and then protoplanets.[^chambers2004] In the zone between Mars and Jupiter that growth was interrupted. Where an asteroid's orbital period is a simple fraction of Jupiter's, the planet's pull repeats at the same point of the orbit and builds up; these mean-motion resonances are densely packed in this zone. As Jupiter formed and its orbit shifted, the resonances swept through the planetesimals and pumped up their eccentricities and inclinations, so that their encounters became violent.[^scott2006][^edgar2004] At such speeds collisions shatter bodies rather than merging them, and no planet could assemble.[^petit2001] The bodies that did form were heated internally, probably by short-lived radioactive isotopes, and many melted at least partly. Some separated into metal cores and rocky mantles, and some may have had magma oceans or explosive volcanism, but because they were small they cooled fast, and the melting was over within a few tens of millions of years.[^taylor1993] Zircon ages from a meteorite attributed to Vesta indicate that at least that body formed within about 10 million years of the Solar System's start.[^kelly2007] ### Evolution Today's asteroids are processed survivors. Beyond early heating they have been melted locally by impacts, weathered by the [[Solar_wind|solar wind]] and cosmic rays, and gardened by micrometeorites, so their surfaces are not direct samples of the primordial disc.[^clark2002][^keil2000] Models of the early belt suggest that it once held on the order of an Earth mass of material; resonances and scattering by embryos removed nearly all of it, and less than 0.1% remains.[^petit2001] Since then the size distribution has changed little, which is why it is sometimes described as fossilised.[^bottke2005] The inner edge is set by the 4:1 resonance with Jupiter, at 2.06 AU; objects that drift there are pushed onto unstable orbits, and bodies that formed farther in were largely swept up or scattered by Mars, whose aphelion lies at 1.67 AU.[^alfven1976] The Hungaria asteroids survive just inside the 4:1 resonance because their high inclinations keep them away from it.[^spratt1990] Temperature also shaped the belt. During its formation the water [[Frost_line_(astrophysics)|frost line]] lay at about 2.7 AU, so planetesimals formed beyond that distance could keep ice.[^lecar2006] A few icy bodies may even be recent arrivals: the quasi-Hilda comet 362P is modelled as a former centaur delivered into the outer belt by an encounter with Jupiter within the past few hundred years.[^delafuente2022] ## Characteristics The belt is mostly empty space. Its members are spread through a volume so large that a spacecraft crossing it at random would be very unlikely to pass close to one.[^knudsen-lucy] The number of bodies nonetheless runs into the millions, depending on the smallest size counted. More than 200 are larger than 100 km, and the Infrared Space Observatory deep survey estimated 700,000 to 1.7 million with diameters of 1 km or more.[^jpl-sbdb][^tedesco2002] Numbers climb steeply toward smaller sizes, roughly following a power law but with excesses near 5 km and near 100 km. Most asteroids larger than about 120 km are thought to be primordial, while most of the smaller ones are fragments of them.[^bottke2005][^obrien2011] Pitjeva and Pitjev's fits to planetary and spacecraft motions give a total of about 2.39 × 10²¹ kg; with the Moon at 7.35 × 10²² kg this is about 3.3% of a lunar mass (derived).[^pitjeva2018][^nasa-fs] Ceres alone holds roughly two-fifths of the belt, and the four largest bodies together a little over three-fifths.[^nasa-ceres][^pitjeva2018] Broadly, silicate-rich asteroids dominate near 2.8 AU and carbon-rich ones near 3.2 AU.[^nasa-bsf] ### Composition Asteroids are sorted by their reflectance spectra into three large groups: the dark carbonaceous C types, the brighter silicate S types, and the X types, whose featureless spectra are split by albedo into metallic M, primitive P and enstatite E types. From the inner edge outward the dominant type shifts from S to C to P and then to the very red, featureless D types, a gradient that records the temperature structure of the early disc.[^demeo2015][^gradie1982] C types are rare in the inner belt but dominate the outer belt and make up the majority of the population; their spectra resemble carbonaceous chondrite meteorites and match the Sun's composition with the most volatile elements removed. S types, concentrated inside about 2.5 AU, show [[Silicon|silicate]] minerals and metal, signs of heating that altered them.[^wiegert2007][^nova2002] M types, which peak near 2.7 AU, look like [[Iron|iron]]–[[Nickel|nickel]] metal, and some are probably exposed cores of shattered differentiated bodies, although the low density measured for 22 Kalliope shows that not every M type is metallic.[^lang2003][^margot2003] Basaltic V-type asteroids are a puzzle. If many bodies melted and differentiated, basalt from their crusts and olivine from their mantles should be common; instead nearly all of the expected basalt is missing.[^than2007] Most known basaltic asteroids are fragments of Vesta, but 1459 Magnya and two outer-belt objects, 7472 Kumakiri and (10537) 1991 RY16, have compositions that point to other parent bodies.[^than2007][^duffard2008] Dust in the belt is cold: infrared observations give about 200 K at 2.2 AU falling to about 165 K at 3.2 AU.[^low1984] ### Main-belt comets Some objects on ordinary asteroid orbits in the outer belt periodically grow dust tails. The first population of these main-belt comets was described in 2006; their orbits cannot be reached by comets captured from the outer Solar System, so they are best read as icy asteroids whose ice is exposed from time to time, perhaps by small impacts.[^hsieh2006][^lakdawalla2006] They extend the evidence for water in the outer belt and have been proposed as one possible source of Earth's [[Water|water]].[^berardelli2006] ### Orbits A typical main-belt [[Orbit|orbit]] is only mildly elongated and tilted. Most members have eccentricities below 0.4 and inclinations below 30°, and the distribution peaks near an eccentricity of 0.07 and an inclination under 4°.[^mpc-dist] The densest part, sometimes called the core, lies between the 4:1 and 2:1 gaps at 2.06 and 3.27 AU, with eccentricities below about 0.33 and inclinations below about 20°. A 2006 count of the Minor Planet Center's orbit database placed 93% of the numbered minor planets there, and the JPL Small-Body Database now lists more than a million main-belt asteroids.[^jpl-mba] The gaps follow directly from [[Kepler's_laws_of_planetary_motion|Kepler's third law]]. With Jupiter at 5.20 AU, an orbit completing exactly three circuits for each of Jupiter's lies at 5.20 × (1/3)^(2/3) ≈ 2.50 AU, and the same formula gives 2.82 AU for 5:2, 2.96 AU for 7:3 and 3.28 AU for 2:1 (derived).[^jpl-t1] Daniel Kirkwood reported the gaps in 1866 and attributed them to Jupiter's perturbations.[^fernie1999] Jupiter's early migration cleared the first occupants of these orbits, and today asteroids drift into them mainly through the slow thermal push of the Yarkovsky effect before being thrown onto eccentric orbits.[^liou1997][^demeo2015] The explorer marks the four gaps at fixed positions; the widths of its empty lanes are illustrative. ## Collisions Collisions have shaped the belt throughout its history. Bodies around 10 km across strike one another roughly once every 10 million years.[^backman1998] A hard enough impact breaks the target into fragments that share its orbit and form a family, while slow encounters can instead merge bodies.[^nesvorny2002] After more than 4 billion years of such events, the population differs greatly from the one that first formed. Evidence from spacecraft and light curves indicates that most asteroids between about 200 m and 10 km are rubble piles: loose aggregates held together mostly by their own gravity, with substantial empty space inside.[^walsh2018] Collisions and micrometeorite impacts also feed a population of dust grains up to a few hundred micrometres across. Radiation pressure and the Poynting–Robertson drag make these grains spiral slowly inward, contributing to the [[Interplanetary_dust_cloud|interplanetary dust cloud]] and to the [[Zodiacal_light|zodiacal light]]; grains near 40 μm dominate the visible glow, and typical lifetimes of about 700,000 years mean that the dust must be continually replaced.[^reach1992] The belt's share of that dust is smaller than once assumed. Modelling by Nesvorný and colleagues attributes about 85% of the zodiacal cloud to Jupiter-family [[Comet|comets]] and at most about 10% to the asteroid belt.[^nesvorny2010] ### Meteorites Some collisional fragments end up on Earth-crossing orbits and fall as [[Meteoroid|meteoroids]]. NASA estimates that of roughly 50,000 meteorites catalogued, about 99.8% come from the asteroid belt.[^nasa-meteorites] ## Families and groups Kiyotsugu Hirayama noticed in 1918 that some asteroids share nearly the same orbital elements, and he interpreted the clusters as families.[^hughes2007b] Roughly a third of main-belt asteroids are now assigned to families, identified by clustering in semi-major axis, eccentricity and inclination and confirmed where the members also share spectral features. About 20 to 30 associations are considered secure.[^lemaitre2004] Prominent examples, from the inner belt outward, include the Flora, Vesta, Nysa, Eunomia, Koronis, Eos and Themis families.[^lang2003] The Flora family, with more than 13,000 known members, may have formed less than a billion years ago.[^martel2004] Vesta is the largest body that genuinely belongs to its family: the Vesta family and the howardite–eucrite–diogenite meteorites are traced to large impacts on it. Ceres, by contrast, only overlaps the Gefion family as an interloper.[^drake2001] Three dust bands detected by the Infrared Astronomical Satellite share inclinations with the Eos, Koronis and Themis families.[^love1992] ### Periphery Inside the main belt, between about 1.78 and 2.0 AU, lie the Hungarias, named for 434 Hungaria. They are cut off from the main belt by the 4:1 gap, move on steeply inclined orbits, and are slowly depleted by Mars.[^spratt1990] The Phocaea group, between 2.25 and 2.5 AU, is another high-inclination population and is dominated by S types, whereas the Hungarias include E types.[^carvano2001][^novakovic2017] Beyond the outer edge, the Cybele group occupies 3.3–3.5 AU, and the [[Hilda_asteroid|Hilda asteroids]] at 3.5–4.2 AU are held in a stable 3:2 resonance with Jupiter. Few asteroids lie between 4.2 AU and Jupiter's orbit, where the two swarms of [[Jupiter_trojan|Jupiter trojans]] roughly equal the main belt in number of bodies larger than 1 km.[^dymock2010] ### New families Some families are young enough that their orbits can be integrated back to the moment of breakup. The Karin cluster formed about 5.7 million years ago from a parent roughly 33 km in radius, and the Veritas family about 8.3 million years ago; dust from the Veritas event has been identified in ocean sediments.[^nesvorny2006k][^mckee2006] The Datura cluster is younger still, about 450,000–530,000 years old, an age derived from the convergence of its members' orbits rather than from physical evidence.[^nesvorny2006d][^vokrouhlicky2009] Young breakups such as these are candidate sources of the dust bands.[^nesvorny2003] ## Exploration Pioneer 10, launched in March 1972, was the first spacecraft to fly through the belt.[^nasa-pioneer10] Mission planners were concerned about impacts, but Pioneer 11, the two Voyagers, Ulysses, Cassini and many others have since passed through without harm; Cassini measured fine dust and plasma during its crossing in 2000.[^schippers2015] The chance that a probe crossing the belt strikes an asteroid has been put at less than one in a billion.[^stern2006] Most main-belt asteroids seen close up were imaged in brief flybys by spacecraft bound elsewhere. Galileo made the first asteroid flybys, of 951 Gaspra in 1991 and 243 Ida in 1993;[^nasa-galileo] NEAR Shoemaker flew by 253 Mathilde in 1997 on its way to orbit and land on the [[Near-Earth_object|near-Earth asteroid]] 433 Eros;[^nasa-near] and Rosetta was targeted at 2867 Šteins and 21 Lutetia, which it passed in 2008 and 2010.[^barucci2007] Dawn was the first mission to orbit main-belt bodies: it arrived at Vesta in July 2011, studied it into 2012, and entered orbit around Ceres in March 2015.[^russell2012][^russell2016] On the way to the Jupiter trojans, Lucy flew past 152830 Dinkinesh in 2023 and 52246 Donaldjohanson in 2025.[^nasa-lucy2023][^nasa-lucy2025] NASA's Psyche spacecraft is due to reach the metal-rich asteroid 16 Psyche in 2029, and China's Tianwen-2 mission plans to visit the main-belt comet 311P/PANSTARRS.[^jpl-psyche2022][^xinhua2019] ## See also - [[Asteroid]] - [[Kirkwood_gap]] - [[Ceres_(dwarf_planet)]] · [[4_Vesta]] · [[2_Pallas]] · [[10_Hygiea]] - [[Hilda_asteroid]] · [[Jupiter_trojan]] - [[Kuiper_belt]] - [[Near-Earth_object]] - [[PORTAL_Solar_System|Solar System portal]] ## References [^williams2015]: Williams, M. 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"Plots of the inner Solar System and orbital-element distributions". https://www.minorplanetcenter.net/iau/lists/MPDistribution.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Asteroid_belt) : [Wikitube](https://en.wikitube.io/wiki/Asteroid_belt) · pinned revision [1373195997](https://en.wikipedia.org/w/index.php?oldid=1373195997) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-013 · explorer state `?obj=belt`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->