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Going Big: Nuclear Rockets
Sails may be able to whisk tiny probes to the stars, but they can’t handle a human mission; you’d need a microwave beam consuming thousands of times more power than the entire world currently generates. The best-developed scheme for human space travel is nuclear pulse propulsion, which the government-funded Project Orion worked on during the 1950s and ’60s.

When you first hear about it, the scheme sounds unhinged. Load your starship with 300,000 nuclear bombs, detonate one every three seconds, and ride the blast waves. Though extreme, it works on the same basic principle as any other rocket—namely, recoil. Instead of shooting atoms out the back of the rocket, the nuclear-pulse system shoots blobs of plasma, such as fireballs of tungsten.

You pack a plug of tungsten along with a nuclear weapon into a metal capsule, fire the capsule out the back of the ship, and set it off a short distance away. In the vacuum of space, the explosion does less damage than you might expect. Vaporized tungsten hurtles toward the ship, rebounds off a thick metal plate at the ship’s rear, and shoots into space, while the ship recoils, thereby moving forward. Giant shock absorbers lessen the jolt on the crew quarters. Passengers playing 3-D chess, or doing whatever else interstellar passengers do, would feel rhythmic thuds like kids jumping rope in the apartment upstairs.

The ship might reach a tenth the speed of light. If for some reason—solar explosion, alien invasion—we really had to get off the planet fast and we didn’t care about nuking the launch pad, this would be the way to go. We already have everything we need for it. “Today the closest technology we have would be nuclear pulse,” Matloff says. If anything, most people would be happy to load up all our nukes on a ship and be rid of them.

Ideally, the bomb blasts would be replaced with controlled nuclear fusion reactions. That was the approach suggested by Project Daedalus, a ’70s-era effort to design a fully equipped robotic interstellar vessel. The biggest problem was that for every ton of payload, the ship would have to carry 100 tons of fuel. Such a behemoth would be the size of a battleship, with a length of 200 meters and a mass of 50,000 tons.

“It was just a huge, monstrous machine,” says Kelvin Long, an English aerospace engineer and co-founder of Project Icarus, a modern effort to update the design. “But what’s happened since then, of course, is microelectronics, miniaturization of technology, nanotechnology. All these developments have led to a rethinking. Do you really need these massive structures?” He says Project Icarus planned to unveil the new design in London in October 2013.

Interstellar designers have come up with all sorts of ways to shrink the fuel tank. For instance, the ship could use electric or magnetic fields to scoop up hydrogen gas from interstellar space. The hydrogen would then be fed into a fusion reactor. The faster the ship were to go, the faster it would scoop—a virtuous cycle that, if maintained, would propel the ship to nearly the speed of light. Unfortunately, the scooping system would also produce drag forces, slowing the ship, and the headwind of particles would cook the crew with radiation. Also, pure-hydrogen fusion is inefficient. A fusion-powered ship probably couldn’t avoid hauling some fuel from

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