On July 21, 1969, Neil Armstrong and Buzz Aldrin launched from the lunar surface and rejoined Michael Collins in orbit before the three men began their trip home.

Ascending from the lunar surface was one of the most important maneuvers on the mission; any problems could leave Aldrin and Armstrong stranded on the moon with no way home. It was a gruesome scenario, but not impossible. In the unlikely event this lunar disaster did happen, NASA had a plan in place.

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In the early days of the Apollo program in the early 1960s, program engineers had to answer a hard but basic question: how safe was safe enough? It wasn’t feasible to build a spacecraft and ensure 100 percent success. Engineers are human and humans make mistake, not to mention the testing program for a perfect success rate was expensive and would take far too long. So they settled on 99.9 percent as the objective, sometimes referred to a “three nines.” If every piece of every spacecraft was proven to worked 99.9 percent of the time, it was safe to fly.

The “three nines” rule did admit that things could go wrong. Some possible failures were minor like a faulty warning lights that were more of a nuisance than anything else. Other possible failures could be catastrophic, like an engine failing to ignite.

On Apollo missions, the engines were possibly the most important piece of hardware. After reaching Earth orbit, the crew fired the Saturn V rocket’s upper stage engine to launch them towards the moon. Four days later, they had to slow their spacecraft to enter into lunar orbit, and this meant firing their Command and Service Module’s (CSM) engine against their direction of travel.

To make a soft landing on the moon, the two astronauts in the Lunar Module (LM) relied on its descent engine. Leaving the lunar surface to meet the third astronaut in orbit, the two lunar astronauts relied on the LM’s ascent engine. Once the CSM and LM were docked, the crew fired the CSM’s engine to leave lunar orbit and return home.

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Throughout the flight, the crew also adjusted their trajectory using the CSM’s engine for mid-course correction burns. These were fairly common; Apollo 11 had four in its mission plan for on way towards the moon alone.

Almost anything on the spacecraft could fail without the mission turning fatal to the crew except the engines. Apollo 13 is the perfect example. When the oxygen tank exploded taking out half the spacecraft’s power, the crew shut down everything in both the CSM and the LM to conserve power for reentry. They let gravity slingshot them around the Moon and bring them back towards Earth, but they did use the LM’s descent engine — the only undamaged engine — for mid-course corrections.

An engine failing to fire was one of the worst things that could happen on an Apollo mission. But there was always the chance, that slim 0.01 percent chance, that it would be the LM ascent engine that failed leaving the crew to die on the lunar surface. NASA knew this was a possibility, so plans were in place in the event of a lunar disaster.

So, what would have happened if, on July 21, 1969, Armstrong and Aldrin packed up from their stay on the lunar surface, fired their LM ascent engine, and nothing happened?

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Collins orbiting in the CSM would have been helpless. There was no contingency for him to recover the LM with the CSM. It didn’t have that capacity. He would have followed orders and returned to Earth alone. President Nixon would have called the widows-to-be, Janet Armstrong and Marion Aldrin. He would then read a prepared statement to the world, and likely to the crew on the Moon as well:

NASA would probably have stayed in contact with the Armstrong and Aldrin afterwards, though the LM’s limited oxygen supply would be running low then. After ending the final communication with the stranded Apollo 11 crew, a clergyman would take over the broadcast from NASA and carry out the procedures of a burial. He would commend the astronauts’ souls to the deep before concluding with the Lord’s Prayer. The world would be looking up at two men on the Moon who would likely be sitting inside the LM looking back at the Earth.

But Apollo 11’s LM ascent engine did fire, as it did on the five lunar landing missions that followed. It’s a good thing, too. Looking up at the Moon and knowing there are two dead astronauts up there would probably turn our natural satellite into something foreboding and sinister. As it stands, it’s pretty beautiful and remains the site of our greatest technological triumph.

Image: United States President Richard M. Nixon welcomes the Apollo 11 astronauts aboard the USS Hornet after Apollo 11 splashed down. Already confined to the Mobile Quarantine Facility (MQF) are (left to right) Neil A. Armstrong, commander; Michael Collins, command module pilot; and Edwin E. Aldrin Jr., lunar module pilot. Credit: NASA