WALKING ON THE MOON
The most famous extravehicular activities of all were the moonwalks carried out by the Apollo astronauts between 1969 and 1972. These were longer than many people realise. On the final mission, Apollo 17, the astronauts spent a total of 22 hours in spacesuits outside the Lunar Module in the course of three separate EVAS. This presented a major challenge in terms of life support, with each astronaut completely reliant on the Portable Life Support System (PLSS) in their backpacks.
Unlike modern spacecraft, the Apollo missions used pure oxygen at reduced pressure rather than an earthlike oxygen-nitrogen mix. This was also true of the lunar EVA suits, where the internal pressure was just a quarter of atmospheric pressure. In addition to the main oxygen tanks, the PLSS contained a backup oxygen purge system and a lithium-hydroxide scrubber to remove excess CO2.