Ever since the President Donald Trump commissioned NASA to get the boots on the moon by 2024, the agency and its partners are hard at work trying to make it happen. Last month, NASA awarded contracts to three companies to build a crewed lunar lander, but the start of the month is just the beginning. The agency also plans to build a permanent moon base before the end of the decade and use it as a stepping-stone to Mars.
If astronauts spend weeks at a time during the month, they will need to know how to live on the land – again, regolith. It is too expensive to ship everything from Earth, which means they will need to be creative with limited resources on the lunar surface. Moon droppings are a great building material and there is water in the form of ice on the south pole that can be fueled. But the hottest commodity of them all can be great to be a crying astronaut.
Earlier this year, a team of researchers in Europe showed that urea, the second most common compound in human urine after water, can be mixed with moon droppings and used for building. The resulting material is a geopolymer, which has similar properties to concrete and can be used to build landing pads, habitats, and other moon structures.
Geopolymers are regularly used on Earth as an eco-friendly alternative to conventional concrete. One of the main components in concrete is cement, which requires a high temperature 22manufacturing process that emits a lot of CO2. But a geopolymer doesn̵7;t need a lot of energy. Instead of cement, it uses pulverized rocks or fly ash, the waste product from burning coal. When it is mixed with water and some reactive compound, it creates such a complex material that it can be thrown into desired shapes before being left to dry.
During the month, most of the infrastructure is likely to be built by industrial 3D printers. The construction of bricks will be way too ineffective and limit the types of structures that can be made. But robotic 3D printers can automatically build more complex dwellings. The Lunar regolith has chemical similarities to fly ash, making geopolymers an attractive choice for building moon objects. The downside is that geopolymers need a lot of water so they can flow through the nozzle of a 3D printer.
“Water is very, very important to the lunar surface,” said Marlies Arnhof, a member of the Advanced Concepts Team at the European Space Agency and a research coauthor. “So one of our main goals in this study is to reduce the amount of water needed to produce a geopolymer.”
Superplasticizers are materials used to reduce the water content of concrete and geopolymers while maintaining their flowability. On Earth, superplasticizers are generally difficult to pronounce substances such as naphthalene and polycarboxylate. But as Arnhof and his colleagues discovered, urea also works and can be easily sourced on the moon. Instead of filtering out astronaut urine contaminants and recycling waste water, the urine can be stored in a tank and harvested for urea.
To test the idea, researchers combined synthetic urea powder with lunar regolith simulant to make cylindrical structures the size of a fist and let them dry under one weight. They then mimic the use of the material in a 3D printer by extruding it into layers through a syringe. They compared the results with conventional geopolymers. “It performed well,” said Anna-Lena Kjøniksen, a materials scientist at Østfold University College and coauthor of the study. “This seems to give the best overall results, especially when it comes to avoiding crack development.”