I think there should be an exploitation that works for asteroids only. As they dont have a liveable atmosphere and are probably only being habited for mining purposes there should be a selection of exploitations for asteroids only that allows it to be mined for one of the rare metals in the game. Change the value for each metal depending on the difficulty to discover/manufacture it. This would allow for the asteroid to gain whichever metal is being harvested as a resource but allow it to be changed later on based on need. Adding this would of course make it impossible for asteroids to have pre existing luxury/strategic resources but that works for me. Pew Pew