If you've got lander upgrades, mine the gas giants. Most of them are loaded.
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It's too bad that you'll have all the RU's you need by the time you can actually do this. This really seemed like a good idea to type up at first. I'll post anyway.
I did not intend to type Gas Giants. Though for a typo, that's pretty damn funny. I really did mean Super Giants are a good place to mine since they all have around 6-10 planets each and many of those planets have really nice stuff on 'em.