Dose Limits and Countermeasures for Mitigating Radiation Risk in Moon and Mars Exploration

Abstract

After decades of research on low-Earth orbit, national space agencies and private entrepreneurs are investing in exploration of the Solar system. The main health risk for human space exploration is late toxicity caused by exposure to cosmic rays. On Earth, the exposure of radiation workers is regulated by dose limits and mitigated by shielding and reducing exposure times. For space travel, different international space agencies adopt different limits, recently modified as reviewed in this paper. Shielding and reduced transit time are currently the only practical solutions to maintain acceptable risks in deep space missions.

Towards sustainable human space exploration—priorities for radiation research to quantify and mitigate radiation risks

“…For example, the average dose received by astronauts inside the ISS is around 0.5–1 mSv/day 37 , while for the Mars mission the RAD instrument (onboard the unmanned MSL “Curiosity”) measured in free space an averaged GCR dose equivalent rate of 1.84 mSv/day 14 , and the equivalent estimate for the Martian surface is 0.64 mSv/day. Based on these dose rates, which are mainly delivered by GCR and can be further increased by potential SPEs, the current estimates for a full Mars mission are critically high, exceeding most space agency limits that are set at 0.6–1 Sv 38 .…”

ICRU Report 98, Stochastic Nature of Radiation Interactions: Microdosimetry

“…A major uncertainty in estimation of the risks is due to uncertainty in the RBE of space radiation compared to the radiations on earth for which there is extensive epidemiological information. Most of the national agencies apply quality factors based on a prescribed relationship to the LET of the components of the radiation field (Boscolo and Durante, 2022; ICRP, 2013). In the United States, however, NASA considers the charge and energy of the particles in the field to evaluate a quality factor that distinguishes also between particles of the same LET but different track structures.…”

“…Special approaches are necessary for radiation protection for space crew because of the unique characteristics of space radiation and the lengthy missions in which crew may be involved. The various national space agencies have adopted differing approaches to setting dose limits, although all are based on judgments of acceptable risk of detrimental health effects (Boscolo and Durante, 2022). A major uncertainty in estimation of the risks is due to uncertainty in the RBE of space radiation compared to the radiations on earth for which there is extensive epidemiological information.…”

“…A major risk from exposure to space radiation is the induction of cancer and it is from estimates of this stochastic risk that the maximum career flight times of NASA space crew are restricted by a Space Permissible Exposure Limit (NASA, 2015). For the purpose of demonstrating compliance with the career limit, NASA has developed a cancer risk projection model for radiation exposure induced death from cancer (REID), in which the formulation and numerical values of the quality factor (QF NASA ) (Cucinotta et al , 2013) are substantially different from those of the radiation weighting factor ( w normalR ) routinely applied for radiation protection on earth (ICRP, 2007) and from those of the quality factor ( Q ) widely used by many space agencies (Boscolo and Durante, 2022; ICRP, 2013). For exposures in space, the quality factor needs to take into account the wide variety of charged particles and the wide spectra of particle energies, particularly within the spacecraft and the crews’ bodies.…”

System of radiological protection: Towards a consistent framework on Earth and in space

“…For these and other reasons, it is evident that the development of a framework of radiological protection is needed, which builds on current knowledge and experience of radiological protection on Earth, takes into account the specific circumstances of exposure to cosmic radiation in space, and contributes towards an approach of radiological protection of astronauts that is harmonized across space agency practices. This is of particular importance given that space agencies are currently using essentially different approaches, for example, in calculating radiation-related risks and dose limits in space [2] , [3] , [4] , [5] , and “non-occupational” exposures to space tourists are looming.…”