Re: Dawn will be orbiting Ceres for a very long time
The ion engine is for getting from place to place.
Don't forget it uses hydrazine as well for its reaction control system - this gives it the thrust it needs to rotate itself (e.g. between imaging Ceres and repositioning itself to send those images home). The job of maintaining or altering its orientation was to be partly down to 3 of its 4 reaction wheels, but two have stopped working, so it'll be using a bit more hydrazine than planned for orientation control now to compensate for only have 2 working wheels - there should be enough in reserve though.