Scientists using the Radiation Assessment Detector (RAD) aboard the Mars Science Laboratory’s Curiosity rover have made detailed measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the surface of the Red Planet.
“Our measurements provide crucial information for human missions to Mars. We’re continuing to monitor the radiation environment, and seeing the effects of major solar storms on the surface and at different times in the solar cycle will give additional important data,” said Dr Don Hassler of Southwest Research Institute, who is the lead author of the paper published online in the journal Science.
“Our measurements also tie into Curiosity’s investigations about habitability. The radiation sources that are of concern for human health also affect microbial survival as well as the preservation of organic chemicals.”
Two forms of radiation pose potential health risks to astronauts: 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.
The radiation on Mars is much harsher than on Earth for two reasons: Mars lacks a global magnetic field and the Martian atmosphere is much thinner than Earth’s, providing little shielding to the surface.
This environmental factor poses a challenge for future human exploration of Mars and is also important in understanding both geological and potential biological evolution on Mars.
Both GCRs and SEPs interact with the atmosphere and, if energetic enough, penetrate into the Martian soil, or regolith, where they produce secondary particles that contribute to the complex radiation environment on the Martian surface, which is unlike anything on Earth.
“The RAD surface radiation data show an average GCR dose equivalent rate of 0.67 millisieverts per day from August 2012 to June 2013 on the Martian surface.”
“In comparison, RAD data show an average GCR dose equivalent rate of 1.8 millisieverts per day on the journey to Mars, when RAD measured the radiation inside the spaceship.”
According to data, most mission radiation exposure will be during outbound and return travel, when the astronauts will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself. The total during just the transit phases of a Mars mission would be approximately 0.66 Sv for a round trip with current propulsion systems and during similar solar activity.
A 500-day mission on the surface would bring the total exposure to around 1 Sv.
Long-term population studies have shown that exposure to radiation increases a person’s lifetime cancer risk; exposure to a dose of 1 Sv is associated with a five percent increase in fatal cancer risk.
Although NASA has generally established a three percent increased risk of fatal cancer as an acceptable career limit for astronauts in low earth orbit, it does not currently have a limit for deep space missions.
Scientists are now working with the National Academies Institute of Medicine to determine appropriate limits for deep space missions, such as a mission to Mars in the 2030s.
Hassler DM et al. Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover. Science, published online December 9, 2013; doi: 10.1126/science.1244797