into kinetic exhaust power. Chemical rockets operate as heat engines, and Carnot's theorem limits the exhaust velocity. Electrostatic thrusters Gridded electrostatic ion thrusters Gridded electrostatic ion thrusters development started in the 1960s and, since then, it has been used for commercial satellite propulsion and scientific missions. Their main feature is that the propellant ionization process is physically separated from the ion acceleration process. The ionization process takes place in the discharge chamber, where by bombarding the propellant with energetic electrons, as the energy transferred ejects valence electrons from the propellant gas's atoms. These electrons can be provided by a hot cathode filament and accelerated through the potential difference towards an anode. Alternatively, the electrons can be accelerated by an oscillating induced electric field created by an alternating electromagnet, which results in a self-sustaining discharge without a cathode (radio frequency ion thruster). The positively charged ions are extracted by a system consisting of 2 or 3 multi-aperture grids. After entering the grid system near the plasma sheath, the ions are accelerated by the potential difference between the first grid and second grid (called the screen grid and the accelerator grid, respectively) to the final ion energy of (typically) 1–2 keV, which generates thrust. Ion thrusters emit a beam of positively charged ions. To keep the spacecraft from accumulating a charge, another cathode is placed near the engine to emit electrons into the ion beam, leaving the propellant electrically neutral. This prevents the beam of ions from being attracted (and returning) to the spacecraft, which would cancel the thrust. Gridded electrostatic ion thruster research (past/present): NASA Solar Technology Application Readiness (NSTAR), 2.3 kW, used on two successful missions NASA's Evolutionary Xenon Thruster (NEXT), 6.9 kW, flight qualification hardware built Nuclear Electric Xenon Ion System (NEXIS) High Power Electric Propulsion (HiPEP), 25 kW, test example built and run briefly on the ground EADS Radio-frequency Ion Thruster (RIT) Dual-Stage 4-Grid (DS4G) Hall-effect thrusters Hall-effect thrusters accelerate ions by means of an electric potential between a cylindrical anode and a negatively charged plasma that forms the cathode. The bulk of the propellant (typically xenon) is introduced near the anode, where it ionizes and flows toward the cathode; ions accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity. The anode is at one end of a cylindrical tube. In the center is a