I’m an engineer born and raised on a spaceship. A spaceship that’s whooshing through the emptiness of space. This particular spaceship is robust and fragile at the same time, barely understood in its inner workings, and yet unbelievably beautiful by design. This spaceship is called planet earth. Most of its crew is trapped on it and being held in place by the pull of gravity. Only a selected few have ever left the earth and seen our blue, marvelous marble from afar. They went to space, turned around, and looked at earth in awe. To each and everyone one of them something magically happened in that very moment. When they came back their lives had been changed forever. They started to think bigger. They became more humble. They, more than everyone else, understood that we are all one global, fate-sharing community. Living on a spaceship. Whooshing through the emptiness of space.
I’m not an astronaut. I’m not even a pilot. Let alone a rocket scientist. I’m a child at heart and an avid dreamer. I honestly believe that going to space is an achievable goal within my lifetime. And once I’m there I will turn around and look at the earth in awe, ready for the magic to happen. Until then, I have to rely on imagination and reports from others.
In 1632 Italian astronomer, physicist, and engineer Galileo Galilei wrote:
If you could see the earth illuminated when you were in a place as dark as night, it would look to you more splendid than the moon.
I don’t know for sure, but I guess it is safe to assume Galilei never went to space himself. That makes it even more impressive that the often called “father of observational astronomy” got it surprisingly correct. That’s the power of imagination!
The first human who went to outer space was Yuri Gagarin, a Soviet Air Force pilot and cosmonaut. He described in great detail what he saw 1961:
What beauty. I saw clouds and their light shadows on the distant dear earth…. The water looked like darkish, slightly gleaming spots…. When I watched the horizon, I saw the abrupt, contrasting transition from the earth’s light-colored surface to the absolutely black sky. I enjoyed the rich color spectrum of the earth. It is surrounded by a light blue aureole that gradually darkens, becoming turquoise, dark blue, violet, and finally coal black.
At that time the so-called space race between the United States of America and the Soviet Union was gaining speed. Only eight years later, Neil Armstrong, United States astronaut was the first person to set foot on the moon. Seeing his home planet from roughly 230.000 miles away was a special moment for him:
It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn’t feel like a giant. I felt very, very small.
In the following Space Shuttle era more and more astronauts had the chance to go to space. One of them was the U.S. Navy test pilot Donald Williams who said:
For those who have seen the Earth from space, and for the hundreds and perhaps thousands more who will, the experience most certainly changes your perspective. The things that we share in our world are far more valuable than those which divide us.
Of all the reports, descriptions, and impressions we have from the courageous men and women who made it into space my favorite one is from William McCool. McCool was an officer, test pilot, aeronautical engineer and, of course, an astronaut. As the pilot of the Space Shuttle Columbia on it’s STS-107 mission, sadly, he was killed when the spacecraft disintegrated during re-entry into the atmosphere. I can’t get his words out of my head as they sum up my expectations just perfectly:
It’s beyond imagination until you actually get up and see it and experience it and feel it.
From William McCool’s words I learned that actually going to space makes all the difference: It’s a full-body, full-mind, all-of-heart-and-soul experience. Being in space and looking at the marvelous marble we call home is beyond imagination. I can’t imagine it. I need to see it for myself, experience it for myself, and feel it for myself. I have no choice but to go to space.
For a long time, Space seemed to be inaccessible and out of reach for common people like me. But is it really, today?
- Aren’t there lucky billionaires building rockets and commercializing space?
- Aren’t there several companies actively working on opening up space for tourism?
- When can I go buy a ticket? What will it cost? Do they accept bitcoin in earth orbit?
- Will there be wifi on the spacecraft? Would I want there to be wifi?
- What are the medical requirements for flying in a rocket? Do I need to ramp up my physical fitness? Maybe I should lift more in preparation for lift-off?
- What’s the closest thing to being in space that I can experience here on earth?
Time for me to do my research.
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…