Here's another thing to factor in: space debris.

BBC TV prog "the Trouble with Space Junk" last night (available for 30 days).

Yea Yea, they are cataloging billions (literally) of things down to quite small pieces. & the probability of strikes will be low. But when you are talking human life, the repercussions are such that the stakes are sooooo much higher. Your Marsonauts will pass through this debris field.

And it was clearly stated that the logistics are way beyond current computers to predict all of the relevant bits. (It took 12 months to process 14 months data for the bigger items on several supercomputers around the world). then there is all of the bit on bit strikes that will occur over the years, generating ever smaller, more numerous debris that is still significant. Clean-up is not a viable option yet, by a long chalk.

We better get to Mars within a decade, because by 2100 the debris field will be denser. Smaller bits maybe, but kinetic energy rules!