What you're talking about is Ion Propulsion.

Ion Propulsion is not propulsion using electricity *only*. There still is fuel.

In normal rockets, we burn tonnes of liquid hydrogen and liquid oxygen to produce a thrust forward. Ion propulsion is slightly different.

In ion propulsion, instead of burning hydrogen and oxygen, we take a kind of gas like Xenon and ionise it using lots of electricity. We only use a tiny bit of gas each time, so the gas lasts very long unlike the Hydrogen+Oxygen combination, but since we use so little, we need to accelerate the gas to very high velocities out the back of the space to generate enough thrust.

The faster you can accelerate the molecules out, the more thrust you get.

There are limitations though. The lifetime of an ion engine is not only limited by the amount of propellant it can carry, but also how long the engine can maintain it's structural integrity. Apparently, the ions will cause erosion in the engine as the engine operates.

Wikipedia has a good explanation on all of this:
http://en.wikipedia.org/wiki/Ion_propulsion

This post has been edited by DeniseLau: Oct 27 2009, 11:55 PM