Kármán line

The Kármán line lies within the lower thermosphere (not to scale).^[1]

The Kármán line (or von Karman line) is an attempt to define a boundary between Earth's atmosphere and outer space, and offers a specific definition set by the Fédération aéronautique internationale (FAI), an international record-keeping body for aeronautics. Defining the edge of space is important for legal and regulatory purposes since aircraft and spacecraft fall under different jurisdictions and are subject to different treaties. International law does not define the edge of space, or the limit of national airspace.^[2][3]

The FAI defines the Kármán line as space beginning 100 kilometres (54 nautical miles; 62 miles; 330,000 feet) above Earth's mean sea level. While experts disagree on exactly where the atmosphere ends and space begins, most regulatory agencies (including the United Nations) accept the FAI Kármán line definition or something close to it.^[4] As defined by the FAI, the Kármán line was established in the 1960s.^[5] Various countries and entities define space's boundary differently for various purposes.^[6][2][7]

The Kármán line is named after Theodore von Kármán (1881–1963), a Hungarian American engineer and physicist who was active in aeronautics and astronautics. In 1957, he was the first person to attempt to derive such an altitude limit.

Definition[]

The FAI uses the term Kármán line to define the boundary between aeronautics and astronautics:^[5]

• Aeronautics — For FAI purposes, aerial activity, including all air sports, within 100 km of Earth's surface.
• Astronautics — For FAI purposes, activity more than 100 km above Earth's surface.

Interpretations of the definition[]

The expression "edge of space", is often used (by, for instance, the FAI in some of their publications)^[8] to refer to a region below the boundary of 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.^[9][10]

There is still no international legal definition of the demarcation between a country's air space and outer space.^[11] In 1963 Andrew G. Haley discussed the Kármán line in his book Space Law and Government.^[12] In a chapter on the limits of national sovereignty, he made a survey of major writers’ views.^{[12]: 82–96} He indicated the inherent imprecision of the Line:

The line represents a mean or median measurement. It is comparable to such measures used in the law as mean sea level, meander line, tide line; but it is more complex than these. In arriving at the von Kármán jurisdictional line, [a] myriad [of] factors must be considered – other than the factor of aerodynamic lift. These factors have been discussed in a very large body of literature and by a score or more of commentators. They include the physical constitution of the air; the biological and physiological viability; and still other factors which logically join to establish a point at which air no longer exists and at which airspace ends.^{[12]: 78, 9}

Kármán's comments[]

In the final chapter of his autobiography Kármán addresses the issue of the edge of outer space:

Where space begins… can actually be determined by the speed of the space vehicle and its altitude above the Earth. Consider, for instance, the record flight of Captain Iven Carl Kincheloe Jr. in an X-2 rocket plane. Kincheloe flew 2000 miles per hour (3,200 km/h) at 126,000 feet (38,500 m), or 24 miles up. At this altitude and speed, aerodynamic lift still carries 98 percent of the weight of the plane, and only two percent is carried by inertia, or Kepler Force, as space scientists call it. But at 300,000 feet (91,440 m) or 57 miles up, this relationship is reversed because there is no longer any air to contribute lift: only inertia prevails. This is certainly a physical boundary, where aerodynamics stops and astronautics begins, and so I thought why should it not also be a jurisdictional boundary? Haley has kindly called it the Kármán Jurisdictional Line. Below this line, space belongs to each country. Above this level there would be free space.^[13]

Technical considerations[]

An atmosphere does not abruptly 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 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 extend the boundary to space to at least 10,000 km (6,200 miles) above sea level. The Kármán line thus is an arbitrary definition based on some technical considerations.

An aircraft can stay aloft only by constantly traveling forward relative to the air (rather than the ground), so that the wings can generate lift. The thinner the air, the faster the plane must go to generate enough lift to stay up. The amount of lift provided (which must equal the vehicle's weight in order to maintain level flight) is calculated by the lift equation:^[14][15]

${\displaystyle L={\tfrac {1}{2}}\rho v^{2}SC_{L}}$

where

L is the lift force

ρ is the air density

v is the aircraft's speed relative to the air

S is the aircraft's wing area,

C_L is the lift coefficient.^[16]

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

In 1956, von Kármán presented a paper in which he discussed aerothermal limits to flight. The faster aircraft fly, the more heat they would generate due to aerodynamic heating from friction with the atmosphere and adiabatic processes. Based on the current state of the art, he calculated the speeds and altitudes at which continuous flight was possible -- fast enough that enough lift would be generated and slow enough that the vehicle would not overheat.^[17] The chart included an inflection point at around 275,000 feet (52.08 mi; 83.82 km), above which the minimum speed would place the vehicle into orbit.^[18][19]

The term "Kármán line" was originally coined by Andrew G. Haley in a 1959 paper,^[20] based on von Kármán's 1956 paper, but Haley acknowledged that the 275,000 feet (52.08 mi; 83.82 km) limit was theoretical and would change as technology improved, as the minimum speed in von Kármán's calculations was based on the speed-to-weight ratio of current aircraft, namely the Bell X-2, and the maximum speed based on current cooling technologies and heat-resistant materials.^[18] Haley also cited other technical considerations for that altitude, as it was approximately the altitude limit for an airbreathing jet engine based on current technology. In the same 1959 paper, Haley also referred to 295,000 feet (55.9 mi; 90 km) as the "von Kármán Line", which was the lowest altitude at which free-radical atomic oxygen occurred.^[18]

Alternatives to the FAI definition[]

Atmospheric gases scatter blue wavelengths of visible light more than other wavelengths, giving the Earth's visible edge a blue halo. The Moon is seen behind the 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 U.S. Armed Forces definition of an astronaut is a person who has flown higher than 50 miles (80 km) above mean sea level, approximately the line between the mesosphere and the thermosphere. NASA formerly used the FAI's 100-kilometre (62-mile) figure, though this was changed in 2005, to eliminate any inconsistency between military personnel and civilians flying in the same vehicle,^[21] when three veteran NASA X-15 pilots (John B. McKay, William H. Dana and Joseph Albert Walker) were retroactively (two posthumously) awarded their astronaut wings, as they had flown between 90 km (56 miles) and 108 km (67 miles) in the 1960s, but at the time had not been recognized as astronauts.^[9] The latter altitude, achieved twice by Walker, exceeds the modern international definition of the boundary of space.

The United States Federal Aviation Administration also recognizes this line as a space boundary:^[22]

Suborbital Flight: Suborbital spaceflight occurs when a spacecraft reaches space but its velocity is such that it cannot achieve orbit. Many people believe that in order to achieve spaceflight, a spacecraft must reach an altitude higher than 100 kilometers (62 miles) above sea level.

2018 works by Jonathan McDowell (Harvard-Smithsonian Center for Astrophysics)^[23] and Thomas Gangale (University of Nebraska-Lincoln)^[18][24] advocate that the demarcation of space should be at 80 km (50 miles; 260,000 feet), citing as evidence von Kármán's original notes and calculations (which concluded the boundary should be 270,000 ft), confirmation that orbiting objects can survive multiple perigees at altitudes around 80 to 90 km, plus functional, cultural, physical, technological, mathematical, and historical factors.^[2][25]

More precisely, the paper summarizes:

To summarize, the lowest possible sustained circular orbits are at of order 125 km altitude, but elliptical orbits with perigees at 100 km can survive for long periods. In contrast, Earth satellites with perigees below 80 km are highly unlikely to complete their next orbit. It is noteworthy that meteors (travelling much more quickly) usually disintegrate in the 70–100 km altitude range, adding to the evidence that this is the region where the atmosphere becomes important.

These findings prompted the FAI to propose holding a joint conference with the International Astronautical Federation (IAF) in 2019 to "fully explore" the issue.^[8]

Another definition proposed in international law discussions defines the lower boundary of space as the lowest perigee attainable by an orbiting space vehicle, but does not specify an altitude.^[26] This is the definition adopted by the U.S. military.^[27]: 13 Due to atmospheric drag, the lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 km (93 miles), whereas an object can maintain an elliptical orbit with perigee as low as about 130 km (81 miles) without propulsion.^{[citation needed]} The U.S. is resisting regulatory movement on this front.^[28][29]

A key consideration is the concept of free molecular flow. The essential property of space is in being a realm where there is significant space between molecules. Hence the name. Under free molecular flow, the path of molecules has transitioned to being straight lines, no longer continually bouncing off of each other. This condition of space can be artificially created in a vacuum chamber, or happens naturally at a high enough altitude. It has been known since at least as early as 1964 that the transition to free molecular flow happens at an altitude above “75-80 Km”.^[30] So the USAF and NASA definitions are supported by this scientifically significant data that the realm of space is entered after the molecules in the atmosphere move in straight lines under free molecular flow, above 80 km (50 miles).

For other planets[]

While the Kármán line is defined for Earth only, if calculated for Mars and Venus it would be around 80 km (50 miles) and 250 km (160 miles) high respectively.^[31]

See also[]

• Exosphere
• Atmosphere of Earth
  • Thermosphere
  • Mesosphere
  • Stratosphere
  • Troposphere
• MW 18014 – the V-2 rocket which was the first human-built object to cross the Kármán line
• Armstrong limit – Altitude above which water boils at human body temperature
• Free molecular flow

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
• Kármán line calculator