Fresh Aire V (Mannheim Steamroller) was written to a dream of sailing to the moon. An odd mix of astral and nautical. The more choral pieces work well. The upbeat synth, not so much.

Between 'Trek, Babylon 5, and the extended Star Wars media, you should be able to find some good pieces that aren't the imperial March.

Dazed and Confused - the live version from Led Zeppelin's Song Remains the Same has a "guitar solo" in the middle (starts about 9 minutes in) that is rambly, echoey, and trippy as hell. Which happens when you use a bow. When I think"Fantasy space" (or Astral Plane), that's the sound I think of.

And shanties are essential.