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Ion propulsion, also known as solar electric propulsion, has been under development since the 1950’s. Harold Kaufman, an engineer at the N.A.S.A. Glenn facility, built the first ion engine in 1959 and in 1960, NASA Glenn undertook a space flight test program called, Space Electric Rocket Test (SERT). In 1964, the N.A.S.A. Glenn facility launched two ion engines on a Scout rocket, one of the two thrusters on board did not work, but the other one operated for nearly thirty minutes. A follow up mission called SERT 2 had two ion thrusters, one operating for three months, and the other one operating for nearly five months. The early ion engines used mercury or cesium as fuel. In SERT 1, one ion engine used mercury and the other used cesium. In SERT 2, both ion engines used mercury. Current ion engines use xenon as fuel, but the overall design of the engines has not changed since the 1950’s. In the 1960’s, the Hughes Research Laboratories continued the ion projects and designed a xenon-fueled ion engine launched in 1979 on the Air Force Geophysics Laboratory's Spacecraft Charging at High Altitude (SCATHA) satellite. In addition, Hughes launched the first commercial ion engine aboard a Ru

http://nmp.jpl.nasa.gov/ds1/tech/ionpropfaq.html

The ion engine’s impressive track record may not only make it the propulsion system of choice for future missions but also for many other potential solar electric propulsion missions. In the future, NASA may have a fleet of robotic spacecraft that have the ability to cruise among the planets like sailboats in space, or perhaps they will be propelled from planet to planet by advanced ion engines. Whatever the future, exploration is the essence of the human spirit.

One the engine is running, a cathode tube, opposite the thrust area, discharge electrons into the main chamber and bombards the xenon atoms. When an electron strikes an atom, it strips away an electron from the atom, thus creating an ion or a positively charged atom. An anode collects the remaining electrons, which is located near the cathode emitter. This alone does not create the thrust. The ions need some “motivation.”

Past the magnetic rings in the main chamber is a pair of metal grids. The grids get charged with about 1280 volts of electricity to attract the ions. The engine charges the first grid positive and the second negative. The result is the creation of an electrostatic force that pulls the xenon ions through the grid and out the back of the engine. The ions exit the thruster at approximately 60,000 mile per hour! This creates the thrust out of the rear of the engine. The culmination of many ions exiting at once gives the spacecraft its push forward, thus making ion engine space travel possible.

This engine’s output is approximately 1/50th of a pound of thrust with a consumption of about 2500 watts of electricity (?). This amount of thrust is equivalent to the downward force of a sheet of paper on a person’s hand. As a result, these engines were not designed for escape velocity. This requires the use of chemical thrusters, like liquid-fuel rockets, to help them exit the atmosphere. However, once in outer space, the craft can accelerate continuously as long as the engine is running. This continuous thrust allows for a shorter travel time in comparison to chemical propelled engines that give a large boost in the beginning and then rely on the ship to coast through space for the rest of the time.

www.bbc.co.uk/science/space/exploration/futurespaceflight/ionengines.shtml

http://www.spaceref.com/news/viewpr.html?pid=9186