Apollo 13’s crisis began long before the crew ever left the ground. An oxygen tank in the spacecraft’s Service Module exploded on April 13, 1970, roughly 56 hours into the mission, crippling the ship and forcing three astronauts to abandon their moon landing and fight to survive the journey home. The root cause was a chain of overlooked errors during manufacturing and pre-launch testing that left damaged wiring inside a pressurized oxygen tank, creating a bomb waiting to go off.
A Two-Inch Drop That Started Everything
The oxygen tank that failed on Apollo 13 wasn’t even built for that mission. It had originally been installed in the Service Module assigned to Apollo 10. In October 1968, engineers at North American Rockwell’s facility in Downey, California, removed the tank assembly to implement design changes. During that process, the tanks were inadvertently dropped about two inches. That small jolt likely damaged the tank’s internal fill and drain system, though the issue wasn’t caught at the time. The tank was reassigned and eventually installed in Apollo 13’s Service Module.
The Voltage Mismatch Nobody Tested For
The damaged drain system caused a serious problem during pre-launch preparations at Kennedy Space Center. When technicians tried to empty the tank of liquid oxygen during a routine procedure called “detanking,” the fluid wouldn’t drain properly. Their workaround was to use the tank’s internal heaters to boil off the remaining oxygen, a process that took hours.
Here’s where the critical design flaw came in. Each heater inside the tank had a safety switch designed to cut power if temperatures rose above 80°F. Those switches were rated for 28 volts, the standard operating power aboard the Apollo spacecraft. But the ground support equipment at Kennedy used 65-volt power. At that higher voltage, the switches failed to open. They simply welded shut and kept the heaters running.
Nobody knew this because qualification testing had never checked whether the switches could handle 65 volts. It was a gap in the testing procedures that no one had flagged. With the safety switches stuck closed, the heaters ran unchecked during the detanking process and reached temperatures as high as 1,000°F. That extreme heat severely damaged the Teflon insulation on the fan motor wiring inside the tank, leaving bare wires surrounded by pressurized liquid oxygen. The tank passed its subsequent checks and was loaded onto the spacecraft. No one had any reason to suspect a problem.
The Explosion at 55 Hours
Two days into the mission, with Apollo 13 roughly 200,000 miles from Earth, Mission Control asked the crew to stir the oxygen tanks. This was a routine procedure: fans inside the tanks churned the super-cold liquid to get accurate quantity readings. When astronaut Jack Swigert flipped the switch to activate the fans in oxygen tank number 2, the bare wires sparked. The spark ignited the Teflon insulation, and the fire rapidly raised pressure inside the sealed tank until it ruptured.
The explosion blew off an entire panel on the side of the Service Module and damaged oxygen tank number 1 as well. Within minutes, the crew watched their second oxygen tank begin bleeding its contents into space. Fuel cell 3 failed almost immediately, since the fuel cells depended on oxygen to generate electricity. Within seconds, the main electrical buses lost power. Astronaut Fred Haise reported readings of zero on the AC and DC systems. Commander Jim Lovell, looking out the window, saw a cloud of gas venting from the ship and realized the situation was far worse than a simple instrument malfunction.
The crew was losing all three of its fuel cells, which meant the Command Module, Odyssey, would soon be completely without power. The moon landing was over. Survival was now the only objective.
Using the Lunar Module as a Lifeboat
With Odyssey dying, the crew powered up the Lunar Module, Aquarius, which had its own oxygen supply, batteries, and engine. Aquarius had been designed to support two astronauts on the lunar surface for about a day and a half. Now it needed to keep three people alive for roughly four days on the trip home.
Three hours after the explosion, the crew fired the Lunar Module’s descent engine to shift their course onto a “free-return trajectory,” a path that would use the Moon’s gravity to slingshot the spacecraft back toward Earth. They swung around the far side of the Moon on April 15, and two hours after their closest lunar approach, they fired the engine again. This second burn shaved nine hours off the return trip and moved the projected splashdown point closer to the U.S. Navy’s recovery fleet in the Pacific. Two more small correction burns followed on April 16 and 17 to fine-tune the approach angle for reentry.
Cold, Thirst, and Carbon Dioxide
Navigating home was only one part of the problem. Living inside Aquarius for four days pushed the crew to their physical limits.
To conserve the Lunar Module’s limited battery power, almost every non-essential system was shut down, including cabin heaters. Temperatures inside the spacecraft dropped to about 38°F, just six degrees above freezing. The astronauts shivered constantly. Condensation formed on the walls and instruments. Sleep came only in short, fitful snatches, and the crew grew increasingly exhausted as the days wore on.
Water was equally scarce, since it was used both for drinking and for cooling the Lunar Module’s electronics. Lovell put the crew on strict rationing of six ounces of water per person per day, a fraction of normal intake. Dehydration is already a risk in space because astronauts naturally lose their sense of thirst, and the rationing made it worse. By the time they neared Earth, all three men were cold, hungry, dehydrated, and running on almost no sleep.
Carbon dioxide buildup posed the most immediately life-threatening problem inside the cabin. Aquarius used lithium hydroxide canisters to scrub CO2 from the air, but the system was sized for two people over a short lunar stay. With three astronauts breathing inside the module, the canisters couldn’t keep up. The Command Module had plenty of spare canisters, but they were square. The Lunar Module’s receptacles were round. Engineers at Mission Control improvised a solution using only materials available on board: plastic bags, plastic-coated cue cards from a reference binder, hoses pulled from the lunar spacesuits, and generous amounts of grey duct tape. They radioed step-by-step assembly instructions to the crew, who built the adapter and connected it. The contraption, nicknamed “the mailbox,” worked and kept CO2 at survivable levels for the rest of the trip.
Reentry and What the Investigation Found
On April 17, the crew transferred back into the Command Module, powered it up using a carefully planned sequence to avoid draining its reentry batteries, jettisoned the Service Module, and separated from Aquarius. When they finally saw the damaged Service Module floating past their window, the extent of the destruction was shocking: an entire panel was missing from one side.
Odyssey splashed down in the South Pacific on April 17, 1970, with all three astronauts alive.
NASA convened a formal review board to determine what had gone wrong. The investigation traced the failure back through the entire chain: the 1968 tank drop that damaged the drain system, the use of 65-volt ground power on switches rated for 28 volts, the missing test requirement that would have caught the voltage mismatch, and the resulting destruction of the Teflon wire insulation during detanking. Each step was individually minor and individually survivable. Stacked together, they created a catastrophic failure. The findings led to redesigns of the oxygen tank assembly, changes to testing procedures, and the addition of a third, independent oxygen tank on future Apollo spacecraft.