 Low-thrust propulsion is essential for all space missions, and NASA's Lewis Research Center is conducting programs to provide a broad range of low-thrust propulsion concepts for both auxiliary and primary functions. Auxiliary propulsion is used for keeping space systems in desired locations or for orientation. Typical examples are the reaction control system for Earth-to-orbit vehicles, drag makeup and attitude control for low-Earth orbit systems such as space station freedom, station keeping for higher orbit systems such as geosynchronous satellites, and finally, retro propulsion functions near planetary bodies. Auxiliary propulsion functions include the moving of space vehicles from point to point in Earth space as well as propulsion between Earth space and various planetary bodies. To understand these various propulsion applications, specific auxiliary and primary missions will be discussed. On Earth-to-orbit vehicles such as the Shuttle Orbiter, auxiliary low-thrust propulsion systems are used to control vehicle orientation or to perform small orbit changes. Examples are orientation of the Shuttle Orbiter to face the sun and rendezvous with low-Earth orbit systems such as the long-duration exposure facility, LEDF. The low-thrust devices for these applications are called reaction control systems, which generally operate at thrust levels from 25 to a few hundred pounds. Space station freedom requires low-thrust propulsion for both orbit and attitude control. Orbit control includes atmosphere drag makeup and collision avoidance. Attitude control includes damping of disturbances such as Shuttle docking and momentum management. Small electric rockets called resisto jets with thrust levels less than a pound and 25 to 100 pound chemical rockets are being considered for orbit and attitude control for space station freedom. Communication satellites are usually first placed in a geosynchronous transfer orbit or GTO. A 100 to 200 pound thrust apogee propulsion system is used to change the GTO to a circular geosynchronous orbit at an altitude of about 22,300 miles. In the geosynchronous orbit, small station keeping rockets are used to overcome gravitational forces from the sun, moon and Earth to maintain the satellite in the desired position. A final example of auxiliary propulsion is the use of 100 to 500 pound retro rockets for orbit change or capture of satellites near planetary bodies. Low-thrust systems are also useful for primary propulsion applications for Earth orbit and planetary missions. Examples in Earth space include the transport of communication satellites from low Earth orbit into geosynchronous orbit and the placement of weather satellites in polar orbits for Earth observations. Planetary missions include transferring of systems beyond Earth space to planets, comets and asteroids. Low-thrust electric propulsion is particularly valuable for very energetic planetary applications such as cargo vehicles for major moon Mars missions. The Lewis Research Center is developing low-thrust chemical and electric propulsion systems. Chemical propulsion includes rockets which use hydrogen, oxygen and storable propellants. Although chemical rockets use various propellants, all involve heating the propellant and its subsequent expansion through a nozzle to produce thrust. Chemical rockets using gaseous hydrogen oxygen have been developed for possible use on the space station. Designs ranging from 25 to 50 pounds thrust have been built and life-tested. Studies indicate that future launch vehicles would benefit from liquid hydrogen-oxygen reaction control systems and optimal approaches are being defined. A breakthrough in storable chemical propulsion technology has been verified with a 5-pound rocket. 100 to 200-pound storable rockets are now under development, which will provide major increases in the life and performance of apogee, retro and orbit change propulsion systems. Specific electric rockets have very different operating principles, but all electric propulsion systems share many features. Energy is derived from a solar or nuclear power source and is converted into electricity and then conditioned for use by the electric rockets which use the power in various ways to accelerate the propellant to produce thrust. Resisto jets, the simplest electric rockets, add energy to a propellant via heat transfer from an electrically heated resistor. A version which uses waste gas from the station modules as the propellant is being developed for drag makeup on space station freedom. In arc jets, the propellant is heated by an electric arc and is then expelled through a nozzle. Arc jets which use about 1 kilowatt of power and hydrazine propellant are under intense development for station keeping on commercial geosynchronous satellites. Electrostatic or ion thrusters and magnetoplasma dynamics or MPD rockets are being developed for Earth orbit and planetary primary propulsion functions. Ion rockets first emit electrons from a cathode to create positively charged ions in a discharge chamber and then electrostatically accelerate those ions through two perforated plates called ion optics. This is a typical discharge chamber along with the ion optics which contain over 20,000 holes to accelerate the ions. This scene shows an ion thruster operating at approximately 10 kilowatts. MPD rockets produce thrust by using an electromagnetic field to accelerate a plasma. Direct measurements of ion and MPD performance and exhaust plumes are necessary. These measurements require large thrust stands and state of the art plume diagnostics. Ion and MPD rockets both require large space simulation facilities with high gas pumping speeds in order to obtain space like vacuums during testing. In summary, advanced low thrust propulsion provides benefits for many applications. These range from reaction control systems for Earth orbit vehicles such as the space shuttle and drag makeup and attitude control devices for large platforms in low Earth orbit such as space station freedom. It also includes significantly reducing the propellant for apogee propulsion and station keeping propulsion for geosynchronous satellites. And then advanced electric propulsion systems enable an order of magnitude reduction in the propellant required for primary propulsion in both Earth orbital and planetary missions. The Lewis Research Center is conducting a low thrust propulsion program utilizing an in-house, university, and industrial team. This blend of skills assures that the program will develop practical devices for near-term applications and also produce more advanced concepts for the longer-term, higher payoff national space missions.