2 Wicky - Hooverphonic
Assignment: Outer Space - Kernfeld
Intelligent Creatures & Technology Eats Its Young - Powerman 5000
Somewhere on the Other Side of Nowhere - Powerman 5000
Wandering Star - Kid Beyond (or the original by Portishead)
Space Oddity - David Bowie
Astro Zombies - Misfits (optionally, listen to the version that's featuring Joan Jett)
Fly Me to the Moon - Frank Sinatra
The Final Countdown - Europe
Space Truckin' - Deep Purple
Man On The Moon - R.E.M.
Telstar - The Tornados
White Light - Gorillaz
Gravity - Gorillaz
Starshine - Gorillaz
Space - M.I.A.
Astronomy - Blue Oyster Cult (or the Metallica version, if you prefer it)
Sometimes when I want a laugh I listen to Electronik Supersonik by Zlad.
Also worth mentioning are the soundtracks to various space-themed tv shows, movies, and games, some of my favorite being Battlestar Galactica (some of these are duds, but the good ones are great!), Descent (the game), and the original Starcraft OST.
You can also pretty much just search for "space ambient" on youtube and come up with a dozen great results.