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Source: http://www.wired.com/science/space/news/2007/10/ion_propulsion#

The key: Ion propulsion only needs a tenth of the fuel of a chemical rocket system to reach the same destination, and that means a smaller rocket — and a lot less fuel — is needed to launch missions, such as the Dawn probe. The miserly use of propellant will allow the Dawn probe to become the first to orbit two separate objects — in this case, the asteroid Vesta in 2011 and the dwarf planet Ceres, where it’s expected to arrive in 2015.

Ion propulsion uses positively charged atoms, or ions, to propel a spacecraft. An electron gun is used to knock electrons from a reservoir of xenon atoms, turning them into ions. Then, two charged plates accelerate the ions and eject them from the back of the rocket engine at speeds of 35 kilometers per second, or about 77,000 mph. To avoid accumulating a negative charge, the probe shoots electrons back into the stream of xenon ions leaving the engine.

Pushing a probe farther into space on a light stream of atoms is a trade-off. While it is 10 times more efficient than chemical propulsion, and thus only requires a tenth of the fuel needed by a chemical thruster, it also lacks power. Marc Rayman, chief engineer on the Dawn mission, has dubbed ion propulsion “acceleration with patience.”