If we spend anything on national defense (and such expenditure is constitutionally authorized), protecting and maintaining cislunar space capabilities falls into the sphere of legitimate federal activity.
I am not so ready to give our president a blank check just because it is, by some loose constructionist Hamiltonian theory, necessary for "national defense." This just gives IMHO, the founders would be rolling in their graves if they knew our military was devoted to maintaining a world empire even bigger than the one they revolted against. They would certainly not see such an empire (and the attempt to extend it, courtesy of our socialist commander in chief, to new planets as "constitutionally authorized" in any meaningfuly sense.