Lunar Orbit Rendezvous
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.
LOR is first known to have been proposed in 1916 by Ukrainian rocket theoretician Yuri Kondratyuk, as the most economical way of landing humans on the Moon. When NASA began actual work in 1961 on President John F. Kennedy’s goal to achieve the first such landing by the end of the 1960s, LOR was proposed by Tom Dolan and championed by John C. Houbolt, but considered controversial, impractical, and possibly dangerous, because space rendezvous had never been done. However, Houbolt’s persistence paid off, convincing NASA’s management, and Administrator James E. Webb publicly announced in July 1962 that Apollo would utilize this method. Even then, Kennedy’s Science Advisor Jerome Wiesner remained opposed to the method, and publicly criticized Webb. As history has shown, the method worked, and allowed NASA to use only one Saturn V per lunar landing mission, something other landing options did not offer.