13 Things That Saved Apollo 13

Part 1: Timing

If the explosion happened earlier (and assuming it would have occurred after Apollo 13 left Earth orbit), the distance and time to get back to Earth would have been so great that there wouldn’t have been sufficient power, water and oxygen for the crew to survive. Had it happened much later, perhaps after astronauts Jim Lovell and Fred Haise had already descended to the lunar surface, there would not have been the opportunity to use the lunar lander as a lifeboat.
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But this was not the first time the crew had been ordered to stir the tank. It was the fifth time during the mission.

Links to the other 12 things are in the post.