NEXT is a solar electric propulsion system where electricity from the spacecraft’s solar panels is used to power a a 7-kW class ion thruster. In this, particles of xenon gas are electrically charged and then accelerated to speeds up to 90,000 mph (145,000 km/h). Such thrusters have already been used on spacecraft, such as NASA’s Dawn probe, and engineers are very interested in them because of their much higher performance compared to conventional chemical rocket engines.

via NASA’s NEXT ion thruster runs five and a half years nonstop to set new record.