Mars Mission’s Invisible Enemy: Radiation

"On a trip to Mars and back, probably every cell in the body would be hit by an ionized particle or a proton, researchers say, and they have very little idea what that would do. 'If every neuron in your brain gets hit, do you come back being a blithering idiot, or not?' asked Dr. Derek I. Lowenstein, the chairman of Brookhaven's collider accelerator department."