Off the top of my head:

For classical: Beethoven's 7th Symphony, second movement; Tchaikovsky The Nutcracker - The Coffee and Symphony 6; Aram Khatchaturian's Gayane ballet suite no. 3;

For classic rock: Pink Floyd's Childhood's End; Roxy Music's For Your Pleasure; Pretty much any Kraftwerk; U2's Zooropa (the title track mainly, but there are some other perfectly scifi songs); Yes's Starship Troopers; Smile's  (1/2 of Queen before they were Queen) Planet Earth; Year of 39 by Queen now that I think about it.

A few other thoughts

More Pink Floyd- Atom Heart Mother, Obscured by Clouds, When You’re In, and if you can find it, the original version of Echoes. It’s space themed.

Giorgio Moroder’s Chase

Maybe something like Visage, Joy Division, or New Order. Something with lots of synths.

I forgot Gary Neuman

Technics 60W amp with matching turntable & cassette deck sporting new for its time Dolby noise reduction running through a pair of EPI loudspeakers, all purchased new in 1980.

Surviving vinyl from that era --- which I define as Moldvay D&D and pre-Zeb's SF --- that I own includes: 

AC/DC Back in Black & Who Made Who

David Bowie Let's Dance

Def Leppard Pyromania

Dio Holy Diver

J. Geils Band Freeze Frame

John Cougar Uh-Huh & American Fool

KISS Creatures of the Night/Lick It Up, & Animalize 

Motley Crue Shout at the Devil

The Pretenders (title album)

Prince 1999

Queen Greatest Hits (1981)

Quiet Riot Metal Health

Ratt Out of the Cellar

Rolling Stones Tattoo You

Run DMC (title album)

Van Halen Diver Down & 1984 

ZZ Top Eliminator & Afterburner 

...and numerous cassette mixes from the above. I never bought any studio cassettes.

Star Trek and Star Wars are out: they're too familiar and closely linked to their series. Actually anything by John Williams is too familiar. Babylon 5 would be a good choice, but if you play it you can only play that; it doesn't mix with anything else. The Last Starfighter is a great choice if your players don't know it by heart (if they do they'll be thinking about that instead of Star Frontiers). Serenity is a surprisingly good fit, since it mostly avoids the space-fiddle of the series. Sky Captain and the World of Tomorrow and Zathura are both good for pulpy, over-the-top drama if your game goes that way.

Actually, thinking about it, you could probably get away with playing Williams's Space Camp, as it's sufficiently removed from Star Wars and Close Encounters that some people might not even recognize it as Williams.

Interestingly, some of the early incidental music from Doctor Who might do well if put sufficiently far in the background.