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Flamingotough t1_j8xkno8 wrote

The Kármán line at 100km is a fairly agreed upon estimate.

But it feels wrong to let anything just hover in place over one's airspace - it might be an idea to include a requirement for the object to be at an orbital flight speed.

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spork3 t1_j91w2zp wrote

What I like about 100 km is that it’s place where two different fields independently agree that space begins. The Karman line is an aeronautic definition, but space physicists also define the space boundary at about 100 km based on composition. It’s about where the ionosphere begins and the mesosphere ends. As a physicist, I had never heard of the Karman line until somewhat recently, so it was interesting to learn that the what many people use to define the boundary for space is the same as what I knew it to be.

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astrongineer t1_j8zpigi wrote

Never heard of geosynchronous artsats?

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apocalypsebuddy t1_j90077a wrote

Geosynchronous orbit is way the fuck out there, not even close to the karman line

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Flamingotough t1_j94i132 wrote

Earth's surface rotates at about 1000mph, and to maintain geosynchronous orbit an object must orbit at about 7000mph - that why I said 'orbital' flight speed as opposed to surface speed.

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astrongineer t1_j94o4fk wrote

Okay, thanks for explaining. I misunderstood what you said.

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cinemascifi t1_j8zk25s wrote

It may seem wrong, but that's how GPS operates... via geosynchronous satellites in orbit all over the world.

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gms01 t1_j8zrqig wrote

Actually, the GPS satellites are not geosynchronous satellites, which would imply orbits at 22,236 miles above sea level for circular orbits. From Earth, only those orbits appear stationary (in equatorial orbit) or at least varying within a relatively small area (if not in equatorial orbit).

The GPS satellites are in Medium Earth Orbit (MEO), around 12,540 miles. The GPS satellites do not hover over one area. They don't have to hover. Each satellite broadcasts its own four dimensional position in spacetime (3 space coordinates and the time, kept by accurate atomic clocks). Based on the calculated time delays from at least 4 satellites, a GPS receiver can uniquely determine the it's position.

See the Wikipedia article on GPS: https://en.wikipedia.org/wiki/Global_Positioning_System

and on geosynchronous orbit:

https://en.wikipedia.org/wiki/Geosynchronous_orbit

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Bu22ard t1_j8zwngb wrote

GPS satellites are not geosynchronous

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Flamingotough t1_j94i4kv wrote

Earth's surface rotates at about 1000mph, and to maintain geosynchronous orbit an object must orbit at about 7000mph - that why I said 'orbital' flight speed as opposed to surface speed.

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cinemascifi t1_j95g4bd wrote

I didnt say anything about speed. Are you responding to the right comment? It seems like a non sequitur.

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Flamingotough t1_j98ap4d wrote

I may have just misunderstood you point?

I thought you critized my idea of taking speed into account, by saying that objects in geosynchronous orbit would be stationary as seen from the ground. I just wanted to note that although that is true enough for this argument, such an object would still travel it's orbit at a significantly different velocity compared to the spot on the ground underneath.

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