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 permenantly move. There are several variants of moonlets, of varying sizes, and with different objects on/in them. Some of the points of interest contained on/in moonlets are the Phage-class Station, the space bar, and the Ganymedean Anarchy base of operations ( aka G4A Baseteroid). There is also a variant that contains uranium caves, which have uranium crystals in them that can be collected and sold for 40K E$ each.
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.