Silent Running [1972] (* * * *)

The idea is interesting in that the earth has already been destroyed (for a change:) and now a few people are sent by govts to nurture trees on satellites.
When the govt gives the order to destroy the experimental forests, one nurturing man can't do it. He takes his satellite to the rings of Saturn and teaches a droid to take care of it. But loneliness gets him and he destroys himself in the end.
Very depressing movie.
But again the theme of sacrificing himself for a larger idea, a forest of birds and animals and trees, appealed to me.