Layers of Atmosphere.¹ (not to scale)

The Kármán line lies at an altitude of 100 km (approx. 62 miles) above the Earth's sea level, and is commonly used to define the boundary between the Earth's atmosphere and outer space.² 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, (1881–1963) 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.^{citation needed}

Many man-made objects are visible from the space immediately beyond that line.

Definition

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.^{citation needed}

An aeroplane only stays in the sky if it is constantly travelling forwards, so that the wings can generate lift. The thinner the air, the faster the plane has to go to generate enough lift to stay up.

If the lift coefficient for a wing at a specified angle of attack is known (or estimated using a method such as thin-airfoil theory), then the lift produced for specific flow conditions can be determined using the following equation

where

L is lift force

ρ is air density

v is true airspeed

A is planform area,

and

CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number

Lift (L) generated is directly proportional to the air density (ρ) . All other factors remaining unchanged, true airspeed (v) has to be increased to compensate for less air density (ρ) at higher altitudes.

An orbiting spacecraft only stays in the sky if the centrifugal component of its movement around the Earth is enough to balance the downward pull of gravity. If it goes any more slowly, the pull of gravity will gradually cause it to accelerate towards the Earth. The required speed is called orbital velocity, and it varies with the height of the orbit. For a space shuttle in low Earth orbit, the orbital velocity is about 17,000 miles per hour (27,000 km per hour).

For an aeroplane that is trying to fly higher and higher, the thinning air gives less and less lift, requiring a higher speed to avoid stalling. There comes an altitude where it needs to fly so fast to generate lift that it reaches orbital velocity. The altitude where the required flying speed is equal to orbital velocity is called the Kármán line.

When studying aeronautics and astronautics in the 1950s, Kármán calculated that above an altitude of roughly 100 kilometers (62 mi), a vehicle would have to fly faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself.

Although the calculated altitude was not exactly 100 km, Kármán proposed that 100 km be the designated boundary to space, since 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.³ However, there is still no international legal definition of the demarcation between a country's air space and outer space.⁴

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 can vary by a factor of five, depending on the time of day, time of year, AP magnetic index, and recent solar flux.^{citation needed}

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):^{citation needed}

• 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.

Interpretations of the definition

Some people (including the FAI in some of their publications) also use the expression "edge of space" to refer to a 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". In such statements, "reaching the edge of space" merely refers to going higher than average aeronautical vehicles commonly would.⁵⁶

Alternatives to the definition

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.⁵

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.⁵

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.

References

External links

• Article on the Kármán line at the FAI website
• Layers of the Atmosphere - NOAA
• The Kármán Line music video featuring NASA footage