Moonwalkers take heart—China’s Chang’e 4 lander has made the first detailed measurements of the intense radiation that blasts the lunar surface and found that it’s safe for human exploration. The results give researchers a better idea of how much protective shielding future crews will need.
Astronauts on the Apollo missions of the 1960s and ’70s carried dosimeters to measure their radiation exposure, but the devices captured total exposure from their entire journey—not merely their time on the Moon’s surface. Ever since, scientists have had to estimate the radiation doses of crews bounding around on the lunar surface “from extrapolation and modeling,” says physicist Robert Wimmer-Schweingruber of the University of Kiel, a co-author of the study. “We’ve never actually measured them exclusively on the Moon.”
But there is renewed interest in taking such measurements, with NASA’s Artemis program intending to land crews for long-term stays by 2024 and the China National Space Administration eying human missions sometime in the 2030s. The robotic Chang’e 4 made history last year when it touched down in Von Kármán crater on the Moon’s far side, bringing a suite of instruments along for the ride.
One of these was a new dosimeter operated by Wimmer-Schweingruber and his colleagues in Germany and China. The device measured hourly radiation rates and found that astronauts would be exposed to roughly 200 times the radiation levels as people on Earth, they report today in Science Advances. The dosimeter’s placement inside the Chang’e 4 probe provides partial shielding, much as an astronaut’s spacesuit would to their body, so the findings are quite applicable to human explorers, Wimmer-Schweingruber says.
The measured dose is about five to 10 times what passengers on an intercontinental flight from New York City to Frankfurt, Germany, receive when the plane is above parts of the protective atmosphere, Wimmer-Schweingruber says. Though high for Earth-based standards, radiation is one of the known dangers of spaceflight. NASA is legally prohibited from increasing the risk of its astronauts dying from cancer by more than 3%, and these levels remain below that.
What’s more, the researchers calculated that Moon bases covered with at least 50 centimeters of lunar soil would be sufficient to protect them. A deeper chamber shielded with about 10 meters of water would be enough to protect against occasional solar storms, which can cause radiation levels to spike dramatically. (Between the Apollo 16 and 17 missions, the Sun flared up in a way that could have caused radiation sickness, vomiting, and possibly death had astronauts been unprotected in space at the time.) Such a chamber would need to be reachable within 30 minutes, the amount of advanced warning time that is now possible with monitoring satellites.
The specialized dosimeter measured both charged and neutral particles. Charged particles, such as protons and heavy nuclei that have been stripped of their electrons, are dangerous because they can penetrate human skin and damage DNA, leading to cancer and other adverse health effects. They come from two main sources: powerful solar flares flung sporadically at the Moon and a constant rain of galactic cosmic rays from deep space.
Unlike Earth, the Moon lacks a magnetic field to deflect this radiation. When it hits the surface, it produces a secondary spray of neutral particles, including neutrons, which can penetrate more deeply than charged particles. The neutrons might smash and knock loose protons in a person’s tissue, transmitting momentum like billiard balls, Wimmer-Schweingruber says. These energetic, charged protons can then wreak havoc on other cells. Measurements of this neutral component didn’t exist before and were difficult to estimate even using sophisticated modeling, Wimmer-Schweingruber says.
The results are nothing that scientists didn’t already suspect, says Francis Cucinotta, who studies the health effects of space radiation at the University of Nevada, Las Vegas, and who wasn’t involved in the work. But it’s good to have the specific measurements, which verify that if astronauts live in shielded bases, they can spend up to 6 months on the Moon without exceeding NASA’s legally mandated limits, he adds.