Pillar · Region · Updated May 2026
The asteroid belt.
The asteroid belt is a region between Mars and Jupiter — roughly 2.2 to 3.2 astronomical units from the Sun — containing millions of rocky bodies. Total mass is about 4% of the Moon, with one-third of that in the dwarf planet Ceres alone. It is mostly empty space, not the dense rock field shown in films.
Open the 3D viewer →Why it exists
The asteroid belt is what remains of the protoplanetary disc material that never managed to coalesce into a planet, because Jupiter's gravity stirred up the velocities of the bodies in that region too quickly. Instead of accreting, they collided and fragmented. Today the belt holds an estimated 1.9 million asteroids larger than 1 km, plus countless smaller fragments down to grain size.
Mostly empty
The total mass of the asteroid belt is small — about 4% of the mass of Earth's Moon. Spread across a torus 1 astronomical unit thick, this means the average distance between large asteroids is millions of kilometres. The Voyager probes, Pioneer 10 and 11, Galileo, Cassini, New Horizons, and Juno have all flown through it without incident.
The largest bodies
Ceres (940 km diameter) is by far the largest, and the only dwarf planet in the belt. Vesta (525 km) is second. Pallas (513 km) is third. Hygiea (430 km) is fourth. Together these four account for about half the belt's total mass.
Missions
NASA's Dawn mission orbited Vesta in 2011-12 and Ceres from 2015-18, the first spacecraft ever to orbit two extraterrestrial bodies. JAXA's Hayabusa missions returned samples from the near-Earth asteroids Itokawa (2010) and Ryugu (2020). NASA's OSIRIS-REx returned a sample from Bennu in 2023. NASA's Lucy mission, launched 2021, is en route to study seven Trojan asteroids that share Jupiter's orbit.
Bodies in this category
Frequently asked questions
How many asteroids are in the asteroid belt?
Roughly 1.9 million larger than 1 km, plus millions more smaller. The belt's total mass is only about 4% of the Moon's.
Where is the asteroid belt?
Between the orbits of Mars and Jupiter, roughly 2.2 to 3.2 astronomical units from the Sun.
Could a spacecraft fly through the asteroid belt?
Easily. Ten or more spacecraft already have, with no collisions. The belt is mostly empty space; large asteroids are typically millions of kilometres apart.