While planning the lunar landings, Nasa had a decision to make: What route would they use to reach the Moon? It was one of many that needed to be made, but little did they know then, it would have an important effect of the survival of the ill-fated Apollo 13 crew.

There are three ways you can reach the Moon

This Sunday, 17 April, marks the safe return of James Lovell, “Jack” Swigert and Fred Haise after almost 88 tense hours on board their critically wounded craft thanks to that stroke of luck.

The drama started unfolding at almost 56 hours Ground Elapsed Time (GET), meaning it was the evening of 13 April in Houston. Shortly after a live television broadcast, Mission Control had asked Apollo 13 to run through some standard checks.

"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.

Without fuel cells, the main source of Oxygen, death would have been imminent – Jerry Woodfill

It’s fortunate that the direct approach was dropped. Had Oxygen Tank 2 exploded at the same point in the journey it would have certainly resulted in death. “If the vehicle's systems were like the Command and Service Module, using fuel cells for power with modest battery reserves for atmospheric entry on return to Earth splashdown, we know the loss of the cryogenic oxygen resulting from the tank explosion would have left at most 12-15 hours of battery power to keep the ship alive,” says Woodfill, who was at the controls monitoring the alarm systems the evening the explosion happened.

“Residual oxygen in the cabin atmosphere and the suit EVA backpacks would have extended the life of the crew but without fuel cells, the main source of oxygen, death would have been imminent.”

Earth orbit rendezvous

The alternative to the direct approach was to perform one of two forms of rendezvous. In a rendezvous, the craft would have had a modular design, with a landing module, and a service/command module.

A small lunar lander would be able to exploit the Moon’s low gravity – saving fuel

These modules would travel to an orbit around either the Earth or the Moon, separate, and then only one, small, craft would have landed on the Moon’s surface. The benefit of this is that you need a smaller rocket to leave Earth’s atmosphere, and the small lunar lander would be able to exploit the Moon’s low gravity – saving fuel.

However, in an ‘Earth orbit rendezvous’ approach, the majority of the journey would have been completed in a single craft. The crew would have transferred from the command module into the lunar module in Earth’s orbit, and then travelled from there directly to the Moon.

“Since the Earth orbit rendezvous has a single vehicle for cruising to the Moon as well as landing and leaving the Moon and re-entering Earth's atmosphere with a re-entry capsule, the results are the same as the direct approach, death to the crew,” says Woodfill.

The Earth orbit rendezvous design did include a separate rocket for retro burn, slowing the craft down as it descended onto the lunar surface. At 56 hours GET this rocket would not yet have been fired, offering the crew a slight advantage – they could use it to speed up their return to Earth.

“But… all the other fatal factors would remain, too little electric power, no fuel cell-produced oxygen to breathe, no power for the guidance system, or the thrusters,” says Woodfill. “The return to Earth would still require more time than firing the extra retro rocket could reduce.”

Lunar orbit rendezvous

The final chosen approach was to travel to the Moon in a small, modular craft, separate while in lunar orbit, come back together again, and then travel back to Earth. In the planning stages of the Apollo programme this wasn’t the obvious choice but it saved the lives of the crew.

An explosion 56 hours into the mission happened before the command module and lunar module had separated, so the crew were able to use the intact lunar module as a lifeboat with its own power sources, rockets and oxygen supply. The command module computers were required for a safe re-entry, but without any power these needed to be recharged using the fuel cells in the lunar module – something only possible with a separate craft.

Going to Mars in a craft that looks similar to the iconic Apollo vehicles is unlikely

As we look to travel to Mars, the likelihood is we will be having the same discussions about approach as the Nasa directors did in the 60s. “Mission planners throughout the space ages will always engage in such debating, which is a good thing. For the rescue of Apollo 13, these sorts of debates were going on in real time,” concludes Woodfill. “Now a mission to Mars, is, indeed, a magnitude more challenging than the rescue of Apollo 13. It is a topic with so many debatable questions that a simple answer at this time is altogether impossible.”

But using a craft that looks similar to the iconic Apollo vehicles is unlikely. “Few espouse an Apollo-like wagon/train approach, bringing everything together on a space caravan.” The use of Mars’ resources mined by robotic precursor missions is more likely, he says.

“For much of mankind the reaches of space had never seemed so infinitely remote as they did when Apollo 13 was crippled nearly a quarter of a million miles from Earth, headed toward the Moon,” read a statement from President Nixon following the safe return of the crew. “To them and to those on the ground who did so magnificent a job of guiding Apollo 13 safely back from the edge of eternity, a grateful nation says ‘well done’.”

He could have also thanked those engineers who so fiercely defended the lunar orbit rendezvous approach when it was an unpopular thing to do.

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William Park is BBC Future’s social media producer. He tweets at @williamhpark.

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