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For nearly 56 hours after the Apollo 13 mission launched on April 11, 1970, it looked to be the smoothest flight of NASA’s Apollo program so far.

The spacecraft ferrying astronauts Jim Lovell, Jack Swigert and Fred Haise to their planned lunar landing had traveled just over 200,000 miles from Earth, and was approaching the moon’s orbit.

Just before 9 pm on April 13, the crew wrapped up a TV broadcast in which they had given a tour of the spacecraft and talked about how they were managing weightlessness. “This is the crew of Apollo 13 wishing everybody there a nice evening,” signed off Mission Commander Lovell, a captain in the U.S. Navy with three other missions (including Apollo 8) under his belt.

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Less than 10 minutes later, after a routine maintenance task went awry and caused the spacecraft’s oxygen tanks to explode, what was supposed to be the U.S. space program’s third landing on the moon turned into a desperate race to save three astronauts’ lives. Working around the clock from Mission Control at the Manned Spacecraft Center (now Johnson Space Center) in Houston, Texas, NASA flight controllers and engineers improvised a series of innovative procedures to bring Lovell, Swigert and Haise safely home on April 17, marking a successful conclusion to one of the most dramatic episodes in the history of the U.S. space program.

WATCH: Apollo 13: Modern Marvels on HISTORY Vault

Missed Warning Signs

In order to power the fuel cells that provided most of the electricity used during the flight, the Apollo spacecraft carried two tanks of liquid hydrogen and two tanks of liquid oxygen. NASA’s subsequent investigation revealed that the No. 2 oxygen tank onboard Apollo 13 had been accidentally dropped during maintenance before the Apollo 10 mission in 1969, causing slight internal damage that didn’t show up in later inspections.

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During testing in March 1970, the reinstalled tank failed to properly empty itself of oxygen. The testing team decided to solve this problem by heating the tank overnight to force the liquid oxygen to burn off. But the surge of power from the high-voltage DC system on the ground caused the automatic shut-off switches on the tank’s heater to fail, and the temperature spiked to more than 1,000 degrees Fahrenheit. Though there was no external indication of the problem, the heat apparently damaged the insulation on the wires inside the tank—effectively turning the tank into a bomb waiting to explode.

Chain Reaction Leads to Explosion

Apollo 13

The Mission Control Center photographed on April 13, 1970, during the fourth television transmission from the Apollo 13 mission. One of four Apollo 13 flight directors views the large screen at front as astronaut Fred Haise, Lunar Module pilot, is seen on the screen.

While in flight, the astronauts had to turn on the fuel tanks’ internal fans periodically in order to stir the super cold oxygen, which tended to stratify, or settle into layers. But when Swigert turned on the fans on the second oxygen tank for a routine “cryo stir” on the night of April 13, the damaged wiring caused a spark, starting a fire. At 9:08 pm, with its internal pressure mounting, the tank exploded.

As Lovell recounts in an upcoming HISTORY This Week podcast, he and Haise were caught completely off guard when they heard the bang. “I looked up at Fred Haise to see if he knew what caused the noise. And I could tell from his expression, he had no idea. Then I...looked down at Jack Swigert in the command module and his eyes were as wide as saucers. And I could see that...this was the start of a long, treacherous journey home.” 

“Houston, we’ve had a problem here,” Swigert said, after noticing a warning light switch on after hearing the bang of the exploding tank. (He would later be famously misquoted.) More blinking lights soon indicated the loss of two of the ship’s three fuel cells, which in addition to electricity provided potable water, used for cooling the spacecraft’s systems as well as hydrating the astronauts.

Then, 13 minutes after the explosion, Lovell glanced out the window and saw something else disturbing. “We are venting something out into the...into space,” he reported. “It’s a gas of some sort.” Because the two oxygen tanks were located in the same segment of the spacecraft, the explosion had damaged the other tank as well, and it had begun leaking oxygen into space.

The Rocky Road to Touchdown

WATCH: Apollo 13 Emergency Radio Transmission

Ground controllers in Houston now mobilized to run an unprecedented survival mission. They ordered the crew to make their way from the spacecraft’s command module, Odyssey, into the separate landing module, Aquarius. If things had gone as planned, Aquarius wouldn’t have been turned on until the astronauts were ready to touch down on the moon. Now, it had to keep Lovell, Swigert and Haise alive for an estimated 90 hours, until they could transfer back to the damaged command module for reentry into Earth’s atmosphere.

The crew turned off all non-critical systems aboard the spacecraft to reduce energy consumption, and cut way back on their consumption of water, in order to have enough to cool the landing module’s overtaxed hardware. At one point, when too much carbon dioxide was building up in Aquarius, Mission Control devised a way for the astronauts to clear the gas out, instructing them to build a “mail box” out of plastic bags, cardboard and tape in order to purge the gas using canisters from the command module.

“They worked out a system and then they relayed it up to us word by word,” Lovell tells HISTORY This Week. “Hose. Duct tape and an old sock and my gosh, time was the one thing that kept us from dying.” 

Apollo 13 Declared a ‘Successful Failure’

HISTORY: Apollo 13

The crew of Apollo 13 waving after their return to Earth. Left to right: Fred W. Haise, James A. Lovell, John L. Swigert. 

On April 17, after the engineers in Houston succeeded in powering Odyssey back up, the crew prepared for the final stages of their journey to Earth by jettisoning the lunar module. Finally, at 11:53 am, what was left of the Apollo 13 spacecraft re-entered the Earth’s atmosphere, touching down in the Pacific Ocean, near Samoa.

Because so much valuable experience was gained in the process of rescuing Lovell, Swiger, and Haise, NASA classified the Apollo 13 mission as a “successful failure.” Starting with Apollo 14, each spacecraft would be supplied with an additional battery as well as a third reserve oxygen tank, located in another section of the service module from the other two, that could be used exclusively to provide air for the astronauts. Over eight more Apollo missions, no such incident ever occurred again. 

READ MORE: 8 Little-Known Facts About the Apollo 11 Moon Landing

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