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and Jupiter.The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter where 98.5% of the known minor planets' orbits can be found.This value was obtained by a simple count up of all bodies in that region using data for 120437 numbered minor planets from the Minor Planet Center orbit database, dated February 8, 2006. Asteroids, or minor planets, are small celestial bodies composed of rock, ice, and some metal that orbit the Sun. This region is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts.

The asteroid belt formed from the primordial solar nebula as a group of planetesimals—the smaller precursors of the planets. However, gravity perturbations by Jupiter impart too much orbital energy to the bodies in this region for them to accretion (astrophysics) into a planet during collisions. Instead, the initial planetesimals have been broken up during the collisions, and the majority of the mass has been lost from this region since the formation of the Solar System. Some fragments from such collisions can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably Perturbation (astronomy) whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into different orbits.

The majority of the mass within the main belt is contained in the largest asteroids. The three largest asteroids in the main belt (individually named 4 Vesta, 2 Pallas and 10 Hygiea) have mean diameters of more than 400 km, while the main belt's only dwarf planet, Ceres (dwarf planet), is about 950 km in diameter. Together these four objects make up nearly half of the total mass in the belt.The remainder form a distribution of smaller bodies that range down to the size of a particle of dust. The asteroid material is so thinly distributed however, that multiple unmanned spacecraft have traversed the belt without incident. Asteroids within the main belt are categorized by their spectrum, and the majority can be grouped into three basic types: carbonaceous (C-type asteroid), silicate (S-type asteroid), and metal-rich (M-type asteroid). Collisions between large asteroids can form an asteroid family, whose members possess similar orbital characteristics and composition. Collisions also produce a fine dust that forms a major component of the zodiacal light.

History of observation .In 1766, the astronomer Johann Daniel Titius von Wittenburg, drawing on the work of earlier writers, such as Christian Wolff and Charles Bonnet, noted an apparent pattern in the layout of the planets. If one began a numerical sequence at 0, then 3, then 6, than 12, than 24, then 48, etc. doubling each time, and then added four to each number and divided by 10, this produced a remarkably close approximation to the orbits of the known planets as measured in astronomical units. This pattern, known as the Titius-Bode Law, predicted the semi-major axes of the six planets of the time (Mercury, Venus, Earth, Mars Jupiter and Saturn) provided one allowed for a "gap" between the orbits of Mars and Jupiter. In 1768, the astronomer Johann Elert Bode made note of Titius's relationship in his Anleitung zur Kenntniss des gestirnten Himmels (though he did not credit Titius, which led to many later referring to it as "Bode's law"), and declared, "Can one believe that the Founder of the universe had left this space empty? Certainly not." When William Herschel discovered Uranus in 1781, the planet's position matched the law almost perfectly, leading astronomers to conclude that there had to be a planet between the orbits of Mars and Jupiter.

In 1800, astronomer Franz Xavier von Zach recruited a number of his fellows into an informal club he dubbed the "Lillienthal Society". Determined to bring the Solar System to order, the group came to be known as the "Himmelspolitzei", or Celestial Police, and eventually included such noted members as Herschel, British astronomer Royal Nevil Maskelyne, Charles Messier and Heinrich Olbers. However, only a few months later, on January 1, 1801, Giuseppe Piazzi, Chair of Astronomy at the University of Palermo, Sicily, who was not a member of the Celestial Police, found, in the exact location predicted by the Titius-Bode Law, a tiny moving object he dubbed Ceres (dwarf planet) after the Ceres (mythology) of the harvest and patron of Sicily. Piazzi initially believed it a comet, but its lack of a coma suggested it was a planet.

Fifteen months later, Olbers discovered a second object in the same region, 2 Pallas, prompting him to suggest to William Herschel that these bodies were the remnants of a destroyed planet. By 1807, further investigation revealed two new planets, 3 Juno and 4 Vesta.{{cite web | author=Staff | year=2002 | url = http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_06.asp | title = Astronomical Serendipity | publisher = NASA JPL | accessdate = 2007-04-20 --> Because of their star-like appearance, William Herschel suggested these objects be named asteroids, after the Greek language root aster- meaning star.{{cite web | author= DeForest, Jessica | date = | url = http://www.msu.edu/~defores1/gre/roots/gre_rts_afx2.htm | title = Greek and Latin Roots | publisher = Michigan State University | accessdate = 2007-07-25 --> However, for several decades it remained common practice to refer to these four objects as planets.

The Napoleonic wars brought the first period of discovery in this region to a close, and it would take until 1845 before another object (5 Astraea) wasdiscovered. Shortly thereafter, however, new objects were found at an increasing rate, and counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list and William Herschel's name for them, asteroids, at last fell into common use. The discovery of Neptune in 1846 led to the eventual discredit of the Titius-Bode Law in the eyes of scientists, as Neptune's orbit was nowhere near the predicted position. To date, no scientific explanation for the Law, and the consensus among astronomers is that it is a coincidence.

By mid-1868, 100 asteroids had been located, and the introduction of astrophotography in 1891 by Max Wolf accelerated the rate of discovery.{{cite web | first=David W. | last=Hughes | year=2007 | url = http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/history.html | title = A Brief History of Asteroid Spotting | publisher = BBC | accessdate = 2007-04-20 --> A total of 1,000 asteroids had been found by 1923, 10,000 by 1951, and 100,000 by 1982.{{cite web | first=Donald K. | last=Yeomans | date = July 13, [ | url = http://ssd.jpl.nasa.gov/sbdb.cgi | title = JPL Small-Body Database Browser | publisher = NASA JPL | accessdate = 2007-04-25 --> — Asteroids are numbered by order of discovery. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies' orbits from the Sun. These gaps were located at positions where their period of revolution about the Sun was an integer fraction of Jupiter's orbital period. Kirkwood proposed that the gravitational Perturbation (astronomy) of Jupiter led to the removal of asteroids from these orbits.

The Japanese astronomer Kiyotsugu Hirayama noticed in 1918 that the orbits of some of the asteroids had similar parameters, forming families or groups. In the 1970s, examination of asteroid colors led to a classification system. The three most common categories were designated C-type asteroid (carbonaceous), S-type asteroid (Silicate) and M-type asteroid (metallic).{{cite web | first=David W. | last=Hughes | year=2007 | url =http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/finding.html | title = Finding Asteroids In Space | publisher = BBC | accessdate = 2007-04-20 -->

In 2006 it was announced that a population of comets had been discovered within the asteroid belt. It has been suggested that comets such as these may have provided a source of water for the formation of the Earth's oceans. According to some models, there was insufficient outgassing of water during the Earth's formulative period to form the oceans, requiring the introduction of an external source such as a cometary bombardment.{{cite web | last = Lakdawalla | first = Emily | date = April 28, [ | url = http://www.planetary.org/blog/article/00000551/ | title = Discovery of a Whole New Type of Comet | publisher = The Planetary Society | accessdate = 2007-04-20 -->

Origin Formation Phaeton (hypothetical planet) of the asteroid belt's origins was that it was originally a planet that was somehow shattered. However, over time this hypothesis has fallen from favor, due to a number of key problems. One is the large amount of energy which would be required to achieve this kind of effect. Another is the low combined mass of the current asteroid belt, which has only a small fraction of the mass of the Earth's moon. Finally, the significant chemical differences between the asteroids is difficult to explain if they come from the same planet.{{cite web | author=Masetti, M.; Mukai, K. | date=December 1, [ | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980810a.html | title=Origin of the Asteroid Belt | publisher=NASA Goddard Spaceflight Center | accessdate=2007-04-25 --> Today, most scientists accept the hypothesis that rather than fragmenting from an original planet, the asteroids never formed a planet at all.

Planetary formation is thought to have occurred via a process comparable to the long-standing nebular hypothesis, which states that a cloud of interstellar dust and gas collapsed under the influence of gravity to form a rotating disk of material that then further condensed to form the Sun and planets.{{cite web | last = Watanabe | first = Susan | date = July 20, [ | url =http://www.jpl.nasa.gov/news/features.cfm?feature=520 | title =Mysteries of the Solar Nebula | publisher = NASA | accessdate = 2007-04-02 --> During the first few million years of the Solar System's history, an Accretion (astrophysics) process of sticky collisions caused clumping together of small particles, formation of larger clumps, and the gradual increase of the size of these bodies. Once the objects reached sufficient mass they could draw in other bodies through gravitational attraction, and become known as planetesimals. The gravitational accretion of these planetesimals led to the formation of the rocky planets and to the gas giants.

In regions where the average velocity of the collisions was too high, the shattering of planetesimals tends to dominate over accretion, preventing the formation of planet-sized bodies. When the orbital period of a planetismal forms an integer fraction of the orbital period of Jupiter, an [orbital resonance occurs that can perturb the object into a different orbit. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter. As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's planetismal population in the process—increasing their velocities relative to each other.{{cite conference | first = E. R. D. | last = Scott | title=Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids | booktitle = Proceedings 37th Annual Lunar and Planetary Science Conference | publisher = Lunar and Planetary Society | date = March 13-17, 2006 | location = League City, Texas | url =http://adsabs.harvard.edu/abs/2006LPI....37.2367S | accessdate = 2007-04-16 --> Planetesimals in this region were (and continue to be) too strongly Perturbation (astronomy) to form a planet. Instead the planetesimals orbit the Sun as before and occasionally collide. The asteroid belt can be considered a relic of the primitive Solar System.

When the main belt was first being formed, the temperatures at a distance of 2.7 A.U. from the Sun formed a "snow line" where the temperatures fell below the condensation point of water. (1 A.U., or astronomical unit, equals the average distance between the Earth and the Sun.) Planetismals formed beyond this radius were able to accumulate ice. [Main-belt comets formed within the belt outside the snow line, and these are a leading candidate for the deposition of water to form the Earth's oceans.{{cite news | first=Phil | last=Berardelli | title=Main-Belt Comets May Have Been Source Of Earths Water | publisher=Space Daily | date=March 23, [ | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2007-04-11 -->

However, a recent hypothesis has partially revived the old "Fifth planet (hypothetical)" model for the asteroids' formation. A 2002 paper suggested that a Planet V formed among the inner planets, but the orbit was destabilized so that it began crossing the inner asteroid belt. As a result of this transition, a number of asteroids would have been ejected from the belt. Later this planet was either absorbed by the Sun or ejected from the system.{{cite web | last = David | first = Leonard | date = March 18, [ | url = http://www.space.com/scienceastronomy/solarsystem/fifth_planet_020318.html | title =Long-Destroyed Fifth Planet May Have Caused Lunar Cataclysm | publisher =Space.com | accessdate = 2007-04-25 -->

Evolution The current asteroid belt is believed to contain only a small fraction (by mass) of the primordial asteroid belt. Based on computer simulations, the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of this material was ejected from the belt within a period of about a million years of formation, leaving behind less than 0.1% of the original mass.

Since their formation, the size distribution of the asteroid belt has remained relatively stable. That is, there has not been a significant increase or decrease in the typical dimensions of the main belt asteroids.{{cite news | first=Lori | last=Stiles | title=Asteroids Caused the Early Inner Solar System Cataclysm | publisher=University of Arizona News |date=September 15, [ | url=http://uanews.org/cgi-bin/WebObjects/UANews.woa/7/wa/SRStoryDetails?ArticleID=11692 | accessdate=2007-04-18 --> However, the asteroids have been affected by many subsequent processes, such as internal heating (in the first few tens of millions of years), surface melting from impacts, and [space weathering from radiation and bombardment by [micrometeorites. Hence, the asteroids themselves are not pristine samples of the early Solar System. By contrast, the objects in the outer [Kuiper belt are believed to have experienced much less change since the Solar System's formation.

The 4:1 orbital resonance with Jupiter, at a radius 2.06 astronomical unit, can be considered the inner boundary of the main belt. Perturbations by Jupiter send bodies straying there onto unstable orbits. Also, most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion out at 1.67 A.U.) or ejected by its gravitational perturbations in the early history of the Solar System.{{cite web | author=Alfvén, H.; Arrhenius, G. | year=1976 | url =http://history.nasa.gov/SP-345/ch4.htm | title =The Small Bodies | work=SP-345 Evolution of the Solar System | publisher = NASA | accessdate = 2007-04-12 --> An exception are the high inclination [Hungaria asteroids which lie slightly closer to the Sun, and were protected from these disturbances by this high inclination.

Characteristics , the first ever imaged by a spacecraft, taken by Galileo (spacecraft) as it passed by it in 1991Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff that is assumed. Over 200 asteroids are known to be larger than 100 kilometre,{{cite web | last = Yeomans | first = Donald K. | date = April 26, [ | url = http://ssd.jpl.nasa.gov/sbdb_query.cgi | title = JPL Small-Body Database Search Engine | publisher = NASA JPL | accessdate = 2007-04-26 --> — search for asteroids in the main belt regions with a diameter >100. while a survey in the infrared wavelengths shows that the main belt has 700,000 to 1.7 million asteroids with a diameter of 1 km or more. The [absolute magnitudes of most of the known asteroids are 11–19, with the median at about 16.

Mass The total mass of the asteroid belt is estimated to be 3.0-3.6 kilograms, which is 4% of the Earth's Moon. By comparison, the Trojan asteroid, two swarms of asteroids located at Jupiter's Lagrange point, to date have revealed a collective mass roughly half that of the asteroid belt,cite web|title=POPULATION AND SIZE DISTRIBUTION OF SMALL JOVIAN TROJAN ASTEROIDS|author=DAVID C. JEWITT AND CHADWICK A. TRUJILLO |year=2007|url=http://www.journals.uchicago.edu/AJ/journal/issues/v120n2/200007/200007.html?erFrom=6876019551608635237Guest|accessdate=2007-10-18--> while the Kuiper belt, a second belt beyond the orbit of Neptune discovered in 1992, may possess a mass up to 200 times its mass. The hypothetical Oort cloud, a great spherical swarm of trillions of cometary bodies estending to beyond 50,000 AU, may possess a total mass six thousand times that of the asteroid belt.

Composition During the early history of the Solar System, minor planets underwent some degree of melting, allowing elements to be partially or completely segregated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. However, because of the relatively small size of these bodies, this period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago, that is in the first few tens to a hundred million years.

is a carbonaceous chondrite meteorite that fell to Earth in Mexico, 1969.The current belt consists primarily of three categories of asteroids. In the outer portion of the belt, closer to Jupiter's orbit, carbon-rich asteroids predominate. These [C-type asteroid ([carbonaceous) asteroids include over 75% of the visible asteroids. They are more red in hue than the other asteroid categories and have a very low [albedo. Their surface composition is similar to [carbonaceous chondrite [meteorites. Chemically, their spectra indicate a match with the primordial composition of the early Solar System, with the lighter elements and volatiles (''e.g.'' ices) removed.

Toward the inner portion of the belt, within 2.5 A.U. of the Sun, S-type asteroid (silicate) chondrite asteroids are more common. The spectra of their surfaces reveal the presence of silicates as well as some metal, but no significant carbonaceous compounds. This indicates that they are made of materials that have been significantly modified from the primordial Solar System composition. The expected mechanism was melting early in their history, which caused mass differentiation. They have a relatively high albedo, and form about 17% of the total asteroid population.

A third category of asteroids, forming about 10% of the total population, is the M-type asteroid (metal-rich). These have a spectrum that resembles metallic iron-nickel, with a white or slightly red appearance and no absorption features in the spectrum. Some M-type asteroids are believed to be formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, there are also some silicate compounds that can produce a similar appearance. Thus, for example, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 A.U.{{cite web | last = Lang | first = Kenneth R. | year=2003 | url = http://ase.tufts.edu/cosmos/print_images.asp?id=15 | title = Asteroids and meteorites | publisher = NASA's Cosmos | accessdate = 2007-04-02 --> Overall it is not yet clear whether all M-types are compositionally similar, or whether it is a label for several varieties which do not fit neatly into the main C and S classes.

One mystery of the asteroid belt is the relative rarity of V-type asteroid, or basaltic asteroids. Theories of asteroid formation predict that objects the size of Vesta or larger should form crusts and mantles, which would be composed mainly of basaltic rock, resulting on more than half of all asteroids being composed either of basalt or olivine. Observations however suggest that 99 percent of the predicted basaltic material is missing. Until 2001, most basaltic bodies discovered in the asteroid belt were believed to originate from the asteroid Vesta (hence their name V-type). However, the discovery of the asteroid (1459) Magnya revealed a slightly different chemical composition to the other basaltic asteroids discovered til then, suggesting a different origin.

The diversity of known basaltic meteorites mandated a diversity in their origins, and in 2007 two asteroids in the outer belt, (7472) Kumakiri and (10537) 1991 RY16, were found to possess a basaltic composition, though they could not have originated from Vesta. They are the only V-type asteroids discovered in the outer belt to date.

The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73°C) at 2.2 A.U. down to 165 K (-108°C) at 3.2 A.U. However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.

Orbits and rotations The large majority of the asteroids within the main belt have orbital eccentricity of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids peak at an eccentricity of around 0.07 and an inclination of less than 4°.{{cite web | last = Williams | first = Gareth |date=April 3, [ | url = http://cfa-www.harvard.edu/iau/lists/MPDistribution.html | title = Distribution of the Minor Planets | publisher = Minor Planets Center | accessdate = 2007-04-15 --> Thus while a typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic, some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.

Sometimes, the term main belt is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 astronomical unit, and at eccentricity (orbit) less than roughly 0.33, along with orbital inclinations below about 20°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.

Measurements of the rotation periods of large asteroids in the main belt show that there is a lower limit. No asteroid with a diameter larger than 100 metres has a period of rotation of less than 2.2 hours. For asteroids rotating faster than approximately this rate, the centrifugal force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that the majority of asteroids with a diameter over 100 metres are actually rubble piles formed through accumulation of debris after collisions between asteroids.{{cite web | last = Rossi | first = Alessandro | date = May 20, [ | url = http://spaceguard.esa.int/tumblingstone/issues/current/eng/ast-day.htm | title = The mysteries of the asteroid rotation day | publisher = The Spaceguard Foundation | accessdate = 2007-04-09 -->

Kirkwood gaps in the "core" of the main belt. Cyan arrows point to the Kirkwood gaps, where orbital resonances with Jupiter destabilize orbits.The semi-major axis of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's orbital period. When considering the semi-major axes of all asteroids, the main belt contains noticeable gaps, called Kirkwood gaps, in its distribution. They occur at the radii at which the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter. This results in mean-motion resonance with the gas giant that is sufficient to perturb an asteroid to new orbital elements. In effect, asteroids that become located in such gap orbits (either primordially because of the migration of Jupiter's orbit, or due to prior perturbations or collisions) are gradually nudged into different, random orbits with a larger or smaller semi-major axis.

However, the gaps are not seen in a simple snapshot of the locations of the asteroids at any one time. This is because asteroid orbits are elliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps is not significantly different than in the neighboring regions.

The main gaps occur at the 3:1, 5:2, 7:3 and 2:1 mean-motion resonances with Jupiter. Thus an asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit. Weaker resonances occur at other values of semi-major axis, such that less asteroids are found with those values than with nearby ones. (For example, a 8:3 resonance for asteroids with a semi-major axis of 2.71 A.U.){{cite conference | first = S. | last = Ferraz-Mello | title = Kirkwood Gaps and Resonant Groups | booktitle = proceedings of the 160th International Astronomical Union | pages = 175-188 | publisher = Kluwer Academic Publishers | date = June 14-18, 1993 | location = Belgirate, Italy | url = http://adsabs.harvard.edu/abs/1994IAUS..160..175F | accessdate = 2007-03-28 -->

The main or "core" population of the asteroid belt is sometimes divided into three zones, based on the most prominent Kirkwood gaps. Zone I lies between the 4:1 resonance (2.06 A.U.) and 3:1 resonance (2.5 A.U.) Kirkwood gaps. Zone II contines from the end of Zone I out to the 5:2 resonance gap (2.82 A.U.). Zone III runs from the outer edge of Zone II to the 2:1 resonance gap (3.28 A.U.).

The main belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 A.U.), and the outer belt formed by those asteroids closer to Jupiter's orbit. (Some authors subdivide the inner and outer belts at the 2:1 resonance gap A.U., while others even define inner, middle and outer belts.)

Largest asteroids See also: List of notable asteroids#Largest known asteroids (out to the orbit of Jupiter)The four largest asteroids are, in decreasing order of mass, 1 Ceres, 4 Vesta, 2 Pallas and 10 Hygiea. Together, they account for almost half of the belt's total mass, with one-third accounted for by Ceres alone.For recent estimates of the masses of Ceres (dwarf planet), 4 Vesta, 2 Pallas and 10 Hygiea, see the references in the infoboxes of their respective articles.

Ceres is the only asteroid large enough for its gravity to force it into a roughly round shape, and so, according to the IAU's 2006 resolution on the definition of planet, is now considered a dwarf planet. Its orbital distance, 2.8 AU, is also the location of the asteroid belt's center of mass. Ceres has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice.{{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--> Like the planets, Ceres is diffrentiated; it has a crust, a mantle and a core.

Vesta, like Ceres, has a diffrentiated interior. Vesta is almost but not quite spherical,P. C. Thomas et al Vesta: Spin Pole, Size, and Shape from HST Images, Icarus, Vol. 128, p. 88 (1997) and the IAU has deferred its decision as to whether or not it qualifies as a dwarf planet. Unlike Ceres however, Vesta formed inside the "frost line", and so is devoid of water.

Collisions , created in part by dust from collisions in the asteroid belt.The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main belt bodies with a mean radius of 10-km are expected to occur about once every 10 million years.{{cite web | last=Backman | first=D. E. | date=March 06, [ | url=http://astrobiology.arc.nasa.gov/workshops/zodiac/backman/backman_toc.html | title=Fluctuations in the General Zodiacal Cloud Density | work=Backman Report | publisher=NASA Ames Research Center | accessdate=2007-04-04 --> A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family). Conversely, collisions that occur at low relative speeds may also join two asteroids together. After more than 4 billion years of such processes, the members of the asteroid belt now bear little resemblance to the original population.

In addition to the asteroid bodies, the main belt also contains bands of dust with particle radii of up to a few hundred micrometres. This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Due to Poynting-Robertson effect, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun.

The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic. Particles that produce the visible zodiacal light average about 40 μm in radius. The typical lifetimes of such particles is on the order of 700,000 years. Thus, in order to maintain the bands of dust, new particles must be steadily produced within the asteroid belt.

Meteorites Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere.{{cite web | last=Kingsley | first=Danny | date=May 1, [ | url=http://abc.net.au/science/news/stories/s843594.htm | title=Mysterious meteorite dust mismatch solved | publisher=ABC Science | accessdate=2007-04-04 --> More than 99.8 percent of the 30,000 meteorites found on Earth to date are believed to have originated in the asteroid belt. A September 2007 study by a joint US-Czech team has suggested that a large-body collision undergone by the asteroid 298 Baptistina sent a number of fragments into the inner solar system. The impacts of these fragments are believed to have created both the Tycho crater on the Moon and the Chicxulub crater in Mexico, the remnant of the massive impact which triggered the extinction of the dinosaurs 65 million years ago.

Families and groups Approximately one third of the asteroids in the main belt are members of an asteroid family. These are asteroids that share similar orbital elements, such as semimajor axis, Orbital eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the main belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families, and likely have a common origin. Additional groupings have been found but these are less certain. Asteroid families can be confirmed when the members display common spectral features.{{cite conference| first=Anne | last=Lemaitre | title=Asteroid family classification from very large catalogues | booktitle=Procedings Dynamics of Populations of Planetary Systems | pages=135-144 | publisher=Cambridge University Press |date=August 31-September 4, 2004 | location=Belgrade, Serbia and Montenegro | url=http://adsabs.harvard.edu/abs/2005dpps.conf..135L | accessdate=2007-04-15 --> Smaller associations of asteroids are called groups or clusters.

Some of the most prominent families in the main belt (in order of increasing semi-major axis) consist of the Flora family, Eunomia family, Koronis family, Eos family and Themis family families. For example, the Flora family, one of the largest, with more than 800 known members, may have formed from a collision less than a billion years ago.{{cite web | last = Martel | first = Linda M. V. | date = March 9, [ | url = http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html | title = Tiny Traces of a Big Asteroid Breakup | publisher = Planetary Science Research Discoveries | accessdate = 2007-04-02 -->The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the Gefion family) is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise the HED meteorites may also have originated from Vesta as a result of this collision.

Three prominent bands of dust have been found within the main belt. These have similar orbital inclinations as the Eos, Koronis and Themis asteroid families, and so are possibly associated with those groupings.

Periphery Skirting the inner edge of the belt (ranging between 1.78 and 2.0 A.U. with a mean semi-major axis of 1.9 A.U.) is the Hungaria family of minor planets. They are named after the main member of this family—434 Hungaria, and the group contains at least 52 named asteroids. The Hungaria group are separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members of this group belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars is a likely factor in reducing the total population of this group.

Another high-inclination group in the inner part of the main belt is the Phocaea family. These are composed primarily of S-type asteroids, where as the neighboring Hungaria family includes some E-type asteroid. The Phocaea family orbit between 2.25 and 2.5 A.U. from the Sun.

Skirting the outer edge of the main belt is the 65 Cybele, orbiting between 3.3 and 3.5 A.U. These have a 7:4 orbital resonance with Jupiter. The Hilda family orbit between 3.5 and 4.2 A.U., and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are relatively few asteroids beyond 4.2 A.U., until reaching Jupiter's orbit. Here the two large groups of Trojan asteroids can be found, although they are not usually considered part of the main asteroid belt.

New families Some asteroid families have formed recently, in astronomical terms. The Karin Cluster apparently formed about 5.7 million years ago from a collision with a 16-km radius progenitor asteroid.{{cite news | title=SwRI researchers identify asteroid breakup event in the main asteroid belt | publisher=SpaceRef.com | date=June 12, [ | url=http://www.spaceref.com/news/viewpr.html?pid=8627 | accessdate=2007-04-15 --> The [490 Veritas formed about 8.3 million years ago, and evidence for this event has been found in the form of interplanetary dust recovered from ocean sediment.{{cite news | first=Maggie | last=McKee | title=Eon of dust storms traced to asteroid smash | publisher=New Scientist Space | date=January 18, [ | url=http://space.newscientist.com/channel/solar-system/comets-asteroids/dn8603 | accessdate=2007-04-15 -->

In the more distant past, the 1270 Datura apparently formed about 450 million years ago from a collision with a main belt asteroid. The age estimate is based on the probability of the members having their current orbits, rather than from any physical evidence. However this cluster may have been a source for some zodiacal dust material. Other recent cluster formations, such as the [4652 Iannini (circa 1–5 million years ago), may have provided additional sources of this asteroid dust.

Exploration image.The first spacecraft to traverse the asteroid belt was Pioneer 10, after entering the belt region on July 16, 1972. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft. Since that time though the belt has been safely traversed by 9 Earth-based craft without incident. These craft include Pioneer 11, Voyager program, Galileo (spacecraft) (which imaged the asteroid 951 Gaspra in 1991 and 243 Ida in 1993}, Cassini–Huygens (which imaged 2685 Masursky in 2000), NEAR Shoemaker (which imaged 253 Mathilde in 1997), Stardust (spacecraft) (which imaged 5535 Annefrank in 2002), New Horizons (which imaged 132524 APL in 2006) and Ulysses probe. Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.{{cite news | first=Alan | last=Stern | title=New Horizons Crosses The Asteroid Belt | publisher=Space Daily | date=June 2, [ | url=http://www.spacedaily.com/reports/New_Horizons_Crosses_The_Asteroid_Belt.html | accessdate=2007-04-14 -->

All spacecraft images of belt asteroids to date have come from brief planetary flyby opportunities by probes headed for other targets. Only the NEAR and Hayabusa missions have studied asteroids for a protracted period in orbit and at the surface, and these were used to study near-Earth asteroids. However, the Dawn Mission has been being dispatched to explore 4 Vesta and Ceres (dwarf planet) in the main belt. If the probe is still operational after examining these two large bodies, an extended mission is possible that could allow additional exploration.{{cite web | author=Staff | date=April 10, [ | url = http://dawn.jpl.nasa.gov/ | title = Dawn Mission Home Page | publisher = NASA JPL | accessdate = 2007-04-14 -->

See also

References Further reading | first=Linda T. | last=Elkins-Tanton | year=2006 | title=Asteroids, Meteorites, and Comets | edition=First edition | publisher=Chelsea House | location=New York | id=ISBN 0-8160-5195-X -->

External links | author=Staff | date=October 31, [ | url = http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html | title = Asteroids | publisher = NASA | accessdate = 2007-04-20 --> | last = Munsell | first = Kirk | date = September 16, 2005 | url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids Asteroids Page | title = Asteroids: Overview | publisher = http://solarsystem.nasa.gov NASA's Solar System Exploration | accessdate = 2007-05-26 --> | last = Arnett | first = William A. | date = February 26, [ | url = http://www.nineplanets.org/asteroids.html | title = Asteroids | publisher = The Nine Planets | accessdate = 2007-04-20 --> | url = http://www.solstation.com/stars/asteroid.htm | title = Main Asteroid Belt | publisher = Sol Company | accessdate = 2007-04-20 --> | last = Hsieh | first = Henry H. | date = March 1, [ | url = http://www.ifa.hawaii.edu/~hsieh/mbcs.html | title = Main-Belt Comets | publisher = University of Hawaii | accessdate = 2007-04-20 --> | author=Staff | year = 2007 | url = http://www.planetary.org/explore/topics/asteroids_and_comets/facts.html | title = Space Topics: Asteroids and Comets | publisher = The Planetary Society | accessdate = 2007-04-20 -->





and Jupiter.The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter where 98.5% of the known minor planets' orbits can be found.This value was obtained by a simple count up of all bodies in that region using data for 120437 numbered minor planets from the Minor Planet Center orbit database, dated February 8, 2006. Asteroids, or minor planets, are small celestial bodies composed of rock, ice, and some metal that orbit the Sun. This region is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts.

The asteroid belt formed from the primordial solar nebula as a group of planetesimals—the smaller precursors of the planets. However, gravity perturbations by Jupiter impart too much orbital energy to the bodies in this region for them to accretion (astrophysics) into a planet during collisions. Instead, the initial planetesimals have been broken up during the collisions, and the majority of the mass has been lost from this region since the formation of the Solar System. Some fragments from such collisions can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably Perturbation (astronomy) whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into different orbits.

The majority of the mass within the main belt is contained in the largest asteroids. The three largest asteroids in the main belt (individually named 4 Vesta, 2 Pallas and 10 Hygiea) have mean diameters of more than 400 km, while the main belt's only dwarf planet, Ceres (dwarf planet), is about 950 km in diameter. Together these four objects make up nearly half of the total mass in the belt.The remainder form a distribution of smaller bodies that range down to the size of a particle of dust. The asteroid material is so thinly distributed however, that multiple unmanned spacecraft have traversed the belt without incident. Asteroids within the main belt are categorized by their spectrum, and the majority can be grouped into three basic types: carbonaceous (C-type asteroid), silicate (S-type asteroid), and metal-rich (M-type asteroid). Collisions between large asteroids can form an asteroid family, whose members possess similar orbital characteristics and composition. Collisions also produce a fine dust that forms a major component of the zodiacal light.

History of observation .In 1766, the astronomer Johann Daniel Titius von Wittenburg, drawing on the work of earlier writers, such as Christian Wolff and Charles Bonnet, noted an apparent pattern in the layout of the planets. If one began a numerical sequence at 0, then 3, then 6, than 12, than 24, then 48, etc. doubling each time, and then added four to each number and divided by 10, this produced a remarkably close approximation to the orbits of the known planets as measured in astronomical units. This pattern, known as the Titius-Bode Law, predicted the semi-major axes of the six planets of the time (Mercury, Venus, Earth, Mars Jupiter and Saturn) provided one allowed for a "gap" between the orbits of Mars and Jupiter. In 1768, the astronomer Johann Elert Bode made note of Titius's relationship in his Anleitung zur Kenntniss des gestirnten Himmels (though he did not credit Titius, which led to many later referring to it as "Bode's law"), and declared, "Can one believe that the Founder of the universe had left this space empty? Certainly not." When William Herschel discovered Uranus in 1781, the planet's position matched the law almost perfectly, leading astronomers to conclude that there had to be a planet between the orbits of Mars and Jupiter.

In 1800, astronomer Franz Xavier von Zach recruited a number of his fellows into an informal club he dubbed the "Lillienthal Society". Determined to bring the Solar System to order, the group came to be known as the "Himmelspolitzei", or Celestial Police, and eventually included such noted members as Herschel, British astronomer Royal Nevil Maskelyne, Charles Messier and Heinrich Olbers. However, only a few months later, on January 1, 1801, Giuseppe Piazzi, Chair of Astronomy at the University of Palermo, Sicily, who was not a member of the Celestial Police, found, in the exact location predicted by the Titius-Bode Law, a tiny moving object he dubbed Ceres (dwarf planet) after the Ceres (mythology) of the harvest and patron of Sicily. Piazzi initially believed it a comet, but its lack of a coma suggested it was a planet.

Fifteen months later, Olbers discovered a second object in the same region, 2 Pallas, prompting him to suggest to William Herschel that these bodies were the remnants of a destroyed planet. By 1807, further investigation revealed two new planets, 3 Juno and 4 Vesta.{{cite web | author=Staff | year=2002 | url = http://dawn.jpl.nasa.gov/DawnCommunity/flashbacks/fb_06.asp | title = Astronomical Serendipity | publisher = NASA JPL | accessdate = 2007-04-20 --> Because of their star-like appearance, William Herschel suggested these objects be named asteroids, after the Greek language root aster- meaning star.{{cite web | author= DeForest, Jessica | date = | url = http://www.msu.edu/~defores1/gre/roots/gre_rts_afx2.htm | title = Greek and Latin Roots | publisher = Michigan State University | accessdate = 2007-07-25 --> However, for several decades it remained common practice to refer to these four objects as planets.

The Napoleonic wars brought the first period of discovery in this region to a close, and it would take until 1845 before another object (5 Astraea) wasdiscovered. Shortly thereafter, however, new objects were found at an increasing rate, and counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list and William Herschel's name for them, asteroids, at last fell into common use. The discovery of Neptune in 1846 led to the eventual discredit of the Titius-Bode Law in the eyes of scientists, as Neptune's orbit was nowhere near the predicted position. To date, no scientific explanation for the Law, and the consensus among astronomers is that it is a coincidence.

By mid-1868, 100 asteroids had been located, and the introduction of astrophotography in 1891 by Max Wolf accelerated the rate of discovery.{{cite web | first=David W. | last=Hughes | year=2007 | url = http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/history.html | title = A Brief History of Asteroid Spotting | publisher = BBC | accessdate = 2007-04-20 --> A total of 1,000 asteroids had been found by 1923, 10,000 by 1951, and 100,000 by 1982.{{cite web | first=Donald K. | last=Yeomans | date = July 13, [ | url = http://ssd.jpl.nasa.gov/sbdb.cgi | title = JPL Small-Body Database Browser | publisher = NASA JPL | accessdate = 2007-04-25 --> — Asteroids are numbered by order of discovery. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies' orbits from the Sun. These gaps were located at positions where their period of revolution about the Sun was an integer fraction of Jupiter's orbital period. Kirkwood proposed that the gravitational Perturbation (astronomy) of Jupiter led to the removal of asteroids from these orbits.

The Japanese astronomer Kiyotsugu Hirayama noticed in 1918 that the orbits of some of the asteroids had similar parameters, forming families or groups. In the 1970s, examination of asteroid colors led to a classification system. The three most common categories were designated C-type asteroid (carbonaceous), S-type asteroid (Silicate) and M-type asteroid (metallic).{{cite web | first=David W. | last=Hughes | year=2007 | url =http://www.open2.net/sciencetechnologynature/planetsbeyond/asteroids/finding.html | title = Finding Asteroids In Space | publisher = BBC | accessdate = 2007-04-20 -->

In 2006 it was announced that a population of comets had been discovered within the asteroid belt. It has been suggested that comets such as these may have provided a source of water for the formation of the Earth's oceans. According to some models, there was insufficient outgassing of water during the Earth's formulative period to form the oceans, requiring the introduction of an external source such as a cometary bombardment.{{cite web | last = Lakdawalla | first = Emily | date = April 28, [ | url = http://www.planetary.org/blog/article/00000551/ | title = Discovery of a Whole New Type of Comet | publisher = The Planetary Society | accessdate = 2007-04-20 -->

Origin Formation Phaeton (hypothetical planet) of the asteroid belt's origins was that it was originally a planet that was somehow shattered. However, over time this hypothesis has fallen from favor, due to a number of key problems. One is the large amount of energy which would be required to achieve this kind of effect. Another is the low combined mass of the current asteroid belt, which has only a small fraction of the mass of the Earth's moon. Finally, the significant chemical differences between the asteroids is difficult to explain if they come from the same planet.{{cite web | author=Masetti, M.; Mukai, K. | date=December 1, [ | url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980810a.html | title=Origin of the Asteroid Belt | publisher=NASA Goddard Spaceflight Center | accessdate=2007-04-25 --> Today, most scientists accept the hypothesis that rather than fragmenting from an original planet, the asteroids never formed a planet at all.

Planetary formation is thought to have occurred via a process comparable to the long-standing nebular hypothesis, which states that a cloud of interstellar dust and gas collapsed under the influence of gravity to form a rotating disk of material that then further condensed to form the Sun and planets.{{cite web | last = Watanabe | first = Susan | date = July 20, [ | url =http://www.jpl.nasa.gov/news/features.cfm?feature=520 | title =Mysteries of the Solar Nebula | publisher = NASA | accessdate = 2007-04-02 --> During the first few million years of the Solar System's history, an Accretion (astrophysics) process of sticky collisions caused clumping together of small particles, formation of larger clumps, and the gradual increase of the size of these bodies. Once the objects reached sufficient mass they could draw in other bodies through gravitational attraction, and become known as planetesimals. The gravitational accretion of these planetesimals led to the formation of the rocky planets and to the gas giants.

In regions where the average velocity of the collisions was too high, the shattering of planetesimals tends to dominate over accretion, preventing the formation of planet-sized bodies. When the orbital period of a planetismal forms an integer fraction of the orbital period of Jupiter, an [orbital resonance occurs that can perturb the object into a different orbit. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter. As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's planetismal population in the process—increasing their velocities relative to each other.{{cite conference | first = E. R. D. | last = Scott | title=Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids | booktitle = Proceedings 37th Annual Lunar and Planetary Science Conference | publisher = Lunar and Planetary Society | date = March 13-17, 2006 | location = League City, Texas | url =http://adsabs.harvard.edu/abs/2006LPI....37.2367S | accessdate = 2007-04-16 --> Planetesimals in this region were (and continue to be) too strongly Perturbation (astronomy) to form a planet. Instead the planetesimals orbit the Sun as before and occasionally collide. The asteroid belt can be considered a relic of the primitive Solar System.

When the main belt was first being formed, the temperatures at a distance of 2.7 A.U. from the Sun formed a "snow line" where the temperatures fell below the condensation point of water. (1 A.U., or astronomical unit, equals the average distance between the Earth and the Sun.) Planetismals formed beyond this radius were able to accumulate ice. [Main-belt comets formed within the belt outside the snow line, and these are a leading candidate for the deposition of water to form the Earth's oceans.{{cite news | first=Phil | last=Berardelli | title=Main-Belt Comets May Have Been Source Of Earths Water | publisher=Space Daily | date=March 23, [ | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2007-04-11 -->

However, a recent hypothesis has partially revived the old "Fifth planet (hypothetical)" model for the asteroids' formation. A 2002 paper suggested that a Planet V formed among the inner planets, but the orbit was destabilized so that it began crossing the inner asteroid belt. As a result of this transition, a number of asteroids would have been ejected from the belt. Later this planet was either absorbed by the Sun or ejected from the system.{{cite web | last = David | first = Leonard | date = March 18, [ | url = http://www.space.com/scienceastronomy/solarsystem/fifth_planet_020318.html | title =Long-Destroyed Fifth Planet May Have Caused Lunar Cataclysm | publisher =Space.com | accessdate = 2007-04-25 -->

Evolution The current asteroid belt is believed to contain only a small fraction (by mass) of the primordial asteroid belt. Based on computer simulations, the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of this material was ejected from the belt within a period of about a million years of formation, leaving behind less than 0.1% of the original mass.

Since their formation, the size distribution of the asteroid belt has remained relatively stable. That is, there has not been a significant increase or decrease in the typical dimensions of the main belt asteroids.{{cite news | first=Lori | last=Stiles | title=Asteroids Caused the Early Inner Solar System Cataclysm | publisher=University of Arizona News |date=September 15, [ | url=http://uanews.org/cgi-bin/WebObjects/UANews.woa/7/wa/SRStoryDetails?ArticleID=11692 | accessdate=2007-04-18 --> However, the asteroids have been affected by many subsequent processes, such as internal heating (in the first few tens of millions of years), surface melting from impacts, and [space weathering from radiation and bombardment by [micrometeorites. Hence, the asteroids themselves are not pristine samples of the early Solar System. By contrast, the objects in the outer [Kuiper belt are believed to have experienced much less change since the Solar System's formation.

The 4:1 orbital resonance with Jupiter, at a radius 2.06 astronomical unit, can be considered the inner boundary of the main belt. Perturbations by Jupiter send bodies straying there onto unstable orbits. Also, most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion out at 1.67 A.U.) or ejected by its gravitational perturbations in the early history of the Solar System.{{cite web | author=Alfvén, H.; Arrhenius, G. | year=1976 | url =http://history.nasa.gov/SP-345/ch4.htm | title =The Small Bodies | work=SP-345 Evolution of the Solar System | publisher = NASA | accessdate = 2007-04-12 --> An exception are the high inclination [Hungaria asteroids which lie slightly closer to the Sun, and were protected from these disturbances by this high inclination.

Characteristics , the first ever imaged by a spacecraft, taken by Galileo (spacecraft) as it passed by it in 1991Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff that is assumed. Over 200 asteroids are known to be larger than 100 kilometre,{{cite web | last = Yeomans | first = Donald K. | date = April 26, [ | url = http://ssd.jpl.nasa.gov/sbdb_query.cgi | title = JPL Small-Body Database Search Engine | publisher = NASA JPL | accessdate = 2007-04-26 --> — search for asteroids in the main belt regions with a diameter >100. while a survey in the infrared wavelengths shows that the main belt has 700,000 to 1.7 million asteroids with a diameter of 1 km or more. The [absolute magnitudes of most of the known asteroids are 11–19, with the median at about 16.

Mass The total mass of the asteroid belt is estimated to be 3.0-3.6 kilograms, which is 4% of the Earth's Moon. By comparison, the Trojan asteroid, two swarms of asteroids located at Jupiter's Lagrange point, to date have revealed a collective mass roughly half that of the asteroid belt,cite web|title=POPULATION AND SIZE DISTRIBUTION OF SMALL JOVIAN TROJAN ASTEROIDS|author=DAVID C. JEWITT AND CHADWICK A. TRUJILLO |year=2007|url=http://www.journals.uchicago.edu/AJ/journal/issues/v120n2/200007/200007.html?erFrom=6876019551608635237Guest|accessdate=2007-10-18--> while the Kuiper belt, a second belt beyond the orbit of Neptune discovered in 1992, may possess a mass up to 200 times its mass. The hypothetical Oort cloud, a great spherical swarm of trillions of cometary bodies estending to beyond 50,000 AU, may possess a total mass six thousand times that of the asteroid belt.

Composition During the early history of the Solar System, minor planets underwent some degree of melting, allowing elements to be partially or completely segregated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. However, because of the relatively small size of these bodies, this period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago, that is in the first few tens to a hundred million years.

is a carbonaceous chondrite meteorite that fell to Earth in Mexico, 1969.The current belt consists primarily of three categories of asteroids. In the outer portion of the belt, closer to Jupiter's orbit, carbon-rich asteroids predominate. These [C-type asteroid ([carbonaceous) asteroids include over 75% of the visible asteroids. They are more red in hue than the other asteroid categories and have a very low [albedo. Their surface composition is similar to [carbonaceous chondrite [meteorites. Chemically, their spectra indicate a match with the primordial composition of the early Solar System, with the lighter elements and volatiles (''e.g.'' ices) removed.

Toward the inner portion of the belt, within 2.5 A.U. of the Sun, S-type asteroid (silicate) chondrite asteroids are more common. The spectra of their surfaces reveal the presence of silicates as well as some metal, but no significant carbonaceous compounds. This indicates that they are made of materials that have been significantly modified from the primordial Solar System composition. The expected mechanism was melting early in their history, which caused mass differentiation. They have a relatively high albedo, and form about 17% of the total asteroid population.

A third category of asteroids, forming about 10% of the total population, is the M-type asteroid (metal-rich). These have a spectrum that resembles metallic iron-nickel, with a white or slightly red appearance and no absorption features in the spectrum. Some M-type asteroids are believed to be formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, there are also some silicate compounds that can produce a similar appearance. Thus, for example, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 A.U.{{cite web | last = Lang | first = Kenneth R. | year=2003 | url = http://ase.tufts.edu/cosmos/print_images.asp?id=15 | title = Asteroids and meteorites | publisher = NASA's Cosmos | accessdate = 2007-04-02 --> Overall it is not yet clear whether all M-types are compositionally similar, or whether it is a label for several varieties which do not fit neatly into the main C and S classes.

One mystery of the asteroid belt is the relative rarity of V-type asteroid, or basaltic asteroids. Theories of asteroid formation predict that objects the size of Vesta or larger should form crusts and mantles, which would be composed mainly of basaltic rock, resulting on more than half of all asteroids being composed either of basalt or olivine. Observations however suggest that 99 percent of the predicted basaltic material is missing. Until 2001, most basaltic bodies discovered in the asteroid belt were believed to originate from the asteroid Vesta (hence their name V-type). However, the discovery of the asteroid (1459) Magnya revealed a slightly different chemical composition to the other basaltic asteroids discovered til then, suggesting a different origin.

The diversity of known basaltic meteorites mandated a diversity in their origins, and in 2007 two asteroids in the outer belt, (7472) Kumakiri and (10537) 1991 RY16, were found to possess a basaltic composition, though they could not have originated from Vesta. They are the only V-type asteroids discovered in the outer belt to date.

The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73°C) at 2.2 A.U. down to 165 K (-108°C) at 3.2 A.U. However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.

Orbits and rotations The large majority of the asteroids within the main belt have orbital eccentricity of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids peak at an eccentricity of around 0.07 and an inclination of less than 4°.{{cite web | last = Williams | first = Gareth |date=April 3, [ | url = http://cfa-www.harvard.edu/iau/lists/MPDistribution.html | title = Distribution of the Minor Planets | publisher = Minor Planets Center | accessdate = 2007-04-15 --> Thus while a typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic, some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.

Sometimes, the term main belt is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 astronomical unit, and at eccentricity (orbit) less than roughly 0.33, along with orbital inclinations below about 20°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.

Measurements of the rotation periods of large asteroids in the main belt show that there is a lower limit. No asteroid with a diameter larger than 100 metres has a period of rotation of less than 2.2 hours. For asteroids rotating faster than approximately this rate, the centrifugal force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that the majority of asteroids with a diameter over 100 metres are actually rubble piles formed through accumulation of debris after collisions between asteroids.{{cite web | last = Rossi | first = Alessandro | date = May 20, [ | url = http://spaceguard.esa.int/tumblingstone/issues/current/eng/ast-day.htm | title = The mysteries of the asteroid rotation day | publisher = The Spaceguard Foundation | accessdate = 2007-04-09 -->

Kirkwood gaps in the "core" of the main belt. Cyan arrows point to the Kirkwood gaps, where orbital resonances with Jupiter destabilize orbits.The semi-major axis of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's orbital period. When considering the semi-major axes of all asteroids, the main belt contains noticeable gaps, called Kirkwood gaps, in its distribution. They occur at the radii at which the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter. This results in mean-motion resonance with the gas giant that is sufficient to perturb an asteroid to new orbital elements. In effect, asteroids that become located in such gap orbits (either primordially because of the migration of Jupiter's orbit, or due to prior perturbations or collisions) are gradually nudged into different, random orbits with a larger or smaller semi-major axis.

However, the gaps are not seen in a simple snapshot of the locations of the asteroids at any one time. This is because asteroid orbits are elliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps is not significantly different than in the neighboring regions.

The main gaps occur at the 3:1, 5:2, 7:3 and 2:1 mean-motion resonances with Jupiter. Thus an asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit. Weaker resonances occur at other values of semi-major axis, such that less asteroids are found with those values than with nearby ones. (For example, a 8:3 resonance for asteroids with a semi-major axis of 2.71 A.U.){{cite conference | first = S. | last = Ferraz-Mello | title = Kirkwood Gaps and Resonant Groups | booktitle = proceedings of the 160th International Astronomical Union | pages = 175-188 | publisher = Kluwer Academic Publishers | date = June 14-18, 1993 | location = Belgirate, Italy | url = http://adsabs.harvard.edu/abs/1994IAUS..160..175F | accessdate = 2007-03-28 -->

The main or "core" population of the asteroid belt is sometimes divided into three zones, based on the most prominent Kirkwood gaps. Zone I lies between the 4:1 resonance (2.06 A.U.) and 3:1 resonance (2.5 A.U.) Kirkwood gaps. Zone II contines from the end of Zone I out to the 5:2 resonance gap (2.82 A.U.). Zone III runs from the outer edge of Zone II to the 2:1 resonance gap (3.28 A.U.).

The main belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 A.U.), and the outer belt formed by those asteroids closer to Jupiter's orbit. (Some authors subdivide the inner and outer belts at the 2:1 resonance gap A.U., while others even define inner, middle and outer belts.)

Largest asteroids See also: List of notable asteroids#Largest known asteroids (out to the orbit of Jupiter)The four largest asteroids are, in decreasing order of mass, 1 Ceres, 4 Vesta, 2 Pallas and 10 Hygiea. Together, they account for almost half of the belt's total mass, with one-third accounted for by Ceres alone.For recent estimates of the masses of Ceres (dwarf planet), 4 Vesta, 2 Pallas and 10 Hygiea, see the references in the infoboxes of their respective articles.

Ceres is the only asteroid large enough for its gravity to force it into a roughly round shape, and so, according to the IAU's 2006 resolution on the definition of planet, is now considered a dwarf planet. Its orbital distance, 2.8 AU, is also the location of the asteroid belt's center of mass. Ceres has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice.{{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--> Like the planets, Ceres is diffrentiated; it has a crust, a mantle and a core.

Vesta, like Ceres, has a diffrentiated interior. Vesta is almost but not quite spherical,P. C. Thomas et al Vesta: Spin Pole, Size, and Shape from HST Images, Icarus, Vol. 128, p. 88 (1997) and the IAU has deferred its decision as to whether or not it qualifies as a dwarf planet. Unlike Ceres however, Vesta formed inside the "frost line", and so is devoid of water.

Collisions , created in part by dust from collisions in the asteroid belt.The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main belt bodies with a mean radius of 10-km are expected to occur about once every 10 million years.{{cite web | last=Backman | first=D. E. | date=March 06, [ | url=http://astrobiology.arc.nasa.gov/workshops/zodiac/backman/backman_toc.html | title=Fluctuations in the General Zodiacal Cloud Density | work=Backman Report | publisher=NASA Ames Research Center | accessdate=2007-04-04 --> A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family). Conversely, collisions that occur at low relative speeds may also join two asteroids together. After more than 4 billion years of such processes, the members of the asteroid belt now bear little resemblance to the original population.

In addition to the asteroid bodies, the main belt also contains bands of dust with particle radii of up to a few hundred micrometres. This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Due to Poynting-Robertson effect, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun.

The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic. Particles that produce the visible zodiacal light average about 40 μm in radius. The typical lifetimes of such particles is on the order of 700,000 years. Thus, in order to maintain the bands of dust, new particles must be steadily produced within the asteroid belt.

Meteorites Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere.{{cite web | last=Kingsley | first=Danny | date=May 1, [ | url=http://abc.net.au/science/news/stories/s843594.htm | title=Mysterious meteorite dust mismatch solved | publisher=ABC Science | accessdate=2007-04-04 --> More than 99.8 percent of the 30,000 meteorites found on Earth to date are believed to have originated in the asteroid belt. A September 2007 study by a joint US-Czech team has suggested that a large-body collision undergone by the asteroid 298 Baptistina sent a number of fragments into the inner solar system. The impacts of these fragments are believed to have created both the Tycho crater on the Moon and the Chicxulub crater in Mexico, the remnant of the massive impact which triggered the extinction of the dinosaurs 65 million years ago.

Families and groups Approximately one third of the asteroids in the main belt are members of an asteroid family. These are asteroids that share similar orbital elements, such as semimajor axis, Orbital eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the main belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families, and likely have a common origin. Additional groupings have been found but these are less certain. Asteroid families can be confirmed when the members display common spectral features.{{cite conference| first=Anne | last=Lemaitre | title=Asteroid family classification from very large catalogues | booktitle=Procedings Dynamics of Populations of Planetary Systems | pages=135-144 | publisher=Cambridge University Press |date=August 31-September 4, 2004 | location=Belgrade, Serbia and Montenegro | url=http://adsabs.harvard.edu/abs/2005dpps.conf..135L | accessdate=2007-04-15 --> Smaller associations of asteroids are called groups or clusters.

Some of the most prominent families in the main belt (in order of increasing semi-major axis) consist of the Flora family, Eunomia family, Koronis family, Eos family and Themis family families. For example, the Flora family, one of the largest, with more than 800 known members, may have formed from a collision less than a billion years ago.{{cite web | last = Martel | first = Linda M. V. | date = March 9, [ | url = http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html | title = Tiny Traces of a Big Asteroid Breakup | publisher = Planetary Science Research Discoveries | accessdate = 2007-04-02 -->The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the Gefion family) is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise the HED meteorites may also have originated from Vesta as a result of this collision.

Three prominent bands of dust have been found within the main belt. These have similar orbital inclinations as the Eos, Koronis and Themis asteroid families, and so are possibly associated with those groupings.

Periphery Skirting the inner edge of the belt (ranging between 1.78 and 2.0 A.U. with a mean semi-major axis of 1.9 A.U.) is the Hungaria family of minor planets. They are named after the main member of this family—434 Hungaria, and the group contains at least 52 named asteroids. The Hungaria group are separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members of this group belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars is a likely factor in reducing the total population of this group.

Another high-inclination group in the inner part of the main belt is the Phocaea family. These are composed primarily of S-type asteroids, where as the neighboring Hungaria family includes some E-type asteroid. The Phocaea family orbit between 2.25 and 2.5 A.U. from the Sun.

Skirting the outer edge of the main belt is the 65 Cybele, orbiting between 3.3 and 3.5 A.U. These have a 7:4 orbital resonance with Jupiter. The Hilda family orbit between 3.5 and 4.2 A.U., and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are relatively few asteroids beyond 4.2 A.U., until reaching Jupiter's orbit. Here the two large groups of Trojan asteroids can be found, although they are not usually considered part of the main asteroid belt.

New families Some asteroid families have formed recently, in astronomical terms. The Karin Cluster apparently formed about 5.7 million years ago from a collision with a 16-km radius progenitor asteroid.{{cite news | title=SwRI researchers identify asteroid breakup event in the main asteroid belt | publisher=SpaceRef.com | date=June 12, [ | url=http://www.spaceref.com/news/viewpr.html?pid=8627 | accessdate=2007-04-15 --> The [490 Veritas formed about 8.3 million years ago, and evidence for this event has been found in the form of interplanetary dust recovered from ocean sediment.{{cite news | first=Maggie | last=McKee | title=Eon of dust storms traced to asteroid smash | publisher=New Scientist Space | date=January 18, [ | url=http://space.newscientist.com/channel/solar-system/comets-asteroids/dn8603 | accessdate=2007-04-15 -->

In the more distant past, the 1270 Datura apparently formed about 450 million years ago from a collision with a main belt asteroid. The age estimate is based on the probability of the members having their current orbits, rather than from any physical evidence. However this cluster may have been a source for some zodiacal dust material. Other recent cluster formations, such as the [4652 Iannini (circa 1–5 million years ago), may have provided additional sources of this asteroid dust.

Exploration image.The first spacecraft to traverse the asteroid belt was Pioneer 10, after entering the belt region on July 16, 1972. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft. Since that time though the belt has been safely traversed by 9 Earth-based craft without incident. These craft include Pioneer 11, Voyager program, Galileo (spacecraft) (which imaged the asteroid 951 Gaspra in 1991 and 243 Ida in 1993}, Cassini–Huygens (which imaged 2685 Masursky in 2000), NEAR Shoemaker (which imaged 253 Mathilde in 1997), Stardust (spacecraft) (which imaged 5535 Annefrank in 2002), New Horizons (which imaged 132524 APL in 2006) and Ulysses probe. Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.{{cite news | first=Alan | last=Stern | title=New Horizons Crosses The Asteroid Belt | publisher=Space Daily | date=June 2, [ | url=http://www.spacedaily.com/reports/New_Horizons_Crosses_The_Asteroid_Belt.html | accessdate=2007-04-14 -->

All spacecraft images of belt asteroids to date have come from brief planetary flyby opportunities by probes headed for other targets. Only the NEAR and Hayabusa missions have studied asteroids for a protracted period in orbit and at the surface, and these were used to study near-Earth asteroids. However, the Dawn Mission has been being dispatched to explore 4 Vesta and Ceres (dwarf planet) in the main belt. If the probe is still operational after examining these two large bodies, an extended mission is possible that could allow additional exploration.{{cite web | author=Staff | date=April 10, [ | url = http://dawn.jpl.nasa.gov/ | title = Dawn Mission Home Page | publisher = NASA JPL | accessdate = 2007-04-14 -->

See also

References Further reading | first=Linda T. | last=Elkins-Tanton | year=2006 | title=Asteroids, Meteorites, and Comets | edition=First edition | publisher=Chelsea House | location=New York | id=ISBN 0-8160-5195-X -->

External links | author=Staff | date=October 31, [ | url = http://nssdc.gsfc.nasa.gov/planetary/planets/asteroidpage.html | title = Asteroids | publisher = NASA | accessdate = 2007-04-20 --> | last = Munsell | first = Kirk | date = September 16, 2005 | url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids Asteroids Page | title = Asteroids: Overview | publisher = http://solarsystem.nasa.gov NASA's Solar System Exploration | accessdate = 2007-05-26 --> | last = Arnett | first = William A. | date = February 26, [ | url = http://www.nineplanets.org/asteroids.html | title = Asteroids | publisher = The Nine Planets | accessdate = 2007-04-20 --> | url = http://www.solstation.com/stars/asteroid.htm | title = Main Asteroid Belt | publisher = Sol Company | accessdate = 2007-04-20 --> | last = Hsieh | first = Henry H. | date = March 1, [ | url = http://www.ifa.hawaii.edu/~hsieh/mbcs.html | title = Main-Belt Comets | publisher = University of Hawaii | accessdate = 2007-04-20 --> | author=Staff | year = 2007 | url = http://www.planetary.org/explore/topics/asteroids_and_comets/facts.html | title = Space Topics: Asteroids and Comets | publisher = The Planetary Society | accessdate = 2007-04-20 -->







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