U.S. researchers have made detailed measurements of the radiation hazards associated with a trip that's long been the subject of science fiction: a manned mission to Mars.
Based on data from the U.S. Curiosity rover, which landed on Mars last August after a 253-day, 560-million-kilometer journey, researchers reported Thursday in the journal Science that astronauts would likely receive "a large fraction of and in some cases greater than currently accepted astronaut career limits" on a round-trip to Mars.
"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," Cary Zeitlin of the U.S. Southwest Research Institute who led the study said in a statement.
Two forms of radiation pose potential health risks to astronauts in deep space: a chronic low dose of galactic cosmic rays (GCRs) and the possibility of short-term exposures to the solar energetic particles (SEPs) associated with solar flares and coronal mass ejections.
Long-term population studies have shown that exposure to radiation increases a person's lifetime cancer risk and exposure to a dose of one sievert is associated with a five percent increase in fatal cancer risk. U.S. space agency NASA has established a three percent increased risk of fatal cancer as an acceptable career limit for its astronauts currently operating in low-Earth orbit.
The new study showed the Curiosity rover was exposed to an average GCR dose equivalent rate of 1.8 milliSieverts per day in cruise. Only about 5 percent of the radiation dose was associated with solar particles because of a relatively quiet solar cycle and the shielding provided by the spacecraft.
In total, the radiation exposure "during just the transit phases of a Mars mission would be approximately 0.66 Sievert for a round trip with current propulsion systems," said Zeitlin.
Time spent on the surface of Mars might add considerably to the total dose equivalent, he said.
Because Zeitlin considered just the radiation exposure on the trip to and from Mars, he said his next step is to continue the radiation measurements from Curiosity as it travels over the Martian surface. "Publishing these results will give the research community additional information to use in evaluating mission scenarios."
Prior to Zeitlin's work, there had been several calculations of the radiation exposure an astronaut on a Mars mission would receive. These predictions were made using models that incorporated educated guesses about the shielding distribution of the vessels used, as well as assumptions about the state of the solar cycle, both of which affect radiation exposures.
Zeitlin said that the results from this study are representative of a trip to Mars under conditions of low to moderate solar activity and fall within the range of previously modeled predictions for radiation exposure on a mission to Mars.
He added a cautionary note to those who want to use the results to make definitive pronouncements about the feasibility of a human mission to Mars.
"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," Zeitlin said. "Those limits depend on our understanding of the health risks associated with exposure to cosmic radiation, and at present, that understanding is quite limited."