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Raving_Lunatic69 t1_j64k77l wrote

The definition is pretty much arbitrary, but it's generally considered the point at which the atmosphere becomes too thin to support conventional flight. In Europe it's considered 62 miles (100km), NASA considers it to be 50 miles (about 80km).

Geography would have a very negligible effect at that altitude. It's a fixed altitude above sea level and doesn't fluctuate

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breckenridgeback t1_j64l9qz wrote

> Geography would have a very negligible effect at that altitude. It's a fixed altitude above sea level and doesn't fluctuate

But just because it's an arbitrary human definition. There are dynamics in the upper atmosphere that matter for some purposes, but the atmosphere in the sense of "has dynamics that matter sometimes" extends far into what we think of as "space".

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TheJeeronian t1_j64u7yn wrote

This. The least arbitrary boundary would probably be the solar wind bow shock, but even that is fuzzy and certainly not spherical.

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Chromotron t1_j66fxvp wrote

But the bow shock is a result of the magnetic field, not atmospheric density or effects. Ultimately the question is "who cares?"; it doesn't matter at all if one is technically inside the atmosphere or not.

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dontmentiontrousers t1_j68cpvi wrote

I kinda like breathing, so there's that.

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Mammoth-Mud-9609 t1_j64p5et wrote

There is a detectable atmosphere which extends out beyond the Moon, it isn't enough to breathe or even protect from the solar wind and solar radiation but it does stretch out into "space"

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slavmememachine t1_j65pngt wrote

I believe they say it’s 100 km because some guy named karman figured out that you would need to be moving at orbital speeds to generate enough lift and it was around 100 km

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