Kármán line

The Kármán line is the most commonly accepted definition of the boundary of outer space. The measurement defines space as beginning 100 kilometers (62.1 miles) above Earth’s surface. In theory, it is the altitude where the atmosphere becomes too thin for conventional aircraft to operate. At this point, the physics of spaceflight take over and the physics of aeronautics no longer apply. The line was named for aerospace pioneer Theodore von Kármán, who determined the original boundary in the mid-twentieth century. While the Kármán line is often used to define the starting point of outer space, it is not a universally accepted measure. Some nations and organizations consider outer space to start at a lower point in the atmosphere, while Earth’s atmosphere itself continues upwards for thousands of miles past the Kármán line.

Background

Earth’s atmosphere is a multi-layered envelope of air surrounding the planet and extending outward for about 6,200 miles (9,978 kilometers). The atmosphere is comprised of five main sections, each differentiated by changes in temperature, composition, density, and movement. Known as the lower atmosphere, almost all weather occurs in this region. The largest and lowest layer of the atmosphere is the troposphere. The troposphere extends from about 4 to 12 miles (6 to 20 kilometers) above Earth’s surface and contains about 75 percent of the atmosphere’s mass and the planet’s breathable air. Earth’s weather occurs in the troposphere.

The stratosphere extends from the troposphere to about 31 miles (50 kilometers) above the surface. Because of the presence of the ozone layer—the protective shield that blocks the sun’s harmful ultraviolet radiation—the temperature of the stratosphere actually increases with height. Most aircraft fly in the stratosphere to avoid the atmospheric turbulence found in the troposphere.

The mesosphere extends up to about 53 miles (85 kilometers) and is the coldest part of the atmosphere. Atmospheric gases start to become denser in this layer, providing a thicker barrier of air that causes meteors and other smaller space objects to slow down and burn up.

The thermosphere is a thin layer of the upper atmosphere that absorbs the sun’s heat energy. The International Space Station (ISS) and many satellites orbit in the thermosphere, which extends upwards to 375 miles (600 kilometers). The topmost layer of the upper atmosphere is the exosphere, where the thinning hydrogen and helium molecules of the atmosphere are extremely spread out. Many scientists consider the top of the exosphere at 6,200 miles (9,978 kilometers) above Earth to be the upper limit of the atmosphere.

From the time of the first airplane flights in the early twentieth century, nations and aeronautics organizations have tried to define the boundary at which the atmosphere ends and space begins. Many of the changing definitions were an attempt to legally define airspace above the world’s national borders. Flying aircraft within a nation’s airspace is considered a violation of international law, but outer space is not covered by international law and is free for use and exploration by all nations.

Overview

Theodore von Kármán was a Hungarian-born physicist who moved to the United States in the 1920s and eventually took over the directorship of the Guggenheim Aeronautical Laboratory at the California Institute of Technology. In 1944, he helped found the Jet Propulsion Laboratory, a national research center for space exploration that became part of the National Aeronautics and Space Administration (NASA) in 1958.

In 1956, Kármán determined that at a certain point above Earth’s surface, atmospheric molecules become too thin for aircraft to fly. To fly, aircraft need to reach speeds where they can generate lift; the thinner the air, the faster an aircraft must travel to maintain flight. Kármán calculated that the atmosphere 52.1 miles (83.8 kilometers) above Earth was too thin to allow aircraft to reach the necessary speeds to achieve lift. At that point, the craft would have to reach orbital velocity to fly or would fall back to Earth. Kármán’s boundary roughly corresponded to the division between the mesosphere and the thermosphere. Although he was not initially trying to define where space began, his measure became known as the Kármán line and was adopted as an unofficial boundary between the atmosphere and outer space.

In the early 1960s, the World Air Sports Federation (FAI), the international governing body of aeronautics and spaceflight records, reset the boundary at 100 kilometers (62.1 miles). By all accounts, the organization chose 100 kilometers as an arbitrary and easily remembered figure. The FAI’s Kármán line was adopted as the legal boundary of outer space by several national governments and aeronautics organizations; however, it was not universally recognized. For example, the US military and NASA consider 50 miles (80.5 kilometers) to be the official boundary of outer space. Pilots who fly above this mark are considered to have earned their astronaut wings and credited with achieving spaceflight.

In 2018, Jonathan McDowell, an astrophysicist at Harvard University, published a study that questioned the 100-kilometer limit of the Kármán line. Based on his observations of about fifty satellites in low-Earth orbit, McDowell determined that the boundary should be set much lower. He found that satellites were able to maintain their orbits above 50 miles (80.5 kilometers), demonstrating that they were still adhering to the physics of outer space more than 10 miles (16 kilometers) below the FAI’s Kármán line. McDowell suggested that the accepted boundary of outer space be moved to 50 miles—a figure closer to Kármán’s original calculations in 1956. In late 2018, the FAI agreed to discuss the matter with the International Astronautical Federation and explore the possibility of moving the Kármán line to a new mark of 80 kilometers.

Bibliography

Clash, Jim. "Who Has Been to Space, and Who Hasn't? Depends on Your Definition." Forbes, 10 May 2024, www.forbes.com/sites/jimclash/2024/03/10/who-has-been-to-space-and-who-hasnt-depends-on-your-definition/. Accessed 7 Nov. 2024.

Drake, Nadia. “Where, Exactly, Is the Edge Of Space? It Depends on Who You Ask.” National Geographic, 20 Dec. 2018, www.nationalgeographic.com/science/2018/12/where-is-the-edge-of-space-and-what-is-the-karman-line/. Accessed 1 Sept. 2020.

“Dr. Theodore von Kármán (1881–1963).” Jet Propulsion Laboratory, www.jpl.nasa.gov/about/bio‗vonkarman.php. Accessed 31 Aug. 2020.

Gangale, Thomas. How High the Sky?: The Definition and Delimitation of Outer Space and Territorial Airspace in International Law. Brill Nijhoff, 2018.

Grush, Loren. “Why Defining the Boundary of Space May Be Crucial for the Future of Spaceflight.” The Verge, 13 Dec. 2018, www.theverge.com/2018/12/13/18130973/space-karman-line-definition-boundary-atmosphere-astronauts. Accessed 1 Sept. 2020.

“Layers of the Atmosphere.” National Weather Service, www.weather.gov/jetstream/layers. Accessed 1 Sept. 2020.

Voosen, Paul. “Outer Space May Have Just Gotten a Bit Closer.” Science, 24 July 2018, www.sciencemag.org/news/2018/07/outer-space-may-have-just-gotten-bit-closer. Accessed 1 Sept. 2020.

Waldek, Stefanie. “Where Does Outer Space Start?” Popular Science, 1 June 2018, www.popsci.com/where-does-space-begin/. Accessed 1 Sept. 2020.

“Where Is Space?” National Oceanic and Atmospheric Administration, 22 Feb. 2016, www.nesdis.noaa.gov/content/where-space. Accessed 1 Sept. 2020.