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{{About||the arcade video game|Asteroids (video game)|other uses}}
{{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. As of 2011 they are, from largest to smallest: ], ], ], ] and its moon Dactyl, ], ], ], ].
| image2 = 4 Vesta 1 Ceres Moon at 20 km per px.png
| caption2 = The largest asteroid in the previous image, ] (left), with ] (center) and Earth's ] (right) shown to scale.
}}

'''Asteroids''' (from ] ἀστεροειδής - ''asteroeidēs'', "star-like",<ref>, Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus</ref> from {{lang|grc|]}} "star" and {{lang|grc|]}} "like, in form") are a class of ] in orbit around the ]. They have also been called '''planetoids''', especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active ], but as small objects in the ] were discovered, their ]-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids.<ref>{{cite web | url=http://ssd.jpl.nasa.gov/?asteroids| title=Asteroids | publisher=NASA – Jet Propulsion Laboratory | accessdate=13 September2010}}</ref> Thus the term ''asteroid'' has come increasingly to refer specifically to the small bodies of the ] out to the orbit of ], which are usually rocky or metallic. They are grouped with the outer bodies—], ]s, and ]s—as ]s, which is the term preferred in astronomical circles.<ref>Asimov, Isaac, and Dole, Stephen H. ''Planets for Man'' (New York: Random House, 1964), p.43</ref> This article will restrict the use of the term 'asteroid' to the minor planets of the inner Solar System.

There are millions of asteroids, many thought to be the shattered remnants of ]s, bodies within the young Sun’s ] that never grew large enough to become ]s.<ref>{{cite web | title=What Are Asteroids And Comets? | url=http://neo.jpl.nasa.gov/faq/#ast | work=Near Earth Object Program FAQ |publisher=] | accessdate=2010-09-13 }}</ref> A large majority of known asteroids orbit in the ] between the orbits of Mars and Jupiter or co-orbital with Jupiter (the ]s). However, other orbital families exist with significant populations, including the ]. Individual asteroids are classified by their characteristic ], with the majority falling into three main groups: ], ], and ]. These were named after and are generally identified with ], ], and ]lic compositions, respectively.

{{TOCLimit|3}}

== Naming ==
{{Main|Minor planet#Naming}}
A newly discovered asteroid is given a ] (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. ]). 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.

=== 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 several variants.<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| page= 80| doi= 10.1086/100212| bibcode=1852AJ......2...80G}}</ref>

{| class="wikitable"
|-
! Asteroid || colspan=2| Symbol
|-
| ] || {{unicode|⚳}} ] ] ] || ] scythe, <br>reversed to double as the letter ''C''
|-
| ] || {{unicode|⚴}} ] ]|| ]'s (Pallas') spear
|-
| ] || {{unicode|⚵}} ] ] ]|| A star mounted on a scepter, <br>for ], the Queen of Heaven
|-
| ] || {{unicode|⚶}} ]] ] ]|| The altar and ]
|-
| ] || ]]|| A scale, or an inverted anchor, <br>symbols of ]
|-
| ] || ]|| ] cup
|-
| ] || ]|| A rainbow (''iris'') and a star
|-
| ] || ]|| A flower (''flora'')<br>(spec. the ])
|-
| ] || ]|| The eye of ] and a star
|-
| ] || ]] || ] serpent and a star, or the ]
|-
| ] || ]]|| A harp, or a fish and a star;<br>symbols of the ]s
|-
| ] || ]|| The ] and a star
|-
| ] || ] || A shield, symbol of ] protection,<br> and a star
|-
| ] || ] ||<small>A dove carrying an olive-branch (symbol of <br>''irene'' 'peace') 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=2006-03-26|url=http://aa.usno.navy.mil/faq/docs/minorplanets.php| date=2001-09-17}} {{Dead link|date=September 2010|bot=H3llBot}}</ref> or <br>an olive branch, a flag of truce, and a star</small>
|-
| ] || ]|| A heart, symbol of good order <br>(''eunomia''), and a star
|-
| ] || ] || A butterfly's wing, symbol of <br> the soul (''psyche''), and a star
|-
| ] || ] || A dolphin, symbol of ], and a star
|-
| ] || ] || The dagger of ], and a star
|-
| ] || ] || The ] and a star
|-
| ] || ] || ]'s pomegranate<!--Webster's (1884) says this is a fruit ('']'') and a star, and is the symbol for ]-->
|-
| ] || ]|| ]'s whip and lance<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| issue=9| page=143| doi=10.1002/asna.18540380907}}</ref>
|-
| ] || ] || The shell of ] and a star
|-
| ] || ]|| A lighthouse beacon, <br>symbol of ]<ref>{{cite journal| last=Rümker| first= G.| year= 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}}</ref>
|-
| ] || ]|| The ] of faith (''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| issue=7| page= 107| doi=10.1002/asna.18550420705| bibcode=1855AN.....42..107L}}</ref>
|}
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=2011-11-06}}</ref> ] 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, though he assigned ① to the fifth, ], the first four continuing with their existing symbols. The numbered-circle convention was quickly adopted by the astronomical community, and no iconic symbols were created after 1855.<ref>Except for Pluto and, in the astrological community, for a few outer bodies such as ]: ⚷</ref> That year Astraea's number was bumped up to ⑤, but Ceres through Vesta would not be listed by their numbers until the 1867 edition. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades, leading to the modern convention.<ref name="hilton"/>

== Discovery ==
] and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.]]
The first asteroid to be discovered, ], was found in 1801 by ], and was originally considered to be a new planet.<ref group=note>Ceres is the largest asteroid and is now classified as a ]. All other asteroids are now classified as ] 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 ] to propose the term "asteroid", from Greek ''αστεροειδής'', ''asteroeidēs'' 'star-like, star-shaped', from ancient Greek ''αστήρ'', ''astēr'' 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the , 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 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".

=== Historical methods ===
Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron ] organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 ] from the Sun by the ], partly because of the discovery, by Sir ] in 1781, of the planet ] at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the ]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.

The first object, ], was not discovered by a member of the group, but rather by accident in 1801 by ], director of the observatory of ] in ]. He discovered a new star-like object in ] and followed the displacement of this object during several nights. His colleague, ], used these observations to find the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets ] and ]. Piazzi named it after ], the Roman goddess of agriculture.

Three other asteroids (], ], and ]) 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, ] persisted, and began searching for more asteroids in 1830. Fifteen years later, he found ], the first new asteroid in 38 years. He also found ] 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 ], ], ], ], ], ], ], ], ], ], ], ], the ] and ].

In 1891, however, ] pioneered the use of ] 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 ], 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{{Citation needed|date=September 2010}}, calling them "vermin of the skies", a phrase due to ].<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. Astr. Soc. Pacific | year=1930 | volume=42 | pages=5–22
| bibcode= 1930PASP...42....5S
| doi= 10.1086/123986+(Dead+links)}}</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 ] by a wide-field ], or ]. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or ] of the same region were viewed under a ]. 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=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 ], 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 ], 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 ].

=== Computerized methods ===
[[Image:Asteroid 2004 FH.gif|framed|right|
] is the center dot being followed by the sequence; the object that flashes by during the clip is an ].]]
There is increasing interest in identifying asteroids whose orbits cross ]'s, and that could, given enough time, collide with Earth (see ]s). The three most important groups of ]s are the ], ], and ]. Various ] have been proposed, as early as the 1960s<!--- ''Project Icarus'' --->.

The ] asteroid ] had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: ], ], ], and finally ], which approached within 0.005 ] of the ] in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of ]' hypothesis that an ] resulted in the ], and the 1994 observation of ] crashing into ]. 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 these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (]) 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 ] (LINEAR) team
* The ] (NEAT) team
* ]
* The ] (LONEOS) team
* The ] (CSS)
* The ] (CINEOS) team
* The ]
* The ] (ADAS)

The LINEAR system alone has discovered 121,346 asteroids, as of March, 2011.<ref>{{cite web|title=Minor Planet Discover Sites|accessdate=2010-08-24|url=http://www.minorplanetcenter.org/iau/lists/MPDiscSites.html}}</ref> Among all the automated systems, 4711 near-Earth asteroids have been discovered<ref>{{cite web|title=Unusual Minor Planets|accessdate=2010-08-24|url=http://www.minorplanetcenter.org/iau/lists/Unusual.html}}<!--- using the "close approach" quote ---></ref> including over 600 more than {{convert|1|km|1|abbr=on}} in diameter.

== Terminology{{anchor|Terminology}} == <!-- Linked from "Comet" -->

Traditionally, small bodies orbiting the Sun were classified as asteroids, ]s or ]s, with anything smaller than ten metres across being called a meteoroid.<ref>{{cite journal | author=Beech, M. |authorlink=Martin Beech | 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=281–284 |bibcode=1995QJRAS..36..281B }}</ref> The term "asteroid" is ill-defined. It never had a formal definition, with the broader term ] being preferred by the ] from 1853 on. In 2006, the term "]" was introduced to cover both most minor planets and comets.<ref>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". (IAU)</ref> Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "]" 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 ] to describe minor planets,<ref>{{cite web | url=http://www.hyperdictionary.com/dict-e/p-44.html | title=English Dictionary&nbsp;– Browsing Page P-44 | publisher=HyperDictionary.com | accessdate=2008-04-15 }}</ref> but is not in common use. The three largest objects in the asteroid belt, ], ], and ], grew to the stage of ]s. Ceres has been classified as a ], 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 ] 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 ] and become asteroids. 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>Weissman, Paul R., William F. Bottke, Jr., and Harold F. Levinson. "Evolution of Comets into Asteroids." ''Southwest Research Institute, Planetary Science Directorate.'' 2002. Web </ref>

For almost two centuries, from the discovery of ] in 1801 until the discovery of the first ], ], in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as ] ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called ], they numbered them among the traditional asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first ], ], was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: ], ], ], 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 ], called "objects" partly to avoid the need to classify them as asteroids or comets.<ref name=KBOasteroids>, "Ask an astronomer", Cornell University</ref> They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids.<ref>, NASA website</ref> Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional ]. (The much more distant ] is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the ] probe, are increasingly blurring the distinction between comets and asteroids,<ref> ''Scientific American'', January 25, 2008</ref> suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.<ref>, ''New Scientist'', 24 January 2008</ref>

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.<ref>For instance, a joint ]-] 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."}} <http://ssd.jpl.nasa.gov/?asteroids></ref>
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 ], ]s, and ]s.

When the IAU introduced the class ] in 2006 to include most objects previously classified as minor planets and comets, they created the class of ]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>, IAU</ref> Currently only the largest object in the asteroid belt, ], at about {{convert|950|km|0|abbr=on}} across, has been placed in the dwarf planet category, although there are several large asteroids (], ], and ]) that may be classified as dwarf planets when their shapes are better known.<ref>, ''New Scientist'', 16 August 2006</ref>

== Formation ==
It is believed that ]s in the asteroid belt evolved much like the rest of the ] until Jupiter neared its current mass, at which point excitation from ]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 {{convert|120|km|0|abbr=on}} in diameter 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 = Durda | last2 = Nesvorny | first2 = Jedicke | last3 = Morbidelli | first3 = Vokrouhlicky | last4 = Levison | first4 = | year = 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 }}</ref> Ceres and Vesta grew large enough to melt and ], 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|year=2000|publisher=Lerner Publications Co.|isbn=0585317631}}</ref>

In the ], many ]s 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 ]s, and possibly include Ceres.<ref>William B. McKinnon, 2008, ''American Astronomical Society,'' DPS meeting #40, #38.03</ref>

== Distribution within the Solar System ==
{{See also|list of minor-planet groups|List of notable asteroids|List of minor planets}}
] (white) and the ] (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 ]. Significant populations include:

=== Asteroid belt ===
{{main|Asteroid belt}}

The majority of known asteroids orbit within the asteroid belt between the orbits of ] and ], generally in relatively low-] (i.e., not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than {{convert|1|km|1|abbr=on}} in diameter,<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> and millions of smaller ones.<ref></ref> These asteroids may be remnants of the ], and in this region the ] of ]s into planets during the formative period of the Solar System was prevented by large gravitational perturbations by ].

=== Trojans ===
{{main|Trojan (astronomy)|l1=Trojan asteroids}}

Trojan asteroids are a population that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two ]s of stability, ], which lie 60° ahead of and behind the larger body.

The most significant population of Trojan asteroids are the ]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.

A couple of ] have also been found orbiting with ].<ref group=note>Neptune also has a few known trojans, and these are thought to actually be much more numerous than the Jovian trojans. However, they are often included in the ] population rather than counted with the asteroids.</ref>

=== 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 the Earth's orbital path are known as ''Earth-crossers''. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500–1,000.

== Characteristics ==
=== Size distribution ===
{{multiple image
| align = right
| direction = vertical
| image1 = Moon and Asteroids 1 to 10.svg
| caption1 = Sizes of the first ten asteroids to be discovered, compared to the Earth's Moon
| image2 = Ceres optimized.jpg
| caption2 = ] image of the dwarf planet Ceres
}}

Asteroids vary greatly in size, from almost 1000 kilometres for the largest down to rocks just tens of metres across.<ref group=note>Below 10 metres, these rocks are by convention considered to be ]s.</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 |last=Schmidt |first=B. |coauthors=Russell, C. T.; Bauer, J. M.; Li, J.; McFadden, L. A.; Mutchler, M.; Parker, J. W.; Rivkin, A. S.; Stern, S. A.; Thomas, P. C. |title=Hubble Space Telescope Observations of 2 Pallas |journal=American Astronomical Society, DPS meeting #39 |volume=39 |page=485 |year=2007 |bibcode=2007DPS....39.3519S}}</ref> and are thought to be surviving ]s. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving ]s or fragments of larger bodies.

The ] ] is by far the largest asteroid, with a diameter of {{convert|975|km|-1|abbr=on}}. The next largest are ] and ], both with diameters of just over {{convert|500|km|abbr=on|-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 ].

The mass of all the objects of the ], lying between the orbits of ] and ], is estimated to be about 2.8-3.2{{e|21}}&nbsp;kg, or about 4 percent of the mass of the Moon. Of this, ] comprises 0.95{{e|21}}&nbsp;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 ] | pages=2014 | year=2004 | url=http://adsabs.harvard.edu/abs/2004cosp.meet.2014P}}</ref> Adding in the next three most massive objects, ] (9%), ] (7%), and ] (3%), brings this figure up to 51%; while the three after that, ] (1.2%), ] (1.0%), and ] (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 ], there are 'bumps' at 5&nbsp;km and 100&nbsp;km, where more asteroids than expected from a ] are found.<ref></ref>

{| class="wikitable" style="margin:auto;"
|+Approximate number of asteroids N larger than diameter D
|-
!D
|100 m || 300 m || 500 m || 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
| ~25,000,000 || 4,000,000 || 2,000,000 || 750,000 || 200,000 || 90,000 || 10,000 || 1,100 || 600 || 200 || 30 || 5 || 3 || 1
|}

====Largest asteroids====
{{See also|Largest asteroids}}
] known,<ref name="Baer2011">. Maintained by Jim Baer. Last updated 2010-12-12. Access date 2011-09-02. 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 |year=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 }}</ref> <br>{{Col-begin}}{{Col-2}}
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Although their location in the asteroid belt excludes them from planet status, the four largest objects, ], ], ], and ], are remnant ]s that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids.

{| class="wikitable" style="margin: 1em auto 1em auto"
|- style="background:#ccf;"
! colspan="12" style="background:#ddd;"| Attributes of protoplanetary asteroids
|- style="font-size: smaller;"
!Name
!Orbital<br>radius (])
!]<br>(years)
!]
!]
! Diameter<br>(km)
! Diameter<br>(% of ])
! Mass<br>({{e|18}} kg)
! Mass<br>(% of Ceres)</sub>
! Rotation<br>period<br>(hr)
! ]
! Surface<br>temperature
|- style="text-align:center;"
! style="text-align:left;"| Vesta
| 2.36
| 3.63
| 7.1°
| 0.089
| 578×560×458<br>(mean 529)
| 15%
| 260
| 28%
| 5.34
| 29°
| 85–270 K
|- style="text-align:center;"
! style="text-align:left;"| Ceres
| 2.77
| 4.60
| 10.6°
| 0.079
| 975×975×909<br>(mean 952)
| 28%
| 940
| 100%
| 9.07
| ≈ 3°
| 167 K
|- style="text-align:center;"
! style="text-align:left;"| Pallas
| 2.77
| 4.62
| 34.8°
| 0.231
| 580×555×500<br>(mean 545)
| 16%
| 210
| 22%
| 7.81
| ≈ 80°
| 164 K
|- style="text-align:center;"
! style="text-align:left;"| Hygiea
| 3.14
| 5.56
| 3.8°
| 0.117
| 530×407×370<br>(mean 430)
| 12%
| 87
| 9%
| 27.6
| ≈ 60°
| 164 K
|}

Ceres is the only asteroid large enough for its gravity to force it into a spheroidal shape, and so, according to the IAU's 2006 resolution on the ], it has been classified as a ].<ref>{{cite web | date = August 24, 2006 | url = http://www.iau.org/public_press/news/detail/iau0602/| title = The Final IAU Resolution on the Definition of "Planet" Ready for Voting | publisher = IAU | accessdate = 2007-03-02 }}</ref> Vesta may eventually be so classified as well. Ceres has a much higher ] than the other asteroids, of around 3.32,<ref>{{cite journal
| author=Parker, J. W.; Stern, S. A.; Thomas, P. C.; Festou, M. C.; Merline, W. J.; Young, E. F.; Binzel, R. P.; and 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
| year=2002
| volume=123
| issue=1
| pages=549–557
| accessdate=2008-09-06 | doi = 10.1086/338093
| bibcode=2002AJ....123..549P
| 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=2007-10-20}}</ref> Like the planets, Ceres is differentiated: it has a crust, a mantle and a core.<ref name="planetary" /> Vesta, too, has a differentiated interior, though it formed inside the Solar System's ], and so is devoid of water;<ref>{{cite web |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/20/image/c |title=Key Stages in the Evolution of the Asteroid Vesta| work=Hubble Space Telescope news release|year=1995|accessdate=2007-10-20}}
{{cite web|title=Dawn mission and operations|author=Russel, C. T.; ''et al.'' |work=NASA/JPL|url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=414750|year=2007|accessdate=2007-10-20}}</ref> its composition is mainly of basaltic rock such as olivine.<ref name="olivine" /> Pallas is unusual in that, like ], it rotates on its side, with one pole regularly facing the Sun and the other facing away.<ref name="Torppa1996">{{cite journal
| author=Torppa, J.; ''et al.''
| title=Shapes and rotational properties of thirty asteroids from photometric data
| journal=Icarus | year=1996
| volume=164 | issue=2 | pages=346–383
| bibcode=2003Icar..164..346T
| doi=10.1016/S0019-1035(03)00146-5 }}</ref> Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated.<ref>{{cite web|title=The composition of asteroid 2 Pallas and its relation to primitive meteorites|author=Larson, H. P.; Feierberg, M. A.; and Lebofsky, L. A. |url=http://adsabs.harvard.edu/abs/1983Icar...56..398L|year=1983|accessdate=2007-10-20}}</ref> Hygiea is a carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the ].<ref>{{cite web|title=10 Hygiea: ISO Infrared Observations|author=Barucci, M. A.; ''et al.'' |url=http://www.lesia.obspm.fr/~crovisier/biblio/preprint/bar02_icarus.pdf|format=PDF|year=2002|accessdate=2007-10-21}}

{{cite web|title=Ceres the Planet|work=orbitsimulator.com|url=http://www.orbitsimulator.com/gravity/articles/ceres.html|accessdate=2007-10-20}}</ref>

===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 inertia 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 ]s formed through accumulation of debris after collisions between asteroids.<ref>{{cite web
| last = Rossi
| first = Alessandro
| date = 2004-05-20
| 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 = 2007-04-09
}}</ref>

=== Composition ===
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 ] core, ] mantle, and basaltic crust.<ref></ref> ], however, which appears to have a uniformly primitive composition of ], 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 ] or are co-orbiting ]: Rubble piles, moons, binaries, and scattered ] are believed to be the results of collisions that disrupted a parent asteroid.

Asteroids contain traces of ]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. (See also ].)<ref> {{Wayback|url=http://web.archive.org/web/20020124092631/http://www.space.com/scienceastronomy/planetearth/meteor_sugar_011219.html|title=|date=20020124092631}}, Space.com, 19 December 2001</ref> In August 2011, a report, based on ] studies with ] found on ], was published suggesting ] and ] components (], ] and related ]) may have been formed on asteroids and ] in ].<ref name="Callahan">{{cite web |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 |url=http://www.pnas.org/content/early/2011/08/10/1106493108 |publisher=] |doi=10.1073/pnas.1106493108 |accessdate=2011-08-15 }}</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=] |date=8 August 2011 |accessdate=2011-08-10 }}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=http://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |publisher=] |accessdate=2011-08-09}}</ref>

Only one asteroid, 4 Vesta, which has a 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 naked-eye visible for a short time.<ref>, Space.com, 4 February 2005</ref>

Composition is calculated from three primary sources: ], 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 ] (268×186×183&nbsp;km), and ] (384×262×232&nbsp;km). Only half a dozen asteroids are ], 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 ], 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>Marchis, Descamps, et al. ''Icarus'', Feb. 2011</ref>

===Surface features===
], a ] measuring about {{convert|50|km|mi|sigfig=1}} across, covered in craters half that size. Photograph taken in 1997 by the ] probe.]]
Most asteroids outside the big four (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50-km ] (shown at right) is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300-km ], one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters.<ref>A.R. Conrad ''et al.'' 2007. "", ''Icarus,'' {{doi|10.1016/j.icarus.2007.05.004}}</ref> Medium-sized asteroids such as Mathilde and ] that have been observed up close also reveal a deep ] 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 ]. 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 not be known until ''Dawn'' arrives in 2015.

== Classification ==
Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance ].

=== 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
| last=Zappalà | first=V.
| title=Asteroid families: Search of a 12,487-asteroid sample using two different clustering techniques
| journal=Icarus | year=1995 | volume=116
| issue=2 | pages=291–314
| bibcode=1995Icar..116..291Z
| doi=10.1006/icar.1995.1127 }}</ref> Families have only been recognized within the ]. They were first recognised by ] in 1918 and are often called ] in his honor.

About 30% to 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 ] {{dp|Haumea}}.

==== Quasi-satellites and horseshoe objects ====

Some asteroids have unusual ]s that are co-orbital with the ] or some other planet. Examples are ] and {{mpl|2002 AA|29}}. The first instance of this type of orbital arrangement was discovered between ]'s moons ] and ].

Sometimes these horseshoe objects temporarily become ]s for a few decades or a few hundred years, before returning to their earlier status. Both Earth and ] are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically ], are a special class of ]s. However, such objects could be associated with outer planets as well.

=== Spectral classification ===
] 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 {{convert|35|m|0|abbr=on}} across can be seen.]]
{{Main|Asteroid spectral types}}

In 1975, an asteroid ] system based on ], ], and ] was developed by ], ], and ].<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 | 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: ]s for dark carbonaceous objects (75% of known asteroids), ]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 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 ] and ]. The former was proposed in 1984 by ], 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=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 ]. 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 | year=2002 | volume=158 | issue=1 | page=146 | doi=10.1006/icar.2002.6856 | bibcode=2002Icar..158..146B}}</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 ====
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 | publisher=Oxford University Press | isbn=0521587514 }}</ref> However, the correspondence between spectral class and composition is not always very good, and a variety of classifications is 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 cannot agree on a better taxonomic system,{{Citation needed|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).

== Exploration ==
{{see also|Asteroid mining|Colonization of the asteroids}}
] is the first asteroid to be imaged in close-up.]]
]]]
Until the age of ], 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 ] 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 ]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). ] 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></ref>

The first close-up photographs of asteroid-like objects were taken in 1971 when the ] probe imaged ] and ], the two small moons of ], which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the ] probes of the small moons of the ]s.

The first true asteroid to be photographed in close-up was ] in 1991, followed in 1993 by ] and its moon ], all of which were imaged by the ] en route to ].

The first dedicated asteroid probe was ], which photographed ] in 1997, before entering into orbit around ], finally landing on its surface in 2001.

Other asteroids briefly visited by spacecraft en route to other destinations include ] (by ] in 1999), and ] (by ] in 2002).

In September 2005, the Japanese ] probe started studying ] in detail and was plagued with difficulties, but returned samples of its surface to earth on June 13, 2010.

The European ] (launched in 2004) flew by ] in 2008 and ], the second-largest asteroid visited to date, in 2010.

In September 2007, ] launched the ], which started orbiting the ] ] in July 2011, and is to orbit ] in 2015. 4 Vesta is the largest asteroid visited to date.

In May 2011, NASA announced the ] sample return mission to asteroid ], and is expected to launch in 2016.

It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on earth (]), or materials for constructing ] (see ]). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for ] and construction.

== 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 spaceships traveling between two other points, and as a threat to life on Earth by potential impact.

== See also ==
{{div col|cols=3}}
* ] (Burst Observer and Optical Transient Exploring System)
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
{{div col end}}

== Notes ==
<references group=note/>

== References ==
{{Reflist|2}}

== External links ==
{{Sister project links|wikt=asteroid|commons=Category:Asteroids|v=no|q=no|s=The New Student's Reference Work/Asteroids|b=General Astronomy/Asteroids}}
*
* (Minor Planet Center)
*
* at
*
* (Institute of Applied Astronomy)
*
*
* Up-to-date ] ] and ] University of Pisa, Italy.
<!--- Older URL, replaced w/ above 28 February 2009. * Up-to date ] ] and ] -->
* Current down-loadable ASCII table of orbit data and absolute mags H for over 200000 asteroids, sorted by number. Caltech/JPL.
*
*
*
*
* Cunningham, Clifford, "Introduction to Asteroids: The Next Frontier", ISBN 0-943396-16-6
* ]:
*
* Schmadel, L.D. (2003). ''Dictionary of Minor Planet Names.'' 5th ed. IAU/Springer-Verlag: Heidelberg.
*
*
*
*
*
*
{{Asteroids}}
{{Solar System}}
{{Minor planets navigator|PageName=]||2 Pallas}}
{{Small Solar System bodies}}
{{Asteroid spacecraft}}

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Revision as of 15:25, 9 May 2012

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