Mekka is actually Earth All Along, or perhaps a human colony world.
It would make a lot of sense, since the Mekkans appear completely human. The astronauts' spaceship could have simply gone into a very long orbit that brought them back to Earth far enough in the future for the big war to have wiped out the historical records, like in Planet of the Apes.
Or, if it's a colony world, perhaps humans discovered Faster-Than-Light Travel after the astronauts left on their sleeper ship, and the astronauts actually arrived at their intended destination only to find a thriving human colony that had totally forgotten about them.
