A billion tons of water sounds like plenty, until someone does the math. According to ABC News, a study in Frontiers in Space Technologies, co-authored by Smithsonian astronomer Martin Elvis and Durham University researchers, has run that math for lunar ice. Small beats big: a village of 1,000, or a town of 10,000, could draw on the polar reserve for centuries, the study finds, while a full city could not. The authors argue the resource is scarce enough to warrant cooperative international management rather than a race between nations to claim it.

The ceiling is lower than it looks. The optimistic estimate: roughly one billion tons of polar ice, enough, ABC News notes, to bury Central Park under 120 meters of it. Scale that up and it empties fast: without recycling, a city of a million residents would drain it in under two years. Push recycling to 98 percent, the rate the International Space Station already approaches, and the city could last around a century, a modest return on the cost of building it. Martin Elvis told ABC News, « I was very surprised that if you don’t do any recycling, then it’s about two years for a city of a million people. That’s still not very much, given the investment in building it. »

Nobody Has Actually Weighed the Ice

Nobody has actually weighed the ice. These projections assume the ice behaves as modeled, which remains unconfirmed. A legged Chinese probe headed for the Moon’s permanently shadowed craters exists to answer exactly that: what form the ice takes, how concentrated it is. Until then, the billion-ton figure is a ceiling drawn from orbit, not a stock counted by hand. Jonathan McDowell told ABC News, « There are physics limits to how much of a lunar society you can end up having. The limits to lunar society are power is really challenging and water is absolutely limiting. »

The real number might be buried deeper. These figures cover only surface ice. Deeper reserves, if they exist, remain unmapped and could ease the constraint or confirm it. Either way, treating lunar water as shared and finite, not a prize to claim, does not depend on which way that unknown breaks.