Milner wants to launch a small “mothership,” filled with hundreds of these thin, disc-like probes. (He thinks each probe can eventually be manufactured at roughly the cost of an iPhone.) Once the mothership reaches orbit, it would release one probe per day. The probe would exit the larger spacecraft, and use its photon thrusters to position itself in the path of a ground-based laser beam.

The laser would be located somewhere in the Southern Hemisphere. “You need to put it high in the mountains,” Milner told me. Too much air or moisture, and the laser will be distorted on its way out of the atmosphere. “An interesting place would be the Atacama desert in Chile,” he said. Apart from the poles, Atacama is the driest place on Earth. Its arid peaks tower more than 16,000 feet, and already, it’s a pilgrimage site for those seeking cosmic communion—Atacama’s more remote perches host some of the world’s most far-seeing observatories.

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Interstellar mission concepts are difficult to vet. Very little research money has been spent on star travel, meaning there isn’t much institutional expertise. But there is an interstellar subculture of sorts, a small community of engineers and scientists who write speculative papers about far-fetched missions in their spare time.

Paul Gilster’s popular Centauri Dreams blog (and its comment section) functions as a kind of salon for people who like to think about star travel. When I asked Gilster about the light-sail mission concept, he said he was surprised that Milner was considering a ground-based laser. Most light-sail research assumed a space-based laser, positioned near Mercury, where huge gobs of solar power are easy to come by. In space, you don’t have to worry about atmospheric turbulence throwing off your laser. And you don’t have to worry about it burning a hole in the ozone layer.

Milner dismissed the idea of a space-based laser. He hinted that the turbulence problem would be solved with adaptive optics, a cutting edge technology that allows observatories to adjust for atmospheric distortion in real time.

“People who talk about lasers in space don’t think about policy issues, and they don’t think about cost,” he said. “Nobody will allow you to build something that you can point in all different directions, as you would be able to in space. This is a very big laser. It can do quite a bit of damage.” On Earth, it could only point in certain directions, and would be much easier for other governments to inspect.

(Later, Milner said, darkly, that one of his big questions is, “whether we are a mature enough civilization to be doing this.” And by “this,” he meant building a starship powered by a superweapon.)

Milner told me that a ground-based laser could run off a giant power plant devoted solely to the mission. It could be a solar array in the Atacama desert, given how much sunlight pours onto its stark landscape. To make it work, the array would have to stretch for tens of miles, and it would need a battery large enough to store fodder for the daily firing of the world’s most powerful laser cannon.

The laser team would need to time its daily blast carefully, to avoid destroying the satellites and planes that pass overhead. When fired, the beam would shoot up through the atmosphere, and slam into the disc-like probe, sending it hurtling toward the edge of the solar system. After only a few minutes, the probe would be traveling at a significant fraction of the speed of light. It would pass Mars in less than an hour. The next day, it would streak by Pluto. (New Horizons took 9 years to achieve this feat.) As the probe headed deeper into the Kuiper belt’s recesses, another one would pop out from the mothership, and float into the laser’s line of sight.