Dennis Tito, space-tourist and uber-rich dude, is planning a Mars Fly-by. The 2018 Launch promises a 501 day Free-Return trajectory – a great big loop past Mars (ain’t stopping) and return to Earth for a high-speed re-entry. Such a mission – if successful – will be a proof of concept that Mars flights aren’t impossible for humans. Actual landing flights will take ~180-130 days in orbital trajectories, either way, so they will require even less zero-gee exposure (if spin-gravity isn’t used) than Tito’s proposal. People have stayed in orbit for much longer than that, and have soaked up even more cosmic-rays than that. So, as Robert Zubrin has noted more than once, there is no real space-medicine case against flights to Mars.