Where does the Earth’s atmosphere end and space begin? The answer to this question is more complex and important than it may seem at first glance. The fact is that, according to the 1967 Outer Space Treaty, outer space is not subject to appropriation and sovereignty. Consequently, outer space is the upper limit of states with all the legal consequences that follow.
Defining the boundaries of space is also quite important for companies organizing suborbital tourist flights. After all, a person who has paid a large sum for such a flight would like to know that he or she has been to space and can call himself or herself an astronaut.
In this material, we will talk about all the complexities involved in defining outer space and what is most often used to determine its boundary.
Where does the Earth’s atmosphere end?
At first glance, the answer to the question of where the boundary of outer space lies is quite simple: space begins where the Earth’s atmosphere ends. But that’s in theory. In reality, everything is quite confusing.
So, our planet’s atmosphere consists of the following five main layers:
- Troposphere (from 0 to 12 km);
- Stratosphere (from 12 to 50 km);
- Mesosphere (from 50 to 80 km);
- Thermosphere (from 80 to 700 km);
- Exosphere (from 700 to 10000 km).
The higher we are, the lower the pressure becomes and the more the conditions become space-like. At an altitude of 15 km, the time of useful consciousness, during which a person in a situation of sudden depressurization will be able to perform some conscious actions, is only 5 seconds. And at an altitude of 18-19 km, the atmospheric pressure becomes so low that liquids boil at the temperature of the human body. This mark is called the Armstrong limit and is considered the boundary above which living organisms (except for some bacteria) cannot exist.
But the fact is that jet airplanes are capable of flying much higher than the Armstrong limit. For example, the SR-71 reconnaissance plane could fly at an altitude of 26 km, and the MiG-25 fighter jet could reach a mark of 37 km. We can also think of stratostats. In 2014, Robert Eustace used it to rise to an altitude of 41 km, after which he made a successful parachute jump.
However, the higher we climb and the thinner the atmosphere becomes, the more difficult the task of maintaining sufficient lift for sustained flight becomes. At about 80-90 km (which corresponds to the mesosphere boundary), the speed required for this starts to exceed the first space velocity. The laws of aerodynamics are replaced by the laws of orbital mechanics.
But does that mean the Earth’s atmosphere ends in this region? Not at all. Although from an ordinary person’s point of view, the conditions there are almost no different from those in space, even the extremely rarefied Earth’s atmosphere still has a significant impact on space equipment located in low orbits with altitudes of several hundred kilometers. Under its influence, satellites gradually descend and, unless regular course corrections are made, will eventually enter the dense atmosphere and burn up. Thanks to this natural mechanism, low-Earth orbits are regularly cleared of space debris.
The size of the spacecraft also plays a role. For example, an average satellite in a 400 km orbit will be able to stay there for about 5-8 years. But the same ISS with its huge solar panels and numerous modules would fall to the Earth in just one year without constant switching on the engines.
Of course, the further we move away from the Earth, the weaker the effects of the atmosphere become. A satellite orbiting at an altitude of 1000 km will be able to stay in space for several thousand years. And geostationary satellites, whose orbits are at an altitude of 36 thousand kilometers (i.e., outside the exosphere), are practically eternal. The time of their orbital life will be measured in millions of years.
50 miles and the Kármán line
To summarize the above, we can conclude that the Earth’s atmosphere itself is not very suitable for use as a space boundary. Although extremely rarefied, it still has a noticeable effect on low orbits, where the vast majority of operational satellites are located.
What, then, should be considered the boundary of space? The Fédération Aéronautique Internationale (FAI), the United Nations, and most space agencies and regulatory bodies have chosen the Kármán line, 100 kilometers above sea level, as the boundary. It was named after the scientist Theodore von Kármán. In the 1950s, he performed calculations to determine the maximum altitude at which continuous flight according to the laws of aerodynamics is still possible, fast enough to generate the necessary lift and slow enough to keep the vehicle from overheating. Kármán obtained a mark of 52.08 miles (83.82 kilometers), above which flight becomes space flight. Other researchers have come to similar conclusions, estimating this value at 80-90 kilometers. So why did FAI accept the figure of 100 km? It’s quite simple: it was more convenient to count, and in general, the number itself is quite beautiful.
The U.S. Air Force also chose its beautiful figure to define the boundary of space. It was 50 miles (80.45 km), which corresponds quite closely to von Kármán’s original calculations. This decision created some confusion, as the Americans granted astronaut status to several pilots who flew the X-15 rocketplane above 50 miles but below the Karman line (of course, the FAI did not recognize them as astronauts).
The history of a peculiar confrontation between 50 miles and 100 km has continued in our time after the beginning of suborbital tourist flights. The fact is that the maximum flight altitude of Virgin Galactic’s SpaceShipTwo is about 90 km, while Blue Origin’s New Shepard flies just above 100 km. In this regard, Blue Origin emphasized in their advertising campaign that the participants of their flights will be considered astronauts all over the world, not just in the United States.
Source: Virgin Galactic
As for NASA, the organization uses two figures to determine the boundary of space. When an astronaut is granted the status of an astronaut, the countdown is from 50 miles. At the same time, to determine the moment of entry of the spacecraft into the atmosphere, NASA uses a mark of 76 miles (122 km). According to engineers’ calculations, here the gas shell of our planet begins to have a significant impact on the spacecraft. It is at this altitude that shuttles, when returning to Earth, switched to maneuvering with the help of controlled surfaces.
Alternative definitions of the space boundary
However, there are other opinions on what exactly to consider as the boundary of outer space. Some scientists propose to use as a height at which a satellite can survive at least one revolution around the Earth. So, a few years ago, a famous astrophysicist, Jonathan McDowell, analyzed data on the orbits of thousands of satellites. He managed to find several examples when spacecraft with a perigee orbit of less than 90 kilometers made several turns around the Earth. He therefore proposed that the 50-mile marker be used as the boundary of space.
However, it is important to note that in these cases, we were talking about vehicles located in elongated orbits with a rather large apogee. If we talk about a satellite in a circular orbit, the minimum possible altitude at which it can make at least one revolution around the Earth is about 150 km.
There are also more unusual opinions. For example, outer space begins 1.5 million kilometers from Earth. This figure corresponds to the Hill sphere – the region where the gravity of our planet is still able to hold a satellite orbiting around it. And already mentioned, Jonathan McDowell, as a joke, once proposed to use the so-called Ripley line, corresponding to the area where you can no longer hear a human scream. This is a reference to the famous slogan of the movie “Alien” – “In space, no one can hear you scream”.
In any case, none of these possible space boundaries is enshrined in international law. And this situation creates preconditions for potential conflicts. In theory, a state can shoot down another country’s aircraft on the grounds that it is not in space, but in its sovereign airspace and is therefore an intruder.
In this regard, we can recall the incident in 2023 when a Chinese balloon was shot down in the skies above the United States. Yes, it was shot down at an altitude of only 20 kilometers. But modern balloons can fly at altitudes of over 50 km. Also, we should not forget about hypersonic aircraft, on which the leading powers are actively working. Some of the promising military developments are capable of flying at altitudes close to the same mark of 50 miles. In this regard, the question of where the airspace ends and space begins is quite capable of moving from the purely theoretical to the practical plane. Only time will tell whether the countries will be able to agree on a generally accepted norm or whether each side will have its definition of space and act based on it.
