We finally know what kind of radiation environment future moonwalkers will be stepping into.
Astronauts hopping about on the lunar surface will soak up about 60 microsieverts of radiation per hour, a new study reports. That's 5 to 10 times higher than the rate experienced on a trans-Atlantic passenger flight and about 200 times what we get on Earth's surface, study team members said.
"In other words, a long-term stay on the moon will expose astronauts' bodies to high doses of radiation," co-author Thomas Berger, a radiation physicist at the German Aerospace Center's Institute of Aerospace Medicine in Cologne, said in a statement.
Those numbers are indeed high — but likely not high enough to impede crewed lunar exploration, as we shall see.
A pioneering result from a pioneering lander
Scientists have long known that radiation levels are relatively high on the moon, which does not have a thick atmosphere or a magnetic field to protect it. (Our Earth, fortunately, possesses both types of shielding.) But precise numbers have proven elusive.
For example, the dosimeters that NASA's Apollo astronauts took to the moon from 1969 through 1972 recorded the cumulative, total-mission exposure, not a detailed breakdown of radiation levels on the lunar surface. The new study gives us that detailed breakdown.
The numbers come courtesy of the Lunar Lander Neutron and Dosimetry instrument (LND), a German-built experiment that rides on China's Chang'e 4 moon mission. Chang'e 4 made history in January 2019 by performing the first-ever soft touchdown on the moon's largely unexplored far side.
Chang'e 4 consists of a rover named Yutu-2 ("Jade Rabbit 2") and a lander, both of which are still going strong. LND is part of the lander's scientific payload, and its partially shielded position provides "a good indication of the radiation inside a spacesuit," Berger said.
Charged particles such as galactic cosmic rays (GCRs), which are accelerated to tremendous speeds by faraway supernova explosions, contribute about 75% to the total lunar-surface dose rate of 60 microsieverts per hour, the LND data indicate.
The GCR exposure rate on the moon is therefore about 2.6 times higher than that experienced by astronauts aboard the International Space Station, according to the new study, which was published online Friday (Sept. 25) in the journal Science Advances. (The space station, while circling above the vast majority of Earth's atmosphere, gains protection from our planet's magnetic field.)
No roadblock for Artemis
NASA is working to land astronauts on the moon in 2024 and establish a sustainable human presence on and around Earth's nearest neighbor by the end of the decade, via a program called Artemis. The lessons learned during Artemis will also help pave the way for the crewed leap to Mars, which NASA aims to achieve in the 2030s, agency officials have said.
The newly reported numbers won't derail any grand Artemis plans, a reading of NASA's radiation-exposure rules suggests. Those rules stipulate that no astronaut receive a career radiation dose that boosts the risk of lifetime cancer mortality by more than 3%. The total equivalent dose that poses this risk depends on the astronaut's gender and age at the start of radiation exposure, among other factors.
Women and spaceflyers who start young are at greater risk. For example, a female astronaut who begins her spaceflight career at the tender age of 25 has a career exposure limit of 1 million microsieverts, whereas the cap is four times that for a man who starts flying at age 55.
But at 60 microsieverts per hour, that 25-year-old female astronaut could spend a total of nearly 700 Earth days exploring the lunar surface before violating her lifetime exposure limit (though this calculation doesn't take into account her time in transit to and from the moon).
And the GCR numbers measured by LND are likely on the high side for any exposure that moonwalkers would experience, the study authors said. That's because the data were gathered during an inactive stretch of the sun's 11-year activity cycle, when relatively more GCRs were able to zoom through the heliosphere, the bubble of charged particles and magnetic fields that the sun blows around itself.
All of this doesn't mean that Artemis astronauts will ship off to the moon for two-year stints, however; NASA will doubtless want to stretch spaceflyers' radiation exposure out over time for safety's sake. Agency astronauts who fly aboard the space station, for example, cannot exceed 50,000 microsieverts of exposure per year.
And NASA will likely still take pains to minimize the radiation risk experienced by Artemis astronauts, especially those who spend a large chunk of time on and around the moon.
"On longer missions to the moon, astronauts will have to protect themselves from it [radiation exposure] — by covering their habitat with a thick layer of lunar rock, for example," study co-author Robert Wimmer-Schweingruber, of the Christian-Albrecht University in Kiel, Germany, said in the same statement.
"This could reduce the risk of cancer and other illnesses caused by long periods of time spent on the moon," said Wimmer-Schweingruber, whose team built LND.
Such measures would also help guard against sporadic but potentially dangerous sun outbursts known as solar particle events (SPEs). LND didn't pick up any SPEs during the stretch covered by the new study, but future lunar explorers could well get hit by one.
Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.