The distance to space is defined by convention rather than a physical barrier. The transition from Earth’s atmosphere to the vacuum of outer space is gradual, not a distinct line. Different organizations and nations use varying altitude thresholds to mark the boundary, often based on the physics of flight or legal considerations. Determining the edge of space is important for setting legal boundaries in international law and for differentiating between aeronautical and astronautical activities.
The International Standard: The Kármán Line
The most widely accepted international standard for the boundary of space is the Kármán Line, set at an altitude of 100 kilometers (62 miles) above mean sea level. This boundary is recognized by the Fédération Aéronautique Internationale (FAI), the world body for air sports and astronautics record-keeping. The line is named after Hungarian-American physicist Theodore von Kármán, who calculated the altitude where the dynamics of flight fundamentally change.
The 100 km figure represents the altitude where the atmosphere becomes so thin that aerodynamic lift is no longer practical for sustaining flight. To generate enough lift, an aircraft would have to travel faster than orbital velocity. A vehicle’s flight thus shifts from relying on air for lift to depending on centrifugal force, marking the theoretical transition from aeronautics to astronautics.
Although von Kármán’s original calculations suggested a slightly lower altitude of about 84 kilometers (52 miles), the FAI officially adopted the round number of 100 km in the 1960s for legal simplicity. This boundary is used by international organizations, including the United Nations, to differentiate between national airspace and outer space, which is generally considered free for all nations.
Alternate Altitude Thresholds
Despite the international acceptance of the Kármán Line, some major space entities use a different, lower altitude to define the start of space. The United States Air Force and NASA have historically used a boundary of 50 miles (approximately 80 kilometers). This threshold has primarily been used for awarding astronaut wings to pilots who have flown above it, such as those in the X-15 rocket-powered aircraft program.
The 50-mile mark aligns closely with von Kármán’s original calculation of where aerodynamic flight becomes impractical. It also roughly corresponds to the mesopause, where the chemical composition begins to change. Some astrophysicists propose that 80–90 kilometers is a more accurate physical boundary based on observations of how low satellites can briefly dip before their orbits decay due to atmospheric drag.
The definition of “space” is influenced by practical and historical considerations rather than a single scientific consensus. The Federal Aviation Administration (FAA) also uses the 50-mile altitude for commercial spaceflight participants to receive astronaut status. The lack of a universally binding treaty allows these alternate definitions to persist for national purposes.
Distances for Common Space Missions
Once a vehicle crosses the boundary of space, the actual distances traveled for common missions are much greater. The primary destination for most crewed and satellite missions is Low Earth Orbit (LEO), defined as the region from about 200 kilometers up to 2,000 kilometers above the surface.
The International Space Station (ISS), the largest human-made structure in space, orbits within the LEO band at approximately 400 kilometers. Satellites in LEO travel at speeds of around 7.8 kilometers per second, completing a full orbit in about 90 minutes. This orbit is favored for imaging, communication, and scientific research because its proximity minimizes launch costs.
Beyond LEO, other orbits require traveling significantly farther. Geostationary Orbit (GEO) is used for communications and weather satellites that remain fixed over a single point on the Earth’s surface. This orbit is located at 35,786 kilometers above the equator. The distance to GEO demonstrates that crossing the 100-kilometer line is only the first step into space travel.