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# Minimum and maximum altitude for Earth orbit

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Hi KSP colleagues,

I have these two questions, please:

• What is the minimum altitude necessary to enter orbit around Earth?
• What is the maximum altitude possible to remain in orbit around Earth?

Thank you.

Stanley

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6 minutes ago, MetricKerbalist said:

Hi KSP colleagues,

I have these two questions, please:

• What is the minimum altitude necessary to enter orbit around Earth?
• What is the maximum altitude possible to remain in orbit around Earth?

Thank you.

Stanley

Minimum altitude orbit is a question of drag. 80-83 km is the point at which you could complete one full orbit without drag immediately taking you down (though probably not more than one).

Maximum Earth orbit is a question of the Hill Sphere, the region where Earth’s gravity well dominates rather than the sun’s. Earth’s Hill Sphere ends at around 1.47 million km.

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It depends a bit on the exact definition of orbit. If it wasn't for the atmosphere, you could orbit at sea level (or maybe slightly higher to barely miss the mountains). Compare it to flying: a commercial airliner operates with mostly lift, as opposed to centripetal force, but a spaceplane starts off relying on lift, and at some point gets into such thin atmosphere that it no longer relies on lift, but rather relies on the centripetal force (the going-fast-sideway force that keeps things in orbit)

So the question becomes: Keeping a spaceplane in a circular orbit, at which height of this orbit will the plane no longer need wings? (this is oversimplified, because there are so many variables at play)
- I think the Karman line is based on such a comparison (rather check the wiki incase i ruined it)

For the highest orbit, would you consider it an orbit if it escapes earth, just to be slingshoted back from Jupiter decades later, in some very unlikely but repeating resonance?
- if it wasn't for all the other gravity source (sun, planets), the maximum orbit is near infinite

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7 minutes ago, Blaarkies said:

- I think the Karman line is based on such a comparison (rather check the wiki incase i ruined it)

Not exactly. The line Von Kàrmàn calculated as the boundary where atmospheric flight is no longer possible at velocities slower than orbital speed, with the X-2 data, is 83.6 km. The FAI Kàrmàn line, 100 km, has absolutely no meaning other than "that's a nice number"

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4 minutes ago, Beccab said:

83.6 km. The FAI Kàrmàn line, 100 km, has absolutely no meaning other than "that's a nice number"

And back to the "Imperial vs Metric". What's a round number for the 'Murican FAI? 50 mi or 100 km? lol.

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1 minute ago, kerbiloid said:

And back to the "Imperial vs Metric". What's a round number for the 'Murican FAI? 50 mi or 100 km? lol.

Make a new system, where 83.6 km is a nice round number

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10 minutes ago, Beccab said:

Make a new system, where 83.6 km is a nice round number

Unlikely it can be exactly 83.6, as the atmosphere is unstable.

Probably 50 mi = 80.45 km is good enough.

And accurately Kerbal.

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3 minutes ago, kerbiloid said:

And accurately Kerbal.

Who knows, maybe 70 km is space on kerbin because it's a round number there too

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17 minutes ago, Beccab said:

Who knows, maybe 70 km is space on kerbin because it's a round number there too

Kerbin's is by definition 80 km.

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1 hour ago, kerbiloid said:

Kerbin's is by definition 80 km.

Kerbin's is by definition 70km*

*Although you can't timewarp above 1x at that altitude, so 70001 isn't that 'useful'. I consider the practical working minimum, 70250m for that reason. And then there's the consideration if you're docking.........etc

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Oops, confused with default LKO.

Anyway. 70 km means that even Kerbals prefer the metric.

Edited by kerbiloid
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4 hours ago, Beccab said:

Not exactly. The line Von Kàrmàn calculated as the boundary where atmospheric flight is no longer possible at velocities slower than orbital speed, with the X-2 data, is 83.6 km. The FAI Kàrmàn line, 100 km, has absolutely no meaning other than "that's a nice number"

Yeah thats the one, your explanation is on point!
What always bugged me about this one is that a single aircraft type is used to define the number. It would certainly be higher if the equation is calculated using a lighter, more streamlined aircraft that provides more lift (the max altitude with better tech probably won't be 10's of km higher anyway)

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On 7/12/2021 at 4:16 PM, Blaarkies said:

What always bugged me about this one is that a single aircraft type is used to define the number. It would certainly be higher if the equation is calculated using a lighter, more streamlined aircraft that provides more lift....

Hypersonic vehicles experience lift from wave drag rather than from airfoil drag, so their lift/drag ratio has an upper limit. The Küchemann relation says that the maximum lift/drag ratio cannot exceed (4M + 12)/M, where M is the Mach number.

So Von Kàrmàn's line is based on a maximally streamlined hypothetical vehicle, since L/D has that upper limit.

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On 7/12/2021 at 6:06 PM, Beccab said:

Not exactly. The line Von Kàrmàn calculated as the boundary where atmospheric flight is no longer possible at velocities slower than orbital speed, with the X-2 data, is 83.6 km. The FAI Kàrmàn line, 100 km, has absolutely no meaning other than "that's a nice number"

100 Km is the legal limit of space as I understand https://en.wikipedia.org/wiki/Border. In short its legal to overfly other countries if above 100 km, this includes suborbital trajectories. Failing an killing cows is bad form.

Orbit has some historical significance. Back in the dawn of the space race before sputnik the US  was already thinking about spy satellites.
However the US was concerned about the legality of orbital flights so they wanted their first satellite to be pure civilian, scientific, high attitude and small, later was also practical.
The US is a bit obsessed with legal issues. None other had tough about it it was no way to intercept an satellite in the 50's anyway so it was not an issue.

Astronaut wings will loose its value over time, once having an gun with rifling implied you was elite soldier.
In 50 years its probably some who does eva a lot. Being an chef with an 10 year practice of making food in zero g does not make you one.
http://freefall.purrsia.com/ff1200/fv01143.htm

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5 hours ago, magnemoe said:

100 Km is the legal limit of space as I understand https://en.wikipedia.org/wiki/Border. In short its legal to overfly other countries if above 100 km, this includes suborbital trajectories. Failing an killing cows is bad form.

Well, to quote the page you posted:

Quote

The vertical boundaries of airspace are not officially set or regulated internationally.

It is only unofficially and presumably to reduce complications that the karman line is usually used there. If it was closer to 83 km, like it will probably be changed to in the near future, it would still be used in the same way

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4 minutes ago, Beccab said:

Well, to quote the page you posted:

It is only unofficially and presumably to reduce complications that the karman line is usually used there. If it was closer to 83 km, like it will probably be changed to in the near future, it would still be used in the same way

Yeah. All this means is that by general consent nobody tries to enforce airspace restrictions on spacecraft. Most countries would have no way of enforcing them anyway, and those that do would not want them enforced on their own spacecraft.

There is no defined limit, really. It's more like "things at 100km are definitely in space" and "things where planes fly" are not, but as we have been discussing, the limits of "space" are fuzzy.

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2 hours ago, mikegarrison said:

Yeah. All this means is that by general consent nobody tries to enforce airspace restrictions on spacecraft. Most countries would have no way of enforcing them anyway, and those that do would not want them enforced on their own spacecraft.

There is no defined limit, really. It's more like "things at 100km are definitely in space" and "things where planes fly" are not, but as we have been discussing, the limits of "space" are fuzzy.

Ok, I thought it was an international agreement like the 12 nm ocean border.
On the other hand it makes some sense as its an huge distance between max attitude for an plane or balloon and the lowest practical one for an satellite.

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10 hours ago, mikegarrison said:

Yeah. All this means is that by general consent nobody tries to enforce airspace restrictions on spacecraft. Most countries would have no way of enforcing them anyway, and those that do would not want them enforced on their own spacecraft.

There is no defined limit, really. It's more like "things at 100km are definitely in space" and "things where planes fly" are not, but as we have been discussing, the limits of "space" are fuzzy.

From what I've heard, the issue wasn't even decided until the "International Geophysical Year (1957)" when the US announced it would put something it orbit (and was 2 months late) and the USSR *did* put something in orbit without serious complaints of border violations.  Before the announcement, NACA/DoD (or whatever existed before NASA, and for all I know it was still Department of War) had no idea what the Soviet reaction to a satellite overflying the USSR would be.

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