Lowest possible altitude for a Satellite

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What is the lowest altitude for a satellite to orbit?
 

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  • #3
DaveC426913
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Virtually any satellite in LEO is slowed by friction with rarefied atmosphere, causing it to lose altitude. They need the ability to boost themselves back up occasionally.
How low an orbit can be depends on how broadly you apply the term 'occasionally'. :biggrin:

At some altitude, its speed will be slowed so much that it needs to boost continually, just to stay at altitude.
In practical terms this too has a limit, due to a limited supply fuel as well as friction/shock heating destroying the craft.

Presumably, if boosting continually, it should no longer be considered 'orbiting'.
 
  • #4
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boosting continually
Single, un-boosted, complete (though decayed) revolution?
 
  • #5
jbriggs444
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  • #6
russ_watters
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Based on the data for Tiangong-1, the orbital decay gets that severe right about 140 km. The graph of altitude versus time gets steep there at the end.

http://www.satflare.com/track.asp?q=37820#TOP
It looks steep based on the scale, but it is only losing about 1-2 km per orbit at that point, which is slower than walking speed (it's about 0.4 m/s). Google tells me an ISS orbit maintenance burn might be 1.3 m/s delta-V over 12 minutes (not sure how typical that is). Yes, if you burned continuously you'd run out of fuel fast, but in terms of the decay rate, at that point it was only about 5% of what a continuous burn could reverse in one orbit.

This is common fodder for sci-fi movies and I'd be curious to know if more can be said. Let's say we have enough fuel for a 1 hour burn, at the above acceleration rate. What is the minimum altitude you could recover from without being back in the same predicament in, say, a week?
 
  • #7
DaveC426913
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This is common fodder for sci-fi movies
You're being generous.

Common fodder for sci-fi movies is that,the moment your engines stop, your orbit immediately starts decaying rapidly, even if you're as far out as the Moon.

:wink:

star_trek_into_darkness_enterprise_n_uss_vengeance_by_velociraptor34-d80xnxc.jpg


GnrA.gif
 

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  • #8
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  • #9
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Virtually any satellite in LEO is slowed by friction with rarefied atmosphere, causing it to lose altitude. They need the ability to boost themselves back up occasionally.
How low an orbit can be depends on how broadly you apply the term 'occasionally'. :biggrin:

At some altitude, its speed will be slowed so much that it needs to boost continually, just to stay at altitude.
In practical terms this too has a limit, due to a limited supply fuel as well as friction/shock heating destroying the craft.

Presumably, if boosting continually, it should no longer be considered 'orbiting'.
Thank you, but at what altitude will it be stable, like it requires extremely low amount of boosting
 
  • #10
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Single, un-boosted, complete (though decayed) revolution?
altitude where boosting is required very little far lower than the ISS for example
 
  • #11
Vanadium 50
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I don't think you're being fair to us. You seem to want an exact answer from us, but are only willing to vaguely specify the problem. "altitude where boosting is required very little" and "extremely low amount of boosting". How long is a piece of string?

The Wikipedia article has examples. You're not going to do any better than that.
 
  • #12
russ_watters
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Thank you, but at what altitude will it be stable, like it requires extremely low amount of boosting
altitude where boosting is required very little far lower than the ISS for example
The ISS requires monthly boosting. Does that satisfy "extremely low" or "very little"? Feels like a fair amount to me...
 
  • #13
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I don't think you're being fair to us. You seem to want an exact answer from us, but are only willing to vaguely specify the problem. "altitude where boosting is required very little" and "extremely low amount of boosting". How long is a piece of string?

The Wikipedia article has examples. You're not going to do any better than that.
I'm sorry if I'm being unfair, i just need an approximate value of range like periodic monthly boosting or something, so I could put in a minor part of project, not serious, so don't spend too much time thinking about it, but thank you anyway for helping out.
 
  • #14
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O
The ISS requires monthly boosting. Does that satisfy "extremely low" or "very little"? Feels like a fair amount to me...
Ok, fine that is fair enough for me, thank you.
 
  • #15
jbriggs444
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I'm sorry if I'm being unfair, i just need an approximate value of range like periodic monthly boosting or something, so I could put in a minor part of project, not serious, so don't spend too much time thinking about it, but thank you anyway for helping out.
This article is light on numbers but does mention the use of ion thrusters at an altitude of 235 km. https://en.wikipedia.org/wiki/Orbital_station-keeping
 
  • #16
russ_watters
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This article is light on numbers but does mention the use of ion thrusters at an altitude of 235 km. https://en.wikipedia.org/wiki/Orbital_station-keeping
That seems like a great application for in engines; I hadn't heard that had been done.

[google]
With a mass of about 1000 kg, that works out to an acceleration of about 0.1 m/s/orbit.

[edit: unit typo fixed]
 
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  • #17
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And even if you can find a number for one satellite, the answer for other satellites depends on the orientation of that satellite and drag vs mass in that orientation.
 
  • #18
Is this a trick question? I think the lowest possible altitude of a satellite orbiting the earth would be just above sea level. The satellite would have to achieve escape velocity, have thrust available to overcome drag, and have to be able to navigate around landmasses, but technically that should qualify as a satellite in orbit.
 
  • #19
bob012345
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Is this a trick question? I think the lowest possible altitude of a satellite orbiting the earth would be just above sea level. The satellite would have to achieve escape velocity, have thrust available to overcome drag, and have to be able to navigate around landmasses, but technically that should qualify as a satellite in orbit.
For a smooth planet with no atmosphere, yes.
 
  • #20
jbriggs444
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Is this a trick question?
Post #13 clarifies the purpose of the question. It is a practical matter -- roughly how high does one need to go so that the station keeping requirements are manageable.
 
  • #21
Flight level 600 is at 60,000 ft which is the upper boundary of controlled airspace, that would be the practical, lowest possible altitude. Search for the relationship between altitude and atmospheric pressure to find that relationship, then go as high as needed to optimize a design based on propulsion energy needed to achieve the desired altitude at orbital/escape velocity with minimum thrust to overcome atmospheric density/drag for the duration of the mission.
 
  • #22
russ_watters
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And even if you can find a number for one satellite, the answer for other satellites depends on the orientation of that satellite and drag vs mass in that orientation.
Yes. The ISS for example is particularly draggy and a google/eye all tells me it is much worse, losing about 7m/orbit or accelerating at 14m/s/orbit.
 
  • #23
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And even if you can find a number for one satellite, the answer for other satellites depends on the orientation of that satellite and drag vs mass in that orientation.
And even for the same satellite, varying solar activity can have an effect on the Earth's atmosphere, which will cause the drag to change.
 
  • #24
russ_watters
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Flight level 600 is at 60,000 ft which is the upper boundary of controlled airspace, that would be the practical, lowest possible altitude. Search for the relationship between altitude and atmospheric pressure to find that relationship, then go as high as needed to optimize a design based on propulsion energy needed to achieve the desired altitude at orbital/escape velocity with minimum thrust to overcome atmospheric density/drag for the duration of the mission.
I don't understand where you are going with this. What is practical? We have craft operating at 60,000 or even 160,000 feet, but they don't use orbital mechanics to stay aloft, they use normal aerodynamic lift or buoyancy. Craft that rely primarily on orbital mechanics to stay aloft are not possible in that altitude range for a couple of reasons: they'd quickly burn up or run out of fuel.
 
  • #25
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Virtually any satellite in LEO is slowed by friction with rarefied atmosphere, causing it to lose altitude. They need the ability to boost themselves back up occasionally.
How low an orbit can be depends on how broadly you apply the term 'occasionally'. :biggrin:

At some altitude, its speed will be slowed so much that it needs to boost continually, just to stay at altitude.
In practical terms this too has a limit, due to a limited supply fuel as well as friction/shock heating destroying the craft.

Presumably, if boosting continually, it should no longer be considered 'orbiting'.
I am in complete agreement but we can say that the stratopause is about the lowest we can have a LEO satellite.
 

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