Earth orbit rendezvous
Earth orbit rendezvous is a method for conducting round trip human flights to the Moon, involving the use of space rendezvous to assemble, and possibly fuel, components of a translunar vehicle in low Earth orbit. It was considered as an alternative to direct ascent but ultimately rejected in favor of lunar orbit rendezvous for NASA's Apollo program of the 1960s and 1970s, mainly because LOR does not require a spacecraft big enough to both make the return trip from Earth orbit to splash down in the ocean and a soft landing on the lunar surface. The two main proposed methodologies were: the in-space assembly of fueled spacecraft modules via docking techniques; and the in-space refueling of fully assembled spacecraft. This was the preferred approach adopted by the Soviet Union for achieving human lunar missions.
Three decades later, it was planned to be used for Project Constellation, until that program's Barack Obama [space policy speech at Kennedy Space Center|cancellation in October 2010].
Gemini and Agena target vehicle
The Agena target vehicle was used for testing Earth orbit rendezvous in the NASA Gemini Program. Gemini 6 and Gemini 7 rendezvoused in orbit in 1965, but without Agena. Next, Gemini 8 successfully docked with the Agena on March 16, 1966. The Agena-Gemini rendezvous also achieved other objectives in later Gemini launches, including docked orbital maneuvering, inspection of the abandoned Gemini 8 ATV and space walks.Apollo
The EOR proposal for Apollo consisted of using a series of small rockets half the size of a Saturn V to put different components of a spacecraft to go to the Moon in orbit around the Earth, then assemble them in orbit. Experiments of Project Gemini involving docking with the Agena target vehicle were designed partly to test the feasibility of this program.In the end, NASA employed the Lunar Orbit Rendezvous for the Apollo Program: a Saturn V would simultaneously lift both the Apollo Command and Lunar Modules into low Earth orbit, and then the Saturn V third stage would fire again to send both spacecraft to the Moon.