Lunar Water Limits Moon City Size

One hundred years. That is the estimated lifespan of a one million person city on the moon, according to a new analysis published in Frontiers in Space Technologies. The figure assumes that settlers can recycle 98 percent of their water, a rate currently achieved on the International Space Station. Without such efficient recycling, the timeline would be even shorter. This finding challenges the ambitious timelines proposed by major space industry leaders who have recently emphasized lunar habitation over other destinations.

In February of this year, Elon Musk, the chief executive of SpaceX, stated on social media that his company had shifted its primary focus from Mars to the moon. He described the goal as building a self-growing city on the lunar surface. Musk suggested this could be achieved in under ten years, a significantly faster timeline than the more than 20 years he previously estimated for Mars settlement. Meanwhile, Jeff Bezos, the head of Blue Origin, has claimed that millions of people will live in space by 2045. These high-profile statements prompted a closer scientific look at whether the moon can actually support such populations.

Martin Elvis, a senior astrophysicist at Harvard University's Smithsonian Astrophysical Observatory, teamed up with Jonathan McDowell, an honorary professor at the Durham University Space Research Centre in England, to evaluate these claims. Their research focused on two critical resources for any large-scale habitat: power and water. The team found that generating energy is not the primary bottleneck. They determined that a combination of solar and nuclear power sources would be sufficient to sustain one million people on the lunar surface indefinitely, provided the infrastructure is built.

Water presents a much more complex challenge. Most of the water believed to exist on the moon is located in permanently shadowed regions at the poles. These areas are cold enough to keep water frozen because sunlight never reaches them. Current estimates suggest there is roughly one billion tonnes of water in these regions. However, the researchers calculated that this amount would only last a million people for about a century. If the water supply is smaller, the settlement would run out of resources much faster.

Elvis offered a comparison to help visualize the scale of this water supply. He noted that one billion tonnes of water is a large number in absolute terms but modest in context. He compared it to an aquarium the size of Central Park filled until the Statue of Liberty was just underwater. He described it as a small lake, noting that such a volume is not enough to supply a big city for a very long time. This perspective highlights the difference between having some water and having enough for a sprawling metropolis.

Smaller settlements face a less urgent problem. The study found that a town of 10,000 people could remain sustainable for centuries under the same assumptions about water recycling and initial supply. McDowell noted that if humanity wants to cover the face of the moon with cities, similar to those depicted in science fiction, it must find a way to transport water there. He emphasized that the resource is not currently available in sufficient quantities on the surface to support massive growth without external input.

The researchers also examined the reliability of the one billion tonne estimate. Elvis suggested that this figure might be 20 or 30 times too big. He estimated that only one-third to one-half of the water present would likely be recoverable. His best guess is that the amount of water that can be reliably counted on is about one percent of the original figure. If this lower estimate is correct, the 100 year sustainability window for a million people shrinks to just one year, making large scale habitation significantly more difficult.

The physical form of the lunar water adds another layer of complexity. Gordon Osinski, a professor of planetary geology at Western University in London, Ontario, pointed out that satellite measurements carry uncertainties. He believes the water is there but wants it confirmed on the ground. He suggested the water could resemble Arctic permafrost, with sub millimetre ice crystals mixed into the soil. This form would be difficult to extract and process for human consumption, potentially increasing the energy and labor costs of settlement.

Despite these constraints, the researchers do not view the findings as a reason to stop trying. McDowell said the truth is never negative. He argued that what is negative is to actually go and try and do something that isn't going to work. The study serves as a realistic baseline for planners and engineers who are designing the next generation of lunar habitats. It suggests that while power is manageable, water management will be the defining engineering challenge for future lunar cities.