Crews traveling to Mars and back with today's technology will face a 5% increase in their chances of developing cancer later in life—higher than is allowed today's NASA astronauts—new data from NASA's Curiosity Mars rover indicates.

Scientists used Curiosity's Radiation Assessment Detector (RAD) instrument to calculate a crew radiation dose over a 500-day roundtrip mission to Mars of about 1,000 millisieverts, including earlier data the RAD collected as Curiosity cruised to Mars.

From August 2012, when the one-ton rover landed in equatorial Gale Crater, and June, it registered an average of 0.67 millisieverts a day, according to a paper published Dec. 9 in connection with the International Geophysical Union in San Francisco. By comparison, a chest X-ray exposes a patient to about 0.02 millisieverts. Overall, NASA allows astronauts a lifetime radiation dose that could increase their cancer risk by 3%.

“That poses a challenge to us with our current radiation standards, which have really been designed, primarily, for low Earth orbit,” says Rich Williams, NASA's chief health and medical officer. “So we are seeking advice from the highest medical authorities in the country at the Institute of Medicine, giving us input about ethics and principles surrounding this level of risk.”

The surface data, reported in the peer-reviewed journal Science, covered a period when the Sun was relatively quiet. In deep space and on the surfaces of planets that, like Mars, lack a global magnetic field, the primary radiation risk to humans comes from galactic cosmic rays and active particles ejected during solar storms. Almost all of the data in the new report involve cosmic rays, according to Donald M. Hassler of the Southwest Research Institute, principal investigator on the RAD instrument, who says a better overall picture will emerge as the Sun becomes more active later in its 11-year cycle.

Even so, the RAD data has helped engineers designing the Orion multipurpose crew vehicle that will take crews to and from Mars. Dan Dumbacher, deputy associate administrator for exploration systems development, says it will inform choices in structural design, materials and even operations that would keep as much structure as possible between crews and radiation sources.

Jason Crusan, who directs NASA's Advanced Exploration Systems Division, says the data will also help in development of future Mars landers and habitats for both the cruise to the planet and the surface. Drugs to mitigate radiation effects on humans, and genetic screening that might identify radiation-resistant crewmembers are under study as well. Williams notes that the latter approach raises ethical issues that extend beyond space travel to the possibility of determining suitability for many jobs based on genetic makeup.

The radiation data may help scientists determine if Mars ever was habitable, too, Hassler notes. “The radiation sources . . . also affect microbial survival, as well as preservation of organic chemicals,” he says.