As Apollo 13 sped toward Earth, mission control was beginning to worry about a new problem. While the lunar module had enough spare oxygen to accommodate Swigert as well as the intended lunar module crew of Lovell and Haise, carbon dioxide was beginning to build up. Normally lithium hydroxide (LiOH) canisters absorbed the gas from the air and prevented it from reaching dangerous levels, but the canisters onboard the Aquarius were being overwhelmed. The Odyssey had more than enough spare LiOH canisters onboard, but these canisters were square and couldn't fit into the holes intended for the lunar modules' round canisters.

Mission control needed a way to put a square peg into a round hole. Fortunately, as with the lunar module activation sequence, somebody was ahead of the game.

As reported in Lost Moon, Lovell's book about the Apollo 13 mission (cowritten by Jeffery Kluger; republished as Apollo 13), Ed Smylie, one of the engineers who developed and tested life support systems for NASA, had recognized that carbon dioxide was going to be a problem as soon as he heard the lunar module was being pressed into service after the explosion.

For two days straight since then, his team had worked on how to jury-rig the Odyssey's canisters to the Aquarius's life support system. Now, using materials known to be available onboard the spacecraft—a sock, a plastic bag, the cover of a flight manual, lots of duct tape, and so on—the crew assembled Smylie's strange contraption and taped it into place. [See photo, Breathing Easy]. Carbon dioxide levels immediately began to fall into the safe range. Mission control had served up another miracle.

Breathing Easy:: To prevent carbon dioxide poisoning, the crew jury-rigged a filter in the lunar module. Astronaut Jack Swigert is on the left.

Although The PC+2 burn had been right on the money, the Trench was increasingly unhappy about the Odyssey and Aquarius's trajectory. Something was pushing the spacecraft off course (afterwards it would be determined that a water vent on the Aquarius had been acting like a little rocket jet, gently sending Apollo 13 in the wrong direction) and they needed another burn to correct the trajectory. But the Trench had given up the navigation system after the PC+2 burn. "We had to come up with some way to align the spacecraft" properly for the corrective burn, Bostick explained to Spectrum.

One of Bostick's controllers, Charles ("Chuck") Dietrich, remembered an alignment technique that had been developed for earlier NASA missions in Earth orbit. A spacecraft could be pointed in the right direction by using a portion of the surface of the Earth as a reference marker—in this case the terminator between night and day.

"All during the Mercury, Gemini and Apollo Earth-orbit programs, that was a technique we had used, but never on a return from the Moon. It was a little more dicey there." In Earth orbit, a small alignment inaccuracy prior to a re-entry would result in the spacecraft landing miles off target, but it usually wasn't life threatening. But if Apollo 13 missed the trajectory it needed to take to re-enter safely—known as the entry corridor—the results would be disastrous as the command module skipped into space or burned up in the atmosphere. "If you missed the entry corridor by a degree, that's a real bad day," says Bostick.

The crew was cold and exhausted by this point—temperatures on board had dropped almost to freezing point. The astronauts had gotten very little sleep since the explosion, and yet they pulled off the course-correction maneuver—and a second one a day later—perfectly.

It Was Now over three days since the explosion in oxygen tank two. It was time to get ready for re-entry. The first step was to recharge the batteries in the command module, which had been significantly depleted before the lunar module came on line.