"Space radiation is one of the most deadly hazards human crews of interplanetary missions face. The Martian Radiation Environment Experiment, known as MARIE, has revealed that space radiation is more intense than scientific models have indicated.

Similar instruments are on space shuttles and the international space station, but MARIE is the first radiation detective to travel beyond Earth's magnetosphere....The exposure to the heavy nuclei, up to and including iron nuclei, is "approximately three times" what the astronauts are receiving on the ISS", reports Cosmiverse

This means manned flights to Mars will be more risky:

"Being away from Earth for three years would mean that every cell of your body would be transversed by a galactic ray, and we just don't know what that would do to people.", reports National Geographic