Understanding Kármán Line
Why 100 km?
The atmosphere does not have a sharp boundary — it gradually thins with altitude. Von Kármán's insight was that at around 100 km, the atmosphere is so tenuous that a vehicle would need to travel at orbital velocity (7.8 km/s) to generate enough aerodynamic lift to support itself. At that speed, centrifugal force alone supports the vehicle — it is effectively in orbit. This makes 100 km a natural physical transition point from aeronautics (lift-based flight) to astronautics (orbit-based flight).
The 80 km vs 100 km Debate
The US Air Force and NASA award astronaut wings at 80 km (50 miles), while the FAI and most international bodies use 100 km. This distinction became commercially significant when Virgin Galactic's SpaceShipTwo began carrying passengers to altitudes of 80–90 km — above the US definition but below the FAI boundary. Blue Origin's New Shepard crosses 100 km, and the company has highlighted this distinction in marketing. For suborbital flights, the definition determines whether passengers are considered "astronauts."
What Happens at 100 km
At the Kármán line, atmospheric pressure is about 0.00003% of sea-level pressure. The sky appears black; stars are visible in daytime. Aerodynamic control surfaces (wings, fins) are ineffective. Temperature varies wildly depending on solar activity. Objects experience near-weightlessness for several minutes during a suborbital flight. Above 160–200 km, the atmosphere is thin enough for short-duration orbital flight (a few days); above 300 km, satellites can operate for months to years depending on solar activity and drag.