Every age has its promised land, and every promised land has a prophet who names the date. In February of this year the prophet was Elon Musk, the date was less than ten years away, and the announcement came the way announcements now come: as a post on social media.
For those unaware, 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.
The reasoning that followed was logistical. Mars can be reached only when the planets line up, every 26 months, with a six-month journey. The moon can be launched to every 10 days, the trip lasting two. A moon city could therefore be iterated faster. Mars was not abandoned; SpaceX, Musk wrote, would begin building there in about five to seven years. But “the overriding priority is securing the future of civilization and the Moon is faster.” Jeff Bezos, who runs Blue Origin, has offered his own horizon: “millions of people” living in space by 2045. In this court, the figures only grow; this month Musk was talking about a trillion dollars in revenue by 2028.
Two men with a calculator
Prophecies invite audits. Martin Elvis, a senior astrophysicist at Harvard University’s Smithsonian Astrophysical Observatory, read the statements and was intrigued, as Nicole Mortillaro reported for CBC News. With Jonathan McDowell, an astrophysicist and honorary professor at the Durham University Space Research Centre in England, he set out to check the numbers. Their paper, published on 14 September in Frontiers in Space Technologies, does not bury its verdict. It is titled “No cities on the Moon: a billion tons of water is not enough for sustainability.”
They asked about the two things without which no settlement anywhere, on any world, has ever survived: energy and water.
Energy, it turned out, can be had. Solar and nuclear together, the pair found, could carry a population of one million on the lunar surface. Power, the abstract says, “is not a strong constraint.”
Then they came to water.
An aquarium the size of Central Park
There is water on the moon, a fact that still surprises people. Most of it is believed to lie in the permanently shadowed regions at the poles, craters where the sun never reaches. Elvis and McDowell assumed roughly one billion tonnes of it, a figure in line with current estimates. Then they did the sums. A city of one million people, recycling 98 per cent of its water as the International Space Station does, would exhaust that supply in roughly 100 years. With no recycling at all, ABC News reported, the same million would use it up in about 2.4 years, a timeline Elvis called “ludicrously short.”
A billion tonnes. The number sounds like an ocean. Elvis offered a way to see it:
If you had a huge aquarium the size of Central Park in Manhattan, you could fill it up to where the Statue of Liberty was just underwater and that’s the amount of water you’ve got. So it’s huge in one way, but … it’s a small lake. And a small lake is not going to supply a big city for very long.
The paper’s conclusion is blunt. Unless recycling improves by a factor of about 10, or a correspondingly larger source of water is found, large-scale, long-term human habitation of the moon is ruled out.
The generous assumption
The century itself is the optimistic version. Elvis does not trust the billion. “Very likely a billion is way overestimating. It’s probably 20 or 30 times too big,” he told CBC. And whatever is there will not all come out of the ground; he expects one-third to half of it could be recovered.
“So my best guess is the real amount of water available that we can be fairly confident about is one per cent of what we said before. So instead of 100 years, it’s one year.”
One year. That is all that is left of the city that was to be built in under ten.
Even that rests on ice nobody has yet touched. Gordon Osinski, a professor of planetary geology at Western University in London, Ontario, points out that the water has been detected from satellites, and “there are uncertainties in those measurements.” “I think it is there, but let’s go and ground-truth that and confirm it,” he said. “And then there’s the bigger unknowns of what form it’s in…. Is it more like the permafrost up in the Arctic, where it’s basically soil with, you know, sub-millimetre size ice crystals, which are hard to get out of?”
What can be built
None of this forbids people from living on the moon. It sets a scale. A town of 10,000, the authors found, could be sustained for centuries. McDowell put it plainly: “Our work shows that, yeah, if you want to have an Antarctic base on the moon for the next few hundred years, no problem. If you want to have a small town, that’s doable. If you want to cover the face of the moon with cities, like in the best science fiction, then you have to find a way to get the water there, because it’s not there right now.”
Elvis has also argued the point in an op-ed for Space.com. And according to ABC News, the two men want nations to cooperate in managing the moon’s scarce water rather than each moving to lock it up for itself. That plea will sound familiar to anyone who knows how rivers have been divided on Earth. A small lake, a few shadowed craters, and several powers hurrying toward the same poles: history has seen this arrangement many times, and it has seldom ended in sharing.
A reflection, then, and only that. Whoever holds the water has always held power over everyone else, in the desert and in the city alike. The men who promise cities rarely do the plumbing sums; that job falls to others, astronomers with spreadsheets, who are then called pessimists for doing it.
McDowell rejects the label. “The truth is never negative,” he said. “What’s negative is to actually go and try and do something that isn’t going to work.”