Freeman Lowell has several cargo containers dumped off the spaceship, to make it appear the ship has suffered an accident. A wide shot shows the ship receding in the distance, with the containers rolling weightlessly in the foreground. As the foreground containers pass slightly in front of the more distant ones, they don't mask them; their images overlap. The containers were apparently composited into the shot with simple multiple exposures.
The space ship "Valley Forge" from Silent Running was re-used in one episode of the television series "Battlestar Galactica" (1978-1979). It was called the "agro-ship," used to grow food for the colonists.