It seems simple and reflective, but in truth, from the mind of group member Brian May, it's the story of a group of space travelers who, in theory, take on a year-long mission of discovery, only to return home to find it is, in fact, 100 years later, thanks to "time dilation" effect of Einstein's Theory of Relativity.
It's a complete and utter dichotomy. A song that seems to take you back in time, especially with its sound, actually propels you a century into the future.
It's my favorite Queen song. For all the copious amounts of variety of music this incredible group made, in the end, simplicity wins out for me. :)