There is no single, sharp line where Earth’s atmosphere ends and space begins. Instead, the air gradually thins out over hundreds of kilometers, and different organizations draw the boundary at different altitudes depending on what they’re measuring. The most widely recognized marker is the Kármán line at 100 km (62 miles) above sea level, but the U.S. military and NASA have historically used 80 km (50 miles), and traces of Earth’s atmosphere actually extend far beyond either of those points.
The Kármán Line: The Most Common Answer
The Fédération Aéronautique Internationale (FAI), the international body that keeps records for aviation and spaceflight, defines the boundary of space at 100 km altitude. This is called the Kármán line, named after physicist Theodore von Kármán, who calculated the approximate altitude where the atmosphere becomes too thin for conventional aircraft to generate lift. At that point, a vehicle would need to travel faster than orbital velocity just to stay aloft using wings, so aeronautics effectively gives way to astronautics.
Von Kármán’s original calculation landed close to 100 km, and the round number was adopted as a practical standard. NASA and the U.S. military, however, have awarded astronaut wings to pilots who crossed 80 km (50 miles), roughly 20 km lower. Neither number reflects a physical wall. They’re administrative lines drawn through a continuous gradient of thinning air.
Why There’s No Hard Edge
Earth’s atmosphere doesn’t stop at a fixed ceiling. It fades out across five layers, each with distinct temperature behavior and density. Starting from the ground:
- Troposphere (0 to 6–20 km): Where weather happens. Temperature drops from an average of about 17°C at the surface to around -51°C at its upper boundary. Its height varies from roughly 6 km at the poles to 20 km at the equator.
- Stratosphere (6–20 km to 50 km): Home to the ozone layer. Temperature actually rises with altitude here, reaching about -15°C at the top, because ozone absorbs ultraviolet radiation and releases heat.
- Mesosphere (50 km to 85 km): Temperature drops again. This is the layer where most meteors burn up.
- Thermosphere (85 km to 500–1,000 km): Temperatures can soar to 2,000°C near the top because molecules absorb intense solar radiation. Despite those extreme temperatures, the air is so thin you wouldn’t feel warm. The International Space Station orbits within this layer at about 400 km.
- Exosphere (600 km to roughly 10,000 km): The outermost layer, where individual atoms drift so far apart that they rarely collide. This is where Earth’s atmosphere gradually merges into the vacuum of interplanetary space.
The Kármán line at 100 km sits near the bottom of the thermosphere, well below where the atmosphere truly fades out. Even the exosphere doesn’t mark a clean cutoff.
Earth’s Atmosphere Reaches the Moon
Observations from the ESA/NASA SOHO spacecraft revealed that the outermost envelope of Earth’s atmosphere, a cloud of hydrogen atoms called the geocorona, extends about 630,000 km from Earth. That’s roughly 50 times the planet’s diameter and well past the Moon’s orbit. At 60,000 km out, there are still about 70 hydrogen atoms per cubic centimeter. At the Moon’s distance, that drops to around 0.2 atoms per cubic centimeter, which is almost nothing, but it isn’t zero. Sunlight compresses these atoms on Earth’s dayside and creates a slightly denser region on the night side.
By any practical definition, this region is space. But technically, it’s still a wisp of Earth’s atmosphere.
Where Air Becomes Too Thin to Matter
For different purposes, the atmosphere “ends” at very different altitudes. A few key thresholds illustrate this:
At 18 to 19 km, you hit the Armstrong limit. This is the altitude where air pressure drops low enough that water boils at body temperature. Above this point, exposed fluids like saliva and tears would boil away without a pressure suit. For the human body, this is where the protective atmosphere functionally disappears, even though you’re still deep within it.
At around 120 km, atmospheric drag becomes strong enough that any orbiting object will inevitably spiral down to reentry. Satellites can’t maintain an orbit below this altitude without continuous thrust. This is the practical floor for spaceflight.
Between 200 and 1,000 km, satellites in low Earth orbit still experience measurable drag from the thin upper atmosphere. A satellite at 300 km may survive only a few months before decaying, while one at 600 km can last years. Above 800 km, orbits can persist for centuries. Solar activity plays a major role here: during solar maximum, the thermosphere heats up and expands, pushing denser air to higher altitudes and dragging satellites down faster. The thermopause, the top of the thermosphere, fluctuates between 500 and 1,000 km or higher depending on solar conditions.
Beyond about 1,000 km, atmospheric drag becomes negligible, and other forces like the Moon’s gravity and Earth’s uneven gravitational field dominate a satellite’s behavior. By this point, for all engineering purposes, you’re in space.
So What Actually Separates Earth From Space?
The honest answer is a gradient, not a border. The atmosphere thins exponentially with altitude, and different boundaries matter for different reasons. The Armstrong limit at 19 km is where your body can no longer survive unprotected. The Kármán line at 100 km is where aerodynamic flight becomes impossible. At 120 km, orbits collapse. At 1,000 km, drag effectively vanishes. And at 630,000 km, the last stray hydrogen atoms of Earth’s geocorona finally give way to interplanetary space.
If someone asks you where space begins, 100 km is the standard answer and a perfectly good one. Just know that it’s a convenient line drawn through a long, gradual fade.