Until now, scientists and engineers didn't really know this critical information. The computer calculations required to simulate the interaction between radiation and spacecraft hulls are way too complicated — with high-energy cosmic rays and solar energetic particles penetrating into the aircraft, interacting and colliding with the molecules in various metal and liquid layers.
But now the mystery is no more: the Mars Curiosity mission has collected and sent that information. NASA installed a Radiation Assesment Detector (RAD) inside the spacecraft, in an strategic place that simulates the future position of the astronauts. It has been measuring the radiation levels for nine months, as the spaceship cruised through millions of kilometres.
Curiosity is riding to Mars in the belly of the spacecraft, similar to where an astronaut would be. This means the rover absorbs deep-space radiation storms the same way a real astronaut would.
This vital data would allow engineers to design a manned spacecraft that could actually travel to other planets in the solar system and beyond. It will be published for all the international community soon.
The mission of RAD is not over yet: when it arrives to Mars, it will start measuring the radiation that future visitors would have to deal with. According to Hassler, this will be a first too — "no one has ever before measured this kind of radiation from the surface of another planet. We're just getting started." [NASA]