Between Mars and Jupiter lies a ring of rocky debris called the asteroid belt. It is not a dense field of tumbling rocks like the movies suggest. The belt is mostly empty space. Its total mass is about 4 percent of the Moon's, and the largest object, Ceres, contains a third of that. If you stood on one asteroid, you would be unlikely to see another without a telescope.

The belt exists because Jupiter's gravity prevented the material there from coalescing into a planet. The giant planet's pull stirred up the region, keeping the planetesimals from settling into stable orbits. What remains is a scattered population of rocky bodies, with distinct groups based on their composition and orbital characteristics.

The belt is not static. Collisions break asteroids apart, and gravitational resonances with Jupiter fling some of them inward toward the inner solar system. Those escapees become near-Earth asteroids and, occasionally, meteorites that land on Earth.