The Kármán line is an imaginary boundary 100 kilometers (62 miles) above sea level that marks where Earth’s atmosphere ends and outer space begins. It’s the most widely recognized definition of “space,” used by the international World Air Sports Federation (FAI) to certify spaceflight records. But the line is less a hard physical boundary than a practical compromise, and not everyone agrees on where it should be drawn.
Why the Boundary Exists
The concept comes from a simple physical problem. As an aircraft flies higher, the atmosphere gets thinner. To generate enough lift to stay airborne, the plane has to fly faster and faster. At a certain altitude, the air is so thin that you’d need to fly at orbital speed just to keep wings working. At that point, aerodynamic flight becomes meaningless because centrifugal force from orbiting the Earth takes over. You’re no longer flying; you’re in orbit.
Theodore von Kármán, a Hungarian American engineer and physicist whose rocket science lab later became NASA’s Jet Propulsion Laboratory, worked out rough calculations for this crossover altitude. His math put it at around 84 kilometers (52 miles). Von Kármán himself never formally published the result. It was his associate, a lawyer named Andrew G. Haley, who proposed using that altitude as the official boundary of space and named it after his colleague.
How 84 Kilometers Became 100
Von Kármán’s original figure was approximate, and when the FAI adopted the concept, they rounded it up to a clean 100 kilometers. That number stuck. It’s easy to remember, sits comfortably above the messy transition zone between atmosphere and space, and gave the international community a single reference point for record-keeping and certification.
The line sits in the thermosphere, the atmospheric layer that begins around 80 kilometers and extends hundreds of kilometers upward. Air still exists at 100 kilometers, just in vanishingly small amounts. The atmosphere doesn’t end with a sharp cutoff. It fades gradually, which is part of why drawing a line at all requires some degree of arbitrariness.
The 80 Kilometer Alternative
Not everyone uses 100 kilometers. NASA and the U.S. military award astronaut wings for flights above 50 miles (about 80 kilometers), a lower threshold that has real consequences. When Virgin Galactic’s suborbital flights cross 80 kilometers but fall short of 100, the passengers qualify as astronauts under U.S. standards but not under the FAI’s international definition.
A 2018 study published in Acta Astronautica by astrophysicist Jonathan McDowell made a detailed case for lowering the line to 80 kilometers. McDowell examined the question from historical, physical, and technological angles. He found that the lowest satellite orbits ever recorded had their closest approach to Earth (called perigee) as low as 80 to 90 kilometers, meaning objects have maintained orbit at altitudes well below 100 kilometers. He also recalculated the original aerodynamic crossover and found it falls between 70 and 90 kilometers depending on the assumptions used, not at 100. His conclusion: 80 kilometers better reflects where space functionally begins, and the result holds regardless of solar activity or other atmospheric variations.
Why It Matters Legally
The Kármán line isn’t just a physics curiosity. It touches on sovereignty, international law, and commerce. The 1967 Outer Space Treaty declares that outer space “is not subject to national appropriation by claim of sovereignty,” but it never defines where outer space starts. Countries control the airspace above their territory, but no nation owns space. Without a universally agreed boundary, the exact altitude where national airspace ends and international space begins remains legally undefined.
Several countries have officially recognized 100 kilometers as the start of outer space, but there’s no binding international agreement. The United States has actively resisted efforts to set a firm legal boundary, arguing that no practical or legal problems have arisen from leaving it undefined. That position reflects the U.S. interest in maintaining flexibility for military and commercial operations at high altitudes.
As commercial spaceflight grows, the question becomes more than academic. Whether a vehicle “reaches space” determines how it’s regulated, whether its passengers count as astronauts, and which legal framework applies to what happens on board. A tourism company advertising a spaceflight experience has a financial stake in where the line sits.
What It Feels Like at the Boundary
From a passenger’s perspective, the Kármán line is invisible. There’s no physical barrier, no color change in the sky, no sudden shift. What you would notice is the view: at 100 kilometers, the curvature of the Earth is clearly visible against the blackness of space, and the thin blue band of the atmosphere is startlingly narrow below you. The sky above is black even during daytime because there aren’t enough air molecules to scatter sunlight.
Gravity at this altitude is only about 3% weaker than at the surface, so the weightlessness astronauts experience isn’t from escaping Earth’s pull. It comes from being in freefall, either on a suborbital arc back toward Earth or in orbit where you’re perpetually falling around the planet. The Kármán line marks a transition in how vehicles stay aloft, not a transition in gravitational force.