The main asteroid belt (shown in white) is located between the orbits of Mars and Jupiter.

The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disk. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered

More than half the mass within the main belt is contained in the four largest objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. Ceres (ˈsɪəriːz TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt TemplateInfobox Planet. --> 2 Pallas (ˈpæləs, or as Παλλάς) is one of the largest All of these have mean diameters of more than 400 km, while Ceres, the main belt's only dwarf planet, is about 950 km in diameter. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded ^[1][2][3][4] The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that multiple unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form an asteroid family whose members have similar orbital characteristics and compositions. An asteroid family is a population of Asteroids that share similar Orbital elements, such as Semimajor axis, eccentricity, and orbital Inclination Collisions also produce a fine dust that forms a major component of the zodiacal light. The zodiacal light is a faint roughly triangular whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the Ecliptic Individual asteroids within the main belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type). A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 C-type asteroids are Carbonaceous Asteroids They are the most common variety forming around 75% of known Asteroids {Fact|date=July 2008}} and an even higher For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI S-type asteroids are of a silicaceous ( stony) composition hence the name The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across M-type asteroids are Asteroids of unknown composition they are moderately bright ( Albedo 0

The asteroid belt formed from the primordial solar nebula as a group of planetesimals, the smaller precursors of the planets. The formation and evolution of the Solar System is estimated to have begun Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the planetesimals with too much orbital energy for them to accrete into a planet. Gravitation is a natural Phenomenon by which objects with Mass attract one another In Astrophysics, the term accretion is used for at least two distinct processes Collisions became too violent, and instead of sticking together, the planetesimals shattered. As a result, most of the main belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. Kirkwood gaps are gaps or dips in the distribution of Main belt Asteroids with Semi-major axis (or equivalently their Orbital period) as seen

History of observation

See also: Definition of planet

Giuseppe Piazzi, discoverer of the first object, Ceres

In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature,^[5] the astronomer Johann Daniel Titius von Wittenburg^[6][7] noted an apparent pattern in the layout of the planets. From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity Giuseppe Piazzi ( July 7 1746 - July 22 1826) was an Italian Theatine monk Mathematician, and Astronomer Charles Bonnet ( March 13, 1720 &ndash May 20, 1793) Swiss naturalist and philosophical writer was born at Geneva Johann Daniel Titius ( January 2 1729 &ndash December 11 1796) was a German Astronomer and a professor at Wittenberg Wittenburg is a town in the district Ludwigslust in Mecklenburg-Western Pomerania, Germany. If one began a numerical sequence at 0, then included 3, 6, 12, 24, 48, etc. , doubling each time, and 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 (one astronomical unit, or AU, equals the average distance between the Earth and the Sun). The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the This pattern, now 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. The Titius–Bode law (sometimes termed just Bode's law) is a hypothesis that the bodies in some orbital systems including Sol 's orbit at semi-major axes In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae In his footnote Titius declared, "But should the Lord Architect have left that space empty? Not at all". ^[6] In 1768, the astronomer Johann Elert Bode made note of Titius's relationship in his Anleitung zur Kenntniss des gestirnten Himmels but did not credit Titius, which led many to refer to it as "Bode's law". Johann Elert Bode ( January 19, 1747 &ndash November 23, 1826) was a German Astronomer known for his reformulation ^[7] 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. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British

In 1800, astronomer Baron Franz Xaver von Zach recruited 24 of his fellows into an informal club he dubbed the "Lilienthal Society". Baron Franz Xaver von Zach ( Franz Xaver Freiherr von Zach) ( June 4, 1754 &ndash September 2, 1832) was a Hungarian astronomer Determined to bring the Solar System to order, the group became known as the "Himmelspolitzei", or Celestial Police. Notable members included Herschel, British astronomer Royal Nevil Maskelyne, Charles Messier, and Heinrich Olbers. The Reverend Dr Nevil Maskelyne ( 6 October[[ 732]] &ndash 9 February[[ 811]] was the fifth English Astronomer Royal. Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an ^[8] Each of the 24 astronomers was assigned a 15° region of the zodiac in which to search for the missing planet. Zodiac denotes an annual cycle of twelve stations along the Ecliptic, the apparent path of the sun across the heavens through the Constellations that divide the ecliptic ^[9]

Only a few months later, a non-member of the Celestial Police confirmed their expectations. On January 1, 1801, Giuseppe Piazzi, Chair of Astronomy at the University of Palermo, Sicily, found a tiny moving object in the exact location predicted by the Titius-Bode Law. New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC Year 1801 ( MDCCCI) was a Common year starting on Thursday (see link for calendar of the Gregorian calendar (or a Common year starting on Tuesday Giuseppe Piazzi ( July 7 1746 - July 22 1826) was an Italian Theatine monk Mathematician, and Astronomer For the University of Palermo in Buenos Aires, Argentina, see University of Palermo at B Sicily ( Italian and Sicilian: Sicilia) is an autonomous region of Italy. He dubbed it Ceres, after the Roman goddess of the harvest and patron of Sicily. Ceres (ˈsɪəriːz Piazzi initially believed it a comet, but its lack of a coma suggested it was a planet. In Astronomy, a coma (from the Latin word for "hair" is the nebulous envelope around the nucleus of a Comet. ^[8] Fifteen months later, Olbers discovered a second object in the same region, Pallas. TemplateInfobox Planet. --> 2 Pallas (ˈpæləs, or as Παλλάς) is one of the largest Unlike the other known planets, the objects remained points of light even under the highest telescope magnifications, rather than resolving into discs. Apart from their rapid movement, they were indistinguishable from stars. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth Accordingly, in 1802 William Herschel suggested they be placed into a separate category, named asteroids, after the Greek asteroeides, meaning "star-like". The Ancient Greek language is the historical stage in the development of the Hellenic language family spanning the Archaic (c ^[10][11] Upon completing a series of observations of Ceres and Pallas, he concluded,^[12]

Neither the appellation of planets, nor that of comets, can with any propriety of language be given to these two stars . . . [They] resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself however the liberty of changing that name, if another, more expressive of their nature, should occur.

Despite Herschel's reservations, for several decades it remained common practice to refer to these objects as planets. ^[5] By 1807, further investigation revealed two new objects in the region: 3 Juno and 4 Vesta. TemplateInfobox Planet. --> Juno (ˈdʒuːnoʊ or as Iūno designated 3 Juno in the Minor Planet ^[13] The Napoleonic wars brought this first period of discovery to a close,^[13] and it was not until 1845 that another object (5 Astraea) was discovered. The Napoleonic Wars (1803-1815 involved Napoleon's French Empire and a shifting set of European allies and opposing coalitions TemplateInfobox Planet. --> 5 Astraea (əˈstriːə, Αστραία, written Astræa Shortly thereafter new objects were found at an accelerating rate, and counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list and William Herschel's choice of nomenclature, asteroids, at last came into common use. ^[5]

The discovery of Neptune in 1846 led to the discrediting of the Titius-Bode Law in the eyes of scientists, as its orbit was nowhere near the predicted position. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. To date, there is no scientific explanation for the law, and the consensus among astronomers is that it is a coincidence. ^[14]

One hundred asteroids had been located by mid-1868, and in 1891 the introduction of astrophotography by Max Wolf accelerated the rate of discovery still further. Astrophotography is a specialized type of Photography that entails making photographs of astronomical objects in the sky such as the Moon, Sun, Planets Maximilian Franz Joseph Cornelius Wolf ( June 21 1863 – October 3 1932) was a German Astronomer, a pioneer of Astrophotography ^[15] A total of 1,000 asteroids had been found by 1923, 10,000 by 1951, and 100,000 by 1982. ^[4] Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

Origin

The asteroid belt (showing inclinations), with the main belt in red and blue ("core" region in red)

Formation

In 1802, Heinrich Olbers suggested to William Herschel that the belt had been formed from a planet that somehow shattered. Year 1802 ( MDCCCII) was a Common year starting on Friday of the Gregorian calendar or a Common year starting on Wednesday of the Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British Phaeton (or Phaëton, less often Phaethon) is the name of a Hypothetical planet posited to once have existed between the Orbits of Mars ^[16] Over time however, this hypothesis has fallen from favor. The large amount of energy that would have been required to achieve this effect and the low combined mass of the current asteroid belt, which is only a small fraction of the mass of the Earth's Moon, do not support the hypothesis. Further, the significant chemical differences between the asteroids are difficult to explain if they come from the same planet. ^[17] Today, most scientists accept that, rather than fragmenting from a progenitor planet, the asteroids never formed a planet at all.

In general in the Solar System, planetary formation is thought to have occurred via a process comparable to the long-standing nebular hypothesis: 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. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. ^[18] During the first few million years of the Solar System's history, an accretion process of sticky collisions caused the clumping of small particles, which gradually increased in size. In Astrophysics, the term accretion is used for at least two distinct processes Once the clumps reached sufficient mass, they could draw in other bodies through gravitational attraction and become planetesimals. Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal This gravitational accretion led to the formation of the rocky planets and the gas giants. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily

Planetesimals within the region which would become the asteroid belt were too strongly perturbed by gravity to form a planet. Instead they continued to orbit the Sun as before, while occasionally colliding. ^[19] In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,^[20] preventing the formation of planet-sized bodies. Orbital resonances occurred where the orbital period of an object in the belt formed an integer fraction of the orbital period of Jupiter, perturbing the object into a different orbit; the region lying between the orbits of Mars and Jupiter contains many such orbital resonances. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other. ^[21]

During the early history of the Solar System, the asteroids melted to some degree, allowing elements within them to be partially or completely differentiated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. Plate tectonics and hotspots Divergent plate boundaries At the Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet However, because of the relatively small size of the bodies, the period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4. 5 billion years ago, in the first tens of millions years of formation. ^[22] In August 2007, a study of zircon crystals in an Antarctic meteorite believed to have originated from 4 Vesta suggested that it, and by extension the rest of the asteroid belt, had formed rather quickly, within ten million years of the Solar System origin. Zircon is a Mineral belonging to the group of nesosilicates. Its chemical name is Zirconium silicate and its corresponding chemical formula is TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt ^[23]

Evolution

The asteroids are not samples of the primordial Solar System. They have undergone considerable evolution since their formation, including internal heating (in the first few tens of millions of years), surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites. Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment A Micrometeoroid (also micrometeorite, micrometeor) is a tiny Meteoroid; a small particle of rock in space usually weighing less than a Gram

• Clark, B. E. ; Hapke, B. ; Pieters, C. ; Britt, D. (2002). Asteroid Space Weathering and Regolith Evolution. University of Arizona. Retrieved on 2007-11-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1519 - Hernán Cortés enters Tenochtitlán and Aztec ruler Moctezuma welcomes him with great a Celebration
• Michael J. Gaffey (1996). The Spectral and Physical Properties of Metal in Meteorite Assemblages: Implications for Asteroid Surface Materials. Retrieved on 2007-11-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1519 - Hernán Cortés enters Tenochtitlán and Aztec ruler Moctezuma welcomes him with great a Celebration
• Keil K. (2000). Thermal alteration of asteroids: evidence from meteorites. Planetary and Space Science. Retrieved on 2007-11-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1519 - Hernán Cortés enters Tenochtitlán and Aztec ruler Moctezuma welcomes him with great a Celebration
• Baragiola, R. A. ; Duke, C. A. ; Loeffler, M. ; McFadden, L. A. ; Sheffield, J. (2003). Impact of ions and micrometeorites on mineral surfaces: Reflectance changes and production of atmospheric species in airless solar system bodies. Retrieved on 2007-11-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1519 - Hernán Cortés enters Tenochtitlán and Aztec ruler Moctezuma welcomes him with great a Celebration

</ref> While some scientists refer to the asteroids as residual planetesimals,^[24] other scientists consider them distinct. ^[25]

The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about a million years of formation, leaving behind less than 0. 1% of the original mass. ^[19] Since their formation, the size distribution of the asteroid belt has remained relatively stable: there has been no significant increase or decrease in the typical dimensions of the main belt asteroids. ^[26]

The 4:1 orbital resonance with Jupiter, at a radius 2. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their 06 AU, can be considered the inner boundary of the main belt. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion at 1. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 67 AU) or ejected by its gravitational perturbations in the early history of the Solar System. ^[27] The Hungaria asteroids lie closer to the Sun than the 4:1 resonance, but are protected from disruption by their high inclination. The Hungaria asteroids are a group of asteroids in the main belt that orbit the Sun between 1 ^[28]

When the main belt was first being formed, the temperatures at a distance of 2. 7 AU from the Sun formed a "snow line" below the condensation point of water. Planetismals formed beyond this radius were able to accumulate ice. ^[29][30] In 2006 it was announced that a population of comets had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans. Main-belt comets are bodies orbiting within the ( main) asteroid belt which show cometary activity during a part of their orbit According to some models, there was insufficient outgassing of water during the Earth's formative period to form the oceans, necessitating an external source such as a cometary bombardment. Outgassing (sometimes called offgassing, particularly when in reference to indoor Air quality is the slow release of a Gas that was trapped ^[31]

Characteristics

The asteroid 951 Gaspra, the first ever imaged by a spacecraft, taken by Galileo as it passed by it in 1991

Contrary to popular imagery, the asteroid belt is mostly empty. TemplateInfobox Planet. --> 951 Gaspra (ˈɡæsprə is an S-type Asteroid that orbits very Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer 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. Over 200 asteroids are known to be larger than 100 km,^[32] 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. ^[33] The apparent magnitudes of most of the known asteroids are 11–19, with the median at about 16. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value ^[34]

The total mass of the asteroid belt is estimated to be 3. 0×10²¹–3. 6×10²¹ kilograms, which is just 4% of the Earth's Moon. ^[1][2] Its four largest objects, 1 Ceres, 4 Vesta, 2 Pallas and 10 Hygiea, account for almost half of the belt's total mass, with one-third accounted for by Ceres alone. ^[3][4] Ceres's orbital distance, 2. 8 AU, is also the location of the asteroid belt's center of mass. ^[35]

Composition

Allende is a carbonaceous chondrite meteorite that fell to Earth in Mexico in 1969. The Allende meteorite is the largest Carbonaceous chondrite ever found on Earth. The United Mexican States ( or commonly Mexico (ˈmɛksɪkoʊ () is a federal constitutional Republic in North America.

The current belt consists primarily of three categories of asteroids: C-type or carbonaceous asteroids, S-type or silicate asteroids, and M-type or metallic asteroids.

Carbonaceous asteroids, as their name suggests, are carbon-rich and dominate the belt's outer regions. ^[36] Together they comprise over 75% of the visible asteroids. They are more red in hue than the other asteroids and have a very low albedo. The albedo of an object is the extent to which it diffusely reflects light from the sun Their surface composition is similar to carbonaceous chondrite meteorites. A carbonaceous chondrite or a C-type chondrite is a type of chondritic Meteorite which contains high levels of Water and Organic compounds A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface Chemically, their spectra match the primordial composition of the early Solar System, with only the lighter elements and volatiles removed. In Planetary science, volatiles, are that group of elements and compounds with low boiling points (see volatile) that are associated with a planet's or moon's

S-type or silicate-rich asteroids are more common toward the inner region of the belt, within 2. S-type asteroids are of a silicaceous ( stony) composition hence the name For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI 5 AU of the Sun. ^[36][37] The spectra of their surfaces reveal the presence of silicates and some metal, but no significant carbonaceous compounds. This indicates that their materials have been significantly modified from their primordial composition, probably via melting and reformation. They have a relatively high albedo, and form about 17% of the total asteroid population.

M-type (metal-rich) asteroids form about 10% of the total population; their spectra resemble that of iron-nickel. M-type asteroids are Asteroids of unknown composition they are moderately bright ( Albedo 0 The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across Some are believed to have 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. For example, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. TemplateInfobox Planet. --> 22 Kalliope (kəˈlaɪəpi Καλλιόπη) is a large Main ^[38] Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2. 7 AU. ^[39] 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. ^[40]

One mystery of the asteroid belt is the relative rarity of V-type, or basaltic asteroids. The spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class (hence the name Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. ^[41] 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 in more than half of all asteroids being composed either of basalt or olivine. The Mineral olivine (when gem-quality also called Peridot) is a Magnesium Iron silicate with the formula ( Mg Observations, however, suggest that 99 percent of the predicted basaltic material is missing. ^[42] 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 from the other basaltic asteroids discovered until then, suggesting a different origin. ^[42] This hypothesis was reinforced by the further discovery in 2007 of two asteroids in the outer belt, (7472) Kumakiri and (10537) 1991 RY16, with differing basaltic composition that could not have originated from Vesta. These latter two are the only V-type asteroids discovered in the outer belt to date. ^[41]

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 AU down to 165 K (−108 °C) at 3. 2 AU^[43] 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 asteroid belt (showing eccentricities), with the main belt in red and blue ("core" region in red)

Most asteroids within the main belt have orbital eccentricities of less than 0. 4, and an inclination of less than 30°. The orbital distribution of the asteroids reaches a maximum at an eccentricity of around 0. 07 and an inclination below 4°. ^[34] 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. The ecliptic is the apparent path that the Sun traces out in the sky during the year

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. Kirkwood gaps are gaps or dips in the distribution of Main belt Asteroids with Semi-major axis (or equivalently their Orbital period) as seen 06 and 3. 27 AU, and at orbital eccentricities less than roughly 0. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape 33, along with orbital inclinations below about 20°. Inclination in general is the Angle between a Reference plane and another plane or axis of direction This "core" region contains approximately 93. 4% of all numbered minor planets within the Solar System. ^[44]

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. In Astronomy, rubble pile is the informal name for an Asteroid that is not a Monolith, consisting instead of numerous pieces of rock that have coalesced ^[45]

Kirkwood gaps

Main article: Kirkwood gap

This chart shows the distribution of asteroid semi-major axes in the "core" of the main belt. Kirkwood gaps are gaps or dips in the distribution of Main belt Asteroids with Semi-major axis (or equivalently their Orbital period) as seen In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae 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. In Geometry, the semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae The orbital period is the time taken for a given object to make one complete Orbit about another object In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies' orbits from the Sun. Daniel Kirkwood ( September 27, 1814 - June 11, 1895) was an American Astronomer. The Sun (Sol is the Star at the center of the Solar System. They 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 perturbations of the planet led to the removal of asteroids from these orbits. ^[46]

When the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter, a mean-motion resonance with the gas giant is created that is sufficient to perturb an asteroid to new orbital elements. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their The elements of an orbit are the parameters needed to specify that Orbit uniquely given a model of two point-masses obeying the Newtonian laws of motion and the In effect, asteroids that become located in the gap orbits (either primordially because of the migration of Jupiter's orbit,^[47] or due to prior perturbations or collisions) are gradually nudged into different, random orbits with a larger or smaller semi-major axis.

The gaps are not seen in a simple snapshot of the locations of the asteroids at any one time 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 does not differ significantly from the neighboring regions. ^[35]

The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit, for instance. Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby. (For example, an 8:3 resonance for asteroids with a semi-major axis of 2. 71 AU. )^[48]

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 AU) and 3:1 resonance (2. 5 AU) Kirkwood gaps. Zone II continues from the end of Zone I out to the 5:2 resonance gap (2. 82 AU). Zone III extends from the outer edge of Zone II to the 2:1 resonance gap (3. 28 AU). ^[49]

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 AU), 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 (3. 3 AU), while others suggest inner, middle, and outer belts. )

Collisions

The zodiacal light, 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). The zodiacal light is a faint roughly triangular whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the Ecliptic Collisions between main belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years. ^[50] A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family). An asteroid family is a population of Asteroids that share similar Orbital elements, such as Semimajor axis, eccentricity, and orbital Inclination 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. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre 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 drag, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun. The Poynting-Robertson effect, also known as Poynting-Robertson drag, named after John Henry Poynting and Howard Percy Robertson, is a process by which The Sun (Sol is the Star at the center of the Solar System. ^[51]

The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. The zodiacal light is a faint roughly triangular whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the Ecliptic This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic. The Sun (Sol is the Star at the center of the Solar System. The ecliptic is the apparent path that the Sun traces out in the sky during the year Particles that produce the visible zodiacal light average about 40 μm in radius. The typical lifetimes of such particles are 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. ^[51]

Meteorites

Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere. ^[52] 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 meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface ^[53] 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. 298 Baptistina is a typical Main belt Asteroid. It was discovered by Auguste Charlois on September 9, 1890 in Nice The Solar System consists of the Sun and those celestial objects bound to it by Gravity. 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. Tycho is a prominent lunar Impact crater located in the southern lunar highlands named after the Danish Astronomer Tycho Brahe. Chicxulub (ʧikʃuˈluːb is a Town, and surrounding municipality of the same name in the Mexican state of Yucatán. The Cretaceous–Tertiary extinction event, which occurred approximately ( Ma) was a large-scale mass extinction of animal and plant species in a geologically ^[54]

Largest asteroids

See also: Largest asteroids

The dwarf planet Ceres

Although their location in the asteroid belt excludes them from planet status, the four largest objects, Ceres, Vesta, Pallas, and Hygiea, hover on the edge of hydrostatic equilibrium, the boundary that separates objects from planethood. The following is a collection of lists of notable asteroids in the Solar system, sometimes also including minor planets beyond the orbit of Jupiter. Ceres (ˈsɪəriːz TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt TemplateInfobox Planet. --> 2 Pallas (ˈpæləs, or as Παλλάς) is one of the largest Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite They share many characteristics common to planets, but also show qualities more akin to rock-like asteroids.

Ceres is the only object in the belt 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 a planet, it is now considered a dwarf planet. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded ^[55] The other three may also eventually be reclassified as well. ^[56][57] Ceres has a much higher absolute magnitude than the other asteroids, of around 3. 32,^[58] and may possess a surface layer of ice. ^[59] Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. ^[59] Vesta, too, has a differentiated interior, though it formed inside the Solar System's "snow line", and so is devoid of water;

• Key Stages in the Evolution of the Asteroid Vesta. Hubble Space Telescope news release (1995). Retrieved on 2007-10-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony

• CT Russel et al. (2007). Dawn mission and operations. NASA/JPL. Retrieved on 2007-10-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony </ref> its composition is mainly of basaltic rock such as olivine. ^[42] Pallas is unusual in that, like Uranus, it rotates on its side, with one pole facing the Sun and the other facing away. ^[60] Its composition is similar to that of Ceres: high in carbon and silicon. ^[61] Hygiea is a carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the ecliptic plane. The ecliptic is the apparent path that the Sun traces out in the sky during the year
• M. A. Barucci et al. (2002). 10 Hygiea: ISO Infrared Observations. Retrieved on 2007-10-21. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1512 - Martin Luther joins the theological faculty of the University of Wittenberg.

• Ceres the Planet. orbitsimulator. com. Retrieved on 2007-10-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1740 - Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony </ref>

Families and groups

Main article: Asteroid family

This plot of orbital inclination (i_p) versus eccentricity (e_p) for the numbered main belt asteroids clearly shows several clumps of asteroid families. An asteroid family is a population of Asteroids that share similar Orbital elements, such as Semimajor axis, eccentricity, and orbital Inclination

In 1918, the Japanese astronomer Kiyotsugu Hirayama noticed that the orbits of some of the asteroids had similar parameters, forming families or groups. was a Japanese Astronomer, best known for his discovery that many Asteroid orbits were more similar to one another than chance would allow leading to the concept ^[62]

Approximately one third of the asteroids in the main belt are members of an asteroid family. These share similar orbital elements, such as semi-major axis, 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. Additional groupings have been found that are less certain. Asteroid families can be confirmed when the members display common spectral features. ^[63] 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 axes) are the Flora, Eunoma, Koronis, Eos, and Themis families. The Flora '''family''' of asteroids is a large grouping of S-type Asteroids in the inner Main belt, whose origin and properties are relatively The Eunomia '''family''' of asteroids is a large grouping of S-type Asteroids named after the Greek goddess Eunomia. The Koronis family is a family of Asteroids in the Main Belt between Mars and Jupiter. The Eos family is a prominent family of Asteroids that is believed to have formed as a result of an ancient catastrophic collision The Themis Asteroid Family is a Hirayama family of asteroids found in the outer portion of the main Asteroid belt, between the orbits of Mars and Jupiter ^[39] 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. ^[64] 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. Properties The members have Proper orbital elements in the approximate ranges Ceres family Minerva family and interlopers Until recently this family The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. The Vesta '''family''' of asteroids is a large and prominent grouping of mostly V-type Asteroids in inner Main belt in the vicinity of 4 Likewise, the HED meteorites may also have originated from Vesta as a result of this collision. The HED meteorites are a grouping of Achondrite meteorite types the Howardites * Eucrites * Diogenites * Dunite ^[65]

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. ^[66]

Periphery

Skirting the inner edge of the belt (ranging between 1. 78 and 2. 0 AU, with a mean semi-major axis of 1. 9 AU) is the Hungaria family of minor planets. The Hungaria asteroids are a group of asteroids in the main belt that orbit the Sun between 1 They are named after the main member, 434 Hungaria; the group contains at least 52 named asteroids. 434 Hungaria is a relatively small inner Main belt Asteroid. It is classified as an E-type (high- Albedo) asteroid The Hungaria group is separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars are likely a factor in reducing the total population of this group. ^[67]

Another high-inclination group in the inner part of the main belt is the Phocaea family. The Phocaea asteroids are a group of asteroids that orbit the Sun between 2 These are composed primarily of S-type asteroids, where as the neighboring Hungaria family includes some E-types. E-type asteroids are Asteroids thought to have Enstatite Achondrite surfaces ^[68] The Phocaea family orbit between 2. 25 and 2. 5 AU from the Sun.

Skirting the outer edge of the main belt is the Cybele group, orbiting between 3. TemplateInfobox Planet. --> 65 Cybele (ˈsɪbɨli sib'-ə-lee) is one of the largest Asteroids 3 and 3. 5 AU. These have a 7:4 orbital resonance with Jupiter. The Hilda family orbit between 3. The Hilda family of asteroids consists of Asteroids with a Semi-major axis between 3 5 and 4. 2 AU, and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are few asteroids beyond 4. 2 AU, until Jupiter's orbit. Here the two large groups of Trojan asteroids can be found; 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. The Karin Cluster is a group of at least 90 Asteroids in the Koronis family in the Main belt. 7 million years ago from a collision with a 16 km radius progenitor asteroid. ^[69] The Veritas family formed about 8. 490 Veritas 3 million years ago; evidence includes interplanetary dust recovered from ocean sediment. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. ^[70]

In the more distant past, the Datura cluster appears to have 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. ^[71] Other recent cluster formations, such as the Iannini cluster (circa 1–5 million years ago), may have provided additional sources of this asteroid dust. 4652 Iannini (1975 QO is a Main-belt Asteroid discovered on, by at ^[72]

Exploration

Artist's concept of the Dawn Mission spacecraft with Vesta (left) & Ceres (right)

The first spacecraft to traverse the asteroid belt was Pioneer 10, which entered the region on July 16, 1972. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Events 622 - The beginning of the Islamic calendar. 1054 - Three Roman legates fractured relations between the Western and Year 1972 ( MCMLXXII) was a Leap year starting on Saturday (link will display full calendar of the Gregorian calendar. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft, but it has since been safely traversed by 9 Earth-based craft without incident. Pioneer 11, Voyagers 1 and 2 and Ulysses passed through the belt without imaging any asteroids. Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and Ulysses is a robotic Space probe designed to study the Sun at all latitudes Galileo imaged the asteroid 951 Gaspra in 1991 and 243 Ida in 1993, NEAR imaged 253 Mathilde in 1997, Cassini imaged 2685 Masursky in 2000, Stardust imaged 5535 Annefrank in 2002, and New Horizons imaged 132524 APL in 2006. Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer TemplateInfobox Planet. --> 951 Gaspra (ˈɡæsprə is an S-type Asteroid that orbits very TemplateInfobox Planet. --> 243 Ida (ˈaɪdə eye'-də) is a Main belt Asteroid The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker renamed after its launch in honor of planetary scientist Eugene M TemplateInfobox Planet. --> 253 Mathilde (məˈtɪldə is a main belt Asteroid that Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its TemplateInfobox Planet. --> The asteroid 2685 Masursky is a main belt Asteroid. Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the 5535 Annefrank ( is an inner main belt Asteroid, and member of the Augusta family. New Horizons is a Robotic spacecraft mission by NASA currently underway 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. ^[73]

All spacecraft images of belt asteroids to date have come from brief flyby opportunities by probes headed for other targets. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the Only the NEAR and Hayabusa missions have studied asteroids for a protracted period in orbit and at the surface and these were near-Earth asteroids. is an Unmanned space mission led by the Japan Aerospace Exploration Agency to return a sample of material from a small Near-Earth asteroid named Near-Earth asteroids (NEAs are Asteroids whose Orbits are close to Earth 's orbit However, the Dawn Mission has been dispatched to explore Vesta and Ceres in the main belt. Dawn, launched on September 27 2007 is a Robotic spacecraft being sent by NASA on a Space exploration mission to the two most massive members TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt Ceres (ˈsɪəriːz If the probe is still operational after examining these two large bodies, an extended mission is possible that could allow additional exploration. ^[74]

See also

• Asteroids in astrology
• Asteroids in fiction
• Centaur (planetoid)
• Colonization of the asteroids
• Debris disk
• List of dwarf planet candidates
• Trojan asteroid

References

Further reading

• Elkins-Tanton, Linda T. (2006). Asteroids, Meteorites, and Comets, First edition, New York: Chelsea House. ISBN 0-8160-5195-X.  

External links

• Arnett, William A. (February 26, 2006). Events 747 BC - Epoch (origin of Ptolemy 's Nabonassar Era 364 - Valentinian I is proclaimed Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Asteroids. The Nine Planets. Retrieved on 2007-04-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII.
• Asteroids Page at NASA's Solar System Exploration
• Fraser Cain. The Asteroid Belt. Universe Today. Retrieved on 2008-04-01. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 527 - Byzantine Emperor Justin I names his nephew Justinian I as co-ruler and successor to the throne
• Hsieh, Henry H. (March 1, 2006). Events 86 BC - Lucius Cornelius Sulla, at the head of a Roman Republic army enters in Athens, removing the Tyrant Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Main-Belt Comets. University of Hawaii. Retrieved on 2007-04-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII.
• Main Asteroid Belt. Sol Company. Retrieved on 2007-04-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII.
• Munsell, Kirk (September 16, 2005). Asteroids Page Asteroids: Overview. NASA's Solar System Exploration. Retrieved on 2007-05-26. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 451 - The Battle of Avarayr between Armenian rebels and the Sassanid Empire takes place
• Plots of eccentricity vs. semi-major axis and inclination vs. semi-major axis at Asteroid Dynamic Site
• Staff (October 31, 2006). Events 445 BC – Ezra reads the Book of the Law to the Israelites in Jerusalem (see Nehemiah 91 NLTse Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Asteroids. NASA. Retrieved on 2007-04-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII.
• Staff (2007). Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Space Topics: Asteroids and Comets. The Planetary Society. Retrieved on 2007-04-20. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1303 - The University of Rome La Sapienza is instituted by Pope Boniface VIII.