Image right: Artist's concept of Deep Space 1 probe with its ion thruster operating at full power. These thrusters must be used in a vacuum to operate at the available power levels, and they cannot be used to put spacecraft in space because large amounts of thrust are needed to escape Earth's gravity and atmosphere. Current ion thrusters can provide only 0.5 newtons (or 0.1 pounds) of thrust, which is equivalent to the force you would feel by holding 10 U.S. The trade-off for the high top speeds of ion thrusters is low thrust (or low acceleration). In comparison, the Space Shuttles can reach speeds around 18,000 mph. Spacecraft powered by these thrusters can reach speeds up to 90,000 meters per second (over 200,000 mph). Several thrusters can be used on a spacecraft, but they are often used just one at a time.

Currently, ion thrusters are used to keep communication satellites in the proper position relative to Earth and for the main propulsion on deep space probes. Chemical rockets have demonstrated fuel efficiencies up to 35 percent, but ion thrusters have demonstrated fuel efficiencies over 90 percent. The ion propulsion system's efficient use of fuel and electrical power enable modern spacecraft to travel farther, faster and cheaper than any other propulsion technology currently available. Ion thrusters, the propulsion of choice for science fiction writers have become the propulsion of choice for scientists and engineers at NASA.

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