Not wishing to stretch the point, but an alien can simply refer to a person from another country. An "alien" does not have to be, as you claim, an 'Extra-terrestrial Being'. But even if Aliens (as per GH's question) were ETs, that does not necessarily equate to their being Astronauts.
It's all to do with your narrow point of reference, not the Queen's English .
Maybe you have watched too many Hollywood movies.