Kármán line

The Kármán line is a conventional and widely accepted boundary at 100 kilometres (62 miles) above Earth’s mean sea level, defining where Earth’s atmosphere transitions into outer space. Named after Hungarian-American engineer Theodore von Kármán, who calculated that above this altitude, the atmosphere becomes so thin that an aircraft would need to travel faster than orbital velocity to generate sufficient aerodynamic lift, rendering it more practical to use rocket propulsion for spaceflight. While the FAI (Fédération Aéronautique Internationale) uses the 100 km standard, some organisations, including elements of the U.S. military, use an 80 km (50 miles) definition.

Since the Kármán line is set at 100 kilometers, it falls within the lower part of the thermosphere. It marks the beginning of what is largely considered the thermosphere, and thus, where the atmospheric properties truly shift towards “space-like” conditions.

Some important aspects about the Kármán line:

• Altitude: The most commonly accepted altitude for the Karman line is 100 kilometres (approximately 62 miles) above mean sea level. This is the definition used by the Fédération Aéronautique Internationale (FAI), the international record-keeping body for aeronautics and astronautics.
• Scientific Basis: Von Kármán’s work suggested that at around this altitude, the atmosphere becomes too thin for conventional aircraft to generate enough aerodynamic lift to stay aloft. In essence, above this line, a vehicle would need to travel so fast to get lift that it would exceed orbital velocity, making it more practical to use rocket propulsion for spaceflight rather than relying on air for lift.
• Not a Hard Boundary: It’s important to understand that the Earth’s atmosphere doesn’t suddenly end at a specific line. It gradually thins out. The Karman line is more of a practical and legal demarcation than a precise physical boundary where conditions drastically change.
• Varying Definitions: While 100 km is the most common, some organisations and countries, like NASA, the U.S. Federal Aviation Administration (FAA), and the U.S. military, consider the boundary of space to be at 80 kilometres (50 miles). This difference often leads to discussions about who truly qualifies as an “astronaut.”
• Purpose: The Karman line serves several important purposes:
  • Legal and Regulatory: It helps define the limits of national airspace versus international space, which are governed by different rules and treaties (like the Outer Space Treaty of 1967).
  • Astronaut Qualification: Crossing the Karman line (typically the 100 km mark, though some US agencies use the 80 km mark) is often a criterion for an individual to be officially considered an “astronaut.”
  • Aerospace Classification: It helps classify whether a craft is an aircraft (operating within the atmosphere) or a spacecraft (operating in outer space).