--- title: Asteroid belt summary: A ring of rocky and icy bodies left over from planet formation, shaped by the resonances of nearby giant planets. The Sun's Main Belt, between Mars and Jupiter, holds about 3 percent of the Moon's mass spread through a volume so large that its asteroids are millions of kilometres apart. science_status: [observed, model, sim] categories: [Asteroid belts, Minor planets, Small bodies, Planetary systems] aliases: [Main Belt, Main asteroid belt, Kirkwood gaps, Kuiper belt, Trojans, Hildas, Debris disc] infobox: type: belt name: Asteroid belt (the Sun's Main Belt) definition: "A broad ring of small bodies orbiting a star where no planet formed, usually bounded and sculpted by mean-motion and secular resonances with a giant planet." location_au: {min: 2.06, max: 3.27, note: "inner edge at Jupiter's 4:1 resonance and the nu6 secular resonance; outer edge at Jupiter's 2:1"} mass: {earth_masses: 4.008e-4, kg: 2.39e21, note: "about 3.3 percent of the Moon's mass (Pitjeva and Pitjev 2018)"} members_over_1_km: {min: 1100000, max: 1900000} largest_member: "Ceres, 939 km across, about 39 percent of the belt's mass" mean_spacing_km: {value: 3000000, note: "order-of-magnitude estimate for bodies over 1 km"} spectral_classes: ["S (stony) dominant in the inner belt", "C (carbonaceous) dominant in the outer belt", "V (basaltic, the Vesta family)", "M and X (metal-rich, e.g. Psyche)", "P and D (dark, in the Hildas and Trojans)"] structure: ["Kirkwood gaps at the 3:1 (2.50 AU), 5:2 (2.82 AU) and 7:3 (2.96 AU) resonances with Jupiter", "Hilda group clustered at the 3:2 resonance (3.97 AU)", "Jupiter Trojans at the L4 and L5 points, 60 degrees ahead of and behind Jupiter"] related_belts: ["Kuiper belt, 39.4 to 47.8 AU, 1.97e-2 Earth masses, about 50 times the Main Belt"] around_other_stars: "Debris discs, detected around about 17 percent of nearby Sun-like stars by Herschel; most belts as faint as the Sun's would be invisible to current surveys" sim_codes: {AF-S1: "inner, main-belt-like", AF-K1: "outer, Kuiper-like", AF-T1: "Trojan swarm", AF-D1: "dense debris"} sim_rule: "Inner belt at 0.397 to 0.630 of the innermost cold giant's semi-major axis; outer belt at 1.31 to 1.59 of the outermost cold giant's, or 10 to 16 snow-line radii with no giant; Trojans at L4 and L5" examples: [Main Belt, Kuiper belt, Jupiter Trojans, Fomalhaut's belts, Epsilon Eridani's belts] sim: entity: belt refs: - id: pitjeva2018 type: article-journal author: [{family: Pitjeva, given: E. V.}, {family: Pitjev, given: N. P.}] title: "Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft" container-title: Astronomy Letters volume: 44 page: 554-566 issued: 2018 DOI: 10.1134/S1063773718090050 - id: murray1999 type: book author: [{family: Murray, given: Carl D.}, {family: Dermott, given: Stanley F.}] title: "Solar System Dynamics" publisher: Cambridge University Press issued: 1999 DOI: 10.1017/CBO9781139174817 - id: tedesco2002 type: article-journal author: [{family: Tedesco, given: Edward F.}, {family: Desert, given: Francois-Xavier}] title: "The Infrared Space Observatory Deep Asteroid Search" container-title: The Astronomical Journal volume: 123 page: 2070-2082 issued: 2002 DOI: 10.1086/339482 - id: gladman1997 type: article-journal author: [{family: Gladman, given: Brett J.}, {family: Migliorini, given: Fabio}, {family: Morbidelli, given: Alessandro}, {family: others}] title: "Dynamical Lifetimes of Objects Injected into Asteroid Belt Resonances" container-title: Science volume: 277 page: 197-201 issued: 1997 DOI: 10.1126/science.277.5323.197 - id: demeo2014 type: article-journal author: [{family: DeMeo, given: Francesca E.}, {family: Carry, given: Benoit}] title: "Solar System evolution from compositional mapping of the asteroid belt" container-title: Nature volume: 505 page: 629-634 issued: 2014 DOI: 10.1038/nature12908 - id: kruijer2017 type: article-journal author: [{family: Kruijer, given: Thomas S.}, {family: Burkhardt, given: Christoph}, {family: Budde, given: Gerrit}, {family: Kleine, given: Thorsten}] title: "Age of Jupiter inferred from the distinct genetics and formation times of meteorites" container-title: Proceedings of the National Academy of Sciences volume: 114 page: 6712-6716 issued: 2017 DOI: 10.1073/pnas.1704461114 - id: petit2001 type: article-journal author: [{family: Petit, given: Jean-Marc}, {family: Morbidelli, given: Alessandro}, {family: Chambers, given: John}] title: "The Primordial Excitation and Clearing of the Asteroid Belt" container-title: Icarus volume: 153 page: 338-347 issued: 2001 DOI: 10.1006/icar.2001.6702 - id: walsh2011 type: article-journal author: [{family: Walsh, given: Kevin J.}, {family: Morbidelli, given: Alessandro}, {family: Raymond, given: Sean N.}, {family: others}] title: "A low mass for Mars from Jupiter's early gas-driven migration" container-title: Nature volume: 475 page: 206-209 issued: 2011 DOI: 10.1038/nature10201 - id: raymond2017 type: article-journal author: [{family: Raymond, given: Sean N.}, {family: Izidoro, given: Andre}] title: "The empty primordial asteroid belt" container-title: Science Advances volume: 3 page: e1701138 issued: 2017 DOI: 10.1126/sciadv.1701138 - id: nesvorny2013 type: article-journal author: [{family: Nesvorny, given: David}, {family: Vokrouhlicky, given: David}, {family: Morbidelli, given: Alessandro}] title: "Capture of Trojans by Jumping Jupiter" container-title: The Astrophysical Journal volume: 768 page: 45 issued: 2013 DOI: 10.1088/0004-637X/768/1/45 - id: park2016 type: article-journal author: [{family: Park, given: R. S.}, {family: Konopliv, given: A. S.}, {family: Bills, given: B. G.}, {family: others}] title: "A partially differentiated interior for (1) Ceres deduced from its gravity field and shape" container-title: Nature volume: 537 page: 515-517 issued: 2016 DOI: 10.1038/nature18955 - id: russell2012 type: article-journal author: [{family: Russell, given: C. T.}, {family: Raymond, given: C. A.}, {family: Coradini, given: A.}, {family: others}] title: "Dawn at Vesta: Testing the Protoplanetary Paradigm" container-title: Science volume: 336 page: 684-686 issued: 2012 DOI: 10.1126/science.1219381 - id: jplsbdb type: webpage author: [{literal: JPL Solar System Dynamics}] title: "Small-Body Database" URL: https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html accessed: 2026-09-27 - id: nasapsyche type: webpage author: [{literal: NASA}] title: "Psyche" container-title: NASA Science URL: https://science.nasa.gov/mission/psyche/ accessed: 2026-09-27 - id: nasalucy type: webpage author: [{literal: NASA}] title: "Lucy" container-title: NASA Science URL: https://science.nasa.gov/mission/lucy/ accessed: 2026-09-27 - id: tanga2023 type: article-journal author: [{literal: Gaia Collaboration}, {family: Tanga, given: P.}, {family: Pauwels, given: T.}, {family: others}] title: "Gaia Data Release 3: The Solar System survey" container-title: Astronomy & Astrophysics volume: 674 page: A12 issued: 2023 DOI: 10.1051/0004-6361/202243796 - id: jewitt1993 type: article-journal author: [{family: Jewitt, given: David}, {family: Luu, given: Jane}] title: "Discovery of the candidate Kuiper belt object 1992 QB1" container-title: Nature volume: 362 page: 730-732 issued: 1993 DOI: 10.1038/362730a0 - id: stern2019 type: article-journal author: [{family: Stern, given: S. A.}, {family: Weaver, given: H. A.}, {family: Spencer, given: J. R.}, {family: others}] title: "Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object" container-title: Science volume: 364 page: eaaw9771 issued: 2019 DOI: 10.1126/science.aaw9771 - id: sibthorpe2018 type: article-journal author: [{family: Sibthorpe, given: B.}, {family: Kennedy, given: G. M.}, {family: Wyatt, given: M. C.}, {family: others}] title: "Analysis of the Herschel DEBRIS Sun-like star sample" container-title: Monthly Notices of the Royal Astronomical Society volume: 475 page: 3046-3064 issued: 2018 DOI: 10.1093/mnras/stx3188 - id: gaspar2023 type: article-journal author: [{family: Gaspar, given: Andras}, {family: Wolff, given: Schuyler Grace}, {family: Rieke, given: George H.}, {family: others}] title: "Spatially resolved imaging of the inner Fomalhaut disk using JWST/MIRI" container-title: Nature Astronomy volume: 7 page: 790-798 issued: 2023 DOI: 10.1038/s41550-023-01962-6 images_wanted: - file: File:Asteroid_belts_illustration_NASA_STScI.jpg subject: "Overhead diagram of the inner Solar System: the Sun, the orbits of Mercury, Venus, Earth and Mars, the Main Belt between Mars and Jupiter, and Jupiter's Trojan swarms ahead of and behind the planet" source: nasa source_page: https://science.nasa.gov/asset/webb/asteroid-belts-illustration/ credit: "Image: NASA, ESA, Joseph Olmsted (STScI)" licence: public domain (NASA; STScI release) - file: File:Kirkwood_gaps_histogram.svg subject: "Histogram of the semi-major axes of numbered main-belt asteroids from 2.0 to 3.5 AU, with the gaps at Jupiter's 4:1, 3:1, 5:2, 7:3 and 2:1 resonances labelled and the Hilda clump at 3:2 marked" source: other note: "shot list: draw from the Minor Planet Center or JPL Small-Body Database orbit file; no existing agency image" - file: File:Fomalhaut_belts_Webb.jpg subject: "Webb MIRI image of the dusty belts around the young star Fomalhaut: an outer ring, an intermediate belt and an inner disc" source: esa-webb source_page: https://esawebb.org/images/weic2312a/ credit: "NASA, ESA, CSA, A. Pagan (STScI), A. Gáspár (University of Arizona)" licence: CC BY 4.0 - file: File:Main_Belt_cluster_sim.png subject: "The view from inside an asteroid cluster in Sol's Main Belt in Pax Abyssi, rocks lit by a distant Sun" source: sim note: "shot list: no render exists yet; take it at a Main Belt cluster once the field streaming rung is signed off" --- An **asteroid belt** is a ring of small rocky and icy bodies orbiting a star where no planet managed to form. The Sun's **Main Belt**, between Mars and Jupiter at 2.06 to 3.27 astronomical units (AU), holds over a million bodies larger than a kilometre, yet their combined mass is only about 3 percent of the Moon's :cite[pitjeva2018] :cite[tedesco2002]. Its edges and gaps are set by resonances with Jupiter, its make-up records how material moved around the young Solar System, and it is the source of most meteorites that reach Earth. ## Where the Main Belt is, and why there The belt's inner edge at about 2.06 AU coincides with Jupiter's 4:1 mean-motion resonance, where an asteroid would complete four orbits for every one of Jupiter's, together with the $\nu_6$ secular resonance with Saturn. Its outer edge, near 3.28 AU, is Jupiter's 2:1 resonance :cite[murray1999]. Kepler's third law turns any resonance into a distance. An asteroid making $p$ orbits for every $q$ of Jupiter's has a period $q/p$ of Jupiter's, and so a semi-major axis $$ a = a_J \left(\frac{q}{p}\right)^{2/3} $$ With Jupiter at $a_J = 5.20$ AU, the 3:1 resonance falls at 2.50 AU, the 5:2 at 2.82 AU and the 7:3 at 2.96 AU. Because the rule depends only on period ratios, the same fractions of a giant's orbit mark the same features around any star. **The Kirkwood gaps.** In the 1860s Daniel Kirkwood noticed that asteroids avoid exactly these distances :cite[murray1999]. At a resonance, Jupiter tugs on an asteroid at the same point of its orbit again and again, and the kicks build up: its eccentricity grows until it crosses the orbit of Mars or Earth, falls into the Sun or is thrown out by Jupiter. Numerical experiments show that bodies pushed into the 3:1 or $\nu_6$ resonances typically last only a few million years, and some of the survivors reach Earth as meteorites :cite[gladman1997]. The gaps are the emptied lanes. Not every resonance empties. The **Hildas**, at the 3:2 resonance near 3.97 AU, are a stable clump whose orbits keep them away from Jupiter, and the **Trojans** share Jupiter's orbit at its L4 and L5 points, 60 degrees ahead and behind. The Trojans are dark, primitive bodies that were probably captured when the giant planets' orbits reshuffled early in the Solar System's history :cite[nesvorny2013]. NASA's Lucy spacecraft, launched in 2021, is due to make its first Trojan flyby in August 2027 :cite[nasalucy]. ::figure{src="File:Asteroid_belts_illustration_NASA_STScI.jpg" size=wide alt="Overhead diagram of the Sun, the orbits of the four inner planets, a broad ring of asteroids between Mars and Jupiter, and two swarms of asteroids sharing Jupiter's orbit" caption="Diagram: the Main Belt between Mars and Jupiter, and Jupiter's Trojans leading and trailing the planet. Credit: NASA, ESA, Joseph Olmsted (STScI)."} ## A belt that is mostly empty Pictures of spaceships dodging tumbling boulders get the belt badly wrong. Its total mass, measured from the pull it exerts on the planets and on spacecraft, is $4.0 \times 10^{-4}$ Earth masses, or $2.4 \times 10^{21}$ kg :cite[pitjeva2018]. Ceres alone, at 939 km across, holds about 39 percent of that :cite[park2016]. The number of bodies larger than a kilometre is 1.1 to 1.9 million :cite[tedesco2002], spread through a doughnut-shaped volume roughly $10^{26}$ cubic kilometres in size. Dividing one by the other puts neighbouring kilometre-sized asteroids some 3 million km apart on average, about eight times the distance from Earth to the Moon. Standing anywhere in the real belt you would almost never see an asteroid as more than a point of light, and spacecraft cross it without having to steer around anything. ## What asteroids are made of Asteroids are sorted into classes by their colour and reflectance spectra. The stony **S types** dominate the inner belt, the dark carbon-rich **C types** the outer belt, and the even darker **P and D types** the Hildas and Trojans. Smaller groups include the basaltic **V types**, chips off Vesta's crust, and the metal-rich **M types** :cite[demeo2014]. The gradient from S to C roughly tracks distance from the Sun, and from the snow line, beyond which water ice could condense. DeMeo and Carry's survey of asteroid colours found far more mixing than a smooth gradient allows, especially among smaller bodies, evidence that material was stirred and moved around after it formed :cite[demeo2014]. Meteorites tell a matching story. They fall into two isotopically distinct families that formed in separate reservoirs, one inside and one outside the orbit of Jupiter, and the separation appears to have been set by Jupiter's early growth within the first million years or so :cite[kruijer2017]. Both families are now mixed in the belt. ## Why no planet formed here The idea that the belt is the rubble of an exploded planet dates from Heinrich Olbers in 1802 and does not survive the numbers: the whole belt would make a body less than half the size of the Moon, and its material comes from at least two different reservoirs with different formation ages. The accepted picture is that a planet never formed. Several models reproduce the belt we see. In one, Jupiter's resonances and wandering planetary embryos stirred up the original belt and cleared out most of its mass :cite[petit2001]. In the "Grand Tack", Jupiter migrated inward toward Mars's present orbit and back out again, truncating the disk and refilling the belt with S types from inside and C types from outside :cite[walsh2011]. In a third, the belt formed nearly empty and was later seeded with material scattered in from both sides :cite[raymond2017]. ## The Kuiper belt and belts around other stars Beyond Neptune lies a second, larger belt. The **Kuiper belt**'s main population orbits between Neptune's 3:2 resonance at 39.4 AU, where Pluto and the other Plutinos sit, and its 2:1 resonance at 47.8 AU. Its mass, about $2 \times 10^{-2}$ Earth masses, is roughly 50 times the Main Belt's :cite[pitjeva2018]. The first Kuiper belt object after Pluto was found in 1992 :cite[jewitt1993], and in 2019 New Horizons flew past Arrokoth, a primitive body about 36 km long made of two lobes that joined gently :cite[stern2019]. Kuiper belt objects are ice-rich, and the belt feeds the short-period comets. Other stars have belts too, seen as the infrared glow of dust ground off colliding bodies. Herschel detected such **debris discs** around 17 percent of nearby Sun-like stars :cite[sibthorpe2018]. Those are only the dustiest: the Sun's own Kuiper belt would sit close to the median of the population, so perhaps half of all stars have a belt at least as bright as ours, and most belts as faint as the Sun's Main Belt are far below any survey's reach :cite[sibthorpe2018]. The James Webb Space Telescope resolved three nested belts around the young star Fomalhaut, reaching about 23 billion km (some 150 AU) from the star, with gaps that may be carved by unseen planets :cite[gaspar2023]. ::figure{src="File:Fomalhaut_belts_Webb.jpg" size=wide alt="Infrared image of a star surrounded by a bright inner disc, a darker gap, an intermediate belt, a second gap and a broad outer ring" caption="Observation: Webb's mid-infrared view of the dust belts around Fomalhaut, 25 light years away. Credit: NASA, ESA, CSA, A. Pagan (STScI), A. Gáspár (University of Arizona)."} ## How we know Most of what is known comes from ground surveys that find asteroids and track their orbits, now numbering in the hundreds of thousands, and from the belt's gravitational pull on the planets and spacecraft, which gives its total mass :cite[pitjeva2018]. The Gaia mission added precise astrometry for more than 150,000 Solar System objects and reflectance spectra of 60,518 asteroids in its third data release :cite[tanga2023]. Spacecraft have visited individual bodies. Dawn orbited Vesta, a differentiated protoplanet with an iron core :cite[russell2012], and Ceres, which turned out to be partly differentiated, with a rocky interior under a volatile-rich shell :cite[park2016]. NASA's Psyche spacecraft, launched in October 2023, is due to begin orbiting the metal-rich asteroid Psyche in August 2029 :cite[nasapsyche]. ## Notable members | Body | Where | Size | Class | Notes | |---|---|---|---|---| | Ceres | 2.77 AU | 939 km | C | Dwarf planet; about 39 percent of the belt's mass | | Vesta | 2.36 AU | 523 km | V | Differentiated protoplanet; source of the HED meteorites | | Pallas | 2.77 AU | 513 km | B | Orbit inclined 35 degrees to the ecliptic | | Hygiea | 3.15 AU | about 410 km | C | Fourth-largest asteroid; largest in the outer belt | | Psyche | 2.93 AU | 222 km | M | Metal-rich; NASA spacecraft arrives 2029 | | 624 Hektor | 5.28 AU | about 225 km | D | Largest Jupiter Trojan | | Arrokoth | 44.1 AU | about 36 km long | Kuiper belt object | Visited by New Horizons in 2019 | Orbital and size values from the JPL Small-Body Database :cite[jplsbdb]. :::callout{type=sim title="In Pax Abyssi"} Every star system in Pax Abyssi carries its belts in its data, placed by a rule built on the science above. An inner, Main-Belt-like belt fills 0.397 to 0.630 of the orbit of the innermost giant planet of at least 10 Earth masses, the span between that giant's 4:1 and 2:1 resonances, with gaps cut at the 3:1, 5:2 and 7:3 resonances and a Hilda clump at 3:2. An outer, Kuiper-like belt sits just beyond the outermost cold giant, or at 10 to 16 times the snow-line distance where there is none. Giants get Trojan swarms at their L4 and L5 points. Composition grades from stony to carbonaceous across the snow line, and outer belts are icy. The odds follow the debris-disc surveys: an outer belt for half of all stars (six in ten for the hotter O, B and A stars), whether or not the star has planets. The rule gives an outer belt to 57.2 percent of systems and an inner belt to only 4.9 percent, since few systems have a cold giant placed to shape one. Sol carries the real Main Belt from 2.06 to 3.27 AU with its gaps and Hildas, Jupiter's and Neptune's Trojans, and the Kuiper belt from 39.4 to 47.8 AU. A belt in the game is the near-empty ring the science describes. Dense rock exists only at a belt's named clusters, each 20 to 60 km across and holding 12,000 to 20,000 rocks, far more crowded than anywhere in a real belt. The game states this as a deliberate departure from the science, made so there is somewhere to fly among asteroids. Two Sol clusters can be visited in the current build, one metal-rich cluster in the Main Belt with a mining station and one icy cluster in the Kuiper belt; both are still in development. Belts are not yet drawn on the orrery or the galaxy map, and their effects on nearby planets, such as impact rates and the delivery of water, are not modelled. ::: ## See also - [[Orbit]] - [[Natural satellite]] - [[Planetary system archetypes]] - [[Star system generation]] - [[Sol]] - [[Gas giant]] - [[Planet occurrence]]