Overall, future lunar travelers face a radiation dose 30 percent to 40 percent higher than originally expected, Spence said.
Galactic cosmic rays, believed to be caused by supernova explosions, are fast-moving electrically charged electrons and atomic nuclei. They are found throughout the solar system, though their numbers are particularly high at the present time due to an unusually quiet period of solar activity.
The lack of solar magnetic fields and reduced solar wind pressure means cosmic rays have been able to more easily penetrate the inner solar system.
"We are in a period when the radiation risks are elevated, but still tolerable," Spence said, adding that the levels were about what an X-ray technician or uranium miner might normally experience in a year.
The sun cycles through periods of high and low magnetic activity about every 11 years. The next period of maximum activity, characterized by increasing number of sunspots and other disruptions of the sun's surface, is expected in 2013.