A resistojet is the simplest form of electric propulsion. It works by super-heating a propellant fluid, such as water or nitrous oxide, over an electrically-heated element and allowing the resulting hot gas to escape through a converging-diverging nozzle. Thrust and specific impulse (a measure of the engine's efficiency) are limited by the material properties of the resistor. Resistojet thrusters, using a variety propellants, are being tested for both low-Earth orbit and deep space missions. For example, the microsatellite UoSAT-12 carries 2.5 kilograms of nitrous oxide, sufficient for 14 hours running. A 60-minute firing of its resistojet would raise its 650-kilometer orbit by 3 kilometers.