I have to admit, though, my tendency toward etymological literalness makes the phrase "space marine" seem kind of oxymoronic to me, since "marine" means "of the sea" in Latin. It just seems to me that if you wanted a word for someone like a military marine, but in space -- i.e. a member of an infantry service specializing in spacecraft-based or interstellar operations -- the appropriate construct would be something like "celestine." Although I guess that doesn't sound very macho.