On April 11, 1970 NASA launched its third mission to the lunar surface. The crew of Apollo 13 consisted of veteran Jim Lovell and first time space flyers Fred Haise and Jack Swigert. While coasting to the Moon, the crew docked the Command-Service Module and Lunar Module, sealing the tunnel that connected the two spacecraft. About 56 hours in, mission control instructed Swigert to perform a procedure called a "cryo stir” which helps monitor oxygen levels. A flip of a switch ended with an explosion.
What they didn’t know at the time was that Apollo 13 launched with the makings of a bomb aboard. The source of the explosion was one of the two cryogenic oxygen tanks located in the Service Module. A preflight test damaged insulation on the wiring inside the tank. When Swigert started the cryo stir, one of the damaged wires caused a spark.
The explosion caused the CSM to quickly lose its power and life sustaining resources. The damaged oxygen tank caused two out of three fuel cells to shutdown. That meant that the spacecraft was flying without its main source of electricity and water. Once mission control figured this out, the Moon was no longer an option. Now, the goal was to salvage enough resources to just get the Command Module back to Earth.
Flight controller, Sy Liebergot, had the unpopular idea to isolate the surge tank - the small reserve of oxygen that the crew would need for re-entry. But doing so would require the crew to ultimately power down every system in the Command-Service Module - making the spacecraft inhospitable. The Lunar Module would then have to be used as a lifeboat.
First, the crew had to power up the Lunar Module. This was supposed to be done using electricity from the CSM which was now on its deathbed. So hundreds of thousands of kilometers away, the lunar module controllers had to figure out how to hotwire the spacecraft using diagrams. After mission control turned on the Lunar Module they then had to figure out how to get the astronauts on a trajectory back home. The team decided the best abort trajectory would utilize the Moon’s gravity to guide the spacecraft back to Earth.
As the spacecraft approached re-entry, the astronauts weren’t in great shape, but after about six days the crew safely made it back onto solid ground.