A Square Filter in a Round Hole
In April 1970, three astronauts were about 320,000 km from Earth, and the air in their spacecraft was slowly turning poisonous. The poison was their own breath: carbon dioxide, the gas every person breathes out.
The spacecraft carried plenty of spare air filters. But the spare filters were square, and the only working air system took round ones.
Make your guess. Engineers in Houston had about a day to solve the filter problem. What did the crew end up using?
- A round filter, rushed up on an emergency rocket
- A square filter, a plastic bag, a hose, a piece of cardboard and tape
- Nothing: the crew held out until the trip home was over
Show answer
B. A square filter, a plastic bag, a hose, a piece of cardboard and tape
Engineers on the ground built a fix from things the crew already had on board, then read the recipe up over the radio, one step at a time. The fix was nicknamed "the mailbox". No rescue rocket was ready to fly, and holding out wasn't an option: without fresh filters, the carbon dioxide would have reached dangerous levels days before Earth.
Start with the fix itself, on the second night, with the gauge climbing.
Build the fix
The crew was living in the lunar module, the small lander meant to touch down on the Moon. The lunar module's round filters were nearly used up, so the carbon dioxide (CO₂) kept rising. The command module next door had plenty of fresh filters, all square.
Your goal: pick the parts in the right order to hook a square filter up to the lunar module's air, before the gauge reaches the emergency limit.
The real crew built the first adapter in about an hour, following instructions read over the radio. Once the square filters were hooked up, the CO₂ fell and stayed low for the rest of the trip.
So a bag and some tape saved three lives. But why were three astronauts living in a lander built for two?
A bang at 56 hours
Apollo 13 launched on April 11, 1970, to land on the Moon. The crew was Jim Lovell, Fred Haise and Jack Swigert. Swigert, the backup pilot, joined the crew two days before launch, after doctors feared the original pilot, Ken Mattingly, might catch German measles.
About 56 hours into the flight, Mission Control asked for a routine stir of the oxygen tanks. Seconds later, the crew heard a loud bang. Swigert radioed, "Okay, Houston, we've had a problem here." (The film line "Houston, we have a problem" is a later rewording.)
One oxygen tank had burst, and the other was leaking into space. That oxygen did two jobs: the crew breathed it, and the ship's fuel cells turned it into electricity. Without oxygen, the command module, named Odyssey, was dying.
The crew moved into the lunar module, named Aquarius, and used the lander as a lifeboat. Aquarius was built to keep two people alive for about two days. Now Aquarius had to keep three people alive for nearly four.
Odyssey still mattered. Only the command module had a heat shield for the plunge back through Earth's air, so the crew shut Odyssey down to save its batteries for the final hour.
A lifeboat still needs a way home, and the Moon lay a day ahead. Should Apollo 13 turn around?
The way home
Turning around sounds fastest. But the ship was racing away from Earth, and stopping takes a big engine.
Your goal: choose a way home, then fire the burns that bring the crew down near the waiting ships.
The Moon did most of the work. A path that swings around the Moon and falls back to Earth with no big burns is called a free return.
The trip home would take nearly four days. Could the lifeboat's batteries last that long?
Ration the power
Aquarius ran on batteries, and nothing on board could recharge them. With every system switched on, the batteries would run flat long before Earth.
Your goal: switch systems off until the batteries last the whole trip home.
Mission Control ran Aquarius on about a third of its usual power. Less power also meant less heat, so the crew used less of the water that cooled the electronics.
The price was cold. Inside the dark command module, the temperature fell to about 3 °C (38 °F), the walls dripped and the windows frosted over. The crew slept little, in turns.
Power and water would now last. But every breath still added CO₂ to the cabin. Why does a gas we breathe out all day become deadly in a closed room?
Why breath turns toxic
Your body burns food with oxygen and breathes out the waste as CO₂. Outdoors, the CO₂ blows away, and the air stays at about 0.04% CO₂. In a sealed spacecraft, the CO₂ has nowhere to go.
About how much CO₂ does one person breathe out in a day?
- About 10 grams, a spoonful
- About 1 kilogram, a bag of sugar
- About 10 kilograms, a packed suitcase
Show answer
B. About 1 kilogram, a bag of sugar
Roughly 1 kg a day, a bit more on a busy day. The carbon in that CO₂ comes from the food you eat. So three people in a small cabin add about 3 kg of CO₂ every day.
As CO₂ builds up, people breathe faster and get headaches, then grow confused and drowsy. So every crewed spacecraft carries a CO₂ scrubber: a filter that grabs CO₂ out of the air.
Apollo's scrubbers were cans of lithium hydroxide (LiOH), a white chemical that reacts with CO₂ and locks the CO₂ into a new solid:
Once all the lithium hydroxide in a can has reacted, what happens?
- The can keeps working, only more slowly
- The can is used up, and CO₂ passes straight through
- The can lets go of the CO₂ it caught
Show answer
B. The can is used up, and CO₂ passes straight through
The reaction uses up the lithium hydroxide, so each can catches a fixed amount of CO₂ and then must be swapped. Three people breathe out CO₂ faster than two, so the lunar module's cans ran out early: NASA's 1970 account says the cans would last only about 50 hours, far short of the trip home.
Flight doctors watched a CO₂ gauge. The usual rule was to swap cans at 7.6 on the gauge. In the emergency, the doctors raised the limit to 15.
Meanwhile, the command module's square cans sat unused, full of fresh lithium hydroxide. How do you hook a square can up to a round socket, with nothing but what is on board?
Houston's recipe
In Houston, Ed Smylie and Jim Correale of NASA's Crew Systems Division worked out a design using only items the crew had on board. Astronaut Tony England then built one from spoken instructions alone, to check that the recipe would work over the radio.
About a day and a half after the explosion, astronaut Joe Kerwin read the recipe up from Mission Control, and Swigert and Lovell built the adapter in the lunar module.
An animation of the finished adapter. Cabin air rises through the square lithium hydroxide can, up the suit hose and through the lunar module's fans, while the carbon dioxide molecules stick inside the can. The animation plays by itself in a loop. Each time a puff of cabin air reaches the can, the motion slows down for a moment, so you can watch the carbon dioxide molecules stick to the lithium hydroxide while the air molecules keep going.
Text version
On the left sits the box that holds the lunar module's fans. On the right, a square can of lithium hydroxide stands inside a plastic bag, with a card bent into an arch over the top of the can to hold the bag up. A suit hose runs from a hole in the top of the bag over to the fans. Cabin air, drawn as pairs of grey dots for oxygen and nitrogen, rises from below into the bottom of the can, with a few carbon dioxide molecules, drawn as rows of three blue dots, mixed in. Inside the can, each carbon dioxide molecule stops and sticks to the lithium hydroxide, and a small ring flashes where it is caught. The air molecules carry on out of the top of the can, along the hose and through the spinning fans, back into the cabin. The caught carbon dioxide stays in the can for a while and then fades out.
Notice the trick: the square can never goes into the round socket. The bag and the hose carry clean air from the square can to the lunar module's air loop, and the fans do the rest.
How it really works: the recipe and the numbers
- The parts. Two square cans from the command module, the roll of grey tape, two plastic bags that had held the crew's cooling underwear, a card from the lunar checklists, and the hoses from the spacesuits.
- Tape belts. Wrap two belts of tape around the can, sticky side out, one near each end. Run strips of tape across the top and bottom to anchor the next parts.
- The arch. Bend the card into an arch over the top of the can and tape the card down, so suction can't pull the bag flat against the can.
- Plug the leak. Plug a small bypass hole in the bottom of the can (the recipe suggested a wet wipe, a piece of sock or crumpled tape) and tape over the plug.
- The bag. Pull a bag over the can, press the bag onto the sticky belts, fold away the slack and tape the bag tight.
- The hose. Cut a small hole in the bag, push a suit hose in, and tape the hose snug. The suit hose leads to the lunar module's fans.
Accounts differ in small details; this order follows the radio transcript in NASA's Apollo Flight Journal.
How much lithium hydroxide? One mole of LiOH weighs about 24 g, and one mole of CO₂ about 44 g. The reaction takes two moles of LiOH (about 48 g) for each mole of CO₂ (44 g), so
\[ \frac{44\text{ g CO}_2}{48\text{ g LiOH}} \approx 0.92 \]A kilogram of lithium hydroxide can catch at most about 0.92 kg of CO₂. Three people breathe out roughly 3 kg of CO₂ a day, so the crew needed well over 3 kg of fresh lithium hydroxide every day, since no can gets used up perfectly.
Why lithium? Lithium is the lightest metal, so lithium hydroxide catches more CO₂ per kilogram launched than similar chemicals, such as the soda lime in hospital breathing machines.
What the gauge read. The gauge showed the CO₂'s partial pressure, in millimetres of mercury (mmHg): the share of the cabin's pressure that comes from CO₂. Earth's air has about 0.3 mmHg of CO₂. The usual limit, 7.6 mmHg, is about 1% of sea-level air pressure, and the emergency limit, 15 mmHg, about 2%. NASA's mission reports say the CO₂ stayed above 7.6 mmHg for about 4 hours, and stayed well below 1 mmHg once the square cans were in use.
The adapter that flew burned up with Aquarius, but museums show copies built on the ground. Space Center Houston gives a close look at one in this short video:
The air was safe. Left was the hardest part: waking up a frozen command module and falling back into Earth's air.
Splashdown
Flight controllers and astronauts in the simulators wrote a brand-new checklist for waking Odyssey, and Mission Control read the checklist up over hours. Power from Aquarius topped up Odyssey's re-entry batteries, the reverse of the normal plan.
Near the end, the crew let go of the service module and saw the damage for the first time: a whole side panel gone. Then the crew let go of Aquarius. "Farewell, Aquarius, and we thank you," Mission Control radioed.
On April 17, 1970, Odyssey splashed down in the Pacific near Samoa, almost 143 hours after launch. Apollo 13 never landed on the Moon, but all three astronauts came home.
One question was left for the investigators: why did a tank explode on a routine stir? Make a guess before you open the answer.
What blew up the oxygen tank?
A chain of small mistakes. In 1968, the shelf holding the tank was dropped about 5 cm (2 inches) at the factory, which may have loosened a tube inside the tank. Before launch, the tank wouldn't empty normally, so workers boiled the oxygen off with the tank's heaters for about 8 hours.
The heaters had safety switches meant to cut the power if the tank got too hot. The switches were built for the spacecraft's 28 volts, but the ground equipment supplied 65 volts, and the switches welded shut. Parts of the tank may have reached about 540 °C (1,000 °F), which damaged the plastic insulation on the wires to the tank's stirring fans.
In flight, the stir switched on the fans. The damaged wires sparked, the insulation caught fire in the pure oxygen, and the tank burst. After Apollo 13, NASA took the fans out of the tanks, added a third oxygen tank and added a spare battery.
Apollo 13 flew more than 50 years ago. Do spacecraft still clean their air with cans of lithium hydroxide?
Where it shows up
- The space station. The International Space Station traps CO₂ in beds of zeolite, a mineral full of tiny pores. Heating the beds and opening them to space drives the CO₂ out, so the beds work again and again. Lithium hydroxide cans still fly as a backup.
- Water from breath. On the space station, a Sabatier reactor combines some of the captured CO₂ with hydrogen to make water, which saves water launched from Earth.
- Orion and Artemis II. NASA's Orion capsule uses an amine swing bed: two beds take turns, one soaking up CO₂ while the other dumps its CO₂ into space. In April 2026, the Artemis II crew flew Orion on a free return around the Moon and broke Apollo 13's record for the farthest any humans have travelled from Earth.
- Submarines. A submarine can stay underwater for months, so a submarine scrubs CO₂ too, often with amine scrubbers, and carries lithium hydroxide for emergencies.
- Divers and hospitals. A diver's rebreather and a hospital's anaesthesia machine both send breath around a loop, through a can of soda lime that catches the CO₂.
The mailbox, explained
Now you can explain the mailbox. People breathe out CO₂, and a sealed cabin keeps all of it, so a spacecraft needs a chemical that grabs CO₂: on Apollo, lithium hydroxide. Three people in a lander built for two used up the round cans early. The fix never forced the square can into the round socket. A bag, a card arch, tape and a hose let the lunar module's fans pull air through the square can instead.
The rest of the rescue ran on the same idea: use what you already have. The Moon's gravity turned the ship around for free, and switching off almost everything stretched the batteries across four days.
Where to go next
Apollo 13 opens the door to life support, orbits and engineering under pressure. Here are the questions this page opens up:
- How does the space station make fresh oxygen from water?
- Why does a free-return path around the Moon bring a ship back to Earth on its own?
- What does too much CO₂ do inside the body, and why does CO₂ make you breathe faster?
- How does a submarine stay underwater for months without running out of air?
- What would a crew on a two-year trip to Mars use to clean its air?
Ask Phy
- Walk me through how the Apollo 13 CO₂ adapter moved air, from the cabin to the fans.
- Balance the lithium hydroxide reaction with me, then work out how many kilograms a crew of three needs for a week.
- Explain a free-return trajectory around the Moon, first without equations, then with them.
- Apollo 13: "Houston, we've got a problem" (NASA EP-76, 1970) — NASA's account of the flight: the routes home, the 50-hour filter limit, the cold.
- Apollo 13 Flight Journal: Building the CO2 adapter (NASA) — The radio transcript of the recipe, step by step.
- Report of Apollo 13 Review Board (NASA, 1970) — The cause of the explosion, and the lunar module's batteries and design limits.
- MSC Apollo 13 investigation team, Panel 3: Flight operations (NASA) — The CO2 limits, the power budget and the burns, hour by hour.
- Conserving the creativity that saved the Apollo 13 astronauts (Smithsonian National Air and Space Museum)
- NASA's Artemis II crew eclipses record for farthest human spaceflight (NASA, 2026)