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[[Category:Asteroid groups and families]]
 
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[[Category:Minor Planets]]
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[[Category:Ceres]]

Revision as of 19:58, 6 May 2012

The asteroid belt is a region of the solar system falling roughly between the orbits of planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found.

It is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts. In this usage, it often refers only to the greatest concentration of bodies with semi-major axes between the 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 AU, with eccentricities less than about 0.33, and with inclinations below about 20°. This region is marked in red in the diagrams below, and contains approximately 93.4% of all numbered minor planets.

The asteroid belt region of space also contains some main-belt comets which may have been the source of Earth's water [1].

Origin

Main belt i vs a

The asteroid belt (showing inclinations), with the main belt in red

A common hypothesis agreed upon by most astronomers, called the nebular hypothesis, is that during the first few million years of the solar system's history, planets formed by accretion of planetesimals. Repeated collisions led to the familiar rocky planets and to the gas giants. However, if the average velocity of the collisions is too high, the shattering of planetesimals dominates over accretion, and planet-sized bodies cannot form. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter, and planetesimals in this region were (and continue to be) kicked around too strongly to form a planet. The planetesimals instead continue to orbit the Sun as before. The inner border of the main belt is determined by the 4:1 orbital resonance with Jupiter at 2.06 AU which sends any bodies straying there onto unstable orbits. Most bodies formed interior of this gap were swept up by Mars (which has an aphelion out at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System.

In this sense the asteroid belt can be considered a relic of the primitive Solar System, but it has been affected by many processes active in later periods, such as internal heating, impact melting, and space weathering. Hence, the asteroids themselves are not particularly pristine. Instead, the objects in the outer Kuiper belt are believed to have experienced much less change since the solar system's formation.

An old hypothesis that is much less favoured these days was that the asteroids in the asteroid belt are the remnants of a destroyed planet. A key problem with such an hypothesis is the staggering amount of energy required to achieve this kind of effect.

Asteroid belt environment

Main belt e vs a

The asteroid belt (showing eccentricities), with the main belt in red

Kirkwood Gaps

Distribution of asteroid semi-major axes in the vicinity of the main belt. Cyan arrows point to the Kirkwood gaps, where orbital resonances with Jupiter destabilize orbits.

Despite 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, tens of thousands of asteroids are currently known, and estimates of the total number range in the millions. About 220 of them are larger than 100 km. The biggest asteroid belt member is Ceres, which is about 1000 km across. The total mass of the Asteroid belt is estimated to be 3.0-3.6×1021 kilograms[1][2], which is 4% of the Earth's Moon. And of that total mass, one-third is accounted for by Ceres alone.

The high population makes for a very active environment, where collisions between asteroids occur very often (in astronomical terms). A collision may fragment an asteroid in numerous small pieces (leading to the formation of a new asteroid family), or may glue two asteroids together if it occurs at low relative speeds. After five billion years, the current Asteroid belt population bears little resemblance to the original one.

Extra-solar belts

Belts of dust or debris have also been detected around stars other than the Sun, including the following:

Star Distance
(ly)
Orbit
(AU)
Epsilon Eridani 10.5 35-75
Vega 25 86-200
AU Microscopii 33 210
HD 69830 41 <1
55 Cancri 41 27-50
HD 139664 57 60-109
HD 53143 60 ?
Beta Pictoris 63 25-550
Zeta Leporis 70 2.5-12.2
HD 107146 88 130
Fomalhaut 133 25
HD 12039 137 5
HR 4796 A 220 200
HD 141569 320 400
HD 113766 430 0.35-5.8

The orbital distance of the belt is an estimated mean distance or range, based either on direct measurement from imaging or derived from the temperature of the belt. The Earth has an average distance from the Sun of 1 AU.

See also

References

  1. Krasinsky, G. A.; Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. (2002). "Hidden Mass in the Asteroid Belt". Icarus 158: 98-105.
  2. Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets - EPM and Determination of Some Astronomical Constants". Solar System Research 39: 176.

External links

The minor planets
Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
 The Solar System v·d·e 
Solar System XXVII
The Sun · Mercury · Venus · Earth · Mars · Ceres* · Jupiter · Saturn · Uranus · Neptune · Pluto* · Haumea* · Makemake* · Eris* · Dwarf Planet Candidates*
Planets · Dwarf Planets · Moons: Terran · Martian · Asteroidal

· Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian

'Ceres * Pluto * Haumea * Makemake * Eris
Small bodies:   Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort Cloud)
Hypothetical Bodies:   Vulcan · Planet 9 · Planet 10 · Tyche · Nibiru · Nemesis · more...
Planets with '*' are dwarf planets.
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.