Lunar Water Shortage Could Limit Moon Cities to Small Towns

Ambitious plans to build large human cities on the moon are colliding with a fundamental resource limit, according to a new study. While high-profile space industry leaders have touted the feasibility of massive lunar settlements within the next few decades, a detailed analysis by astrophysicists suggests the moon lacks enough water to support such populations for long. The findings, published in Frontiers in Space Technologies, highlight a significant gap between current technological optimism and the physical constraints of the lunar environment.

The tension stems from recent public statements by major space executives. In February, Elon Musk of SpaceX declared that his company had pivoted its focus from Mars to creating a self-growing city on the moon, a project he estimated could be completed in under ten years. Jeff Bezos of Blue Origin has similarly predicted that millions of people will live in space by 2045. Martin Elvis, a senior astrophysicist at the Smithsonian Astrophysical Observatory, and Jonathan McDowell, a professor at Durham University, sought to verify if the lunar surface could actually support these projected population sizes.

The researchers identified power and water as the two critical resources for settlement. Power proved to be the more manageable challenge. Their analysis indicates that a combination of solar and nuclear energy sources would be sufficient to provide electricity for one million residents. However, water presents a much steeper hurdle. Most of the moon's water is estimated to reside in permanently shadowed polar regions. Even assuming a total supply of roughly one billion tonnes, a population of one million would exhaust that supply in approximately 100 years, provided that 98 percent of the water is recycled, a rate comparable to that of the International Space Station.

Elvis put the scale of the available water into perspective by comparing it to an aquarium the size of Central Park filled until the Statue of Liberty was just submerged. He described it as a small lake, noting that such a volume is not enough to sustain a large city indefinitely. A much smaller settlement of 10,000 people, however, could remain sustainable for centuries under the same assumptions. McDowell noted that if the goal is to cover the moon with cities as depicted in science fiction, engineers will need to find a way to transport water from off-world sources, as the local supply is insufficient.

The study's assumptions may even be generous. Elvis suggested that the one-billion-tonne estimate is likely 20 or 30 times too high. He further argued that only one-third to one-half of the existing water would be recoverable, with his best guess for usable water being just one percent of the original figure. This would shrink the 100-year sustainability window for a million people to roughly one year. Gordon Osinski, a planetary geology professor at Western University, added that the physical form of the water remains uncertain. Satellite data has uncertainties, and the water might exist as sub-millimetre ice crystals mixed into soil, similar to Arctic permafrost, making extraction difficult.

Despite the constraints, the researchers do not view the findings as a reason to abandon lunar ambitions. McDowell emphasized that the truth is not inherently negative. He argued that the negative outcome is attempting to build something that is destined to fail. The study suggests that while mega-cities may be out of reach for now, smaller, carefully managed communities are a viable path forward for lunar habitation.