Lunar water is not that exciting

Part of my series on countering misconceptions in space journalism.

Water, the staff of life. What a shame, then, that the Earth’s Moon always seemed to be so dry! So dry, in fact, that in most places if there was concrete available it would be a better source of water than average moon dirt.

Not everywhere, though. For decades now, permanently shadowed craters near the pole have been thought to trap volatiles, including water, in their intense cold. More recently a series of missions have confirmed the presence of some water. Scarcely a month goes by without some breathless headline exclaiming NASA’s (re)discovery of moon water, complete with some Kerbal speculative art.

According to some enthusiasts, moon water changes everything. Yes, the moon is still a barren remote frozen boiling irradiated cratered hellscape, but since there’s some water near the poles, we can build bases with fountains and swimming pools, and make money by selling the water!

I’m actually good with all of this, except the last part. The problem is that there’s not that much water and it’s not that valuable.

First, the quantity. No-one actually knows how much water is there, but if we assume that the bottom of the relevant craters could be lined with 100m of frozen comet rubble, with a water concentration of 10%, then there could be a billion tons, or one gigaton, at each pole.

When I think of water I think of a nice lake full of fish. Instead, we have some cryogenically cold frozen mud that’s full of shards of metals, including heavy metals, and other crud. Still, it’s a lot better than the rest of the moon, which has water abundance of around 200ppm, and is drier than fresh dessicant.

One gigaton of water sounds like a lot, but it’s only a cubic kilometer. For comparison, there are many reservoirs on Earth that hold more water. Converted into hydrogen and oxygen it could fuel all of Earth’s combustion-powered vehicles for a couple of months. And then it’s gone. This fossil water took billions of years to accumulate, and it’s a non-renewable resource.

The second problem is that it’s not that valuable. Lunar water mining advocates envision a mine that produces rocket fuel and transports it to other places where it’s needed more, such as LEO. Here a tug could be fueled to transport geostationary satellites to their final orbits.

It’s worth noting here that water isn’t rocket fuel, it’s rocket exhaust. Turning water back into rocket fuel requires putting all the energy that fabulously energetic fuel generated back into the water, plus some extra for inefficiency’s sake. At 10 kWh/kg of hydrogen and oxygen, generating enough fuel to transport 100 T/month back to Earth would require an average of 30 MWh per day, or about 1.2 MW continuous power. For comparison, the ISS solar arrays generate about 100 kW when in full sun.

Unfortunately for this business model, the assumption that GEO launch cost would remain above $35k/kg has proven false. Indeed, continuous launch innovation on Earth has driven launch cost down by an order of magnitude, crushing the lunar water export model. For a more thorough analysis, see my blog on the topic. Fundamentally, the value added per kg of water or hydrolox fuel is much too low to be a good business, even on Earth!

Despite the hype, the fundamental economics are well understood by the mining industry. We don’t have to worry about a resource rush suddenly occurring, complete with moustache twirling monocle-clad capitalists and grieving moon environmentalists.

That aside, major conceptual work on lunar mining continues to this day. A source of water (and other light elements) is important for a lunar base, and lunar water fuel is great for flying hoppers around on the moon. Reducing mass requirements and shipping costs for a lunar base is awesome, but it won’t pay for the base itself. For that, we need something that has a very high demand and value density on Earth that can only be obtained on the Moon.

So when you see articles about lunar water, remember that it’s a very limited resource in a very particular place, and there’s no market demand for it.

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