According to Physics World’s Obama’s Dream of Mars At Risk From Radiation, any human mission to Mars during the 2020-2040 timeframe will be a bit radioactively hot. As noted in the article, “The result of two separate solar-activity cycles, which are both predicted to hit their maximum during the period, the increased radiation could cause radiation sickness and an increased cancer risk for any astronauts venturing away from the safety of the Earth’s atmosphere.“