This is a weak trademark, it has no secondary meaning that I can discern. You have marines. They're in space. Thus, "space marines." That would seem to fall under the weaker classes of trademark protection[1] and even though I have long known about Warhammer 40K, the words "space marines" do not make me (or most other people I know) think of Warhammer 40K. Personally, that term makes me think of the Aliens movie and Starcraft. They're nowhere near as famous.
Of course, the problem here is that they have more lawyers and wherewithal to contest this than most. Your average person does not have the resources to make a federal case out of this and their lawyers have a perverse incentive to be assholes when trying to prop up a weak trademark claim.
This is all true and the interesting part of the story. My point is that just crying "prior art" doesn't work in trademark. Observations like yours do.
So, putting "Space Marine" in a title might a bad idea, but having marines in space (with or without mechanized body suits) should be fine. To reduce legal risks, you might want to call them "Space Core Maines" or "Space Marine Core".
