A method to predict space radiation biological effectiveness for non-cancer effects following intense Solar Particle Events

In addition to the continuous exposure to cosmic rays, astronauts in space are occasionally exposed to Solar Particle Events (SPE), which involve less energetic particles but can deliver much higher doses. The latter can exceed several Gy in a few hours for the most intense SPEs, for which non-stochastic effects are thus a major concern. To identify adequate shielding conditions that would allow respecting the dose limits established by the various space agencies, the absorbed dose in the considered organ/tissue must be multiplied by the corresponding Relative Biological Effectiveness (RBE), which is a complex quantity depending on several factors including particle type and energy, considered biological effect, level of effect (and thus absorbed dose), etc. While in several studies only the particle-type dependence of RBE is taken into account, in this work we developed and applied a new approach where, thanks to an interface between the FLUKA Monte Carlo transport code and the BIANCA biophysical model, the RBE dependence on particle energy and absorbed dose was also considered. Furthermore, we included in the considered SPE spectra primary particles heavier than protons, which in many studies are neglected. This approach was then applied to the October 2003 SPE (the most intense SPE of solar cycle 23, also known as "Halloween event") and the January 2005 event, which was characterized by a lower fluence but a harder spectrum, i.e., with higher-energy particles. The calculation outcomes were then discussed and compared with the current dose limits established for skin and blood forming organs in case of 30-days missions. This work showed that the BIANCA model, if interfaced to a radiation transport code, can be used to calculate the RBE values associated to Solar Particle Events. More generally, this work emphasizes the importance of taking into account the RBE dependence on particle energy and dose when calculating equivalent doses.

Cancer and circulatory disease risks for the largest solar particle events in the space age

In this paper we use the NASA Space Cancer Risk (NSCR version 2022) model to predict cancer and circulatory disease risks using energy spectra representing the largest SPE's observed in the space age. Because tissue dose-rates behind shielding for large SPE's lead to low dose-rates (<0.2 Gy/h) we consider the integrated risk for several historical periods of high solar activity, including July-November, 1960 events and August-October 1989 events along with the February 1956 and August 1972 events. The galactic cosmic ray (GCR) contribution to risks is considered in predictions. Results for these largest historical events show risk of exposure induced death (REID) are mitigated to < 1.2 % with a 95 % confidence interval with passive radiation shielding of 20 g/cm2 aluminum, while larger amounts would support the application of the ALARA principle. Annual GCR risks are predicted to surpass the risks from large SPEs by ∼30 g/cm2 of aluminum shielding.

A methodology for investigating the impact of medical countermeasures on the risk of exposure induced death

The space radiation environment is composed of ionizing particles that may pose health risks to crew members during Low Earth Orbit (LEO) and deep space missions. NASA has established astronaut career radiation limits for cancer of 3% Risk of Exposure Induced Death (REID) at the 95% confidence level. The REID is the increased lifetime risk of death from cancer due to radiation exposure in comparison to an unexposed background population and has been traditionally mitigated by passive shielding design concepts and limiting safe days in space. Additional reduction in radiation exposure risk may be achieved with Medical Countermeasures (MCM). Recent meta-analyses have demonstrated the efficacy of aspirin in the reduction of the background colorectal cancer incidence and mortality rates for specific cohorts. Additional studies of warfarin in patients greater than 50 years of age have indicated statistically significant decreases in stomach, bladder, brain, prostate, and lung cancer incidence as compared to control groups. While ultimate selection of suitable countermeasures will be the responsibility of flight surgeons, this paper presents a general methodology for incorporating MCM into the NASA Space Radiation Cancer Risk model and includes modifications of the background mortality rates (hazard rates) and the radiation risk coefficients to numerically quantify the benefits of MCM. As examples of the method, aspirin and warfarin will be employed as MCM in a sensitivity analysis to compute the REID for astronauts embarking on a one-year deep space mission scenario.

A commentary on the impact of modelling results to inform mission planning and shield design

A correspondence has been received in reference to a recently published article titled "On the decision making criteria for cis-lunar reference scenarios". The intent of the paper was to demonstrate: (i) a novel methodology for calculating the dose from solar particle events (SPEs), and (ii) the impact of the SPE parametric model, shield thickness, dose metric, and radiation transport code on choosing a worst-case scenario. This effort assumed a spherical, aluminum spacecraft with an internal diameter of 3.8 m and with varying wall thickness ranging from 2 to 10 cm. A brief component of this article compared the dose from several solar particle events (SPEs) inside the spherical spacecraft geometry as calculated with Monte Carlo radiation transport code MCNPX and the on-line tool OLTARIS. In this comparison, the MCNPX simulation parameters assumed a volume-averaged dose while OLTARIS calculations assumed a point-dose estimate at the center of the spherical geometry. These modeling assumptions were detailed in the initial publication. The differences in the neutron, proton, and light-ion fluences and doses obtained between both codes were generally attributed to differences transport methodologies, nuclear physics models, boundary condition setup and detector regions. The commentary received demonstrated when both codes used a point-detector geometry and/or volume-averaged geometries, the two would yield similar proton fluences. This is a worthwhile observation that further emphasizes the impact of modeling assumption. The commentary further suggested however that the volume-averaged dose results "artificially reduced" estimates and that it was both "misleading" and "not-applicable" for use in storm shelter design. The response presented here will reiterate the context of the initial assumptions made, demonstrate the variability in point-dose estimates relative to a volume-averaged dose estimate, state why a volume-averaged estimate is equally applicable in this context, and lastly reference other factors that can give rise to increased uncertainty.

Acute radiation risk assessment and mitigation strategies in near future exploration spaceflights

As more exploration spaceflights are planned to travel beyond the protective Earth magnetosphere to deep space destinations, acute health risks due to possible high radiation doses during severe Solar Particle Events (SPEs) are of greater concern to mission planners and management teams. It is expected that some degree of Acute Radiation Syndromes (ARS) symptoms may be observed, but the specific list of health risks that are relevant to exploration missions has been ambiguous and debatable in the past. This mini-review gives a brief summary of the features of radiation exposure if astronauts encounter severe SPEs beyond Low Earth Orbit (LEO), the evidence of ARS radiobiological studies at exposure levels close to recommended limits, and the shortcomings of previous dose projection approaches for ARS risk assessment. Some ARS biomathematical models, particularly those pertinent to the dose ranges that severe SPEs beyond LEO could generate, are reviewed and evaluated, focusing on their capability to predict the incidence of performance incapacitation and time-phased health effects with subsequent medical care recommendations. Using onboard active dosimeter input for estimating organ doses and likely clinical outcomes for SPEs in real time, a new strategy for ARS assessment and mitigation is described to cope with the potential threats of severe SPEs for planned deep space missions.

A water-filled garment to protect astronauts during interplanetary missions tested on board the ISS

As manned spaceflights beyond low Earth orbit are in the agenda of Space Agencies, the concerns related to space radiation exposure of the crew are still without conclusive solutions. The risk of long-term detrimental health effects needs to be kept below acceptable limits, and emergency countermeasures must be planned to avoid the short-term consequences of exposure to high particle fluxes during hardly predictable solar events. Space habitat shielding cannot be the ultimate solution: the increasing complexity of future missions will require astronauts to protect themselves in low-shielded areas, e.g. during emergency operations. Personal radiation shielding is promising, particularly if using available resources for multi-functional shielding devices. In this work we report on all steps from the conception, design, manufacturing, to the final test on board the International Space Station (ISS) of the first prototype of a water-filled garment for emergency radiation shielding against solar particle events. The garment has a good shielding potential and comfort level. On-board water is used for filling and then recycled without waste. The successful outcome of this experiment represents an important breakthrough in space radiation shielding, opening to the development of similarly conceived devices and their use in interplanetary missions as the one to Mars.

Shielding evaluation for solar particle events using MCNPX, PHITS and OLTARIS codes

Detailed analyses of Solar Particle Events (SPE) were performed to calculate primary and secondary particle spectra behind aluminum, at various thicknesses in water. The simulations were based on Monte Carlo (MC) radiation transport codes, MCNPX 2.7.0 and PHITS 2.64, and the space radiation analysis website called OLTARIS (On-Line Tool for the Assessment of Radiation in Space) version 3.4 (uses deterministic code, HZETRN, for transport). The study is set to investigate the impact of SPEs spectra transporting through 10 or 20 g/cm(2) Al shield followed by 30 g/cm(2) of water slab. Four historical SPE events were selected and used as input source spectra particle differential spectra for protons, neutrons, and photons are presented. The total particle fluence as a function of depth is presented. In addition to particle flux, the dose and dose equivalent values are calculated and compared between the codes and with the other published results. Overall, the particle fluence spectra from all three codes show good agreement with the MC codes showing closer agreement compared to the OLTARIS results. The neutron particle fluence from OLTARIS is lower than the results from MC codes at lower energies (E<100 MeV). Based on mean square difference analysis the results from MCNPX and PHITS agree better for fluence, dose and dose equivalent when compared to OLTARIS results.