Forget lunar metropolises: Water limits could cap moon’s population, scientists say

Visions of bustling lunar metropolises and heavy industrial hubs on Earth’s nearest celestial neighbor face a hard physical limit: water. While orbital missions have mapped potential resources in the Moon’s polar regions, new mathematical models reveal stark constraints on how long future human colonies can survive on the available supply.

The discovery of water ice inside permanently shadowed impact craters has driven a surge of interest in lunar settlement. Asteroids bombarded the Moon shortly after its formation, depositing water that remains trapped in pits of eternal darkness near the lunar poles, where temperatures stay below 110K. Because these crater floors receive no direct sunlight, water ice remains stable, losing less than a millimeter of thickness every billion years.

Harvard-Smithsonian Models Test Limits of One-Million-Person Lunar Cities

Since 2013, successive lunar-orbiting missions have mapped these cold traps, inspiring commercial ambitions for long-term human habitation. Tech executives have floated plans ranging from independent settlements to self-growing cities, imagining a future where colonists farm, run full-service plumbing, and manufacture products on-site rather than importing supplies from Earth.

To test these ambitions, researchers at the Harvard-Smithsonian Center for Astrophysics modeled the resource demands of large human populations ranging from 100,000 to one million people. Their calculations assumed generous baseline estimates of up to one billion tons of available lunar water ice alongside advanced recycling efficiencies matching the 98% rate achieved on the International Space Station.

Even in the best-case scenario, with 98% efficient recycling of water — the same as achieved on the ISS — a one-million-person city will run out of water in just over a century.

Dr Martin Elvis and Dr Jonathan McDowell, via Frontiers

Without recycling, a million-person city would exhaust the supply in just a few years. Furthermore, scientists note that current estimates of lunar water reserves run roughly 30 times lower than the most optimistic one-billion-ton projection. Under more conservative estimates, even a small city would deplete its water source within a decade.

Solar Power Towers and the Prospect of a Lunar Economy

While water supplies present a formidable bottleneck, energy generation appears far more achievable. Crater rims surrounding the polar dark zones sit in near-permanent illumination, earning them the description of peaks of eternal light.

Researchers calculated that kilometer-tall towers fitted with photovoltaic arrays could generate up to three gigawatts of electricity without relying on nuclear reactors. Because silicon is abundant on the lunar surface, future colonists could manufacture solar panels directly on-site. The resulting power generation could support massive artificial intelligence data centers, laying the groundwork for a true lunar economy, according to the research team.

Smaller Settlements and Future Exploration Strategies

The mathematical models offer a more optimistic outlook for modest outposts. Scaling down population targets changes the math entirely; a settlement supporting 1,000 to 10,000 individuals could sustain itself on the available water ice for centuries.

Overcoming the water barrier for larger populations will require technological breakthroughs.

• Improving water recycling efficiency by a factor of five or higher.
• Adopting agricultural methods such as vertical farming to reduce consumption.
• Importing water supplies from accessible asteroids.
• Deploying advanced surveying techniques to discover deeper reserves.

Current surveying techniques can only probe the top few meters of surface regolith. Because rocky rubble extends tens of meters downward into the cold traps, undiscovered ice deposits may lurk beneath the surface. Until deeper resources are confirmed, large-scale urbanization on the Moon remains confined to the realm of science fiction.