[mabbo]

The difference is 'momentum gained per gram of propellant'.

Traditional engines burn fuel, and the expanding, hot fuel pushes itself out the back of the engine at high speed, pushing the rest of the spacecraft forward. The fuel containing the energy is also the propellant.

Ion engines are the opposite in almost every way. You spend a lot of electrical energy (solar, nuclear, whatever) speeding up a tiny amount of propellant (which is usually something non-reactive like argon). You shoot a little bit of fuel at insanely high speeds out of the engine, and it pushes the spacecraft forward a little bit- so you do it for a very long time.

Yes, ion engines still need propellant, but they need a whole lot less of it.

[mreithub]

Thanks to you and the others for clarifying. For some reason it didn't occur to me they were talking about an ion engine. I've heard of them and their slow-but-steady acceleration characteristics before but never actually knew how they work (and for some reason didn't quite make the connection).