The Kármán Line is a boundary 100 kilometers above mean sea level that borders the Earth’s atmosphere and the beginning of space. However, defining exactly where the gap begins can be quite tricky and depends on who you ask. This is due to the fact earth’s atmosphere does not end abruptly but gets thinner and thinner at higher altitudes, meaning there is no definite upper limit.
International law states that “outer space shall be free to exploration and use by all” according to NOAA (opens in a new tab). But due to a variety of definitions of where space actually begins and no definitive law that confirms the true boundary. The door “where space begins” has been left wide open, inviting a host of different interpretations.
For Nasa and the US military, for example, space begins at an altitude of 50 miles (about 80 kilometers), according to NOAA. However, for the international community, including the International Aeronautical Federation (opens in a new tab) (FAI), space begins a little higher, at 100 km, on the Kármán line.
Related: The Earth’s Layers: Exploring Our Planet Inside Out
The theory behind the Kármán line
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As an aircraft climbs to a higher and higher altitude, the density of the surrounding air decreases more and more. This means the cabin air must be pressurized to allow people to breathe, but it also has an effect on how the aircraft flies.
The aircraft is held aloft by an aerodynamic force called lift, which must counterbalance the downward pull of gravity. The lower the air density, the faster the aircraft must move for its wings to generate the necessary lift.
But there is a second way to travel at high speed to counteract gravity. It was discovered by Isaac Newton in the 17th century, long before the birth of the science of aerodynamics. In fact, Newton completely ignored atmospheric effects and simply wondered what would happen if a cannonball was fired horizontally at higher and higher velocity.
The answer is that it travels further and further before falling Earth. Eventually, when it reaches “orbital velocity“, the cannonball circles the planet without ever touching the ground.
Fast forward to the middle of the 20th century, when a Hungarian-American aerospace engineer named Theodore von Kármán posed a simple question. At what altitude does the speed needed to keep an airplane aloft with aerodynamic lift become so high that it exceeds orbital speed?
Kármán did the necessary calculations, then rounded the answer to that memorable figure of 100 kilometers (62 miles). This elevation is now known as the “Kármán line” in his honor.
Given the controversy surrounding whether to start at around 50 miles (80 km) or 62 miles (100 km), some people wonder if it would be easier to simply define space as the absolute point at which space ends. earth’s atmosphere. But this definition would complicate things even more.
Traveling beyond the limits of Earth’s atmosphere would take you approximately 10,000 km above the Earth’s surface to the top of the highest layer (opens in a new tab) of the Earth’s atmosphere — the exosphere. The exosphere marks the limit of our atmosphere, so why wouldn’t it also mark the beginning of space?
The international space station (ISS) orbits the Earth at an average altitude of 248 miles (400 kilometers) and in low Earth orbit satellites stay at altitudes of less than 620 miles (opens in a new tab) (1000 kilometers). With the space frontier at this new height of 6,000 miles (10,000 km), most of our spacecraft orbiting Earth would no longer be considered “spacecraft” and all visitors to the ISS, for example, would no longer be called astronauts.
This new definition would only blur the definition of space waters even more than the two definitions we currently have of 50 miles (80 km) and 62 miles (100 km). So for now, these are our best options.
For more information on Theodore von Kármán’s work and the Kármán line, see the NASA biography (opens in a new tab) on the pioneer and “Theodore Von Kármán, 1881-1963 (opens in a new tab)by Sidney Goldstein.
Nasa. (2019, October 2). Earth’s atmosphere: a multi-layered cake — climate change: the planet’s vital signs. Nasa. Retrieved November 11, 2022, from https://climate.nasa.gov/news/2919/earths-atmosphere-a-multi-layered-cake/ (opens in a new tab)
Nasa. Earth’s atmosphere. Nasa. Retrieved November 11, 2022, from https://www.grc.nasa.gov/WWW/K-12/airplane/atmosphere.html (opens in a new tab)
Types of orbits. ESA. Retrieved November 11, 2022, from https://www.esa.int/Enabling_Support/Space_Transportation/Types_of_orbits (opens in a new tab)
Where is the space? NESDIS. Retrieved November 11, 2022, from https://www.nesdis.noaa.gov/news/where-space (opens in a new tab)
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