HOUSTON  –  If, like Elon Musk, you have been dreaming of a future in which large populations of humans can live on the moon, you’re likely to be disappointed. Even though there could be trillions of pounds of water trapped inside tiny glass beads spread across the lunar surface, scientists estimate that this won’t be enough to sustain a big city for very long, according to a study published Monday in the journal Frontiers in Space Technologies. Musk announced in February that his aerospace company, SpaceX, had shifted its focus from developing a settlement on Mars to “building a self-growing city on the Moon,” because it would be quicker to achieve, potentially “in less than 10 years.”  That raised the question, “Well, how big a city could you support on the moon with that water supply? And the answer was very surprising,” lead study author Martin Elvis, a senior astrophysicist at the Harvard and Smithsonian Center for Astrophysics. Taking 1 billion tons of water as a “generous” estimate, Elvis and his colleague, astrophysicist Jonathan McDowell, who is now an honorary professor at the UK’s Durham University Space Research Centre (SPARC), calculated how long populations of various sizes could last if they were using water for drinking, hygiene, growing food, rocket fuel and even breathing.

They found that a population of 1 million people — about the number of people who live in Sweden’s capital, Stockholm — would finish the water supply in under two and a half years, according to the study. However, if the water was efficiently recycled, such as at the 98% water recovery rate that the International Space Station has achieved, it would take a city of 1 million people 100 years to deplete it. And “that depends on people being very nice” about their water usage, Elvis said, including “always using rather dubious toilets and not doing laundry,” and this behavior is “unlikely to be sustainable, I would think.”  “For such a large investment, a lifetime of about a century seems to fall short of the sustainable, long-term settlement beyond Earth that some advocate,” the researchers noted. They calculated that a small city of about 100,000 people — slightly fewer than the number of people living in Sandy Springs, Georgia — could survive for 1,000 years, which “seems long enough to justify being called sustainable.” “If you have a small town of only 10,000 people… then you’ve got… 10,000 years’ worth of water, and that starts feeling like, okay, on a 10,000-year timescale, we can find another solution,” McDowell told mediamen.

For example, there is “a lot of water in the solar system,” including water in the asteroid belt and on the moons of Saturn, he said. So, maybe in the future, those on the moon could import “deliveries of water in the quantities required to sustain a large population. It’s hard to see practically how to do that with current technology, but in the long term, maybe you can do that.”

In the short term, he suggested that instead of a city on the moon in 10 years, as Musk proposed, “I would say within 30 years we might have a town on the moon. That would be the way that I would translate what Elon said.”  “This is an important study. I think the authors have done a valuable service by injecting some realism into the discussion of longer-term plans for establishing large human settlements on the Moon,” astronomer Ian Crawford, a professor of planetary science and astrobiology at Birkbeck, University of London, who was not involved in the research.