An artist's conception of proposed craft destined for Neptune, powered by ion propulsion thrusters. Bob Sauls

NASA’s Ion Engine

1. Charge the Fuel

Xenon is an inert gas, seemingly useless for rocketry. Before it´s used as fuel, the engine must convert it into an electrically charged gas, also called a plasma. An electron emitter fires electrons at the xenon gas. When an electron hits a xenon atom, it strips off an additional electron from the atom´s shell to create a positively charged xenon ion.

2. Accelerate the gas

Once the xenon is charged, it moves through two grids held less than a millimeter apart. The first is a positively charged grid that repels the plasma. The second is a negatively charged grid that attracts it. Net effect: The xenon ions shoot downstream and out the back of the engine at tens of thousands of miles an hour.

3. Speed ahead

The force of this high-speed plasma propels the spacecraft forward, like a kid on a skateboard shooting a fire extinguisher. Although at any moment the amount of thrust is small-equivalent to the weight of four quarters-the net effect of this thrust continuously applied for years at a time could accelerate a spacecraft to more than 32,000 miles an hour.