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Space radiation is a notable hazard for long-duration human spaceflight 1 . Associated risks include cancer, cataracts, degenerative diseases 2 and tissue reactions from large, acute exposures 3 . Space radiation originates from diverse sources, including galactic cosmic rays 4 , trapped-particle (Van Allen) belts 5 and solar-particle events 6 . Previous radiation data are from the International Space Station and the Space Shuttle in low-Earth orbit protected by heavy shielding and Earth's magnetic field 7,8 and lightly shielded interplanetary robotic probes such as Mars Science Laboratory and Lunar Reconnaissance Orbiter 9,10 . Limited data from the Apollo missions 11-13 and ground measurements with substantial caveats are also available 14 . Here we report radiation measurements from the heavily shielded Orion spacecraft on the uncrewed Artemis I lunar mission. At differing shielding locations inside the vehicle, a fourfold difference in dose rates was observed during proton-belt passes that are similar to large, reference solar-particle events. Interplanetary cosmic-ray dose equivalent rates in Orion were as much as 60% lower than previous observations 9 . Furthermore, a change in orientation of the spacecraft during the proton-belt transit resulted in a reduction of radiation dose rates of around 50%. These measurements validate the Orion for future crewed exploration and inform future human spaceflight mission design., (© 2024. The Author(s).)