Measurements made by the Radiation Assessment Detector (RAD) on board Curiosity show that radiation in deep space poses a “significant” risk to human space travellers.

During the 253-day, 560-million-km trip of the spacecraft to Mars from November 26, 2011 to August 6, 2012, the detector recorded 466 millisevert (mSv) radiation with an error margin of 84 mSv. Even if a faster transit of 180 days is chosen when astronauts are travelling to Mars, a roundtrip to the planet will expose them to 662 mSV with an error margin of 108 mSv. The calculation is based on average galactic cosmic ray dose equivalent rate of 1.8 mSv per day in cruise.

But a fact of particular significance is that the trip to Mars was during a quieter part of the solar cycle and hence the spacecraft was less exposed to radiation from solar energy particles (SEP) associated with solar flares and coronal mass ejections. Spacecraft shielding also cut a large part of the radiation. Both factors combined resulted in SEP contributing just 5 per cent of the total radiation dose. But the SEP contribution can increase “many times in a different time frame.”

Despite the quieter solar cycle, the radiation exposure during the cruise period alone is a large fraction of NASA’s maximum of one Sv exposure during an astronaut’s entire career. “Radiation exposure at the level we measured is right at the edge, or possibly over the edge, of what is considered acceptable in terms of career exposure limits defined by NASA and other space agencies,” Cary Zeitlin from Southwest Research Institute, Boulder, the first author of the paper published on May 31 in Science , was quoted as saying in a release.

Aside from radiation during the cruise period, the time spent on Mars, which would be about a year or more, will add up to the total. Thus the trip and the stay on Mars combined will “add considerably to the total dose equivalent, depending on shielding conditions and duration of stay,” notes the paper. The central message from the radiation measurement is that a round trip to Mars would expose astronauts to a “hefty dose of damaging radiation,” says a perspective piece in the same issue of the journal. Data from RAD will help scientists to build spacecraft with better radiation protection while planning Mars missions.