There are trillions of bodies in the asteroid belt. A lot of them could be mined for valuable minerals and resources needed for the development of space colonies. Comets that come close to the Sun could also be mined for the most valuable resource in space: water

The asteroid belt is not only a good place for mining, but also for space colonies. At first these colonies would be simple mining outposts, but would fast grow into large space cities. A proposed model for such cities consists in linking by a cable two asteroids of about the same size and spinning the system in order to generate artificial gravity on the upper side of both asteroids.

Another type of asteroid colonies would consist in mining the interior of an asteroid. When the interior would be completely excavated, the asteroid should be spined up in order to provide gravity on the interior surface of the excavated asteroid. This type of colony resembles the O'Neil cylinder.