In one possible future, great maglev lines cross the lunar surface. But these rails don’t carry trains. Instead, like space catapults, these machines accelerate cargo to supersonic speeds and fling it into the sky. The massive catapults have one task: throwing mounds of moondust off-world. Their mission is to halt climate change on Earth, 250,000 miles away.

All that dust will stream into deep space, where it will pass between Earth and the sun—and blot out some of the sun’s rays, cooling off the planet. As far-fetched as the idea is, it’s an idea that received real scientific attention. In a paper published in the journal PLOS Climate on February 8, researchers simulated just how it might go if we tried to pull it off. According to their computer modeling, a cascade of well-placed moondust could shave off a few percent of the sun’s light. 

It’s a spectacular idea, but it isn’t new. Filtering the sunlight that reaches Earth in the hope of cooling off the planet, blunting the blades making the thousand cuts of global warming, is an entire field called solar geoengineering. Designers have proposed similar spaceborne concepts: swarms of mirrors or giant shades, up to thousands of miles across, strategically placed to act as a parasol for our planet. Other researchers have suggested dust, which is appealing because, as a raw material, there’s no effort or expense to engineer it.

“We had read some accounts of previous attempts,” inspiring them to revisit the technique, says Scott Kenyon, an astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and one of the study’s authors.

Kenyon and his colleagues don’t usually dream up ways to chill planets. They study a vastly different type of dust: the kind that coalesces around distant, newly forming stars. In the process, the astrophysicists realized that the dust had a shading effect, cooling whatever lay in its shadow. 

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“So we began to experiment with collections of dust that would shield Earth from sunlight,” says Kenyon. They turned methods that let them simulate distant dust disks to another problem, much closer to home.

Most solar engineering efforts focus on altering Earth’s atmosphere. We could, for instance, spray aerosols into the stratosphere to copy the cooling effects from volcanic eruptions. Altering the air is, predictably, a risky business; putting volcanic matter in the sky could have unwanted side effects such as eroding the ozone layer or seeding acid rain.

“If you could just reduce the amount of incoming sunlight reaching the Earth, that would be a cleaner intervention than adding material to the stratosphere,” says Peter Irvine, a solar geoengineer at University College London, who was not an author of the paper.

Even if you found a way that would leave the skies ship-shape, however, the field is contentious. By its very nature, a solar geoengineering project will impact the entire planet, no matter who controls it. Many observers also believe that promises of a future panacea remove the pressure to curb carbon emissions in the present. 

It’s for such reasons that some climate scientists oppose solar geoengineering at all. In 2021, researchers scrubbed the trial of a solar geoengineering balloon over Sweden after activists and representatives of the Sámi people protested the flight, even though the equipment test wouldn’t have conducted any atmospheric experiments.

But perhaps there’s a future where those obstacles have been cast aside. Perhaps the world hasn’t pushed down emissions quickly enough to avoid a worsening catastrophe; perhaps the world has then come together and decided that such a gigaproject is necessary. In that future, we’d need a lot of dust—about 10 billion kilograms, every year, close to 700 times the amount of mass that humans have ever launched into space, as of this writing. 

That makes the moon attractive: With lower gravity, would-be space launchers require less energy to throw mass off the moon than off Earth. Hypothetical machines like mass drivers—those electromagnetic catapults—could do the job without rocket launches. According to the authors, a few square miles of solar panels would provide all the energy they need.

That moondust isn’t coming back to Earth, nor is it settling into lunar orbit. Instead, it’s streaming toward a Lagrange point, a place in space where two objects’ respective gravitational forces cancel each other out. In particular, this moondust is headed for the sun and Earth’s L1, located in the direction of the sun, about 900,000 miles away from us.

There, all that dust would be in a prime position to absorb sunlight on a path to Earth. The 10 billion kilograms would drop light levels by around 1.8 percent annually, the study estimates—not as dramatic as an eclipse, but equivalent to losing about 6 days’ worth of sunlight per year.

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Although L1’s gravitational balance would capture the dust, enough for it to remain for a few days, it would then drift away. We’d need to keep refilling the dust, as if it were a celestial water supply—part of why we’d need so much of it.

That dust wouldn’t come back to haunt Earth. But L1 hosts satellites like NASA’s SOHO and Wind, which observe the sun or the solar wind of particles streaming away from it. “The engineers placing dust at L1 would have to avoid any satellites to prevent damage,” says Kenyon.

Of course, this is one hypothetical, very distant future. Nobody can launch anything from the moon, let alone millions of tons of moondust, without building the infrastructure first. While market analysts are already tabulating the value of the lunar economy in two decades’ time, building enough mass drivers to perform impressive feats of lunar engineering probably isn’t in the cards.

“If we had a moonbase and were doing all sorts of cool things in space, then we could do this as well—but that’s something for the 22nd century,” says Irvine. Meanwhile, a far more immediate way to blunt climate change is to decarbonize the energy grid and cull fossil fuels, with haste. “Climate change,” Irvine says, “is a 21st century problem.”