An illustration depicts a NASA spacecraft approaching the metal-rich asteroid Psyche. Though there are no plans to mine Psyche, such asteroids are being eyed for their valuable resources. Credit: NASA/JPL-Caltech/ASU Each electric thruster on Psyche generates just 250 milli-newtons of thrust, roughly equivalent to the weight of three quarters. But they can operate for months at a time, and over the course of a multi-year cruise, these thrusters provide a more efficient means of propulsion than c...