Examine individual changes

'{{other uses}} {{Use dmy dates|date=June 2013}} [[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 Shoemaker]]'' probe.]] [[File:PIA21597 fig1.gif|right|thumb|[[2014 JO25]] imaged by radar during its 2017 Earth flyby]] '''Asteroids''' are [[minor planet]]s, especially those of the [[Solar System#Inner Solar System|inner Solar System]]. The larger ones have also been called '''planetoids'''. These terms have historically been applied to any astronomical object orbiting the [[Sun]] that did not show the disc of a planet and was not observed to have the characteristics of an active [[comet]]. As [[Distant minor planet|minor planets in the outer Solar System]] were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the [[asteroid belt]].<ref>{{cite web |url=http://ssd.jpl.nasa.gov/?asteroids |title=Asteroids |website=NASA – Jet Propulsion Laboratory |accessdate=13 September 2010}}</ref> In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with [[Jupiter]]. There are millions of asteroids, many thought to be the shattered remnants of [[planetesimal]]s, bodies within the young Sun's [[solar nebula]] that never grew large enough to become [[planet]]s.<ref>{{cite web |website=CNEOS |title=Frequently Asked Questions (FAQs) — What Are Asteroids And Comets? |url=http://neo.jpl.nasa.gov/faq/#ast |accessdate=13 September 2010 |archiveurl= https://web.archive.org/web/20100909210213/http://neo.jpl.nasa.gov/faq/ |archivedate= 9 September 2010 <!--DASHBot-->| deadurl= no}}</ref> The large majority of known asteroids orbit in the asteroid belt between the orbits of [[Mars]] and Jupiter, or are co-orbital with Jupiter (the [[Jupiter trojan]]s). However, other orbital families exist with significant populations, including the [[near-Earth object]]s. Individual asteroids are classified by their characteristic [[Emission spectrum|spectra]], with the majority falling into three main groups: [[C-type asteroid|C-type]], [[M-type asteroid|M-type]], and [[S-type asteroid|S-type]]. These were named after and are generally identified with [[Carbon|carbon-rich]], [[metal]]lic, and [[silicate]] (stony) compositions, respectively. The size of asteroids varies greatly; the largest is almost {{cvt|1000|km|round=25}} across. Asteroids are differentiated from [[comet]]s and [[meteoroid]]s. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice.<ref>{{cite web |title=What is the difference between an asteroid and a comet?|url=http://coolcosmos.ipac.caltech.edu/ask/181-What-is-the-difference-between-an-asteroid-and-a-comet- |website=Cool Cosmos |publisher=Infrared Processing and Analysis Center |accessdate=13 August 2016}}</ref> The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter.<ref name=Rubin2010>{{cite journal |last1=Rubin |first1=Alan E. |last2=Grossman |first2=Jeffrey N. |title=Meteorite and meteoroid: new comprehensive definitions |journal=Meteoritics and Planetary Science |date=January 2010 |volume=45 |issue=1 |pages=114–122 |bibcode=2010M&PS...45..114R |doi=10.1111/j.1945-5100.2009.01009.x |doi-access=free}}</ref> Finally, meteoroids can be composed of either cometary or asteroidal materials.<ref name="Universe Today">{{cite web |title=What is the difference between asteroids and meteorites? |url=http://www.universetoday.com/36398/what-is-the-difference-between-asteroids-and-meteorites/ |website=Universe Today |last=Atkinson |first=Nancy |date=2 June 2015 |accessdate=13 August 2016}}</ref> 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.<ref>{{cite web |url=http://www.space.com/spacewatch/050204_2004_mn4.html |title=Closest Flyby of Large Asteroid to be Naked-Eye Visible |website= SPACE.com |first=Robert Roy |last=Britt |date=4 February 2005}}</ref> {{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.<ref>{{cite web |url=http://minorplanetcenter.net/mpc/summary |title=Latest Published Data |access-date= October 11, 2017 |website= International Astronomical Union Minor Planet Center}}</ref> The [[United Nations]] declared 30 June 2018 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.<ref>{{cite press release |url=http://www.unoosa.org/oosa/en/informationfor/media/2016-unis-os-478.html |title=United Nations General Assembly proclaims 30 June as International Asteroid Day |date=7 December 2016 |id=UNIS/OS/478 |website=United Nations Office for Outer Space Affairs}}</ref><ref>{{cite web |url=https://www.un.org/ga/search/view_doc.asp?symbol=A/71/492 |others=Rapporteur: Awale Ali Kullane |title=International cooperation in the peaceful uses of outer space |date=25 October 2016 |access-date=6 December 2016 |website=United Nations}}</ref> In April 2018, the [[B612 Foundation]] reported "It's a 100 per cent certain we'll be hit [by a devastating asteroid], but we're not 100 per cent sure when."<ref name="DS-20180428">{{cite news |last=Harper |first=Paul |title=Earth will be hit by asteroid with 100% CERTAINTY – space experts warn - EXPERTS have warned it is "100pc certain" Earth will be devastated by an asteroid as millions are hurling towards the planet undetected. |url=https://www.dailystar.co.uk/news/world-news/699177/Asteroids-earth-space-apocalypse-meteor-B612-Foundation |date=28 April 2018 |work=[[Daily Star]] |accessdate=22 June 2018 }}</ref><ref name="INQ-20180428">{{cite news |last=Homer |first=Aaron |title=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. |url=https://www.inquisitr.com/4881237/earth-will-be-hit-by-an-asteroid-with-100-percent-certainty-says-space-watching-group-b612/ |date=28 April 2018 |work=[[Inquisitr]] |accessdate=22 June 2018 }}</ref> == Discovery == [[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 (asteroid)|Dactyl]]. Dactyl is the first satellite of an asteroid to be discovered.]] The first asteroid to be discovered, [[Ceres (dwarf planet)|Ceres]], was originally considered to be a new planet.<ref group=note>Ceres is the largest asteroid and is now classified as a [[dwarf planet]]. All other asteroids are now classified as [[Small Solar System body|small Solar System bodies]] along with comets, centaurs, and the smaller trans-Neptunian objects.</ref> 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 [[William Herschel|Sir William Herschel]] to propose the term "asteroid",{{refn |group=note |In an oral presentation,<ref>{{cite web |title=HAD Meeting with DPS, Denver, October 2013 - Abstracts of Papers |url=http://had.aas.org/meetings/2013bAbstracts.html#HADII |access-date=14 October 2013 |archive-url=https://web.archive.org/web/20140901143955/http://had.aas.org/meetings/2013bAbstracts.html#HADII |archive-date=1 September 2014 |dead-url=yes |df=dmy-all}}</ref> Clifford Cunningham presented his finding that the word was coined by Charles Burney, Jr., the son of a friend of Herschel,<ref>{{cite news |first=Robert |last=Nolin |title=Local expert reveals who really coined the word 'asteroid' |url=http://www.sun-sentinel.com/news/broward/fl-asteroid-word-origin-20131008,0,501498,full.story |date=8 October 2013 |access-date=10 October 2013 |newspaper=SunSentinel}}</ref><ref>{{cite web |url=http://www.space.com/10593-post-william-herschel-coin-term-asteroid.html |title=Who Really Invented the Word 'Asteroid' for Space Rocks? |last=Wall |first=Mike |website=SPACE.com |date=10 January 2011 |access-date=10 October 2013}}</ref>}} coined in Greek as ἀστεροειδής, or ''asteroeidēs'', meaning 'star-like, star-shaped', and derived from the Ancient Greek {{lang|grc|[[wikt:ἀστήρ|ἀστήρ]]}} ''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. <ref group=note>For example, the [https://books.google.com/books?id=NAMAAAAAMAAJ&pg=PA316&dq=%22planets%22+asteroids ''Annual of Scientific Discovery for 1871''], page 316, reads "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 [[110 Lydia|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".</ref> === Historical methods === {{unreferenced 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 [[Astronomical unit|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. This task required that hand-drawn sky charts be prepared for all stars in the [[zodiac]]al 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 (dwarf planet)|Ceres]] and [[4 Vesta|Vesta]]) from [[Mars]] – viewed by [[Curiosity (rover)|''Curiosity'']] (20 April 2014).]] The first object, [[Ceres (dwarf planet)|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 (constellation)|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 (Roman mythology)|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 [[John Russell Hind|J. R. Hind]], [[Annibale de Gasparis]], [[Karl Theodor Robert Luther|Robert Luther]], [[Hermann Mayer Salomon Goldschmidt|H. M. S. Goldschmidt]], [[Jean Chacornac]], [[James Ferguson (astronomer)|James Ferguson]], [[Norman Robert Pogson]], [[Ernst Wilhelm Leberecht Tempel|E. W. Tempel]], [[James Craig Watson|J. C. Watson]], [[Christian Heinrich Friedrich Peters|C. H. F. Peters]], [[Alphonse Louis Nicolas Borrelly|A. Borrelly]], [[Johann Palisa|J. Palisa]], the [[Paul Henry and Prosper Henry|Henry brothers]] and [[Auguste Charlois]]. In 1891, [[Maximilian Franz Joseph Cornelius Wolf|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",<ref>{{cite web|url=http://www.planetary.org/blogs/guest-blogs/lou-friedman/20130219-vermin-of-the-sky.html |website=The Planetary Society|title=Vermin of the Sky|author=Friedman, Lou}}</ref> a phrase variously attributed to [[Eduard Suess]]<ref>{{cite magazine | last=Hale | first=George E. | authorlink = George Ellery Hale | title= Address at the semi-centennial of the Dearborn Observatory: Some Reflections on the Progress of Astrophysics | magazine=Popular Astronomy | date=1916 | volume=24 | pages=550–558, at p 555 | bibcode= 1916PA.....24..550H |bibcode-access=free}}</ref> and [[Edmund Weiss]].<ref>{{cite journal | last=Seares |first=Frederick H. | title= Address of the Retiring President of the Society in Awarding the Bruce Medal to Professor Max Wolf | journal=Publ. Astron. Soc. Pac. | date=1930 | volume=42 | pages=5–22, at p 10 | bibcode= 1930PASP...42....5S |bibcode-access=free | doi= 10.1086/123986 |doi-access=free}}</ref> 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 [[Astrophotography|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 [[photographic plate|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.<ref>{{cite web | last=Chapman | first=Mary G. | date=17 May 1992 | url=https://astrogeology.usgs.gov/About/People/CarolynShoemaker | title=Carolyn Shoemaker, Planetary Astronomer and Most Successful 'Comet Hunter' To Date | publisher=USGS | accessdate=15 April 2008}}</ref> These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a [[provisional designation in astronomy|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 [[satellite|artificial satellite]].]] There is increasing interest in identifying asteroids whose orbits cross [[Earth]]'s, and that could, given enough time, collide with Earth ''(see [[List of Earth-crossing minor planets|Earth-crosser asteroids]])''. The three most important groups of [[near-Earth asteroid]]s are the [[Apollo asteroid|Apollos]], [[Amor asteroid|Amors]], and [[Aten asteroid|Atens]]. Various [[asteroid deflection strategies]] have been proposed, as early as the 1960s<!--- ''Project Icarus'' --->. The [[near-Earth object|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 [[Astronomical unit|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 event|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 satellite]]s, 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 ([[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.<ref name=nasa_neo/> A list of teams using such systems includes:<ref>{{cite web | last=Yeomans | first=Don | url=http://neo.jpl.nasa.gov/programs/ | title=Near Earth Object Search Programs | publisher=NASA | accessdate=15 April 2008 | archiveurl= https://web.archive.org/web/20080424093951/http://neo.jpl.nasa.gov/programs/| archivedate= 24 April 2008 <!--DASHBot-->| deadurl= no}}</ref> * [[Lincoln Near-Earth Asteroid Research]] (LINEAR) * [[Near-Earth Asteroid Tracking]] (NEAT) * [[Spacewatch]] * [[LONEOS|Lowell Observatory Near-Earth-Object Search]] (LONEOS) * [[Catalina Sky Survey]] (CSS) * [[Campo Imperatore Near-Earth Object Survey]] (CINEOS) * [[Japanese Spaceguard Association]] * [[Asiago-DLR Asteroid Survey]] (ADAS) * [[Pan-STARRS]] {{as of|2013|09|20}}, the LINEAR system alone has discovered 138,393 asteroids.<ref>{{cite web |title=Minor Planet Discover Sites |website= International Astronomical Union Minor Planet Center |accessdate=24 August 2010 |url=http://www.minorplanetcenter.org/iau/lists/MPDiscSites.html}}</ref> Among all the surveys, 4711 near-Earth asteroids have been discovered<ref>{{cite web|url=http://www.minorplanetcenter.org/iau/lists/Unusual.html|title=Unusual Minor Planets|website=International Astronomical Union Minor Planet Center|accessdate=24 August 2010}}<!--- using the "close approach" quote ---></ref> including over 600 more than {{cvt|1|km|1}} in diameter. == Terminology{{anchor|Terminology}} == <!-- Linked from "Comet" --> [[File:Euler diagram of solar system bodies.svg|thumb|upright=1.5|right|[[Euler diagram]] showing the types of bodies in the Solar System. (see [[Small Solar System body]])]] {{pp-move-indef}}{{multiple image | align = right | direction = vertical | image1 = Asteroidsscale.jpg | caption1 = A composite image, to scale, of the asteroids that have been imaged at high resolution except [[Ceres (dwarf planet)|Ceres]]. {{As of|2011}}, they are, from largest to smallest: [[4 Vesta]], [[21 Lutetia]], [[253 Mathilde]], [[243 Ida]] and its moon [[Dactyl (asteroid)|Dactyl]], [[433 Eros]], [[951 Gaspra]], [[2867 Šteins]], [[25143 Itokawa]]. | image2 = 4 Vesta 1 Ceres Moon at 20 km per px.png | caption2 = The largest asteroid in the previous image, [[4 Vesta|Vesta]] (left), with [[Ceres (dwarf planet)|Ceres]] (center) and the [[Moon]] (right) shown to scale. }} Traditionally, small bodies orbiting the Sun were classified as [[comet]]s, asteroids, or [[meteoroid]]s, with anything smaller than ten meters across being called a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition including size limits.<ref name=Beech1995>{{cite journal | last1=Beech |first1=M. |authorlink=Martin Beech |date=September 1995 |title=On the Definition of the Term Meteoroid |quote=''Meteoroid: A solid object moving in space, with a size less than 10 m, but larger than 100 μm.''|journal=Quarterly Journal of the Royal Astronomical Society |volume=36 |issue=3 |pages=281–284 |url=http://adsabs.harvard.edu/full/1995QJRAS..36..281B |last2=Steel |first2=D. |bibcode=1995QJRAS..36..281B |bibcode-access=free |access-date=16 December 2017}}</ref><ref>{{cite journal | last1 = Czechowski | first1 = L. | date = 2006 | title = Planetology and classification of the solar system bodies | url = | journal = Adv. Space Res. | volume = 38 | issue = | pages = 2054–2059 | doi = 10.1016/j.asr.2006.09.004 |bibcode = 2006AdSpR..38.2054C }}</ref> 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&nbsp;[[micrometer|µm]] and 1&nbsp;meter in size in order to maintain the distinction between asteroids and meteoroids.<ref name=Rubin2010/> The smallest asteroids discovered (based on [[Absolute magnitude#Solar System bodies (H)|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&nbsp;meter.<ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2011CQ1;cad=1#cad |title=JPL Small-Body Database Browser: (2011 CQ1) |type=2011-02-04 last obs}}</ref> In 2006, the term "[[small Solar System body]]" was also introduced to cover both most minor planets and comets.<ref group=note>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".</ref><ref name=IAU2006/> 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 planet]]s as well as an alternative for asteroids since they are not star-like.<ref>{{cite web |url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/text/text_plan_1.html |title=Solar System Modeling |publisher=Harvard University |language=English |accessdate=9 April 2016}}</ref> 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,<ref>{{cite web |url=http://www.hyperdictionary.com/dictionary/planetule |title=Meaning of PLANETULE |website=hyperdictionary | accessdate=15 April 2008}}</ref> but is not in common use. The three largest objects in the asteroid belt, [[Ceres (dwarf planet)|Ceres]], [[2 Pallas|Pallas]], and [[4 Vesta|Vesta]], grew to the stage of [[protoplanet]]s. 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 [[Outgassing|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 [[volatiles|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.<ref>{{cite web|authors=Weissman, Paul R.; Bottke, William F. Jr.; Levinson, Harold F. |title=Evolution of Comets into Asteroids |website=Southwest Research Institute, Planetary Science Directorate |date=2002 |url=http://www.boulder.swri.edu/~hal/PDF/asteroids3.pdf |accessdate= August 3, 2010}}</ref> For almost two centuries, from the discovery of [[Ceres (dwarf planet)|Ceres]] in 1801 until the discovery of the first [[centaur (minor planet)|centaur]], [[2060 Chiron|Chiron]] in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as [[944 Hidalgo|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. <ref>{{cite news |url=https://trove.nla.gov.au/newspaper/page/2368062 |title=The Asteroids |newspaper=[[The Queenslander]] |accessdate = 25 June 2018 |date=16 June 1932 |author=D. and A.C. Eglinton |department=Astronomy (column)}}</ref> When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called [[centaur (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|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|Kuiper-belt objects]], called "objects" partly to avoid the need to classify them as asteroids or comets.<ref name=KBOasteroids>{{cite news |url=http://curious.astro.cornell.edu/question.php?number=601 |title=Are Kuiper Belt Objects asteroids? |work=Ask an astronomer |publisher=Cornell University |deadurl=yes |archiveurl=https://web.archive.org/web/20090103110110/http://curious.astro.cornell.edu/question.php?number=601 |archivedate=3 January 2009 }}</ref> They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.<ref>{{cite web |url=http://rst.gsfc.nasa.gov/Sect19/Sect19_22.html |author=Nicholas M. Short, Sr. |title=Asteroids and Comets |website=NASA.gov |deadurl=yes |archiveurl=https://web.archive.org/web/20080925014037/http://rst.gsfc.nasa.gov///Sect19/Sect19_22.html|archivedate=25 September 2008}}</ref> Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional [[Comet nucleus|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 (spacecraft)|''Stardust'']] probe, are increasingly blurring the distinction between comets and asteroids,<ref>{{cite news |url=http://www.sciam.com/podcast/episode.cfm?id=ADD0878B-D6C3-3B70-7B5BC373545BB82D |title=Comet Dust Seems More Asteroidy |work=Scientific American |date=25 January 2008}}</ref> suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.<ref>{{cite news |url=https://www.newscientist.com/channel/solar-system/comets-asteroids/dn13224-comet-samples-are-surprisingly-asteroidlike.html |title=Comet samples are surprisingly asteroid-like |work=New Scientist |date=24 January 2008}}</ref> 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]] [http://ssd.jpl.nasa.gov/?asteroids public-outreach website] states: {{quote|"We include Trojans (bodies captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more correctly be called "minor planets" instead of asteroids."}}}} However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System.<ref name=KBOasteroids/> Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the [[asteroid belt]], [[Jupiter trojan]]s, and [[near-Earth object]]s. When the IAU introduced the class [[Small Solar System body|small Solar System bodies]] in 2006 to include most objects previously classified as minor planets and comets, they created the class of [[dwarf planet]]s for the largest minor planets—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."<ref>[http://www.iau.org/public/themes/pluto/ Questions and Answers on Planets], IAU</ref> Currently only the largest object in the asteroid belt, [[Ceres (dwarf planet)|Ceres]], at about {{cvt|975|km|0}} across, has been placed in the dwarf planet category. == Formation == [[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 dwarf]].<ref>{{cite web|title=The Glowing Halo of a Zombie Star|url=http://www.eso.org/public/news/eso1544/|accessdate=16 November 2015}}</ref>]] It is thought that [[planetesimal]]s 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 resonance]]s 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 [[Accretion (astrophysics)|accreted]] during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.<ref>{{cite journal |last1= Bottke |first1= William F. Jr. |last2= Durda |first2= Daniel D. |last3= Nesvorny |first3= David |last4= Jedicke |first4= Robert |date= 2005 |title= The fossilized size distribution of the main asteroid belt |url= http://astro.mff.cuni.cz/davok/papers/fossil05.pdf |journal= Icarus |volume= 175 |issue= |page= 111 |bibcode= 2005Icar..175..111B |doi= 10.1016/j.icarus.2004.10.026 |display-authors=3 |last5= Morbidelli |first5= Alessandro |last6= Vokrouhlicky |first6= David |last7= Levison |first7= Hal}}</ref> Ceres and Vesta grew large enough to melt and [[Planetary differentiation|differentiate]], with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.<ref name=ACM>{{cite book |title=Asteroids, Comets, and Meteors |author=Kerrod, Robin |date=2000 |publisher=Lerner Publications Co. |isbn=0-585-31763-1}}</ref> In the [[Nice model]], many [[Kuiper belt|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 asteroid]]s, and possibly include Ceres.<ref>{{cite journal|author=McKinnon, William |author2=B. McKinnon|date= 2008|bibcode=2008DPS....40.3803M |title=On The Possibility Of Large KBOs Being Injected Into The Outer Asteroid Belt|journal=Bulletin of the American Astronomical Society |volume=40 |page=464 }}</ref> == 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 asteroids]] (green)]] Various 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 === {{main|Asteroid belt}} The majority of known asteroids orbit within the asteroid belt between the orbits of [[Mars]] and [[Jupiter]], generally in relatively low-[[orbital eccentricity|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,<ref> {{cite press release | first=Edward | last=Tedesco | author2=Metcalfe, Leo | title=New study reveals twice as many asteroids as previously believed | publisher=European Space Agency | date=4 April 2002 | url=http://www.spaceref.com/news/viewpr.html?pid=7925 | accessdate=21 February 2008}} </ref> and millions of smaller ones. These asteroids may be remnants of the [[protoplanetary disk]], and in this region the [[accretion (astrophysics)|accretion]] of [[planetesimal]]s into planets during the formative period of the Solar System was prevented by large gravitational perturbations by [[Jupiter]]. === Trojans === {{main|Trojan (astronomy)}} [[Trojan (astronomy)|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 point]]s of stability, [[Trojan points|L4 and L5]], which lie 60° ahead of and behind the larger body. The most significant population of trojans are the [[Jupiter trojan]]s. 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 trojan|Venus]], [[Earth trojan|Earth]], [[Mars trojan|Mars]], [[Uranus trojan|Uranus]], and [[Neptune trojan|Neptune]]. === Near-Earth asteroids === {{main|Near-Earth object#Near-Earth asteroids|l1=Near-Earth asteroids}} Near-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 known<ref name=nasa_neo>{{cite web |title=Discovery Statistics |url=http://neo.jpl.nasa.gov/stats/ |website=CNEOS |accessdate=15 June 2016}}</ref> and the number over one kilometer in diameter is estimated to be 900–1,000. [[File:Asteroids-KnownNearEarthObjects-Animation-UpTo20180101.gif|thumb|center|600px|<center>Known [[Near-Earth objects]] – as of January 2018<br>[https://www.youtube.com/watch?v=vfvo-Ujb_qk Video (0:55; July 23, 2018)]</center>]] [[File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|thumb|upright=2|center|Frequency of [[bolide]]s, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.]] == Characteristics == === Size distribution === [[File:Asteroids by size and number.svg|thumb|right|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&nbsp;meter across.<ref group=note>Below 1&nbsp;meter, these are considered to be [[meteoroid]]s. 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.</ref> The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors,<ref name=Schmidt2007>{{cite journal |title=Hubble Space Telescope Observations of 2 Pallas |journal=Bulletin of the American Astronomical Society |volume=39 |page=485 |date=2007 |display-authors=3 |author1=Schmidt, B. |author2=Russell, C.T. |author3=Bauer, J.M. |author4=Li, J. |author5=McFadden, L.A. |author6=Mutchler, M. |author7=Parker, J.W. |author8=Rivkin, A.S. |author9=Stern, S.A. |author10=Thomas, P.C. |bibcode=2007DPS....39.3519S}}</ref> and are thought to be surviving [[protoplanet]]s. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving [[planetesimal]]s or fragments of larger bodies. The [[dwarf planet]] [[Ceres (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 (dwarf planet)|Ceres]] comprises {{val|.95|e=21|u=kg}}, a third of the total.<ref>{{cite conference | first=E. V. |last=Pitjeva | authorlink=Elena V. Pitjeva | title=Estimations of masses of the largest asteroids and the main asteroid belt from ranging to planets, Mars orbiters and landers | booktitle=35th COSPAR Scientific Assembly. Held 18–25 July 2004, in Paris, France | pages=2014 | date=2004 | bibcode=2004cosp...35.2014P}}</ref> Adding in the next three most massive objects, [[4 Vesta|Vesta]] (9%), [[2 Pallas|Pallas]] (7%), and [[10 Hygiea|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.<ref>Davis (2002) ''Asteroids III'', cited by {{cite web |url=http://www.astro.washington.edu/users/ivezic/Astr598/lecture4.pdf |title=Lecture 4: Moving Objects Detected by SDSS |first=Željko |last=Ivezić |date=2004 |dead-url=yes |archive-url=https://web.archive.org/web/20110720111753/http://www.astro.washington.edu/users/ivezic/Astr598/lecture4.pdf |archive-date=20 July 2011}}</ref> {| 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) |- !D | 0.1&nbsp;km || 0.3&nbsp;km || 0.5&nbsp;km || 1&nbsp;km || 3&nbsp;km || 5&nbsp;km || 10&nbsp;km || 30&nbsp;km || 50&nbsp;km || 100&nbsp;km || 200&nbsp;km || 300&nbsp;km || 500&nbsp;km || 900&nbsp;km |- !N | {{val|25000000}} || {{val|4000000}} || {{val|2000000}} || {{val|750000}} || {{val|200000}} || {{val|90000}} || {{val|10000}} || {{val|1100}} || 600 || 200 || 30 || 5 || 3 || 1 |} ====Largest asteroids==== {{See also|Largest asteroids}} [[File:masses of asteroids vs main belt.png|thumb|upright=1.75|The relative masses of the twelve [[List of notable asteroids#Largest by mass|largest asteroids]] known,<ref name="Baer2011">[http://home.earthlink.net/~jimbaer1/astmass.txt "Recent Asteroid Mass Determinations"]. Maintained by Jim Baer. Last updated 2010-12-12. Retrieved 2 September 2011.</ref><ref group=note>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.</ref> compared to the remaining mass of the asteroid belt.<ref name="Pitjeva05"> {{cite journal |last=Pitjeva |first=E. V. |authorlink=Elena V. Pitjeva |title=High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants |journal=Solar System Research |date=2005 |volume=39 |issue=3 |page=184 |url=http://iau-comm4.jpl.nasa.gov/EPM2004.pdf |format=PDF |doi=10.1007/s11208-005-0033-2 |bibcode=2005SoSyR..39..176P |deadurl=yes |archiveurl=https://web.archive.org/web/20140703074335/http://iau-comm4.jpl.nasa.gov/EPM2004.pdf |archivedate=3 July 2014 }}</ref> {| style="width: 100%; margin-bottom: 4px;" |- | valign=top width=30% | {{legend2|#3D6599|[[Ceres (dwarf planet)|1 Ceres]]}}<br /> {{legend2|#A44142|[[4 Vesta]]}}<br /> {{legend2|#779B4C|[[2 Pallas]]}}<br /> {{legend2|#6C4D83|[[10 Hygiea]]}} | valign=top width=30% | {{legend2|#278AA0|[[31 Euphrosyne]]}}<br /> {{legend2|#D27B40|[[704 Interamnia]]}}<br /> {{legend2|#4A7BBA|[[511 Davida]]}}<br /> {{legend2|#C84F50|[[532 Herculina]]}} | valign=top width=30% | {{legend2|#91BD5C|[[15 Eunomia]]}}<br /> {{legend2|#845E9F|[[3 Juno]]}}<br /> {{legend2|#30A9C3|[[16 Psyche]]}}<br /> {{legend2|#FF964E|[[52 Europa]]}}<br /> {{legend2|#A7B6D5|all others}} |} ]] <!-- end image --> Although their location in the asteroid belt excludes them from planet status, the three largest objects, [[Ceres (dwarf planet)|Ceres]], [[4 Vesta|Vesta]], and [[2 Pallas|Pallas]], are intact [[protoplanet]]s that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids. The fourth largest asteroid, [[10 Hygiea|Hygiea]], has an undifferentiated interior, 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]].<ref name=IAU2006>{{cite press release |date=24 August 2006 |url= http://www.iau.org/news/pressreleases/detail/iau0602/ |title=The Final IAU Resolution on the Definition of "Planet" Ready for Voting |website= International Astronomical Union |accessdate=2 March 2007}}</ref> It has a much higher [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] than the other asteroids, of around 3.32,<ref> {{cite journal |display-authors=3 |author1=Parker, J. W. |author2=Stern, S. A. |author3=Thomas, P. C. |author4=Festou, M. C. |author5=Merline, W. J. |author6=Young, E. F. |author7=Binzel, R. P. |author8=Lebofsky, L. A. | title=Analysis of the First Disk-resolved Images of Ceres from Ultraviolet Observations with the Hubble Space Telescope | journal=The Astronomical Journal | date=2002 | volume=123 | issue=1 | pages=549–557 | doi = 10.1086/338093 |doi-access=free | bibcode=2002AJ....123..549P |bibcode-access=free | arxiv=astro-ph/0110258 }}</ref> and may possess a surface layer of ice.<ref name="planetary">{{cite web|title=Asteroid 1 Ceres|work=The Planetary Society|url=http://www.planetary.org/explore/topics/asteroids_and_comets/ceres.html |accessdate=20 October 2007| archiveurl= https://web.archive.org/web/20070929092440/http://planetary.org/explore/topics/asteroids_and_comets/ceres.html| archivedate= 29 September 2007 <!--DASHBot-->| deadurl= no}}</ref> Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.<ref name="planetary" /> 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 (astrophysics)|frost line]], and so is devoid of water;<ref>{{cite press release |id=STScI-1995-20 |title=Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta |url=http://hubblesite.org/news_release/news/1995-20 |website=Hubble Space Telescope |date=19 April 1995 |access-date=16 December 2017}}<br />{{cite press release |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |title=Key Stages in the Evolution of the Asteroid Vesta |website=Hubble Space Telescope |date=19 April 1995 |accessdate=20 October 2007 |archiveurl= https://web.archive.org/web/20080907192327/http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c| archivedate=7 September 2008<!--DASHBot-->| deadurl=no}}</ref><ref> {{cite journal |title=Dawn mission and operations |last1=Russel |first1=C. |last2=Raymond |first2=C. |last3=Fraschetti |first3=T. |last4=Rayman |first4=M. |last5=Polanskey |first5=C. |last6=Schimmels |first6=K. |last7=Joy |first7=S. |journal=Proceedings of the International Astronomical Union |volume=1 |issue=S229 |pages=97–119 |url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=414750 |date=2005 |accessdate=20 October 2007 |display-authors=3 |doi=10.1017/S1743921305006691}}</ref> its composition is mainly of basaltic rock such as olivine.<ref>{{cite journal |last=Burbine |first=T.H. |title=Where are the olivine asteroids in the main belt? |journal=Meteoritics |date=July 1994 |volume=29 |issue=4 |page=453 |bibcode=1994Metic..29..453B |bibcode-access=free}}</ref> 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 asteroid]]s, and is the source of the [[HED meteorite]]s, 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.<ref name="Torppa1996">{{cite journal |first1=J. |last1=Torppa |first2=M. |last2=Kaasalainen |first3=T. |last3=Michałowski |first4=T. |last4=Kwiatkowski |first5=A. |last5=Kryszczyńska |first6=P. |last6=Denchev |first7=R. |last7=Kowalski | title=Shapes and rotational properties of thirty asteroids from photometric data | journal=Icarus | date=1996 | volume=164 | issue=2 | pages=346–383 | bibcode=2003Icar..164..346T | doi=10.1016/S0019-1035(03)00146-5 |display-authors=3}}</ref> Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.<ref>{{cite journal |title=The composition of asteroid 2 Pallas and its relation to primitive meteorites |author=Larson, H.P. |author2=Feierberg, M.A. |author3=Lebofsky, L.A. |last-author-amp=yes |date=1983 |bibcode=1983Icar...56..398L |volume=56 |issue=3 |page=398 |journal=Icarus |doi=10.1016/0019-1035(83)90161-6}}</ref> Pallas is the parent body of the [[Palladian family]] of asteroids. Hygiea is the largest carbonaceous asteroid<ref>{{cite web |title=10 Hygiea: ISO Infrared Observations |author=Barucci, M. A. |url=http://www.lesia.obspm.fr/~crovisier/biblio/preprint/bar02_icarus.pdf |format=PDF |date=2002 |accessdate=21 October 2007 |archiveurl=https://web.archive.org/web/20071128200223/http://www.lesia.obspm.fr/~crovisier/biblio/preprint/bar02_icarus.pdf |archivedate=28 November 2007 |deadurl=yes |display-authors=etal |df=dmy-all}}</ref> and, unlike the other largest asteroids, lies relatively close to the [[plane of the ecliptic]].<ref> {{cite web |title=Ceres the Planet |work=orbitsimulator.com |url=http://www.orbitsimulator.com/gravity/articles/ceres.html |accessdate=20 October 2007 |archiveurl=https://web.archive.org/web/20071011154140/http://orbitsimulator.com/gravity/articles/ceres.html |archivedate=11 October 2007 <!--DASHBot-->|deadurl=no}}</ref> It is the largest member and presumed parent body of the [[Hygiean family]] of asteroids. {| class="wikitable" |- style="background:#ccf;" ! colspan="13" style="background:#ddd;"| Attributes of largest asteroids |- style="font-size: smaller;" !Name !Orbital<br />radius<br />([[Astronomical unit|AU]]) ![[Orbital period|Orbital<br />period]]<br />(years) ![[Inclination|Inclination<br />to ecliptic]] ![[Orbital eccentricity|Orbital<br />eccentricity]] ! Diameter<br />(km) ! Diameter<br />(% of [[Moon]]) ! Mass<br />({{e|18}} kg) ! Mass<br />(% of Ceres) ! Density<ref>{{cite web | url = http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3022.pdf | title = Asteroid Density, Porosity, and Structure | publisher = lpi.usra.edu |accessdate=3 January 2013}}</ref><br />(g/cm³) ! Rotation<br />period<br />(hr) ! [[Axial tilt]] ! Surface<br />temperature |- style="text-align:center;" ! style="text-align:left;"| [[4 Vesta|Vesta]] | 2.36 | 3.63 | 7.1° | 0.089 | 573×557×446<br />(mean 525) | 15% | 260 | 28% | 3.44 ± 0.12 | 5.34 | 29° | 85–270 K |- style="text-align:center;" ! style="text-align:left;"| [[Ceres (dwarf planet)|Ceres]] | 2.77 | 4.60 | 10.6° | 0.079 | 975×975×909<br />(mean 953) | 28% | 940 | 100% | 2.12 ± 0.04 | 9.07 | ≈ 3° | 167 K |- style="text-align:center;" ! style="text-align:left;"| [[2 Pallas|Pallas]] | 2.77 | 4.62 | 34.8° | 0.231 | 580×555×500<br />(mean 545) | 16% | 210 | 22% | 2.71 ± 0.11 | 7.81 | ≈ 80° | 164 K |- style="text-align:center;" ! style="text-align:left;"| [[10 Hygiea|Hygiea]] | 3.14 | 5.56 | 3.8° | 0.117 | 530×407×370<br />(mean 435) | 12% | 87 | 9% | 2.76 ± 1.2 | 27.6 | ≈ 60° | 164 K |} === Rotation === Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. No asteroid with a diameter larger than 100 meters has a rotation period smaller than 2.2 hours. 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 pile]]s formed through accumulation of debris after collisions between asteroids.<ref>{{cite web |last=Rossi |first=Alessandro |date=20 May 2004 |url=http://spaceguard.iasf-roma.inaf.it/tumblingstone/issues/current/eng/ast-day.htm |title=The mysteries of the asteroid rotation day |publisher=The Spaceguard Foundation |accessdate=9 April 2007 |deadurl=yes |archiveurl=https://web.archive.org/web/20060512060350/http://spaceguard.iasf-roma.inaf.it/tumblingstone/issues/current/eng/ast-day.htm |archivedate=12 May 2006 |df=dmy }}</ref> === Composition === [[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.<ref>{{cite web |url=http://hubblesite.org/newscenter/archive/releases/1995/20/image/ |publisher=HubbleSite – NewsCenter |title=Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta – Release Images |date=19 April 1995 |accessdate=27 January 2015}}</ref> [[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 [[Asteroid moon|moons]] or are co-orbiting [[binary asteroid|binaries]]: Rubble piles, moons, binaries, and scattered [[asteroid family|asteroid families]] are thought to be the results of collisions that disrupted a parent asteroid, or, possibly, a [[disrupted planet|planet]].<ref name="ARX-20060816">{{cite journal |last=Soter |first=Steven |title=What is a Planet? |url=https://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf |date=16 August 2006 |format=[[PDF]] |accessdate=25 December 2017 }}</ref> Asteroids contain traces of [[amino acid]]s 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]])''.<ref>{{cite web |url=http://www.space.com/scienceastronomy/planetearth/meteor_sugar_011219.html |title=Life is Sweet: Sugar-Packing Asteroids May Have Seeded Life on Earth |website=SPACE.com |date=19 December 2001 |archive-url=https://web.archive.org/web/20020124092631/http://www.space.com/scienceastronomy/planetearth/meteor_sugar_011219.html |archive-date=24 January 2002}}</ref> In August 2011, a report, based on [[NASA]] studies with [[meteorite]]s found on [[Earth]], was published suggesting [[DNA]] and [[RNA]] components ([[adenine]], [[guanine]] and related [[organic molecules]]) may have been formed on asteroids and [[comet]]s in [[outer space]].<ref name="Callahan">{{cite journal |display-authors=3 |last1=Callahan |first1=M.P. |last2=Smith |first2=K.E. |last3=Cleaves |first3=H.J. |last4=Ruzica |first4=J. |last5=Stern |first5=J.C. |last6=Glavin |first6=D.P. |last7=House |first7=C.H. |last8=Dworkin |first8=J.P. |date=11 August 2011 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |journal=[[PNAS]] |doi=10.1073/pnas.1106493108 |doi-access=free |pmid=21836052 |pmc=3161613 |volume=108 |pages=13995–8|bibcode = 2011PNAS..10813995C }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=[[NASA]] |date=8 August 2011 |accessdate=10 August 2011}}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=https://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |publisher=[[ScienceDaily]] |accessdate=9 August 2011}}</ref> [[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&nbsp;km in diameter, and include [[121 Hermione]] (268×186×183&nbsp;km), and [[87 Sylvia]] (384×262×232&nbsp;km). Only half a dozen asteroids are [[List of notable asteroids#Largest by diameter|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.<ref>{{cite journal |journal=Icarus |volume=211 |issue=2 |date=February 2011 |pages=1022–1033 |title=Triplicity and physical characteristics of Asteroid (216) Kleopatra |display-authors=3 |first1=P. |last1=Descamps |first2=F. |last2=Marchis |first3=J. |last3=Berthier |first4=J.P. |last4=Emery |first5=G. |last5=Duchêne |first6=I. |last6=de Pater |first7=M.H. |last7=Wong |first8=L. |last8=Lim |first9=H.B. |last9=Hammel |first10=F. |last10=Vachier |first11=P. |last11=Wiggins |first12=J.-P. |last12=Teng-Chuen-Yu |first13=A. |last13=Peyrot |first14=J. |last14=Pollock |first15=M. |last15=Assafin |first16=R. |last16=Vieira-Martinsa |first17=J.I.B. |last17=Camargoi |first18=F. |last18=Braga-Ribas |first19=B. |last19=Macomberk |doi=10.1016/j.icarus.2010.11.016|arxiv=1011.5263 |bibcode=2011Icar..211.1022D }}</ref> On 7 October 2009, the presence of [[ice|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 [[Sublimation (phase transition)|sublimated]], it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.<ref>{{cite news | first=Ron | last=Cowen | date=8 October 2009 | title=Ice confirmed on an asteroid | publisher=Science News | url=http://www.sciencenews.org/view/generic/id/48174/title/Ice_confirmed_on_an_asteroid | accessdate=9 October 2009 | archiveurl= https://web.archive.org/web/20091012075224/http://www.sciencenews.org/view/generic/id/48174/title/Ice_confirmed_on_an_asteroid| archivedate= 12 October 2009 <!--DASHBot-->| deadurl= no}}</ref><ref>{{cite web | last = Atkinson | first = Nancy | date = 8 October 2009 | title = More water out there, ice found on an asteroid | work = International Space Fellowship | accessdate = 11 October 2009 | url = http://spacefellowship.com/2009/10/08/more-water-out-there-ice-found-on-an-asteroid/ | archiveurl= https://web.archive.org/web/20091011051040/http://spacefellowship.com/2009/10/08/more-water-out-there-ice-found-on-an-asteroid/| archivedate= 11 October 2009 <!--DASHBot-->| deadurl= no}}</ref><ref name="Campins2010">{{ Cite journal |display-authors=3 |author1=Campins, H. |author2=Hargrove, K |author3=Pinilla-Alonso, N |author4=Howell, E.S. |author5=Kelley, M.S. |author6=Licandro, J. |author7=Mothé-Diniz, T. |author8=Fernández, Y. |author9=Ziffer, J. |title=Water ice and organics on the surface of the asteroid 24 Themis |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |date=2010 |doi=10.1038/nature09029 |pmid=20428164 |pages=1320–1321 |bibcode= 2010Natur.464.1320C}}</ref><ref>{{Cite journal |last=Rivkin |first=Andrew S. |last2=Emery |first2=Joshua P. |title=Detection of ice and organics on an asteroidal surface |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |pages=1322–1323 |date=2010 |doi=10.1038/nature09028 |pmid=20428165 |bibcode= 2010Natur.464.1322R}}</ref> 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.<ref name="Campins2010" /> In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the [[white dwarf]] [[GD 61]].<ref>{{cite web |last=Mack |first=Eric |title=Newly spotted wet asteroids point to far-flung Earth-like planets |url=http://news.cnet.com/8301-17938_105-57607077-1/newly-spotted-wet-asteroids-point-to-far-flung-earth-like-planets/ |publisher=CNET}}</ref> On 22 January 2014, [[European Space Agency]] (ESA) scientists reported the detection, for the first definitive time, of [[water vapor]] on [[Ceres (dwarf planet)|Ceres]], the largest object in the asteroid belt.<ref name="KüppersO’Rourke2014">{{cite journal |display-authors=3 |last1=Küppers |first1=Michael |last2=O’Rourke |first2=Laurence |last3=Bockelée-Morvan |first3=Dominique |last4=Zakharov |first4=Vladimir |last5=Lee |first5=Seungwon |last6=von Allmen |first6=Paul |last7=Carry |first7=Benoît |last8=Teyssier |first8=David |last9=Marston |first9=Anthony |last10=Müller |first10=Thomas |last11=Crovisier |first11=Jacques |last12=Barucci |first12=M. Antonietta |last13=Moreno |first13=Raphael |title=Localized sources of water vapour on the dwarf planet (1) Ceres |journal=Nature |volume=505 |issue=7484 |date=2014 |pages=525–527 |doi=10.1038/nature12918 |bibcode = 2014Natur.505..525K |pmid=24451541}}</ref> The detection was made by using the [[Far-infrared astronomy|far-infrared abilities]] of the [[Herschel Space Observatory]].<ref name="NASA-20140122">{{cite web |last1=Harrington |first1=J.D. |title=Herschel Telescope Detects Water on Dwarf Planet – Release 14-021 |url=http://www.nasa.gov/press/2014/january/herschel-telescope-detects-water-on-dwarf-planet |date=22 January 2014 |work=[[NASA]] |accessdate=22 January 2014 }}</ref> 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."<ref name="NASA-20140122" /> In May 2016, significant asteroid data arising from the [[Wide-field Infrared Survey Explorer]] and [[Wide-field Infrared Survey Explorer#NEOWISE|NEOWISE]] missions have been questioned.<ref name="ARX-20160523">{{cite arXiv |last=Myhrvold |first=Nathan |title=Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data |date=23 May 2016 |arxiv=1605.06490}}</ref><ref name="NYT-20160523">{{cite news |last=Chang |first=Kenneth |title=How Big Are Those Killer Asteroids? A Critic Says NASA Doesn’t Know. |url=https://www.nytimes.com/2016/05/24/science/asteroids-nathan-myhrvold-nasa.html |date=23 May 2016 |work=[[The New York Times]] |accessdate=24 May 2016 }}</ref><ref name="SA-20160527">{{cite web |last=Billings |first=Lee |title=For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling |url=http://www.scientificamerican.com/article/for-asteroid-hunting-astronomers-nathan-myhrvold-says-the-sky-is-falling1/ |date=27 May 2016 |work=[[Scientific American]] |accessdate=28 May 2016 }}</ref> Although the early original criticism had not undergone peer review,<ref name="NASA-20160525">{{cite news |author=NASA Administrator |title=NASA Response to Recent Paper on NEOWISE Asteroid Size Results |url=https://www.nasa.gov/feature/nasa-response-to-recent-paper-on-neowise-asteroid-size-results |date=25 May 2016 |work=[[NASA]] |accessdate=29 May 2016 }}</ref> a more recent peer-reviewed study was subsequently published.<ref name="ICARUS-20180522">{{cite journal |last=Myhrvold |first=Nathan |authorlink=Nathan Myhrvold |title=An empirical examination of WISE/NEOWISE asteroid analysis and results |url=https://www.sciencedirect.com/science/article/pii/S0019103516307643 |date=22 May 2018 |journal=[[Icarus (journal)|Icarus]] |doi=10.1016/j.icarus.2018.05.004 |accessdate=14 June 2018 |bibcode=2018Icar..314...64M }}</ref><ref name="NYT-20180614c" /> === Surface features === Most asteroids outside the "[[List of exceptional asteroids#Largest by mass|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.<ref>{{cite journal |display-authors=3 |first1=A.R. |last1=Conrad |first2=C. |last2=Dumas |first3=W.J. |last3=Merline |first4=J.D. |last4=Drummonf |first5=R.D. |last5=Campbell |first6=R.W. |last6=Goodrich |first7=D. |last7=Le Mignant |first8=F.H. |last8=Chaffee |first9=T. |last9=Fusco |first10=S.H. |last10=Kwok |first11=R.I. |last11=Knight |date=2007 |journal=Icarus |doi=10.1016/j.icarus.2007.05.004 |url=http://www2.keck.hawaii.edu/inst/people/conrad/research/pub/d511.pdf |title=Direct measurement of the size, shape, and pole of 511 Davida with Keck AO in a single night |archive-url=https://web.archive.org/web/20070811073647/http://www2.keck.hawaii.edu/inst/people/conrad/research/pub/d511.pdf |archive-date=11 August 2007|bibcode=2007Icar..191..616C }}</ref> 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)|Dawn spacecraft]]'', which entered Ceres orbit on 6 March 2015.<ref>{{cite news |url=http://www.nbcnews.com/.../dawn-spacecraft-slips-quietly-orbit-around-dwarf-planet-ceres-n318371 | title= Dawn Spacecraft Slips Quietly Into Orbit Around Dwarf Planet Ceres |date=March 6, 2015 |author=Boyle, Alan |publisher=NBCUniversal Media, LLC |work=NBCNews.com |accessdate=March 11, 2015}}</ref> === Color === Asteroids become darker and redder with age due to [[space weathering]].<ref>{{cite web |title=University of Hawaii Astronomer and Colleagues Find Evidence That Asteroids Change Color as They Age |work=University of Hawaii Institute for Astronomy |date=19 May 2005 |url=http://www.ifa.hawaii.edu/info/press-releases/Jedicke_asteroids5-17-04.html |accessdate=27 February 2013}}</ref> 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.<ref>{{cite web |title=Sun damage conceals asteroids' true ages |author=Rachel Courtland |work=[[New Scientist]] |date=30 April 2009 |url=https://www.newscientist.com/article/dn17062-sun-damage-conceals-asteroids-true-ages.html |accessdate=27 February 2013}}</ref> == Classification == Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance [[visible spectrum|spectrum]]. === Orbital classification === {{Main|Asteroid group|Asteroid family}} 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.<ref>{{cite journal |display-authors=3 | last=Zappalà | first=V. |first2=Ph. |last2=Bendjoya |first3=A. |last3=Cellino |first4=P. |last4=Farinella |first5=C. |last5=Froeschlé | title=Asteroid families: Search of a 12,487-asteroid sample using two different clustering techniques | journal=Icarus | date=1995 | volume=116 | issue=2 | pages=291–314 | bibcode=1995Icar..116..291Z | doi=10.1006/icar.1995.1127 }}</ref> Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the [[Jupiter trojan]]s.<ref name="Jewitt2004">{{cite book |last=Jewitt |first=David C. |last2=Sheppard |first2=Scott |last3=Porco |first3=Carolyn |chapter=Jupiter's Outer Satellites and Trojans |title=Jupiter: The Planet, Satellites and Magnetosphere |date=2004 |publisher=Cambridge University Press |editor=Bagenal, F. |editor2=Dowling, T.E. |editor3=McKinnon, W.B. |url=http://www.dtm.ciw.edu/users/sheppard/pub/Sheppard04JupChapter.pdf |format=pdf}}</ref> 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 orbit]]s 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 (moon)|Epimetheus]] and [[Janus (moon)|Janus]]. Sometimes these horseshoe objects temporarily become [[quasi-satellite]]s 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 (planet)|Mercury]], are a special class of [[Aten asteroid]]s. However, such objects could be associated with outer planets as well. === Spectral classification === {{Main|Asteroid spectral types}} [[File:433eros.jpg|thumb|This picture of [[433 Eros]] shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as {{cvt|35|m|0}} across can be seen.]] In 1975, an asteroid [[Taxonomy (general)|taxonomic]] system based on [[color]], [[albedo]], and [[spectral line|spectral shape]] was developed by [[Clark R. Chapman]], [[David Morrison (astrophysicist)|David Morrison]], and [[Ben Zellner]].<ref>{{cite journal | first=C. R. | last=Chapman | title=Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry | journal=Icarus | volume=25 | issue=1 | pages=104–130 | bibcode=1975Icar...25..104C | date=1975 | doi=10.1016/0019-1035(75)90191-8 | last2=Morrison | first2=David | last3=Zellner | first3=Ben}}</ref> These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: [[C-type asteroid|C-types]] for dark carbonaceous objects (75% of known asteroids), [[S-type asteroid|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.<ref>{{cite conference | last=Tholen | first=D. J. | title=Asteroid taxonomic classifications | booktitle=Asteroids II; Proceedings of the Conference | pages=1139–1150 | publisher=University of Arizona Press | date=1989 | bibcode=1989aste.conf.1139T}}</ref> 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 asteroid|M-type]]. There are also several smaller classes.<ref>{{cite journal | last=Bus | first=S. J. | title=Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy | journal=Icarus | date=2002 | volume=158 | issue=1 | page=146 | doi=10.1006/icar.2002.6856 | bibcode=2002Icar..158..146B}}</ref> 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. {| class="wikitable" |+Summary of asteroid taxonomic classes !Tholen Class !SMASSII (Bus Class) !Albedo !Spectral Features |- |[[A-type asteroid|A]] |A |moderate |Very steep red slope shortward of 0.75&nbsp;µm; moderately deep absorption feature longward of 0.75&nbsp;µm. |- |[[B-type asteroid|B]], [[C-type asteroid|C]], [[F-type asteroid|F]], [[G-type asteroid|G]] |B, C, Cb, Ch, Cg, Chg |low |Linear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7&nbsp;µm. |- |[[D-type asteroid|D]] |D |low |Relatively featureless spectrum with very steep red slope. |- |[[E-type asteroid|E]], [[M-type asteroid|M]], [[P-type asteroid|P]] |[[X-type asteroid|X]], Xc, Xe, Xk |from low (P) to very high (E) |Generally featureless spectrum with reddish slope; differences in subtle absorption features and/or spectral curvature and/or peak relative reflectance. |- |[[Q-type asteroid|Q]] |Q |moderate |Reddish slope shortward of 0.7&nbsp;µm; deep, rounded absorption feature longward of 0.75&nbsp;µm. |- |[[R-type asteroid|R]] |R |moderate |Moderate reddish slope downward of 0.7&nbsp;µm; deep absorption longward of 0.75&nbsp;µm. |- |[[S-type asteroid|S]] |S, Sa, Sk, Sl, Sq, Sr |moderate |Moderately steep reddish slope downward of 0.7&nbsp;µm; moderate to steep absorption longward of 0.75&nbsp;µm; peak of reflectance at 0.73&nbsp;µm. Bus subgroups intermediate between S and A, K, L, Q, R classes. |- |[[T-type asteroid|T]] |T |low |Moderately reddish shortward of 0.75&nbsp;µm; flat afterward. |- |[[V-type asteroid|V]] |V |moderate |Reddish shortward of 0.7&nbsp;µm; extremely deep absorption longward of 0.75&nbsp;µm. |- |— |[[K-type asteroid|K]] |moderate |Moderately steep red slope shortward of 0.75&nbsp;µm; smoothly angled maximum and flat to blueish longward of 0.75&nbsp;µm, with little or no curvature. |- |— |[[L-type asteroid|L]], Ld |moderate |Very steep red slope shortward of 0.75&nbsp;µm; flat longward of 0.75&nbsp;µm; differences in peak level. |- |— |[[O-type asteroid|O]] |— |Peculiar trend, known so far only for asteroid 3628. |} ==== Problems ==== Originally, spectral designations were based on inferences of an asteroid's composition.<ref>{{cite book | first=Harry Y. | last=McSween Jr. | date=1999 | title=Meteorites and their Parent Planets | edition=2nd | publisher=Oxford University Press | isbn=0-521-58751-4 }}</ref> 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. == Naming == {{Main|Minor planet#Naming}} [[File:Asteroid20130318-full.jpg|thumb|right|[[2013 EC]], shown here in radar images, has a provisional designation]] A newly discovered asteroid is given a [[Provisional designation in astronomy|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.<ref>{{cite web|title=The Naming of Asteroids|url=http://www.open.edu/openlearn/science-maths-technology/science/physics-and-astronomy/astronomy/the-naming-asteroids|website=Open Learn|publisher=The Open University|accessdate=14 August 2016}}</ref> In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.<ref>{{cite web|title=Asteroid Naming Guidelines|url=http://www.planetary.org/get-involved/contests/osirisrex/guidelines.html|website=The Planetary Society|publisher=The Planetary Society|accessdate=14 August 2016}}</ref> === Symbols === {{main|Astronomical symbols}} 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.<ref>{{cite journal| last=Gould| first=B. A.| authorlink=Benjamin Apthorp Gould | date=1852| title= On the Symbolic Notation of the Asteroids| journal= Astronomical Journal| volume= 2| page= 80| doi= 10.1086/100212 |doi-access=free | bibcode=1852AJ......2...80G |bibcode-access=free}}</ref> {| class="wikitable" |- ! Asteroid || colspan=2| Symbol || Year |- | [[Ceres (dwarf planet)|1 Ceres]] || ⚳ [[File:Ceres symbol.svg|x20px|Old planetary symbol of Ceres]] [[File:Ceres2.svg|x20px|Variant symbol of Ceres]] [[File:Ceres3.svg|x20px|Other sickle variant symbol of Ceres]] || [[Ceres (mythology)|Ceres']] scythe, reversed to double as the letter ''C'' || 1801 |- | [[2 Pallas]] || ⚴ [[File:2Pallas symbol.svg|30x20px|Old symbol of Pallas]] [[File:2 Pallas.svg|x20px|Variant symbol of Pallas]]|| [[Athena]]'s (Pallas') spear || 1801 |- | [[3 Juno]] || ⚵ [[File:Juno symbol.svg|x20px|Old symbol of Juno]] [[File:3 Juno (1).svg|x20px|Other symbol of Juno]] [[File:Symbol 3.jpg|x20px]]|| A star mounted on a scepter, for [[Juno (mythology)|Juno]], the Queen of Heaven || 1804 |- | [[4 Vesta]] || ⚶ [[File:Vesta symbol.svg|x20px|Modern astrological symbol of Vesta]] [[File:4 Vesta (0).svg|x20px|Old symbol of Vesta]] [[File:Simbolo di Vesta.svg|x20px|Old planetary symbol of Vesta]] [[File:4 Vesta Unsimplified Symbol.svg|x20px]]|| The altar and [[sacred fire of Vesta]] || 1807 |- | [[5 Astraea]] || [[File:5 Astraea symbol alternate.svg|x20px]] [[File:5 Astraea Symbol.svg|x20px]]|| A scale, or an inverted anchor, symbols of [[Astraea (mythology)|justice]] || 1845 |- | [[6 Hebe]] || [[File:6 Hebe Astronomical Symbol.svg|x20px]]|| [[Hebe (mythology)|Hebe's]] cup || 1847 |- | [[7 Iris]] || [[File:7 Iris Astronomical Symbol.svg|x20px]]|| A rainbow (''iris'') and a star || 1847 |- | [[8 Flora]] || [[File:8 Flora Astronomical Symbol.svg|x20px]]|| A flower (''flora''), specifically the [[Rose of England]] || 1847 |- | [[9 Metis]] || [[File:9 Metis symbol.svg|x20px]]|| The eye of [[Metis (mythology)|wisdom]] and a star || 1848 |- | [[10 Hygiea]] || [[File:10 Hygeia symbol alternate.svg|x20px]] [[File:10 Hygiea Astronomical Symbol.svg|x20px]] || [[Hygieia|Hygiea's]] serpent and a star, or the [[Rod of Asclepius]] || 1849 |- | [[11 Parthenope]] || [[File:11 Parthenope symbol alternate.svg|x20px]] [[File:11 Parthenope symbol.svg|x20px]]|| A harp, or a fish and a star; symbols of the [[Siren (mythology)|sirens]] || 1850 |- | [[12 Victoria]] || [[File:12 Victoria symbol.svg|x20px]]|| The [[laurels of victory]] and a star || 1850 |- | [[13 Egeria]] || [[File:13 Egeria symbol.svg|x20px|Astronomical symbol of 13 Egeria]] || A shield, symbol of [[Egeria (mythology)|Egeria's]] protection, and a star || 1850 |- | [[14 Irene]] || [[File:Symbol 14 Irene.png|x30px]] ||A dove carrying an olive branch (symbol of ''irene'' 'peace')<br />with a star on its head,<ref name="hilton">{{cite web|title=When Did the Asteroids Become Minor Planets|authorlink=James L. Hilton| first=James L.| last=Hilton |accessdate=26 March 2006 |url=http://aa.usno.navy.mil/faq/docs/minorplanets.php| date=17 September 2001 |archiveurl=https://web.archive.org/web/20071106124911/http://aa.usno.navy.mil/faq/docs/minorplanets.php |archivedate=2007-11-06}}</ref> or an olive branch, a flag of truce, and a star || 1851 |- | [[15 Eunomia]] || [[File:15 Eunomia symbol.svg|x20px]]|| A heart, symbol of good order (''eunomia''), and a star || 1851 |- | [[16 Psyche]] || [[File:16 Psyche symbol.svg|x20px]] || A butterfly's wing, symbol of the soul (''psyche''), and a star || 1852 |- | [[17 Thetis]] || [[File:17 Thetis symbol.png|x20px]] || A dolphin, symbol of [[Thetis]], and a star || 1852 |- | [[18 Melpomene]] || [[File:18 Melpomene symbol.svg|x20px]] || The dagger of [[Melpomene]], and a star || 1852 |- | [[19 Fortuna]] || [[File:19 Fortuna symbol.svg|x20px]] || The [[Rota Fortunae|wheel of fortune]] and a star || 1852 |- | [[26 Proserpina]] || [[File:26 Proserpina symbol.svg|x20px]] || [[Proserpina]]'s pomegranate<!--Webster's (1884) says this is a fruit (''[[Pomona|pomum]]'') and a star, and is the symbol for [[32 Pomona]]--> || 1853 |- | [[28 Bellona]] || [[File:28 Bellona symbol.svg|x20px]]|| [[Bellona (goddess)|Bellona]]'s whip and lance<ref>{{cite journal| last=Encke| first= J. F.| date= 1854| title=Beobachtung der Bellona, nebst Nachrichten über die Bilker Sternwarte| journal= Astronomische Nachrichten| volume= 38| issue=9| page=143| doi=10.1002/asna.18540380907 |doi-access=free |bibcode = 1854AN.....38..143. |bibcode-access=free}}</ref> || 1854 |- | [[29 Amphitrite]] || [[File:29 Amphitrite symbol.svg|x20px]] || The shell of [[Amphitrite]] and a star || 1854 |- | [[35 Leukothea]] || [[File:35 Leukothea symbol.png|x20px]]|| A lighthouse beacon, symbol of [[Leucothea]]<ref>{{cite journal| last=Luther | first= R| date= 1855| title=Name und Zeichen des von Herrn R. Luther zu Bilk am 19. April entdeckten Planeten| journal= Astronomische Nachrichten| volume= 40| issue=24| page= 373| doi=10.1002/asna.18550402405 |doi-access=free |bibcode=1855AN.....40Q.373L |bibcode-access=free}}</ref> || 1855 |- | [[37 Fides]] || [[File:37 Fides symbol.svg|x20px]]|| The [[crucifix|cross]] of faith (''fides'')<ref>{{cite journal| last=Luther| first= R.| date= 1855 | title=Schreiben des Herrn Dr. R. Luther, Directors der Sternwarte zu Bilk, an den Herausgeber| journal= Astronomische Nachrichten| volume= 42| issue=7| page= 107| doi=10.1002/asna.18550420705 |doi-access=free | bibcode=1855AN.....42..107L |bibcode-access=free}}</ref> || 1855 |} In 1851,<ref>{{cite web| url=http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/minor-planets| title=When did the asteroids become minor planets?| publisher=Naval Meteorology and Oceanography Command| accessdate=6 November 2011| deadurl=yes| archiveurl=https://web.archive.org/web/20120406222551/http://www.usno.navy.mil/USNO/astronomical-applications/astronomical-information-center/minor-planets/| archivedate=6 April 2012| df=dmy-all}}</ref> after the fifteenth asteroid ([[15 Eunomia|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, [[5 Astraea|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]].<ref group=note>Except for Pluto and, in the astrological community, for a few outer bodies such as [[2060 Chiron]]</ref> 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.<ref name="hilton"/> == Exploration == {{see also|Sample return mission|Asteroid mining|Colonization of the asteroids}} Until the age of [[Spaceflight|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 curve]]s (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.<ref>{{cite web |url=http://ti.arc.nasa.gov/m/project/neo/pdf/NEO_feasibility.pdf |title=A Piloted Orion Flight to a Near-Earth Object: A Feasibility Study |display-authors=3 |author1=Rob R. Landis |author2=David J. Korsmeyer |author3=Paul A. Abell |author4=Daniel R. Adamo |website=[[American Institute of Aeronautics and Astronautics]]}}</ref> The first close-up photographs of asteroid-like objects were taken in 1971, when the ''[[Mariner 9]]'' probe imaged [[Phobos (moon)|Phobos]] and [[Deimos (moon)|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 program|Voyager]] probes of the small moons of the [[gas giant]]s. 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 (asteroid)|Dactyl]], all of which were imaged by the [[Galileo (spacecraft)|''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 (spacecraft)|Stardust]]'' in 2002). From September to November 2005, the Japanese ''[[Hayabusa (spacecraft)|Hayabusa]]'' probe studied [[25143 Itokawa]] in detail and was plagued with difficulties, but [[Sample return mission|returned samples]] of its surface to Earth on 13 June 2010. The European [[Rosetta (spacecraft)|''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)|''Dawn'' spacecraft]], which orbited [[4 Vesta]] from July 2011 to September 2012, and has been orbiting the dwarf planet [[Ceres (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 avoidance|asteroid impact event]], and has developed and released the ''"[https://www.whitehouse.gov/wp-content/uploads/2018/06/National-Near-Earth-Object-Preparedness-Strategy-and-Action-Plan-23-pages-1MB.pdf National Near-Earth Object Preparedness Strategy Action Plan]"'' to better prepare.<ref name="WH-20180621">{{cite web |author=Staff |title=National Near-Earth Object Preparedness Strategy Action Plan |url=https://www.whitehouse.gov/wp-content/uploads/2018/06/National-Near-Earth-Object-Preparedness-Strategy-and-Action-Plan-23-pages-1MB.pdf |format=[[PDF]] |date=21 June 2018 |work=[[White House]] |accessdate=22 June 2018 }}</ref><ref name="GIZ-20180621">{{cite news |last=Mandelbaum |first=Ryan F. |title=America Isn't Ready to Handle a Catastrophic Asteroid Impact, New Report Warns |url=https://gizmodo.com/america-isnt-ready-to-handle-a-catastrophic-asteroid-im-1827014709 |date=21 June 2018 |work=[[Gizmodo]] |accessdate=22 June 2018 }}</ref><ref name="ICARUS-220180522">{{cite journal |last=Myhrvold |first=Nathan |authorlink=Nathan Myhrvold |title=An empirical examination of WISE/NEOWISE asteroid analysis and results |url=https://www.sciencedirect.com/science/article/pii/S0019103516307643 |date=22 May 2018 |journal=[[Icarus (journal)|Icarus]] |doi=10.1016/j.icarus.2018.05.004 |accessdate=22 June 2018 |bibcode=2018Icar..314...64M }}</ref><ref name="NYT-20180614c">{{cite news |last=Chang |first=Kenneth |title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks - Relevant Comments |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html#permid=27500228:27506217 |date=14 June 2018 |work=[[The New York Times]] |accessdate=22 June 2018 }}</ref> === Planned and future missions === In May 2011, NASA selected the [[OSIRIS-REx]] sample return mission to asteroid [[101955 Bennu]]; it launched on September 8, 2016. 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.<ref>{{cite web |url=http://www.space.com/22993-nasa-slam-captured-asteroid-moon.html |title=NASA May Slam Captured Asteroid Into Moon (Eventually) |website=SPACE.com |first=Mike |last=Wall |date=30 September 2013}}</ref> 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.<ref name="AP-20140619">{{cite news |last=Borenstein |first=Seth |title=Rock that whizzed by Earth may be grabbed by NASA |url=http://apnews.excite.com/article/20140619/us-sci-nasa-asteroid-4e7bba0551.html |date=19 June 2014 |work=[[AP News]] |accessdate=20 June 2014 }}</ref> 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 habitat]]s ''(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)|''Psyche'' spacecraft]] proposal to [[16 Psyche]] and [[Lucy (spacecraft)|''Lucy'' spacecraft]] to [[Jupiter trojan]]s made it to the semifinalist stage of mission selection. == Fiction == {{Main|Asteroids in fiction}} 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. == Gallery == <gallery mode="packed" heights="200px"> File:951 Gaspra.jpg|[[951 Gaspra]] is the first asteroid to be imaged in close-up, imaged by ''[[Galileo (spacecraft)|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 (spacecraft)|Dawn]]'' on {{nowrap|1=9 July 2011}} File:Ceres processed.jpg|[[Ceres (dwarf planet)|Ceres]] imaged by ''[[Dawn (spacecraft)|Dawn]]'' on {{nowrap|1=4 February 2015}} </gallery> == See also == {{Div col|colwidth=20em}} * [[Amor asteroid]] * [[Apollo asteroid]] * [[Asteroid Day]] * [[Asteroid impact avoidance]] * [[Aten asteroid]] * [[Atira asteroid]] * [[BOOTES]] (Burst Observer and Optical Transient Exploring System) * [[:Category:Asteroids|Category:Asteroids]] * [[:Category:Asteroid groups and families|Category:Asteroid groups and families]] * [[:Category:Binary asteroids|Category:Binary asteroids]] * [[Centaur (minor planet)]] * [[Chang'e 2|Chang'e 2 lunar orbiter]] * [[Constellation program]] * [[Dawn (spacecraft)|Dawn spacecraft]] * [[Dwarf planet]] * [[Impact event]] * [[List of asteroid close approaches to Earth]] * [[List of minor planets named after people]] * [[List of minor planets named after places]] * [[List of minor planets]] * [[List of notable asteroids]] * [[List of impact craters on Earth]] * [[List of unconfirmed impact craters on Earth]] * [[Lost asteroid]] * [[Marco Polo (spacecraft)]] * [[Meanings of minor planet names]] * [[Mesoplanet]] * [[Minor planet]] * [[Near-Earth object]] * [[NEOShield]] * NEOSSat ([[Near Earth Object Surveillance Satellite]]) Canada's new satellite * [[Pioneer 10|Pioneer 10 spacecraft]] * [[Rosetta (spacecraft)|Rosetta spacecraft]] {{Portal|Space|Astronomy}} {{Div col end}} == Notes == <references group=note/> == References == {{Reflist|30em}} == External links == {{Sister project links|voy=no|wikt=asteroid|commons=Category:Asteroids|v=no|q=no|s=The New Student's Reference Work/Asteroids|b=General Astronomy/Asteroids}} * [http://www.minorplanetcenter.org/iau/lists/MPNames.html Alphabetical list of minor planet names (ASCII)] (Minor Planet Center) * [http://www.psrd.hawaii.edu/Archive/Archive-Asteroids.html Asteroid articles in Planetary Science Research Discoveries] * [http://www.esa.int/Our_Activities/Operations/Space_Situational_Awareness/Near-Earth_Objects_-_NEO_Segment ESA Space Situational Awareness: Near-Earth Object Segment] * [http://www.ss.astro.umd.edu/IAU/csbn/ IAU Committee on Small Body Nomenclature] * [http://www.jpl.nasa.gov/asteroidwatch/ JPL Asteroid Watch Site] * [http://www.nasa.gov/asteroid-and-comet-watch NASA Asteroid and Comet Watch Site] * [http://neat.jpl.nasa.gov/ Near Earth Asteroid Tracking (NEAT)] * [http://newton.dm.unipi.it/neodys2/ Near Earth Objects Dynamic Site] * [http://szyzyg.arm.ac.uk/~spm/neo_map.html NEO MAP] ([[Armagh Observatory]]) * [http://spaceguardcentre.com/ Spaceguard Centre] * [http://www.brera.mi.astro.it/sormano/teca.html TECA Table of next close approaches to the Earth] * [https://www.webcitation.org/5msUtFmJu?url=http://aa.usno.navy.mil/faq/docs/minorplanets.php When Did the Asteroids Become Minor Planets?] * [https://www.bbc.co.uk/programmes/p003k9kh Asteroids], BBC Radio 4 discussion with Monica Grady, Carolin Crawford & John Zarnecki (''In Our Time'', Nov. 3, 2005) {{Asteroids}} {{Ceres}} {{Comets}} {{Solar System}} {{Minor planets navigator|PageName=[[Ceres (dwarf planet)|1 Ceres]]||2 Pallas|state=autocollapse}} {{Small Solar System bodies}} {{Asteroid spacecraft}} {{Planetary defense}} {{Authority control}} [[Category:Asteroids| ]] [[Category:Minor planets]]'