The Kármán Line marks the intriguing border between Earth’s atmosphere and the beginnings of outer space, situated approximately 100 kilometers above the Earth’s surface.
Although 100 kilometers might seem remarkably close to Earth, it remains well within the grasp of Earth’s gravity and the extended atmosphere. Nonetheless, this relatively nearby location has been accepted as the dividing line between Earth and space. But why?
The determination of the Kármán Line is based on physical reality, signifying the altitude where traditional aircraft can no longer effectively operate due to changes in the physical laws governing a vehicle’s ability to fly.
Despite the atmosphere extending above the Kármán Line, the air becomes exceedingly thin at these elevated altitudes. Therefore, anything traveling above the Kármán Line necessitates a propulsion system independent of the atmospheric lift.
Theodore von Kármán at the Jet Propulsion Laboratory, California Institute of Technology, USA, in 1950 (Getty Images)
Engineer, physicist, and aerospace pioneer Theodore von Kármán proposed the establishment of the Kármán Line at an altitude of 100 kilometers (Getty Images)
Defining the Kármán Line
Determining precisely where space begins can be somewhat challenging because Earth’s atmosphere doesn’t abruptly end but gradually thins at higher altitudes, meaning there are no clearly defined upper boundaries.
According to “Astronomy,” engineer, physicist, and aerospace pioneer Theodore von Kármán suggested that the most logical edge of space would be where orbital forces exceed aerodynamic air forces. He set this at 100 kilometers, a practical boundary.
The Kármán Line received its name from this Hungarian-born scientist, born in 1881, who was among the first to calculate that at this altitude, the dynamic lift could no longer keep an aircraft aloft. He worked on early helicopter designs in the years following World War I.
Although his name is now associated with the boundaries of space, von Kármán himself never published this idea. The concept emerged through discussions at a conference.
Andrew G. Halliday, the world’s first space lawyer, published the first comprehensive study on the bounds of space in the early 1960s, following von Kármán’s criteria more precisely. He determined the actual space boundary to be approximately 84 kilometers above the Earth’s surface, an altitude also used by the U.S. Air Force in the 1950s when issuing astronaut permits for pilots who flew above 80 kilometers.
The international community, including the International Civil Aviation Organization, regards space as commencing at the Kármán Line, precisely at 100 kilometers. However, NASA and the U.S. military consider space to begin at an altitude of around 80 kilometers.
Why Can’t Aircraft Fly Above the Kármán Line?
The Kármán Line represents the point at which the physical laws governing a vehicle’s ability to fly undergo a transformation. It describes how Earth’s atmosphere’s properties at different altitudes affect an aircraft’s flying capabilities.
Aircraft typically generate lift primarily through their wings during flight, which counteracts the weight of the aircraft and keeps it airborne. But with increasing altitude in our atmosphere, the air becomes significantly thinner.
At a certain point, the air becomes too thin for traditional aircraft to generate sufficient lift, meaning there is no aerodynamic force to keep them aloft. Any aircraft above this altitude requires a propulsion system, such as a rocket, to stay in flight. This is why spacecraft don’t resemble conventional airplanes in form.
Why Is It Important to Define Earth and Space Boundaries?
According to international law, the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space stipulates that “outer space shall be free for exploration and use by all.” Due to varying definitions of where space truly begins, this can have legal implications.
Air traffic is typically regulated at the national level, with individual countries controlling the airspace above their territories. Crossing this boundary inadvertently could lead to international conflicts. Thus, defining the distinction between Earth’s airspace and outer space has become crucial, as space is a global domain, while airspace is the sovereign territory of individual nations.
In his posthumously published autobiography, “The Wind and Beyond,” von Kármán himself wrote about the boundary between Earth and space, saying, “These are material boundaries, to be sure, where air dynamics end and space navigation begins. Under this line, the area belongs to each country. Beyond this level, there will be free space.”
It’s worth noting that while the Kármán Line is defined as the boundary, it doesn’t necessarily function as a solid demarcation where one side is entirely space and the other entirely Earth’s atmosphere. Earth’s atmosphere doesn’t disappear abruptly but gradually thins with increasing altitude, eventually blending into space.
The definition of the Kármán Line as the invisible boundary where space begins is indeed an essential concept when it comes to space travel. Even though the exact altitude where space starts remains a subject of debate, this idea assists in defining whether something exists in space or within Earth’s atmosphere. This also applies to the classification of who qualifies as an astronaut, as one doesn’t receive a spacefarer’s license unless they venture above this line.
Image: A photograph capturing Earth’s atmospheric layers taken by an astronaut aboard the International Space Station in 2011 (NASA)
Image: A photograph capturing Earth’s atmospheric layers taken by an astronaut aboard the International Space Station in 2011 (NASA)
Why Doesn’t Space Begin When the Atmosphere Ends?
According to “Space,” some people wonder if it might be easier to define space as the absolute point where Earth’s atmosphere ends. However, this definition could introduce complexity.
Traveling beyond Earth’s atmospheric envelope would take us to an altitude of about 10,000 kilometers above Earth’s surface, reaching the highest layer of Earth’s atmosphere, known as the exosphere.
The International Space Station orbits Earth at an average altitude of 400 kilometers, while satellites orbit at altitudes less than 1,000 kilometers. By defining space at this new altitude of 10,000 kilometers, most spacecraft and satellites orbiting Earth would no longer be considered “spacecraft.” Visitors to the International Space Station, for example, would not be called astronauts. Therefore, the best options for the starting point of space remain 80 or 100 kilometers, the Kármán Line.
This threshold, defining space as the region beyond the Kármán Line, is indeed crucial for space travel. Even if the exact altitude where space begins is debatable, establishing the threshold at this line is helpful in determining whether something is in space or within Earth’s atmosphere, and this definition applies to the classification of astronauts, as an individual is only granted astronaut status if they traverse this boundary.
