Astronaut
The astronaut embodies humanity's quest to solve mysteries rather than to constitute one. We know precisely what happens to human muscle tissue in zero gravity, how cosmic radiation affects cellular DNA. The only remaining mysteries concern what astronauts might discover, not what they fundamentally are.
Time
Time remains one of the most profound mysteries in physics. We cannot adequately explain why Time moves in only one direction. Philosophers debate whether Time actually flows or whether past, present, and future exist simultaneously in a 'block universe.' Physicists cannot reconcile quantum mechanics with general relativity partly because of Time's peculiar behaviour.