Lesson 3: Where Does Space Start?

Why isn't there a clear, natural line between Earth and space?

The atmosphere doesn't stop suddenly—it slowly gets thinner and thinner the higher you go. There's no obvious wall, so we have to pick a line ourselves.

At what height do humans start struggling to breathe without help?

About 6 km (20,000 ft).

How high is Mount Everest and why do climbers need oxygen tanks?

Mount Everest is about 8-9 km high. Climbers need oxygen because at ~6 km humans start struggling to breathe without help.

At what height do commercial airplanes fly?

About 10 km (35,000 ft).

At what height do military airplanes fly?

About 20 km (65,000 ft).

At what height do weather balloons and spy balloons fly?

About 30 km (100,000 ft).

What's the highest height any balloon or plane has ever flown?

About 40 km (130,000 ft).

Who defined the Kármán line and what is it?

Theodore von Kármán, a scientist in the 1950s, defined it as 100 km above Earth's surface—the point where air gets so thin that an airplane simply can't fly anymore, no matter how fast it goes.

What is the internationally accepted starting point of space?

The Kármán line at 100 km above Earth's surface.

Is there still atmosphere above the Kármán line?

Yes, traces of atmosphere exist up to 1,000 km.

Why does the Kármán line matter legally?

Airplanes and satellites are governed by different regulations, and the switch happens right at 100 km.

Does NASA use the same number as the Kármán line for defining space?

No. NASA uses 80 km for deciding who counts as an "astronaut." So someone could be an astronaut by NASA's rules but not by international rules.