Humans traveling to Mars aboard spacecraft developed with current technologies face a significant radiation threat, based on first-ever measurements of galactic cosmic radiation and solar electron protons gathered aboard NASA’s Mars Science Laboratory as it delivered the Curiosity rover to the red planet in 2011-12.

The measurements suggest a roundtrip exposure of 0.66 Sieverts, plus or minus 0.12 Sieverts, over the 180-day outbound and inbound legs of the deep-space voyage using current shielding and propulsion techniques, according to a team of 17 researchers from the Southwest Research Institute (SwRI), Germany’s Christian Albrechts University and NASA.

The exposure approaches two-thirds of the 1 Sievert career exposure limit that carries a 5% increased risk of fatal cancer recognized by the Russian, European and Canadian space agencies. NASA observes a stricter, individually tailored standard that equates to a 3% increased risk of fatal cancer over a career. The study measurements do not include radiation measurements from the Martian surface, which Curiosity is currently gathering during a nominal two-year mission that began with the rover’s Aug. 6, 2012, landing in Gale Crater. Under current human mission scenarios, explorers may spend up to 500 days on the Martian surface as they wait for the red planet and the Earth to align favorably for the return leg of the mission.

The measurements en route to Mars, made with the SwRI-managed radiation assessment detector (RAD), were the first made in deep space by sensors protected by shielding comparable to that available to human spacecraft, such as the Orion crew exploration vehicle.

“Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep-space destinations is critical for planning future crew missions,” said Cary Zeitlin, SwRI scientist and the lead author of the report, published in the May 31 edition of the journal Science. “Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself.”

NASA’s current human deep-space planning is focused on a 2021 mission to an asteroid robotically corralled into lunar orbit as a stepping-stone toward Mars voyages in the 2030s.

“Before we can send astronauts, we need to understand the environment and hazards they would face,” Chris Moore, NASA’s deputy director of advanced exploration systems, told a NASA news briefing that accompanied the Science paper. “The RAD data will help us to design deep-space habitats in which the astronauts live on the trip to Mars and it will help us to improve the shielding to protect them from the harmful effects of space radiation.”

In addition, NASA is currently looking to solar electric and nuclear thermal propulsion technologies as most promising to meet a 2030s Mars mission goal.

A spacecraft “storm shelter” would likely prove effective in thwarting solar electric protons associated with the “max” period of the 11-year solar cycle.