Asteroid 791 226185302 2008-07-17T06:00:23Z Ultra two 2789682 added {{see also}} template to alleviate confusion {{Merge|Minor planet|Talk:Asteroid#Merge proposal|date=May 2008}} {{otheruses}} {{see also|Minor planet}} [[Image:(253) mathilde.jpg|thumb|260px|right|[[253 Mathilde]], a [[C-type asteroid]] measuring about 50 km across. Photograph taken in 1997 by the [[NEAR Shoemaker]] probe.]] '''Asteroids''', also called '''[[minor planets]]''' or '''planetoids''', are [[Solar System]] bodies smaller than planets but larger than [[meteoroid]]s (which are commonly defined as being 10 meters across or less),<ref>{{cite journal | author=Beech, M. |authorlink=Martin Beech | coauthors=[[Duncan I. Steel|Steel, D. I.]] | year=1995 | month=September | title=On the Definition of the Term Meteoroid | journal=Quarterly Journal of the Royal Astronomical Society | volume=36 | issue=3 | pages=281&ndash;284 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44b52c369007834 |accessdate=2006-08-31 }}</ref> and that are not [[comet]]s. The distinction between asteroids and comets is made on visual appearance when discovered: comets must show a perceptible [[coma (cometary)|coma]] (a fuzzy "atmosphere"), while asteroids do not. Asteroids vary greatly in size, from a few hundreds of kilometres in diameter down to rocks just tens of metres across. A few of the largest are roughly spherical and are very much like miniature planets. The vast majority, however, are much smaller and are irregularly shaped. The physical composition of asteroids is varied and in many cases poorly understood. Some are solid rocky bodies, with a greater or lesser metallic content, while others are piles of rubble held together loosely by gravity. Only one asteroid&mdash;[[4 Vesta|Vesta]]&mdash;is visible to the naked eye, and this only in very dark skies when it is favourably positioned. The first named minor planet, [[Ceres (dwarf planet)|Ceres]], was discovered in 1801 by [[Giuseppe Piazzi]], and was originally considered a new planet.<ref>Ceres, originally considered a new planet, was the largest asteroid known but is now classified as a [[dwarf planet]]. All other asteroids are now classified as [[Small Solar System body|small solar system bodies]].</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", from Greek ''αστεροειδής'', ''asteroeidēs'' = star-like, star-shaped, from ancient Greek ''Aστήρ'', ''astēr'' = star. The vast majority of known asteroids are found within the main [[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 estimated to contain more than 750,000 asteroids larger than 1 kilometer across, and millions of smaller ones.<ref>[http://www.nasa.gov/worldbook/asteroid_worldbook.html World Book at NASA]</ref> It is thought that these asteroids are 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]]. Some asteroids have [[Asteroid moon|moons]] or are found in co-orbiting pairs known as [[binary asteroid|binary systems]]. Minor planets have more recently been found to cross the orbits of planets, from [[Mercury (planet)|Mercury]] to [[Neptune]]&mdash;with hundreds of trans-Neptunian objects ([[Trans-Neptunian object|TNOs]]) now known to exist well past Neptune's orbit. (Using indirect methods, the total number of TNOs has been estimated in the hundreds of millions or even billions.) Asteroids are given a provisional designation by year in the order of discovery, and a designation (a sequential number) and name if their existence is well established and an [[orbit]] has been determined. == Terminology == <!-- Linked from "Comet" --> The term "asteroid" is used to describe any of a diverse group of small celestial bodies orbiting the Sun&mdash;traditionally in the inner Solar System, since those were the only ones known. In English it is the most commonly used word for a [[minor planet]],{{Fact|date=May 2008}} which was the term preferred by the [[International Astronomical Union]] (IAU) prior to 2006. Other languages prefer "planetoid" (Greek for "planet-like"). The word "[[planetesimal]]" has a similar meaning, but often refers specifically to small bodies that existed at the time the Solar System was forming. The term "planetule" was coined by the geologist Conybeare to describe minor planets,<ref>{{cite web | url=http://www.hyperdictionary.com/dict-e/p-44.html | title=English Dictionary - Browsing Page P-44 | publisher=HyperDictionary.com | accessdate=2008-04-15 }}</ref> but is not in common use. Traditionally, small bodies orbiting the Sun were classified as asteroids, [[comets]] or [[meteoroids]], with anything smaller than, say, ten metres across being called a meteoroid. 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 eventually are depleted of their volatile ices and then appear as point-like objects, i.e. asteroids. A further distinction is that comets typically have more eccentric orbits than asteroids (though some objects classified as asteroids also have notably eccentric orbits). In recent years, the situation has been complicated by the discovery of [[trans-Neptunian object]]s (TNOs). 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. The innermost of these are the [[Kuiper Belt Objects]] (KBOs), called "objects" partly to avoid the need to classify them as asteroids or comets.<ref>[http://curious.astro.cornell.edu/question.php?number=601 "Are Kuiper Belt Objects asteroids?"], "Ask an astronomer", Cornell University</ref> KBOs are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.<ref>[http://rst.gsfc.nasa.gov/Sect19/Sect19_22.html "Asteroids and Comets"], NASA website</ref> Furthermore, they do not necessarily have the highly eccentric orbits usually associated with comets, and there are significant numbers very much larger than traditional [[Comet nucleus|comet nuclei]]. The much more distant [[Oort cloud]] is also hypothesised to be a 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>[http://www.sciam.com/podcast/episode.cfm?id=ADD0878B-D6C3-3B70-7B5BC373545BB82D "Comet Dust Seems More Asteroidy"] ''Scientific American'', January 25, 2008</ref> suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.<ref>[http://space.newscientist.com/channel/solar-system/comets-asteroids/dn13224-comet-samples-are-surprisingly-asteroidlike.html "Comet samples are surprisingly asteroid-like"], ''New Scientist'', 24 January 2008</ref> In late [[2006 definition of planet|August 2006]], the IAU introduced the class [[small solar system body|small solar system bodies]] (SSSB) to include most objects previously classified as minor planets and [[comets]]. At the same time, the class [[dwarf planet]]s was created for the largest minor planets&mdash;those which have sufficient mass to have become more-or-less spherical 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_press/news/release/iau0603/questions_answers/ Questions and Answers on Planets], IAU</ref> Currently only the largest object in the asteroid belt, [[Ceres (dwarf planet)|Ceres]], at about 950 km across, is in the dwarf planet category, although there are several relatively large near-spherical asteroids ([[4 Vesta|Vesta]], [[2 Pallas|Pallas]] and [[10 Hygiea|Hygiea]]) that may be reclassified as dwarf planets in future.<ref>[http://space.newscientist.com/article.ns?id=dn9761&feedId=online-news_rss20F53 "Three new planets may join solar system"], ''New Scientist'', 16 August 2006</ref> == Distribution within the Solar System == {{see also|Asteroid group|List of noteworthy asteroids|List of asteroids}} [[Image:InnerSolarSystem-en.png|250px|thumb|left|The [[Asteroid belt|Main asteroid belt]] (white) and the [[Trojan asteroids]] (green)]] Hundreds of thousands of asteroids have been discovered within the Solar System, with the rate of discovery currently running at around 5,000 per month. Of the more than 400,000 registered minor planets, 189,005 have orbits known well enough to be assigned [[astronomical naming conventions|permanent official numbers]].<ref>{{cite web | author=JPL | title=How Many Solar System Bodies | url=http://ssd.jpl.nasa.gov/?body_count | work=JPL Solar System Dynamics | publisher=NASA | accessdate=2008-06-19}}</ref><ref>{{cite web | title=Minor Planet Statistics | publisher=Minor Planet Center | url=http://www.cfa.harvard.edu/iau/lists/ArchiveStatistics.html | accessdate=2008-06-19}}</ref> Of these, 14,574 have official names.<ref>{{cite web | title=Minor Planet Names | publisher=Minor Planet Center | url=http://www.cfa.harvard.edu/iau/lists/MPNames.html | accessdate=2008-06-19}}</ref> The lowest-numbered, unnamed minor planet is {{mpl|(3708) 1974 FV|1}};<ref>{{cite web | title=Discovery Circumstances: Numbered Minor Planets (1)-(5000) | publisher=Minor Planet Center | url=http://www.cfa.harvard.edu/iau/lists/NumberedMPs000001.html | accessdate=2006-10-11}}</ref> the highest-numbered named minor planet is [[184878 Gotlib]].<ref>{{cite web | title=Discovery Circumstances: Numbered Minor Planets (180001)-(185000)| publisher=Minor Planet Center | url=http://www.cfa.harvard.edu/iau/lists/NumberedMPs180001.html | accessdate=2008-06-19}}</ref> Current estimates put the total number of asteroids above 1 km in diameter in the Solar System to be between 1.1 and 1.9 million.<ref>{{cite press release | first=Edward | last=Tedesco | coauthors=Metcalfe, Leo | title=New study reveals twice as many asteroids as previously believed | publisher=European Space Agency | date=[[April 4]], [[2002]] | url=http://www.spaceref.com/news/viewpr.html?pid=7925 | accessdate=2008-02-21}}</ref> [[Ceres (dwarf planet)|Ceres]], with diameters of 975 × 909 km, was once considered the largest asteroid in the inner solar system, but it has since been recategorized as a [[dwarf planet]]. That distinction now falls to [[2 Pallas]] and [[4 Vesta]]; both have diameters of about 500 km. Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some very rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see [[99942 Apophis]]. [[Image:4 Vesta 1 Ceres Moon at 20 km per px.png|thumb|300px|right|Left to right: [[4 Vesta]], [[Ceres (dwarf planet)|1 Ceres]], Earth's [[Moon]]]] The mass of all the objects of the [[Asteroid Belt|Main asteroid belt]], lying between the orbits of [[Mars]] and [[Jupiter]], is estimated to be about 3.0-3.6{{e|21}}&nbsp;kg, or about 4 percent of the mass of the [[Moon]]. Of this, [[Ceres (dwarf planet)|Ceres]] comprises 0.95{{e|21}}&nbsp;kg, some 32 percent of the total.<ref>{{cite journal | authorlink=Georgij A. Krasinsky | first=G. A. | last=Krasinsky | coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. | url=http://adsabs.harvard.edu/abs/2002Icar..158...98K | title=Hidden Mass in the Asteroid Belt | journal=Icarus | volume=158 | issue=1 | pages=98–105 | month=July | year=2002 | doi=10.1006/icar.2002.6837}}</ref><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 | year=2004 | url=http://adsabs.harvard.edu/abs/2004cosp.meet.2014P}}</ref> Adding in the next three most massive asteroids, [[4 Vesta]] (9%), [[2 Pallas]] (7%), and [[10 Hygiea]] (3%), brings this figure up to 51%; while 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. Various classes of asteroid have been discovered outside the main asteroid belt. [[Near-Earth asteroids]] have orbits in the vicinity of Earth's orbit. [[Trojan asteroids]] are gravitationally locked into synchronisation with a planet, either leading or trailing the planet in its orbit. The majority of Trojans are associated with [[Jupiter]], but a few have been found orbiting with [[Mars]] or [[Neptune]]. Asteroids orbiting between [[Jupiter]] and [[Neptune]] are called [[Centaur (planetoid)|Centaurs]], and beyond this lie swarms of [[trans-Neptunian objects]]. A group of asteroids called [[Vulcanoid asteroid|Vulcanoids]] are hypothesised by some to lie very close to the Sun, within the orbit of [[Mercury (planet)|Mercury]], but none has so far been found. == Classification == Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance [[visible spectrum|spectrum]]. === Orbit groups and families === {{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 much tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past.<ref>{{cite journal | last=Zappalà | first=V. | coauthors=Bendjoya, Ph.; Cellino, A.; Farinella, P.; Froeschle, C. | title=Asteroid families: Search of a 12,487-asteroid sample using two different clustering techniques | journal=Icarus | year=1995 | volume=116 | pages=291–314 | url=http://adsabs.harvard.edu/abs/1995Icar..116..291Z | accessdate=2007-04-15 | doi=10.1006/icar.1995.1127 }}</ref> Families have only been recognized within the [[main belt|main asteroid belt]]. They were first recognised by [[Kiyotsugu Hirayama]] in 1918 and are often called [[Hirayama families]] in his honor. About 30% to 35% of the bodies in the main 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 Trans-Neptunian Object [[(136108) 2003 EL61]]. ===Quasi-satellites and horseshoe objects=== Some asteroids have unusual [[horseshoe orbit]]s that are co-orbital with the [[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 prior 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 ===<!-- This section is linked from [[4769 Castalia]] --> [[Image:433eros.jpg|thumb|right|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 35&nbsp;m across can be seen.]] {{main|Asteroid spectral types}} In 1975, an asteroid [[taxonomy|taxonomic]] system based on [[colour]], [[albedo]], and [[spectral line|spectral shape]] was developed by [[Clark R. Chapman]], [[David Morrison]], and [[Ben Zellner]].<ref>{{cite journal | first=C. R. | last=Chapman | coauthors=Morrison, D.; Zellner, B. | title=Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry | journal=Icarus | volume=25 | pages=104–130 | url=http://adsabs.harvard.edu/abs/1975Icar...25..104C | year=1975 | doi=10.1016/0019-1035(75)90191-8}}</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-type]]s for dark carbonaceous objects (75% of known asteroids), [[S-type asteroid|S-type]]s 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 a number of other asteroid types. The number of types continues to grow as more asteroids are studied. The two most widely used taxonomies currently 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=pp. 1139-1150 | publisher=University of Arizona Press | date=March 8-11, 1988 | location=Tucson, AZ | url=http://adsabs.harvard.edu/abs/1989aste.conf.1139T | accessdate=2008-04-14 }}</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 a number of smaller classes.<ref>{{cite journal | last=Bus | first=S. J. | coauthors=Binzel, R. P. | title=Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy | journal=Icarus | year=2002 | volume=158 | pages=146 | doi=10.1006/icar.2002.6856 }}</ref> Note that 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. ==== Problems with spectral classification ==== Originally, spectral designations were based on inferences of an asteroid's composition.<ref>{{cite book | first=Harry Y. | last=McSween Jr. | year=1999 | title=Meteorites and their Parent Planets | edition=2nd edition | publisher=Oxford University Press | isbn=0521587514 }}</ref> However, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While 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. At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system{{Fact|date=July 2008}}, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful). == Discovery == [[Image:243 ida.jpg|thumb|right|[[243 Ida]] and its moon Dactyl, the first satellite of an asteroid to be discovered.]] === Historical methods === 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 as a consequence 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. Ironically, the first asteroid, [[Ceres (dwarf planet)|1 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. His colleague, [[Carl Friedrich Gauss]], used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets [[Mars]] and [[Jupiter]]. Piazzi named it after [[Ceres (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. 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 year 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, however, [[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 previous visual methods: Wolf alone discovered 248 asteroids, beginning with [[323 Brucia]], whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies". === 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 [[photograph]]ed 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 [[film]]s 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 appear 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=[[May 17]], [[1992]] | url=http://astrogeology.usgs.gov/About/People/CarolynShoemaker | title=Carolyn Shoemaker, Planetary Astronomer and Most Successful 'Comet Hunter' To Date | publisher=USGS | accessdate=2008-04-15 }}</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 week of discovery, and finally a letter and a number indicating the discovery's sequential number (example: {{mp|1998 FJ|74}}). The final 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 previous 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 === [[Image: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 [[Earth-crosser asteroid]]s). 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 the [[Earth]] in 1937. Astronomers began to realize the possibilities of Earth impact. Two events in later decades increased the level of alarm: the increasing acceptance of [[Walter Alvarez]]' hypothesis that an [[impact event]] resulted in the [[Cretaceous-Tertiary extinction event|Cretaceous-Tertiary extinction]], and the 1994 observation of [[Comet Shoemaker-Levy 9]] crashing into [[Jupiter]]. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to 10 metres across. All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device ([[Charge-coupled device|CCD]]) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated 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=2008-04-15 }}</ref> * The [[Lincoln Near-Earth Asteroid Research]] (LINEAR) team * The [[Near-Earth Asteroid Tracking]] (NEAT) team * [[Spacewatch]] * The [[LONEOS|Lowell Observatory Near-Earth-Object Search]] (LONEOS) team * The [[Catalina Sky Survey]] (CSS) * The [[Campo Imperatore Near-Earth Objects Survey]] (CINEOS) team * The [[Japanese Spaceguard Association]] * The [[Asiago-DLR Asteroid Survey]] (ADAS) The LINEAR system alone has discovered 84,764 asteroids, as of [[August 28]], [[2007]].<ref>{{cite web|title=Minor Planet Discover Sites|accessdate=2007-08-31|url=http://www.cfa.harvard.edu/iau/lists/MPDiscSites.html}}</ref> Between all of the automated systems, 4711 near-Earth asteroids have been discovered<ref>{{cite web|title=Unusual Minor Planets|accessdate=2007-08-31|url=http://www.cfa.harvard.edu/iau/lists/Unusual.html}}<!--- using the "close approach" quote ---></ref> including over 600 more than 1 km in diameter. == Naming == === Overview: naming conventions === 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. Asteroids that have been given a number but not a name keep their provisional designation, e.g. [[(29075) 1950 DA]]. As modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to be left unnamed was for a long time [[(3360) 1981 VA]], now [[3360 Syrinx]]; as of November 2006, this distinction is now held by (3708){{mpl| 1974 FV|1}}. On rare occasions, a small body's [[Provisional designation in astronomy|provisional designation]] may become used as a name in itself: the still unnamed {{mpl|(15760) 1992 QB|1}} gave its name to a group of [[Kuiper belt]] objects which became known as [[cubewano]]s. === Numbering === Asteroids are awarded with an official number once their orbits are confirmed. With the increasing rapidity of asteroid discovery, asteroids are currently being awarded six-figure numbers. The switch from five figures to six figures arrived with the publication of the [[Minor Planet Circular]] (MPC) of [[October 19]], [[2005]], which saw the highest numbered asteroid jump from 99947 to 118161. This change caused a small [[Year 2000 problem|Y2K]]-like crisis for various automated data services, since only five digits were allowed in most data formats for the asteroid number. Most services have now widened the asteroid number field. For those which did not, the problem has been addressed in some cases by having the leftmost digit (the ten-thousands place) use the alphabet as a digit extension. A=10, B=11,..., Z=35, a=36,..., z=61. A high number such as 120437 is thus cross-referenced as C0437 on some lists. === Sources for names === {{main|Meanings of asteroid names}} The first few asteroids were named after figures from [[Graeco-Roman mythology]], but as such names started to dwindle the names of famous people, literary characters, discoverer's wives, children, and even television characters were used. The first asteroid to be given a non-mythological name was [[20 Massalia]], named after the city of [[Marseille]]s. For some time only female (or feminized) names were used; [[Alexander von Humboldt]] was the first man to have an asteroid named after him, but his name was feminized to [[54 Alexandra]]. This unspoken tradition lasted until [[334 Chicago]] was named; even then, oddly feminised names show up in the list for years afterward. As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were [[482 Petrina]] and [[483 Seppina]], named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of [[2309 Mr. Spock]] (the name of the discoverer's cat). Although the [[International Astronomical Union|IAU]] subsequently banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as [[4321 Zero]], [[6042 Cheshirecat]], [[9007 James Bond]], [[13579 Allodd]], [[24680 Alleven]], or [[26858 Misterrogers]]. === Special naming rules === Asteroid naming is not always a free-for-all: there are some types of asteroid for which rules have developed about the sources of names. For instance [[Centaur (planetoid)|Centaurs]] (asteroids orbiting between Saturn and Neptune) are all named after mythological [[centaur]]s, [[Trojan asteroid|Trojans]] after heroes from the [[Trojan War]], and [[trans-Neptunian objects]] after underworld spirits. Another well-established rule is that comets are named after their discoverer(s), whereas asteroids are not. One way to circumvent this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other. A particular exception to this rule is [[96747 Crespodasilva]], which was named after its discoverer, [[Lucy d'Escoffier Crespo da Silva]], because she died shortly after the discovery, at age 22.<ref>{{cite web|title=Citation from MPC 55988|accessdate=2006-06-05|url=http://scully.cfa.harvard.edu/~cgi/ShowCitation.COM?num=96747}}</ref>{{Dubious|date=April 2008}}<ref>{{cite news | author=Staff | date=[[November 28]], [[2000]] | title=Lucy Crespo da Silva, 22, a senior, dies in fall | publisher=Hubble News Desk | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2008-04-15 }}</ref> A few objects are also cross-listed as both comets and asteroids, such as [[4015 Wilson-Harrington]] and [[107P/Wilson-Harrington]]. === Symbols === The first few asteroids discovered were assigned symbols like the ones traditionally used to designate Earth, the Moon, the Sun and planets. The symbols quickly became ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol(s).<ref>{{cite journal| last=Gould| first=B. A.| authorlink=Benjamin Apthorp Gould | year=1852| title= On the Symbolic Notation of the Asteroids| journal= Astronomical Journal| volume= 2| pages= 80| doi= 10.1086/100212}}</ref> {| class="wikitable" |- align="center" style="align:center; background:#ffc0c0" ! Asteroid || Symbol |- | [[Ceres (dwarf planet)|Ceres]] || [[Image:Ceres symbol.svg|30x20px|Old planetary symbol of Ceres]] [[Image:Ceres2.svg|30x20px|Variant symbol of Ceres]] [[Image:Ceres symbol.svg|30x20px|Sickle variant symbol of Ceres]] [[Image:Ceres3.svg|30x20px|Other sickle variant symbol of Ceres]] |- | [[2 Pallas]] || [[Image:2Pallas symbol.svg|30x20px|Old symbol of Pallas]] [[Image:2 Pallas.svg|30x20px|Variant symbol of Pallas]] |- | [[3 Juno]] || [[Image:Juno symbol.svg|30x20px|Old symbol of Juno]] [[Image:3 Juno (1).png|30x20px|Other symbol of Juno]] [[Image:Symbol 3.jpg|30x20px]] |- | [[4 Vesta]] || [[Image:4 Vesta (0).svg|30x20px|Old symbol of Vesta]] [[Image:Simbolo di Vesta.svg|30x20px|Old planetary symbol of Vesta]] [[Image:Vesta symbol.svg|30x20px|Modern astrological symbol of Vesta]][[Image:4 Vesta Unsimplified Symbol.svg|30x20px]] |- | [[5 Astraea]] || [[Image:5 Astraea Symbol.svg|30x20px]] |- | [[6 Hebe]] || [[Image:6 Hebe Astronomical Symbol.svg|30x20px]] |- | [[7 Iris]] || [[Image:7 Iris Astronomical Symbol.svg|30x20px]] |- | [[8 Flora]] || [[Image:8 Flora Astronomical Symbol.svg|30x20px]] |- | [[9 Metis]] || [[Image:9 Metis symbol.svg|30x20px]] |- | [[10 Hygiea]] || [[Image:10 Hygiea Astronomical Symbol.svg|30x20px]] |- | [[11 Parthenope]] || [[Image:11 Parthenope symbol.svg|30x20px]] |- | [[12 Victoria]] || [[Image:12 Victoria symbol.svg|30x20px]] |- | [[13 Egeria]] || <small>Never assigned. |- | [[14 Irene]] || <small>"A dove carrying an olive-branch, with a star on its head," never drawn.</small><ref name="hilton">{{cite web|title=When Did the Asteroids Become Minor Planets|authorlink=James L. Hilton| first=James L.| last=Hilton |accessdate=2006-03-26|url=http://aa.usno.navy.mil/faq/docs/minorplanets.php| date=2001-09-17}}</ref> |- | [[15 Eunomia]] || [[Image:15 Eunomia symbol.svg|30x20px]] |- | [[28 Bellona]] || [[Image:28 Bellona symbol.svg|30x20px]] |- | [[35 Leukothea]] || [[Image:35 Leukothea symbol.png|30x20px]] |- | [[37 Fides]] || [[Image:37 Fides symbol.svg|30x20px]] |} [[Johann Franz Encke]] made a major change in the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook) for 1854. He introduced encircled numbers instead of symbols, although his numbering began with [[5 Astraea|Astraea]], the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1855), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. A few more asteroids ([[28 Bellona]],<ref>{{cite journal| last=Encke| first= J. F.| year= 1854| title=Beobachtung der Bellona, nebst Nachrichten über die Bilker Sternwarte| journal= Astronomische Nachrichten| volume= 38| pages=143| doi=10.1002/asna.18540380907}}</ref> [[35 Leukothea]],<ref>{{cite journal| last=Rümker| first= G.| coauthors= Peters, C. A. F.| year= 1855| title=Name und Zeichen des von Herrn R. Luther zu Bilk am 19. April entdeckten Planeten| journal= Astronomische Nachrichten| volume= 40| pages= 373| doi=10.1002/asna.18550402405}}</ref> and [[37 Fides]]<ref>{{cite journal| last=Luther| first= R.| year= 1856| title=Schreiben des Herrn Dr. R. Luther, Directors der Sternwarte zu Bilk, an den Herausgeber| journal= Astronomische Nachrichten| volume= 42| pages= 107| doi=10.1002/asna.18550420705}}</ref>) would be given symbols as well as using the numbering scheme. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.<ref name="hilton">{{cite web|title=When Did the Asteroids Become Minor Planets|authorlink=James L. Hilton| first=James L.| last=Hilton |accessdate=2006-03-26|url=http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html| date=2001-09-17}}</ref> == Exploration == 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, as well as the Earth-orbiting [[Hubble Space Telescope]], can resolve a small amount of detail on the surfaces of the very 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. 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 subsequent images from the [[Voyager program|Voyager]] probes of the small moons of the [[gas giant]]s. [[Image:951 Gaspra.jpg|thumb|right|[[951 Gaspra]], the first asteroid to be imaged in close up.]] 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). In September 2005, the Japanese [[Hayabusa]] probe started studying [[25143 Itokawa]] in detail and may return samples of its surface to earth. The Hayabusa mission has been plagued with difficulties, including the failure of two of its three control wheels, rendering it difficult to maintain its orientation to the sun to collect solar energy. Following that, the next asteroid encounters will involve the European [[Rosetta space probe|Rosetta probe]] (launched in 2004), which will study [[2867 Šteins]] and [[21 Lutetia]] in 2008 and 2010. In September 2007, [[NASA]] launched the [[Dawn (spacecraft)|Dawn Mission]], which will orbit the dwarf planet [[Ceres (dwarf planet)|Ceres]] and the asteroid [[4 Vesta]] in 2011-2015, with its mission possibly then extended to [[2 Pallas]]. It has been suggested that asteroids might be used in the future as a source of materials which may be rare or exhausted on earth ([[asteroid mining]]), or materials for constructing [[space habitats]] (see [[Colonization of the asteroids]]). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for [[space manufacturing]] and construction. == In fiction == {{main|Asteroids in fiction}} Asteroids and asteroid belts are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places which human beings might colonize; as resources for extracting minerals; as a hazard encountered by spaceships travelling between two other points; and as a threat to life on Earth due to potential impacts. == See also == {{wiktionary|asteroid}} {{div col|cols=3}} * [[Asteroid belt]] * [[Asteroid mining]] * [[BOOTES]] (Burst Observer and Optical Transient Exploring System) * [[:Category:Asteroid groups and families|Category:Asteroid groups and families]] * [[:Category:Asteroids|Category:Asteroids]] * [[:Category:Binary asteroids|Category:Binary asteroids]] * [[Centaur (planetoid)]] * [[Dwarf planet]] * [[Impact event]] * [[List of asteroids]] * [[List of asteroids named after important people]] * [[List of asteroids named after places]] * [[List of minor planets]] * [[List of noteworthy asteroids]] * [[Meanings of asteroid names]] * [[Mesoplanet]] * [[Minor planet]] * [[Minor Planet Center]] * [[Near-Earth object]] * [[Pronunciation of asteroid names]] * [[Asteroid deflection strategies]] {{div col end}} == References == {{reflist|2}} == External links == {{Commonscat|Asteroids}} * [http://www.cfa.harvard.edu/iau/lists/MPNames.html Alphabetical list of minor planet names (ASCII)] (Minor Planet Center) * [http://neat.jpl.nasa.gov/ Near Earth Asteroid Tracking (NEAT)] * [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids Asteroids Page] at [http://solarsystem.nasa.gov NASA's Solar System Exploration] *[http://www.colorado.edu/physics/2000/applets/satellites.html Asteroid Simulator with Moon and Earth] * [http://www.newscientistspace.com/channel/solar-system/comets-asteroids Everything you wanted to know about comets and asteroids] - Provided by ''[[New Scientist]]''. * [http://www.ipa.nw.ru/PAGE/DEPFUND/LSBSS/englenam.htm Alphabetical and numerical lists of minor planet names (Unicode)] (Institute of Applied Astronomy) * [http://www.armageddononline.org/content/view/46/67/ Known Asteroid Impacts & Their Effects] * [http://www.theregister.co.uk/2008/03/31/kofels_asteroid/ Assyrian clay tablet points to 'Sodom and Gomorrah' asteroid] * [http://www.fair-society.org Future Asteroid Interception Research] * [http://newton.dm.unipi.it/cgi-bin/neodys/neoibo Near Earth Objects Dynamic Site] * [http://hamilton.dm.unipi.it/cgi-bin/astdys/astibo Asteroids Dynamic Site] Up-to date [[osculating orbit|osculating]] [[orbital elements]] and [[proper orbital elements]] * [http://quasar.ipa.nw.ru/PAGE/DEPFUND/LSBSS/statmpn.htm Asteroid naming statistics] * [http://www.spaceguarduk.com/ Spaceguard UK] *[http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm Large amount of information on asteroid groups collected by Gérard Faure], translation Richard Miles. * [http://michaelgr.wordpress.com/2007/04/14/near-earth-objects-are-we-whistling-in-the-dark/ Near Earth Objects and Asteroids: Are We Whistling in the Dark?] * [http://www.cnn.com/2008/TECH/space/01/29/asteroid.recalculated.ap/index.html 1908 Siberian asteroid] * [http://www.ss.astro.umd.edu/IAU/csbn/ Committee on Small Body Nomenclature] * [http://www.projectpluto.com/mp_group.htm List of minor planet orbital groupings and families from ProjectPluto] * Cunningham, Clifford, "Introduction to Asteroids: The Next Frontier", ISBN 0-943396-16-6 * [[James L. Hilton]]: [http://aa.usno.navy.mil/faq/docs/minorplanets.php When Did the Asteroids Become Minor Planets?] * [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1874MNRAS..35...61K&amp;db_key=AST&amp;high=40daf3f6f901929 Kirkwood, Daniel; ''Relations between the Motions of some of the Minor Planets'' (1874).] * Schmadel, L.D. (2003). ''Dictionary of Minor Planet Names.'' 5th ed. IAU/Springer-Verlag: Heidelberg. *[http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm Large amount of information on asteroid groups collected by Gérard Faure], translation Richard Miles. *[http://www.psrd.hawaii.edu/Archive/Archive-Asteroids.html Asteroid articles in Planetary Science Research Discoveries] {{Solar System}} {{MinorPlanets Navigator|PageName=[[Ceres (dwarf planet)|1 Ceres]]||2 Pallas}} {{Small Solar System bodies}} [[Category:Asteroids| ]] [[Category:Minor planets]] [[Category:Space exploration]] {{Link FA|la}} [[als:Asteroid]] [[ar:كويكب]] [[an:Asteroide]] [[ast:Asteroide]] [[bn:গ্রহাণু]] [[zh-min-nan:Sió-he̍k-chheⁿ]] [[be:Астэроід]] [[be-x-old:Астэроід]] [[bs:Asteroidi]] [[bg:Астероид]] [[ca:Asteroide]] [[cv:Астероид]] [[cs:Asteroid]] [[da:Asteroide]] [[de:Asteroid]] [[et:Asteroid]] [[el:Αστεροειδής]] [[es:Asteroide]] [[eo:Asteroido]] [[eu:Asteroide]] [[fa:سیارک]] [[fr:Astéroïde]] [[gl:Asteroide]] [[ko:소행성]] [[hr:Asteroidi]] [[io:Asteroido-zono]] [[id:Asteroid]] [[is:Smástirni]] [[it:Asteroide]] [[he:אסטרואיד]] [[jv:Asteroid]] [[pam:Asteroid]] [[kn:ಕ್ಷುದ್ರ ಗ್ರಹ]] [[ka:მცირე ცთომილები]] [[la:Asteroides]] [[lv:Asteroīds]] [[lb:Asteroid]] [[lt:Asteroidas]] [[li:Planetoïed]] [[hu:Kisbolygó]] [[mk:Астероид]] [[mt:Asterojde]] [[ms:Asteroid]] [[nl:Planetoïde]] [[ja:小惑星]] [[no:Asteroide]] [[nn:Asteroide]] [[oc:Asteroïde]] [[nds:Asteroid]] [[pl:Planetoida]] [[pt:Asteróide]] [[ksh:Asteroid]] [[ro:Asteroid]] [[qu:Puriq quyllurcha]] [[ru:Астероид]] [[scn:Astiròidi]] [[simple:Asteroid]] [[sk:Asteroid]] [[sl:Asteroid]] [[sr:Астероид]] [[sh:Asteroid]] [[su:Astéroid]] [[fi:Asteroidi]] [[sv:Asteroid]] [[tl:Asteroyd]] [[ta:சிறுகோள்]] [[tt:Asteroidlar]] [[th:ดาวเคราะห์น้อย]] [[vi:Tiểu hành tinh]] [[tr:Asteroit]] [[uk:Астероїд]] [[ur:سیارچے]] [[vec:Asteroide]] [[yi:אסטערויד]] [[zh:小行星]]