Science1 publisher2 min readPublished
ISS-grade water recycling buys a million-person moon city just over a century
A feasibility estimate in Frontiers in Space Technologies divides the lunar polar ice by what a large population would consume. At the inventory the authors call current, the settlements that last for centuries hold 1,000 to 10,000 people.
The Scientist · Science desk
What happened
- A feasibility study in Frontiers in Space Technologies finds that a lunar city of a million people, recycling water at the 98% efficiency achieved on the International Space Station, runs out in just over a century.
- Without recycling, the same generous starting inventory of a billion tons of polar ice supports that city for only a few years.
- Current best estimates of lunar water are about 30 times smaller than a billion tons, and the authors say that shortens every exhaustion time by the same factor, so a small city runs dry in about a decade.
- At that reduced inventory the study puts a 1,000-person village, or a 10,000-person town, on a supply that lasts several centuries or more.
- Electricity is the easy input: the near-permanently sunlit crater rims could carry kilometre-tall photovoltaic towers generating 3 gigawatts, with no nuclear reactors.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Loss per cycle in the water loop sets how many people a polar base can hold for a century. That puts recycling engineering at the front of any city-scale design.
- contradiction The paper's headline inventory and the inventory it calls current differ by a factor of 30. The same method then answers either a century or a few years, depending on which figure a planner adopts.
- decision A programme that wants city scale has to pay for the inventory measurement first. Settling the population number takes better data than the orbital mapping already in hand.
- capability If panels can be made from lunar silicon, adding gigawatts becomes a manufacturing job on the surface. The authors put data centres near the sunlit peaks on that basis.
A settlement's lifetime in this model is set by what the loop loses. The authors take 98% recovery, the figure achieved on the International Space Station, as the best case, so 2% of the water passing through the loop is gone for good each cycle [4].
Spread 910 million tonnes across a million people for a hundred years and each resident accounts for about 9 tonnes of unrecovered water a year [17]. If all of that loss comes out of the loop, a 2% leak implies roughly 455 tonnes cycling through per person per year, about 1.2 tonnes a day [18].
The estimate is deliberately coarse. "We realized that we could make a first feasibility estimate simply by looking at how much power and water a large population would use on the moon, and how long a city of 100,000 or 1 million people could survive there," the authors wrote in phys.org [11]. The ice enters as a stock being drawn down [20], and the setting supports that treatment. Dozens of polar crater floors have gone about 4 billion years without direct sunlight, many sit below 110 K, and ice in them sublimates less than a millimetre in a billion years [9]. The water came in with the asteroids that made the craters [10].
The inventory is the softer input. A billion tons is the upper figure from orbiter mapping since 2013 [2], while the authors say today's best estimates are about 30 times lower [5], or roughly 33 million tons [15]. Divide the century by 30 and a million-person city at ISS-grade recycling has under four years [16]. The phys.org article does not identify the surveys behind the lower number [14].
Four fixes are on the authors' list: better recycling, lower demand through techniques such as vertical farming, imported water from accessible asteroids, and more water found in place [12]. Only the first can be priced from what is already on the table. Read a fivefold improvement as cutting per-cycle loss from 2% to 0.4%, and every lifetime multiplies by five, so the million-person city on the reduced inventory goes from under four years to roughly 17 [19]. The authors call finding more water the most promising route. It is a drilling problem: current surveying reaches no more than a few metres down, while the rubble-like regolith typically extends tens of metres and may hold ice in the same cold traps [13].
What to watch
- Whether the authors or a follow-up paper tie the 30x-lower inventory figure to named orbiter or lander surveys.
- Any prospecting result from below a few metres of polar regolith that narrows the factor-of-30 spread in the inventory.
- Closed-loop life support demonstrations that push water recovery past the 98% the study uses as its best case.