The main asteroid belt is a broad region between the orbits of Mars and Jupiter containing many rocky and metallic bodies. It is part of the solar system’s early history: material left over from planet formation was shaped by gravitational interactions, especially with Jupiter. Despite the familiar illustrations of crowded rocks, the objects are spread over an enormous volume of space.
Where is the belt?
Most main-belt asteroids travel around the Sun between Mars and Jupiter, often at distances of roughly 2 to 3.5 astronomical units (AU) from the Sun. An AU is the average Earth–Sun distance. The belt is a region of many individual orbits, not a solid band or a single continuous object. Spacecraft have crossed it repeatedly on their way to the outer planets.
What is in it?
Asteroids vary widely in size, composition and shape. Some are rich in silicate minerals, while others contain more carbon-bearing material or metals. Ceres is the largest object in the main belt and is classified as a dwarf planet; Vesta is another large, differentiated body. Small fragments from asteroids can become meteoroids, and some eventually reach Earth as meteorites.
Why is there no planet there?
The belt does not hold enough mass to form a planet today. In the early solar system, Jupiter’s gravity disturbed the material between Mars and Jupiter and impeded growth into a single large planet. Collisions and changes in orbit continued to reshape the population. Astronomers study asteroid families and their compositions to trace those events.
Spacecraft and related populations
NASA’s Dawn mission orbited Vesta and Ceres, revealing two very different worlds. Lucy is studying Jupiter’s Trojan asteroids, which share Jupiter’s orbit rather than belonging to the main belt; it also encounters main-belt objects along the way. Near-Earth asteroids are another population whose paths bring them close to Earth, so “asteroid” does not always mean “main belt.”
Explore NASA images and sources
NASA asteroid, comet and meteor image gallery · NASA asteroid multimedia · NASA asteroid facts · NASA asteroid belt illustration. The last image is an illustration, not a photograph of a crowded belt.
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