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Asteroid belt

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. The term asteroid belt can also generally describe a region of concentrated asteroids with similar orbits.


It is believed that, during the first million years of the solar system history, planets formed by accretion of planetesimals. Repeated collisions led to the familiar rocky planets and to the gas giants cores.

However, in this zone of the system the strong gravity of Jupiter inhibited the final stages and the planetesimals could not form a single planet. The planetesimals instead continued to orbit the Sun as before. In this sense, the asteroid belt can be considered a relict of the primitive Solar System, but many observations point to an active evolution of the physical conditions. Instead, the outer Kuiper belt is believed to have had little change.

Asteroid belt environment

Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a high volume that it would be highly improbable to reach an asteroid without aiming carefully.

Despite this, some 9,000 of asteroids are currently known, and estimates on the total number are in millions range. About 220 of them are lager than 100 km. The biggest asteroid belt member is Ceres, about 1000 km across. The total mass of the Asteroid belt is estimated to be 2.3×1021 kilograms (of which more than a third is accounted by Ceres), which is less than that of Pluto.

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 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 classic of science fiction stories, since they are always portrayed as being so dense that adventurous measures must be taken to avoid an impact. In reality, the asteroids are spread over such a high volume that it would be highly improbable to even pass close a random asteroid. For example, the numerous space probes sent to the outer solar system, just across the main asteroid belt, never had any major problem.

See also: Asteroid, Asteroid family\n