Text settings Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only Learn more Minimize to nav Solar storms, like the one in August 1972 that hit during the gap between the Apollo 16 and Apollo 17 missions, throw bursts of protons intense enough to raise an astronaut’s cancer risk or even cause radiation sickness. Earth’s atmosphere and magnetic field absorb this radiation, but crews heading to the Moon or Mars won’t have that protection, and no spacecraft built so far has enough shielding to stop it.
A team led by Jordan Houri and Oren Milstein of StemRad, an Israeli-American startup developing personal protective equipment against radiation, proposed we could solve this by shielding the astronauts instead of shielding the spacecraft.
To test this idea, StemRad’s team flew a wearable radiation-shielding vest called AstroRad to the Moon and back aboard NASA’s uncrewed Artemis I mission, then used the flight data to calculate how it would perform during an actual solar storm. It turns out the vest would perform roughly as well as the Orion’s heavily shielded onboard shelter the crew was supposed to hide in to wait out a storm.
Spacecraft designers have spent decades weighing shielding options, from aluminum hulls to water-filled walls to superconducting magnets that would deflect charged particles before they reach the crew. All of them run into the same challenge. “The question was how to use mass in a very efficient way,” Milstein says. “Mass is really the bottleneck—every gram counts.”
A shielding garment an astronaut could wear is simultaneously sensible and ridiculous. The ridiculous part was that, for a long time, people thought that a protective garment would need to look like full-plate armor in order to offer meaningful protection. It would presumably need to be made of lead or other high-density materials that would add mass and make moving around nearly impossible.
StemRad engineers, though, figured dressing as a medieval knight doesn’t really get you all that much. The human body, Milstein explains, isn’t uniformly vulnerable to radiation. “Tissues like the bone marrow are a lot more sensitive to radiation compared to the brain,” he says. Following this idea, StemRad developed a belt for nuclear first responders, worn around the hips, which hold roughly half the body’s bone marrow, the tissue that makes blood cells. Protecting even a fraction of the bone marrow lets a person regrow it and survive a high-dose exposure.
So, StemRad, working with Lockheed Martin, expanded this idea into a female vest that, aside from the hips, also covered the breasts, stomach, colon, and reproductive organs. These are all less immediately life-threatening when irradiated, but carry a long-term cancer risk.
“It still gets people surprised,” Milstein says. “Everybody asks, what about the head? But we’re actually able to reduce the effective dose by 60 percent without protecting the head, the arms, or even the legs.” But choosing where to put shielding was just one part of the problem. Picking the material and making a design that would not hinder the astronaut’s movements was another.
“The primary factor in how effective a shielding material is is its atomic number divided by its atomic mass,” Houri says. Hydrogen, which has no neutrons, has roughly double that ratio of any other element, which is why water is often cited as a good space radiation shield. High-density polyethylene (HDPE), an ordinary plastic, packs even more hydrogen by mass than water, and, unlike water, it’s a solid, so you don’t have to worry about leaks.
The problem with a solid, rather stiff material like HDPE is that when made thick enough to offer good radiation protection, it can compromise comfort. To go around it, StemRad’s team broke it apart into hexagonal rods of varying lengths and cross-sections. “We divided up the shielding panels into thousands of hexagonal tessellated rods of HDPE,” Houri says. These rods, sandwiched between two layers of elastic fabric, made the vest extremely flexible and fluid-like. “It almost behaves the same way that water would, while still remaining solid,” Houri claims.
The rods run 9 to 60 millimeters long. Engineers calculated their length using the Bethe-Bloch formula, which describes how charged particles lose energy while moving through matter. This way, the team could predict how far a particle with a given energy would travel through their shielding before stopping.
Of course, calculations, even highly precise ones, are not enough. To get hard data, StemRad sent its AstroRad vest on a round trip around the Moon.
Most of the radiation all these detectors recorded, though, came from the mission’s passage through Earth’s inner Van Allen belt, a region of high-energy trapped protons. There was no solar storm during the Artemis I flight. “If we did have [a solar particle event], that would have been the end of the analysis—we could have just used that data,” Houri said. Instead, the team used the belt crossing as a proxy, since its proton energies roughly overlap with those present during a typical storm. They built Monte Carlo simulations of the spacecraft, phantoms, and vest and checked them against the dosimeters. The output of the models was, for each dosimeter, within five percent of the actual measured data.
With that confidence established, they swapped the belt’s radiation spectrum for the spectra of two historical solar storms, from August 1972 and October 1989, and let the simulated phantoms tumble freely rather than sit fixed facing forward, since a real astronaut wouldn’t stay seated for an entire event. The vest cut the effective radiation dose by around 60 percent for the 1972-style event but only about 40 percent for the 1989-like storm.
“The 1989 storm was much more energetic—the spectrum was harder and shifted towards higher energies. It was more of a worst-case scenario. The storm from 1972 was more of a typical large event,” Houri explains. Solar storm particle flux is usually dominated by lower-energy protons, with high-energy ones exponentially rarer, which is why the vest performed as it did. “The bulk of that radiation is in the lower energy range, where you can shield it with a reasonable amount of material,” Houri adds.
Translated into mission planning, wearing the vest during a single major storm could extend an astronaut’s career by roughly 40 to 193 days, as measured against NASA’s 600-millisievert lifetime dose limit. That’s comparable to Orion’s dedicated storm shelter, a heavily shielded cabin area astronauts could retreat to, but with a key difference: the shelter is restrictive. Astronauts hiding in the shelter wouldn’t be able to move around or perform mission operations. “With the vest, they’d be able to enjoy the same amount of protection while continuing to move around the cabin. We see it as a protection complementary to the storm shelter,” Houri says.
The vest, though, does nearly nothing against galactic cosmic rays (GCR), which are the other major radiation source astronauts face. Unlike solar storm radiation, GCR arrive continuously at much higher energies, which makes them harder to shield against. “Using the vest against these rays would not be reasonable,” Houri says. “You’d have to wear it at all times.”
The problem is, the AstroRad vest is unlikely to be iterated in a way that could offer more protection in the future, and the protection we think it offers as it is today rests mainly on computer models.
The uncertainties baked into the computer models StemRad’s team used to simulate the vest’s performance against solar storms include a simplified two-material stand-in for Orion’s hull and a simplified Van Allen belt spectrum, which used a statistical prediction rather than a direct recording of the radiation.
Getting real, not simulated, data on the vest’s performance would mean sending it onboard an Orion spacecraft all the way to deep space and waiting for a solar storm to happen, which is hardly feasible.
But an even bigger constraint for this design, the team says, is dealing with its weight. As flown on the Artemis I, the vest weighed in at 26 kilograms, which is roughly as much as a full suit of medieval plate armor. “It will be hard [to improve protection] without adding more mass,” Milstein says. So, the team wants to go the opposite direction.
Rather than chase more protection, StemRad’s engineers’ near-term focus is preserving what the vest already does, while cutting its weight. Using the Artemis I data, they already managed to cut its weight from 26 kilograms to 16. “That reduction came without losing a large part of the protection,” Milstein claims. The goal, he says, is a lighter vest astronauts could wear not just for a storm’s peak, but for days around it.
The study on AstroRad’s test during the Artemis I mission is published in Science Advances: http://dx.doi.org/10.1126/sciadv.adz1892
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