Supersonic Man

January 4, 2020

the edge of space

Filed under: Hobbyism and Nerdry,science! — Supersonic Man @ 11:32 am

There’s a controversy about where “space” begins. The internationally accepted standard is the altitude of 100 km, which is known as the Kármán line. But in the USA, many advocate for the more lenient definition which says you’ve been to space if you rise to an altitude of only 80 km, or more traditionally, 50 miles. Which view is more correct? Well, when an orbiter reenters the atmosphere, the point when reentry heating starts to get significant is around 120 km, so in my view the 80 km line is definitely the less valid of the two.

In the end, I say both are bogus: you aren’t really in space until you get to at least 200 km up, high enough so that it’s possible to orbit the Earth a few times without promptly falling down from drag. You can’t orbit for very long at 200 km up — useful satellites start at about 300 km — but it is at least possible to orbit for a little while, like a day or so, at that altitude. At 100 km altitude, you are in the ionosphere, not in space, and probably won’t make it around the Earth even once. In fact, the ionosphere actually extends above most satellites… which is intentional, as this means they will eventually come down. Keeping most satellites this low is a policy which reduces the long term risk of “space junk”. But I would say that if you can orbit, you’re in space.

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