En route to the Martian surface, NASA's Curiosity rover was buffeted five times by distinct blasts of protons jettisoned from the sun. The spacecraft was also bombarded by a steady stream of galactic cosmic rays, charged particles emanating from far outside the solar system. All of this was documented by the Curiosity's Radiation Assessment Detector, mounted on the top deck of the rover.
Now the team responsible for the experiment has tallied up the results and estimated the sort of risk the unfriendly radiation environment beyond low-Earth orbit might pose to human explorers. The numbers, reported this week in Science, are in line with previous estimates. All told, the authors find, an astronaut on a 360-day round trip to the Red Planet would be exposed to 662 millisieverts of radiation from galactic cosmic rays alone (solar protons tack on a bit more).
That's a somewhat encouraging number. The European Space Agency, the Russian Space Agency, and the Japanese Space Agency set the current career limit for astronauts at 1000 millisieverts (pdf). NASA sets its career limit at no more than a 3 percent risk of developing and dying from a cancer caused by radiation exposure: the actual dose will vary by age and gender, but it's in that same 1000 millisievert ballpark.
There are a few caveats to the new results. Protons from the sun amounted to just 5 percent of the radiation dose measured during Curiosity's cruise to the planet, but the mission coincided with a relatively inactive period of the sun's 11-year cycle. The solar contribution, the paper says, could potentially be "many times larger in a different time frame." Then, of course, there's the radiation astronauts will be exposed to while on the Martian surface, which the team has yet to calculate. Since Mars has considerably less atmospheric and magnetic shielding than the Earth, the flux there could also be quite high.
On the other hand, new developments might reduce radiation exposure on missions beyond the moon. The shielding around Curiosity during its flight "is probably not drastically different" from what will be on the Crew Exploration Vehicle, NASA's planned spacecraft for deep space missions, the researchers write. But alternate shielding materials could help, and some are eyeing the possibility of using superconducting coils to create a protective magnetic bubble around a spacecraft.