The first den on the Moon
Image credit: ESA Foster + Partners
Living on the Moon and Mars means living off the Moon and Mars. In preparation for the return of astronauts to other worlds, engineers are busy trying to build with lunar dust.
The first humans to live on the Moon and Mars will make their home in life-sustaining modules built on Earth. But there is no question of continually filling rockets with food, cement, and other consumables to support new human colonies. Living on other worlds hinges on our ability to live off those lands.
“The initial habitat should be built on Earth; it is way more expensive, but it is safer,” says Dr Christiane Heinicke, who leads a team designing a pre-fabricated Moon and Mars base at the University of Bremen. “In the long term, if we send more people there and really want to have a permanent presence there, I think it makes much more sense to use local materials to construct the habitat. [...] On the surface of the Moon and Mars you have this regolith and you have so much of it you can use it for construction.”
Lunar regolith – mineral and glass dust generated by the unrelenting bombardment of the surface by radiation and meteorites – is one of those substances that can be immensely irritating. The particles are electrostatically charged (so everything gets coated in it) and jagged (so wiping it off causes scratching). This is what we will have to build with.
Work on in-situ resource utilisation (ISRU) has been under way since the 1960s, mainly focused on extracting water and oxygen. Given how far we are from humans living on other worlds, let alone expanding their little prefabricated homes, building with regolith may be considered a problem for the far-flung future. However, Nasa and ESA have been enthusiastically exploring ISRU for construction; Russia’s Roscosmos has announced it will support its long-term missions with 3D-printed structures; and in 2019 China’s National Space Administration said it will 3D-print a base on the Moon.
Dr Advenit Makaya, an advanced manufacturing engineer at ESA, cautions that we are “in the preparatory phase” of ISRU, in which researchers are browsing approaches as widely as possible to see which are most promising. However, he acknowledged that we may need to start employing ISRU for construction sooner than expected: “On paper, you don’t need concrete construction with regolith in the near term. However, you have two problems that need to be solved relatively soon. The first one is radiation protection. If you want to send people for extended durations to the Moon, you need to protect them. The second is: how the heck do you land relatively large spacecraft on the Moon without getting dust everywhere? One way to avoid that is to make landing pads, ideally from local resources. So, construction might come earlier in the picture than we thought.”
There are two broad approaches to construction with lunar regolith: sintering and binding. Sintering involves heating regolith to below melting point, causing bonds to form across grain boundaries to create a solid piece of glass or ceramic.
Despite studies into spectacular sci-fi concepts – including rovers with mounted lasers sintering Martian roads as humans follow behind – lasers probably have limited applications in large-scale construction. “[A laser] is very power-hungry and also the focal point is very small,” Makaya explains. “So, you can make small things with very high accuracy, but you cannot make large things because it takes forever. Lasers will be useful for very specific things.”
Concentrated sunlight is perhaps the most elegant approach, as it does not call for an additional power source. A 2020 paper co-authored by University of Arizona astronomer Professor Roger Angel imagines using sun-sintered blocks to build a Pantheon-like structure in just three years with similarly Pantheon-like longevity, while a laboratory near Cologne is growing ‘artificial stone’ from regolith using a solar oven.
The most promising approach for sintering-based construction may use microwaves; it is possible to melt lunar regolith in an ordinary microwave oven in minutes. This has been demonstrated with lunar soil from the Apollo 17 mission. While microwaving has failed to produce usable building bricks from regolith, researchers have proposed that it could be an ideal approach for preparing foundations, roads, and launch and landing pads on the Moon.
The other approach to lunar construction is to make a concrete mixture via ISRU. Just as with terrestrial concrete, the basic ingredients for lunar concrete – known as ‘mooncrete’ or ‘lunarcrete’ – are aggregate and cement. The former would be regolith and the latter would be something of a challenge given the Moon’s scarce water resources. Still, wouldn’t solving this challenge be instructive for us here on Earth as we exhaust our natural resources?
Much R&D in this area is focused on sulfur (or, on Mars, phosphoric acid). Sulfur could be extracted from lunar troilite and heated to melting point, 140°C, to bind the regolith, setting very quickly to a rock-like solid. As there is little or no chemical reaction, it would not strictly result in concrete, and the properties fall short of terrestrial concrete.
Scientists are also exploring biological binders, inspired by the historical use of tree resin, potato starch, hoof collagen and animal blood as binders. University of Manchester researchers, for instance, discovered that a common protein in human blood can be mixed with simulant lunar regolith to produce concrete with compressive strength comparable to terrestrial concrete. This is thanks to a curdling or clotting process through which the proteins mesh to form an extended structure holding the material together.
Engineer Dr Aled Roberts, who worked on the project, explained that it will be necessary to use whatever resources are available on Moon and Mars missions. “We thought could you, you know, potentially farm the humans? That’s how it came about,” he says, although he gave reassurance that bloodletting is not the future of large-scale space construction.
“I don’t think using human blood is going to be feasible at all for significant construction,” he says. “There might be some sort of niche application. Maybe in an emergency you’d need to seal something and don’t have any binder, so you can use human blood.”
This blood-based concrete, like some other proposed lunar concretes, has 3D printing potential. Scientists expect 3D printing will play a major role in space construction, particularly given the limitations on human and robotic activity in these extreme environments. A recent ESA study combined lunar regolith simulant with a liquid binder applied along pre-defined paths to turn the material into a ‘stone-like’ solid, producing closed-cell structures, which have the potential to shield astronauts from cosmic rays and solar flares.
Meanwhile, ICON is developing the first full-scale 3D printer for space applications for Nasa. The hope is that the printer could extrude a regolith-based concrete to form construction-scale structures.
Other engineers are testing the limits of regolith to see just how little binder we could use without the structures collapsing. “People are looking at different binders produced on-site from human waste,” says Professor Yu Qiao of the University of California at San Diego. “But you look at traditional materials that are 15 to 20 per cent binder and, you know... how much can you produce?”
Qiao is exploring ultra-low-binder concrete: a field of R&D also critical for the sustainable future of construction on Earth. He and his colleagues, working with lunar regolith simulant, have shown it is possible to produce a substance stronger than steel-reinforced concrete with just 4 per cent binder combined with a compaction process. What he discovered when repeating the experiment with Martian regolith was even more extraordinary.
“We wanted to borrow this understanding for Martian soil. So, we began running the task using the Martian soil simulant. We put in 4 per cent binder [a standard epoxy glue] and it worked perfectly. ‘Wow’, we said, ‘great’, then we say ‘let’s test the limit; let’s reduce it to 3 per cent’. Still works. Two per cent? Still works. One per cent? Still works! Then I say: ‘wait a minute, something is interesting. This shouldn’t work’.”
Qiao and his colleagues tried compacting the simulant again, this time with no binder at all. It still formed a solid. Their hypothesis is that applying pressure causes the iron in the regolith to break apart and leave “almost atomically flat” surfaces. When two very smooth surfaces are brought together in a vacuum, the atoms in contact do not ‘know’ they belong to different surfaces and form atomic bonds. This phenomenon, cold welding, has also been observed on the ISS. For now, Qiao is waiting for ongoing missions to refine our understanding of Martian regolith so he can learn if this is a quirk of regolith simulant, or a phenomenon that could permit future generations to build on Mars with no binder at all.
Circular (lunar) economy
We are many decades away from sintering a lunar civilisation, in part because sintering tends to produce heterogenous solids with distinctly un-concretey properties.
“They were failures,” says Dr Milad Hamidi, an additive manufacturing scientist at École Polytechnique Fédérale de Lausanne, of these approaches. “It’s not so promising because first we are processing a material that has oxygen. Oxygen you can use, but as soon as you lose this oxygen, you lose a large part of [the regolith] and oxides are very hard, very brittle. And, during oxygen removal, a large part of the oxygen tries to [remove itself] from the molten material and during the solidification you get a lot of porosity. Usually, these are origin points for failures.”
Hamidi is leading a project at EPFL on sintering regolith once it has been treated to extract oxygen, which leaves behind a metallic mixture: an alloy. The hope is that these by-products could be sintered (specifically using laser powder bed fusion) to build structural applications on the lunar surface. This approach could create more homogenous materials: “You can melt these elements that are randomly organised all over the place and the heterogeneity vanishes.”
The work is complex and highly experimental, and the researchers are wrestling with myriad unexpected challenges caused by the infuriating material, although Hamidi reports promising results. The approach, which is at very high resolution, would be suitable not as a construction technique but for manufacturing small load-bearing elements such as disposable robot hands, as well as for mending in situ.
Construction on the Moon and Mars will necessitate a range of approaches: some parts will use binders and other parts will be sintered; there will be building blocks from which swarms of autonomous robots will construct structures, and there will be applications for additive manufacturing.
“There is no super material out there yet,” says Hanna Läkk, a space architecture expert. “It’s very similar to construction on Earth. Whatever it is you are going to construct, there is not going to be one material that’s going to apply to everything. You’re going to build in layers, different functions need different materials, so it’s sort of necessary to look into these different methods.”
ISRU has been under discussion since the 1960s – and since the 1980s with regard to construction – but every past wave of interest has been aborted by shifting priorities for space agencies. Researchers are hopeful that this current wave may be different: “At the moment, because of commercial space – which is a factor that was not there 20 years ago and in the Apollo era – there is a chance for things to still go through [...] whatever political changes there may be,” says Makaya. “[Interest in ISRU] has gone by waves, and we are hoping this wave will actually bear fruit in terms of implementation.”
Through its Artemis mission, Nasa aims to return astronauts to the Moon in the coming years and establish permanent in-orbit and surface presences – akin to Arctic research laboratories – as stepping stones to Mars and beyond. In 2020, it made headlines by announcing it would buy lunar regolith from companies mining on the Moon for ISRU. Writing in a blog post, then-Nasa administrator Jim Bridenstine said: “We know a supportive policy regarding the recovery and use of space resources is important to the creation of a stable and predictable investment environment for commercial space innovators and entrepreneurs.”
This has been generally interpreted to mean that Nasa hopes to ride the success of the growing space economy from low-Earth orbit to the Moon. So, what would the lunar economy look like?
“I think this is an expression of an ideal [...] that there’s a responsibility to extend capitalism and the so-called American way of life out to the Moon as a counter to what people would see as the expansion of China’s presence,” says Casey Dreier, chief advocate at The Planetary Society. “But in terms of the details of a lunar economy, nobody really knows what that looks like, because if it was obvious or straightforward it would be happening already.”
Construction and other forms of ISRU – for oxygen and water – as necessities for survival, are the obvious starting places for hopeful participants in the lunar economy. However, Dreier opines that a lunar economy would need more “bootstrapping” than the low-Earth orbit space sector as there are no comparable human-centred business opportunities. With its announcement, however, Nasa hopes to encourage imaginative investors by guaranteeing a core buyer for their quarry, withdrawing its presence once a lunar construction sector is thriving – however long that takes.
“Which, ironically, isn’t very capitalist at the end of the day,” Dreier says.
Fungus-based materials can have a staggering range of properties; whether transparent and filmy, leathery, or rock-like. Hanna Läkk, a researcher at Rhea System BV for ESA, was involved with a study examining the potential of fungi to build structures in space.
“If you have a settlement there, you need to feed astronauts. They would need to grow their own food. One by-product of this is organic waste,” she says. “We can combine the organic waste with fungi that you would bring from Earth. So, you would just have to bring a couple of starter cultures of fungi and then you would cultivate it on the Moon.”
The concept would involve combining the ‘root’ of the fungi (the mycelium) with organic waste, loosely comparable to the cement and aggregate, respectively. As the mycelium grows through the organic waste, it binds it together into a solid block. Once the structure is complete, the fungus is killed with extreme heat or cold. This produces an insulating, fireproof material comparable to cork.
Although this remains highly experimental, there are many potential avenues to take this research. The organic waste substrate could be placed in brick-shaped moulds or 3D-printed, and there is also interest in investigating whether the mycelium could be used as a regolith binder; this would use a mixture of regolith and organic waste as the aggregate.
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