Really big space rocks (a mile or wider across) crash into Earth about once every 500,000 years. That’s rare, but we might not spot the next one until it’s too late for existing technology to stop it—especially if it’s a comet. Because comets can travel twice as fast as an asteroid, we’d need something 20 times more powerful than anything in our arsenal to fend it off. What’s humanity to do? One scientist has an answer: fusion rockets.

Glen Wurden, a plasma physicist at Los Alamos National Laboratory by day and an amateur astronomer by night, has conceived a comet-buster that would work like this: Harnessing the tremendous energy of fusion, the process in which two atomic nuclei collide to form a new nucleus, could propel a rocket to more than 100 kilometers per second. That’s 100 times faster than the fastest cruise missile. The science of controlling the reaction on board—in a plasma surrounded by a magnetic ring—has already been proved possible in labs.

fusion rocket flies around Mars
A concept drawing of a fusion-powered spacecraft flying around Mars. University of Washington, MSNW

Once the rocket approaches the comet, the idea isn’t to score a direct hit, but rather to set off a nuclear bomb close enough to boil away surface material, changing the comet’s mass and trajectory. Wurden thinks it’d be worth spending up to $40 billion over 40 years to build it. Sure beats becoming a fossil record.

This article was originally published in the July/August 2016 issue of Popular Science. Check out other insane ideas that just might save the world.