It looks like space was created specifically for scenes in space, while outer_space was perhaps meant for depictions of detailed stars in the sky or seeing space through a portal or somesuch. However, the latter only has 15 images under it and space has evolved to cover both cases.
On the other hand, outer_space is a much more descriptive tag (as "space" can mean...well...a space between things). Even though space has the majority of the entries at the moment, I would vote to alias space -> outer_space.
If either case comes about, I'll edit the wiki of the survivor to clarify the tag, especially in cases where sky should be used instead or at the same time.
Updated by OOZ662