Apollo spacecraft. Cutaway illustration of the Command/Service Module (CSM) spacecraft, used during the Apollo programme which landed astronauts on the Moon. The Command Module (conical section, top) housed a crew of three astronauts. The cylindrical Service Module contained scientific instruments, fuel tanks, and propulsion systems. It also housed oxygen and hydrogen tanks, seen here, connected to the fuel cells (upper compartment) which provided electrical power. Once at the Moon, two astronauts used the Lunar Module (not shown) to descend to the Moon's surface and return to the CSM. The Command Module was then used to re-enter the Earth's atmosphere, with the other sections being jettisoned.