Space Radiation Cancer Risk Projections for Exploration Missions: Uncertainty Reduction and Mitigation

Late effects from the high charge and energy (HZE) ions present in the galactic cosmic rays (GCR) including cancer and the poorly understood risks to the central nervous system constitute the major risks for exploration missions. Methods used to project risk in low Earth orbit (LEO) are viewed as highly uncertain for projecting risks on exploration missions because of the limited radiobiology data available for estimating risks from HZE ions. For the first-time we make a quantitative assessment of the uncertainties in cancer risk projections for space radiation exposures. Cancer risk projections are described as a product of many biological and physical factors, each of which has a differential range of uncertainty due to lack of data and knowledge. We use Monte-Carlo sampling from subjective error distributions that represent the lack of knowledge in each factor to quantify the overall uncertainty in risk projections. Cancer risk analysis is applied to several exploration mission scenarios including lunar station, deep space outpost, and Mars missions of duration of 360, 660, and 1000 days. At solar minimum, the number of days in space where career risk less than the limiting 3% excess cancer mortality can be assured at a 95% confidence level is found to be only of the order of 100 days. The current uncertainties would only allow a confidence level of less than 50% for a 1000-day class Mars mission, this is considered insufficient for assuring crew radiation safety at this time. A further result of this analysis is the quantification of the dominant role of biological factors in comparison to physical factors in the overall uncertainties for cancer risk projections. Approaches to reduce these uncertainties and mitigate risks that will enable the human exploration of space are discussed.