Moon bases will need to be 3 metres underground to avoid radiation

To protect astronauts on the Moon from harmful radiation, a long-term lunar base would need to be protected by 2-3 metres of regolith – the layer of rock and dust on the Moon’s surface.

Guo Jingnan and Mikhail Dbinde Researchers at the Hefei University of Science and Technology in China analyzed how the thickness of shielding for a lunar base would affect radiation doses for people living on the Moon. Their study included using lunar soil as shielding as well as additional artificial shielding.

According to Guo, there are two main types of radiation that are dangerous to astronauts: the ubiquitous background levels of galactic cosmic rays, which pose a long-term cancer risk, and solar energetic particles (SEPs) from sporadic solar activity. These SEPs can cause more serious symptoms of radiation exposure, such as skin damage and damage to bone marrow and lymphatic tissue, which are involved in the production of blood cells and platelets. In severe cases, exposure to SEPs can lead to death.

“The amount of radiation on the lunar surface is not constant,” Guo said. “For a short lunar stay that does not encounter a strong SEP, the radiation impact should be small.”

But if a solar explosion were to occur, the danger could be enormous. For example, at the end of the Apollo program, Apollo 16 landed astronauts on the Moon for a few days in April 1972. It was followed by Apollo 17 in December of the same year. During that time, there was a massive radiation storm.

“The SEP event was so large during September that it could have killed any unprotected astronauts on the lunar surface,” Guo said.

The study also found that too little regolith shielding could be worse than no shielding at all, because lunar soil scatters radiation and produces various types of secondary particles, including neutrons. This secondary radiation peaks about 50 centimetres deep on the lunar surface, but drops off rapidly beyond that.

“Neutrons have a large biological impact because they interact efficiently with the human body and induce radiation effects in internal organs,” Guo said. “With about 50 centimeters of shielding, neutrons can account for more than 90 percent of the total effective radiation dose.”

The study found that a three-metre deep base could safely house the same crew for more than 20 years without exceeding lifetime and annual radiation limits set by the space agency.

NASA and the Russian Space Agency impose different occupational radiation dose limits for astronauts, at 600 millisieverts and 1000 mSv, respectively. Both agencies also impose annual radiation exposure limits. The annual natural radiation exposure on Earth is approximately 2.4 mSv..

Guo says two metres of shielding would be enough for a single crew member to stay on the moon for no more than a few months, and that crew members could return to Earth frequently enough to avoid exceeding lifetime and annual radiation health limits, she says.

The most practical way to reduce radiation exposure would be to give the base natural shielding, by building it in an existing cave or lava tube, or by constructing habitats below the surface, Guo said.

Better prediction of solar storms is also important to keep astronauts safe when they leave base and conduct surface activities, she said.