Washington - Astronauts who travel on future missions to Mars would probably be exposed to their lifetime limit of radiation during the trip, not to mention time spent on the Red Planet, scientists said on Thursday.
The measurements were made aboard the Mars Science Laboratory, an unmanned Nasa rover and mobile lab that set off for Mars in 2011 before landing 253 days later in August 2012, said the report in the US journal Science.
"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Cary Zeitlin, a principal scientist in Southwest Research Institute's (SwRI) Space Science and Engineering Division.
"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 more study is needed to determine the actual health risks - including the likelihood of developing cancer - associated with exposure to cosmic radiation before any human trip to Mars can take place.
The US space agency has said it is aiming for the first-ever astronaut mission to Mars sometime in the 2030s.
Until now, only robotic rovers have been able to tour the dry planet, where scientists believe some form of life once existed.
Previous radiation estimates did not benefit from the latest technology aboard the Mars Science Laboratory, which is fitted with a radiation detector shielded by a spacecraft likely similar to one that would carry humans on the 560-million-kilometer journey to Mars.
Nasa estimates an outbound flight would take around 180 days, followed by a stint on the planet that could last 500 days, and then the trip home.
Zeitlin and colleagues measured radiation on the trip - which occurred during low to moderate solar activity - and future studies will use data from the Curiosity rover to estimate radiation exposure on the planet.
Galactic cosmic rays
"Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions," Zeitlin said.
The Radiation Assessment Detector (RAD) was built by SwRI and Christian Albrechts University in Kiel, Germany with funding from the Nasa and Germany's national aerospace research centre.
"Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself," Zeitlin said.
Astronauts risk radiation exposure from galactic cosmic rays (GCRs) and solar energetic particles (SEPs) from the Sun's flares and coronal mass ejections.
In the general population, exposure to radiation is measured in units of Sievert (Sv) or milliSievert (1/1000 Sv).
Exposure to a dose of 1 Sv is associated with a five percent increase in the risk of deadly cancer.
"The RAD data show an average GCR dose equivalent rate of 1.8 milliSieverts per day in cruise," said Zeitlin.
"The total during just the transit phases of a Mars mission would be approximately .66 Sv for a round trip with current propulsion systems."
Time spent on the Red Planet might add considerably to the radiation dose, he added.