The
Armstrong limit, often called
Armstrong's line, is the
altitude that produces an
atmospheric pressure so low (0.0618
atmosphere or ) that water boils at the normal temperature of the human body: 37 °C (98.6 °F). It is named after
Harry George Armstrong, who founded the U.S. Air Force’s Department of Space Medicine in 1947 at
Randolph Field, Texas. Armstrong was the first to recognize this phenomenon, which occurs at an altitude beyond which humans absolutely cannot survive in an unpressurized environment. On Earth, the altitude is variously reported as being between 18,900 and 19,350 meters (62,000 and 63,500 feet, or about ).