Operational Space Radiation Environment: Analogs, Pathogenesis, and Translation Into Clinical Outcomes in Humans
Texas A&M University, Department of Physics and Astronomy
The study of human health risks of spaceflight typically involves analogs that closely represent the space en- vironment. In most cases, theory, models, and study outcomes can be validated with available spaceflight data or, at a minimum, observation of humans subjected to analog terrestrial stresses. In contrast, space radiation research is limited to the use of analogs or models that for many reasons do not accurately represent the opera- tional space radiation environment or the complexity of human physiology. For example, studies on the effects of space radiation generally use mono-energetic beams and acute, single-ion exposures (including protons, lithium, carbon, oxygen, silicon, iron, etc.) instead of the complex energy spectra and diverse ionic composi- tion of the space radiation environment. In addition, a projected, cumulative mission dose is often delivered in one-time, or rapid and sequential, doses delivered to experimental animals. In most cases, these dose-rates are several orders of magnitude higher than actual space environment exposures. Even the use of animal models introduces error, as studies make use of a variety of animal species with differing responses and sensitivity to radiation that may not represent human responses to similar exposures. Further, studies do not challenge multi- ple organ systems to respond concurrently to the numerous stressors seen in an operational spaceflight scenario. These disparities and numerous other environmental considerations contribute to the large uncertainties in the outcomes of space radiobiology studies and the applicability of such studies for extrapolation and prediction of clinical health outcomes in future spaceflight crews. Here we present a novel modeling approach of the GCR environment by utilizing large-scale multi-core, high-performance computing and Monte Carlo methods to simulate 3D nuclear and subnuclear interactions. We show that the linear energy transfer spectrum of the intravehicular environment of, e.g., spaceflight vehicles can be accurately generated experimentally by perturb- ing the intrinsic properties of hydrogen-rich crystalline materials in order to instigate specific nuclear spallation and fragmentation processes when placed in an accelerated mono- energetic heavy ion beam. Modifications to the internal geometry and chemical composition of the materials allow for the shaping of the emerging field to specific spectra that closely resemble the intravehicular field. Validation of these results with beam-line mea- surements, both from the peer-reviewed literature and as performed herein, demonstrate reasonable agreement with model predictions. Our approach can be generalized to other radiation spectra and is therefore of wide applicability for biological exposures as well as general radiation studies, such as the deployment of shielding, electronics, and other materials in a space environment. This provides the first instance of a true ground-based analog for characterizing the effects of space radiation.