Kármán line
The Kármán line (or von Kármán 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. The FAI defines the Kármán line as space beginning 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. As defined by the FAI, the Kármán line was established in the 1960s. Various countries and entities define space's boundary differently for various purposes. 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: Interpretations of the definition. The expression "edge of space" is often used (by, for instance, the FAI in some of their publications) 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. There is still no international legal definition of the demarcation between a country's air space  and outer space. In 1963, Andrew G. Haley discussed the Kármán line in his book "Space Law and Government". In a chapter on the limits of national sovereignty, he made a survey of major writers' views. 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. 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. Technical considerations. An atmosphere does not abruptly end at any given height but becomes progressively less dense 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 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: where Lift ("L") generated is directly proportional to the air density ("ρ"). All other factors remaining unchanged, true airspeed ("v") must increase to compensate for lower air density 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. The chart included an inflection point at around , above which the minimum speed would place the vehicle into orbit. The term "Kármán line" was originally coined by Andrew G. Haley in a 1959 paper, based on von Kármán's 1956 paper, but Haley acknowledged that the 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. 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 as the "von Kármán Line", which was the lowest altitude at which free-radical atomic oxygen occurred. Alternatives to the FAI definition. The U.S. Armed Forces definition of an astronaut is a person who has flown higher than above mean sea level, approximately the line between the mesosphere and the thermosphere. NASA formerly used the FAI's figure, though this  was changed in 2005, to eliminate any inconsistency between military personnel and civilians flying in the same vehicle, 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 and in the 1960s, but at the time had not been recognized as astronauts. 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: Works by Jonathan McDowell (Harvard-Smithsonian Center for  Astrophysics) and Thomas Gangale (University of Nebraska-Lincoln) in 2018 advocate that the demarcation of space should be at , 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. More precisely, the paper summarizes: These findings prompted the FAI to propose holding a joint conference with the International Astronautical Federation (IAF) in 2019 to "fully explore" the issue. 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. This is the definition adopted by the U.S. military. 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 , whereas an object can maintain an elliptical orbit with perigee as low as about without propulsion. The U.S. is resisting regulatory movement on this front. 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”. 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 and high respectively.