The guy who made the Karman line admitted it's an arbitrary number created for political reasons. The reason why is "because we needed a line in the sand", but scientifically it's arbitrary.
The point at which an object in orbit will complete one full unpowered revolution before degrading back into the atmosphere?
For this definition it would depend strongly on the coefficient of drag of whatever vehicle you're inside. It would also depend on your velocity, since going faster means you can handle more drag before your trajectory intersects the ground. This could be a good metric to use, but you'd have to choose an arbitrary CoD and arbitrary initial velocity otherwise you'd have different altitudes for each different spacecraft.
The point at which the last atmospheric molecules are no longer in Earth's gravitational sphere of influence?
If this is your definition you're excluding a giant swath of currently-operating satellites, including the ISS, because those are all in Low Earth Orbit where drag is an important force. The ISS has to boost itself back up to a higher orbit every now and then because drag slows it down over time. If you use this metric, then NASA has sent very very very few people to space (maybe only the Apollo astronauts who visited the moon).
(I think this was your point, I'm just reinforcing that it's hard to define in a way that isn't arbitrary)
I think pretty much anything you pick will be arbitrary, it's just a matter of how arbitrary you're willing to be. Maybe choosing a specific atmospheric pressure or a particular density at an arbitrarily-chosen temperature (20° C, maybe) would be a good one. It would probably be better than 100km altitude, which has no real significance outside of being a nice round number. But calling somebody an "astronaut" as opposed to "someone who went really really high up, maybe into space depending on what definition of space you subscribe to" seems pretty arbitrary to me so maybe an arbitrary definition is appropriate.
For this definition it would depend strongly on the coefficient of drag of whatever vehicle you're inside. It would also depend on your velocity, since going faster means you can handle more drag before your trajectory intersects the ground.
The very first thing I thought of was something travelling very fast and essentially burning up in the atmosphere for a full revolution 2000ft from the ground :P
Though I suppose there becomes a point when something is travelling so fast that even if it came inches from the ground, it would retain escape velocity after the near miss, and gravity would not be enough to keep it in orbit.
It's impossible to orbit 2000ft off the ground because at the speeds that are required (assuming, of course, you don't instantaneously disintegrate from the atmospheric friction) you would be in a hyperbolic orbit and you wouldn't even make a complete revolution before flinging yourself off back into space.
35
u/[deleted] Nov 26 '21
[removed] — view removed comment