The Edge Of Space

Welcome to Episode 2 of “On My Way To Space”. I’m your host, Dan, and today we’ll be talking about… big surprise… space!

My favorite smartphone wallpaper, yes, that’s a thing nowadays, is one that came with my phone’s operating system. It’s a dynamic picture of our beautiful earth from space. Dynamic, as in the globe is slowly rotating. Furthermore, the wallpaper is simulating daytime and nighttime by rendering corresponding satellite imagery onto the sphere. There are even simulated clouds, which I believe are based on actual weather radar data. In any way, a lot is happening on my phone’s screen even when I’m not using it actively. But one thing always made me wonder: On my phone’s wallpaper space begins more or less where earth ends. As if there is a clear line for where earth ends and where space begins.

And this is how I started wondering: Where exactly does space start and our home planet end? Would I be in space if I climb the highest mountain on earth, Mount Everest, and jump into the air? Of course not. Similarly, riding a passenger airplane doesn’t make one an astronaut. But somewhere high up the line must be drawn. I wanted to find out where exactly the boundary between earth and outer space is and how it is defined. I wanted to learn about the edge of space.


My research brought up several new terms and even more questions. To name few:

  • I learned there’s a thing called space weather
  • I smiled at nations being unable to agree on when to call a person an astronaut
  • I stumbled over pictures of atmospheric layers not being to scale and therefore harder to grasp dimensionally

And so much more… But ain’t nobody got time for that.

I’d like to share the three approaches of defining the boundary between earth and space that I found most appealing. Coincidentally, they’re a bit sciency. But be aware, I’m not a rocket scientist and I heavily rely on others getting the math right when they shoot me into space eventually. So please double check the numbers I’ll mention if your life depends on them.

Let’s take a deep breath. Air, a mix of gases including oxygen, nitrogen, and carbon dioxide, is what it all begins with. To be more precise, we’ll be using the term atmosphere going forward. Earth’s atmosphere is held in place by gravity, although other factors, such as temperature, play a role in preventing atmospheric gases from evaporating into outer space.

On the surface level the atmospheric pressure is around 101,325 Pascals, commonly referred to as 1 atmosphere, or for the fans of good old non-standardized units: 29.92 inches of mercury. Generally speaking, atmospheric pressure is higher closer to the ground and decreases the further away we move from the center of gravity. However, not linearly so, as other effects such as temperature and density come into play. At around 100 km altitude atmospheric pressure drops to .032 Pa. The atmospheric pressure becomes insignificant at this altitude compared to the radiation pressure from the sun and dynamic pressure from solar winds and maybe space weather.

As the density of the atmosphere gradually decreases at higher altitudes we observe an interesting effect. Aerodynamic lift begins to lose its power. Aerodynamic lift is a force exerted perpendicular to the direction of movement of an object moving through air. Think of wing-based aircraft like a jet-powered passenger plane. Their wings, and with them the whole aircraft, move fast through the surrounding air thus lifting the aircraft up overcoming the force of gravity. This effect works best at certain air densities. At roughly 100 km altitude the density of earth’s atmosphere isn’t much good anymore for lift to occur. At this altitude it gets easier to maneuver a vehicle using thrusters. Orbital space flight, however, is a topic for another time.

Another way of defining the boundary between earth and space is to look at the mass of earth’s atmosphere and how much of it is present below a certain altitude. Considering how essential the atmosphere is for our survival I think we should look at those numbers:

  • 50% of earth’s atmospheric mass is below 5.6 km altitude (18,000 ft)
  • 90% of earth’s atmospheric mass is below 16 km altitude (52,000 ft)
  • and 99.99997% of earth’s atmospheric mass is below 100 km altitude (330,000 ft or 62 mi)

Give or take a few atoms, almost the entire mass of earth’s atmosphere is just sitting around below 100 km of altitude, minding its own business, occasionally being disturbed by a passing rocket or returning astronauts.

If you spotted a pattern here: Congratulations! You identified the so-called Karman line. It’s defined to be at 100 km altitude. Below the Karman line space belongs to each country. Above the line, we find outer space.


When I look at my phone these days I do not wonder anymore where earth ends. It may not be clearly visible on a 5.7 inch screen, but with the Karman line we have a widely accepted, mostly accurate definition of the edge of space. Humans traveling above the Karman line are also generally considered to be astronauts, with some nations already awarding the title to everyone who made it beyond 80 km altitude. What does that mean for my own journey to space? Well, I guess I have to cross the Karman line for truly being on my way to space…