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

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