There’s been this hand-wave, this assumption, this yada yada at the core of our long-term space programs. We’ll mine that, combine it with the carbon dioxide in the atmosphere, and yada yada, we’ll use that to fly the astronauts back. But here’s the thing: No one has ever successfully turned water into rocket fuel, not for a spaceship of any significant size. A startup called General Galactic, led by a pair of twentysomething engineers, is aiming to be the first. That’s called electric propulsion—again, I’m way oversimplifying.