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Thread: Near-earth gravity

  1. #1 Near-earth gravity 
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    Hello, new here

    I think I've got a problem.

    I have an orbital space station connected to an earth-like planet via a space tether (elevator). Well, I don't, per se, it's integral to a story.

    Some of the action in that story revolves around possible sabotage to the tether. The question is, what sort of gravity would be affecting this station? At this point, the orbiter creates its own gravity (did I mention Space Opera?) a la Star Trek, not extending much past the station itself. It is mentioned that gravity in the outer construction (rings) is weaker (it's a sort of sky ranch and solar power collector) and that space ships coming and going from there have to deal with gravity shifts.

    Is this even feasible? Would an orbital station such as this still be affected by the planet's gravity, in terms of how people move around on the station (and of course how it affects the efficiency of the tether climbers.) If so, it would not need to generate its own gravity.

    For this plot to work, I really need there to be so little gravity from the planet that a detached crawler close to the station would NOT crash back down to earth, but simply drift off.
    Doable? Basically, at what point away from an earth-like planet do things start to float.

    If not, I'm going to have to blow it up somehow without breaking the tether (graphene nanotubes).
    Any comments much appreciated. This is sci-fi Space Opera so it just needs to be plausible - I don't need the math (cause that'll just hurt my brain)


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  3. #2  
    Universal Mind John Galt's Avatar
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    Things never float in space, they fall towards the most dominant local gravitational source. Objects in orbit fall continuously around the object they are orbiting.

    Sky hoooks would work with a large mass, your orbiting station, located in geosynchronous orbit and a balancing mass extending beyond that to counteract the mass of the tether.

    That's simplified, perhaps overly so. Others may offer more insightful comments.


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  4. #3  
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    Thanks,
    What I need to know, though, is: if the crawler, say a cargo pod, becomes disconnected from the tether close to the counterweight (the station), would it immediately return to the planet, or would it move slowly, as if it were drifting/floating.
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  5. #4  
    Universal Mind John Galt's Avatar
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    It would move away from the tether. I need to revisit my basic orbital mechanics equations to figure out exactly how. I'm just hoping some other member will post a proper solution for you. Some of them can do that stuff in their sleep. I need to go back to basics and now is not the time.
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  6. #5  
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    What I need to know, though, is: if the crawler, say a cargo pod, becomes disconnected from the tether close to the counterweight (the station), would it immediately return to the planet, or would it move slowly, as if it were drifting/floating.
    if it was within the geosynchronous orbit then it wouldn't have enough speed to keep in orbit and would fall back to earth. the higher up it was the longer it would take.

    i reckon anyway.

    :-)
    Sometimes it is better not knowing than having an answer that may be wrong.
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  7. #6  
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    If the pod became separated from the elevetor at or near the station then it would be at near orbital velocity and would appear to drift away. The actual orbital speed would be a function of how high the crawler was when it lost its grip on the cable. The cable is making one orbit in 24 hours. C=2r(pi), earth radius is approx 4000 miles, so the "orbit of the crawler is = 4000 + hight of crawler pod *2*(pi). the speed can be calculated by dividing that number by 24hrs.

    The energy requirements of a space elevator have to allow for the lifting of the pod up the elevetor and for excellerating it to higher orbitital speeds. It is always making one orbit in 24 hrs but the orbit gets longer the higher it climbs so effectively it goes faster. The energy for the increased speed must come from somewhere.
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  8. #7  
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    That works

    So now I have another problem.

    If the crawler is close to the orbital station and the power to it was cut, would it continue to climb due to centrifugal force? Slow down? Stop?
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  9. #8  
    Forum Freshman overthelight's Avatar
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    Quote Originally Posted by ChrisReher View Post
    That works

    So now I have another problem.

    If the crawler is close to the orbital station and the power to it was cut, would it continue to climb due to centrifugal force? Slow down? Stop?
    I like easy maintenance to facility.

    BTW, it stands by Kepler's law.
    Kepler's laws of planetary motion - Wikipedia, the free encyclopedia
    Simply description, when the material getting faster than the station, material will get upper orbit. when getting slow, the material will get lower orbit.

    Completely it relies the condition before cutting electricity.
    Science gives people the hope always.
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  10. #9  
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    Something of note, unless the station orbits the equatior of that planet, you would need a tether that extends and retracts during the orbital period. Geostationary orbits is a special geosynchronous orbit that goes around the equator. Any geo-sync orbit not around the equator draw a figure 8 in the sky over the point it is tethered to the ground.

    Cargo going up the tether will always be going at the same angular velocity as the space station. Orbits near Earth are going at a higher angular velocity. As you climb up into higher orbits, the angular velocity decreases. For example, the ISS orbits the Earth every 90 minutes or so, but a geostationary space station orbits the Earth every 24 hours. The angular velocity is basically the RPM, or how fast it goes around the center of the circle. This is not to be confused with the instantenous velocity tangential to the orbit, which is always higher in a higher orbit.

    To answer your question, cargo that doesn't make it to the station (and aren't secured to the tether) will fall because its angular velocity is not high enough to achieve orbit at that altitude. It won't fall straight down, but rather in a parabolic curve.

    If the cargo is secured to the tether but can move freely up and down, then it will basically fall like a ball would on Earth. If it's really really high up then the initial (downward) acceleration will be much less than that on Earth. For example, if the cargo is say, 10,000 km high, the gravitational acceleration is only 1.5 m/s. Note that the geostationary orbit of an Earth-size planet is about 35,800 km in altitude. A falling cargo that's only say, 50 km from the station would accelerate at merely 0.22 m/s (basically slowly floating away). Also don't forget to account for the cargo's velocity prior to power failure / tether breakage.
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