Illustration of a theoretical spacecraft powered by solar thermal propulsion (STP). STP uses the Sun's energy to heat a fluid, such as hydrogen, to very high temperatures (up to 2,800 Kelvin), causing it to expand and become pressurised. The pressurised fluid is fed through a nozzle, generating thrust for the spacecraft. STP is more efficient than conventional chemical propulsion systems and negates the need to carry an electrical generator unlike other solar-powered systems.