You’re going where?: A look at where Earth ends and space begins
As more people, probes, cameras and countries head ‘into space’, see how far they must travel for that to hold true - and other lines they cross along the way.
They call it the final frontier, but where exactly does Earth end and space begin?
As more people, probes, cameras and countries head “into space”, what barriers must they breach in order for that phrase to hold true?
Eventually, Google Maps may jot all this down for the tourists, but for now, here goes. The most widely accepted boundary between Earth and space is the Karman line, named after the Hungarian physicist Theodore von Karman (1881-1963).
It’s an imagined boundary about 100 km above sea level — more or less the point at which traditional aircraft begin to falter.
The reason they falter is that the medium they are travelling through, which is essentially compressed air, starts to dissipate at this altitude. In some of his early work, Karman is believed to have fixed his line closer to 80 km from sea level, because atmospheric fluctuations can make this entire band hard to traverse.
A big part of what creates that compressed air — what we call atmosphere — is the pull of Earth’s gravity drawing atmospheric gasses (including oxygen) towards its surface. As one moves away from the surface, that force weakens.
At about 100 km, the pull now barely registers on the gasses. Thus, while atmospheric pressure stands at 14.7 psi (pound-force per sq inch) at sea level, it is considered to be more or less zero at the Karman line.
And so, for the purposes of space travel, the international community, including the US National Aeronautics and Space Administration (NASA), considers the 100-km mark the point at which one officially leaves Earth.
Once satellites cross this line, they enter Low-Earth Orbit, a zone that stretches for about 2,000 km. Beyond 2,000 km, one can consider oneself in outer space.
The International Space Station (ISS) — and most of the planned labs, hotels and other tourist ventures — will be limited to “space” or Low-Earth Orbit. ISS, for instance, currently orbits at about 400 km from Earth’s surface: high enough to avoid most of our planet’s atmospheric drag and low enough to avoid space debris such as fragments of asteroids and other natural and manmade junk.
There’s an interesting boundary in the other direction too.
The Armstrong limit, at about 19 km above sea level, marks the altitude at which human blood would boil if unprotected by a suit. It bubbles up at this point not because the temperature rises, but because the low atmospheric pressure reduces its boiling point.
The line was defined by US Air Force major general Harry Armstrong (1899-1983), an early aerospace physician. And isn’t that just the stuff of space nightmares?
