Space radiation risks for astronauts on multiple International Space Station missions

Abstract

Mortality and morbidity risks from space radiation exposure are an important concern for astronauts participating in International Space Station (ISS) missions. NASA's radiation limits set a 3% cancer fatality probability as the upper bound of acceptable risk and considers uncertainties in risk predictions using the upper 95% confidence level (CL) of the assessment. In addition to risk limitation, an important question arises as to the likelihood of a causal association between a crew-members' radiation exposure in the past and a diagnosis of cancer. For the first time, we report on predictions of age and sex specific cancer risks, expected years of life-loss for specific diseases, and probability of causation (PC) at different post-mission times for participants in 1-year or multiple ISS missions. Risk projections with uncertainty estimates are within NASA acceptable radiation standards for mission lengths of 1-year or less for likely crew demographics. However, for solar minimum conditions upper 95% CL exceed 3% risk of exposure induced death (REID) by 18 months or 24 months for females and males, respectively. Median PC and upper 95%-confidence intervals are found to exceed 50% for several cancers for participation in two or more ISS missions of 18 months or longer total duration near solar minimum, or for longer ISS missions at other phases of the solar cycle. However, current risk models only consider estimates of quantitative differences between high and low linear energy transfer (LET) radiation. We also make predictions of risk and uncertainties that would result from an increase in tumor lethality for highly ionizing radiation reported in animal studies, and the additional risks from circulatory diseases. These additional concerns could further reduce the maximum duration of ISS missions within acceptable risk levels, and will require new knowledge to properly evaluate.

Figure 1. Distribution of the percent risk of exposure induced death (%REID) for solid cancer and leukemia versus particles type as represented by Z*²/β² for 45-y female never smokers.

Results are for exposure to space radiation on an 1-year ISS mission near solar minimum with 20 g/cm² aluminum shielding.

Figure 2. Central estimates of the percent risk of exposure induced cancer (% REIC) (left panel) and percent risk of exposure induced death (%REID) (right panel) versus age at exposure for 1-year ISS missions near solar minimum.

Predictions for F and M of the U.S average population or a never-smoker (NS) population are shown.

Figure 3. Predictions of tissue (disease) specific percent risk of exposure induced cancer (%REIC) or percent risk of exposure induced death (%REID) for participation in two ISS missions at age 40-y and age 45-y (6-months at solar median and 1-year near solar minimum) for F and M never-smoker populations.

Figure 4. Predictions of tissue (disease) specific percent risk of exposure induced cancer (%REIC) or percent risk of exposure induced death (%REID) for participation in three ISS missions at age 40-, 45, and 50-y (6-month at solar median, 6-months near solar maximum, and 1-year near solar minimum) for F and M never-smoker populations.

Figure 5. Predictions of tissue (disease) specific %PC at age 55-y (left panel) and age 65-y (right panel) following participation in two ISS missions at age 40-y and age 45-y (6-month at solar median, and 1-year near solar minimum) for F and M never-smoker populations.

Figure 6. Predictions tissue (disease) specific percent risk of exposure induced cancer (%REIC) or percent risk of exposure induced death (%REID) or percent probability of causation (%PC) at age 65-y (right panels) following participation in1-y ISS missions at age 45-y for F and M never-smoker populations.

Figure 7. Predictions of the PDF for percent risk of exposure induced death (%REID) for cancer in NSCR-2012 model , and current results for the total (cancer and circulatory disease combined) and the modification of total %REID for the case of increased tumor lethality for highly densely ionizing particle.

Predictions are for participation in two ISS missions at age 40-y and age 45-y (6-month at solar median and 1-year near solar minimum) for F never-smoker population.