Space is expensive. To generate more power and more thrust, rockets need to carry more fuel, increasing the amount of weight the same rocket engine needs to lift. It’s a vexing equation, which might explain why much of recent spaceflight is about putting objects into orbit, where they can earn their keep by relaying communications on the ground below. Maybe what interplanetary travel needs is just a better engine to take people there. Rosatom State Atomic Energy Corporation, Russia’s nuclear agency, thinks nuclear engines are just the ticket to get to Mars.