An ion thruster is a kind of spacecraft engine. It works by using static electricity or magnetism to push ions (electrically charged atoms) out the back of the spacecraft. It takes a very long time to speed up, but its fuel lasts much longer than a rocket engine. It has to be carried into outer space by a regular rocket, because the ions don't push hard enough for it to escape the Earth's gravity, but after it gets to outer space it can travel a very long way. An ion thruster needs electricity to run, so the spacecraft has to also carry solar panels or a nuclear reactor.