Artemis I found radiation near the Moon lower than expected, but higher than in Earth orbit

Radiation measured around the Moon on NASA’s uncrewed Artemis 1 flight came in higher than in Earth orbit, but by less than many scientists expected. Prof. Pawel Bilski of the Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN) in Krakow and four co-authors report the result from small lithium fluoride crystals that flew inside two human-shaped mannequins on the Orion capsule. Separate dosimetry published in Nature put the galactic cosmic ray dose equivalent inside Orion at 0.96 to 1.24 millisieverts per day.

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Orion flew around the Moon in late November and early December 2022 on a 25-day mission. The experiment, called MARE, placed more than 10,000 crystals in two mannequins named Helga and Zohar. Several dozen were experimental fluorescent lithium fluoride detectors from IFJ PAN and nearly 600 were thermoluminescent detectors. Zohar also wore an AstroRad protective vest, so the two mannequins allowed a direct comparison of a shielded and an unshielded body flying the same trajectory. Poland’s National Science Centre funded the IFJ PAN part of the work, and the detectors had to return to Krakow for readout before any dose could be calculated.

Trapped Van Allen protons explain the Earth-orbit gap

In the summary of the IFJ PAN result, Bilski says the difference between lunar and Earth-orbit radiation is not as large as intuition suggests. The paper, published in Radiation Measurements, explains why. In low Earth orbit, low-energy protons trapped in the Van Allen belts can account for most of the dose. Near the Moon that contribution largely disappears, leaving high-energy galactic particles that pass through the whole body and deposit energy fairly uniformly. The earlier ISS MATROSHKA experiment, by contrast, showed uneven dose patterns because of the trapped protons.

The new detectors agreed within 15% with conventional track detectors across the overlapping energy range, and they recorded tracks below the threshold of older devices. They are passive, like photographic film, and are read under a fluorescence microscope without chemical etching. Each crystal is only a few millimetres across, gives sharper track images than plastic detectors, and can register neutrons when enriched with lithium-6. A separate methods analysis of the same flight detectors found the fluorescent crystals overstated dose by up to 30% when tracks above 100 keV per micrometre were included, and agreed within a few percent when the analysis was limited to lower values. The five authors of the Radiation Measurements paper are Bilski, A. Kilian, M. Sankowska, T. Horwacik and Berger, who is also the principal investigator of MARE at DLR in Cologne, so the Polish and German teams share both the hardware and the interpretation.

Dose figures from the Orion capsule

The Nature paper, led by Stuart P. George with Thomas Berger of the German Aerospace Center (DLR) Institute of Aerospace Medicine as last author, supplies the absolute numbers. Galactic cosmic ray dose equivalent rates inside Orion were 0.96 to 1.24 millisieverts per day, as much as 60% lower than previous observations. More lightly shielded instruments recorded 1.58 millisieverts per day (the Mars rover’s RAD instrument) and 1.55 (the lunar orbiter’s CRaTER). Whole-mission dose equivalents were 26.7 to 35.4 millisieverts, of which 1.80 to 3.94 came from the radiation belt passes. NASA’s career limit for astronauts, the paper notes, is 600 millisieverts.

Location and orientation mattered. Through the inner proton belt, dose rates differed up to fourfold between the most-shielded sensor, at 69 microgray per minute, and the least-shielded ones, at 240 and 287. A 90-degree rotation of the spacecraft during the belt transit cut the measured dose rate by about 50%. DLR’s account adds that Berger sees the manoeuvre as a way to reduce crew exposure significantly, and that the storm shelter caps the total dose from large solar particle events at about 150 millisieverts.

Expected versus measured

The word expected covers two baselines. Bilski’s result is lower than intuition about the gap with Earth orbit. The Nature paper’s 60% figure is lower than earlier measurements by lightly shielded instruments, while DLR reports that the Orion measurements largely match model calculations, which suggests radiation simulations have improved. The Nature authors also state that their values are similar to the 1.40 millisieverts per day calculated for interplanetary space from high-latitude measurements inside the International Space Station.

That 1.40 figure is a calculation for interplanetary space, not a dose rate recorded in Earth orbit, and the ScienceDaily summary of the Radiation Measurements paper gives no low-Earth-orbit number to set beside the lunar range. The open item is that numeric comparison. Per DLR, future Artemis flights lasting days to weeks are unlikely to exceed NASA’s astronaut limits under similar conditions, a judgment that rests on the 26.7 to 35.4 millisievert mission totals.

Orion’s detectors flew on a 25-day mission with no one aboard; the mannequins carried the crystals, and the NASA photo release shows the mannequins returning from the flight for detector removal at Kennedy Space Center before the torsos went to Germany for analysis.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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