Airlocks are vital for protecting the crew of a space-craft. In a submerged submarine, if there was no airlock, the vessel would instantly be flooded with water as soon as the hatch was opened. In the same way, a space station with no airlock would depressurize the instant the door was opened, killing anybody on board who was not wearing a spacesuit. This is because air always tries to remain at a level pressure. If the pressure inside a spacecraft is greater than the pressure outside, as soon as the hull is breached, air will rush out into outer space.
Airlocks, typified by the International Space Station’s (ISS) primary Quest Joint Airlock, are designed to permit safe passage of people and objects between a pressurised vessel and its surrounding environment. Further, they are designed to minimise pressure and air-level changes within the host craft.
The Quest airlock of the ISS is split into two main sections: an equipment chamber and crew lock chamber. The former connects to the ISS and supplies an auxiliary holding bay for any essential equipment, such as Extravehicular Mobility Units (EMUs – or spacesuits), as well as other key gear. It also supplies a staging area where astronauts can prepare for a spacewalk -namely get in and out of their spacesuits.
Connected to the equipment compartment is the crew lock, a smaller cylindrical chamber into which astronauts enter prior to any spacewalk. Once inside this section, the interior hatch between the equipment lock and the crew lock is shut. This provides a sealed environment for the suited astronaut and allows depressurisation to proceed. When the crew lock is fully depressurised, an external hatch becomes operational, providing an exit for the astronaut to enter space.
Importantly, before any spacewalk is attempted, astronauts must ‘camp out’ within the equipment chamber of the airlock in a reduced-nitrogen environment in order to purge nitrogen from their blood stream. This ensures that astronauts avoid decompression sickness in the low pressure experienced within the pure-oxygen atmosphere of the spacesuit. Nitrogen and oxygen are supplied and replenished via four externally mounted gas tanks, which ensures that the lock does not need to draw upon the host ISS’s own gas supplies.