“Any future Mars mission would be fraught with difficulty, including the threat from high-levels of radiation once astronauts have landed, and Stofan claims NASA are still trying “to figure out how to adequately protect them.”
Another challenge is presented by Mars’ thin atmosphere, which makes landing a colossal spaceship an incredibly difficult proposition. Scientists are still researching a “way of slowing yourself down really rapidly” under such conditions.”

via NASA Plans Manned Mars Mission By 2035 | DARK MATTER SPACE.