Other inert gases, such as krypton and argon, also can be used. The majority of thrusters use xenon, which is chemically inert, colorless, odorless, and tasteless. Modern ion thrusters use inert gases for propellant, so there is no risk of the explosions associated with chemical propulsion. Image left: Xenon ion discharge from the NSTAR ion thruster of Deep Space 1. Large amounts of the gas push out at relatively low speeds to propel the spacecraft. Conventional chemical rockets burn a fuel with an oxidizer to make a gas propellant. This is like air escaping from the end of a balloon and propelling it forward. Sir Isaac Newton's third Law states that every action has an equal and opposite reaction. Since much less fuel must be carried into space, smaller, lower-cost launch vehicles can be used. Deep Space 1 used less than 159 pounds of fuel in over 16,000 hours of thrusting. Acceleration continues throughout the flight, however, so tiny, constant amounts of thrust over a long time add up to much shorter travel times and much less fuel used if the destination is far away. To compensate for low thrust, an ion thruster must be operated for a long time for the spacecraft to reach its top speed. 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.

Mit mini helicon plasma thruster