How safe is safe enough? Radiation risk for a human mission to Mars

Abstract

Astronauts on a mission to Mars would be exposed for up to 3 years to galactic cosmic rays (GCR)--made up of high-energy protons and high charge (Z) and energy (E) (HZE) nuclei. GCR exposure rate increases about three times as spacecraft venture out of Earth orbit into deep space where protection of the Earth's magnetosphere and solid body are lost. NASA's radiation standard limits astronaut exposures to a 3% risk of exposure induced death (REID) at the upper 95% confidence interval (CI) of the risk estimate. Fatal cancer risk has been considered the dominant risk for GCR, however recent epidemiological analysis of radiation risks for circulatory diseases allow for predictions of REID for circulatory diseases to be included with cancer risk predictions for space missions. Using NASA's models of risks and uncertainties, we predicted that central estimates for radiation induced mortality and morbidity could exceed 5% and 10% with upper 95% CI near 10% and 20%, respectively for a Mars mission. Additional risks to the central nervous system (CNS) and qualitative differences in the biological effects of GCR compared to terrestrial radiation may significantly increase these estimates, and will require new knowledge to evaluate.

Figure 1. GCR organ dose characterization at average solar minimum.

(A) The attenuation of annual GCR organ averaged doses versus depth of aluminum shielding for deep space, the martian surface, and for combined GCR and trapped protons in the ISS orbit. Calculations are for males absorbed dose (D) (mGy), solid cancer and leukemia risks (H) (mSv), and non-cancer effects (G) (mGy-Eq). Calculations on the martian surface consider its atmosphere with an 18 g/cm² CO₂ vertical height.

Figure 2. Distribution of %REID for solid cancer for particles represented by Z*²/β² for deep space, martian surface and ISS orbit for 20 g/cm² shield.

The full GCR spectrum that traverse astronauts in deep space are more biologically damaging compared to the higher energy GCR that occur in low Earth orbit.

Figure 3. Estimates of the GCR organ doses over recent solar cycles at 0 and 20/cm² of aluminum shielding (left axis) and the log of the 100 MeV integral proton fluence, which was shown to be a useful predictor of SPE organ doses after considering their variable energy spectra (11) (right axis).

Figure 4. Estimates of tissue specific %REIC, %PC for incidence and death and %REIC and %REID for 940-d Mars mission with average solar minimum conditions.

(A) Values are for an average 45-y female never smokers and (B) for an average 45-y male never smokers. Calculations assume 20 g/cm² and 10 g/cm² aluminum shielding for transit and martian surface, respectively. PC estimations are for 20 years post mission. All point assessments are bracketed by 95% confidence intervals.

Figure 5. Comparison of %REID from cancer and circulatory diseases combined for several space exploration missions.

(A) Estimates are shown for 45-y old female and (B) male never-smokers. Calculations assume 20 g/cm² and 10 g/cm² aluminum shielding for transit vehicle and martian surface habitat, respectively. Error bars reflect the 95% confidence intervals.