The asteroid belt is a region of the solar system falling roughly between the 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. 98.5% of all numbered minor planets lie in this region ^[1].

Sometimes, the term main belt is used to refer only to a 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 AU, and at eccentricities less than roughly 0.33, along with inclinations below about 20°. This "core" region is marked in red in the diagrams below, and contains approximately 93.4% of all numbered minor planets ^[1]. Several prominent Kirkwood gaps are sometimes used to divide this region into three or four sections.

Origin

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, much less favored nowadays, was that the asteroids in the asteroid belt are the remnants of a destroyed planet called Phaeton. The key problems with this hypothesis are the staggering amount of energy required to achieve this kind of effect, and the low combined mass of the asteroid belt (less than that of Earth's moon).

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

Asteroid belt environment

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, and the only dwarf planet found there, 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^[2][3], which is 4% of the Earth's Moon. 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.

The asteroid belt in fiction and film

Asteroid belts are a staple of 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. One of the best-known examples of this is the Hoth system in Star Wars: The Empire Strikes Back. Proto-planets in the process of formation and planetary rings may look like that, but the Sun's asteroid belt does not. (The asteroid belt in the HD 69830 system may, however.) 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 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 2001: A Space Odyssey is unusual in that it does portray realistically the ship's "encounter" with a lone asteroid pair).

On the other hand, written depictions of human encounters with asteroids, their mining and their colonization - an increasingly frequent science fiction theme since the late 1940s - are more often scientifically accurate. In Ben Bova's The Rock Rats series, men live in the asteroid belt and mine the asteroids for metals.