I think that you can set some resources in some planets, which can be discovered by players. And, players can collect them to produce fuel of their crafts. Also, they can be just set as some science discoveries with funny texts.
For example, you can set a series of water ice mineral occurrence in Permanently shaded regions in Lunar, and players can gain hydrolox fuel by some kind of instrument in these regions. Of course, you must discover it by some kind of instrument firstly.
It will make the game be more playable like Minecraft (Lunar caves? Mars caves? :) )
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