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Encyclopedia > Main 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. It is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts.


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 relic of the primitive Solar System, but many observations point to an active evolution of the physical conditions so the asteroids themselves are not particularly pristine. Instead, the objects in the outer Kuiper belt are believed to have had little change since the solar system's formation.

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.

Despite this, tens of thousands of asteroids are currently known, and estimates of the total number are in millions range. 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 2.31021 kilograms (of which more than a third is accounted for 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 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 science fiction stories less concerned with realism than with drama, since they are always portrayed as being so dense that adventurous measures must be taken to avoid an impact. Proto-planets in the process of formation may look like that, but asteroid belts don't. 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 movie 2001: A Space Odyssey is unusual in that it does portray realistically the ship's "encounter" with a lone asteroid.

See also

The Minor Planets
Vulcanoids | Main belt | Groups and Families | Near-Earth objects | Jupiter Trojans
Centaurs | Trans-Neptunians | Damocloids | Comets | Kuiper Belt | Oort Cloud
(For other objects and regions, see: Binary asteroids, Asteroid moons and the Solar system)
(For a complete listing, see: List of asteroids)

The Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroids
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass

  Results from FactBites:
New Horizons Crosses The Asteroid Belt (1740 words)
The first spacecraft to transit the asteroid belt was NASA's Pioneer 10, which made its epic crossing in 1972 on the way to the historic first encounter of a spacecraft with Jupiter.
Fortunately, the asteroid belt is so huge that, despite its large population of small bodies, the chance of running into one is almost vanishingly small - far less than one in a billion.
In addition to main belt asteroid flybys, NASA's NEAR and the Japanese Hayabusa mission both have made orbital rendezvous and landings on asteroids closer to Earth.
Asteroid Facts - Explore the Cosmos | The Planetary Society (1207 words)
Main Belt: Most minor planets orbit the Sun in the main asteroid belt, a vast ring of hundreds of thousands of rocky objects circling between the orbits of Mars and Jupiter, from 2 to 4 AU from the Sun.
In 2006, scientists reported that at least three bodies in the outer part of the main belt demonstrate classical cometary behavior, and defined a new class of bodies called the main belt comets.
Many main belt asteroids may in fact contain a lot of ice, especially in the outer part of the belt; the line separating asteroids and comets is growing increasingly fuzzy.
  More results at FactBites »



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