I have vivid memories of watching Silent Running as a child, of it making me feel awfully sad and teary-eyed. I also vividly remember how moving I thought Joan Baez's songs in the film were.
Listening to the soundtrack now, I could take or leave the Baez songs, but the actual soundtrack itself is wonderful. It reminds me of other '70s-dystopian-y sci-fi film soundtracks, like Fred Myrnow's Soylent Green score, or Jerry Goldsmith's Logan's Run score.
Here are a couple of tracks from Silent Running, plus the complete album, which was rereleased in 1984 but isn't available anymore, it seems.
(Saturn especially is an interesting track - the 'scary violin' part that sets in at about 39 seconds, sounds like doo-wop with screechy Psycho strings..)
Tending to Huey
the whole Huey, Dewey and Louie