A new analysis finds that even a generous estimate of one billion tons of accessible lunar water would support a city of one million people for only about 2.4 years without recycling, or roughly a century with International Space Station-level recovery. Smaller settlements appear far more sustainable: a population of about 100,000 could potentially stretch the same water supply for roughly 1,000 years at 98% recycling, while a village of 1,000 people could use lunar water much more freely. Power appears to be a weaker constraint than water, making better prospecting, highly efficient recycling and the discovery of additional underground ice critical if large permanent lunar settlements are ever attempted. The Moon contains water, but that does not necessarily mean it contains enough to support the vast cities sometimes imagined for humanity’s future in space. A new analysis estimates that even under generous assumptions, a lunar metropolis containing one million people could consume its available water surprisingly quickly. Without recycling, a billion-ton supply would last only about 2.4 years. Even with recycling efficiency comparable to …