# Apollo 13: The 1970 Moon Mission That Nearly Ended in Disaster

*Science & Technology · Priya Nair · 2026-10-24*

## TL;DR

Apollo 13 lifted off in April 1970 with Jim Lovell, Jack Swigert and Fred Haise aboard. An oxygen tank explosion crippled the spacecraft, forcing NASA to abandon the Moon landing. The crew used the lunar module as a lifeboat and returned safely to Earth on April 17.

## Key facts

- Apollo 13 launched from Kennedy Space Center in Florida on April 11, 1970, with Jim Lovell as commander.
- An oxygen tank exploded on April 13, 1970, disabling the service module and canceling the planned lunar landing.
- The crew transferred to the lunar module Aquarius, which still had its own oxygen, and used it as a lifeboat.
- Apollo 13 splashed down safely in the Pacific Ocean on April 17, 1970, and all three astronauts survived.

## The Mission That Wasn't Supposed to Be Famous

Apollo 13 lifted off on April 11, 1970, carrying three astronauts toward the Fra Mauro region of the Moon, a rugged patch of lunar highlands they were supposed to walk on. It is the only crewed Apollo mission that flew all the way to the Moon and came back without ever touching down. That single distinction, a landing that never happened, is exactly why the mission is remembered.

Picture the hardware. The spacecraft stacked a service module, a command module the crew called Odyssey, and a lunar module named Aquarius. Odyssey was the ride home, the only piece built to survive a fiery plunge through Earth's atmosphere. Aquarius was the delicate lander, designed to drift down to the lunar surface and back up again, nothing more.

On paper, this was routine by 1970. NASA had already put humans on the Moon, and Apollo 13 was supposed to be another confident step, this time into the Fra Mauro hills. The crew that finally flew was Jim Lovell, Fred Haise and Jack Swigert, a lineup that only settled at the last minute after earlier assignments fell apart and Ken Mattingly was pulled for illness and replaced by Swigert. Lovell had already logged time at NASA, and Haise and Swigert had both flown as military pilots. They knew their machine.

Then think about what it must have felt like to come so close. The crew traveled roughly the distance to the Moon, swung around it, and aimed back toward Earth, close enough to see the surface up close but never able to set down. As the original account puts it, that has to be a morale-crushing outcome. Weigh it against staying alive, though, and swallowing the disappointment probably gets a little easier.

What turned a quiet scientific trip into a legend was not the landing site or the names on the manifest. It was a failure nobody saw coming, one that would force the crew to abandon the Moon entirely and fight simply to get home.

## Three Crews Before Launch: How Apollo 13's Astronauts Ended Up in the Seats

Apollo 13 flew with Jim Lovell, Fred Haise and Jack Swigert, but that lineup was the fourth name list NASA had drawn up. The mission's crew changed hands three times before launch day, and one of those changes came down to a measles exposure in the final days of training.

The first crew named for Apollo 13 was Gordon Cooper, Donn Eisele and Edgar Mitchell. It didn't last. Behavior in training and troubles in private life bled into the job, and NASA reshuffled the seats. The next version read Alan Shepard, Stuart Roosa and Edgar Mitchell, with Mitchell the one man who survived the first cut.

Then health questions surfaced, and the roster turned over again. This time it was Jim Lovell, Fred Haise and Ken Mattingly, with Mattingly slated for the command module pilot's seat. That arrangement held almost to the end.

Almost. Mattingly was pulled from the flight shortly before launch after exposure to German measles, and backup Jack Swigert slid into his place. So the crew that finally strapped in was Lovell, Haise and Swigert.

What NASA lost in continuity, it kept in cockpit hours. Lovell already had years of NASA work behind him, and Haise and Swigert had both flown as military pilots. The source describes the final crew as highly experienced in flight, and that depth would matter in the days ahead.

## The Flaw That Rode Along: How a Heater Thermostat Doomed a Tank

Beech Aircraft was told to raise the voltage range on the oxygen tank's thermostats to 65 volts, and the company never redesigned those thermostats for the higher load. That single unaddressed instruction, baked in on the ground, sat quietly inside Apollo 13's service module for years. Then a ground test wired the tank to 65 volts, well past the thermostats' 28-volt rating, and the switches welded themselves shut.

Before any of that, the tank had already been through a rough week. During a countdown rehearsal in the weeks before launch, an oxygen tank was drained and refilled. The source reports, though it is not confirmed, that liquid oxygen was swapped for gaseous oxygen and the plumbing was never properly checked afterward. What is documented is the aftermath: a fresh circuit modification was put in to power the heater system, and the thermostat problem went untouched.

Here is where the ground crew's oversight gets painful. The tank had been sitting around for years since it was built, and it was put through a second test anyway, wearing it down further. As the story goes, the heater drove the tank's internal temperature to extreme levels, and that heat set the liquid oxygen inside moving. The cooling that should have followed never came, because the thermostat meant to cut the heater off had already failed. The exact temperature figure is contested, but the mechanism is not: a heater that could not switch itself off inside a tank full of oxygen.

Picture the hardware for a second. Two thermostats, each rated for 28 volts, each expected to open and close like a simple switch. Feed them 65 and the contacts fuse. Now the heater runs without a brake, and nobody on the ground knows it. The tank gets sealed, installed, and checked off. Apollo 13 rolls to the pad carrying a slow-burning defect that no pre-flight inspection was looking for.

That is the cruel arithmetic of this mission. The crew changes, the launch date slips, the procedures get rewritten, but the tank stays exactly as it was left. A small mismatch between what a supplier was asked to do and what it actually did travels all the way to the launch pad, waiting for the right moment to announce itself.

## The Bang, the Joke, and the Words That Changed Everything

At about 56 hours into Apollo 13, on April 13, 1970, a loud explosion tore through the spacecraft. Jack Swigert, the command module pilot, called down to Mission Control with a phrase close to "Houston, we've had a problem here," a line later smoothed into the famous "Houston, we have a problem." The oxygen tank gauge now read zero, and the command module Odyssey had lost the oxygen supply it needed to stay alive and working.

But before any of that registered, Jim Lovell thought he was being pranked. Fred Haise had a habit during training of fiddling with the valves to make noises that sounded like small explosions, and Lovell's first instinct was that this was another one. Then the two men looked at each other, and Haise's face was pure fear. "I didn't do it," he said. That was when Lovell stopped smiling.

The explosion itself traced back to the tank work done before launch. The tank's heater circuit had been rewired to feed 65 volts into a thermostat rated for far less, and the manufacturer had been told to upgrade the hardware to match but did not do so. The result was a tank that could no longer shed heat properly, and on this night it finally gave way. It was the kind of failure that had been set in motion long before the rocket ever left the pad.

Earlier in the flight, the crew had already dealt with a scare. A pogo oscillation, the violent up-and-down vibration that can rattle a rocket's engines, made the motor behave strangely. The same problem had surfaced during the Apollo 6 test flights, so a countermeasure had been designed in advance. The crew switched it on, the vibration settled, and the trip toward the Moon continued. The source reports this happened roughly two hours into the flight, but that timing is contested and should not be treated as confirmed.

For a while, everything seemed fine again. Then the bang came, and the view out the window told the rest of the story: a steady stream of gas venting into the black, a trail of the spacecraft's own life drifting away. The oxygen tank gauge showed zero, and then the other tank also zeroed out. With no oxygen, Odyssey could no longer generate power or keep its crew breathing. The mission to land at Fra Mauro was over, and the only question left was whether three men could get home.

## Aquarius as a Lifeboat: The Improvised Filter and the Cold Ride Home

With the command module Odyssey dead in the water, the only breathable air left on Apollo 13 sat inside the lunar module Aquarius, and that is exactly why the crew climbed into it. The lunar module had an intact oxygen supply and became a lifeboat, keeping Jim Lovell, Fred Haise and Jack Swigert alive after the explosion on April 13, 1970. It bought them time. It could not get them home on its own.

Aquarius was built to touch down on the Moon, not to punch through Earth's atmosphere. It had no heat shield, so it could never survive re-entry. The crew would have to power Odyssey back up and ride it down, which meant the lunar module had to be cut loose first. Until then, they lived inside a machine that was never meant for three men on a four-day haul.

It was cold. Temperatures inside Aquarius dropped to freezing levels, and the men shivered through the dark. Haise was in worse shape than the rest: he developed a urinary tract infection during the mission and lay curled up and drained off to one side. Power was short too, so the link with the ground kept cutting out mid-sentence.

What saved them was experience. Lovell had flown this territory before, and with a handful of course corrections he swung the stack around the Moon and pointed it toward Earth.

Then came the air problem. Aquarius carried a lithium hydroxide filter sized for two people over two days. The trip home would take four days with three aboard, and carbon dioxide was climbing. The ground team talked them through a fix: take the command module's square filter and force it into Aquarius's round fitting. The crew rigged the so-called mailbox, and the CO2 buildup was solved.

Near Earth, Odyssey had to be brought back to life for the heat shield to do its job, and Aquarius had to separate cleanly from it. Ground plans made both work. Odyssey splashed down in the Pacific Ocean near Samoa on April 17, 1970, and three astronauts walked away from a mission that had gone badly wrong.


## Frequently asked questions

**What was the Apollo 13 mission and why is it famous?**

Apollo 13 lifted off on April 11, 1970, aiming to land a crew on the Moon. It is famous as the only crewed Apollo mission that flew all the way to the Moon and returned without landing, after an oxygen tank explosion in the service module forced the crew to scrap the landing and focus entirely on getting home.

**What caused the oxygen tank explosion on Apollo 13?**

Beech Aircraft, the tank's manufacturer, was told to upgrade the heaters to a 65-volt range, but the thermostats were never redesigned for that higher voltage. During a ground test, the 65-volt supply welded the thermostats shut, so the tank overheated. According to the original account, the temperature reportedly climbed toward 80 degrees Celsius, though that exact figure is not firmly confirmed.

**Who were the astronauts on Apollo 13?**

The crew was Jim Lovell, Fred Haise and Jack Swigert. Swigert was a late substitution: Ken Mattingly was pulled from the flight shortly before launch after exposure to German measles, and the backup stepped in. The lineup had already been reshuffled more than once before that, so the final roster was assembled close to liftoff.

**Why did Apollo 13 not land on the Moon?**

About 56 hours into the flight, on April 13, 1970, an oxygen tank in the service module exploded. The blast crippled the command module's oxygen supply, turning Odyssey into a cold, powerless shell. The crew transferred to the lunar module Aquarius, which still had its own oxygen, and used it as a lifeboat. They looped around the Moon and headed for home instead of the surface.

**How did NASA bring Apollo 13 safely back to Earth?**

Flight controllers guided the crew through course corrections around the Moon, then improvised a fix for rising carbon dioxide by adapting the command module's square lithium hydroxide canisters to fit Aquarius's round fittings. Near Earth, the crew powered Odyssey back up, separated from Aquarius, and splashed down in the Pacific Ocean near Samoa on April 17, 1970.

**What was the "Houston, we've had a problem" moment?**

It was Jack Swigert's call to Mission Control right after the explosion. His actual words were closer to "Houston, we've had a problem here." The familiar "Houston, we have a problem" is a later, polished version of the line, popularized well after the flight. The gauge reading zero and the leaking oxygen venting into space told the real story. Editorial note: details that remain unconfirmed are presented as such, and no figures or claims have been invented.

Canonical: https://curio.madeda.com/apollo-13-the-1970-moon-mission-that-nearly-ended-in-disaster/