Thirty-five years ago this evening, a spherical oxygen tank in the Apollo 13 spacecraft exploded, with a loud bang that rocked the spacecraft.

Flight controllers on the ground didn’t hear the bang, but they noticed sudden changes in numbers displayed on the computer terminals where they monitored the many systems that made up the spacecraft.

The controllers talk to each other with headsets wired into a communications channel called the loop. Many of the controllers are listening to two loops: the main flight controller loop, and a second loop connecting specialists in a particular mission sub-system. Thirty-five years ago tonight, the real action was on the EECOM loop, where the experts monitoring the environmental and electrical systems of the spacecraft’s command and service modules worked to understand the most serious problem America had yet faced in space.

Astronaut Swigert: Okay, Houston, we’ve had a problem here.

Unidentified voice #1: What’s the matter with the data, EECOM?

Unidentified voice #2: We’ve got more than a problem.

EECOM: Okay, listen, listen, you guys. We’ve lost fuel cell one and two pressure.

Unidentified voice #2: We lost O₂ tank two pressure. And temperature.

Astronaut Lovell: Uh, Houston, we’ve had a problem.

EECOM: Okay.

Unidentified voice #2: Standby, they’ve got a problem.

Astronaut Lovell: Main B bus undervolt.

Capcom: Roger, main B undervolt.

Later, EECOM Sy Liebergot realized, with dread, exactly what had happened:

Unidentified voice #3: I want to psych out what those fuel cells are doing here. We might have a pressure problem in the fuel cells, it looks like.

EECOM: Yeah, I see the N₂…

Unidentified voice #3: Two fuel cells simultaneously.

EECOM: That can’t be.

Unidentified voice #3: I can’t believe that, right off the bat, but—but they’re not feeding current.

EECOM: Yeah, if you believe that N₂ pressure, we blew a sphere.

Apollo 13’s lunar landing mission was doomed from the moment the oxygen tank exploded. Through the heroic efforts of the astronauts and the ground crew, the three astronauts were returned safely to earth.

For years, NASA considered the Apollo 13 mission a failure, and tried to sweep it under the rug. It took years to understand that Apollo 13 was not a failure. It was the most severe test imaginable of the people and the processes of the Apollo program; they passed the test. The other Apollo missions were engineering triumphs. Apollo 13 was more than that. The safe splashdown of the Apollo 13 command module was the greatest moment of the entire Apollo program.

Years later, Apollo 13 commander Jim Lovell wrote a book about the experience, called Lost Moon. Hollywood turned the book into the blockbuster movie Apollo 13, and Lovell’s book was renamed Apollo 13 to take advantage of the publicity. The book is well worth reading. (The movie is entertaining, but overly melodramatic.)

The best book about Apollo 13 is 13: The Flight That Failed, by Henry S. F. Cooper. I think I’ve read it five times. It’s never taken me much more than a day to read it, because I just can’t put it down once I start reading. If you have any interest in the Apollo program, Cooper’s book is the best place to start reading.