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Earth orbit rendezvous (EOR) 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.[1] It was considered as an alternative to direct ascent but ultimately rejected in favor of lunar orbit rendezvous (LOR) for NASA's Apollo program of the 1960s and 1970s,[2] 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.[3] 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.[4] American in 1965 believed that this was the preferred approach adopted by the Soviet Union for achieving human lunar missions.[5]
Three decades later, it was planned to be used for Project Constellation, until that program's cancellation in October 2010.[6]
The Agena target vehicle (ATV) 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 (Gemini 10 and Gemini 11), inspection of the abandoned Gemini 8 ATV (Gemini 10) and space walks (Gemini 12).[7]

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 (Trans-lunar injection) to send both spacecraft to the Moon.[8]