# Asteroid <!-- 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 Jupiter trojans in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=trojans&embed=1" data-title="The Jupiter trojans in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=trojans`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 year/s and watch the two trojan swarms travel round the Sun with Jupiter, one 60° ahead of it and one 60° behind, in the planet's 12-year period; under show, choose small bodies to compare the swarms with the main belt inside them; press l to label the bodies and find Hektor, the one real trojan drawn among the sampled points.* An **asteroid** is a small rocky, metallic or icy body orbiting the [[Sun]] in the inner [[PORTAL_Solar_System|Solar System]], at least about a metre across, that is neither a planet nor an active [[Comet|comet]].[^nasa-asteroids][^harris2011] Most known asteroids travel in the [[Asteroid_belt|asteroid belt]] between [[Mars]] and [[Jupiter]], roughly 2 to 4 [[Astronomical_unit|AU]] from the Sun; others share Jupiter's orbit as [[Jupiter_trojan|Jupiter trojans]] or pass close to [[Earth]] as [[Near-Earth_object|near-Earth asteroids]].[^nasa-asteroids][^nssdc-near] More than 1.5 million minor planets had been catalogued by 2026, yet the whole main belt holds only about 3% of the mass of the [[Moon]].[^mpc-summary][^pitjeva2018] Asteroids range from boulders to [[Ceres_(dwarf_planet)|Ceres]], a [[Dwarf_planet|dwarf planet]] about 940 km across, and are grouped by their spectra into broad carbonaceous, stony and metallic classes.[^nasa-ceres-exp][^bus2002] They are leftovers and fragments of the planetesimals from which the planets formed. Spacecraft have flown past, orbited, landed on, sampled and deliberately struck them, and those near Earth are tracked because an impact could be catastrophic.[^michel2014][^nasa-dart2022] The explorer at the top of this page is set on the Jupiter trojans: two swarms of ILLUSTRATIVE points, a sample rather than a catalogue, gathered 60° ahead of and behind Jupiter at its L4 and L5 points, with the large trojan 624 Hektor drawn on its real orbit.[^jpl-sbdb] ## Terminology The term has never been formally defined. In 2006 the International Astronomical Union introduced "small Solar System body" for everything orbiting the Sun that is not a planet, dwarf planet or moon, and said that "minor planet" may still be used; "asteroid" survives as the everyday word.[^iau-b5][^iau-pluto] William Herschel coined it in 1802 from the Greek for "star-like", because the first finds showed no discs in telescopes.[^herschel1802] By convention, an object smaller than about a metre is a [[Meteoroid|meteoroid]], and a body that shows a coma when heated is a comet. The boundary is blurred: some asteroids have turned out to be active, many comets end as inert, asteroid-like nuclei, and highly eccentric asteroids are probably extinct comets.[^weissman2002] Dust returned by the Stardust mission from a comet also proved surprisingly asteroid-like.[^shiga2008] The word is increasingly reserved for the inner Solar System. Bodies farther out, [[Centaur_(small_Solar_System_body)|centaurs]] from 1977 onward and [[Trans-Neptunian_object|trans-Neptunian objects]] from 1992, are usually called "objects", partly to avoid deciding whether they are asteroids or comets.[^cornell-kbo] Since 2006 the largest round minor planets have been classed as dwarf planets; in the asteroid belt only Ceres qualifies.[^iau-b5][^nasa-ceres-exp] ## History of observations Although asteroids are numerous, none was known before 1801, and only [[4_Vesta|Vesta]] is normally visible to the naked eye.[^cunningham2001][^britt2005] Counts have since grown enormously: the Minor Planet Center's summary lists more than 1.5 million minor planets, almost 900,000 of them numbered.[^mpc-summary] ### Discovery of Ceres In 1772 Johann Elert Bode published the numerical pattern of Johann Daniel Titius, which matched the planets' distances except for a gap near 2.8 AU. The discovery of [[Uranus]] near the next predicted distance lent it weight, and in 1800 Franz Xaver von Zach recruited 24 astronomers, the "celestial police", to search for the missing planet.[^hogg1948][^hoskin1992] Before his invitation arrived, Giuseppe Piazzi at Palermo found a slowly moving object on 1 January 1801. He followed it until February and at first reported it as a comet. After it was lost in the Sun's glare, Carl Friedrich Gauss computed its orbit, and von Zach and Heinrich Olbers recovered it on 31 December 1801.[^forbes1971][^cunningham2001] Piazzi named it Ceres Ferdinandea, after Sicily's patron goddess and his king.[^fodera2002] ### Further search Pallas, Juno and Vesta followed by 1807. After a gap of 38 years, Karl Ludwig Hencke found 5 Astraea in 1845, and fifteen asteroids were known by the end of 1851.[^hogg1948][^dawn-fb09] In 1891 Max Wolf began photographic searches, in which asteroids appear as short trails on long exposures; he alone found 248, beginning with 323 Brucia.[^dawn-fb09][^dawn-wolf] Many astronomers regarded the flood of faint objects as "vermin of the skies".[^friedman2013] ### 19th and 20th centuries Through the twentieth century, discovery usually meant photographing a field twice about an hour apart and comparing the plates under a stereoscope, where a moving body appeared to float above the stars; its positions were then measured against catalogued stars.[^chapman1992] A first sighting only earns a provisional designation. The Minor Planet Center links separate sightings into a single orbit, after which the object receives a number and the discoverer may propose a name.[^esa-names] ## Naming As discoveries accelerated, a system of numbers was introduced alongside names in the early 1850s. From 1892 unconfirmed objects were labelled by year and letter (1892A, 1892B and so on), and the present scheme of provisional designations began in 1925.[^dawn-fb09][^mpc-desig] Today a new asteroid first receives a provisional designation giving its year, half-month and sequence of discovery; once its orbit is secure it is numbered and may be named, as in (433) Eros, usually written 433 Eros. Names are proposed by discoverers under IAU guidelines.[^openlearn][^planetary-names] ### Symbols The first asteroids received individual symbols like those of the planets; there were about two dozen by 1852, many with several variants.[^gould1852] In 1851 Johann Franz Encke introduced a numbered circle as a generic symbol, and 16 Psyche, found in 1852, was the first asteroid designated that way at discovery. No new individual symbols were created after 37 Fides in 1855.[^hilton2001] ## Formation Many asteroids are fragments of planetesimals, the building blocks of the planets, that never grew into planets.[^cneos-faq] In the belt, growth stopped once Jupiter approached its present mass: its resonances stirred the region and removed more than 99% of the planetesimals. Asteroids larger than about 120 km are thought to have accreted in that early era, while most smaller ones are later collision fragments.[^bottke2005] The largest bodies grew big enough to melt and differentiate into metal cores and rocky crusts, as Vesta did.[^kerrod2000] In models of giant-planet migration such as the [[Nice_model|Nice model]], some Kuiper-belt bodies were captured into the outer belt beyond about 2.6 AU; the survivors may be the dark D-type asteroids.[^mckinnon2008] ## Distribution within the Solar System Asteroid orbits are shaped by the gravity of the planets and by the Yarkovsky effect, a slow thrust from uneven heat emission, and several distinct populations result.[^morbidelli2002] ### Asteroid belt Most known asteroids orbit between Mars and Jupiter on orbits of low eccentricity. The belt holds an estimated 1.1–1.9 million bodies larger than 1 km, and more than 200 larger than 100 km.[^tedesco2002esa][^jpl-sbdb] Its total mass, about 2.39 × 10²¹ kg, is roughly 3% of the Moon's, and the four largest bodies hold about 62% of it, Ceres alone some 39%.[^pitjeva2018] ### Trojans Trojans share a planet's orbit at the two stable Lagrange points 60° ahead of and behind it. Most known ones accompany Jupiter, and more than a million Jupiter trojans larger than 1 km may exist.[^yoshida2005] Much smaller populations of trojans are known for Mars, Earth, Uranus and [[Neptune]]; Neptune's may be numerous.[^sheppard2006] The explorer's two swarms show where Jupiter's trojans gather, but their points are illustrative. ### Near-Earth asteroids Near-Earth asteroids have orbits that come close to Earth's. By April 2022 about 28,800 were known, 878 of them at least 1 km across.[^cneos-totals] A few are extinct comets; most were driven out of the main belt by resonances with Jupiter.[^morbidelli2002] They are divided by orbit: Atiras lie wholly inside Earth's orbit, Atens have semi-major axes under 1 AU and cross it, Apollos have larger semi-major axes and cross it, and Amors approach from outside with perihelia between 1.017 and 1.3 AU.[^cneos-groups][^delafuente2019] Some have moons: radar during the close approach of 3122 Florence in 2017 revealed two, each 100–300 m across.[^benner2017] ### Martian moons [[Moons_of_Mars|Phobos and Deimos]] resemble dark C- or D-type asteroids, which suggests they might be captured main-belt bodies.[^nasa-mro2007] Their near-circular, equatorial orbits are hard to reconcile with capture, however.[^cazenave1980] Phobos's low density implies 25–35% empty space inside, and its infrared spectrum matches no chondrite class.[^andert2010][^giuranna2010] Canup and Salmon model both moons as forming from debris of a giant impact on Mars.[^canup2018] ## Characteristics ### Size distribution Sizes range from almost 1,000 km down to about a metre. Ceres, about 940 km, is followed by Vesta and [[2_Pallas|Pallas]] at just over 500 km, then [[10_Hygiea|Hygiea]] at about 430 km. These largest bodies are roughly round and at least partly differentiated, and are thought to be surviving protoplanets.[^schmidt2007][^vernazza2021] Numbers rise steeply toward smaller sizes, with excesses near 5 km and 100 km, and the primordial belt probably held about 200 times its present population.[^bottke2005][^obrien2011] Unlike most asteroids, the large, differentiated bodies have no known moons.[^mcfadden2018] ### Rotation Very few asteroids larger than about 100 m spin faster than once every 2.2 hours. At that rate, material at the surface of a loose body would be flung off, which suggests that most such asteroids are rubble piles held together by gravity rather than solid rock.[^alcdef][^rossi2004] ### Color Space weathering by the [[Solar_wind|solar wind]] and micrometeorites darkens and reddens asteroid surfaces over time. Most of that change seems to happen within about a million years, which limits colour as an age indicator.[^uh2004][^courtland2009] ### Surface features Apart from the four largest, asteroids are generally irregular and heavily cratered. The 50 km 253 Mathilde is saturated with craters as large as its own radius.[^conrad2007] Dawn found that Ceres lacks the dozen or so basins over 400 km that models had predicted; its largest confirmed crater, Kerwan, is 284 km across.[^marchi2016][^williams2018] ### Composition Asteroids are classed spectrally into three main groups: carbon-rich C types, stony S types and metallic M types. Ceres has a rocky interior beneath an icy, salty crust, while Vesta has an [[Iron|iron]]–[[Nickel|nickel]] core, an olivine mantle and a basaltic crust.[^bus2002][^savage1995] Vernazza and colleagues find two broad populations in the belt: dark, volatile-rich C and P types with densities under about 2.2 g/cm³, and denser S and M types.[^vernazza2021] Asteroids with moons, whose densities can be measured, have often proved to be porous rubble piles, even at sizes near 300 km.[^descamps2011] [[Water]] is widespread. Ice was detected on the surface of 24 Themis in 2010, water vapour escaping from Ceres was reported in 2014, and grains returned from Itokawa show that the solar wind can make water in surface minerals.[^campins2010][^rivkin2010][^kuppers2014][^daly2021] Carbonaceous meteorites contain nucleobases and sugars, including ribose, formed in space. This suggests that asteroids could have supplied some of the ingredients of life to early Earth.[^callahan2011][^furukawa2019] ## Classification ### Orbital classification Asteroids are grouped by their orbits. Groups are loose dynamical associations; families are tighter clusters formed by the break-up of a parent body. Kiyotsugu Hirayama first recognised families in the main belt in 1918, and about a third of belt asteroids are family members.[^zappala1995] Some asteroids follow horseshoe or quasi-satellite paths co-orbital with Earth, such as 3753 Cruithne.[^morbidelli2002] ### Spectral classification Chapman, Morrison and Zellner introduced the first taxonomy in 1975, with dark C types, stony S types and a catch-all U class.[^chapman1975] David Tholen's 1984 scheme, based on an eight-colour survey, has 14 classes, and the SMASS survey of 2002 extended it to 24, grouped into C, S and X complexes.[^tholen1989][^bus2002] Spectral class does not map neatly onto composition, and survey biases mean the observed proportions do not represent the true population.[^mcsween1999] ### Active asteroids Active asteroids have asteroid-like orbits but lose mass like comets. David Jewitt and Henry Hsieh first described them in 2006 as main-belt comets, a name later broadened because not all are icy.[^jewitt2015] The first recognised was 133P/Elst–Pizarro, found to have a tail in 1996.[^hsieh2004] Hubble saw six tails on 311P/PANSTARRS, probably from material shed by a fast-spinning rubble pile.[^jewitt2013] The DART impact in 2022 made Dimorphos an active asteroid under known conditions. The collision released about a million kilograms and produced a tail about 10,000 km long.[^li2023][^witze2023] ### Dark comets First identified in 2024, dark comets are asteroid-like bodies that show the non-gravitational accelerations of comets without any visible coma. Two groups are known: larger objects on Jupiter-family-like orbits and smaller, darker ones on nearly circular orbits.[^sciam2025] The interstellar object [[1I/ʻOumuamua]] showed the same combination.[^hoang2020] ## Observation and exploration From the ground, light curves give rotation periods, pole directions and rough shapes, spectra give surface composition, stellar occultations give sizes, and radar gives shapes and precise orbits of near-Earth asteroids. These methods sample only the outermost micrometres of a surface.[^michel2014] ### Ground-based observations Brightness measurements yield absolute magnitudes and so rough sizes. Radar from the Arecibo and Goldstone dishes has mapped craters and boulders on near-Earth asteroids and refined their orbits and spins.[^michel2014] ### Space-based observations Space telescopes avoid the atmosphere and can survey more of the sky. NEOWISE observed more than 100,000 main-belt asteroids, and Spitzer more than 700 near-Earth asteroids, though mostly for sizes.[^michel2014][^myhrvold2018] Hubble has recorded asteroid collisions and break-ups and imaged mission targets such as Vesta.[^hubble2010][^hubble2014][^hubble1997] ### Space probe missions Galileo made the first close-up images, of 951 Gaspra in 1991 and of 243 Ida and its moon Dactyl in 1993.[^nasa-galileo] NEAR Shoemaker, the first dedicated asteroid mission, orbited 433 Eros and landed on it in 2001.[^nasa-near] Japan's Hayabusa studied Itokawa in 2005 and returned grains of it to Earth in 2010.[^nasa-hayabusa] Dawn orbited Vesta and Ceres.[^russell2016] NASA's OSIRIS-REx delivered a sample of Bennu in September 2023.[^nasa-osirisrex] In 2022, DART struck Dimorphos and shortened its orbit around Didymos by about 32 minutes.[^nasa-dart2022] Lucy, launched in 2021, has flown past two main-belt asteroids on its way to the Jupiter trojans.[^nasa-lucy] Psyche launched in October 2023 toward the metal-rich asteroid 16 Psyche, ESA's Hera followed in October 2024 to study the DART impact site, and China's Tianwen-2 launched in May 2025 to sample the near-Earth asteroid Kamoʻoalewa.[^nasa-psyche][^esa-hera][^astronomy-tw2] ## Asteroid mining Asteroid mining was proposed in the 1970s as a source of metals and volatiles, either for return to Earth or for construction and fuel in space.[^oleary1979][^anderson2019] Water extracted from ice could supply propellant depots, and a Keck Institute study examined retrieving a small asteroid into lunar orbit.[^brophy2012] Forgan and Elvis have suggested that mining by other civilisations might leave detectable signatures in the debris discs of other stars.[^forgan2011] ## Threats to Earth Near-Earth asteroids have been known since the early twentieth century, but concern about impacts grew mainly with acceptance of the impact explanation for the Cretaceous–Paleogene extinction and the 1994 collision of Comet Shoemaker–Levy 9 with Jupiter.[^mellor2007] Automated CCD surveys followed. By 2011 an estimated 89–96% of near-Earth asteroids 1 km or larger had been found.[^cneos-totals] In 2013 testimony to the US Congress, NASA was said to need at least five years of preparation before launching an interception mission.[^house2013] ### Asteroid deflection strategies Proposed defences fall into two families: fragmentation, which breaks an impactor into pieces small enough to miss or burn up, and delay, which changes its arrival time.[^hall1997][^solem2000] Earth, about 12,750 km across and moving at about 30 km/s, travels its own diameter in roughly 425 seconds (derived), so shifting an impactor's arrival by a few minutes can turn a hit into a miss.[^ross2001][^nasa-fs] An early study, Project Icarus in 1967, planned six Saturn V launches with nuclear warheads against the asteroid 1566 Icarus.[^portree2012] DART demonstrated the kinetic-impactor approach in 2022.[^nasa-dart2022] ## Fiction Asteroids are a staple of science fiction: settings for colonies and mines, hazards for spaceships crossing the belt, and threats to life on Earth. Mellor traces how such impact stories, and the films built on them in the 1990s, have been used to frame public arguments for asteroid defence and for weapons in space.[^mellor2007] ## See also - [[Asteroid_belt]] · [[Kirkwood_gap]] - [[Jupiter_trojan]] · [[Hilda_asteroid]] · [[Near-Earth_object]] - [[Ceres_(dwarf_planet)]] · [[4_Vesta]] · [[2_Pallas]] · [[10_Hygiea]] - [[Small_Solar_System_body]] · [[Meteoroid]] · [[Comet]] - List of minor planets ## Notes The time for Earth to cross its own diameter is derived from its diameter and orbital speed in the NASA fact sheet. ## References [^nasa-asteroids]: NASA Science (26 October 2017, updated). "Asteroids". https://science.nasa.gov/solar-system/asteroids/ [^harris2011]: Harris, A. W. (2011). "Asteroid". In Gargaud, M.; et al. (eds.), *Encyclopedia of Astrobiology*. Springer, pp. 102–112. https://doi.org/10.1007/978-3-642-11274-4_116 [^nssdc-near]: NASA NSSDCA. 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"Asteroids". https://science.nasa.gov/solar-system/asteroids/ - NASA JPL Center for Near-Earth Object Studies. https://cneos.jpl.nasa.gov/ - IAU Minor Planet Center. https://minorplanetcenter.net/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Asteroid) : [Wikitube](https://en.wikitube.io/wiki/Asteroid) · pinned revision [1374401326](https://en.wikipedia.org/w/index.php?oldid=1374401326) · 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-014 · explorer state `?obj=trojans`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->