Kármán line

The Kármán line is a special height above Earth that many people use to mark where the sky ends and space begins. It is set at 100 kilometres (about 62 miles) above sea level. This line is used by groups that keep track of flying and space records, such as the FAI, which is short for the Fédération aéronautique internationale.

The line is named after a smart scientist named Theodore von Kármán, who figured out that airplanes can’t fly very high because the air gets too thin. Even though this line isn’t a real wall between Earth and space, it helps countries decide which rules apply to airplanes and which ones apply to spacecraft. Different countries might pick a slightly different height for this line, but most agree it’s close to where the FAI sets it.

This line matters because it helps our world know when something is flying in the sky or traveling in space. It’s high enough that regular airplanes and balloons can’t reach it, but low enough that some satellites might still come back down to Earth before they can go all the way around the planet.

Definition

The FAI uses the Kármán line to mark the boundary between aeronautics and astronautics. Activities within 100 kilometers of Earth's surface are considered aeronautics, while those above this line are part of astronautics.

Interpretations of the definition

"Edge of space" redirects here. For other uses, see Edge of space (disambiguation).

People sometimes talk about the "edge of space" or "near space" to describe areas below where space officially starts. This can include places much lower than where most airplanes fly. For example, some balloon or airplane flights might be called "reaching the edge of space" just because they flew very high.

There isn’t a worldwide rule about where a country’s air ends and space begins. In 1963, Andrew G. Haley wrote about this in his book Space Law and Government. He explained that the Kármán line is like an average measurement, similar to "mean sea level" or a "tide line". Many things must be thought about to decide where air stops and space starts, such as the air’s condition and where flying becomes very hard.

Kármán's comments

Theodore von Kármán talked about where space begins in his book. He used an example of a pilot named Iven Carl Kincheloe Jr., who flew very fast and high in a special plane called the X-2. At one height, the plane still needed air to stay up, but higher up, there wasn’t enough air, so only the force of motion kept the plane moving. Kármán thought this height could be a line to separate where countries’ rules apply from where there are no rules for space. This idea was called the Kármán Jurisdictional Line.

outer space
Iven Carl Kincheloe Jr.
X-2 rocket plane
aerodynamic lift
Kepler force
aerodynamics
astronautics
Andrew G. Haley

Technical considerations

The air around Earth doesn't suddenly stop; it just gets thinner and thinner as you go higher. Because of this, deciding where space really begins can be tricky. The Kármán line is one way to mark this boundary, but it's mostly a made-up line for technical reasons.

Airplanes stay in the sky by moving forward fast enough to create lift with their wings. But as the air gets thinner, they need to go faster to stay up. At very high speeds, the force from turning around Earth helps keep airplanes in place, like how satellites stay in orbit. Scientists figured out that at a certain height, airplanes would need to go so fast that they'd basically become satellites. This height is part of why the Kármán line is set where it is.

A scientist named von Kármán looked at how hot airplanes get when they fly fast. He found that above a certain height, airplanes would get too hot to fly safely. Another scientist later used this idea to help pick a height for the Kármán line, but he knew this height might change as new technology develops.

Alternatives to the FAI definition

Different groups use different heights to mark the edge of space. The U.S. Armed Forces say that anyone who flies higher than 50 miles (80 km) above sea level is an astronaut. This is about the line between the mesosphere and the thermosphere.

NASA used to follow the same 100-kilometre (62-mile) rule as the international group FAI, but changed it in 2005 so military and civilian pilots would have the same rules. Three pilots from the X-15 aircraft were later given astronaut wings because they had flown between 90 km and 108 km in the 1960s.

Some scientists, like Jonathan McDowell and Thomas Gangale, think the line should be lower, at 80 km. They say this is where the atmosphere really starts to end, based on old notes from Theodore von Kármán and new space technology. They also point out that objects orbiting at 80 km to 90 km can survive, while those lower usually break up.

The U.S. military uses a different idea for the edge of space, looking at the lowest point an orbiting object can reach. They don’t pick a specific height, but say it’s about 150 km (93 miles) before an object can’t stay in orbit without help.

For other planets

The Kármán line is usually talked about for Earth, but some scientists have thought about similar lines for other planets like Mars and Venus. One scientist, Isidoro Martínez, suggested these lines would be at 80 km (50 miles) for Mars and 250 km (160 miles) for Venus. Another scientist, Nicolas Bérend, estimated them to be at 113 km (70 miles) for Mars and 303 km (188 miles) for Venus.

In popular culture

In 2014, Oscar Sharp made a short film called The Kármán Line. The film stars Olivia Colman as Sarah, a mother who begins to float upwards. She slowly rises until she reaches the Kármán line and enters outer space.