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] 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]