The Kármán Line is an internationally designated altitude commonly used to define outer space. According to definitions by the Fédération Aéronautique Internationale (FAI), the Karman or Kármán line lies at a height of 100 km (about 62 miles) above Earth's surface (ie. in technical terms 100 km above mean sea level). It was named after Theodore von Kármán.
Around this altitude the Earth's atmosphere becomes negligible for aeronautic purposes, and there is an abrupt increase in atmospheric temperature and interaction with solar radiation.
Strictly speaking, there is no such thing as an end to Earth's atmosphere: An atmosphere does not technically end at any given height, but becomes progressively thinner with altitude. Also, depending on how the various layers that make up the space around the Earth are defined (and depending on whether these layers are considered as part of the actual atmosphere), the definition of the edge of space could vary considerably: If one were to consider the thermosphere and exosphere only part of the atmosphere and not of space, one might have to place the boundary to space as high as about 10,000 km (6200 miles) up. When studying aeronautics and astronautics in the 1950's, Karman calculated that above an altitude of roughly 100 km, a vehicle would have to fly faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself. Though the calculated altitude was not exactly 100km, Karman proposed that 100km be designated the boundary to space as the round number is more memorable and the calculated altitude varies minutely as certain parameters are varied. An international committee recommended the 100km line to the FAI, and upon adoption it became the internationally accepted boundary to space.
Another hurdle to strictly defining the boundary to space is the dynamic nature of Earth's atmosphere. For example; at an altitude of 1000 km, the atmosphere's density may vary by a factor of five, depending on the time of day, time of year, AP magnetic index, and recent solar flux.
The FAI apparently doesn't itself use the precise words "boundary to space" or "edge of space"; the FAI uses the term "Kármán line" or speaks of a "100 km altitude boundary for astronautics", as also reflected in their following two definitions (quoted verbatim from their website):
- Aeronautics -- For FAI purposes, aerial activity, including all air sports, within 100 kilometres of Earth's surface.
- Astronautics -- For FAI purposes, activity more than 100 kilometres above Earth's surface.
Also see the article on the Kármán line under External links below, which has an excellent explanation on how this boundary was determined.
A diluted definition
Some people (including the FAI in some of their publications) also use the expression "edge of space" to refer to a very vaguely defined (essentially undefined) region below the actual 100 km boundary to space, which is often meant to include substantially lower regions as well. Thus, certain balloon or airplane flights might be described as "reaching the edge of space", when they really don't even go half as high as 100 km up. In such statements, "reaching the edge of space" merely refers to going somewhat higher than average aeronautical vehicles would commonly.
The U.S. definition
U.S. authorities define the boundary to space to lie at a height of 50 miles (about 80 km) above mean sea level, about where the mesosphere ends. This definition is thought by some to be outdated, and is not commonly accepted internationally.