NASA Glenn has been developing the next generation of ion thrusters for future missions. NASA’s Evolutionary Xenon Thruster (NEXT) Project has developed a 7-kilowatt ion thruster that can provide the capabilities needed in the future. 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 are currently used for stationkeeping on communication satellites and for main propulsion on deep space probes. Ion thrusters expel ions to create thrust and can provide higher spacecraft top speeds than any other rocket currently available.
NASA explains how an ion thruster works:
“In an ion thruster, ions are accelerated by electrostatic forces. The electric fields used for acceleration are generated by electrodes positioned at the downstream end of the thruster. Each set of electrodes, called ion optics or grids, contains thousands of coaxial apertures. Each set of apertures acts as a lens that electrically focuses ions through the optics. NASA’s ion thrusters use a two-electrode system, where the upstream electrode (called the screen grid) is charged highly positive, and the downstream electrode (called theaccelerator grid) is charged highly negative. Since the ions are generated in a region of high positive and the accelerator grid’s potential is negative, the ions are attracted toward the accelerator grid and are focused out of the discharge chamber through the apertures, creating thousands of ion jets. The stream of all the ion jets together is called the ion beam. The thrust force is the force that exists between the upstream ions and the accelerator grid. The exhaust velocity of the ions in the beam is based on the voltage applied to the optics. While a chemical rocket’s top speed is limited by the thermal capability of the rocket nozzle, the ion thruster’s top speed is limited by the voltage that is applied to the ion optics (which is theoretically unlimited). Because the ion thruster expels a large amount of positive ions, an equal amount of negative charge must be expelled to keep the total charge of the exhaust beam neutral. A second hollow cathode called the neutralizer is located on the downstream perimeter of the thruster and expels the needed electrons.”
An ion thruster produces small levels of thrust relative to chemical thrusters, but does so at higher specific impulse (or higher exhaust velocities), which means that an ion thruster has a fuel efficiency of 10-12 times greater than a chemical thruster. The higher the rocket’s specific impulse (fuel efficiency), the farther the spacecraft can go with a given amount of fuel. Given that an ion thruster produces small levels of thrust relative to chemical thrusters, it needs to operate in excess of 10,000 hours to slowly accelerate the spacecraft to speeds necessary to reach the asteroid belt or beyond.
As of last month, the NEXT ion thruster has been operated for over 43,000 hours (five years), which for rocket scientists means that the thruster has processed over 770 kilograms of xenon propellant and can provide 30 million-newton-seconds of total impulse to the spacecraft. This is an important development, as ion thrusters are pegged as one of the best ways to power long-term deep-space missions to other planets and solar systems. This demonstrated performance permits future science spacecraft to travel to varied destinations, such as extended tours of multi-asteroids, comets, and outer planets and their moons.