It appears that a heater and stirrer in the oxygen tank did not turn off correctly creating explosive pressures leading to the accident. Fortunately it exploded out into space rather than into the spaceship but it meant the astronauts were now in a critical situation. The crew moved into the Lunar Module (LM) and used these resources as a 'lifeboat'. Without this option they would almost certainly have died.
After considering all the possible return paths they fired the available engines to get the correct trajectory - it would take four days to get home. The LM was designed for two people for two days not three people for a four day trip home, so they had very limited electrical power, heating and drinking water. They had enough oxygen for the trip but the critical issue was carbon dioxide.
Normally about 0.04 % of air is CO2. As it rises it causes our respiratory rate to increase but high levels can lead to headaches, confusion and eventually loss of consciousness . About 3% of expired air is CO2, so in a small space such as the LM, the levels will quickly rise. The LM used cylindrical canisters of Lithium hydroxide (anhydrous) in the air circulating system to remove carbon dioxide (lithium carbonate is produced):
2 LiOH + CO2 → Li2CO3 + H2O
However, they did not have enough canisters to support the crew for four days. The CM had adequate supply but unfortunately these units didn't fit the equipment in the LM, they were the wrong shape. The crew had to 'lash-up' a device to solve this problem using a space suit hose, cardboard, tape and the extra canisters. After a nail biting blackout period as the lunar module re-entered the Earth's atmosphere, the LM and her crew splashed-down safely in the Pacific. Despite all the complex trajectory calculations, engine firing and computer 'reboots' on the way home, the crew ultimately survived due to the little canisters of LiOH.
References and Links
 Apollo 13, Columbia, 1995.
 Life at the Extremes, Frances Ashcroft, Flamingo, 2001
How teachers can use these articles in a lesson
Why Hollywood Science
Open University Hollywood Science web site
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