First humans to leave Earth orbit

The first humans to leave Earth orbit were American astronauts Tom Stafford, John Young and Eugene Cernan on board the Apollo 10 spacecraft. The record-breaking flight took off on 18 May 1969 from the Kennedy Space Center in Florida, USA, and crossed the equigravisphere (the point at which the gravitational pull of the Earth and the Moon are equal) at 06:39 (UTC) on 21 May 1969.

Apollo 10 was the last of the Apollo preparatory missions, designated internally as a "Type F" mission (the actual Moon landing was a "Type G" mission). The crew were to conduct a full dress rehearsal for the upcoming Moon landing, going through every stage of the mission plan short of actually landing the lunar module. It had been preceded by various uncrewed tests and the crewed Apollo missions 7 (testing Command Module systems in Earth orbit), 8 (orbiting the moon) and 9 (flying the Command Module and Lunar Module in Earth orbit).

The crew chosen for this mission comprised three veterans of Project Gemini: Tom Stafford, John Young and Gene Cernan. These three already had five launches and around 250 spaceflight hours between them. They also subjected themselves to a gruelling training schedule, doing five hours of extra training (on top of the usual briefings) for every hour of their 8-day mission (including 300 hours of simulator time). Thanks to their exhaustive preparations, they completed all the objectives of the mission, demonstrating the various manoeuvres their colleagues on the Apollo 11 crew would need to perform in a few months time.

The mission was originally scheduled for launch on 16 May, but it was moved back to 18 May on the suggestion of Apollo program geologist (and future Apollo 17 astronaut) Harrison Schmitt. He suggested this change would allow the crew to observe the ALS-2 potential landing site (which would become Apollo 11's Tranquillity Base) under ideal lighting conditions. This shift also had the effect of moving the spacecraft's arrival in lunar orbit from the day after the Moon was at apogee (farthest point from Earth) to three days after, lowering its maximum distance from Earth by a few hundred kilometres.

Because the Moon was getting closer to the Earth at the time that Apollo 10 launched, the spacecraft was at its most distant point at the moment of its arrival in lunar orbit. Over the two and half days Apollo 10 spend in lunar orbit, its average distance from the Earth reduced by around 8,000 km.