On the Moon, astronaut pee will be a hot commodity

Ever since President Donald Trump directed NASA to get boots on the Moon by 2024, the agency and its partners have been hard at work trying to make it happen. Late last month, NASA awarded contracts to three companies to develop a crewed lunar lander, but getting to the Moon is just the start. The agency also plans to build a permanent Moon basebefore the end of the decade and use it as a stepping stone to Mars.

If astronauts are going to spend weeks at a time on the Moon, they’re going to have to figure out how to live off the land—er, regolith. It’s too expensive to ship everything from Earth, which means they’ll have to get creative with the limited resources on the lunar surface. Moon dirt is a great building material and there’s water in the form of ice at the south pole that can be turned into rocket fuel. But the hottest commodity of them all may very well turn out to be an astronaut’s own pee.

Earlier this year, a team of European researchers demonstrated that urea, the second-most common compound in human urine after water, can be mixed with Moon dirt and used for construction. The resulting material is a geopolymer, which has similar properties to concrete and could potentially be used to build landing pads, habitats, and other structures on the Moon.

Geopolymers are regularly used on Earth as an eco-friendly alternative to conventional concrete. One of the main ingredients in concrete is cement, which requires a high-temperature ₂₂ manufacturing process that releases a lot of CO₂. But a geopolymer doesn’t require much energy at all. Instead of cement, it uses pulverized rocks or fly ash, the waste product from burning coal. When this is mixed with water and certain reactive compounds, it creates a putty-like material that can be cast into desired shapes before it’s left to dry.

On the Moon, most infrastructure is likely going to be built by industrial 3D printers. Building with bricks would be way too inefficient and would limit the types of structures that could be made. But robotic 3D printers could autonomously build more complex habitats. Lunar regolith has chemical similarities to fly ash, which makes geopolymers an attractive option for building stuff on the Moon. The downside is that geopolymers require a lot of water so they’ll flow through the nozzle of a 3D printer.

“Water is very, very valuable on the lunar surface,” says Marlies Arnhof, a member of the Advanced Concepts Team at the European Space Agency and a coauthor of the research. “So one of our main goals with this study was to reduce the amount of water necessary to produce a geopolymer.”

Superplasticizers

Superplasticizers are materials that are used to reduce the water content of concrete and geopolymers while maintaining their flowability. On Earth, superplasticizers are typically hard-to-pronounce substances like naphthalene and polycarboxylate. But as Arnhof and her colleagues discovered, urea works just as well and could easily be sourced on the Moon. Rather than filtering out contaminants in astronaut urine and recycling the wastewater, the pee could be stored in a tank and harvested for urea.

To test the idea, the researchers mixed synthetic urea powder with lunar regolith simulant to make cylindrical structures the size of a fist and let them dry under a weight. They then simulated using the material in a 3D printer by extruding it in layers through a syringe. They compared the results with conventional geopolymers. “It performed quite well,” says Anna-Lena Kjøniksen, a materials scientist at Østfold University College and coauthor of the study. “It seemed to give the best overall results, especially when it came to avoiding crack formation.”

The team’s pee-powered polymer may also help create shelters that shield astronauts from dangerous ionizing radiation on the lunar surface. The Moon doesn’t have an atmosphere or a strong magnetic field to deflect the radiation streaming from the Sun, which means astronauts have an increased risk of cancer from long stays on the lunar surface. Geopolymers have emerged as a promising candidate for containing nuclear waste on Earth, and Arnhof says the same principle could protect astronauts on the Moon. But instead of keeping radiation from leaking out of a structure, it would keep it from coming in.

The European Space Agency is working on a follow-up study that will help determine whether the urea-based geopolymer is effective in shielding against radiation. The agency also needs to prove that manufacturing the stuff is possible on the Moon. In their most recent study, Arnhof and her colleagues made the geopolymer at room temperature in an open-air environment. But 3D printing is a lot more challenging in the low-gravity vacuum environment found on the lunar surface. The geopolymer also needs to be able to withstand the large temperature swings on the Moon, which range between 260 and -280 degrees Fahrenheit.

Sintering

Whether or not they include urea, geopolymers are a promising alternative to sintering, the other main approach being explored for lunar construction, says Phil Metzger, a planetary scientist at the University of Central Florida and an expert on lunar resources. Sintering involves blasting regolith with high amounts of energy so that it melts together. Power is hard to come by on the Moon, and building large structures would require megawatt-scale solar farms that feed all their power to a sintering facility. It’s easier to scale polymer-based construction, but Metzger is skeptical that urine will end up being the superplasticizer of choice on the Moon. Instead, he says that polymers will likely be brought from Earth or mined from the lunar ice.

“I don’t know if making pee would scale well,” Metzger says. “I just don’t think you’re going to have that many people peeing on the Moon.”

It’s unlikely that NASA will start sending rockets full of pee to the Moon to make up the difference. But there are plenty of other uses for astronaut urine other than cement. One option is to recycle it and drink it, just like astronauts do on the International Space Station. Although there’s frozen water at the Moon’s south pole, it’s still unclear how hard it will be to extract and use. It also contains toxic substances like mercury and hydrogen sulfide, which means it’d have to be cleaned up before it’s safe for human consumption.

Even if humans do end up drinking Moon water instead of recycled pee, using astronaut urine for geopolymers may still not be the best choice. Instead, it might be better for growing food. Urea decomposes into ammonia and carbon dioxide, and certain types of microbes are great at converting ammonia into nitrate salts, a common type of fertilizer. This means that it could be used in a closed-loop life support system where the water from urine is recycled and the urea is used as feedstock for vegetable fertilizer. Researchers at the German Aerospace Center have been successfully growing vegetables in human urine for years. In late 2018, they launched a satellite to test a miniature version of their pee-fueled greenhouses in Earth orbit.

In the barren lunar environment, every drop of liquid matters. So whether it’s used as a building material, a fertilizer, or life support, it looks like pee is going to have a big role to play as humans venture out into the solar system. Space travel has never seemed so glamorous.

This story originally appeared on wired.com.