Reason: The wiki for space uses outer_space in its definition of the tag, the two tags seem to be used in the same manner on images, and "outer space" is a little more clear a tag on what is meant than just "space."

What about th innes space of the earth?
(Joking, but that concept exist, some people actually belive on the existence of a similar environment on the inner parts of the earth.)

Weeeelll. If an image depicts that, I guess it gets the inner_space tag lol.