Findings by an instrument aboard the Mars transit vehicle that carried the Curiosity rover show that radiation exposure for a mission of permanent settlement will be well within space agencies' astronaut career limits.
Radiation on the way to Mars
A study published in the journal Science in May 2013 calculates 662 +/ 108 millisieverts (mSv) of radiation exposure for a 360 day return trip, as measured by the Radiation Assessment Detector (RAD). The study shows that ninety five percent of the radiation received by the RAD instrument came from Galactic Cosmic Rays or GCRs, which are hard to shield against without use of prohibitive shielding mass (1).
The 210-day journey Mars One settlers will take, amounts to radiation exposure of 386 +/- 63 mSv, considering these recent measurements as standard. This exposure is below the upper limits of accepted standards for an astronaut career: European Space Agency, Russian Space Agency and Canadian Space Agency limit is 1000 mSv; NASA limits are between 600-1200mSv, depending on sex and age (1).
Mars transit habitat radiation shelter
On the way to Mars, the crew will be protected from solar particles by the structure of the spacecraft. The crew will receive general protection of 10-15 g/cm2 shielding from the structure of the Mars transit vehicle. In case of a solar flare or Solar Particle Event (SPE), this shielding will not suffice and the crew will retreat to a dedicated radiation shelter in Mars Transit habitat, taking their cue from the onboard radiation monitoring and alert system. The dedicated radiation shelter located in the hollow water tank, will provide additional shielding to the level of 40 g/cm2. The astronauts should expect one SPE every two months on average and a total of three or four during their entire trip, with each one usually lasting not more than a couple of days.
Radiation on Mars
Mars's surface receives more radiation than the Earth's but still blocks a considerable amount. Radiation exposure on the surface is 30 µSv per hour during solar minimum; during solar maximum, dosage equivalent of this exposure is reduced by the factor two (2).
If the settlers spend on average three hours every three days outside the habitat, their individual exposure adds up to 11 mSv per year.
The Mars One habitat will be covered by several meters of soil, which provides reliable shielding even against galactic cosmic rays. Five meters of soil will provide the same protection as the Earth's atmosphere-- equivalent to 1,000 g/cm2 of shielding.
With the help of a forecasting system, taking shelter in the habitat can prevent radiation exposure from SPEs.
Total radiation exposure
The 210-day trip results in radiation exposure of the crew of 386 +/- 61 mSv. On the surface, they will be exposed to about 11 mSv per year during their excursions on the surface of Mars. This means that the settlers will be able to spend about sixty years on Mars before reaching their career limit, with respect to ESA standards.
1. Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory C. Zeitlin, D. M. Hassler, F. A. Cucinotta, B. Ehresmann, R. F. Wimmer-Schweingruber, D. E. Brinza, S. Kang, G. Weigle, S. Böttcher, E. Böhm, S. Burmeister, J. Guo, J. Köhler, C. Martin, A. Posner, S. Rafkin, and G. Reitz Science 31 May 2013: 340 (6136), 1080-1084.
2. F. A. Cucinotta, L. Chappell, M. Y. Kim, Space Radiation Cancer Risk Projections and Uncertainties–2012 (NASA Technical Paper 2013-217375, NASA STI Program, Hampton, VA, 2013)