Just one return trip to Mars would eat up about two-thirds of an astronaut's lifetime "safe" dose of radiation, a new study has shown.
Using the radiation detector mounted on the Curiosity rover that was sent to Mars aboard the Mars Science Laboratory (MSL) probe last year, Southwest research Institute scientist Cary Zeitlin, together with other NASA researchers, recorded how much cosmic and solar radiation the rover was exposed to as it made the 253 day, 560 million kilometre journey to its rendezvous with our red neighbour.
The average daily dose of radiation an astronaut would receive, the data reveal, is about 2 millisieverts, equivalent to a barium meal medical investigation on a daily basis.
Over the entire journey, this level of exposure would add up to about 660 millisieverts for the round trip; and even if the venture was one way, it would still be 330 millisieverts, which is a substantial chunk of an astronaut's safe career exposure limit of 1 sievert, which nonetheless carries a 3% cancer risk. As the time point out in their paper in Science, "It is clear that the exposure from the cruise phases alone is a large fraction of (and in some cases greater than) currently accepted astronaut career limits. Time spent on Mars might add considerably to the total dose..."
33/66 rem (330/660 mSv) is quite a dose. The normal "occupational limit" is normally 1 to 5 rem per year (10 to 50 mSv). Of course, that is all without radiation shielding. I believe there is significant effort to find better radiation shielding, or to design the ship so that various non-living compartments would also be oriented to provided substantial shielding. Food, waste, water, etc.