It was lashed together from a cardboard flight-plan cover, two plastic stowage bags, a length of hose pulled out of a spacesuit, a roll of gray duct tape, and a single sock. Mission Control called it “the mailbox,” for the squat shape it made. It is one of the ugliest objects ever flown to the Moon, and for the better part of a day it was the only thing standing between three men and a death they could not see, taste, or feel coming.
The men were Jim Lovell, Jack Swigert, and Fred Haise, the crew of Apollo 13. The thing the mailbox had to do was pull their own breath back out of the air before it killed them.
The radio call that changed the mission
Fifty-five hours and fifty-five minutes into the flight, about 200,000 miles out and still coasting toward the Moon, Swigert keyed his microphone and said, “Okay, Houston, we’ve had a problem here.” Lovell repeated it a moment later, and added the detail that mattered: a Main B Bus undervolt. An oxygen tank in the service module had exploded. The blast vented both tanks to space, and with them went the command module’s electricity, its water, and its air.
The date was April 13, 1970. Apollo 13 was the third mission meant to land men on the Moon. It would not land. Within hours the only question left was whether its three astronauts could be kept alive long enough to fall back to Earth.
The lifeboat that was the wrong size
The command module, Odyssey, was dying. The lunar module bolted to its nose, Aquarius, was not. It had its own oxygen, its own batteries, its own water, all of it untouched because the Moon landing hadn’t happened yet. So the crew crawled through the tunnel into the lander and shut Odyssey down to save what little it had left for the final plunge through the atmosphere.
Aquarius became a lifeboat. The trouble was the arithmetic. The lander had been built to keep two men alive on the lunar surface for about forty-five hours. It now had to keep three men alive for roughly four days. Stretch the oxygen, stretch the water, stretch the power — the engineers in Houston worked all of it. But one number would not stretch, and it had nothing to do with what the crew was running out of. It had to do with what they could not stop making.
Every breath a person takes out is carbon dioxide. Sealed in a small metal can in space, three men exhale enough of it to kill themselves in a matter of hours unless something pulls it back out of the air. Each spacecraft did that with canisters of lithium hydroxide, a chemical that absorbs CO2 as the cabin air is blown through it. Aquarius carried its own supply — sized, like everything else aboard, for two men and a short stay. With three men breathing, those canisters loaded up and saturated fast.
Odyssey had plenty of spare canisters. They were useless. The command module had been built by one contractor and the lunar module by another, and nobody had ever imagined needing to swap filters between the two. Odyssey’s lithium hydroxide cartridges were square. The sockets in Aquarius were round. A square peg, a round hole, and a rising tide of carbon dioxide that the gauges were already starting to show.
The junk-drawer machine
The man who first saw it coming was Ed Smylie, chief of the crew systems division at the Manned Spacecraft Center in Houston — the division responsible for everything that kept an astronaut alive in a sealed suit or a sealed cabin. It was around one in the morning, a few hours after the explosion, when it struck him that the CO2 was going to become the thing that killed them.
Smylie’s team had one hard constraint: they could use only what the crew already had aboard. There was no resupply 200,000 miles up. So they assembled, on a table on the ground, a duplicate of every item known to be on the spacecraft, and set about building an adapter out of the contents of a junk drawer.
This is where the sock and the flight-plan cover come in. The goal was to force the square command-module canister to do the lander’s job. They took the cardboard cover off a flight plan to make a stiff frame. They used plastic stowage bags, a length of hose pulled from one of the spacesuits, and — stuffed into the canister’s own bypass hole so the air couldn’t sneak around the filter instead of through it — a sock. The whole thing was lashed together with gray duct tape. The mailbox was, in the most literal sense, a machine built from a junk drawer to keep three men breathing.
Read up from the ground
None of it would help unless three exhausted men in a cold, dim cabin could build the same thing from the same parts. So the instructions were read up step by step from Houston, the CAPCOM talking the crew through each fold of tape and bag while the astronauts assembled it in orbit around their own slow problem.
It took the better part of an hour. When the mailbox was finished, they taped it over the lander’s air system and turned it on, and the cabin air began to flow through Odyssey’s square canister at last. The carbon dioxide reading, which had been climbing toward the danger line, started to fall. Then it kept falling.
Lovell would later call the fix a fine example of cooperation between the ground and the spacecraft. Smylie, who lived to ninety-five and died in 2025, was more wry about it. “Everybody could understand a filter,” he said.
What the mailbox bought
The mailbox solved only one of the things trying to kill Apollo 13, but it was the one with the shortest fuse. The crew still had to swing around the Moon on a free-return trajectory, fire the lander’s engine to aim themselves home, ration water until Haise came down with a fever, and power up the frozen command module one switch at a time for reentry.
On April 17, 1970, four days after a routine stir of an oxygen tank tore the side off their spacecraft, Odyssey dropped under its parachutes into the South Pacific. All three men climbed out alive. NASA called it a successful failure — the only mission that never reached its destination and is still remembered as a triumph, because the destination changed, in the space of one radio call, to simply getting home. A sock, a flight-plan cover, and a roll of duct tape are part of the reason there was anyone left aboard to bring back.