Earth orbit rendezvous

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. 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, 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 cancellation in October 2010.