Lunar Orbit Rendezvous (LOR)

Direct Ascent would have required a rocket the size of a skyscraper. Earth Orbit Rendezvous was too complex. LOR was the elegant, risky solution.

Proposed by John Houbolt, Lunar Orbit Rendezvous required sending two separate vehicles to the Moon: a Command/Service Module (CSM) to stay in orbit, and a lightweight Lunar Module (LM) to land. This drastically reduced the total mass that needed to be injected into a trans-lunar trajectory.

The risk was that the two craft had to dock in lunar orbit, 240,000 miles from Earth. If the LM ascent engine failed, the crew was stranded.