Comment by ravi-delia

Propellant is still used for rotation control. Reaction wheels can "saturate" if they compensate for rotation more in one direction than the other on net, so propellant is needed to get them back down. Ion engines, generally speaking, do not use background dust. They still carry propellant, they just eject it electromagnetically. An photon engine, basically just a laser pointed backwards, uses pure electricity to produce thrust. But of course the numbers all work out, since photons have momentum. They're extremely weak though, even lasers of staggering power produce very little force. There's no way you could put one on a satellite