The Planets is a good resource because of the variety of moods is presents. Other pieces of composed music you could use are Star Wars, Close encounters of the third kind, Also sprach Zarathustra by Strauss (used for 2001Space Odyssey, but not written as space music). You can sometimes get all of these on one CD. There are anumber of musical plays Blast Off! is a good one if you can get hold of it. There is a short play in Phantasmagoria, First Kids in Space pub by Golden Apple . Out of the Ark are certain to have one too. It is a wonderful subject for creative music which the kids always enjoy and can produce some great pieces. Try using Fantasia with the sound off - the section about how the earth was formed and get the children to make up a soundtrack (tricky) or use it as a stimulus.