In previous lunar orbit missions, the spacecraft was on a free-return trajectory. This meant if for some reason they did not fire their engine to slow into lunar orbit, they would loop around the moon and return to Earth. At 30:40 MET, the crew performed a mid-course correction firing of their engine. This lowered their closest approach point to 60 miles; this was more efficient for getting into lunar orbit, but it meant if something went wrong, they would have to alter their course to get back to Earth.

At 55 hours, 54 minutes, 53 seconds into the mission, an oxygen tank exploded in the Service Module, critically damaging other oxygen tanks and starving the electricity generating fuel cells to a point where they became useless. Looking out the window, the astronauts could see the oxygen venting out into space. Within 90 minutes, the crew and Mission Control accepted the fact that the Command Service Module was dead and the Lunar Module systems had to be used to get the crew home.

To get the crew home, Mission Control devised a way for the crew to use the Lunar Module systems to a) keep 3 people alive for 86 hours and b) perform rocket burns to place the spacecraft on the correct trajectory to return to a safe splashdown in the Pacific Ocean.