The asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found.
Despite popular imagery, the asteroid belt is mostly empty.
The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully.
Nonetheless, tens of thousands of asteroids are currently known, and estimates of the total number range in the millions.
About 220 of them are larger than 100 km.
The biggest asteroid belt member, and the only dwarf planet found there, is Ceres, which is about 1000 km across.
The total mass of the Asteroid belt is estimated to be 3.0 to 3.6×1021 kilograms, which is 4 percent of the Earth's Moon.
Of that total mass, one third is accounted for by Ceres alone.
The high population makes for a very active environment, where collisions between asteroids occur very often (in astronomical terms).
A collision may fragment an asteroid in numerous small pieces (leading to the formation of a new asteroid family), or may glue two asteroids together if it occurs at low relative speeds.
After five billion years, the current Asteroid belt population bears little resemblance to the original one.