The asteroid belt is so empty that spacecraft fly through it unharmed
Films show the asteroid belt as a crowded field of tumbling rocks. In reality its known members sit about a million kilometres apart on average, and many uncrewed probes have crossed it without a scrape. The belt is what remains of a planet that never formed, and it lost almost all its original material.
It began as planetesimals, small building blocks drifting in the primordial solar nebula. Gravity from Jupiter and Mars stirred them up, so instead of gently merging they smashed each other and most of the emerging protoplanets apart. Within the first 100 million years, 99.9 percent of the original mass was gone, and some fragments fell inward to strike the rocky planets. Jupiter still sculpts what is left: wherever an orbit would line up in a simple rhythm with the giant planet, asteroids get nudged away, leaving empty lanes called Kirkwood gaps.
The survivors are lopsided. Ceres, about 950 kilometres across, is the only one big enough to count as a dwarf planet, and together with Vesta, Pallas and Hygiea it holds about 60 percent of the belt's mass. The rest range down to boulders a few metres wide. Large collisions still occur, creating families of fragments that share similar orbits and makeup. Most asteroids fall into carbon-rich, silicate or metal-bearing X-type classes. The belt is the innermost and smallest disc of debris circling the Sun, far smaller than the Kuiper belt and Oort cloud farther out.
People guessed something lay there long before seeing it. In 1596 Johannes Kepler, puzzling over Tycho Brahe's data, felt the gap between Mars and Jupiter was too wide and wrote that he would place a planet in it. In 1766 Johann Daniel Titius slipped a numerical pattern for planetary distances into an anonymous footnote of a book he was translating, noting a blank between Mars and Jupiter and asking whether the Lord Architect would have left that space empty.
When Giuseppe Piazzi found Ceres on 1 January 1801, it sat right where the pattern pointed. He named it after the Roman harvest goddess and patron of Sicily and first took it for a comet, but it lacked the fuzzy coma. Heinrich Olbers spotted Pallas about 15 months later.
Source: Asteroid belt
