"13, we've got one more item for you, when you get a chance. We'd like you to stir up the cryo tanks,” said capsule communicator Jack Lousma at Mission Control. The purpose of stirring the cryogenic tanks containing the hydrogen and oxygen was to give more accurate readings of how much gas was left. But because of an electrical fault, one of the oxygen tanks exploded. Initially the crew thought a meteoroid had hit them, but it soon became apparent they were losing oxygen. “It looks to me… that we are venting something,” replied Jack Swigert. “It’s a gas of some sort.” The clock was ticking.

The explosion on board Apollo 13 marked the start of one of the greatest rescue missions in human exploration, but the three crewmembers owed their lives to a decision made years previously.

Direct approach

There are three ways you can reach the Moon. A ‘direct approach’ means taking off from Earth in a one-size-fits-all rocket, heading straight to the Moon, landing vertically and then returning to Earth in the same craft.

This would have required an enormous rocket. At the time, two large rockets were proposed – the Saturn and Nova. Initially, the Nova designs were much larger than the Saturn designs and might have been capable of a direct approach.

This was favoured by many senior engineers, like Wehrner von Braun and President Kennedy’s scientific advisor, says Jerry Woodfill, the spacecraft warning system engineer for Apollo 11 and 13 (and the only engineer to ever have this responsibility). But the enormous size of the rockets needed to go directly to the Moon would have meant using an incredible amount of fuel, and made the mission more costly. As a result, the direct approach was discarded, but only after a heated debate.