Where does the atmosphere end and outer space begin? This is a common question, whose simplest answer is the Kármán line, but the matter becomes complicated when trying to establish an exact point where the boundary between the planet and the void of outer space is located.
Where Is the Kármán Line?
We often look at the planet as an orb with well-defined limits, but when studied in depth, it is not that simple. By definition, we know that the Kármán line marks the boundary between the Earth’s atmosphere and outer space, although the real question is where the boundary is located.
The line is named after the Hungarian-American aeronautical engineer Theodore von Kármán, who in the 1950s dedicated himself to finding the boundary of the atmosphere. However, it was not until 1975 that the term “Kármán line” was formally established by the lawyer Andrew G. Haley, the first to delve into space law issues and who became interested in the matter because the Kármán line would also mark the end of the national airspace and therefore, laws would not be applicable beyond the threshold.
As a good aeronautical engineer, Kármán calculated that the boundary of the atmosphere would rise to approximately 100 kilometers above the average sea level. In this way, the altitude would be consistent with engineering calculations where orbital forces surpass aerodynamic forces. In this part of the planet, the density of the atmosphere is so tenuous that aircraft would have to reach a force equivalent to the orbital speed of the Earth to continue flying, an impressive speed that is impossible for winged and propeller-driven craft, one of the reasons they do not reach such heights.
Years later, the International Aeronautical Federation (FAI) accepted the limit stipulated by Kármán. Today, this institution and many others point out that the Kármán line, that is, the boundary between the atmosphere and space, is located at 100 kilometers altitude above the average sea level. But there is no clear consensus, and some argue that the boundary would be slightly lower.
The Atmosphere Is Not That Simple
The Earth’s atmosphere is not a simple bubble of gases that covers the planet, each of its layers is essential to maintain the balance of life. But such complexity also applies to its formation and termination; the atmosphere does not end abruptly, but becomes thinner at higher altitudes and therefore, does not have a completely defined upper limit.
It is precisely this characteristic that has posed a dilemma for meteorologists, physicists, and engineers to determine exactly where the atmosphere ends and outer space begins. While the International Aeronautical Federation establishes that the Kármán line rises to 100 kilometers above the average sea level, what do other institutions say about it?
According to the National Aeronautics and Space Administration (NASA), the Kármán line rises to 80 kilometers above the average sea level. To this calculation, the US military also places the planet’s limit at the same altitude as NASA. But the atmosphere fades so complexly that even today the matter is under discussion, and the most important institutions in the world for exploration and space law are trying to come to an agreement.