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The Kármán line is the altitude at which the atmospheric pressure is so thin a craft would need to achieve orbital velocity in order to maintain enough aerodynamic lift and control. Therefore, logically the Kármán line itself should belong to outer space already, according to that definition. However, this document states that you have to go beyond 100 km to be recognized an astronaut by the FAI. While the actual Kármán line is lower anyway, logically you should be recognized a space traveller if you reached an apogee of 100,000 meters. But if I understand the document correctly, for the FAI you wouldn't; you have to reach e.g. 100,001 meters to be recognized an astronaut by the FAI.

I believe the U.S. space definition is that you have to go above 50 miles (or 80,467.2 meters) to be recognized an astronaut but that's logical since it may be tied to a certain atmospheric pressure. But if you set the space border according to the Kármán line, the line itself should belong to space, or did I miss something?

Questions like this might play and have played a role in flights of spaceplanes. Mike Melvill reached an apogee of 100,124 meters in SpaceShipOne flight 15P. That's the lowest FAI-recognized spaceflight.

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    $\begingroup$ This Kármán line thing always bored me. There are a lot of questions about it. The answer is the same for all: somewhere a limit should have been drawn, to make international agreements clear and valid. But the atmosphere won't stop at a fixed height, like the oceans, where we can point to. So, a fixed height was drawn and so is it. Relevant read. Sorry for the tone, I hope you will get an useful answer (I will vote for "leave open" your question), ideally with some insight into the internal affairs of the decision mechanism. $\endgroup$
    – peterh
    Commented Aug 11, 2020 at 16:22
  • $\begingroup$ @peterh-ReinstateMonica Perhaps they should never have established a fixed boundary in the first place. During reentry the Space Shuttle flew lower than 80 km above foreign countries and nobody cared. Above 150 km (93 mi) stable orbits are possible and noone will doubt you're in space. Up to the Armstrong line at 19 km (12 mi) noone will doubt you're in the atmosphere and national airspace. $\endgroup$
    – Giovanni
    Commented Aug 11, 2020 at 16:30
  • $\begingroup$ The orbits lower than about 200 km height do decay very fast due to atmospheric drag. If there is so much drag at 200 km, above 100 km may not be considered to be in outer space. It is still very close to Earth and its high atmosphere. $\endgroup$
    – Uwe
    Commented Aug 11, 2020 at 16:52
  • $\begingroup$ @Uwe But still you manage multiple revolutions around the Earth at even 160 km. And even at about 100 if the orbit is highly elliptical. $\endgroup$
    – Giovanni
    Commented Aug 11, 2020 at 17:56

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I do not think there is a useful distinction to make here.

Any border you designate will be somewhat arbitrary. Any time you set one, someone else will argue. And setting the border to be exclusive or inclusive is just one of those points you can argue, but it doesn't have any use.

"Above" 100km" is also 100,000.001 metre. Unless you are ever in a situation where your instrumentation reads exactly 100,000.000000000000 metre (as many zeroes as your instruments allow), you will never be in a situation where this distinction matters. It is an absolute non-issue in practice.

And that also applies to the US space definition. Unless you are expecting to read exactly 50 miles on your instruments, down to the last fraction, this is an utterly irrelevant point.

If you set the border to be inclusive (50 miles or higher), you have changed absolute nothing in practice, but someone else will make a similar argument like yours, but the other way around.

"Above 50 miles" is just shorter then "50 miles or above" and since the difference is utterly irrelevant, people stick with the easier definition.

Since atmospheric height changes with many factors - e.g. solar wind and ionization of the upper layers, the uncertainty inherent in the line is already far greater then this infinitesimal small difference. Plus, the change you are advocating is below the tolerances of the instruments, anyways.

Plus, both lines (50 miles or 100km) have been chosen to be round numbers somewhat close to the actually calculated value, but nevertheless arbitrary in the first place. Arguing about the accuracy of lines chosen to be nice round numbers really isn't the best use of time. They were already chosen that way for convenience.

The relevant document is Section 8 of the FAI Sporting code and states:

2.18.1 All flights must exceed an altitude of 100 km in order to qualify for records.

So yes, if the FAI strictly adheres to their rules, you must exceed 100km.

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  • $\begingroup$ A related issue is rounding of the altimeter measurement. Some digital readouts round up. Dial measurements can appear "on the line". If they allowed 100 km to count, then flights at 99.999 km might be technically counted because the instruments could not distinguish it from the Karman line, even though that wouldn't meet the intention of the rule. Making the criteria "exceed" removes such doubt. $\endgroup$
    – DrSheldon
    Commented Aug 12, 2020 at 2:24

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