Where does “outer space” begin?

View from Apollo 11 shows the Earth rising above the Moon’s horizon on 7/20/1969. (NASA)

Last week we read about Felix Baumgarten’s “skydive from space.” That’s what they called it. But the 24-mile altitude at which his free-fall began was nowhere near space. Explanation? Everyone loves a catchy headline, and who cares about reality?

Earth’s atmosphere slowly thins with altitude, so there is no precise elevation where space begins. But interesting things do happen as you head upward.


At 18,000 feet, half of all air is below you. Unless you’re well-conditioned, you’ll soon go unconscious. At 5 ½ miles or 29,000 feet – the height of Everest – only a few people have trained themselves to be able to function for a while. At 12 miles, blood, saliva and eye-wetness boil away.

The International Aeronautical Federation cites 62 miles as the boundary of space. It’s the so-called Kármán line, above which an airplane would have to travel faster than orbital velocity to attain sufficient aerodynamic lift to support itself.

NASA’s mission control uses 76 miles as its official reentry altitude (called the Entry Interface): the boundary where atmospheric drag becomes noticeable. It’s the height at which Space Shuttles had to switch from steering with thrusters to using movable surfaces that deflect air.

In 2009, Canadian scientists established the space boundary at 73 miles, the rough transition location between Earth’s atmospheric winds and the violent flows of high-speed charged particles of space. But this is still not a vacuum. Meteors burn up at a height of 80 miles, revealing air’s presence there, while the Aurora glows at about 100 miles, where our atmosphere’s ionized gases fluoresce.

Moreover, no satellite can orbit below 120 miles, because drag from the sparse air atoms would rapidly slow it down and destroy its ability to stay aloft. The very first satellite, Sputnik 1, had an oval orbit that never dipped below 134 miles, letting it stay aloft for three months before its orbit decayed.

Thus, one could call the “edge of space” anywhere from 62 to 120 miles up.

Another question is: How high must you be to see the whole Earth? In reality, only the Apollo astronauts did: 40 years ago, en route to the Moon. Earth’s curvature is quite subtle, even from the 220-mile height of the Space Station. Those astronauts routinely use ultra-wide-angle lenses to create the televised illusion of curvature. The horizon appears flat from the ISS through a “normal” 50mm lens.

To determine “How far is the horizon?” the best method is the simple formula d=1.23√h, where d is the distance to the horizon in miles when h is your height in feet. As an example, if you sit on a beach chair at the water’s edge, your eyes four feet above the ocean, the square root of four is two, multiplied by 1.23 equals 2 ½ miles. So that very sharp horizon edge is a mere 2½ miles away. Small objects beyond that point lie hidden beyond the curve of Earth.

From that skydiver’s height of 128,000 feet, the horizon was 440 miles away. Had his balloon been over our region, Cleveland would have sat on his horizon.

From the ISS, the horizon lies 1,326 miles away. So when the Space Station passes over us, its horizon is marked by Kansas City to the west and Miami to the south. Far from seeing an entire hemisphere, those on board cannot see a trace of Greenland or Mexico when they’re over us.

Draw a circle. Make a dot one circle-radius outside of it. This represents a 4,000-mile-high orbit. Place another dot halfway between this orbit and the circle circumference to depict a height of 2,000 miles. Bisect this to get 1,000, and divide it again for 500 miles. Take half of that and you’ve accurately represented the ISS orbit of 230 miles.

This quick exercise is eye-opening. Now mark off one-tenth of this ISS height, and you’ve shown that skydiver’s altitude to scale. This says it all.

Don’t get me wrong: I approve of the Bored Idle Rich doing things like jumping out of balloons. But hey, space is space.

There is one comment

  1. vinod ekbote

    We can go back and check when colombia shuttle crashed at @ 85 km above earth surface. That time friction with atomosphere started with shuttle. Thus we can say that edge of space is @ 100 km-110km above earth surface.

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