Illustration showing a spacecraft enabled by nuclear thermal propulsion. Nuclear propulsion systems can provide high thrust and double the propellant efficiency of chemical rockets, making them a potential viable option for crewed missions to Mars. The system works by transferring heat from a fission reactor directly to a gaseous hydrogen propellant. Heated hydrogen expands through a nozzle to provide thrust to propel a spacecraft. NASA in conjunction with industry partners are currently developing nuclear propulsion systems. A test flight of a prototype is set for 2027 to orbit the Earth.