Future manned missions to Mars and other remote targets will require internal shielding and advanced propulsion systems to shorten transit times, minimizing exposure to cancer-causing radiation from the sun and deep space, scientists said Thursday.
Data collected by the Radiation Assessment Detector, or RAD, instrument during the Curiosity Mars rover's cruise to the Red Planet last year generally confirmed the results from earlier studies showing space radiation is a major problem that must be overcome before manned trips into deep space are attempted.
"NASA's very excited to get this new cruise data to help us refine and improve our radiation environment models we use to estimate crew exposure and risks for various mission scenarios," Eddie Semones, spaceflight radiation health officer at the Johnson Space Center, told reporters.
"Cruise data (is) critical to the understanding of the impacts of galactic cosmic rays and solar particle events inside a platform similar to vehicles we're developing for human exploration missions."
The RAD instrument, mounted on the upper deck of the Curiosity rover, measured the radiation environment for seven months during the cruise to Mars, recording impacts from charged particles blasted away by the sun during solar storms as well as galactic cosmic rays generated by supernova explosions and other high-energy events. The data was presented Thursday in the journal Science.
Radiation exposure is measured in units called Sieverts. In a news release, NASA said exposure to 1 Sievert over time translates into a 5 percent increase in the risk that an individual might develop a fatal cancer. NASA's current safety guidelines permit a 3 percent increased risk for astronauts in low-Earth orbit.
Not counting solar particles, which made up only about 5 percent of the radiation recorded during Curiosity's flight to Mars, the RAD instrument showed that an astronaut flying along with the rover would have been exposed to more than three times the equivalent radiation dose experienced by space station crews.
The average annual exposure at Earth's surface from all sources is less than 10 milliSieverts per year. Space station astronauts are exposed to about 100 milliSieverts in six months, while Curiosity's RAD instrument showed an exposure of 330 milliSieverts during the half-year cruise to Mars, or about 1.8 milliSieverts per day.
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Astronauts in low-Earth orbit are protected in part by Earth's magnetic field, which deflects charged particles. Earth's atmosphere provides an additional buffer for most of the planet's surface.
But that protection is not available in deep space, and the levels recorded by the RAD instrument are comparable to getting a whole-body CT scan every five or six days, said Cary Zeitlin, a principal researcher at the Southwest Research Institute in San Antonio, Texas.
"The radiation environment in deep space is several hundred times more intense than it is on Earth, even inside a shielded spacecraft," he said.
Chris Moore, deputy director of advanced exploration systems at NASA headquarters, said shorter transit times and improved shielding will be needed to protect future deep space crews.
"To get really fast trip times to cut down on radiation exposure we'd probably need nuclear thermal propulsion, and we're working with the U.S. Department of Energy to look at various types of fuel elements for these rockets," Moore said.
"But it's a long-range technology development activity and it will probably be many years before that is ready. But it is part of our design reference mission architecture for sending humans to Mars.... That could probably cut the (one-way) trip time down to around 180 days."
Semones said on-board shielding also will be required. One option would be to surround the crew module with water, using hydrogen to protect against charged particles from the sun. Another option would be to develop shields, or panels, that could be deployed inside the spacecraft when solar storms are detected.
"The shields that we're developing, the deployable shields, are very effective in reducing or eliminating the effects of solar particle events," he said. "For cosmic rays, generally the thicknesses required to have any substantial reduction exceed the (capabilities of the) spacecraft we can effectively launch."
Shields impervious to galactic cosmic rays would be "very, very thick -- meters thick -- to make an effect," he said. "We're not going to be able to solve it with passive shielding for galactic cosmic rays."
"We need to get there faster to reduce the impact of galactic cosmic rays; we need to have local shielding on board to eliminate the effects of solar particles."
Moore said data collected by the RAD instrument since Curiosity's landing last August will be presented in an upcoming paper.