Sciency Words: Karman Line

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


If I may begin on a personal note, I spent most of 2018 essentially grounded by real life problems.  So for 2019, I’m dusting off the old imaginary spaceship, and I’m ready to launch myself back into outer space.  It seems I have a whole lot of space research I need to catch up on!  But first, where exactly is space?  How far away is it?

In the early 1960’s, Hungarian-American physicist Theodore von Kármán proposed an idea that has come to be known as the Karman line. Basically, the Karman line can be defined as the altitude where you need to stop thinking in terms of aerodynamics and start thinking in terms of orbital mechanics.

A traditional aircraft flying above the Karman line will no longer get enough lift to stay aloft, and a satellite or other space vehicle that dips below the Karman line will experience too much atmospheric drag to maintain its orbit.  Technically speaking, there are still more layers of Earth’s atmosphere above that line, but still this seems like a sensible enough place to define the beginning of outer space.

So how high up is the Karman line?  According to the Fédération Aéronautique Internationale (F.A.I.), which is sort of like the Guinness Book of World Records specifically for air and space flight, the Karman line is 100 km above sea level.  This is the value that seems to be most commonly accepted around the world, but it is not the value accepted by one noteworthy space agency: NASA.

According to NASA, space begins 50 miles above sea level. This 50 miles number is not merely a result of America’s famous disdain for the metric system.  As explained in this paper from Acta Astronautica, calculating the exact altitude where aircraft can no longer fly and satellites can no longer maintain their orbits has been a challenge for many decades; however, an estimate of 80 km (approximately 50 miles) may be closer to the real Karman line than the 100 km estimate set by the F.A.I.

A lot may depend on your spacecraft’s design, the parameters of your orbit, and solar activity, which causes Earth’s atmosphere to puff up slightly at times.  But to quote from that Acta Astronautica paper:

[…] elliptical orbits with perigees at 100 km can survive for long periods. In contrast, Earth satellites with perigees below 80 km are highly unlikely to complete their next orbit.

In other words, a satellite can safely dip below an altitude of 100 km, but if it gets as low as 80 km, that satellite is toast.

So when I climb back into my imaginary spaceship, how far up do I need to go to reach space?  50 miles?  100 km?  Or is there some other number I should be aiming for?

I’m still not sure.  But given the places I’m planning to go with my research in the coming year, maybe it doesn’t really matter.  Me and my imaginary spaceship will be flying well beyond the Karman line, wherever precisely that line is.

4 thoughts on “Sciency Words: Karman Line

  1. The problem is that any line will be arbitrary to some extent. Even the ISS has to periodically boost itself to stay in orbit due to atmospheric drag, and it’s 400 km up. Maybe the standard should be how high you have to be up to stay in orbit for a day, although someone could argue that if you can orbit the Earth at least once, that should count.

    Liked by 2 people

    1. That’s sort of the argument behind the 50 mile (80 km) definition. If you dip below that altitude, you won’t be able to complete even one orbit. But you still have to travel a very long way up before you’re truly free of Earth’s atmosphere. Some of the other definitions I looked at said the I.S.S. is not actually in space, and I get why someone might want to argue that.

      Liked by 1 person

    1. It is kind of hard to know where, exactly, it is. I saw some estimates that placed it lower, and some that placed it higher. But it seems like the 100 km line that the World Air Sports Federation uses and the 50 mile line that NASA uses are the most commonly cited estimates, so I focused on those.


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