Astronaut Radiation Exposure

Hello!

Thanks for a great question; it was nice to have an excuse to dig into this subject, which turned out to be a bit more interesting than I’d expected. Here’s what I was able to find out; most of this is from a 2024 article in Nature, one of the world’s premier scientific journals.

Let me start with the Artemis 1 mission. The findings from that mission are relevant to the radiation that will be faced by all future missions to the Moon. I’m going to start with the radiation doses that were measured because, when we’re looking for radiation health effects, dose is where it all begins.

(link to NASA map of Artemis flight)

Artemis 1 was a long and complex mission; over the course of 25 ½ days the unmanned craft lifted off and transited to the Moon, passing through Earth’s radiation belts on the way. After looping around the Moon for a week or so, including some adjustments to its orbit, Artemis 1 made one more passage through the radiation belts and returned to Earth. And, although no passengers were onboard, the craft did carry two mannequins, Helga and Zohar, who were instrumented to monitor their radiation exposure during the flight. In addition to the radiation protection designed into the craft itself, Zohar wore a radiation protection suit while Helga was not protected as such, to show how much protection the suit provided. I’ll come back to Helga’s and Zohar’s radiation exposure shortly – but first let’s take a look at the shielding provided by the spacecraft itself.

The Orion module is designed with radiation shielding in mind; the structure of the module, the location of storage lockers and tanks for fluids are carefully arranged not only for radiation shielding, but so that the weights are distributed for maximum stability and balance throughout the mission. On top of that, the spacecraft has a small, shielded “storm cellar” where astronauts can hunker down during large solar flares or coronal mass ejections. This enclosure is designed to keep astronaut doses to less than 150 mSv (15 rem) under the worst conditions the Sun is likely to produce. Also, the spacecraft itself can be reoriented in space to present the greatest amount of shielding to oncoming radiation; thus reorienting the capsule by 90° during the ship’s transit of the Earth’s proton belt cut radiation exposures in half.

Back to Helga and Zohar. While there were no solar particle events during this flight, there was the garden-variety cosmic radiation from the Sun and from the rest of the galaxy, providing enough radiation to make it possible to test new protective gear on Zohar.  The gear, known as AstroRad, made by a company called StemRad. Most of the radiation exposure from solar cosmic rays is in the form of protons, and protons are best-shielded by materials that contain a lot of hydrogen.  AstroRad is built around high-density polyethylene (HDPE) that contains the highest number of hydrogen atoms of any solid material known. It turns out that Zohar received only about 40% as much radiation exposure as the unshielded Helga. AstroRad wouldn’t need to be worn all the time, but it could be worn, for example, if an astronaut needs to leave the storm cellar to operate the capsule in the midst of a solar storm.

So, Artemis 1 lowered potential astronaut doses by taking the lowest-dose path through the Earth’s radiation belts, orienting the capsule to reduce dose even further, designing the space capsule to help shield cosmic radiation, including a storm cellar to shield radiation from solar flares, and including garments to provide even more shielding.  Based on measurements made on the Artemis 1 mission, these should keep astronaut doses to less than 250 mSv (25 rem) during even a large solar particle event and, if the astronauts are wearing their body shielding, to less than 150 mSv (15 rem). These doses are lower than the NASA limits and, even at the higher dose of 250 mSv (25 rem) the added risk of cancer is only a little more than 1%.

Best,

Norman Dickinson, PhD, Certified Health Physicist