What is the asteroid belt?
The asteroid belt is a broad region between the orbits of Mars and Jupiter where most known asteroids orbit the Sun. Despite its fame, it is mostly empty space and contains surprisingly little mass.
Picture the Solar System from above. Inside the orbit of Mars lie the rocky planets; outside Jupiter lie the giants. Between Mars and Jupiter, roughly 2.1 to 3.3 times Earth's distance from the Sun, is the main asteroid belt, home to most of the more than a million asteroids discovered so far.
Films often show spacecraft dodging tumbling boulders, but the real belt is extremely sparse. The average distance between sizeable asteroids is enormous, typically hundreds of thousands of kilometres or more, and probes such as Dawn and Juno have crossed it without difficulty. The total mass of the belt is only around 3 per cent of the mass of our Moon, and the dwarf planet Ceres, about 940 kilometres across, holds a large share of it, about two-fifths. Vesta, Pallas and Hygiea are the next largest bodies.
Why is there no planet there? Jupiter's powerful gravity is the main reason. It stirred up the orbits of the bodies forming in this region, raising their collision speeds so that they tended to shatter rather than merge, and over time much of the original material was scattered away. Jupiter still shapes the belt today: at certain distances, where an asteroid's orbital period would be a simple fraction of Jupiter's, its regular tugs clear out zones known as Kirkwood gaps.
The belt is the source of most meteorites. Collisions produce fragments, and the slow push of sunlight on a spinning rock, the Yarkovsky effect, makes them drift until they reach one of these resonant zones. Jupiter's influence then stretches their orbits until they cross Earth's path. Families of fragments from past collisions, such as the one associated with Vesta, can still be traced in the belt today.