Why is the Kármán line at 100 km and not some other altitude?

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.

What is orbital velocity and how does it relate to the Kármán line?

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.

Does the Kármán line have a legal status in international law?

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.

🌌 Kármán Line — Where Does Space Begin?

Drag the altitude slider to explore how atmospheric density, aerodynamic lift, and orbital speed change with height. The Kármán Line (~100 km) marks where the air is too thin to provide meaningful aerodynamic lift at any speed — and any aircraft would need to travel orbital velocity just to stay aloft.

Probe Settings

Altitude 80 km Wing area (m²) 30 m² Aircraft mass (t) 15 t Lift coeff C_L 0.50

Zone

—

Telemetry

Altitude—

Air density ρ—

Pressure p—

Orbital speed—

Min. lift speed—

Lift ≥ orbit speed—

Physics:
ρ(h) = ρ₀·exp(−h/8500)
L = ½·ρ·C_L·A·v²
v_orbit = √(GM/r)
Kármán: v_lift = v_orbit

Legend

━ Density profile
━ Min. lift speed
━ Orbital speed
✦ Kármán Line ≈100 km
⊕ Your probe altitude

The Kármán Line Explained

Hungarian-American physicist Theodore von Kármán calculated that above approximately 100 km altitude the atmosphere becomes so thin that an aircraft would need to travel at orbital velocity (≈7.9 km/s) just to generate enough lift to support itself. At that point, it is no longer "flying" aerodynamically — it is orbiting. The Fédération Aéronautique Internationale (FAI) adopted 100 km as the official boundary of space. Below this line, lift > gravity is possible with sub-orbital speeds. Above it, only rocket thrust can maintain altitude. The US Air Force uses 80 km (50 miles) as its own boundary, which is why some X-15 pilots were awarded astronaut wings for reaching that altitude.

About Kármán Line Simulator

The Kármán line is the internationally recognised boundary between Earth's atmosphere and outer space, set at 100 km (62 miles) altitude by the Fédération Aéronautique Internationale. At this altitude, the atmosphere is so thin that a vehicle travelling fast enough to generate aerodynamic lift equal to its weight would need to exceed orbital velocity, making conventional aeronautics impractical and orbital dynamics dominant. It was named after Theodore von Kármán, who first computed this boundary.

The physical basis of the Kármán line involves balancing aerodynamic lift, which scales as ρv² (where ρ is air density and v is velocity), against gravity. As altitude increases, air density decreases exponentially with a scale height of ~8.5 km. At 100 km altitude, density is approximately 2.2 × 10⁻⁷ kg/m³ — a million times less than at sea level. For a vehicle to generate lift, it would need v ≈ 8 km/s, which is orbital velocity for circular orbits around Earth (~7.9 km/s at sea level altitude).

The Kármán line is a legal and regulatory boundary: above it, the 1967 Outer Space Treaty applies (free access, no national sovereignty) rather than national airspace law. The US Air Force originally awarded astronaut wings above 50 miles (80 km), which is the basis for some commercial spaceflight companies' definitions. International law around national airspace versus outer space lacks a firm treaty definition, creating ambiguity as high-altitude flights become more common.