An airlock is a special chamber used to enter and exit pressurized environments, such as submarines or spacecraft. It consists of two airtight doors and a central chamber in which air pressure and other factors can be controlled as needed. Only one airlock door is open at any given time; the other remains closed until the first door has been sealed. This creates a buffer zone to maintain pressure and atmosphere within the enclosed environment. It also prevents decompression sickness, a potentially fatal condition caused by rapid changes in pressure on the human body.
The engineering advances of the 19th century included the first submarines as well as ambitious underwater construction projects such as suspension bridges. Work on these structures was complicated by caisson disease, later known as decompression sickness. The human body is unable to adapt to rapid changes in external pressure; in extreme cases, death can result. Eventually, visitors to underwater environments such as deep-sea divers learned to employ special chambers that slowly changed the external pressure, allowing the body to adjust. Refinements in this technology led to the modern airlock.
With the advent of space travel in the 20th century, the airlock had a new purpose. The harsh environment of outer space presents numerous hazards, including radiation, extremes of temperature, and a lack of atmospheric pressure. Spacecraft and space stations are equipped with airlocks so that astronauts can safely leave the craft for spacewalks or transfers to other vehicles. Prior to a spacewalk, astronauts will sometimes "camp out” in an airlock overnight. Environmental pressure is slowly reduced during this time, preventing the onset of decompression sickness.
Airlocks have other specialized uses in terrestrial environments. The “clean rooms” used to manufacture high-grade electronics often have airlocks to keep out dust and contaminants. Chemical and radioactive laboratories use airlocks to prevent dangerous materials from escaping controlled areas. Many public and private buildings have a double-door construction for well-used entryways. While not truly airtight, these passages are sometimes called airlocks, as they help isolate the building’s internal environment from outside conditions.
Airlocks are familiar to most people from their portrayals in popular fiction. Undersea film dramas like The Hunt for Red October and The Abyss set suspenseful scenes in or around airlocks. Television shows such as Babylon 5 and Battlestar Galactica reference a science-fiction concept sometimes called “spacing.” This is a procedure for killing someone by throwing him or her out of an airlock without a protective suit. Perhaps the most famous airlock sequence appears in the film 2001: A Space Odyssey, in which an astronaut attempts to manually enter a spaceship that has been taken over by a renegade computer.