The number 13 had a bad reputation long before 1970, but it only got worse on April 13 of that year. Just eight months earlier, Neil Armstrong set foot on the moon. Apollo 12 had a successful mission to the moon in November. But Apollo 13 had that number. Lift off two days earlier was at 1:13 PM, or 13:13 in military time. Apollo 13 commander Jim Lovell noticed the coincidence of the time, but he is not a superstitious man. Besides, lift-off was on April 11th. He, Jack Swigert, and Fred Haise were on their way to the moon to explore the Fra Mauro highlands and bring back moon samples.

On the 13th, two days into the flight, there was a loud bang. Lovell and Swigert looked at Haise, who had startled the crew members before by actuating a lunar module relief valve. But this was no prank. A liquid oxygen tank had exploded, which blew out the side of the service module. Power went down. Oxygen leaked out.

Swigert and Lovell radioed Mission Control with those understated words, "Houston, we've had a problem." The crew at Mission Control who had complained of boredom over the previous two days would never do so again. The original mission was aborted; the new mission was to get the three crewmen back to earth alive. Flight director Gene Kranz and his team hunkered down to figure out how to stretch the spaceship's available resources.

To conserve energy, all but essential systems were shut down. The astronauts moved into the lunar module, which had enough resources for two days for two men. Now, those resources would have to last four days for three men. There was backup oxygen on board, but power and water had to be conserved. And carbon dioxide buildup would be a problem.

Mission Control worked on the filter problem. The canisters of lithium hydroxide used to filter carbon dioxide in the lunar module were a different size from the available canisters in the command module. On directions from Houston, the crew rigged up a system using duct tape and hosing, to adapt the command module's canisters for lunar module use. The contraption they built was called the "mailbox".

Also to conserve energy, on board television cameras were turned off, and NASA used miniatures and animations to keep the public informed of Apollo 13's progress.

With critically low resources, there was no way the crew could land on the moon, but they continued on the trajectory to the moon in order to pass around it and gain momentum from the moon's gravity to coast back to earth. This picture was taken from Apollo 13's lunar flyby.

On approach to earth, the crew jettisoned the damaged service module, then moved into the command module and jettisoned their "lifeboat", the lunar module. They splashed down in the Pacific on April 17th, four days after the tank explosion.

An investigation into what went wrong with Apollo 13 found that a heating wire inside the liquid oxygen tank had lost its insulation, which led to the explosion. The cause was a combination of mistakes in design, testing, and implementation. The report is available online. The entire Apollo 13 operations team was awarded the Presidential Medal of Freedom as a group.

Mission number 13 may have been unlucky to have experienced the explosion, but an entire series of lucky breaks and intelligent decisions contributed to the crew's safe return. Lovell wrote a book about the mission, Lost Moon in 1994.Â It became a best seller. Ron Howard made the book into the 1995 movie Apollo 13.

Tom Hanks was nervous about his role as commander Jim Lovell, because an actorÂ rarely gets the chance to play one of his childhood heroes. But the movie introduced a new generation to the real-life suspense of the Apollo 13 mission.

Jim Lovell and Fred Haise got together yesterday with Gene Krantz and other members of the Apollo 13 team at the Adler Planetarium in Chicago to mark the 40th anniversary of the flight. Jack Swigert died in 1982. Jim Lovell, now 82, owns a restaurant in Lake Forest, Illinois. Fred Haise, now 76, is building a science museum in Mississippi. See a video of the two retired astronauts speaking about the mission.

For more of the story, follow the highlighted links in this overview. Almost all images courtesy of NASA.