Boundary of space altitude
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Defining the Boundary of Space Altitude: The Kármán Line and Other Criteria
The Kármán Line as the Conventional Space Boundary
The most widely recognized boundary between Earth's atmosphere and outer space is the Kármán line, set at an altitude of 100 kilometers (km) above sea level. This definition was established in the early 1950s by Theodore von Kármán, who argued that at this altitude, the atmosphere becomes too thin to support conventional aeronautical flight, and vehicles must travel at orbital velocity to generate enough lift from aerodynamic forces. This boundary was adopted by the International Federation of Astronautics (IAF) and is also used by the Fédération Aéronautique Internationale (FAI), which governs records for both aircraft and spacecraft. The FAI refers to this as the "Kármán ellipsoid," marking the nominal transition from aeronautics to astronautics, and thus, from aircraft to spacecraft operation .
Alternative Space Boundary Proposals
While the Kármán line at 100 km is the most accepted, some experts and organizations have proposed different boundaries. For example, an 80 km altitude has been suggested as a practical boundary based on the balance between gravity and aerodynamic forces. At this altitude, aerodynamic lift becomes negligible, and the environment is more characteristic of space than of the atmosphere. This 80 km threshold is sometimes used in the United States for awarding astronaut wings and is justified by the physical transition from aeronautical to astronautical domains .
Physiological and Atmospheric Considerations
Other significant altitude markers include the Armstrong limit at about 19 km, where human blood would boil without a pressure suit due to low atmospheric pressure. However, this is far below the operational boundary of space and is more relevant for human survival than for defining the edge of space . Measurements of the solar spectrum at altitudes up to 35 km show that the atmosphere is still present and affects solar radiation, indicating that true "space" conditions are not reached until much higher altitudes .
Gravity and Geophysical Boundaries
From a geophysical perspective, studies of Earth's gravity field show that as altitude increases, the characteristics of the gravity field anomalies change significantly. At satellite altitudes, such as 500 km, these anomalies are much less pronounced than at sea level, reflecting the transition to the external gravitational field of the Earth, which is a key feature of the space environment .
Practical Implications for Spacecraft and Debris
The definition of the space boundary also has practical implications for spacecraft design, space debris removal, and mission planning. For example, the choice of disposal orbits for space debris and the design of spacecraft propulsion systems depend on the altitude at which space operations are considered to begin .
Conclusion
The boundary of space altitude is most commonly set at the Kármán line, 100 km above sea level, based on physical, operational, and regulatory considerations. Alternative boundaries, such as 80 km, are also used in some contexts, but the 100 km mark remains the international standard for distinguishing between aeronautics and astronautics. This definition is supported by both the physical properties of the atmosphere and the practical needs of space operations 132.
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