And if a manned mission to Mars, or anywhere for that matter, gets hit by a CME? Ouch. Toasties for astronauts.
We are such small, pityful creatures to go off exploring the universe...
A.C. Clarke had it right in 2001...we will explore space when we can shed these bodies of fragile flesh for metal constructs that are in themselves ships. Until we can do that, all we can do is potter about in the backyard swimming pool of the inner solar system, never reaching the sea of the larger universe. The key to space isn't ever-faster space drives - it is making ourselves into constructs far more suited that we are.
N.B. - Clarke wasn't the only prophet of that view, but he is the one that I would expect most to know...