Asteroids

​Asteroids:

Ben Laktin:

Why are Asteroids important?

Asteroids are important because they are the likely thing that formed the way that Earth is now on the surface. Asteroids are one of the most likely ways that water got to Earth. Asteroids are responsible for the creation and evolution of life but they are also one of the reasons that lots of life has ended. There have been many meteorite impacts over the many millions of years that have killed animals and other creatures such as the Asteroid in Yucatan, Mexico that killed the Dinosaurs.

What would happen if an asteroid crashed into Earth?

If an asteroid hit Earth, then the effects would be catastrophic. Firstly, as the asteroid entered the atmosphere, the asteroid would suddenly be compressed by the gravity of the Earth and would begin to get part of it ripped off. The asteroid would also start to shrink as parts of it are burned up and turned into dust. Also, many asteroids are also made up of ice that melts in the atmosphere. If an asteroid hit the Earth's surface (assuming that it is 8 kilometers in diameter) it would destroy a supercity in the main blast. The debris from the blast would go up into the Earth's atmosphere and block out the sun over the entire surface of the Earth. Events like this, have caused ice-ages in Earth's history. An event involving anything bigger than an 8 kilometer asteroid would likely destroy 99% or more of all life on Earth.

​Asteroids: What they are made of. The composition of asteroids are varied and in most cases poorly understood. Some have rocky cores with an icy mantle while other have a nickel-iron core but most asteroids are just piles of rubble held together loosely by gravity. Overall asteroids are mostly made of iron, ice and dust.

=Asteroids= [|253 Mathilde], a [|C-type asteroid] measuring about 50 kilometres (30 mi) across. Photograph taken in 1997 by the [|NEAR Shoemaker] probe. There are millions of asteroids, and like most other small Solar System bodies the asteroids are thought to be remnants of [|planetesimals], material within the young Sun’s [|solar nebula] that have not grown large enough to form [|planets].[|[][|2][|]] The large majority of known asteroids orbit in the [|main asteroid belt] between the orbits of Mars and Jupiter, however many different orbital families exist with significant populations including [|Jupiter trojans] and [|near-Earth asteroids]. Individual asteroids are categorized by their characteristic [|spectra], with the majority falling into three main groups: [|C-type], [|S-type], and [|M-type]. These are generally identified with [|carbon-rich], [|stony], and [|metallic] compositions respectively.
 * Asteroids** (from [|Greek], [|ἀστήρ] ”star” + [|εἶδος] “like”, in form), sometimes grouped with [|Centaurs], [|Neptune trojans] and [|trans-Neptunian objects] into **[|minor planets]** or **planetoids**, are a class of [|small Solar System bodies] in orbit around the [|Sun]. The term "asteroid" was historically applied to any astronomical object orbiting the Sun that was not observed to have the characteristics of an active [|comet] or a planet, but it has increasingly come to particularly refer to the small rocky and metallic bodies of the [|inner Solar System] and out to the orbit of [|Jupiter]. As small objects in the [|outer Solar System] have begun to be discovered their observed composition differs from the objects historically termed asteroids. Harbouring predominantly [|volatiles]-based material similar to comets rather than the more familiar rocky or metallic asteroids, they are often distinguished from them.[|[][|1][|]]

Discovery
[|243 Ida] and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered. The first named minor planet, [|Ceres], was discovered in 1801 by [|Giuseppe Piazzi], and was originally considered a new planet. This was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light, like stars, showing little or no planetary disc (though readily distinguishable from stars due to their apparent motions). This prompted the astronomer [|Sir William Herschel] to propose the term "asteroid", from Greek //αστεροειδής//, //asteroeidēs// = star-like, star-shaped, from ancient Greek //Aστήρ//, //astēr// = star. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the [|//Annual of Scientific Discovery for 1871//], page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of 8 new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered 2 planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter" (emphasis added).

Symbols
Main article: [|Astronomical 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 recognize. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol(s).[|[][|3][|]] [|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 [|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], [|35 Leukothea], and [|37 Fides] ) would be given symbols and numbers. The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.
 * ~ Asteroid ||~ Symbol ||
 * [|Ceres] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Ceres_symbol.svg/20px-Ceres_symbol.svg.png width="20" height="20" caption="Old planetary symbol of Ceres" link="http://en.wikipedia.org/wiki/File:Ceres_symbol.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Ceres2.svg/10px-Ceres2.svg.png width="10" height="20" caption="Variant symbol of Ceres" link="http://en.wikipedia.org/wiki/File:Ceres2.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/8/88/Ceres3.svg/13px-Ceres3.svg.png width="13" height="20" caption="Other sickle variant symbol of Ceres" link="http://en.wikipedia.org/wiki/File:Ceres3.svg"]] ||
 * [|2 Pallas] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/2Pallas_symbol.svg/13px-2Pallas_symbol.svg.png width="13" height="20" caption="Old symbol of Pallas" link="http://en.wikipedia.org/wiki/File:2Pallas_symbol.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/5e/2_Pallas.svg/13px-2_Pallas.svg.png width="13" height="20" caption="Variant symbol of Pallas" link="http://en.wikipedia.org/wiki/File:2_Pallas.svg"]] ||
 * [|3 Juno] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/Juno_symbol.svg/20px-Juno_symbol.svg.png width="20" height="20" caption="Old symbol of Juno" link="http://en.wikipedia.org/wiki/File:Juno_symbol.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/3_Juno_%281%29.png/11px-3_Juno_%281%29.png width="11" height="19" caption="Other symbol of Juno" link="http://en.wikipedia.org/wiki/File:3_Juno_%281%29.png"]] [[image:http://upload.wikimedia.org/wikipedia/en/thumb/8/89/Symbol_3.jpg/12px-Symbol_3.jpg width="12" height="19" caption="Symbol 3.jpg" link="http://en.wikipedia.org/wiki/File:Symbol_3.jpg"]] ||
 * [|4 Vesta] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/4_Vesta_%280%29.svg/13px-4_Vesta_%280%29.svg.png width="13" height="20" caption="Old symbol of Vesta" link="http://en.wikipedia.org/wiki/File:4_Vesta_%280%29.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/e/e6/Simbolo_di_Vesta.svg/20px-Simbolo_di_Vesta.svg.png width="20" height="20" caption="Old planetary symbol of Vesta" link="http://en.wikipedia.org/wiki/File:Simbolo_di_Vesta.svg"]] [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Vesta_symbol.svg/20px-Vesta_symbol.svg.png width="20" height="20" caption="Modern astrological symbol of Vesta" link="http://en.wikipedia.org/wiki/File:Vesta_symbol.svg"]][[image:http://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/4_Vesta_Unsimplified_Symbol.svg/14px-4_Vesta_Unsimplified_Symbol.svg.png width="14" height="20" caption="4 Vesta Unsimplified Symbol.svg" link="http://en.wikipedia.org/wiki/File:4_Vesta_Unsimplified_Symbol.svg"]] ||
 * [|5 Astraea] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/b/bf/5_Astraea_Symbol.svg/16px-5_Astraea_Symbol.svg.png width="16" height="20" caption="5 Astraea Symbol.svg" link="http://en.wikipedia.org/wiki/File:5_Astraea_Symbol.svg"]] ||
 * [|6 Hebe] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/6_Hebe_Astronomical_Symbol.svg/15px-6_Hebe_Astronomical_Symbol.svg.png width="15" height="19" caption="6 Hebe Astronomical Symbol.svg" link="http://en.wikipedia.org/wiki/File:6_Hebe_Astronomical_Symbol.svg"]] ||
 * [|7 Iris] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/7_Iris_Astronomical_Symbol.svg/30px-7_Iris_Astronomical_Symbol.svg.png width="30" height="15" caption="7 Iris Astronomical Symbol.svg" link="http://en.wikipedia.org/wiki/File:7_Iris_Astronomical_Symbol.svg"]] ||
 * [|8 Flora] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/8_Flora_Astronomical_Symbol.svg/15px-8_Flora_Astronomical_Symbol.svg.png width="15" height="20" caption="8 Flora Astronomical Symbol.svg" link="http://en.wikipedia.org/wiki/File:8_Flora_Astronomical_Symbol.svg"]] ||
 * [|9 Metis] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/d/dc/9_Metis_symbol.svg/25px-9_Metis_symbol.svg.png width="25" height="20" caption="9 Metis symbol.svg" link="http://en.wikipedia.org/wiki/File:9_Metis_symbol.svg"]] ||
 * [|10 Hygiea] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/10_Hygiea_Astronomical_Symbol.svg/9px-10_Hygiea_Astronomical_Symbol.svg.png width="9" height="20" caption="10 Hygiea Astronomical Symbol.svg" link="http://en.wikipedia.org/wiki/File:10_Hygiea_Astronomical_Symbol.svg"]] ||
 * [|11 Parthenope] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/11_Parthenope_symbol.svg/9px-11_Parthenope_symbol.svg.png width="9" height="19" caption="11 Parthenope symbol.svg" link="http://en.wikipedia.org/wiki/File:11_Parthenope_symbol.svg"]] ||
 * [|12 Victoria] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/6/65/12_Victoria_symbol.svg/16px-12_Victoria_symbol.svg.png width="16" height="19" caption="12 Victoria symbol.svg" link="http://en.wikipedia.org/wiki/File:12_Victoria_symbol.svg"]] ||
 * [|13 Egeria] || Never assigned. ||
 * [|14 Irene] || "A dove carrying an olive-branch, with a star on its head," never drawn. [|[][|4][|]] ||
 * [|15 Eunomia] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/15_Eunomia_symbol.svg/11px-15_Eunomia_symbol.svg.png width="11" height="19" caption="15 Eunomia symbol.svg" link="http://en.wikipedia.org/wiki/File:15_Eunomia_symbol.svg"]] ||
 * [|28 Bellona] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/28_Bellona_symbol.svg/21px-28_Bellona_symbol.svg.png width="21" height="20" caption="28 Bellona symbol.svg" link="http://en.wikipedia.org/wiki/File:28_Bellona_symbol.svg"]] ||
 * [|35 Leukothea] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/93/35_Leukothea_symbol.png/25px-35_Leukothea_symbol.png width="25" height="20" caption="35 Leukothea symbol.png" link="http://en.wikipedia.org/wiki/File:35_Leukothea_symbol.png"]] ||
 * [|37 Fides] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/4/45/37_Fides_symbol.svg/12px-37_Fides_symbol.svg.png width="12" height="19" caption="37 Fides symbol.svg" link="http://en.wikipedia.org/wiki/File:37_Fides_symbol.svg"]] ||
 * [|2060 Chiron] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Chiron_symbol.svg/20px-Chiron_symbol.svg.png width="20" height="20" caption="Chiron symbol.svg" link="http://en.wikipedia.org/wiki/File:Chiron_symbol.svg"]] ||

Naming
Main article: [|Minor planet#Naming] A newly discovered asteroid is given a [|provisional designation] (such as [|2002 AT] ) 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.

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 [|AU] from the Sun by the [|Titius-Bode law], partly because of the discovery, by Sir [|William Herschel] in 1781, of the planet [|Uranus] at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the [|zodiacal] band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers. The first asteroid, [|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] and followed the displacement of this object during several nights. His colleague, [|Carl Friedrich Gauss], used these observations to find 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], 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 [|J. R. Hind], [|Annibale de Gasparis], [|Robert Luther], [|H. M. S. Goldschmidt], [|Jean Chacornac], [|James Ferguson], [|Norman Robert Pogson], [|E. W. Tempel], [|J. C. Watson], [|C. H. F. Peters], [|A. Borrelly], [|J. Palisa], the [|Henry brothers] and [|Auguste Charlois]. In 1891, however, [|Max Wolf] pioneered the use of [|astrophotography] to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with [|323 Brucia], whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies", a phrase due to [|Edmund Weiss]. Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting
Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was [|photographed] by a wide-field [|telescope], or [|Astrograph]. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two [|films] of the same region were viewed under a [|stereoscope]. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations. These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a [|provisional designation], made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74 ). The last step of discovery is to send the locations and time of observations to the [|Minor Planet Center], where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the [|International Astronomical Union].

Computerized methods
[|2004 FH] is the center dot being followed by the sequence; the object that flashes by during the clip is an [|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 asteroids]). The three most important groups of [|near-Earth asteroids] are the [|Apollos], [|Amors], and [|Atens]. Various [|asteroid deflection strategies] have been proposed, as early as the 1960s. The [|near-Earth] asteroid [|433 Eros] had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: [|1221 Amor], [|1862 Apollo], [|2101 Adonis], and finally [|69230 Hermes], which approached within 0.005 [|AU] of the [|Earth] in 1937. Astronomers began to realize the possibilities of Earth impact. Two events in later decades increased the alarm: the increasing acceptance of [|Walter Alvarez]' hypothesis that an [|impact event] resulted in the [|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 these considerations helped spur the launch of highly efficient automated systems that consist of 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: The LINEAR system alone has discovered 97,470 asteroids, as of September 18, 2008. Among all the automated systems, 4711 near-Earth asteroids have been discovered including over 600 more than 1 km (0.6 mi) in diameter. The rate of discovery peaked in 2000, when 38,679 minor planets were numbered, and has gone down steadily since then (719 minor planets were numbered in 2007).
 * The [|Lincoln Near-Earth Asteroid Research] (LINEAR) team
 * The [|Near-Earth Asteroid Tracking] (NEAT) team
 * [|Spacewatch]
 * The [|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)

Terminology
Traditionally, small bodies orbiting the Sun were classified as asteroids, [|comets] or [|meteoroids], with anything smaller than ten metres across being called a meteoroid. The term "asteroid" is ill-defined. It never had a formal definition, with the broader term [|minor planet] being preferred by the [|International Astronomical Union] from 1853 on. In 2006, the term "[|small Solar System body]" was introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "[|planetesimal]" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist [|William Daniel Conybeare] to describe minor planets, but is not in common use. When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to [|sublimation] of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface [|volatile ices] and become 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. For almost two centuries, from the discovery of the first asteroid, [|Ceres], in 1801 until the discovery of the first [|centaur], [|2060 Chiron], in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as [|944 Hidalgo] 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 [|centaurs], 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 [|trans-Neptunian object], [|1992 QB1], was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: [|Kuiper belt object], [|trans-Neptunian object], [|scattered-disc object], and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids. The innermost of these are the [|Kuiper belt objects], called "objects" partly to avoid the need to classify them as asteroids or comets. They are believed to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional [|comet nuclei]. (The much more distant [|Oort cloud] is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the [|Stardust] probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line. The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the [|main asteroid belt], [|Jupiter trojans], and [|near-Earth objects]. When the IAU introduced the class [|small solar system bodies] in 2006 to include most objects previously classified as minor planets and comets, they created the class of [|dwarf planets] for the largest minor planets—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." Currently only the largest object in the asteroid belt, [|Ceres], at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids ([|Vesta], [|Pallas], and [|Hygiea]) that may be classified as dwarf planets when their shapes are better known.

Formation
It is believed that [|planetesimals] in the main asteroid belt evolved much like the rest of the [|solar nebula] until Jupiter neared its current mass, at which point excitation from [|orbital resonances] with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. At least two asteroids, Ceres and Vesta, grew large enough to melt and [|differentiate], with heavy metallic elements sinking to the core, leaving rocky minerals in the crust. In the [|Nice model], many [|Kuiper Belt objects] are captured in the outer Main Belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the [|D-type asteroids], and possibly include Ceres.

Distribution within the Solar System
The [|Main asteroid belt] (white) and the [|Trojan asteroids] (green) Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Significant populations include;

Main asteroid belt
Main article: [|Asteroid belt] The majority of known asteroids orbit within the main asteroid belt between the orbits of [|Mars] and [|Jupiter], generally in relatively low-[|eccentricity] (i.e., not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, and millions of smaller ones. These asteroids may be remnants of the [|protoplanetary disk], and in this region the [|accretion] of [|planetesimals] into planets during the formative period of the solar system was prevented by large gravitational perturbations by [|Jupiter].

Trojans
Main article: [|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 [|Lagrangian points] of stability, [|L4 and L5], which lie 60° ahead of and behind the larger body. The most significant population of Trojan asteroids are the [|Jupiter Trojans]. Although fewer Jupiter Trojans have been discovered as of 2010, it is thought that there are as many as there are asteroids in the main belt. A couple [|trojans] have also been found orbiting with [|Mars].

Near-Earth asteroids
Main article: [|Near-Earth asteroids] Near-Earth asteroids, or NEA's, 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.

Size distribution
Asteroid [|Vesta] (left) and dwarf planet [|Ceres] (center), with Earth's [|Moon] (right) shown to scale Objects in the main asteroid belt vary greatly in size, from almost 1000 kilometres for the largest down to rocks just tens of metres across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving [|protoplanets]. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving [|planetesimals] or fragments of larger bodies. The relative masses of the nine largest main-belt asteroids for which precise data is available, compared to the remaining mass of the main belt.[|[][|32][|]][|[][|33][|]] The [|dwarf planet] [|Ceres] is the largest object in the asteroid belt, with a diameter of 975 km (610 mi). The next largest are the asteroids [|2 Pallas] and [|4 Vesta], both with diameters of just over 500 km (300 mi). Normally Vesta is the only main belt asteroid that can, on occasion, become visible to the naked eye. However, on some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see [|99942 Apophis]. The mass of all the objects of the [|Main asteroid belt], lying between the orbits of [|Mars] and [|Jupiter], is estimated to be about 3.0-3.6 × 1021 kg, or about 4 percent of the mass of the Moon. Of this, [|Ceres] comprises 0.95 × 1021 kg, some 32 percent of the total.[|[][|34][|]][|[][|35][|]] Adding in the next three most massive objects, [|Vesta] (9%), [|Pallas] (7%), and [|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. The number of asteroids decreases markedly with size. Although this generally follows a [|power law], there are 'bumps' at 5 km and 100 km, where more asteroids than expected from a [|logarithmic distribution] are found. Approximate number of asteroids N larger than diameter D||~ D || 100 m || 300 m || 500 m || 1 km || 3 km || 5 km || 10 km || 30 km || 50 km || 100 km || 200 km || 300 km || 500 km || 900 km ||
 * [|Ceres] [|4 Vesta] [|2 Pallas] [|10 Hygiea] [|704 Interamnia] || [|511 Davida] [|15 Eunomia] [|3 Juno] [|16 Psyche] All others ||
 * ~ N || ~25,000,000 || 4,000,000 || 2,000,000 || 750,000 || 200,000 || 90,000 || 10,000 || 1100 || 600 || 200 || 30 || 5 || 3 || 1 ||

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 nickel-iron core, [|olivine] mantle, and basaltic crust. [|10 Hygiea], however, which appears to have a uniformly primitive composition of [|carbonaceous] [|chondrite], is thought to be the largest undifferentiated asteroid. Many, perhaps most, of the smaller asteroids are piles of rubble held together loosely by gravity. Some have [|moons] or are co-orbiting [|binary asteroids]. The rubble piles, moons, binaries, and scattered [|asteroid families] are believed to be the results of collisions that disrupted a parent asteroid. Asteroids contain traces of amino-acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth. (See also [|panspermia].) 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. The orbits of asteroids are often influenced by the gravity of other bodies in the solar system or the [|Yarkovsky effect].

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

Orbital classification
Main articles: [|Asteroid group] and [|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. Families have only been recognized within the [|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 plutoid [|dwarf planet] [|Haumea].

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

Spectral classification
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 m (115 ft) across can be seen. Main article: [|Asteroid spectral types] In 1975, an asteroid [|taxonomic] system based on [|colour], [|albedo], and [|spectral shape] was developed by [|Clark R. Chapman], [|David Morrison], and [|Ben Zellner]. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: [|C-types] for dark carbonaceous objects (75% of known asteroids), [|S-types] for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied. The two most widely used taxonomies now used are the [|Tholen classification] and [|SMASS classification]. The former was proposed in 1984 by [|David J. Tholen], and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the [|M-type]. There are also several smaller classes. 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. 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, 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
Vesta, imaged by the Hubble Space Telescope [|951 Gaspra] is the first asteroid to be imaged in close-up. Until the age of [|space travel], objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting [|Hubble Space Telescope] can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their [|light curves] (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occulations (when an asteroid passes directly in front of a star). [|Radar] imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. The first close-up photographs of asteroid-like objects were taken in 1971 when the [|Mariner 9] probe imaged [|Phobos] and [|Deimos], the two small moons of [|Mars], which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the [|Voyager] probes of the small moons of the [|gas giants]. The first true asteroid to be photographed in close-up was [|951 Gaspra] in 1991, followed in 1993 by [|243 Ida] and its moon [|Dactyl], all of which were imaged by the [|Galileo probe] en route to [|Jupiter]. The first dedicated asteroid probe was [|NEAR Shoemaker], which photographed [|253 Mathilde] in 1997, before entering into orbit around [|433 Eros], finally landing on its surface in 2001. Other asteroids briefly visited by spacecraft en route to other destinations include [|9969 Braille] (by [|Deep Space 1] in 1999), and [|5535 Annefrank] (by [|Stardust] in 2002). In September 2005, the Japanese [|Hayabusa] probe started studying [|25143 Itokawa] in detail and was plagued with difficulties, but returned samples of its surface to earth on June 13, 2010. The European [|Rosetta probe] (launched in 2004) flew by [|2867 Šteins] in 2008 and [|21 Lutetia], the largest asteroid visited to date, in 2010. In September 2007, [|NASA] launched the [|Dawn Mission], which will orbit the [|protoplanet] [|4 Vesta] in 2011 and the [|dwarf planet] [|Ceres] in 2015. It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on earth ([|asteroid mining]), or materials for constructing [|space habitats] (see [|Colonization of the asteroids]). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for [|space manufacturing] and construction.

Fiction
Main article: [|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.