GURPS Space has at least 3 editions prior to GURPS 4th edition, those being:
- GURPS Space
- GURPS Space 2e
- GURPS Space 3e
Where the edition refers to the book, and Not the edition of GURPS it was written for, as far as I know.
I have 2e and 3e in my bookshelf at home. The 2e is the classic 128-page GURPS book, and the 3e is a fatter book that has more content, but can't check the differences as I'm not with the books at the moment.