Ion propulsion systems generate thrust by ionizing a propellant (typically xenon or krypton) and accelerating the ions through electric fields to extremely high exhaust velocities. Gridded ion engines and Hall-effect thrusters are the two most common variants. While their thrust is measured in millinewtons, their specific impulse of 1,500-5,000 seconds allows spacecraft to achieve large delta-v changes over time with minimal propellant mass.
Ion propulsion has been proven on missions from NASA's Dawn spacecraft to commercial GEO satellites that use electric orbit-raising. SpaceX's Starlink satellites each carry a krypton-fueled Hall-effect thruster for orbit maintenance and deorbit maneuvers. The technology is also central to proposed missions for asteroid redirect, deep-space cargo delivery, and maintaining large satellite constellations.