Lunar orbit rendezvous (LOR) is a key concept for landing humans on the Moon and returning them to Earth and was first utilized for the
Project Apollo missions in the 1960s and 1970s. In a LOR mission, a main
spacecraft (such as the
Apollo CSM) and a smaller lunar lander (such as the
Apollo LM) travel together into
lunar orbit. The lunar lander then independently descends to the surface of the Moon, while the main spacecraft remains in lunar orbit. After completion of the mission there, the lander returns to lunar orbit to
rendezvous and re-
dock with the main spacecraft, then is discarded after transfer of crew and payload. Only the main spacecraft returns to Earth.