The scenario is now just a gleam in an engineer’s eye: An ambitious mission to the outermost reaches of the solar system is ready to leave Earth orbit. After a flawless launch, a final rocket motor ignites. When it falls away spent after a few minutes, ground controllers check the heading of the craft, and with a punch of a button, activate a nuclear reactor the size of a small trash can. The reactor represents NASA’s technological declaration of independence from gravity as a tool for propelling interplanetary spacecraft. Whereas today, a trip to the outer solar system relies on five to 10 minutes of burning chemicals and months or years of coasting, nuclear propulsion holds the promise of faster, more direct, more experiment-packed missions to places where sunlight is too feeble to power spacecraft. Indeed, some say that manned missions to Mars and beyond are unthinkable without nuclear propulsion.