Editing Asteroid belt

The '''asteroid belt''' is a region of the [[Solar System]] falling roughly between the [[planet|planets]] [[Mars|Mars]] and [[Jupiter|Jupiter]] where the greatest concentration of [[w:Asteroid|asteroid]] [[Orbit|orbits]] can be found.  

It is termed the '''main belt''' when contrasted with other concentrations of [[w:Minor planet|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 axis|semi-major axes]] between the 4:1 and 2:1 [[w:Kirkwood gap|Kirkwood gaps]] at 2.06 and 3.27 [[astronomical unit|AU]], with [[Eccentricity|eccentricities]] less than about 0.33, and with [[Inclination|inclinations]] below about 20°. This region is marked in red in the diagrams below, and contains approximately 93.4% of all numbered [[w:Minor planet|minor planets]].  

The asteroid belt region of space also contains some [[w:Active asteroid|main-belt comets]] which may have been the source of Earth's water [http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html see Article at spacedaily.com].

A common theory agreed upon by most astronomers, called the [[w:Formation and evolution of the Solar Systam|nebular theory]], is that during the first few million years of the solar system's history, planets formed by accretion of [[w:Planetisimal|planetesimals]]. Repeated collisions led to the familiar rocky planets and to the [[w:Gas giant|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 [[w:Orbital resonance|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 [[apoapsis|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 [[w:Space weathering|space weathering]]. Hence, the asteroids themselves are not particularly pristine. Instead, the objects in the outer [[w:Kuiper belt|Kuiper belt]] are believed to have experienced much less change since the solar system's formation.

[[Image:Kirkwood Gaps.png|250px|thumb|Distribution of asteroid [[Semi-major axis|semi-major axes]] in the vicinity of the main belt. Cyan arrows point to the [[w:Kirkwood gaps|Kirkwood gaps]], where [[w:Orbital resonance|orbital resonances]] with [[Jupiter]] destabilize orbits.

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 [[w:Kilometre|km]]. The biggest asteroid belt member is [[w:Ceres (dwarf planet)|Ceres]], which is about 1000 km across. The total mass of the Asteroid belt is estimated to be {{e|3.0-3.6|21}} kilograms, which is 4% of the [[Earth|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 [[w:Asteroid family|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.

Asteroid belts are a staple of [[w:Science fiction|science fiction]] stories less concerned with realism than with drama, since they are frequently portrayed as being so dense that adventurous measures must be taken to avoid an impact. The best-known example is probably the [[w:Hoth|Hoth]] system in ''Star Wars: [[w:The Empire Strikes Back|The Empire Strikes Back]]''. [[w:Protoplanet|Protoplanets]] in the process of formation and planetary rings may look like that, but asteroid belts do not. In reality, the asteroids are spread over such a high volume that it would be highly improbable even to pass close to a random asteroid. For example, the numerous [[w:Space probe|space probes]] sent to the outer solar system, just across the main asteroid belt, have never had any problems, and asteroid rendezvous missions have elaborate targeting procedures.  

The inaccurate image of an overcrowded Asteroid Belt is especially frequent in science fiction films, apparently because it makes for dramatic visual images which the true nearly empty space does not provide. (The movie ''[[w:2001: A Space Odyssey (film)|2001: A Space Odyssey]]'' is unusual in that it does portray realistically the ship's "encounter" with a lone asteroid pair).  

*[[w:Asteroids in fiction|Asteroid]]

*[[w:Asteroids in fiction|Asteroids in fiction]].

* [[w:Asteroid|Asteroid]]

* [[w:Asteroid family|Asteroid family]]

* [[w:Minor planet|Minor planet]]

* [[w:Centaur (small Solar System body)|Centaur]]

* [[w:Kirkwood gap|Kirkwood gap]]