The Armstrong Limit represents an atmospheric condition where pressure is so low that water in the human body begins to boil at normal body temperature. Understanding this boundary is central to ensuring safety in high-altitude flight and space exploration. This physiological threshold has shaped the design of protective equipment and spacecraft.
Defining the Armstrong Limit
The Armstrong Limit, also known as Armstrong’s Line, is the altitude where atmospheric pressure drops to a point that water boils at the average human body temperature of 98.6°F (37°C). This occurs at approximately 63,000 feet (19,000 meters) above sea level. As pressure decreases, the boiling point of water also decreases. It is important to recognize that this is a matter of pressure, not temperature, as the surrounding air at such altitudes is extremely cold.
This physiological boundary is named after Harry G. Armstrong, a United States Air Force General and pioneer in aviation medicine. In the late 1940s, Armstrong identified this altitude as a limit beyond which human survival in an unpressurized environment is impossible. His work contributed to the understanding of high-altitude flight and human protection in near-space conditions. The Armstrong Limit changed how engineers and scientists approached high-altitude aviation and space exploration.
Physiological Impact on the Unprotected Body
Without adequate protection, exposure to the Armstrong Limit leads to severe physiological consequences. The primary effect is ebullism, the formation of water vapor bubbles in bodily fluids due to reduced external pressure. While blood within the circulatory system does not boil due to internal pressure, exposed fluids like saliva, tears, urine, and the liquid lining the lungs’ alveoli will vaporize.
This rapid vaporization causes tissues to swell significantly, as the gases expand within the body. Simultaneously, the extreme lack of atmospheric pressure means there is insufficient oxygen to sustain consciousness, leading to rapid hypoxia. An unprotected person would typically lose consciousness within 60 to 90 seconds. Unless pressure is quickly restored, these combined effects, including impaired blood circulation and lung function due to bubble formation, would lead to death.
Overcoming the Limit for Space Travel
To safely operate at or beyond the Armstrong Limit, technological solutions are employed to create a habitable environment for humans. The primary methods involve either enclosing individuals in pressurized cabins or equipping them with specialized pressure suits.
Pressurized cabins, used in commercial airliners and spacecraft, maintain an internal air pressure equivalent to a much lower, survivable altitude, typically between 6,000 and 8,000 feet. This allows occupants to breathe normally without supplemental oxygen.
For situations where individuals must exit a pressurized environment, such as during spacewalks or high-altitude military flights, full-pressure suits are used. These suits act as personal spacecraft, encasing the wearer in an artificial atmosphere that mimics sea-level or near sea-level pressure. Early pressure suits, developed in the 1930s and 1940s for military pilots, were precursors to the space suits used by astronauts. Over time, these protective garments evolved from basic modified flight suits to highly sophisticated systems with integrated life support, enabling extended operations in the harsh vacuum of space.