Lunar orbit rendezvous (LOR) is a process for landing humans on the Moon and returning them to Earth. It was utilized for the Apollo program missions in the 1960s and 1970s. In a LOR mission, a main spacecraft and a smaller lunar lander travel to 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.
Lunar orbit rendezvous was first proposed in 1919 by Ukrainian engineer Yuri Kondratyuk, as the most economical way of sending a human on a round-trip journey to the Moon.
O'Neill began researching high-energy particle physics at Princeton in 1954 after he received his doctorate from Cornell University.
Two years later, he published his theory for a particle storage ring. This invention allowed particle physics experiments at much higher energies than had previously been possible. In 1965 at Stanford University, he performed the first colliding beam physics experiment.
While teaching physics at Princeton, O'Neill became interested in the possibility that humans could live in outer space. He researched and proposed a futuristic idea for human settlement in space, the O'Neill cylinder, in "The Colonization of Space", his first paper on the subject. He held a conference on space manufacturing at Princeton in 1975. Many who became post-Apollo-era space activists attended. O'Neill built his first mass driver prototype with professor Henry Kolm in 1976. He considered mass drivers critical for extracting the mineral resources of the Moon and asteroids. His award-winning book The High Frontier: Human Colonies in Space inspired a generation of space exploration advocates. He died of leukemia in 1992.