> What concerns me most is the radiation risk of travelling close to light speed. Surely we'd pass by some ionising radiation, or weakly-interacting neutrinos.

> What concerns me most is the radiation risk of travelling close to light speed. Surely we'd pass by some ionising radiation, or weakly-interacting neutrinos. It's much worse than that. At those speeds, the cosmic background radiation would get blue-shifted to the point that you'd be constantly bathed in x-rays and gamma rays.