They spend so much time out there with the high energy particles shooting holes in their brains, by the time they get back they might as well be from space.

(That and, NASA asks them questions like, would you be willing to give up your family and your job and put your life on hold for 5 years for the chance to go into space? 10 years? 20 years? And if you flinch, you're out.

...might be selecting for a certain kind of personality, there.)