🌍 Kármán Line — Where Does Space Begin?

Theodore von Kármán calculated that at ~100 km, the air is too thin for a vehicle to generate aerodynamic lift without exceeding orbital velocity. This makes 100 km a physically meaningful transition between flight and orbital mechanics. The FAI adopted this as the official boundary, though the US and some space companies use 80 km (50 miles) instead.

Orbital velocity is the speed at which centrifugal force exactly balances gravity, keeping a satellite in circular orbit (~7.9 km/s at Earth's surface, decreasing slightly with altitude). The Kármán line is where the velocity needed for aerodynamic lift equals orbital velocity — above this, wings are useless and rockets or orbital mechanics must provide the trajectory.

The 1967 Outer Space Treaty establishes freedom of access and no sovereignty above a certain altitude, but does not define the exact boundary. No binding international treaty sets the precise altitude of the space boundary. The 100 km FAI definition is widely used in practice, but the legal ambiguity matters for routing satellites over other nations' territories and for regulating commercial suborbital flights.

At 100 km, air pressure is about 0.032 Pa (3×10⁻⁷ atmospheres), temperature is around −70°C, and the mean free path of air molecules is ~30 cm. Spacecraft experience negligible drag but satellites in very low orbits (180–200 km) decay significantly. The ionosphere's D-layer, which absorbs HF radio waves, begins around this altitude. Meteors typically ablate at 80–120 km.

The region from about 50–100 km is the mesosphere; the thermosphere begins at ~80–100 km. Between 80–100 km lies the mesopause, the coldest region of Earth's atmosphere (~−85°C), where noctilucent clouds form. Just above 100 km, the International Space Station orbits at ~400 km, and low Earth orbit (LEO) satellites operate from ~200–2000 km.