Cosmic Rays: Hurdles on the Road to Mars

Space radiation has long been recognized as a major health hazard for human space exploration. Unlike terrestrial radiation, space radiation comprises high energy protons and high charge and energy (HZE) nuclei, which produce distinct forms of biological damage to biomolecules, cells, and tissue compared to terrestrial radiation, making risk predictions highly uncertain [1]. While the crews in low Earth orbit (LEO), such as the International Space Station (ISS), are partially protected by the Earth's magnetic field, for interplanetary missions in deep space the risk of acute effects caused by solar particle events (SPE) and late effects induced by protons and HZE nuclei in the galactic cosmic rays (GCR) is significant. Intense SPE can be lethal for unprotected crews, but shielding is effective against solar protons. Chronic exposure to GCR represents instead a very serious risk of carcinogenesis [2]. It is not clear if the health risks associated with long-term exposure to HZE nuclei can be adequately estimated by epidemiological studies, as done for radiation protection on Earth, due to both quantitative and qualitative differences in biological damage. Therefore, risk estimates, mostly based on ground-based cell or animal studies, are affected by large uncertainties. Moreover, shielding from very energetic CGR nuclei tends to be poor given the weight constraints of spacecraft.