From ΔV: Wiki

While exploring the rings there is a possibility that you'll come across Moonlets. Moonlets are some of the largest objects in the game world, and are impossible to mine, or move. There are several variants of moonlets, of varying sizes, and with different objects on them. Including the Phage-class Station, the space bar, and the Ganymedean Anarchy base of operations. There is also a variant that has uranium caves, which have uranium crystals in them that can be collected and sold for 40K E$.

Finding a moonlet gives you the "This is no Moon" achievement.

Finding a moonlet with uranium caves gives you the "Fissile Goods" achievement.

Finding a moonlet with the space bar will give you the "A little break" achievement.

Finding a moonlet with the Ganymedean Anarchy Station will give you the "Inside Job" achievement.