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{{other uses}}{{short description|Minor planet that is not a comet}}{{Use dmy dates|date=April 2019}}File:(253) mathilde.jpg|thumb|upright=1.35|253 Mathilde, a C-type asteroid measuring about {{cvt|50|km|mi|-1}} across, covered in craters half that size. Photograph taken in 1997 by the NEAR ShoemakerNEAR ShoemakerFile:PIA21597 - New Radar Images of Asteroid 2014 JO25 (cropped).gif|right|thumb|2014 JO252014 JO25(File:Asteroid Belt.svg|thumb|Diagram of the Solar System's asteroid belt)Asteroids are minor planets, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt.WEB,weblink Asteroids, NASA – Jet Propulsion Laboratory, 13 September 2010, In this article, the term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter.There exist millions of asteroids, many the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.WEB, CNEOS, Frequently Asked Questions (FAQs) – What Are Asteroids And Comets?,weblink 13 September 2010,weblink" title="">weblink 9 September 2010, live, The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively. The sizes of asteroids varies greatly; the largest, Ceres, is almost {{cvt|1000|sigfig=1|km}} across.Asteroids are somewhat arbitrarily differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are primarily composed of dust and ice. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice.WEB, What is the difference between an asteroid and a comet?,weblink Cool Cosmos, Infrared Processing and Analysis Center, 13 August 2016, The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter.JOURNAL, Rubin, Alan E., Grossman, Jeffrey N., Meteorite and meteoroid: new comprehensive definitions, Meteoritics and Planetary Science, January 2010, 45, 1, 114–122, 2010M&PS...45..114R, 10.1111/j.1945-5100.2009.01009.x, free, Finally, meteoroids can be composed of either cometary or asteroidal materials.WEB, What is the difference between asteroids and meteorites?,weblink Universe Today, Atkinson, Nancy, 2 June 2015, 13 August 2016, Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.WEB,weblink Closest Flyby of Large Asteroid to be Naked-Eye Visible,, Robert Roy, Britt, 4 February 2005, {{As of|2017|10}}, the Minor Planet Center had data on almost 745,000 objects in the inner and outer Solar System, of which almost 504,000 had enough information to be given numbered designations.WEB,weblink Latest Published Data, 11 October 2017, International Astronomical Union Minor Planet Center, The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908.PRESS RELEASE,weblink United Nations General Assembly proclaims 30 June as International Asteroid Day, 7 December 2016, UNIS/OS/478, United Nations Office for Outer Space Affairs, WEB,weblink Rapporteur: Awale Ali Kullane, International cooperation in the peaceful uses of outer space, 25 October 2016, 6 December 2016, United Nations, In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent sure when."NEWS, Homer, Aaron, Earth Will Be Hit by an Asteroid With 100 Percent Certainty, Says Space-Watching Group B612 – The group of scientists and former astronauts is devoted to defending the planet from a space apocalypse.,weblink 28 April 2018, Inquisitr, 26 November 2018, Also in 2018, physicist Stephen Hawking,in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet.NEWS, Stanley-Becker, Isaac, Stephen Hawking feared race of 'superhumans' able to manipulate their own DNA,weblink 15 October 2018, The Washington Post, 26 November 2018, WEB, Haldevang, Max de, Stephen Hawking left us bold predictions on AI, superhumans, and aliens,weblink 14 October 2018, Quartz (publication), Quartz, 26 November 2018, NEWS, Bogdan, Dennis, Better Way To Avoid Devastating Asteroids Needed?,weblink 18 June 2018, The New York Times, 26 November 2018, In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.NEWS, Chang, Kenneth, Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks, Two years ago, NASA dismissed and mocked an amateur's criticisms of its asteroids database. Now Nathan Myhrvold is back, and his papers have passed peer review.,weblink 14 June 2018, The New York Times, 26 November 2018, According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.WEB, U.S.Congress, Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II) – Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session,weblink 19 March 2013, 147, United States Congress, 26 November 2018,


(File:Moon and Asteroids 1 to 10.svg|thumb|Sizes of the first ten asteroids to be discovered, compared to the Moon)File:243 ida.jpg|thumb|243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.]]The first asteroid to be discovered, Ceres, was originally considered to be a new planet.Ceres is the largest asteroid and is now classified as a dwarf planet. All other asteroids are now classified as small Solar System bodies along with comets, centaurs, and the smaller trans-Neptunian objects. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid",{{refn |group=note |In an oral presentation,WEB, HAD Meeting with DPS, Denver, October 2013,weblink 14 October 2013,weblink" title="">weblink 1 September 2014, dead, Clifford Cunningham presented his finding that the word was coined by Charles Burney, Jr., the son of a friend of Herschel,NEWS, Robert, Nolin, Local expert reveals who really coined the word 'asteroid',weblinkweblink" title="">weblink dead, 30 November 2014, 8 October 2013, 10 October 2013, SunSentinel, WEB,weblink Who Really Invented the Word 'Asteroid' for Space Rocks?, Wall, Mike,, 10 January 2011, 10 October 2013, }} coined in Greek as ἀστεροειδής, or asteroeidēs, meaning 'star-like, star-shaped', and derived from the Ancient Greek astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably. For example, the Annual of Scientific Discovery for 1871, quote, p. 316: "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".Overview of discovery timeline:WEB,weblink List of asteroids classified by size, Simoes, Christian,, 2018-11-07,
  • 10 by 1849
    • 1 Ceres, 1801
    • 2 Pallas – 1802
    • 3 Juno – 1804
    • 4 Vesta – 1807
    • 5 Astraea – 1845
    • in 1846, planet Neptune was discoveredWEB,weblink Today in science: Discovery of Neptune {{!,||language=en-US|access-date=2018-11-13}}
    • 6 Hebe – July 1847
    • 7 Iris – August 1847
    • 8 Flora – October 1847
    • 9 Metis – 25 April 1848
    • 10 Hygiea – 12 April 1849 tenth asteroid discovered
  • 100 asteroids by 1868
  • 1,000 by 1921
  • 10,000 by 1989
  • 100,000 by 2005JOURNAL, 6, Tichá, Jana, Marsden, Brian G., Bowell, Edward L.G., Williams, Iwan P., Marsden, Brian G., Green, Daniel W.E., Aksnes, Kaare, Schulz, Rita M., A'Hearn, Michael F., Fernández, Julio A., Kilmartin, Pamela, Kozai, Yoshihide, Lazzaro, Daniela, Nakano, Syuichi, Noll, Keith S., Schmadel, Lutz D., Shor, Viktor A., West, Richard M., Williams, Gareth V., Yeomans, Donald K., Zhu, Jin, Division III / Working Group Committee on Small Bodies Nomenclature, Proceedings of the International Astronomical Union, 4, T27A, 2009, 187–189, 1743-9213, 10.1017/S1743921308025489, 2009IAUTA..27..187T,
  • ~700,000 by 2015WEB,weblink Fig 1. The cumulative number of known asteroids and the yearly,

Historical methods

{{More citations needed section|date=January 2018}}Asteroid discovery methods have dramatically improved over the past two centuries.In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law.BOOK,weblink The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds, McCall, Gerald Joseph Home, Bowden, A. J., Howarth, Richard John, 2006, Geological Society of London, 978-1-86239-194-9, en, This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.File:PIA17937-MarsCuriosityRover-FirstAsteroidImage-20140420.jpg|thumb|right|First asteroid image (Ceres and Vesta) from Mars – viewed by Curiosity (20 April 2014).]]The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.{{citation needed|date=October 2017}}However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime years 1944 and 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies",WEB,weblink The Planetary Society, Vermin of the Sky, Friedman, Lou, a phrase variously attributed to Eduard SuessMAGAZINE, Hale, George E., George Ellery Hale, Address at the semi-centennial of the Dearborn Observatory: Some Reflections on the Progress of Astrophysics, Popular Astronomy, 1916, 24, 550–558 [555], 1916PA.....24..550H, free, and Edmund Weiss.JOURNAL, Seares, Frederick H., Address of the Retiring President of the Society in Awarding the Bruce Medal to Professor Max Wolf, Publ. Astron. Soc. Pac., 1930, 42, 245, 5–22 [10], 1930PASP...42....5S, free, 10.1086/123986, free, Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.WEB, Chapman, Mary G., 17 May 1992,weblink Carolyn Shoemaker, Planetary Astronomer and Most Successful 'Comet Hunter' To Date, USGS, 15 April 2008, These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: {{mp|1998 FJ|74}}).The last step of discovery is to send the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Computerized methods

File:Asteroid 2004 FH.gif|framed|right|2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.]](File:Known NEAs.png|thumb|upright=1.2|Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2017)There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.All these considerations helped spur the launch of highly efficient surveys that consist of charge-coupled device (CCD) cameras and computers directly connected to telescopes. {{As of|2011}}, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered. A list of teams using such systems includes:WEB
, Yeomans, Don
, Near Earth Object Search Programs
, NASA, 15 April 2008,weblink" title="">weblink 24 April 2008, live,
, Discovery Statistics – by Survey (all)
, 27 December 2018
, 27 December 2018
, 28 December 2018
, live,
{{as of|2018|10|29}}, the LINEAR system alone has discovered 147,132 asteroids.WEB, Minor Planet Discover Sites, International Astronomical Union Minor Planet Center, 27 December 2018,weblinkiau/lists/MPDiscSites.html, Among all the surveys, 19,266 near-Earth asteroids have been discoveredWEB, https:minorplanetcenter.neiau/lists/Unusual.html, Unusual Minor Planets, International Astronomical Union Minor Planet Center, 27 December 2018, including almost 900 more than {{cvt|1|km|1}} in diameter.WEB,, Discovery Statistics – Cumulative Totals, 20 December 2018, 27 December 2018,


File:Euler diagram of solar system bodies.svg|thumb|upright=1.4|Euler diagram showing the types of bodies in the Solar System. (see Small Solar System bodySmall Solar System body{{pp-move-indef}}{{multiple image| align = right| direction = vertical| width =| image1 = Asteroidsscale.jpg!D!N
Ceres (dwarf planet)>Ceres. {{As of4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl (asteroid)>Dactyl, 433 Eros, 951 Gaspra, 2867 Å teins, 25143 Itokawa.| image2 = Ceres and Vesta, Moon size comparison.jpg4 Vesta>Vesta (left), with Ceres (center) and the Moon (right) shown to scale.}}Traditionally, small bodies orbiting the Sun were classified as comets, asteroids, or meteoroids, with anything smaller than one meter across being called a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition including size limits.JOURNAL, Beech, M., September 1995, On the Definition of the Term Meteoroid, Meteoroid: A solid object moving in space, with a size less than 10 m, but larger than 100 μm., Quarterly Journal of the Royal Astronomical Society, 36, 3, 281–284, Steel, D., 1995QJRAS..36..281B, free, JOURNAL, Czechowski, L., 2006, Planetology and classification of the solar system bodies, Adv. Space Res., 38, 9, 2054–2059, 10.1016/j.asr.2006.09.004, 2006AdSpR..38.2054C, The term "asteroid", from the Greek word for "star-like", never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union.However, following the discovery of asteroids below ten meters in size, Rubin and Grossman's 2010 paper revised the previous definition of meteoroid to objects between 10 Âµm and 1 meter in size in order to maintain the distinction between asteroids and meteoroids. The smallest asteroids discovered (based on absolute magnitude H) are {{mp|2008 TS|26}} with H = 33.2 and {{mpl|2011 CQ|1}} with H = 32.1 both with an estimated size of about 1 meter.WEB,weblink JPL Small-Body Database Browser: (2011 CQ1), 2011-02-04 last obs, In 2006, the term "small Solar System body" was also introduced to cover both most minor planets and comets.The definition of "small Solar System bodies" says that they "include most of the Solar System asteroids, most trans-Neptunian objects, comets, and other small bodies". Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English especially for larger minor planets such as the dwarf planets as well as an alternative for asteroids since they are not star-like.WEB,weblink Solar System Modeling, Harvard University, English, 9 April 2016, The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,WEB,weblink Meaning of Planetule, hyperdictionary, 15 April 2008, but is not in common use. The three largest objects in the asteroid belt, Ceres, Pallas, and Vesta, grew to the stage of protoplanets. Ceres is a dwarf planet, the only one in the inner Solar System.When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.WEB, Weissman, Paul R., Bottke, William F. Jr., Levinson, Harold F., Evolution of Comets into Asteroids, Southwest Research Institute, Planetary Science Directorate, 2002,weblink 3 August 2010, For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, Chiron in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as Hidalgo ventured far beyond Jupiter for part of their orbit. Those located between the orbits of Mars and Jupiter were known for many years simply as The Asteroids.NEWS,weblink The Asteroids, The Queenslander, 25 June 2018, 16 June 1932, D. and A.C. Eglintoncentaur (minor planet)>centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered asteroids or as a new type of object. Then, when the first trans-Neptunian object (other than Pluto), 15760 Albion, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper belt>Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets.NEWS,weblink Are Kuiper Belt Objects asteroids?, Ask an astronomer, Cornell University, dead,weblink" title="">weblink 3 January 2009, They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.WEB,weblink Nicholas M. Short, Sr., Asteroids and Comets,, dead,weblink" title="">weblink/Sect19/Sect19_22.html, 25 September 2008, Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids,NEWS,, Comet Dust Seems More Asteroidy, Scientific American, 25 January 2008, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.NEWS,weblink Comet samples are surprisingly asteroid-like, New Scientist, 24 January 2008, The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.{{refn |group=note |For instance, a joint NASA–JPL public-outreach website states: }} However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets â€“ those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally the term 'Small Solar System Body' will be preferred."Questions and Answers on Planets, IAU Currently only the largest object in the asteroid belt, Ceres, at about {{cvt|975|km|0}} across, has been placed in the dwarf planet category.


File:Artist’s impression of the glowing disc of material around the white dwarf SDSS J1228+1040.jpg|thumb|Artist's impression shows how an asteroid is torn apart by the strong gravity of a white dwarfwhite dwarfIt is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately {{cvt|120|km|0}} in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.JOURNAL, Bottke, William F. Jr., Durda, Daniel D., Nesvorny, David, Jedicke, Robert, 2005, The fossilized size distribution of the main asteroid belt,weblink Icarus, 175, 1, 111, 2005Icar..175..111B, 10.1016/j.icarus.2004.10.026, 3, Morbidelli, Alessandro, Vokrouhlicky, David, Levison, Hal, Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.BOOK, Asteroids, Comets, and Meteors, Kerrod, Robin, 2000, Lerner Publications Co., 978-0-585-31763-2, registration,weblink In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.JOURNAL, McKinnon, William, B. McKinnon, 2008, 2008DPS....40.3803M, On The Possibility of Large KBOs Being Injected into The Outer Asteroid Belt, Bulletin of the American Astronomical Society, 40, 464,

Distribution within the Solar System

{{See also|List of minor-planet groups|List of notable asteroids|List of minor planets}}File:InnerSolarSystem-en.png|thumb|right|The asteroid belt (white) and Jupiter's trojan asteroidstrojan asteroidsVarious dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than {{cvt|1|km|1}} in diameter,PRESS RELEASE
, Edward, Tedesco, Metcalfe, Leo
, New study reveals twice as many asteroids as previously believed
, European Space Agency, 4 April 2002
, 21 February 2008,
and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.


Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L4 and L5, which lie 60° ahead of and behind the larger body.The most significant population of trojans are the Jupiter trojans. Although fewer Jupiter trojans have been discovered ({{as of|2010}}), it is thought that they are as numerous as the asteroids in the asteroid belt. Trojans have been found in the orbits of other planets, including Venus, Earth, Mars, Uranus, and Neptune.

Near-Earth asteroids

File:Asteroids-KnownNearEarthObjects-Animation-UpTo20180101.gif|thumb|upright=1.7|Known (Near-Earth objects]] – as of January 2018Video (0:55; July 23, 2018))File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|thumb|upright=1.7|Frequency of bolidebolideNear-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. {{As of|2016|06}}, 14,464 near-Earth asteroids are knownWEB, Discovery Statistics,weblink CNEOS, 15 June 2016, and the number over one kilometer in diameter is estimated to be 900–1,000.


Size distribution

(File:Asteroids by size and number.svg|thumb|upright=1.5|The asteroids of the Solar System, categorized by size and number)Asteroids vary greatly in size, from almost {{val|1000|u=km}} for the largest down to rocks just 1 meter across.Below 1 meter, these are considered to be meteoroids. The definition in the 1995 paper (Beech and Steel) has been updated by a 2010 paper (Rubin and Grossman) and the discovery of 1-meter asteroids. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,JOURNAL, Hubble Space Telescope Observations of 2 Pallas, Bulletin of the American Astronomical Society, 39, 485, 2007, 3, Schmidt, B., Russell, C.T., Bauer, J.M., Li, J., McFadden, L.A., Mutchler, M., Parker, J.W., Rivkin, A.S., Stern, S.A., Thomas, P.C., 2007DPS....39.3519S, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.The dwarf planet Ceres is by far the largest asteroid, with a diameter of {{cvt|975|km|0}}. The next largest are 4 Vesta and 2 Pallas, both with diameters of just over {{cvt|500|km|-2}}. Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be about 2.8–{{val|3.2|e=21|u=kg}}, or about 4% of the mass of the Moon. Of this, Ceres comprises {{val|.95|e=21|u=kg}}, a third of the total.CONFERENCE, E.V., Pitjeva, Elena V. Pitjeva, Estimations of masses of the largest asteroids and the main asteroid belt from ranging to planets, Mars orbiters and landers, 35th COSPAR Scientific Assembly. Held 18–25 July 2004, in Paris, France, 2014, 2004, 2004cosp...35.2014P, Adding in the next three most massive objects, Vesta (9%), Pallas (7%), and Hygiea (3%), brings this figure up to 51%; whereas the three after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), only add another 3% to the total mass. The number of asteroids then increases rapidly as their individual masses decrease.The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at {{val|5|u=km}} and {{val|100|u=km}}, where more asteroids than expected from a logarithmic distribution are found.Davis (2002) Asteroids III, cited by WEB,weblink Lecture 4: Moving Objects Detected by SDSS, Željko, Ivezić, 2004, dead,weblink" title="">weblink 20 July 2011, {| class="wikitable" style="text-align:right;"! colspan="15" style="background:#ddd; text-align:center;"| Approximate number of asteroids (N) larger than a certain diameter (D)
| 900 km
25000000}} {{val2000000}} {{val200000}} {{val10000}} {{val| 1

Largest asteroids

(File:The Four Largest Asteroids.jpg|thumb|upright=1.35|The four largest asteroids: 1 Ceres, 4 Vesta, 2 Pallas and 10 Hygiea){{See also|Largest asteroids}}Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly-shaped asteroids. The fourth largest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior{{cn|date=November 2019}}, like the majority of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a dwarf planet.PRESS RELEASE, 24 August 2006,weblink The Final IAU Resolution on the Definition of "Planet" Ready for Voting, International Astronomical Union, 2 March 2007, It has a much higher absolute magnitude than the other asteroids, of around 3.32,JOURNAL
, 3, Parker, J.W., Stern, S.A., Thomas, P.C., Festou, M.C., Merline, W.J., Young, E.F., Binzel, R.P., Lebofsky, L.A.
, Analysis of the First Disk-resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope
, The Astronomical Journal
, 2002
, 123
, 1
, 549–557
, 10.1086/338093, free
, 2002AJ....123..549P, free
, astro-ph/0110258
, and may possess a surface layer of ice.WEB, Asteroid 1 Ceres, The Planetary Society,weblink archiveurl=weblink" title="">weblink url-status= live, Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth.Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water;PRESS RELEASE, STScI-1995-20, Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta,weblink Hubble Space Telescope, 19 April 1995, 16 December 2017, PRESS RELEASE,weblink Key Stages in the Evolution of the Asteroid Vesta, Hubble Space Telescope, 19 April 1995, 20 October 2007,weblink" title="">weblink 7 September 2008, live, JOURNAL, Dawn mission and operations, Russel, C., Raymond, C., Fraschetti, T., Rayman, M., Polanskey, C., Schimmels, K., Joy, S., Proceedings of the International Astronomical Union, 1, S229, 97–119, 2005, 3, 10.1017/S1743921305006691, 2006IAUS..229...97R, its composition is mainly of basaltic rock such as olivine.JOURNAL, Burbine, T.H., Where are the olivine asteroids in the main belt?, Meteoritics, July 1994, 29, 4, 453, 1994Metic..29..453B, free, Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth.Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane.JOURNAL, J., Torppa, M., Kaasalainen, T., Michałowski, T., Kwiatkowski, A., Kryszczyńska, P., Denchev, R., Kowalski
, Shapes and rotational properties of thirty asteroids from photometric data
, Icarus, 1996
, 164, 2, 346–383
, 2003Icar..164..346T
, 10.1016/S0019-1035(03)00146-5, 3, Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.JOURNAL, The composition of asteroid 2 Pallas and its relation to primitive meteorites, Larson, H.P., Feierberg, M.A., Lebofsky, L.A., yes, 1983, 1983Icar...56..398L, 56, 3, 398, Icarus, 10.1016/0019-1035(83)90161-6, Pallas is the parent body of the Palladian family of asteroids.
Hygiea is the largest carbonaceous asteroidJOURNAL, 10 Hygiea: ISO Infrared Observations, Icarus, 156, 1, 202–210, Barucci, M.A.,weblink 2002, 21 October 2007,weblink" title="">weblink 28 November 2007, dead, etal, 2002Icar..156..202B, 10.1006/icar.2001.6775, and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic.WEB, Ceres the Planet,,weblink 20 October 2007,weblink" title="">weblink 11 October 2007, dead, It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family, and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope's SPHERE imager in 2017 and 2018, and announced in late 2019, revealed that Hygiea has a nearly spherical shape, which is at least consistent with it being in hydrostatic equilibrium and thus a dwarf planet.JOURNAL
, etal
, Vernazza, P.
, Jorda, L.
, Ševeček, P.
, Brož, M.
, Viikinkoski, M.
, Hanuš, J.
, Nature Astronomy
, 10.1038/s41550-019-0915-8
, 2019-10-28
, A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea, Supplementary Information
, 2019-10-30, WEB
, It's an asteroid! No, it's the new smallest dwarf planet in our solar system
, Strickland, A.
, 2019-10-28
, 2019-10-28,
{| class="wikitable" style="background:#ccf;"! colspan="13" style="background:#ddd;"| Attributes of largest asteroids style="font-size: smaller;"!Name!Orbitalradius(AU)!Orbitalperiod(years)!Inclinationto ecliptic!Orbitaleccentricity! Diameter(km)! Diameter(% of Moon)! Mass({{e|18}} kg)! Mass(% of Ceres)! Density(g/cm3)! Rotationperiod(hr) style="text-align:center;"! style="text-align:left;"| Ceres| 2.77| 4.60| 10.6°| 0.079| 964×964×892(mean 939.4)| 27%| 938| 100%| 2.16±0.01| 9.07 style="text-align:center;"! style="text-align:left;"| Vesta| 2.36| 3.63| 7.1°| 0.089| 573×557×446(mean 525.4)| 15%| 259| 28%| 3.46 ± 0.04| 5.34 style="text-align:center;"! style="text-align:left;"| Pallas| 2.77| 4.62| 34.8°| 0.231| 550×516×476(mean 512±6)| 15%| 201±13| 21%| 2.57±0.19| 7.81 style="text-align:center;"! style="text-align:left;"| Hygiea| 3.14| 5.56| 3.8°| 0.117| 450×430×424(mean 434±14)| 12%| 83.4±0.8| 9%| 1.94±0.19| 13.8{{clear}}File:masses of asteroids vs main belt.png|thumb|upright=1.75|left|The relative masses of the twelve largest asteroids known,"Recent Asteroid Mass Determinations". Maintained by Jim Baer. Last updated 2010-12-12. Retrieved 2 September 2011.The values of Juno and Herculina may be off by as much as 16%, and Euphrosyne by a third. The order of the lower eight may change as better data is acquired, but the values do not overlap with any known asteroid outside these twelve. compared to the remaining mass of the asteroid belt.JOURNAL
, Pitjeva
, E.V.
, Elena V. Pitjeva
, High-Precision Ephemerides of Planets – EPM and Determination of Some Astronomical Constants
, Solar System Research
, 2005
, 39
, 3
, 184
, 10.1007/s11208-005-0033-2
, 2005SoSyR..39..176P
, dead
,weblink" title="">weblink
, 3 July 2014
, {| style="width: 100%; margin-bottom: 4px;"{{legend2|#3D6599|1 Ceres}}{{legend2|#A44142|4 Vesta}}{{legend2|#779B4C|2 Pallas}}{{legend2|#6C4D83|10 Hygiea}}{{legend2|#278AA0|31 Euphrosyne}}{{legend2|#D27B40|704 Interamnia}}{{legend2|#4A7BBA|511 Davida}}{{legend2|#C84F50|532 Herculina}}{{legend2|#91BD5C|15 Eunomia}}{{legend2|#845E9F|3 Juno}}{{legend2|#30A9C3|16 Psyche}}{{legend2|#FF964E|#A7B6D5|all others}}{{Clear}}


Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period smaller than 2.2 hours.WEB,weblink Asteroid Lightcurve Photometry Database – About Lightcurves, 4 December 2018, 27 December 2018, ALCDEF, For asteroids rotating faster than approximately this rate, the inertial 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 most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.WEB
, Rossi
, Alessandro
, 20 May 2004
, The mysteries of the asteroid rotation day
, The Spaceguard Foundation
, 9 April 2007
, dead
,weblink" title="">weblink
, 12 May 2006
, {{further|List of fast rotators (minor planets)|List of slow rotators (minor planets)}}


(File:Vesta Cratered terrain with hills and ridges.jpg|thumb|right|Cratered terrain on 4 Vesta)The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust.WEB,weblink HubbleSite – NewsCenter, Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta – Release Images, 19 April 1995, 27 January 2015, 10 Hygiea, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: Rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or, possibly, a planet.JOURNAL, Soter, Steven, What is a Planet?,weblink 16 August 2006, 25 December 2017, Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (also see panspermia).WEB,weblink Life is Sweet: Sugar-Packing Asteroids May Have Seeded Life on Earth,, 19 December 2001,weblink" title="">weblink 24 January 2002, WEB,weblink Harvard study suggests asteroids might play key role in spreading life, Reuell, Peter, 2019-07-08, Harvard Gazette, en-US, live, 2019-09-26, In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.JOURNAL, 3, Callahan, M.P., Smith, K.E., Cleaves, H.J., Ruzica, J., Stern, J.C., Glavin, D.P., House, C.H., Dworkin, J.P., 11 August 2011, Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases, PNAS, 10.1073/pnas.1106493108, free, 21836052, 3161613, 108, 34, 13995–13998, 2011PNAS..10813995C, WEB, Steigerwald, John, NASA Researchers: DNA Building Blocks Can Be Made in Space,weblink NASA, 8 August 2011, 10 August 2011, WEB, ScienceDaily Staff, DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests,weblink 9 August 2011, ScienceDaily, 9 August 2011, (File:PIA18469-AsteroidCollision-NearStarNGC2547-ID8-2013.jpg|thumb|right|Asteroid collision – building planets (artist concept).)Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.JOURNAL, Icarus, 211, 2, February 2011, 1022–1033, Triplicity and physical characteristics of Asteroid (216) Kleopatra, 3, P., Descamps, F., Marchis, J., Berthier, J.P., Emery, G., Duchêne, I., de Pater, M.H., Wong, L., Lim, H.B., Hammel, F., Vachier, P., Wiggins, J.-P., Teng-Chuen-Yu, A., Peyrot, J., Pollock, M., Assafin, R., Vieira-Martinsa, J.I.B., Camargoi, F., Braga-Ribas, B., Macomberk, 10.1016/j.icarus.2010.11.016, 1011.5263, 2011Icar..211.1022D, On 7 October 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.MAGAZINE, Ron, Cowen, 8 October 2009, Ice confirmed on an asteroid url= archiveurl=weblink" title="">weblink url-status= live, WEB, Atkinson, Nancy, 8 October 2009, More water out there, ice found on an asteroid, International Space Fellowship, 11 October 2009,weblinkweblink" title="">weblink> archivedate= 11 October 2009 LAST2=EMERY TITLE=DETECTION OF ICE AND ORGANICS ON AN ASTEROIDAL SURFACE NATURE (JOURNAL)>NATURE ISSUE=7293 DATE=2010 PMID=20428165, 2010Natur.464.1322R, Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory.In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61.WEB, Mack, Eric, Newly spotted wet asteroids point to far-flung Earth-like planets,weblink CNET, On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.JOURNAL, 3, Küppers, Michael, O’Rourke, Laurence, Bockelée-Morvan, Dominique, Zakharov, Vladimir, Lee, Seungwon, von Allmen, Paul, Carry, Benoît, Teyssier, David, Marston, Anthony, Müller, Thomas, Crovisier, Jacques, Barucci, M. Antonietta, Moreno, Raphael, Localized sources of water vapour on the dwarf planet (1) Ceres, Nature, 505, 7484, 2014, 525–527, 10.1038/nature12918, 2014Natur.505..525K, 24451541, The detection was made by using the far-infrared abilities of the Herschel Space Observatory.WEB, Harrington, J.D., Herschel Telescope Detects Water on Dwarf Planet – Release 14-021,weblink 22 January 2014, NASA, 22 January 2014, The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." In May 2016, significant asteroid data arising from the Wide-field Infrared Survey Explorer and NEOWISE missions have been questioned.JOURNAL, Myhrvold, Nathan, Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data, Icarus, 303, 91–113, 23 May 2016, 1605.06490, 10.1016/j.icarus.2017.12.024, 2018Icar..303...91M, NEWS, Chang, Kenneth, How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know,weblink 23 May 2016, The New York Times, 24 May 2016, WEB, Billings, Lee, For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling,weblink 27 May 2016, Scientific American, 28 May 2016, Although the early original criticism had not undergone peer review,NEWS, NASA Administrator, NASA Response to Recent Paper on NEOWISE Asteroid Size Results,weblink 25 May 2016, NASA, 29 May 2016, a more recent peer-reviewed study was subsequently published.JOURNAL, Myhrvold, Nathan, Nathan Myhrvold, An empirical examination of WISE/NEOWISE asteroid analysis and results, 22 May 2018, Icarus (journal), Icarus, 314, 64–97, 10.1016/j.icarus.2018.05.004, 2018Icar..314...64M, In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of panspermia.NEWS, Steigerwald, Bill, Jones, Nancy, Furukawa, Yoshihiro, First Detection of Sugars in Meteorites Gives Clues to Origin of Life,weblink 18 November 2019, NASA, 18 November 2019, JOURNAL, Furukawa, Yoshihiro, et al., Extraterrestrial ribose and other sugars in primitive meteorites,weblink 18 November 2019, Proceedings of the National Academy of Sciences of the United States of America, 10.1073/pnas.1907169116, 18 November 2019,

Surface features

Most asteroids outside the "big four" (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km (31-mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km (186-mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.JOURNAL, 3, A.R., Conrad, C., Dumas, W.J., Merline, J.D., Drummonf, R.D., Campbell, R.W., Goodrich, D., Le Mignant, F.H., Chaffee, T., Fusco, S.H., Kwok, R.I., Knight, 2007, Icarus, 191, 2, 616–627, 10.1016/j.icarus.2007.05.004,weblink Direct measurement of the size, shape, and pole of 511 Davida with Keck AO in a single night,weblink" title="">weblink 11 August 2007, 2007Icar..191..616C, Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will be expanded with the Dawn spacecraft, which entered Ceres orbit on 6 March 2015.NEWS,weblink Dawn Spacecraft Slips Quietly into Orbit Around Dwarf Planet Ceres, 6 March 2015, Boyle, Alan, NBCUniversal Media, LLC,, 11 March 2015,


Asteroids become darker and redder with age due to space weathering.WEB, University of Hawaii Astronomer and Colleagues Find Evidence That Asteroids Change Color as They Age, University of Hawaii Institute for Astronomy, 19 May 2005,weblink 27 February 2013, However evidence suggests most of the color change occurs rapidly, in the first hundred thousands years, limiting the usefulness of spectral measurement for determining the age of asteroids.WEB, Sun damage conceals asteroids' true ages, Rachel Courtland, New Scientist, 30 April 2009,weblink 27 February 2013,


File:Kirkwood-20060509.png|thumb|Showing Kirkwood gapsKirkwood gapsAsteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbital classification

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.JOURNAL
, 3, Zappalà, V., Ph., Bendjoya, A., Cellino, P., Farinella, C., Froeschlé
, Asteroid families: Search of a 12,487-asteroid sample using two different clustering techniques
, Icarus, 1995, 116
, 2, 291–314
, 1995Icar..116..291Z
, 10.1006/icar.1995.1127, Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the Jupiter trojans.BOOK, Jewitt, David C., Sheppard, Scott, Porco, Carolyn, Jupiter's Outer Satellites and Trojans, Jupiter: The Planet, Satellites and Magnetosphere, 2004, Cambridge University Press, Bagenal, F., Dowling, T.E., McKinnon, W.B.,weblink Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.
About 30–35% of the bodies in the asteroid belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet {{dp|Haumea}}.

Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or some other planet. Examples are 3753 Cruithne and {{mpl|2002 AA|29}}. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.

Spectral classification

File:433eros.jpg|thumb|This picture of 35|m|0}} across can be seen.In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.JOURNAL, C.R., Chapman, Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry, Icarus, 25, 1, 104–130, 1975Icar...25..104C, 1975, 10.1016/0019-1035(75)90191-8, Morrison, David, Zellner, Ben, These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories.CONFERENCE, Tholen, D.J., Asteroid taxonomic classifications, Asteroids II; Proceedings of the Conference, 1139–1150, University of Arizona Press, 1989, 1989aste.conf.1139T, In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.JOURNAL, Bus, S.J., Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy, Icarus, 2002, 158, 1, 146, 10.1006/icar.2002.6856, 2002Icar..158..146B, The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.


Originally, spectral designations were based on inferences of an asteroid's composition.BOOK, Harry Y., McSween Jr., 1999, Meteorites and their Parent Planets, 2nd, Oxford University Press, 978-0-521-58751-8, However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.


File:Asteroid20130318-full.jpg|thumb|right|2013 EC2013 ECA newly discovered asteroid is given a provisional designation (such as {{mpl|2002 AT|4}}) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number (e.g. (433) Eros), but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.WEB, The Naming of Asteroids,weblink Open Learn, The Open University, 14 August 2016, In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.WEB, Asteroid Naming Guidelines,weblink The Planetary Society, The Planetary Society, 14 August 2016,


The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.JOURNAL, Gould, B.A., Benjamin Apthorp Gould, 1852, On the Symbolic Notation of the Asteroids, Astronomical Journal, 2, 80, 10.1086/100212, free, 1852AJ......2...80G, free, {| class="wikitable"! Asteroid || colspan=2| Symbol || YearCeres (dwarf planet)>1 Ceres ⚳ (File:Ceres symbol.svgOld planetary symbol of Ceres) (File:Ceres2.svgVariant symbol of Ceres) (File:Ceres3.svgOther sickle variant symbol of Ceres) Ceres (mythology) scythe, reversed to double as the letter C >| 18012 Pallas >30x20pxx20pxAthena's (Pallas') spear >| 18013 Juno >x20pxx20pxx20px) A star mounted on a scepter, for Juno (mythology), the Queen of Heaven >| 18044 Vesta >x20pxx20pxx20pxx20px) The altar and sacred fire of Vesta 18075 Astraea >x20px) (File:5 Astraea Symbol.svgAstraea (mythology)>justice 18456 Hebe >x20px) Hebe (mythology) cup >| 18477 Iris >x20px) A rainbow (iris) and a star 18478 Flora >x20px) A flower (flora), specifically the Rose of England 18479 Metis >x20px) The eye of Metis (mythology) and a star >| 184810 Hygiea >x20px) (File:10 Hygiea Astronomical Symbol.svgHygieia>Hygiea's serpent and a star, or the Rod of Asclepius 184911 Parthenope >x20px) (File:11 Parthenope symbol.svgSiren (mythology)>sirens 185012 Victoria >x20px) The laurels of victory and a star 185013 Egeria >x20pxEgeria (mythology)>Egeria's protection, and a star 185014 Irene >x30px) A dove carrying an olive branch (symbol of irene 'peace')with a star on its head,WHEN DID THE ASTEROIDS BECOME MINOR PLANETS>AUTHORLINK=JAMES L. HILTON LAST=HILTON URL=HTTP://AA.USNO.NAVY.MIL/FAQ/DOCS/MINORPLANETS.PHPARCHIVEURL=HTTPS://WEB.ARCHIVE.ORG/WEB/20071106124911/HTTP://AA.USNO.NAVY.MIL/FAQ/DOCS/MINORPLANETS.PHP | 185115 Eunomia >x20px) A heart, symbol of good order (eunomia), and a star 185116 Psyche >x20px) A butterfly's wing, symbol of the soul (psyche), and a star 185217 Thetis >x20px) A dolphin, symbol of Thetis, and a star 185218 Melpomene >x20px) The dagger of Melpomene, and a star 185219 Fortuna >x20px) The Rota Fortunae and a star >| 185226 Proserpina >x20px) Proserpina's pomegranate 185328 Bellona >x20px) Bellona (goddess)'s whip and lanceENCKE DATE= 1854 JOURNAL= ASTRONOMISCHE NACHRICHTEN ISSUE=9 DOI=10.1002/ASNA.18540380907 BIBCODE = 1854AN.....38..143. | 185429 Amphitrite >x20px) The shell of Amphitrite and a star 185435 Leukothea >x20px) A lighthouse beacon, symbol of LeucotheaLUTHER > FIRST= R TITLE=NAME UND ZEICHEN DES VON HERRN R. LUTHER ZU BILK AM 19. APRIL ENTDECKTEN PLANETEN VOLUME= 40 PAGE= 373DOI-ACCESS=FREE BIBCODE-ACCESS=FREE, 185537 Fides >x20px) The crucifix of faith (fides)LUTHER DATE= 1855 JOURNAL= ASTRONOMISCHE NACHRICHTEN ISSUE=7 DOI=10.1002/ASNA.18550420705 BIBCODE=1855AN.....42..107L | 1855In 1851,WEB,weblink When did the asteroids become minor planets?, Naval Meteorology and Oceanography Command, 6 November 2011, dead,weblink" title="">weblink 6 April 2012, dmy-all, after the fifteenth asteroid (Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years (see chart above). 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.Except for Pluto and, in the astrological community, for a few outer bodies such as 2060 Chiron That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.


{{see also|Sample return mission|Asteroid mining|Colonization of the asteroids}}(File:Eros rotation Dec. 3-4 2000.gif|thumb|Eros as seen by visiting spacecraft)Until the age of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta-v and propellant requirements, NEOs are more easily accessible than the Moon.WEB,weblink A Piloted Orion Flight to a Near-Earth Object: A Feasibility Study, 3, Rob R. Landis, David J. Korsmeyer, Paul A. Abell, Daniel R. Adamo, American Institute of Aeronautics and Astronautics, The first close-up photographs of asteroid-like objects were taken in 1971, when the Mariner 9 probe imaged Phobos and Deimos, the two small moons of Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the Voyager probes of the small moons of the gas giants.The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter.The first dedicated asteroid probe was NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001.Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), and 5535 Annefrank (by Stardust in 2002).From September to November 2005, the Japanese Hayabusa probe studied 25143 Itokawa in detail and was plagued with difficulties, but returned samples of its surface to Earth on 13 June 2010.The European Rosetta probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the third-largest asteroid visited to date, in 2010.In September 2007, NASA launched the Dawn spacecraft, which orbited 4 Vesta from July 2011 to September 2012, and has been orbiting the dwarf planet 1 Ceres since 2015. 4 Vesta is the second-largest asteroid visited to date.On 13 December 2012, China's lunar orbiter Chang'e 2 flew within {{cvt|2|mi|km|order=flip}} of the asteroid 4179 Toutatis on an extended mission.The Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa2 probe in December 2014, and plans to return samples from 162173 Ryugu in December 2020.In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.WEB, Staff, National Near-Earth Object Preparedness Strategy Action Plan,weblink 21 June 2018, White House, 22 June 2018, NEWS, Mandelbaum, Ryan F., America Isn't Ready to Handle a Catastrophic Asteroid Impact, New Report Warns,weblink 21 June 2018, Gizmodo, 22 June 2018, JOURNAL, Myhrvold, Nathan, Nathan Myhrvold, An empirical examination of WISE/NEOWISE asteroid analysis and results, 22 May 2018, Icarus (journal), Icarus, 314, 64–97, 10.1016/j.icarus.2018.05.004, 2018Icar..314...64M, NEWS, Chang, Kenneth, Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks,weblink 14 June 2018, The New York Times, 22 June 2018, (File:Bennu 330km 181029.jpg|thumb|100px|left|Bennu)In September 2016, NASA launched the OSIRIS-REx sample return mission to asteroid 101955 Bennu, which it reached in December 2018. {{As of|2019|06}}, the probe is in orbit around the asteroid.NEWS, OSIRIS-REx spacecraft captures closest ever image of asteroid Bennu, 18 June 2019, New Scientist,weblink 8 September 2019,

Planned and future missions

(File:Lucy--mission-13--v3.png|thumb|Planned Lucy spacecraft)In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon.WEB,weblink NASA May Slam Captured Asteroid into Moon (Eventually),, Mike, Wall, 30 September 2013, On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s.NEWS, Borenstein, Seth, Rock that whizzed by Earth may be grabbed by NASA,weblink 19 June 2014, AP News, 20 June 2014, It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth (asteroid mining), or materials for constructing space habitats (see Colonization of the asteroids). Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.In the U.S. Discovery program the Psyche spacecraft proposal to 16 Psyche and Lucy spacecraft to Jupiter trojans made it to the semi-finalist stage of mission selection.In January 2017, Lucy and Psyche mission were both selected as NASA's Discovery Program missions 13 and 14 respectively.WEB,weblink NASA Selects Two Missions to Explore the Early Solar System, Karen, Northon, NASA, 4 January 2017, {{further|List of missions to minor planets}}Location of Ceres (within asteroid belt) compared to other bodies of the Solar System{{Distance from Sun using EasyTimeline}}


Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.


File:951 Gaspra.jpg|951 Gaspra is the first asteroid to be imaged in close-up, imaged by Galileo on {{nowrap|1=29 October 1991}} (enhanced color)File:PIA02475 Eros' Bland Butterscotch Colors.jpg|Several views of 433 Eros in natural color, imaged by NEAR on {{nowrap|1=12 February 2000}}File:Dawn-image-070911.jpg|Vesta imaged by Dawn on {{nowrap|1=9 July 2011}}File:Ceres processed.jpg|Ceres imaged by Dawn on {{nowrap|1=4 February 2015}}

See also

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Further reading

Further information about asteroids

External links

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