It can be much better than that. The ion thrusters in Dawn have a specific impulse of 3,100 seconds (30 km/s exhaust velocity). Later thrusters designed for the

It can be much better than that. The ion thrusters in Dawn have a specific impulse of 3,100 seconds (30 km/s exhaust velocity). Later thrusters designed for the Jupiter Icy Moons Orbiter achieved 9,600 seconds in a lab (94 km/s e.v.) JIMO itself is designed for a delta-v of 40 km/s. https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf https://www.lpi.usra.edu/meetings/jimo2003/hartman.pdf delta-v is proportional to the specific impulse (or exhaust velocity), and logarithmic in the propellant ratio. For instance, with a propellant ratio of e = 2.7, you can get a delta-v equal to the exhaust velocity. For a delta-v twice that, you need a propellant ratio of e^2 = 7.4. Obviously it's not practical to go much faster the exhaust velocity; the propellant demand grows exponentially. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation