The space agency limits astronauts to a 3% increased risk of fatal cancer. This translates to different levels of radiation exposure, depending on an astronaut's age and gender.
But according to a paper published in Friday's edition of the journal Science, radiation exposure in a nonstop round-trip to Mars, which would take about a year, would ring in at about 662 millisieverts.
One sievert, or 1,000 millisieverts, of radiation over time is generally associated with a 5% increase in fatal cancer risk.
"It is clear that the exposure from the cruise phases alone is a large fraction of (and in some cases greater than) currently accepted astronaut career limits," the authors wrote.
As the Obama administration calls for a manned trip to the Red Planet by the mid-2030s, mission planners will have to address the challenge, experts said.
"It's not a show-stopper," said Lewis Dartnell, an astrobiologist at the University of Leicester in England who was not involved in the work. "But it does mean if we want to do a human mission to Mars as safely as we can, we perhaps need to start thinking about how to better shield and protect these astronauts against radiation in space."
Radiation detectors on most unmanned spacecraft are exposed directly to space and pick up all the energetic protons, helium ions and heavier particles flying around out there. But because Curiosity's Radiation Assessment Detector was huddled inside its protective spacecraft, its readings are the first to show what kind of radiation risk humans might face while traveling inside a shielded spacecraft.
Curiosity's RAD instrument was designed to measure the radiation on the surface of Mars. But late in the game, scientists realized they could also use it in flight to test how much radiation got through the vessel's shielding.
"It actually wasn't really planned until about a year before the launch, and then we realized we had the opportunity to do this," said first author Cary Zeitlin, a physicist at the Southwest Research Institute in Boulder, Colo.
At a news conference Thursday, RAD lead scientist and Southwest Research Institute physicist Donald Hassler compared Curiosity's protective spacecraft to the similarly designed Orion capsule, a manned craft Lockheed Martin is building that could carry humans to the moon and Mars.
Given the ships' similar silhouettes, measurements of radiation within Curiosity's spacecraft could help shed light on the exposure astronauts might face traveling in Orion, he said.
Scientists are concerned about the radiation dose humans might get once on Mars, but they think that exposure during the trip from Earth would be much more severe because particles are coming from all directions and not just from above, Zeitlin said.
The researchers used RAD to detect two types of radiation: galactic cosmic rays, which are particles hurled from supernova explosions and other high-energy phenomena; and solar energetic particles, shot out from the sun after a flare or a coronal mass ejection.
The RAD instrument picked up an average of about 1.8 millisieverts per day of galactic cosmic rays, according to the study. The total radiation was the equivalent of getting a CT scan every five to six days for the duration of the trip, Zeitlin said.
Over the 348 million miles Curiosity traveled to Mars, the rover only experienced five solar energetic particle events, which lasted up to a few days. Though levels of radiation shot up during those events, overall they contributed only about 5% of the total radiation exposure during the trip.
Still, the researchers noted, the sun has been in a very weak solar maximum recently. If NASA someday sends Mars-bound astronauts into outer space, it could be during a much more active solar season.
University of Leicester's Dartnell said that engineers would have to figure out how to make the spacecraft get to Mars much faster — or would have to find creative ways to shield the spacecraft.
For example, some researchers have proposed using a ship's water and wastewater supplies to line a small protective chamber for astronauts. Water serves as a very effective shield against radiation.
Or, Dartnell added, it may be time to consider "reassessing what level of risk we think is acceptable" for astronauts.