13 April 2005—"Houston, we've had a problem."

Thirty-five years ago today, these words marked the start of a crisis that nearly killed three astronauts in outer space. In the four days that followed, the world was transfixed as the crew of Apollo 13—Jim Lovell, Fred Haise, and Jack Swigert—fought cold, fatigue, and uncertainty to bring their crippled spacecraft home.

But the crew had an angel on their shoulders—in fact thousands of them—in the form of the flight controllers of NASA's mission control and supporting engineers scattered across the United States.

To the outsider, it looked like a stream of engineering miracles was being pulled out of some magician's hat as mission control identified, diagnosed, and worked around life-threatening problem after life-threatening problem on the long road back to Earth.

From the navigation of a badly damaged spacecraft to impending carbon dioxide poisoning, NASA's ground team worked around the clock to give the Apollo 13 astronauts a fighting chance. But what was going on behind the doors of the Manned Spacecraft Center in Houston—now Lyndon B. Johnson Space Center—wasn't a trick, or even a case of engineers on an incredible lucky streak. It was the manifestation of years of training, teamwork, discipline, and foresight that to this day serves as a perfect example of how to do high-risk endeavors right.

Many people are familiar with Apollo 13, thanks to the 1995 Ron Howard movie of the same name. But as Howard himself was quick to point out when the movie was released, it is a dramatization, not a documentary, and many of the elements that mark the difference between Hollywood and real life are omitted or altered. For this 35th anniversary of Apollo 13, IEEE Spectrum spoke to some of the key figures in mission control to get the real story of how they saved the day.

First, A Little Refresher on moon-shot hardware: a powerful, 85-meter tall, three-stage Saturn V booster launched each mission from Cape Canaveral in Florida [see photo, To The Moon]. Atop the Saturn V rode the Apollo stack, which was composed of two spacecraft: a three-person mother ship to go to the moon and back, called the command and service module, or CSM; and a two-person lander, called the lunar module, or LM, to travel between the CSM and the surface of the moon.

To The Moon:: The Apollo 13 Saturn V prior to launch on April 11, 1970

The two spacecraft each, in turn, had two parts. The CSM divided into a cylindrical service module (SM) and a conical command module (CM). The service module housed the main engine and supplied all the oxygen, electricity, and water the crew needed for the long voyage—it took about six days for a round trip between the Earth and the moon. The crew lived in the cramped command module, which housed the flight computer and navigation equipment. The command module was the only part of the Apollo stack that was designed to come back safely to Earth. It would plummet through the atmosphere, the blunt end of its cone designed to withstand the immense heat generated by the descent, and then deploy parachutes and splash down in the ocean.

The lunar module consisted of an ascent stage and a descent stage. The descent stage had a powerful engine used to land the lunar module on the moon. After the lunar expedition was complete, it served as a launch pad for the ascent stage, which housed the astronauts, to blast off and rendezvous with the command and service module in lunar orbit.

For most of the way to the moon, the command and service module and the lunar module—dubbed the Odyssey and Aquarius, respectively, on the Apollo 13 mission—were docked nose to nose. But the astronauts generally remained in the command module, because the lunar module was turned off to preserve power.

Most of that power came from a cluster of three fuel cells in the service module. The fuel cells were fed hydrogen and oxygen from two pairs of cryogenic tanks, combining them to produce electricity and water [see diagram, Module Map].

Module Map:: A cutaway diagram of the service module. The fuel cells, in green, provided water and electricity by combining oxygen and hydrogen stored in cryogenic tanks, marked in red and blue respectively. Oxygen tank 2, bright red, exploded during the Apollo 13 mission, almost killing the crew.

There were some batteries on board the command module, but these were intended for only a few hours use during re-entry, after the service module was jettisoned close to Earth.