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

For centuries, people on Earth have imagined what it would be like to live on the moon. Scientists and sci-fi authors have envisioned massive cities and settlements on the lunar surface.

In February, Elon Musk, founder and CEO of SpaceX, posted on X that "SpaceX has already shifted focus to building a self-growing city on the Moon, as we can potentially achieve that in less than 10 years, whereas Mars would take 20+ years." A month later, NASA laid out plans for a $20 billion moon base that will depend heavily on SpaceX technology.

But according to a new research report, those grand lunar ambitions may need to be tempered because of a shortage of lunar water and the challenge of generating enough power to sustain a large city.

In the report, published Monday in the journal Frontiers in Space Technologies, Martin Elvis, an astronomer at the Smithsonian Astrophysical Observatory and astrophysicist and space sustainability expert Jonathan McDowell investigated how large a lunar city could grow and how long it could be sustained based on the amount of water believed to be available on the moon. They also looked at how challenging it would be to meet the power requirements.

"There's been so much talk and enthusiasm about a future in which the moon is a part of our civilization and abundantly populated, and science fiction shows with extensive cities on the moon and space traffic around it, and so we just wanted to, I guess, throw some cold water, literally, on some of that overoptimism," McDowell told ABC News.

McDowell believes there will eventually be settlements on the moon but "there are physics limits to how much of a lunar society you can end up having," he said.

On the moon, there are areas at the lunar poles that are always in the dark. Known as permanently shadowed regions, these lunar craters never get above minus 250 degrees Fahrenheit, according to NASA's Jet Propulsion Laboratory (JPL). JPL says most of the moon's water ice can be found in these areas. NASA's Artemis IV mission plans to land astronauts near the lunar South Pole as early as 2028. The U.S. and China also have robotic missions designed to search for water ice in that region scheduled for as early as next year.

"I've been looking at places to put astronomical telescopes on the moon, and one of the favorite places are these permanently shadowed regions," Elvis told ABC News. "So, I got to look at the amount of water that was there, and at first I was very impressed, because the first estimates were going on towards a billion tons, and that sounded like a lot. But then I got to thinking about how there's no such thing as perfect recycling. So, how much would you lose? How long will that last?"

Elvis and McDowell, who used other researchers' lunar water estimates, point out that a billion tons of water ice is an optimistic projection and that the actual amount is likely to be much less. But even when using a billion tons as a baseline, which they say could fill Manhattan's Central Park, to a depth of nearly 400 feet, they found it wasn't enough to sustain a large settlement.

"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, which is ludicrously short. You couldn't even build the city in that time," said Elvis.

Elvis says that if a lunar city of a million people could recycle water with the same efficiency as the International Space Station, with its 98% recovery rate, that would give the city at least 100 years of water.

"That's still not very much, given the investment in building it," said Elvis.

While a large city may not be realistic, Elvis and McDowell say that doesn't mean humans can't live and work on the moon for a long time and achieve important scientific discoveries.

"If the settlement remains a 'Moon Village' at a population of about 1,000, comparable to the winter-over population of Antarctica, then we can be relatively carefree in our use of lunar water," they wrote in their report. "If the population reaches 100,000, then we need to start being careful."

The scientists say there are potential ways to overcome the limited water supply and build larger cities on the moon.

More efficient recycling could extend the lifespan of a million-person city, although they say "that goal will be hard to reach and to maintain." The duo says techniques like vertical farming and switching to a low- or no-meat diet could reduce water use. They say water could also be imported from other places, such as an asteroid. And while their estimate of a billion tons of water is much higher than what's believed to exist, Elvis and McDowell say finding additional water could extend a large city's lifespan.

When it comes to generating enough power for a large city, Elvis and McDowell say it would be challenging, but not a deal breaker. They found that a city of a million people could use either solar or nuclear power. If the population grew to millions, the scientists say nuclear power would be necessary.

"The limits to lunar society are power is really challenging and water is absolutely, limiting," said McDowell.

Elvis says the goal of the research is not "to put a damper on anyone's dreams" but to provide a reality check about the challenges of building large lunar settlements. He says he hopes that their analysis will encourage people to ask questions about what's possible and come up with solutions.

Elvis and McDowell are also encouraging countries to cooperate in managing the moon's limited water resources, rather than trying to secure what water exists for themselves.

"Without cooperation between China and the U.S. or between the Artemis partners and the lunar research station partners, then the only thing that we have is a free-for-all, and I don't think that's a good solution," said Elvis.