The Kármán line: Separating space from sky

How do you know where the sky ends and space begins?
Raymond Aguilar
Kármán line
Kármán line

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The idea of space travel fascinates almost everyone. Sci-fi movies that involve space often include scenes inside spaceships roaming the universe from one planet to another. Although it all seems impossible at a young age, we eventually learn that space travel is not entirely fiction and is, in fact, possible. But what exactly is meant by space, and where exactly does it begin?

There is actually no clear agreement on exactly where space begins. Various heights have been used, including 50 miles (80 km), 62 miles (100km), and more than 6,000 miles (10,000km) above the Earth's surface. Based on this, it might still be unclear what space is.

Knowing exactly where space begins may seem like a question of no real importance. However, according to international law, the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space states that “outer space shall be free for exploration and use by all.” Because there are different definitions of where space actually begins, this could bring a number of legal consequences.

There might also be other questions that have come across our minds regarding space and space travel. Regardless of the time of the day, you’ll see the sky as you go outside and look up. Typically, you’d see airplanes in the sky, but perhaps you have wondered if airplanes can go into space, just like spaceships in sci-fi movies. How do you know when an object is in space? All these curious questions can be answered by none other than – “The Kármán line.”

What is the Kármán line?

The Kármán line is an invisible boundary that is used by some to delimit where Earth’s space begins. Therefore, knowing where this is is essential in identifying which is space and which is the sky.

The Kármán line: Separating space from sky
Kármán line

The sky as we know it can better be understood in the context of the Earth’s atmosphere and its many different layers. Although the Kármán line is defined as a boundary, it doesn’t necessarily mean that it acts like a solid boundary line where one side would all be space and the other side would be all atmosphere. This is because the Earth’s atmosphere does not disappear suddenly but gets thinner as it gets higher and eventually blends into space. In addition, the sky does not necessarily equate to the Earth’s atmosphere but rather is a term in relation to everything we can see when we look up.

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The Earth’s atmosphere is composed of five major layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere (and several secondary). The layers are divided according to altitude and temperature. The first layer, the troposphere, can be found on average within 7.5 miles (12km) of the Earth’s surface, and this is where clouds are usually found. This is the densest layer. Next to the troposphere is the stratosphere, where the UV-shielding ozone layer can be found. In contrast with the troposphere, the temperature in the stratosphere increases as the altitude increases because of the presence of the ozone layer, while temperature decreases with altitude in the troposphere.

The Kármán line: Separating space from sky
Layers of the atmosphere

The next layer is the mesosphere, located between about 31 to 50 miles (50 to 80 kilometers) above the Earth’s surface, where again, temperatures decrease as the altitude increases – it is the coldest among the layers of the Earth’s atmosphere. Next comes the thermosphere, which starts at around 50 miles (80km) and reaches around 700 miles (440 km). This is where temperatures start to rise again with the altitude – due to the low density of molecules in this layer and to solar radiation. This is also the layer where the International Space Station orbits.

Lastly, the final layer, the exosphere, extends to around 6,214 miles (10,000km) and eventually merges with the solar wind. In this layer, molecules are of low density and escape into space. Therefore, due to this behavior, the exosphere is not actually defined as a definitive upper boundary of the Earth's atmosphere. This is also why the atmosphere is said to be getting thinner and thinner as it goes up and eventually merges with space. You can think of it like a two-color gradient where one color smoothly blends into another. There is no definitive boundary in which you can tell where each color stops and the next begins. It just smoothly transitions from one color to another – similar to how the atmosphere mixes with space.

The Kármán line lies in the lower thermosphere, the fourth layer of the atmosphere, as you move away from the Earth’s surface. All things passing this are generally considered as part of space – per the definition of the Kármán line. In relation to the earlier question, airplanes cannot go into space as they typically fly in the troposphere, the atmospheric layer closest to Earth. Above the Kármán line, the air is too thin to support a craft that relies on the lift generated by Earth’s atmosphere. Similarly, only those that fly beyond the Kármán line can generally be considered spacecraft.

Origin of the Kármán line

This line that defines space is named after Hungarian-American engineer and physicist Theodore von Kármán. He is known for his work and contribution to aerospace and aeronautics. He moved to the U.S. in 1930 and became an expert in rockets and supersonic flights. Together with colleagues, he helped found NASA’s Jet Propulsion Laboratory. As part of his work, von Kármán was interested in the question of at what is the altitude at which aerodynamic lift can no longer be possible to keep an aircraft aloft. 

To answer this question, he calculated what is now known as the Kármán Line.

How high is the Kármán line?

The line was established in the 1960s by the Fédération Aéronautique Internationale (FAI) and has been used by members, countries, and organizations affiliated with FAI as one definition of “space.” The FAI definition of the Kármán line is around 62 miles (100km) above sea level. The originally computed number was difficult to remember, so the rounded value of 100 km is used as it is easier to remember and was later accepted as the height of the line.

Similarly, the National Oceanic and Atmospheric Administration (NOAA) sets this imaginary boundary where space begins at 62 miles (100km). However, not everyone recognizes the definition of space given by FAI using the Kármán line.

The Federal Aviation Administration, the U.S. Air Force, and NASA use 50 miles (80 km) above sea level as the definition of where space begins. Those who pass this mark are given their astronaut badge, which is also known as astronaut wings due to the wing design feature of the badge itself. These astronaut wings are formally known as either the FAA Commercial Astronaut Wings, the U.S. Astronaut Badge, or the NASA Civilian Astronaut Wings, depending on the badge-giving body. The differences in how space is defined between organizations lie behind the idea of the possible removal of the astronaut status of some badge earners.

Nevertheless, both altitudes used in defining the beginning of space are still within the thermosphere.

How many humans have crossed the Kármán line?

Since space travel was made possible, many humans have crossed the Kármán line.

Astronauts are the ones predominantly traveling in space, and crossing the Kármán line means that an object or a person is in space. By this definition, humans crossing the line would be considered astronauts, hence the wings. Yet, it should be important to understand that the Kármán line, as defined by FAI, shouldn’t be the sole basis for determining one’s astronaut status. This is because other organizations mentioned earlier, like NASA, use the 50-mile mark in defining space and grant astronaut wings to whoever passes this mark.

The Kármán line: Separating space from sky
Atlantis undocking ISS

Perhaps an interesting topic in relation to humans crossing the line is the billionaire space race. This was between the space companies Blue Origin, owned by Jeff Bezos, and Virgin Galactic, owned by Richard Branson, both of which are competing, in part, to develop commercial and space tourism. Blue Origin aimed to travel into space, flying past the Kármán line using FAI’s definition at 62 miles (100km) above sea level. Meanwhile, space planes from Virgin Galactic transport people into space using the definition of 50 miles (80 km) above sea level, as recorded on Branson’s flight reaching 53.5 miles (85km).

The billionaire space race between these two companies re-egnited the conversation on how space is defined, whether by using FAI’s Kármán line or the 50-mile mark as used by FAA, U.S Airforce, and NASA.

Importance of the Kármán line

The definition of the Kármán line as the invisible boundary where space begins is indeed an important aspect in relation to space-related travel. Even if the altitude where space begins is debatable (62m Kármán line and 50-mile mark), the idea given by this line still provides an idea of how we can say something is in space.

Focusing on the Kármán line in defining space, the line would be beneficial in relation to how specific rules on flying are applied, like in the international space law.

Now, whenever someone says your goals are unattainable because you are aiming for the sky, tell them that you’re reaching for space instead. There's a great chance they will believe you’ll achieve your goal, as space is a much more precise concept than the sky, thanks to the Kármán line.