Kármán line

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. [cite web|url=http://www.fai.org/press_releases/2004/documents/12-04_100km_astronautics.doc|title=The 100 km Boundary for Astronautics|publisher=Fédération Aéronautique Internationale Press Release|date=24 June 2004|format=DOC|accessmonthday=October 30 |accessyear=2006] 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 (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.Fact|date=February 2007

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 (~6210 miles) up.

When studying aeronautics and astronautics in the 1950s, Kármán calculated that above an altitude of roughly km to mi | 100, 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. [citeweb|title=A word about the definition of space|url=http://www.nasa.gov/centers/dryden/news/X-Press/stories/2005/102105_Schneider.html|publisher="NASA"|accessdate=2008-04-29] However, there is still no international legal definition of the demarcation between a country's air space and outer space. [ [http://books.google.com/books?id=NR7mFXCB-wgC&pg=PA239&lpg=PA239&dq=legal+boundary+of+space&source=web&ots=Q56qiINocX&sig=bP5Ye5YBYytJpikkfp-lUznLJUk&hl=en "International Law: A Dictionary"] , by Boleslaw Adam Boczek; Scarecrow Press, 2005; page 239: "The issue whether it is possible or useful to establish a legal boundary between airspace and outer space has been debated in the doctrine for quite a long time. . . . no agreement exists on a fixed airspace - outer space boundary . . ."]

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 (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"; 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."

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.cite web|url=http://www.nasa.gov/centers/dryden/news/X-Press/stories/2005/102105_Wings.html|title=A long-overdue tribute|publsher=NASA|date=21 October 2005|accessmonthday=October 30 |accessyear=2006] [cite web|url=http://www.nasa.gov/worldbook/astronaut_worldbook.html|title=World Book @ NASA|publisher=NASA|accessmonthday=October 18 |accessyear=2006]

Controversy

Although the United States does not officially define a "boundary of space", the US definition of an astronaut, which is still held today, is a person who has flown above 80 km (50 miles) 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, two veteran X-15 pilots were retroactively awarded their astronaut wings, as they had flown higher than 110 km in the 1960s, but at the time had not been recognized as astronauts.

ee also

*Astronaut wings
*Exosphere
*Mesosphere
*Outer space
*Stratosphere
*Thermosphere
*Troposphere

References

External links

* [http://www.fai.org/astronautics/100km.asp Article on the Kármán line] at the FAI website
* [http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm Layers of the Atmosphere - NOAA]