It was one of the cryogenic tanks that would reveal itself as the Odyssey's Achilles' heel. On 13 April 1970, around 9 p.m. Houston time, almost 56 hours into Apollo 13's flight, mission control [see photo, Calm Before The Storm] asked the crew to turn on fans in all the cryogenic tanks to stir the contents in order to get accurate quantity readings. Due to a series of pre-launch mishaps, turning on the fan sparked a short circuit between exposed wires within oxygen tank two [see sidebar, "The Devil's in the Details"].

Calm Before The Storm:: Mission control a few minutes before the explosion that would cripple Apollo 13's spacecraft. The back of flight director Gene Kranz can be seen in the foreground, while astronaut Fred Haise appears on the wall screen during a television broadcast.

The Odyssey was dying, but no one knew it yet.

Even The Crew were unaware of the gravity of the situation. In the Ron Howard movie, the oxygen tank two explosion is accompanied by a whole series of bangs and creaks while the astronauts are tossed around like ping-pong balls. But in real life, "there was a dull but definite bang—not much of a vibration though...just a noise," said Apollo's 13's commander, Lovell, afterward. Then the Odyssey's caution and warning lights lit up like a Christmas tree.

On the ground, mission control was initially unperturbed. During the cryogenic tank stir, the flight controller in charge of the fuel cells and the tanks, Sy Liebergot, had his attention focused on oxygen tank one. Liebergot was an EECOM, a job title that dated back to the Mercury program days of the early 1960s. It originally meant the person was responsible for all Electrical, Environmental, and COMunications systems onboard the CSM. The communications responsibilities had recently been split out of the EECOM's job, but the name remained.

In an unfortunate coincidence, oxygen tank two's quantity sensor had failed earlier, but the two tanks were interconnected, so Liebergot was watching the quantity that tank one reported to get an idea what was in tank two.

As he sat in mission control at his console, with its mosaic of push buttons and black-and-white computer displays, Liebergot wasn't alone in tending to the Odyssey's electronic and life support systems. He was in voice contact with three other controllers in a staff support room across the hall. Each flight controller in mission control was connected via so-called voice loops—pre-established audio-conferencing channels—to a number of supporting specialists in back rooms who watched over one subsystem or another and who sat at similar consoles to those in mission control.

Liebergot's wingmen that day were Dick Brown, a power-systems specialist, and George Bliss and Larry Sheaks, both life support specialists. As the pressure rapidly rose in oxygen tank two and then abruptly fell within seconds, their eyes were fixed on the other cryogenic tank readouts, and they all missed the signs that tank two had just exploded.

Suddenly The Radio Link from the crew crackled to life. "Okay Houston, we've had a problem here," reported command module pilot Swigert as he surveyed the Odyssey's instruments. "Houston, we've had a problem," repeated Lovell a few seconds later, adding that the voltage of one of the two main power-distribution circuits, or buses, that powered the spacecraft's systems, was too low. But a few seconds later the voltage righted itself, so the crew began chasing down what seemed to be the big problems: the jolt of the explosion had caused their computer to reset and had knocked a number of valves closed in the attitude-control system that kept the Odyssey pointed in the right direction.

In mission control though, things weren't adding up. The spacecraft's high-gain directional antenna had stopped transmitting, and the Odyssey had automatically fallen back to its low-gain omnidirectional antennas. Liebergot and his team were seeing a lot of screwy data, dozens of measurements out of whack. Fuel cells one and three had lost pressure, and were no longer supplying current, leaving only fuel cell two to pick up the load; oxygen tank two's pressure was reading zero; the pressure in oxygen tank one was rapidly failing; and Odyssey had completely lost one of its electrical distribution buses along with all the equipment powered by it. The crew connected one of their re-entry batteries to the remaining bus in a bid to keep the command module's systems up and running.