Apollo 13 was a NASA Moon mission that nearly ended in disaster when an oxygen tank exploded inside the spacecraft on April 13, 1970, about 56 hours after launch. The three astronauts, Jim Lovell, Jack Swigert, and Fred Haise, never landed on the Moon. Instead, they spent four days in a freezing, powered-down spacecraft, using their lunar lander as a lifeboat to survive the journey home. They splashed down safely in the South Pacific on April 17.
A Damaged Tank That Should Have Been Replaced
The root cause of the Apollo 13 accident traces back to events on the ground, months before launch. Oxygen tank number 2, installed in the service module, had a troubled history from the start. During manufacturing and testing, the tank failed to meet specifications for heat leak rates. Engineers submitted a formal waiver to use it “as is.”
The real damage happened during a pre-launch test at Kennedy Space Center. When technicians tried to drain the tank, it wouldn’t empty. They decided to use the tank’s internal heaters to boil off the remaining liquid oxygen, connecting them to a 65-volt ground power supply instead of the spacecraft’s normal 28-volt system. The heaters had a safety switch designed to cut power at 80°F, but at the higher voltage, the switch failed to open. The heaters ran for eight hours. Post-accident testing showed temperatures inside the tank could have reached 1,000°F, severe enough to melt the Teflon insulation off the internal wiring. Nobody caught it. Pre-launch discussions focused on other issues and never addressed what the extended heating had done inside the tank.
The Explosion
Apollo 13 launched on April 11, 1970, at 2:13 PM Eastern time. The first two days of flight were uneventful. Then, at 55 hours and 54 minutes into the mission, the crew was asked to turn on the fans inside the oxygen tanks to stir the contents, a routine procedure. When current flowed through the damaged wiring in tank number 2, it sparked. That spark ignited the pure oxygen filling the tank.
The tank ruptured, blowing off an entire panel on the side of the service module and damaging the adjacent oxygen tank number 1. Within minutes, the spacecraft was losing oxygen from both tanks. The command module’s fuel cells, which generated electricity by combining hydrogen and oxygen, began failing. The crew watched their power supplies die one by one. What had been a routine flight to the Moon’s Fra Mauro highlands became a survival mission 200,000 miles from Earth.
Using the Lunar Module as a Lifeboat
With the command module losing power and oxygen, the crew moved into the lunar module, called Aquarius. It was never designed for this. Built to support two astronauts for 45 hours on the lunar surface, it now had to keep three men alive for roughly 90 hours on the trip home.
Oxygen turned out to be the least of the problems. The full descent tank alone held enough, and there were additional reserves in the ascent engine tanks and two unused backpacks meant for moonwalks. Power was a different story. The lunar module’s batteries held 2,181 ampere hours. Ground controllers worked out a plan to shut down every nonessential system, cutting power consumption to one-fifth of normal levels. They even figured out how to charge the command module’s batteries using lunar module power, so the command module could function during reentry. By the time Aquarius was jettisoned, 20 percent of its battery power remained.
Water was the most critical concern. The lunar module’s systems used water for cooling, and engineers calculated the crew would run out about five hours before reaching Earth. The astronauts cut their water intake to six ounces per day each, one-fifth of the normal amount, and relied on fruit juices and wet-pack foods like hot dogs for additional moisture. Lovell lost 14 pounds during the mission. The crew lost a combined 31.5 pounds, nearly 50 percent more than any other Apollo crew. Those extreme measures worked: they finished with 28.2 pounds of water to spare, about 9 percent of their total supply.
Freezing Conditions and a CO2 Problem
With most systems powered down, the spacecraft became bitterly cold. Cabin temperatures in the lunar module hovered between 54°F and 60°F, while the powered-down command module dropped to around 43°F. Moisture condensed on windows and interior surfaces. The crew couldn’t sleep well, shivered constantly, and had no way to warm up. Total power draw for most of the return trip sat between 350 and 400 watts, less than what most household appliances use.
A second life-threatening problem emerged as carbon dioxide built up inside the lunar module. Aquarius had its own CO2 scrubbing canisters, but they were running out. The command module had plenty of spare canisters, but they were square. The lunar module’s system accepted round ones. Engineers on the ground designed an adapter using only materials available on the spacecraft, then talked the crew through building it. The improvised fix worked, keeping CO2 at safe levels for the rest of the trip.
Getting Home
The crew couldn’t simply turn around. They were already on a path toward the Moon and had to swing around it to head back to Earth. Commander Jim Lovell fired the lunar module’s descent engine for 30 seconds, adding 43 mph to their speed of 3,500 mph. This put them on a “free return trajectory,” a path that would use the Moon’s gravity to sling them back toward Earth without requiring another major engine burn.
After rounding the Moon, controllers decided on an additional burn to speed up the return and shift the landing point to the Pacific Ocean, where recovery ships were positioned. Navigation was a challenge with most instruments offline, but the crew managed each correction using the lunar module’s engine.
Before reentry, the astronauts powered up the command module using the battery charge transferred from Aquarius. They jettisoned the damaged service module first, getting their first look at the damage. An entire panel was missing, and the interior was gutted. They then released the lunar module that had kept them alive. The command module hit the atmosphere and splashed down in the South Pacific at 12:07 PM Central time on April 17, 1970. The crew was picked up by the USS Iwo Jima.
What the Investigation Found
NASA convened a review board led by Edgar Cortright to determine what went wrong. The board’s conclusion was blunt: the accident “was not the result of a chance malfunction in a statistical sense but, rather, it was the result of an unusual combination of mistakes coupled with a somewhat deficient and unforgiving design.” The damaged wiring from the overheated tank was the direct cause, but the deeper failures were organizational. Warning signs during testing had been dismissed or overlooked.
The board recommended a complete redesign of the service module’s oxygen storage system, a review of all Apollo systems using cryogenic oxygen with special attention to materials compatibility, and stronger management structures to ensure that anomaly reviews included the full history of every component. These changes were implemented before Apollo 14 flew ten months later. No similar failure occurred on any subsequent mission.