“I really want to see a mass driver on the moon that is shooting AI satellites into deep space,” Elon Musk said last week when he announced his plan to go to the moon. “It’s going to be incredibly exciting to see it happen.” He’s right. I want to see it too, although probably we will both be dead before his vision is realized.

The lunar mass driver—essentially a cannon that uses magnetic power to accelerate an object—is a key component to launch the million satellites Musk wants to put in orbit around the Earth. But Musk wasn’t the first person to come up with the idea. Smarter people than him thought about this in the 1970s as the solution to a key problem for human exploration.

Launching spacecraft from Earth is extremely expensive. Every pound lifted from Cape Canaveral to low Earth orbit costs thousands of dollars in fuel, hardware, and operational complexity. The farther you want to go in space, the more massive and complex the rocket has to be, increasing costs. Chemical rockets must carry their own oxidizer and propellant, which means most of the vehicle’s mass is just fuel to lift more fuel. This tyranny of the rocket equation has strangled space development for seven decades, only slightly eased by the economics of reusable rockets like the Falcon 9.

A mass driver could break that stranglehold by using electricity instead of explosives, turning launches into a utility-scale operation rather than a high-wire act. On the Moon, where gravity is one-sixth of Earth’s and there’s no atmosphere to create drag, this technology could launch payloads at a fraction of the cost—a few dollars per pound in electricity. Compare that to the $1,200 per pound it currently costs to launch a payload on a reusable Falcon 9 rocket.

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[Image: Xai]

An elegant design

American physicist Gerard O’Neill and MIT physicist Henry Kolm built the first prototype of a mass driver in 1976 with a $2,000 budget. The Mass Drive 1 could fire objects at 131 feet per second while experiencing 33 times Earth’s gravity. Their next version achieved 10 times greater acceleration with a comparable funding increase. University of Texas researchers subsequently priced a serious version at $47 million for a device capable of launching a 22-pound payload at 13,400 miles per hour.

A mass driver is basically a very long track stretching across the lunar surface, angled gently skyward at its far end. The track is lined from end to end with hundreds of electromagnetic coils, which are simply loops of wire that snap into powerful magnets the instant electricity runs through them. A payload sits inside a magnetizable carrier called a bucket.

To move the bucket, the coils fire in a precise sequence, one after another, each energizing at exactly the right moment as the bucket reaches it, grabbing it forward, then cutting off the instant it passes. The result is a cascade of invisible magnetic hands, each passing the bucket to the next. The bucket never makes mechanical contact with any surface: It is held aloft and guided entirely by the interplay of magnetic fields, which is why these systems have a theoretical operational lifespan of up to millions of launches with negligible wear.

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