Why is it difficult to identify the exact boundary between airspace and outer space?

In the context of air law, identifying the exact boundary between airspace and outer space is difficult primarily because Earth’s atmosphere does not have a sharp, defined cutoff; instead, it thins out gradually with altitude.12

Because international law grants nations sovereignty over their own air space while declaring outer space to be free and not subject to national appropriation, the lack of a precise legal boundary creates significant ambiguity. Various conventions exist, but none are universally accepted:134

• The Kármán line: Often cited as the boundary, this estimate (frequently placed at 80–100 km) was originally based on the altitude where aircraft can no longer rely on aerodynamic lift.12
• Conflicting Standards: Organizations like NASA and the Fédération Aéronautique Internationale have used different altitude markers (80 km vs. 100 km), and air density fluctuates even at consistent altitudes, making a fixed line technically fluid.12

Ultimately, the United Nations has refrained from designating a formal boundary, leaving the legal transition point between national territory and international space unresolved.24