|Picture of Space Shuttle Atlantis, from NASA web site|
From the linked article: "a 180-day flight to the Red Planet followed by a 600-day stay on Mars and a 180-day flight back to Earth would expose astronauts to about 1.01 sieverts (radiation units)." According to the Nuclear Regulatory Committee, the maximum "total effective dose equivalent' is .05 sieverts (or 5 rems. Oh, nuclear dose coversions are fun) OR "the sum of the deep-dose equivalent and the committed dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (.5 sv.) OR "(2) The annual limits to the lens of the eye, to the skin of the whole body, and to the skin of the extremities, which are:
(i) A lens dose equivalent of 15 rems (0.15 Sv), and
(ii) A shallow-dose equivalent of 50 rem (0.5 Sv) to the skin of the whole body or to the skin of any extremity."
I'm not a scientist. But a maximum of 5 vs. a maximum of 50 seems....rather different. Just sayin'. But that's yearly. 180 days plus 600 days plus another 180 days is...2.63 years. So, within regulation, regardless of which one of those dose limits you're looking at. Though according to the XKCD radiation dose chart I linked recently, 8 Sv is a fatal dose. Everything is just so close.
NASA also has a neat interactive thinger on their site called "How Astronauts are Affected by Space Exploration". You can click around on different body parts and systems, and Radiation Risks is one of the segments on the widget. For more dose comparison, 1 mSv (millisievert) is equal to "about 3 chest x-rays". Also, on the topic of shielding, it says "Shielding is not effective against GCR [Giant Costmic Rays](penetrating protons and heavy nuclei), but it can be against SPEs [Solar Particle Events](largely medium energy protons)." All things to keep in mind when I'm trying to write realistic and yet still compelling space science fiction (especially after I look up giant cosmic rays and solar particle events in more detail).