Kármán line

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Layers of Atmosphere - not to scale (NOAA)[1]

Atmospheric gases scatter blue wavelengths of visible light more than other wavelengths, giving the Earth’s visible edge a blue halo. At higher and higher altitudes, the atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space.

The Kármán line lies at an altitude of 100 km above the Earth's surface, and is commonly used to define the boundary between the Earth's atmosphere and outer space.^[1] This definition is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard setting and record-keeping body for aeronautics and astronautics.

The line was named after Theodore von Kármán, a Hungarian-American engineer and physicist who was active primarily in the fields of aeronautics. He first calculated that around this altitude the Earth's atmosphere becomes too thin for aeronautic purposes. Also, there is an abrupt increase in atmospheric temperature and interaction with solar radiation.^{[citation needed]}

1 Overview
2 A diluted definition
3 The U.S. definition
4 See also
5 References
6 External links

[edit] Overview

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 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 1950s, Karman calculated that above an altitude of roughly 100 kilometers (62.1 mi), 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 100 km, Karman proposed that 100 km be the designated 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 100 km 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.

The article on the Kármán line under External links below has a more detailed explanation on how this boundary was determined.

[edit] 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 commonly would.

[edit] The U.S. definition

Although the United States does not officially define a "boundary of space", the definition of an astronaut is one who has flown above 50 miles above mean sea level, which is still held today.^[2]^[3] This is approximately the line between the mesosphere and the thermosphere. This definition of an astronaut had been somewhat controversial, due to differing definitions between the United States military and NASA. In 2006, two X-15 pilots were awarded their astronaut wings, as they flew higher than 50 miles in the 1960s, but at the time they were not considered to be astronauts.^[2]