A representative photo of the Moon: Guillermo Ferla/Unsplash.
A Chinese-German team of researchers, using data from the Lunar Lander Neutrons and Dosimetry (LND) experiment onboard China’s Chang’e 4 lander on the Moon’s far side, has obtained the first-ever comprehensive radiation measurements from the Moon’s surface.
Many missions have gone to the Moon before but none of them have recorded daily data that could help scientists determine how much radiation an astronaut on the Moon might be exposed to during a long-term stay. In their new study, the Chinese-German team reports dose-rate measurements “with previously unseen accuracy from the surface of the Moon”. And the results aren’t good news.
They found that radiation levels on the Moon’s surface are 200- to 1,000-times more than that on Earth’s surface – and 2.6-times more than what astronauts onboard the International Space Station (ISS) are exposed to.
The Chang’e 4 lander landed in the von Karman crater on the Moon’s far side on January 3, 2019. Scientists used data recorded by its LND sensor in the first two lunar days since – from January 3 to January 12 and again from January 31 to February 10, 2019 (in Earth days).
The study’s findings assume greater significance considering many teams at NASA are working towards landing “the first woman and the next man” on the Moon as early as 2024, through the agency’s much-touted Artemis programme. The findings could inform NASA scientists’ efforts to protect the astronauts selected for this programme, in the short-term by designing more effective protective gear and in the long-term by planning mission profiles and capabilities that account for the higher exposure.
In fact, based on the current level of protection available to astronauts, AFP quoted Robert Wimmer-Schweingruber, an astrophysicist at the University of Kiel and a study co-author, as saying that their stay on the Moon is currently limited “to approximately two months on the surface”, accounting for exposure during the journeys to the Moon and back.
“This is an immense achievement in the sense that now we have a data set which we can use to benchmark our radiation” and better understand the potential risk to people on the Moon, Thomas Berger, the study’s coauthor and a physicist with the German Space Agency’s Institute of Aerospace Medicine, told Associated Press.
One important suggestion that arises from this study is that protective shelters for visitors to the Moon could be made of lunar regolith – Moon dirt – and should have walls that are 80 cm (30 inches) thick. Lunar regolith is obviously abundant on the Moon, which means missions need to carry less construction material up from Earth.
Under the Artemis programme, NASA plans to send astronauts back to the Moon to stay for about a week, and subsequently also for one to two months after it has set up a functional base camp. The first mission, the uncrewed Artemis I, is expected to launch in 2021.
The two main sources of radiation in the Solar System, including on the Moon, are galactic cosmic rays (GCRs) and solar particle events (SPEs).
GCRs are very high energy particles travelling through space that bombard Earth’s upper atmosphere, the Moon and all heavenly bodies in the universe. They consist of 87% protons, 12% helium nuclei and 1% other heavier nuclei. These nuclei have a lot of energy and can penetrate materials to a greater extent than other particles. They are formed as a result of explosive cosmic events like supernovae.
After a star has gone supernova, the principal remains of the explosion is a large cloud of gas with strong magnetic fields and which can last for thousands of years. Particles, like electrons and ions, move about randomly within this cloud, guided by the magnetic fields. Over time, some of these particles could become accelerated to such great energies that they break free from the cloud and shoot off through space.
The Italian physicist Enrico Fermi had worked out the math of this phenomenon in 1949. Many years later, the NASA Fermi gamma-ray space telescope, named for him, played an important role in proving his work right.
On the other hand, SPEs – also known as solar proton events – are thought to occur when protons emitted by the Sun are accelerated, either close to the Sun because of a solar flare or in interplanetary space by even more powerful stellar outbursts called coronal mass ejections.
GCRs contribute to a low radiation dose rate but SPEs can be very intense, and haphazard. Both forms could also affect Earth. But fortunately, Earth’s surface is shielded by the planet’s magnetosphere, which deflects most space radiation away. Even the ISS is partly shielded by the magnetosphere.
But no such shields are available on the Moon, leaving the natural satellite’s naked surface exposed.
In fact, when GCRs smash into Moon dirt, the collision leads to tiny nuclear reactions that release neutrons and gamma rays, which become secondary sources of radiation. So in a way, exposing too much of the regolith to radiation could be more dangerous than having no regolith at all. This is why the recommended thickness of regolith shelters is 80 cm. Anything more and the walls will emit more radiation of their own.
“Radiation levels should be pretty much the same all over the Moon, except near the walls of deep craters. Basically, the less you see of the sky, the better. That’s the primary source of the radiation,” study coauthor Robert Wimmer-Schweingruber of the Christian-Albrechts-University in Kiel, Germany, told Associated Press.
He and his colleagues estimated that GCR contributed to a radiation dose of 1,369 micro-sievert per day. One sievert is equal to 1 J/kg, so the unit effectively measures the amount of radiation absorbed. And 1,369 micro-sievert – or 1.3 milli-sievert – is comparable to receiving an abdominal X-ray but stretched out over a day. (Note, however, that different forms of radiation from different sources have varying biological effectiveness.)
In the period for which the researchers analysed data, they didn’t detect any SPEs from the Moon’s surface. They attribute this to the Sun currently going through an extended solar minimum period. And “because the Sun is currently still in an extended activity minimum,” they wrote in their paper, “the dose rate from GCRs reported here may be considered as an upper limit for human exploration of the Moon during conditions of low solar activity.”
Radiation is known to have many harmful effects on humans. Chronic exposure to GCRs can cause cataract, cancer and/or degenerative diseases of the central nervous system.
A study published in 2017 examined potential cancer risks for astronauts on Mars due to prolonged exposure to cosmic rays.
“Galactic cosmic ray exposure can devastate a cell’s nucleus and cause mutations that can result in cancers,” Francis Culcinotta, a space physics expert at the University of Nevada, Las Vegas, and one of the study’s authors, said in a press release. “We learned the damaged cells send signals to the surrounding, unaffected cells and likely modify the tissues’ micro-environments. Those signals seem to inspire the healthy cells to mutate, thereby causing additional tumours or cancers.”
Unnati Ashar got her MS degree in space physics from CESSI in 2019. She is currently a temporary research staff. She writes as a hobby and is passionate about sharing the wonders of astronomy with people.