Target Pillar: Key Milestones in the History of Space Exploration

Last Updated: August 13, 2026 Author: Sarah Mitchell

Introduction

The root cause of one of spaceflight’s most famous near-disasters traces back five years before the mission even launched, to a single, minor-seeming engineering decision. In 1965, NASA upgraded the voltage rating for the heaters inside the Apollo command module’s oxygen tanks from 28 volts to 65 volts — but the thermostatic switches controlling those heaters on at least one tank were never updated to match.[^1]

That mismatch sat dormant for years until April 13, 1970, when it caused an explosion 200,000 miles from Earth that forced NASA to abandon a Moon landing and improvise, in real time, a way to bring three astronauts home alive. NASA itself has always described the outcome with a term now inseparable from the mission: a “successful failure.”[^2]

A Routine Mission, Until It Wasn’t

Apollo 13 launched April 11, 1970, NASA’s third crewed lunar landing attempt, targeting the Fra Mauro highlands with Commander Jim Lovell, Command Module Pilot Jack Swigert, and Lunar Module Pilot Fred Haise aboard.[^3] Swigert’s presence on the crew was itself a last-minute change — original CMP Ken Mattingly was pulled from the mission just days before launch after exposure to German measles left him without immunity, and backup pilot Swigert took his place.[^4]

The mission’s first two days were, by NASA’s own account, unusually smooth — smoother than any prior Apollo flight. At 46 hours, 43 minutes into the mission, Capcom Joe Kerwin radioed the crew: “The spacecraft is in real good shape as far as we are concerned. We’re bored to tears down here.”[^1] It would be the last time anyone involved used the word “boredom” for a long while.

What Actually Exploded, and Why

At roughly 56 hours into the mission — the evening of April 13 — Mission Control asked the crew to perform a routine “cryo stir,” activating fans inside the oxygen tanks to prevent the supercooled liquid oxygen inside from stratifying into unusable layers.[^5] Because of the years-old voltage mismatch, when the fan motor inside oxygen tank No. 2 switched on, damaged wiring insulation inside the tank sparked and ignited the oxygen around it.[^1]

The resulting explosion over-pressurized the tank until it ruptured, blowing an entire panel off the side of the service module and damaging the adjacent oxygen tank No. 1 badly enough to start a slow leak of its own.[^6][^1] With both tanks compromised, the spacecraft’s main oxygen supply, electrical power, and water production — the fuel cells that generated all three depended on that same oxygen — began failing simultaneously.[^7] Commander Lovell’s transmission to Mission Control, condensed in popular memory to “Houston, we have a problem,” set off one of the most consequential improvisation efforts in NASA’s history.[^8]

The Lunar Module Becomes a Lifeboat

With the command module Odyssey’s power and oxygen draining fast, the crew and Mission Control made a decision that had been discussed as a theoretical contingency before but never used in practice: powering down the Command Module almost entirely and moving into the Lunar Module Aquarius, using it as a survival lifeboat for the return trip rather than its intended purpose of landing on the Moon.[^7][^9] The Command Module’s own remaining battery and oxygen reserves were preserved untouched, specifically so they’d be available for the final hours of reentry, when the crew would need to move back into Odyssey — the only module equipped with a heat shield capable of surviving atmospheric reentry.[^9]

Aquarius was built to sustain two astronauts for roughly two days on the lunar surface — not three astronauts for the four-plus days a return trip around the Moon and back to Earth would now require.[^6] Nearly every resource on board was stretched or rationed accordingly: power, water, and breathable air all had to last far longer than the module was ever designed to provide.

The Carbon Dioxide Problem and Its Duct-Tape Solution

One of the mission’s more urgent improvised fixes involved carbon dioxide, not oxygen. Aquarius’s onboard lithium hydroxide canisters — the filters designed to scrub CO2 from the air — were sized for two people, and three astronauts breathing inside the cramped module quickly began producing more carbon dioxide than the lunar module’s filters could handle.[^6] The command module carried compatible replacement canisters, but they were the wrong physical shape to fit Aquarius’s square receptacles.[^10]

Engineers on the ground worked out a fix using only materials the crew already had on board: plastic bags, cardboard, and duct tape, assembled into an adapter — half-jokingly nicknamed “the mailbox” by the crew — that let the square Command Module canisters connect to the lunar module’s round air-scrubbing system.[^10][^6] It’s a genuinely representative example of the broader Apollo 13 rescue effort: not a single dramatic breakthrough, but a series of resourceful, occasionally improvised fixes built from a strictly limited inventory of onboard materials.

Getting Home: A Free Ride Around the Moon

Rather than attempting a dangerous direct turnaround, Mission Control opted to let the crippled spacecraft continue on its existing trajectory around the far side of the Moon, using lunar gravity to slingshot Apollo 13 back toward Earth — a free return trajectory that required only a modest engine burn to properly target reentry, minimizing the demand on the lunar module’s limited engine and power reserves.[^11] The crew endured the return trip in genuinely harsh conditions: cabin temperatures dropped close to freezing with the Command Module powered down, potable water was severely rationed, and the crew lost significant body weight and endured real physical hardship for the several remaining days of the journey.[^2]

Despite all of this, the crew successfully splashed down safely on April 17, 1970 — four days after the explosion.[^3] NASA’s official review board later concluded the direct technical cause was exactly the damaged, unrated wiring insulation inside oxygen tank No. 2, sparked by the routine fan activation.[^12] As a direct engineering response, later Apollo spacecraft were modified to include a third, isolated oxygen tank along with additional emergency battery capacity — a redundancy specifically designed to prevent a single point of failure like this from ever threatening a crew again.[^4]

Why “Successful Failure” Is the Accurate Description

The phrase isn’t spin — it’s a precise summary of what actually happened. As a lunar landing mission, Apollo 13 unambiguously failed; the crew never reached the lunar surface, and the mission’s core scientific objectives at Fra Mauro went unmet.[^1] But as a test of NASA’s ability to diagnose a genuine life-threatening emergency in real time, adapt hardware never designed for its improvised purpose, and bring a crew home alive from nearly a quarter-million miles away, it succeeded completely — and NASA has treated the mission’s real legacy accordingly, as a demonstration of engineering resourcefulness and crisis management at least as valuable as a successful landing would have been.[^2]

Frequently Asked Questions

Sources

1. NASA — Apollo 13: Mission Details
2. NASA — A Successful Failure (Mission Control, Houston, April 13, 1970)
3. History.com — Apollo 13 Oxygen Tank Explodes, April 13, 1970
4. Lunar and Planetary Institute — Apollo 13 Mission
5. NASA Technical Reports Server — A Case Study of the Failure on Apollo 13
6. Smithsonian National Air and Space Museum — Apollo 13
7. History.com — Apollo 13 Oxygen Tank Explodes (Lovell transmission)
8. Noiser History Podcasts — The Incredible Story of the Apollo 13 Disaster
9. NASA — Mission Control, Houston, April 13, 1970 (Command Module power preservation)
10. Noiser History Podcasts — The Incredible Story of the Apollo 13 Disaster (“mailbox” CO2 fix)
11. Britannica — Apollo 13
12. The Planetary Society — Apollo 13

Note on methodology: technical and mission details above are drawn from NASA’s official mission pages, NASA’s technical accident review documentation, and Smithsonian archival sources. This is a well-documented historical event with consistent figures across primary NASA sources as of mid-2026.