Apollo 13 was supposed to be NASA’s third moon-landing mission. Instead, in an instant, the spacecraft pivoted from a moon-bound landing unit to a crippled vessel. The spaceflight stands today as a demonstration of NASA innovation saving lives on the fly, and vividly illustrates the dangers of working in space as well.

The Apollo 13 astronauts

First-time flyer Jack Swigert, 38, had been an astronaut since 1966, and had previously been part of the support crew for Apollo 7. He was initially Apollo 13’s backup command module pilot but joined the crew just 48 hours before the launch after prime crew member Ken Mattingly was unwittingly exposed to the German measles. Since Mattingly had no immunity, NASA doctors yanked him from the mission over commander Jim Lovell‘s protests.

Lovell, 42, was the world’s most traveled astronaut. He had three missions and 572 spaceflight hours of experience. Lovell participated in Apollo 8, the first mission to circle the moon, and flew two Gemini missions — including a 14-day endurance run.

Rounding out the crew was Fred Haise, 36. Haise was in the same astronaut class as Swigert and had previously been a backup crew member on Apollo 8 and 11.

The entire crew had test flight experience before they became astronauts, meaning they were used to dealing with in-flight problems. That experience would come in handy on Apollo 13.

 ‘Houston, we’ve had a problem’

Apollo 13 launched on April 11, 1970. The Apollo spacecraft was made up of two independent spacecraft joined by a tunnel: orbiter Odyssey, and lander Aquarius. The crew lived in Odyssey on the journey to the moon.

On the evening of April 13, when the crew was 200,000 miles from Earth and closing in on the moon, mission controller Sy Liebergot saw a low-pressure warning signal on a hydrogen tank in Odyssey.

The signal could have shown a problem, or could have indicated the hydrogen just needed to be resettled by heating and fanning the gas inside the tank. That procedure was called a “cryo stir,” and was supposed to stop the supercold gas from settling into layers.

Swigert flipped the switch for the routine procedure. A moment later, the entire spacecraft shuddered around the startled crew. Alarm lights lit up in Odyssey and in Mission Control as oxygen pressure fell and power disappeared. The crew notified Mission Control, with Swigert famously uttering, “Houston, we’ve had a problem.” (The 1995 movie “Apollo 13” took some creative license with the phrase, changing it to “Houston, we have a problem” and having the words come out of Apollo 13 commander James Lovell’s mouth.)

Much later, a NASA accident investigation board determined wires were exposed in the oxygen tank through a combination of manufacturing and testing errors before flight. That fateful night, a spark from an exposed wire in the oxygen tank caused a fire, ripping apart one oxygen tank and damaging another inside the spacecraft.

Since oxygen fed Odyssey’s fuel cells, power was reduced as well. The spacecraft’s attitude control thrusters, sensing the venting oxygen, tried to stabilize the spacecraft through firing small jets. The system wasn’t very successful given several of the jets were slammed shut by the explosion.

Luckily for Apollo 13, the damaged Odyssey had a healthy backup: Aquarius, which wasn’t supposed to be turned on until the crew was close to landing on the moon. It didn’t have a heat shield to survive the trip back to Earth, but it could keep the crew alive long enough to get there. Then, the astronauts could switch to Odyssey for the rest of the trip home.

Haise and Lovell frantically worked to boot Aquarius up in less time than designed, while Swigert remained in Odyssey to shut down its systems to keep power for splashdown.

Cold days before splashdown

The crew now had to balance the challenge of getting home with the challenge of preserving power on Aquarius. After they performed a crucial burn to point the spacecraft back towards Earth, the crew powered down every nonessential system in the spacecraft.

Without a source of heat, cabin temperatures quickly dropped down close to freezing. Some food became inedible. The crew also rationed water to make sure Aquarius — operating for longer than it was designed — would have enough liquid to cool its hardware down.

On Earth, flight director Gene Kranz pulled his shift of controllers off regular rotation to focus on managing consumables like water and power. Other mission control teams helped the crew with its daily activities. Spacecraft manufacturers worked around the clock to support NASA and the crew.

It was a long few days back home; the entire crew lost weight, and Haise developed a kidney infection. In the hours before splashdown, the exhausted crew powered up Odyssey (which had essentially been in a cold soak for days, and could have shorted out if they were unlucky). They prepared for splashdown, not knowing if the explosion had damaged the heat shield. [Video: What If Apollo 13 Failed to Return Home?]

Lovell, Haise and Swigert returned safely to the Pacific Ocean on April 17. The spacecraft design was reconfigured with better wires and an extra tank, and subsequent missions did not face the same problem.

Apollo 13 legacy

Numerous design changes were made to the Apollo service module and command module on subsequent missions in the Apollo program. According to former mission controller Sy Liebergot on the website collectSPACE, these changes included:

• Another cryo oxygen tank that could be isolated to only supply the crew.
• Removing all cryo tank fans and wiring.
• Removing the thermostats from cryo tanks, and changing the type of heater tube.
• Adding a 400-amp-hour lunar module descent stage battery.
• Adding water storage bags to the command module.

As for the astronauts, Haise was assigned to command the Apollo 19 moon mission. However, it and two other missions were canceled after NASA’s budget was cut. He later piloted the space shuttle Enterprise during its test flights.

In 1982, Swigert was elected to Congress in his home state of Colorado. However, during the campaign, he learned that he had bone cancer, and he died before he could be sworn in.

In 1994, Lovell and journalist Jeffrey Kluger co-wrote a book about Lovell’s spaceflight career that primarily focused on the events of the Apollo 13 mission. The book was called “Lost Moon: The Perilous Voyage of Apollo 13”, and spurred the movie “Apollo 13” (1995), which starred Tom Hanks. The movie won two Academy Awards and was filmed in cooperation with NASA.

The agency gave the movie crew access to the 1960s-era Mission Control in Houston to reconstruct the site as a set, and also let the actor “astronauts” fly aboard NASA’s Vomit Comet airplane to simulate weightlessness. Lovell made a cameo at the end of the film as the captain of the U.S.S. Iwo Jima; Marilyn Lovell and Gene Kranz made short appearances as well, according to the Internet Movie Database.

Other biographical accounts of the Apollo 13 mission include Liebergot and David Harland’s “Apollo EECOM: Journey of a Lifetime” (2003) and Kranz’s “Failure Is Not An Option” (2000). Several non-fiction books have also examined Apollo 13; a notable but older example was Andrew Chaikin’s “A Man On The Moon” (1994), which included interviews with all of the surviving Apollo astronauts.

Notable fictional accounts of Apollo 13 include:

• “Houston, We’ve Got A Problem” (1974), which focuses on the stress on ground personnel during the crisis;
• “From The Earth to the Moon” (1998), which had an entire episode about Apollo 13 called “We Interrupt This Program.” The episode focused mostly on television reporters on the ground, rather than the crew in space.
• The interactive theater show “Apollo 13: Mission Control” debuted in 2008 in New Zealand, and went on tour in the United States.