Layers of Atmosphere. (not to scale)
The
Kármán line lies at an
altitude of 100 km (62.1 miles) above the
Earth's
sea level, and is commonly used to define the boundary between the
Earth's atmosphere and
outer space. 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 and
astronautics. He first calculated that around this altitude the
Earth's atmosphere becomes too thin for aeronautical purposes (because any vehicle at this altitude would have to travel faster than
orbital velocity in order to derive sufficient
aerodynamic lift from the atmosphere to support itself). Also, there is an abrupt increase in atmospheric
temperature and interaction with solar radiation.
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 (6,215 miles) above sea level.
When studying aeronautics and astronautics in the 1950s, Kármán calculated that above an altitude of roughly , 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, Kármán 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 widely accepted as the boundary to space for many purposes. However, there is still no international legal definition of the demarcation between a country's air space and outer space.
Another hurdle to strictly defining the boundary to space is the dynamic nature of Earth's atmosphere. For example, at an altitude of 1,000 km (621 miles), 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 does not itself use the precise words "
boundary to space" or "
edge of space"; however, 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):
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 conventional 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.
Controversy
Although the
United States does not officially define a "
boundary of space," the U.S. definition of an
astronaut, which is still held today, is a person who has flown more than 50 miles (~80 km)
above mean sea level. (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, three veteran NASA
X-15 pilots were retroactively (two
posthumously) awarded their
astronaut wings, as they had flown between 90 and 108 km in the
1960s, but at the time had not been recognized as astronauts.
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.
See also
