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B Lowest possible altitude for a Satellite

  1. Jun 24, 2018 #1
    What is the lowest altitude for a satellite to orbit?
     
  2. jcsd
  3. Jun 24, 2018 #2

    jim mcnamara

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  4. Jun 24, 2018 #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'.
     
  5. Jun 24, 2018 #4

    Bystander

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    Single, un-boosted, complete (though decayed) revolution?
     
  6. Jun 24, 2018 #5

    jbriggs444

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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
     
  7. Jun 24, 2018 #6

    russ_watters

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    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?
     
  8. Jun 24, 2018 #7

    DaveC426913

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    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
     
  9. Jun 25, 2018 #8
    I did check that page, and It doesn't say anything about it, only examples of low earth orbiting satellites like ISS
     
  10. Jun 25, 2018 #9
    Thank you, but at what altitude will it be stable, like it requires extremely low amount of boosting
     
  11. Jun 25, 2018 #10
    altitude where boosting is required very little far lower than the ISS for example
     
  12. Jun 25, 2018 #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.
     
  13. Jun 25, 2018 #12

    russ_watters

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    The ISS requires monthly boosting. Does that satisfy "extremely low" or "very little"? Feels like a fair amount to me...
     
  14. Jun 25, 2018 #13
    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.
     
  15. Jun 25, 2018 #14
    O
    Ok, fine that is fair enough for me, thank you.
     
  16. Jun 25, 2018 #15

    jbriggs444

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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
     
  17. Jun 25, 2018 #16

    russ_watters

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    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]
     
    Last edited: Jun 25, 2018
  18. Jun 25, 2018 #17
    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.
     
  19. Jun 26, 2018 #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.
     
  20. Jun 26, 2018 #19
    For a smooth planet with no atmosphere, yes.
     
  21. Jun 26, 2018 #20

    jbriggs444

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    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.
     
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