The Asteroid Belt
The Asteroid Belt is a huge ring of rocky leftovers that orbits the Sun between Mars and Jupiter. It contains more than a million known asteroids, plus probably billions of smaller ones. The belt is so wide that even though it sounds packed, you could fly a spaceship through it and never come close to a single rock.
- Where is it?Between Mars and JupiterAbout 2 to 3 times further from the Sun than Earth
- How wide?About 200 million kmBigger than the gap between Earth and the Sun
- How many asteroids?Over 1 million knownPlus probably billions of smaller ones
- Biggest objectCeres940 km across, classed as a dwarf planet
- Total massHalf the mass of the MoonSpread across an enormous volume
- First foundCeres in 1801By the Italian astronomer Giuseppe Piazzi
The four biggest asteroids
Just these four make up roughly half of all the mass in the belt.
Ceres is so big that it is also classed as a dwarf planet (the same category as Pluto). The other three are giants of the belt but still much smaller than any moon worth speaking of.
What is the Asteroid Belt?
The Asteroid Belt is a flat doughnut-shaped region of space between the orbits of Mars and Jupiter. Inside it are millions of rocky bodies called asteroids, ranging from the dwarf planet Ceres (940 km across) down to grains of dust. Despite the huge number of objects, the belt is mostly empty space because it is spread over such a vast volume.
Why is there no planet here?
When the Solar System was forming, the area between Mars and Jupiter had enough material to make a small planet. So why didn't it? The answer is Jupiter.
Jupiter is the biggest planet by far, with very strong gravity. Its pull constantly disturbed the orbits of the building-block bodies in this region, smashing them into each other instead of letting them settle into a single planet. Over billions of years most of the material was either flung out of the Solar System or fell into the Sun. What remains today is just a tiny fraction of the original belt: less than one thousandth of the mass of Earth.
Visiting the belt
Several spacecraft have crossed the Asteroid Belt on their way to the outer Solar System: Pioneer 10 and 11 in the 1970s, Voyager 1 and 2, Galileo, Cassini, New Horizons. NASA's Dawn mission was the first to deliberately study belt objects up close. It orbited Vesta from 2011 to 2012 and Ceres from 2015 to 2018, taking detailed photos and gathering data on what they are made of.
Future missions are planned to study small metal-rich asteroids like Psyche, which is thought to be the leftover iron core of a small world that broke apart in the early Solar System.
Deeper dive: Kirkwood gaps, asteroid families and the Late Heavy Bombardment
The Asteroid Belt is not uniform. There are several almost-empty rings within it called Kirkwood gaps, discovered by American astronomer Daniel Kirkwood in 1866. These gaps form where the orbital period of an asteroid is a simple fraction of Jupiter's, creating a gravitational resonance that destabilises any object trying to settle there. Resonances at 3:1, 5:2 and 7:3 of Jupiter's orbit are the most prominent. Asteroids that wander into these gaps are eventually flung onto Mars-crossing or Earth-crossing orbits.
Asteroids in the belt are not randomly distributed. They cluster into asteroid families, groups of bodies that share similar orbits and compositions because they are the fragments of a single parent body that broke up in a giant collision long ago. The Vesta family, for example, includes thousands of small rocky bodies thought to have been blasted off Vesta in one or more huge impacts that left a 500 km wide crater on its south pole.
About 4 billion years ago, the inner Solar System may have gone through a violent period called the Late Heavy Bombardment, during which large numbers of asteroids and comets struck the Moon, Earth, Venus, Mercury and Mars. The dark "seas" you can see on the Moon's near side are the lava-filled basins of giant impact craters from that era. The cause may have been a rearrangement of the outer planets that flung huge quantities of small bodies into the inner Solar System. Asteroid and comet impacts during this period probably delivered much of Earth's water, and the chemical building blocks for life soon followed.
For another rocky leftover region way beyond Neptune, see the Kuiper Belt. For more on individual asteroids, read what is an asteroid.