For such a futuristic word, astronaut sure has old origins. It was popularized by NASA for the Mercury missions, but lingered in the English language among the stories of science fiction writers, arguably as far back as the nineteenth century. Whoever did create it obviously combined two Greek words, astro (also the source of astral), meaning "of the stars", and naut (also the source of nautical), meaning "of the sea". Astro is a combining form of the earlier Greek word astron, which meant "star". This comes from the Ancient Greek word aster, from Proto-Indo-European hster, both still meaning "star". Here it gets interesting: hster is reconstructed as having come from the even earlier PIE word hehs, which meant "to burn", though it wasn't until thousands of years later that we realized that the stars are continuously burning. Naut, on the other hand, comes from nautes ("sailor"), which itself derives from naus, ("ship"). This probably traces to the Proto-Indo-European word nau, which meant "boat" and may or may not come from the earlier PIE word meaning "swim". So while we can definitely say that an astronaut is a "star sailor", it's only a possibility that they are also "fire swimmers".
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.