For decades, there has been relative agreement in the world on an uncomplicated response to the question: “Where does space begin?” We generally hear that the boundary, the Kármán line, is at 100km (62 miles) above sea level. This altitude has become the normalised cut-off, perceived as the point where terrestrial flight ends and spaceflight begins. In practice, this figure provides easy reference points for everything from world records to astronaut wings. But, much like with many too coincidentally round numbers, it is not a direct consequence of scientific precision. The 100km Kármán line is more a convention than a scientifically exact boundary[1].
The line is named after a brilliant guy, Theodore von Kármán, but his original math never landed on 100 km. He figured the real boundary was closer to 84 km (about 52 miles). That's the altitude where a vehicle would have to go so fast to stay airborne; it might as well just be in orbit. The 100 km number? That was the world's air sports federation (the FAI), just rounding up for simplicity's sake[2].
This article calls for a reassessment of the 100km boundary, arguing that it muddies both legal and technical discussions about the frontier between sky and space. By looking at the scientific background, the evolution of flight technology, and the legal frameworks involved, we’ll see that the original, lower figure has a stronger foundation in physics - and that clinging to the higher line creates unnecessary ambiguity. For a future where space activity is increasing, clarity on this point isn’t merely academic; it’s essential.
A definitive boundary of space cannot be separated from Theodore von Kármán[3], a brilliant physicist and engineer who, in the mid-20th century, laid down the basic theory for the "Kármán line." He was not looking to identify a legal limit, but to find a scientific answer to a simple question: at what altitude does an aircraft stop being an aircraft and become a spacecraft?
Von Kármán devoted the most effort to the relationship between the amount of aerodynamic lift and the speed to achieve lift. He determined that for an aircraft to ascend further into the atmosphere, it must reach a point where it is moving at a considerably faster pace to generate enough lift for it to remain in the air. He established that roughly at an altitude of 84 km (52 miles), a vehicle's speed needed to achieve lift would equal or exceed the speed needed to reach orbit. In these conditions, the primary force maintaining the vehicle in the air is not lift, but the momentum and physics of a ballistic trajectory.
This is the main difference. Based on the physics of fluid dynamics, von Kármán's research produced a figure of about 84 km. The world's record-keeping organisation for astronautics and aeronautics, the Fédération Aéronautique Internationale (FAI), eventually adopted a different figure, though. The FAI decided to define the edge of space for its record books using the round number of 100 km for convenience and international standardisation. Although this choice established a straightforward and unambiguous border, it was a pragmatic concession that overshadowed the scientific accuracy of von Kármán's initial computations. The current argument over the actual boundary of space is based on this purposeful rounding off of a significant scientific figure.
The operational realities of flight technology and atmospheric physics both provide compelling evidence in favour of a lower Kármán line. Significant physical differences in the 80–85 km region highlight why this is a more scientifically sound boundary than the arbitrary 100 km mark.
The upper limits of the mesosphere and the mesopause, dividing the mesosphere from the thermosphere, fall within this altitude range as per atmospheric physics[4]. The density of the atmosphere decreases steeply here. The air is so very thin that it cannot supply sufficient lift or control for conventional aerodynamic surfaces to fly effectively, even though there is residual gas present. This region is also where a critical transition in atmospheric behaviour occurs, from an atmosphere under control of mixing (the homosphere) to one under control of diffusion (the heterosphere).
Flight at lower altitudes, below approximately 84 kilometres, depends heavily on aerodynamics. Winged aircraft generate lift by shaping the flow of air over their wings to create pressure differences that enable them to fly and manoeuvre at lower speeds[5]. This requires an atmosphere with relatively large amounts of air (i.e., dense air). At higher altitudes, there is less atmosphere; consequently, there are also far fewer air molecules to support generating lift in this manner. Eventually, at high enough altitudes, there is no longer enough air density for aerodynamic flight to be possible.
Once these critical thresholds are exceeded, the rules of projectile and rocket flight take over. Rockets and spacecraft, for example, take thrust by producing equal and opposite reactive mass momentum and create an overall flight path governed by a mixture of momentum and gravity. When a vehicle crosses around 84 km, it is no longer "flying" in the aerodynamic sense; its flight path is determined by its initial velocity and gravitational effects and has more or less become a ballistic free-fall flight path[6], even if that path is still within the limits of the atmosphere.
It is important to consider the historical precedent in flying technologies. The X-15 program[7], a collaborative effort between NASA and the U.S. Air Force in the 1950s and 60s, was a game changer in high altitude flying. The U.S. Air Force awarded astronaut wings from the Astronaut Qualification Program to their pilots who flew the X-15 above 50 miles (approximately 80 km) because they felt that at that altitude, the aircraft was no longer in aerodynamic flight, and the pilot was merely the pilot of a space vehicle considered an astronaut. This acknowledgement by a leading aerospace power at their time provides further evidence of the scientific and technological rationale for establishing a space boundary more in the range of 80-85 km than 100 km.
Agreed by scientists and regulators, the Kármán line at 100 km is arbitrary and has caused a legal vacuum, creating severe difficulties for international law and national sovereignty. The foundation of international aviation law, the Chicago Convention[8], specifically states that countries have "complete and exclusive sovereignty over the airspace above their territory." It notably does not state how high sovereignty applies and therefore has created an ambiguous "vertical grey area" where new aerospace vehicles fall.
If a lower scientifically-based boundary, such as 84 km (rounded to 80/85 km), were adopted, it would provide a rational and logical solution to the problem. It would provide a clear physical parameter for where sovereign airspace ends and outer space begins, whereby outer space is specifically recognised as a global commons under the 1967 Outer Space Treaty[9] and cannot be nationalised or appropriated. The clear limit would remove legal ambiguity[10] altogether and reduce options for disputes over overflight permissions, jurisdiction, and applicability of domestic law concerning suborbital flights.
The lack of a defined boundary creates significant legal ambiguity for nascent commercial enterprises, primarily suborbital tourism and point-to-point travel. In the absence of an agreed-upon demarcation, the legal status of a vehicle moves along a continuum that is, at best, unclear, which creates uncertainty surrounding the liability in the event of accidents, as well as regulatory uncertainty for companies and investors. A boundary that is scientifically-based and agreed upon internationally would have a substantial positive impact in providing legal clarity that supports innovation and robust investment in the commercial space industry; it would also serve as the basis for new agreements that clarify and define new common international "rules of the road" for the next generation of aerospace vehicles.
Despite the scientific case for 84 km, many governments prefer the 100 km boundary. The reason is straightforward: a higher line extends a nation’s jurisdiction by another 15 km. It gives states more control over vertical space, more authority to manage traffic, and more leeway to apply domestic law to suborbital activities.
Politically, this is convenient. The 100 km convention is widely recognised, and it avoids reopening difficult international negotiations. But it prioritises sovereignty over science. By entrenching a “round number” compromise, the world risks building the next generation of space law and regulation on shaky foundations.
The 100 km Kármán line has served as a convenient convention, but it is scientifically imprecise and legally problematic. Von Kármán’s original calculations, the behaviour of the atmosphere, and the operational realities of high-altitude flight all point to a lower boundary, around 84 km.
Redefining the line would do more than correct history. It would give nations a clear, scientifically justified limit to sovereignty, while marking outer space as a shared global commons. It would also reduce uncertainty for the rapidly growing commercial space sector, from tourism to suborbital transport.
In short, the future of safe, predictable, and innovative spaceflight depends on replacing political convenience with scientific clarity. The real edge of space is not at 100 km, it is at 84.
[1] https://www.skadden.com/-/media/files/publications/2019/03/wheredoesspacebeginthedecadeslonglegalmissiontofin.pdf
[2] Sec 2.18.1 https://naa.aero/wp-content/uploads/2023/11/FAI-Sporting-Code-Astronautics.pdf
[3] https://www.jpl.nasa.gov/who-we-are/faces-of-leadership-the-directors-of-jpl/dr-theodore-von-karman-1881-1963/
[4] https://www.noaa.gov/jetstream/atmosphere/layers-of-atmosphere
[5] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/density.html
[6] https://www.scirp.org/journal/paperinformation?paperid=97186
[7] https://www.nasa.gov/reference/x-15/
[8] https://www.refworld.org/legal/agreements/icao/1944/en/16932
[9] https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/outerspacetreaty.html
