The first attempt at really covering Science Fiction as a whole outside of the GURPS Basic Set.
From the intro:
"There's this about space: It's big. Sometimes entirely too big. This project has been tough and long-delayed. But now that it's done, it's probably the best worldbook yet, and it's certainly the biggest ... it just kept growing.
"There's just so much to cover. In fact this book "spun off" several other projects."
The spin offs became the GURPS Space Atlas series, GURPS Ultra-Tech, and GURPS Aliens and later GURPS Space Bestiary.
Certainly the most ambitious of all the GURPS worldbooks. It laid the foundation for all the science-fiction oriented expansions that were released for GURPS over the years.