Photo: NASA · Public domain
100 km
Space is often said to begin just 100 kilometers above your head.
5 min readLast updated
The short answer
Yes. A common definition puts the edge of space at the Kármán line, 100 kilometers above sea level. Some organisations, including the US Air Force and NASA for astronaut wings, use about 80 kilometers instead.
Why it's true
Earth's atmosphere does not stop at a clear line. It gets thinner and thinner with height until it fades into space. To decide where space begins, people needed a convenient boundary. The Fédération Aéronautique Internationale, which keeps aviation and spaceflight records, uses 100 kilometers, known as the Kármán line.
The idea is linked to the engineer Theodore von Kármán. He reasoned that as air thins, an aircraft must fly faster and faster to get enough lift from its wings. At some height, it would need to go as fast as an orbiting satellite, so wings stop being useful and spaceflight takes over. That height is roughly where the line sits.
One hundred kilometers is surprisingly close, about the distance from London to Brighton and back, but vertical.
By the numbers
100 km
Kármán line altitude
~80 km
US astronaut wings threshold
~400 km
Space station altitude
~10,000 km
Approximate outer reach of the exosphere
The backstory
Von Kármán's own calculations in the 1950s suggested a boundary somewhat lower than 100 kilometers, near 80 to 90 kilometers. The round figure of 100 was later adopted for simplicity. Meanwhile, the United States awarded astronaut wings to pilots who flew above 50 miles, about 80 kilometers, a tradition dating back to rocket plane flights such as the X-15 in the 1960s.
The debate came back into the news with commercial space tourism. Some suborbital flights cross 100 kilometers, while others reach just above 80. Astronomer Jonathan McDowell has argued, based on satellite orbits, that 80 kilometers is a more physically meaningful line. Both definitions are still used, depending on who is counting.
A short trip, a huge speed
Reaching 100 kilometers is relatively easy for a rocket. Staying in space is the hard part. An orbit needs a sideways speed of about 28,000 kilometers per hour.
That is why suborbital tourist flights can cross the line and return within minutes, while orbital missions need far larger rockets.
Worth remembering
- The Kármán line puts the edge of space at 100 kilometers.
- Some organisations use about 80 kilometers instead.
- The atmosphere has no sharp edge; it thins gradually.
In context
Most of the air you breathe is packed close to the ground. About half of the atmosphere's mass lies below roughly 5.5 kilometers, lower than the summit of Mount Everest. Airliners cruise around 10 to 12 kilometers up, already above most of the air.
From orbit, the atmosphere looks like a thin blue film wrapped around the planet. Astronauts often say this view made them realise how fragile and precious it is.
Air pressure at 100 kilometers is less than a millionth of its value at sea level. Without a pressurised suit or cabin, a human would lose consciousness within seconds. Yet the air there is still thick enough to heat incoming meteors until they glow, which is why most shooting stars appear between about 70 and 120 kilometers up, close to where many people say space begins.
A closer look
Layers of the sky
Scientists divide the atmosphere into layers. The troposphere holds weather, the stratosphere holds the ozone layer, and the mesosphere is where most meteors burn up.
Above that lies the thermosphere, where the Kármán line sits and where the station orbits, then the exosphere fading into space.
Auroras near the line
The northern and southern lights glow mostly between about 100 and 300 kilometers up, as charged particles from the Sun hit thin air.
That puts the lower edge of many auroras right around the edge of space.
Why the definition matters
Records, astronaut titles and some legal discussions depend on where space begins. Treaties govern outer space differently from national airspace.
Yet there is no single international law fixing the boundary, which is why different numbers persist.
Related facts
For life far above the line astronauts see 16 sunrises a day.
Space is closer than you think astronauts can be the nearest people to Point Nemo.
For what the atmosphere is made of nitrogen makes up most of the air.
The thin blue line has a scientific explanation why the sky is blue.
Key terms
- Kármán line
- The 100 kilometer altitude commonly used as the boundary of space.
- Suborbital flight
- A flight that reaches space but falls back without completing an orbit.
- Exosphere
- The outermost layer of Earth's atmosphere, fading into space.
Myth vs reality
Myth
There is a sharp edge where the atmosphere ends and space begins.
Reality
The atmosphere thins gradually. The Kármán line is a convention, not a physical wall.
People also ask
What is the Kármán line?
It is a boundary 100 kilometers above sea level, widely used to mark the start of space. It is named after engineer Theodore von Kármán.
Why 100 kilometers?
Around that height, an aircraft would need to fly close to orbital speed to stay up using its wings. The round figure of 100 was chosen for convenience.
Does everyone agree space starts at 100 kilometers?
No. The US military and NASA have long used about 80 kilometers for astronaut wings, and some scientists argue 80 kilometers is more meaningful.
Is there any air above the Kármán line?
Yes, but very little. Traces of atmosphere extend thousands of kilometers higher, in a layer called the exosphere.
How high do satellites orbit?
Most low Earth orbit satellites fly from a few hundred to about 2,000 kilometers up. Below about 160 kilometers, air drag makes orbits decay quickly.
