Grayson Hoffman and Luke Kamphuis
One of the largest issues with a permanent lunar habitat is radiation. On Earth, our natural magnetic field and ozone layer protects us from the worst of the solar radiation that Earth is bombarded with. The moon however, has no such protection. Radiation comes in many forms, and while some are easy to stop, others, are not. Why is radiation itself such a big issue? Quite simply, whatever the temperature difference, lack of oxygen or atmospheric pressure, etc., does not kill, radiation absolutely will. In the simplest of terms, radiation is the emission of high energy particles. It can be thought of as trillions upon trillions of subatomic bullets, flying through space at or near the speed of light, and can only be halted by placing a dense enough material between yourself and it. These bullets can and will smash anything that cannot stop it. This includes DNA, the master instructions to our very bodies. Any amount of damage done to our DNA dramatically increases our risk of cancer or organ failure, and with enough of these subatomic bullets, we can die within days, even hours. Radiation does not stop there, however. Just as these bullets can destroy our own flesh, they can destroy our electronics too. In high enough doses, radiation can fry our computers and data storage devices, and computer systems would be absolutely critical to any lunar habitat. The destruction or damaging of these systems is absolutely unacceptable. If its anything more complex than a light switch, radiation can and will destroy it.
How do we protect ourselves and our astronauts from this threat?
We put something between us and it. Dense enough materials in enough thickness wll stop these bullets in their tracks. If you have ever gone to the dentist, this is the exact reason you wear a leaded vest, to protect your central organs. Lead itself is a great material to defend from radiation, but it is heavy, and thus, would be insanely expensive to ship to the moon. However, we have options beyond just lead. These are what we will look at below.
Any moon base will likely have to utilize local resources to support itself. Water can be harvested from ice hidden deep in craters, the possibility of mining the moon for metals also exist. Why not harvest the materials for our radiation protection from the moon? Present in vast quantities across the lunar surface is what we call regolith. Across the moons many millions of years of existence, the rocky surface has been weathered down by the solar wind, impacts, and other such events. This has left much of the surface covered in a fine layer of regolith, a mixture of dust, rock particulate, and other ground down minerals. Regolith is incredibly fine, and can cause issues with equipment due to its fine nature, the regolith often sticking to much of what it comes in contact with. Should it be tracked into any human habitat, it could even cause problems with human health. However, due to its abundance, it can be utilized and turned useful, rather than continue to be a minor hindrance to our habitation efforts.
When it comes to shelter, a possible lunar habitat has many options. We can build modules either on Earth or in orbit, then deploy them to the surface in sections, similar to how the ISS is constructed. We could also use inflatable habitats, in essence tents, formed and held by an internal supporting structure and atmospheric pressure. Both of these options, which are the most likely type of structure we would deploy, would be located out on the surface of the moon, among the regolith. To take advantage of the local abundance of materials, we could essentially bury these structures within the regolith itself, digging out pits to place our structures, then covering them with the excavated regolith. This process could further be enhanced by the usage of drones and other unmanned vehicles, deployed months ahead of human arrival, to prepare the subsurface layout of the base, and to assist humans in setting up their habitation modules and burying them. Other construction methods could include utilizing lunar soil in a form of 3-D printing, in which case we could possibly construct shells of our radiation shielding regolith, then inflate our structures within to form our habitats.
Building off current/former studies into the concept, our regolith layer would need to be around 50 centimeters at a density of 1.5 gm/m^3. This should provide adequate protection from the normal background radiation that poses the largest long term health risks to human habitation. With on average, a thickness of five meters before yielding to the crust of the moon, there should be no shortage of material to utilize in this effort. Additionally, our regolith layer would serve as a thermal barrier, assisting in heat retention inside the base. We can also further enhance the shielding properties of regolith by layering the material with other radiation shielding/absorbing materials, such as aluminum.
Below is a possible layout for a buried design, with all necessary facilities protected by a thick shell of regolith.
Taking a long term settlement into account, it is possible our habitat will have to protect our inhabitants from radiation much higher than that of traditional background radiation. Be it a solar flare, a stray gamma-ray burst, however unlikely, we cannot build to protect just from the minimum, standard levels of radiation. As such, we either need to add extra protection in the form of a thicker layer of regolith, or introduce secondary measures.
Water is very effective at shielding items of delicacy from radiation. Many unused/retired nuclear reactor cores are initially submerged in pools of the life giving liquid, both due to its protective and cooling abilities. While we cannot ship up significant amounts of water to the Moon without tremendous expense, the option exists to mine and source it locally, from deep within craters and tunnels, protected from the Suns rays that would have evaporated the ice. However, water is heavy, and there is no guarantee our shelter could support the weight of water and regolith piled above. Lead shielding presents a similar issue, without the option to source it locally.
Additionally, as stated previously, regolith itself is messy, to say the least. Its jagged nature could also present issues such as our layers wearing down any pressurized shelter underneath. Regolith is also electrically charged, which cases the aforementioned desire to stick to most surfaces it comes into contact with, and also, along with building up on and inside said devices, issues with powered equipment.
While water and lead have their advantages, their disadvantages are high, and while keeping cost effectiveness in mind, are not entirely practical. To help protect from increased radiation levels should the worst happen, we can dig further into the crust of the moon underneath our base. Around 3 meters below ground level should be sufficient to protect from most natural phenomena that would cause a spike in radiation levels, and we can further increase this margin of safety by surrounding these emergency shelters with layers of lead, or by digging deeper should a larger safety margin be desired. With these shelters designed for uses in extreme emergencies, and with them likely having a small relative footprint compared to the size of our base, shipping up the needed lead, while expensive, should not prove as immense a challenge than if we were to utilize lead shielding for the entirety of the base. To further enhance shielding, we could attempt to surround these shelters with layers of concrete as well, perhaps even a mixture using regolith to utilize available materials, given its effectivness against all forms of known radiation.
With regolith's natural messiness, there are few alternatives besides strict cleaning and maintenance practices. Air locks can be equipped with pressurized air hoses to blow the regolith off into collection receptacles for later use, or perhaps a form of vacuum hoses to suck up the regolioth. With equipment such as rovers, cameras, etc., there is only one identifiable option. Our astronauts/colonists will have to maintain a strict cleaning and inspection regimen, and an adequate supply of spare parts will either have to be shipped to the base regularly, or the base must be outfitted with the capability to locally manufacture spares.
It would be possible to bury an inflatable structure in lunar regolith. The atmospheric pressure alone is enough to support the regolith above it.
The density of lunar regolith is about 1.5 g/cm3. The moon’s gravity is 1.62 m/s². This means that the weight per square centimeter is .00243 N per cm of height of the regolith. Atmospheric pressure is 10.1 N/cm2. Atmospheric pressure alone is enough to hold up to 41 m of regolith.
https://www.grainger.com/product/55YM61?gucid=N:N:PS:Paid:GGL:CSM-2295:4P7A1P:20501231&gad_source=1&gclid=CjwKCAjw7-SvBhB6EiwAwYdCAfPiNMMo4I2pWIZ4VY0v_b0z96gjqPzAEMYDNyLeaPVli82qXlxg-hoCoG0QAvD_BwE&gclsrc=aw.ds
This PVC tubing is 1/8 of an inch thick and rated for up to 20 psi (atmospheric is about 14psi). It also works over a large temperature range and is compatible with many common chemicals. This is one example of the many materials that could be used for this application. However, something like this that is safe for food would be preferred. Many plastics can release harmful chemicals into the air. Normally this is not an issue but if we are going to be living in a plastic room we will want to be cautious about this.
These inflatable structures can be cylindrical with fittings to connect them as well as fittings for water, air, and waste. There could also be tunnel segments to connect groupings of these rooms. Generally, any dimensions would work. Multiple variations with different shapes, sizes, and fitting arrangements could be produced. Their compressibility would make them easy to produce on earth and ship to the moon in large quantities.
To place, a few meters of regolith could be excavated in an area. The inflatable structure would be placed, connected, and inflated and the regolith would be filled back in. To deploy on the surface they could be surrounded by regolith bricks for shielding on the sides and then covered in a few meters of regolith. This makes them very compatible with other construction ideas within the project as a cheap form of bulk shelter.
Radiation shielding properties of lunar regolith and ... (n.d.). https://www.lpi.usra.edu/meetings/nlsc2008/pdf/2028.pdf
“Imagining a Moon Base.” ESA, www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Imagining_a_Moon_base. Accessed 6 Apr. 2024.
Akisheva, Yulia, and Yves Gourinat. “Utilisation of Moon Regolith for Radiation Protection and Thermal Insulation in Permanent Lunar Habitats.” MDPI, Multidisciplinary Digital Publishing Institute, 24 Apr. 2021, www.mdpi.com/2076-3417/11/9/3853.