Space radiation measurements during the Artemis I lunar mission

Human spaceflight beyond low Earth orbit presents significant challenges, one of the most critical being exposure to space radiation. This radiation, originating from galactic cosmic rays (GCRs), trapped radiation belts (Van Allen belts), and solar particle events (SPEs), poses considerable health risks to astronauts. These risks include increased cancer incidence, cataracts, degenerative diseases, and acute radiation sickness from high-dose exposures. Previous research has relied on data gathered from the International Space Station (ISS) and Space Shuttle missions, both operating within the protective shield of Earth's magnetic field and relatively heavy shielding. Data from robotic probes like the Mars Science Laboratory and Lunar Reconnaissance Orbiter, while valuable, are from lightly shielded spacecraft. Limited information from the Apollo missions also exists, however, these data points have significant limitations and are not representative of the challenges faced during modern, longer duration deep space missions. The Artemis I mission, with its extended duration and deep-space journey to the vicinity of the Moon, provided a unique opportunity to gather comprehensive radiation data within a heavily shielded spacecraft, specifically the Orion spacecraft. This research aims to comprehensively characterize the space radiation environment within the Orion crew cabin during the Artemis I mission, thereby enhancing our understanding of radiation exposure in deep space and informing the design of future crewed missions to the Moon and beyond. The importance of this research stems from the need to accurately assess and mitigate the risks associated with long-duration space travel, paving the way for safer and more sustainable human exploration of the solar system.

Extensive research on space radiation has highlighted its detrimental effects on human health. Studies have shown a strong link between space radiation exposure and an increased risk of cancer, cardiovascular disease, and central nervous system disorders (Patel et al., 2020; Sishc et al., 2022). Modeling of space radiation environments has advanced significantly, with models like AP9-IRENE providing estimates of particle fluxes and dose rates (O'Brien et al., 2018). However, the accuracy of these models in heavily shielded environments remains a subject of ongoing investigation. Prior measurements from the ISS and robotic missions, while providing valuable data, have limitations due to shielding differences and the protection afforded by Earth's magnetic field (Zeitlin et al., 2013, 2023; Berger et al., 2017). Apollo mission data offers some insight into radiation exposure during lunar missions, but these measurements were limited in scope and methodology (Schaefer et al., 1972; English et al., 1973; Fleischer et al., 1973). The current study directly addresses these gaps by providing extensive radiation measurements from a heavily shielded spacecraft during a deep space mission.

The Artemis I mission incorporated a comprehensive radiation monitoring system within the Orion spacecraft. This system utilized several instruments at various locations within the spacecraft, offering a multi-layered assessment of radiation exposure. The instruments used included the NASA Hybrid Electronic Radiation Assessor (HERA), the European Space Agency (ESA) Active Dosimeter (EAD), the German Aerospace Center (DLR) M-42, and the NASA Crew Active Dosimeter (CAD). HERA and EAD provided estimates of both absorbed dose (in gray, Gy) and dose equivalent (in sievert, Sv), while M-42 and CAD focused primarily on absorbed dose. The key difference lies in the biological effectiveness of radiation, as expressed by dose equivalent, which considers the varying radiosensitivity of different human organs. The Matroshka AstroRad Radiation Experiment (MARE), using life-size instrumented anthropomorphic phantoms (Helga and Zohar), provided measurements of organ doses. These phantoms replicate the radiation transport properties of the human body. The instruments were strategically placed in Orion at locations with varying degrees of shielding, allowing for the assessment of shielding effectiveness. The mission profile involved transit through the Earth's inner and outer radiation belts, followed by an extended period in the interplanetary environment dominated by GCRs, and two lunar flybys. During the mission, data were continuously collected, allowing for detailed analysis of radiation exposure during different phases of the flight. Post-mission analysis involved comparing the measured data with predictions from radiation transport models, such as HZETRN and Geant4, using the Badhwar-O'Neill model of GCR flux. These comparisons aimed to validate the accuracy of radiation modeling tools in heavily shielded deep space environments. Further analysis included spectral analysis of energy deposition and linear energy transfer (LET), providing insights into the radiation quality and biological effectiveness.

The Artemis I radiation measurements revealed several key findings. A significant difference in dose rates was observed during the inner proton belt passes, with a fourfold variation between the most and least shielded locations within Orion. This variation validates the effectiveness of Orion's shielding design in reducing exposure during large SPEs, which have similar peak fluxes and spectral shapes to the inner belt. Measurements revealed that interplanetary cosmic-ray dose equivalent rates in Orion were as much as 60% lower than those reported in previous studies from less-shielded instruments. This suggests that increased shielding can significantly reduce biological impact, even though it may not dramatically affect the absorbed dose. Furthermore, a 90° rotation of the Orion spacecraft during inner-belt transit led to a 50% reduction in radiation dose rate. This unexpected finding highlights the importance of spacecraft orientation in minimizing radiation exposure. Measurements using the M-42 instrument inside and outside the MARE Helga phantom showed that inner proton belt doses varied roughly twofold between different organs, and that the front skin dose was threefold higher than that of the spine. The GCR cumulative organ doses varied only by a few percent, with the back skin (highest shielding) showing the highest dose. Comparisons between Artemis I measurements and radiation transport models (HZETRN and Geant4) showed broad agreement, validating the models’ accuracy in this heavily shielded environment. Spectral analysis revealed the dominance of secondary Bremsstrahlung (X-rays) in the outer electron belt radiation environment. The calculated dose equivalent rates from GCRs were lower than those reported for lightly shielded instruments during a similar solar cycle, confirming that increased shielding can reduce the biological harm of GCRs. The total mission dose equivalents were calculated to be 26.7–35.4 mSv, with a small portion of this attributable to the belt passes.

The findings from the Artemis I radiation measurements address several crucial aspects of space radiation protection. The observed fourfold difference in dose rates during inner-belt passes highlights the importance of shielding design and placement in minimizing radiation exposure. The significant reduction in GCR dose equivalent rates compared to previous studies underscores the effectiveness of increased shielding in mitigating biological harm. This is particularly significant given that radiation protection policy strives to keep astronaut exposures ‘as low as reasonably achievable’ (ALARA). The unexpected effect of spacecraft orientation on dose rates demonstrates that even subtle changes in mission design and operations can have a measurable impact on astronaut radiation exposure. The validation of radiation transport models using the Artemis I data provides confidence in their ability to accurately predict radiation environments for future missions. However, ongoing refinement and validation of these models is crucial. The combination of detailed dosimetry data with validated computational tools will allow for the development of more accurate and comprehensive radiation risk assessments for future missions. These insights will help inform the development of advanced shielding strategies and mission operational procedures for future crewed deep space missions.

The Artemis I mission provided invaluable radiation data during a deep space mission inside a heavily shielded spacecraft. The study confirms the effectiveness of Orion's shielding design and the impact of spacecraft orientation on radiation exposure. The lower-than-expected GCR dose equivalent rates indicate that substantial radiation shielding can reduce the biological impact of GCRs. This research validates existing models and informs future mission planning, emphasizing the need for careful consideration of shielding, spacecraft orientation, and trajectory optimization to mitigate space radiation risks for future crewed missions, particularly longer duration ones like missions to Mars. Further research should focus on refining radiation transport models, exploring advanced shielding materials and techniques, and investigating the long-term health effects of space radiation exposure.

While the Artemis I data provides significant insights, certain limitations exist. The mission did not encounter any solar particle events, limiting the assessment of Orion's shielding performance during these high-intensity radiation events. The study focused primarily on absorbed dose and dose equivalent, and further research is needed to fully characterize the biological effects of different radiation types and energies. The findings are specific to the Orion spacecraft design and might not be directly generalizable to other spacecraft with different shielding configurations or materials. Long-term health effects of the observed radiation exposures are not addressed in this study and will require continued monitoring of astronauts participating in future deep space missions.