Space begins at 100 kilometers (62 miles) above Earth’s surface, at least by the most widely used definition. This invisible line, called the Kármán line, marks the altitude where the atmosphere becomes so thin that conventional flight is physically impossible. But not everyone agrees on this number, and the real answer depends on who’s drawing the line and why.
The Kármán Line at 100 Kilometers
The most recognized boundary between Earth’s atmosphere and outer space is the Kármán line, set at 100 kilometers. The World Air Sports Federation, which is the international body that certifies aeronautic and astronautic records, uses this as its official boundary. NASA also references it as the standard transition point.
The reasoning is rooted in physics. As you climb higher in altitude, the atmosphere gets thinner. An aircraft needs to move faster and faster to generate enough lift to stay airborne. At a certain point, the air is so sparse that a vehicle would need to travel at orbital velocity just to maintain lift. At that speed, you’re no longer flying through the atmosphere. You’re orbiting. Centrifugal force has replaced aerodynamic lift, and traditional flight has lost all meaning. After extensive calculations in the mid-20th century, scientists settled on 100 kilometers as the altitude where this crossover happens.
To put the thinness of the air in perspective: 99.99997 percent of Earth’s atmosphere sits below the Kármán line. At 100 kilometers, air density is roughly one two-millionth of what it is at sea level. There’s still technically gas up there, but it’s negligible for any practical purpose related to flight.
The 80-Kilometer Alternative
The United States has long used a lower boundary. The U.S. Air Force and the Federal Aviation Administration award astronaut wings to anyone who flies above 50 miles (about 80 kilometers). This is the standard that military and commercial pilots in the U.S. have historically been measured against.
This lower line got a significant scientific boost in 2018, when astrophysicist Jonathan McDowell at the Harvard-Smithsonian Center for Astrophysics published a detailed analysis revisiting the Kármán line. After examining historical, physical, and technological evidence, including the trajectories of actual space vehicles, McDowell concluded that 80 kilometers is a more appropriate boundary than 100 kilometers. His argument is that the functional edge of space, where satellites can briefly sustain an orbit and where the atmosphere stops meaningfully affecting objects, sits closer to 80 kilometers than the round number everyone adopted decades ago.
Why the 20-Kilometer Gap Matters
The difference between 80 and 100 kilometers might sound academic, but it has real consequences for the people and companies flying near these altitudes. When Richard Branson’s Virgin Galactic launched its SpaceShipTwo, the vehicle reached a peak altitude of around 90 kilometers. That’s above the U.S. 80-kilometer line but below the internationally recognized Kármán line. Jeff Bezos’s Blue Origin, by contrast, designed its New Shepard rocket to fly just past 100 kilometers, clearing the Kármán line by a slim margin.
Both companies can technically claim their passengers reached space, but by different definitions. It’s the same trip to the edge of the atmosphere, judged by two different rulers. For passengers paying hundreds of thousands of dollars, and for pilots hoping to earn astronaut wings, which line you use determines whether you’ve been to space or just very, very high up.
The Atmosphere Doesn’t Simply Stop
Part of the reason there’s no single answer is that Earth’s atmosphere doesn’t have a hard edge. It fades gradually over hundreds of kilometers. The thermosphere, which begins around 80 kilometers, extends up to about 700 kilometers. Above that is the exosphere, stretching from roughly 700 to 10,000 kilometers, where individual gas molecules drift so far apart they rarely collide with each other. At the very top of the exosphere, the atmosphere effectively merges with the solar wind flowing from the sun.
So depending on how you define “atmosphere,” you could argue it extends more than 6,000 miles into space. But almost none of that gas has any practical effect. For satellites, the relevant question is how low they can orbit before atmospheric drag pulls them back to Earth. The European Space Agency notes that satellites generally don’t fly below 180 kilometers because the thin wisps of atmosphere at that altitude are still enough to decay an orbit rapidly. Even at the Kármán line, there’s far too much drag for anything to stay in orbit for more than a brief pass.
No Legal Line in the Sky
International law hasn’t settled the question either. The 1967 Outer Space Treaty, the foundational agreement governing activities in space, establishes that outer space is not subject to national sovereignty. No country can claim ownership of territory in space. But the treaty never specifies where outer space actually starts. Nations have sovereign control over their airspace, and space belongs to everyone, yet no treaty draws the line between the two.
This legal ambiguity has persisted for decades. The United Nations Office for Outer Space Affairs oversees the treaty, but defining the boundary remains an unresolved point of international discussion. As more commercial vehicles fly in the zone between 80 and 100 kilometers, the question of where airspace ends and outer space begins carries increasing legal weight for issues like overflight rights and liability.
The Practical Answer
If you’re looking for a single number, 100 kilometers is the standard most international organizations and space agencies use. It’s the Kármán line, the altitude where aerodynamic flight gives way to orbital mechanics. The U.S. uses 80 kilometers for its own purposes, and there’s credible scientific support for that lower figure. Both numbers mark a real physical transition: the point where Earth’s atmosphere becomes so thin it essentially ceases to exist for anything trying to fly through it. The air doesn’t vanish at either altitude, but for all practical purposes, the sky ends and space begins somewhere in that 20-kilometer window.