For Ken Mattingly, the last couple of weeks had been an emotional roller coaster. He'd been through the anguish of being bumped from a moon flight, because the doctors were worried he'd come down with German Measles. (He still hadn't.)

Mattingly had been in Mission Control when Jim Lovell and Fred Haise went off to the moon without him, taking Jack Swigert in his place. He'd followed the progress of the mission, in the midst of the worst depression of his life.

And then, he'd witnessed Apollo 13's sudden transformation into a life-or-death drama for his crew mates. Since then, Mattingly had joined hundreds of other astronauts, flight controllers and engineers in the effort to bring Apollo 13 home.

In Mission Control, Deke Slayton displays the makeshift air purifier to be constructed by the astronauts.

Then there was the intensive, multi-dimensional effort going on in the back rooms of Mission Control. One team studied ways to conserve power aboard the lunar module Aquarius. Another was busy trying to figure out the checklist for reviving the command module Odyssey before the reentry into the Earth's atmosphere on Friday, April 17. Still others grappled with a host of exotic problems.

The most pressing issue, on the morning of April 15, was the disturbing fact that the atmosphere inside Apollo 13 was becoming dangerously overloaded with carbon dioxide. That was a consequence of using the lunar lander as a lifeboat: it harbored three men instead of two, all of whom were exhaling carbon dioxide (CO2). There were chemical "scrubbers" that used lithium hydroxide to remove CO2, but Aquarius' supply of the special canisters was running out.

There were more of the purifiers in Odyssey, but they were the wrong shape to be used in Aquarius' environmental control system. It was up to Mission Control to figure out how to use them in the lander. Otherwise, Lovell and his crew would suffocate from their own breath.

By Wednesday morning, one of the many "tiger teams" in the back rooms had come up with a fix. Using nothing but materials aboard the spacecraft -- cardboard, a plastic cover from a flight plan, a hose from one of the spacesuits, and even a sock -- they had figured out how to adapt Odyssey's air scrubbers for use in Aquarius.

The air purifier as made by the astronauts, in place in the Apollo 13 lunar module.

In Mission Control, the instructions for building the contraption -- it ended up looking something like a mailbox -- were radioed to Apollo 13 by Capcom Joe Kerwin, and the astronauts went to work building it. When they were finished, their handiwork looked exactly like the one built by the engineers in Houston. Once it was hooked up to Aquarius environmental control system, the carbon dioxide problem was solved.

The "mailbox" fix typified the ingenuity being brought to bear on every problem facing Apollo 13. What amazed Ken Mattingly, as he thought about it, was how many of the solutions had been dreamed up in "what-if" planning for previous missions. For example, the Apollo 9 mission in 1969 had tested the procedures for firing the lunar module's descent engine while it was still docked to the command module.

And on Wednesday night, Lovell and Haise were ready to try yet another fix dreamed up by the back room teams -- this one for a "midcourse correction" rocket firing. With the computer and the guidance system turned off, the astronauts needed another way to orient the spacecraft for the burn. And there was a way: Lovell would use the crescent Earth as a reference, lining up the two points of the crescent with the crosshairs in a special sighting device.

Lovell's reaction wasn't exactly enthusiastic when he heard the plan. "I hope the guys in the back room knew what they were doing," he told Houston. But when Lovell thought about it, he realized he'd heard the scheme before: It had been dreamed up during a "what-if" session before his first moon mission, Apollo 8.

When the time came, and Lovell had the "horns" of Earth sitting on the crosshairs, he and Haise took the controls for a brief firing of the descent engine. With Swigert eyeing a stopwatch, the men gave a 14-second blast, then shut the engine off. Lovell wondered if the unorthodox procedure had worked to put Apollo 13 back on course. When Jack Lousma in Mission Control radioed, "Nice work," Lovell could only say in response, "Let's hope it was."