While the sun can pose a danger to astronauts, especially during solar flares, there is another radiation problem few know about. Galactic Cosmic Rays (GCRs) are a major radiation hazard for astronauts traveling beyond Earth’s magnetosphere. These highly energetic charged particles-mainly protons and heavy ions-have energies ranging from hundreds of MeV up to tens of GeV or more. The average flux of GCR particles near Earth orbit is roughly 3 particles per square centimeter per second, with energies often around 1 GeV per nucleon. However, the radiation dose measurements taken inside NASA’s Orion spacecraft during Artemis I do not directly reflect this full flux of high-energy particles, and understanding why reveals important realities about spacecraft shielding and radiation detection.
Why Artemis I Radiation Readings Don’t Match the Full GCR Flux
The Artemis I mission carried over 5,600 passive sensors and 34 active detectors inside Orion, including human-tissue-equivalent mannequins, to measure radiation exposure during the 25.5-day lunar mission. These instruments recorded dose rates and particle fluxes that were significantly lower than what a simple count of 3 particles/cm²/s at ~1 GeV might suggest. This discrepancy arises because:
1. Shielding and Detector Sensitivity:
Orion’s multi-layered aluminum and polyethylene hull provides substantial shielding that attenuates many lower-energy particles and reduces the flux of particles reaching the cabin. However, the most energetic GCR particles (above ~1 GeV/nucleon) penetrate the spacecraft with minimal energy loss. These high-energy particles often pass through detectors quickly, depositing less energy per interaction, making them harder to detect or distinguish from background noise in some instruments.
2. Detector Energy Thresholds and Saturation:
Active detectors like the Timepix sensors used in Artemis I are calibrated to measure energy deposition (dE/dx) within certain ranges. Extremely high-energy particles can traverse detectors with minimal interaction or cause overlapping signals during intense radiation events, complicating accurate flux measurement. As a result, some of the highest-energy GCR particles may be undercounted or not fully resolved in the data.
3. Secondary Radiation and Dose Contribution:
While high-energy GCRs penetrate shielding, they also interact with spacecraft materials to produce secondary particles (neutrons, protons, lighter ions) that contribute significantly to the measured dose inside the cabin. The dose equivalent recorded thus reflects a combination of primary GCRs and secondary radiation, not just the raw GCR flux.
4. Measurement Location and Shielding Variation:
Radiation levels vary inside the spacecraft depending on local shielding thickness and geometry. Sensors placed behind thicker structural components or storm shelters detect lower fluxes. The Artemis I data show dose rates varying by location, so the average flux measured inside does not represent the unshielded GCR flux in free space.
The Challenge of Shielding High-Energy GCRs
GCR particles with energies around 1 GeV/nucleon and above have ranges of many centimeters of aluminum-equivalent material (e.g., 12.6 cm or 34 g/cm² for 1 GeV/nucleon iron ions). This means:
– No practical spacecraft shielding can fully stop these particles without adding prohibitive mass.
– Increasing shielding thickness reduces lower-energy particles but can increase secondary radiation, which also contributes to dose.
– The Artemis I mission’s shielding strategy balances mass constraints with dose reduction, resulting in roughly 50% dose reduction during Van Allen belt crossings and effective protection during solar particle events.
Implications for Artemis Crew Safety and Vision Protection
– Radiation doses recorded during Artemis I (26.7–35.4 mSv over ~25 days) are consistent with effective shielding and operational protocols, keeping exposures below NASA career limits for lunar missions.
– High-energy GCRs still penetrate the spacecraft and human tissue, posing long-term risks including vision impairment. Artemis uses additional countermeasures such as the AstroRad radiation vest to protect radiosensitive organs, including eyes, from some radiation exposure.
– Operational strategies like using storm shelters and limiting extravehicular activities during solar storms further reduce risk.
Conclusion
The apparent mismatch between the average GCR flux (~3 particles/cm²/s at ~1 GeV) and the Artemis I radiation measurements inside Orion arises because the highest-energy GCRs are difficult to fully detect and cannot be completely stopped by spacecraft shielding. Instead, Artemis’s multi-layer shielding, combined with advanced detectors and operational protocols, effectively reduces astronaut radiation exposure to safe levels for lunar missions while acknowledging the unavoidable penetration of high-energy cosmic rays.
This nuanced understanding is critical as NASA prepares for longer-duration deep-space missions, where improved shielding materials, active magnetic shielding concepts, and biological countermeasures will be essential to protect crews from the full spectrum of space radiation.
References
– Space radiation measurements during the Artemis I lunar mission, Nature (2024)[1]
– Artemis I Radiation Measurements Validate Orion Safety for Astronauts, NASA (2023)[2]
– Comparison of Artemis I Radiation Measurements with Orion EFT-1 and ISS Data, Texas Tech University (2023)[5][7]
– Measurements of Materials Shielding Properties with 1 GeV/nuc 56Fe, Lawrence Berkeley National Laboratory (2010)[8]
Read More
[1] https://www.nature.com/articles/s41586-024-07927-7
[2] https://www.nasa.gov/missions/artemis/artemis-1/artemis-i-radiation-measurements-validate-orion-safety-for-astronauts/?rand=772197
[3] https://pubmed.ncbi.nlm.nih.gov/39294379/
[4] https://www.swsc-journal.org/articles/swsc/full_html/2025/01/swsc240037/swsc240037.html
[5] https://ttu-ir.tdl.org/bitstreams/04c19d52-3192-4770-911c-542c4f128f21/download
[6] https://www.nasa.gov/wp-content/uploads/2017/07/niac_2011_phasei_tripathi_electrostaticactivespace_tagged.pdf
[7] https://wrmiss.org/workshops/twentysixth/Laramore.pdf
[8] https://www.osti.gov/servlets/purl/901821
[9] https://e-docs.geo-leo.de/bitstreams/0e16c031-1b9c-4a7e-a51d-536d82f65963/download