The Asteroid Field Biome is a relatively desolate biome, in which small chunks of asteroids exist, spaced out quite far away from each other. These asteroids are made out of Meteorite Rock and Magmarock, containing Copper ore, Silver ore, and Gold ore in huge veins.
From what it seems, these areas are for cosmetic use only, seeing as the large-scale construction planets provide much better terrain to build, and there is no need to obtain anything from this biome.
One very important thing about this biome is that, while it may appear that there is no floor at first, there is. DO NOT fall too far or come within 2 blocks of this floor. If you touch this floor you will die and be unable to recover any lost items as you can't get close enough even if you build all the way down to it.