Every action creates an equal and opposite reaction. It’s perhaps the best known law of physics, and Guido Fetta thinks he’s found a way around it.

According to classical physics, in order for something—like a spaceship—to move, conservation of momentum requires that it has to exert a force on something else. A person in roller skates, for example, pushes off against a wall; a rocket accelerates upward by propelling high-velocity combusted fuel downward. In practice, this means that space vessels like satellites and space stations have to carry up to half their weight in propellant just to stay in orbit. That bulks up their cost and reduces their useful lifetime.