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B Stationary Orbit

  1. Aug 6, 2017 #1
    Is it possible to park a ship say halfway between the earth and the moon and stay there for about a year and watch the earth and moon go around the sun and comeback to the same place you are?
     
  2. jcsd
  3. Aug 6, 2017 #2

    mfb

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    Where is "there"? You mean without orbiting the sun? It would have to fire its rocket engines constantly in order to avoid falling towards the sun. The amount of fuel needed would be completely unrealistic.

    "There" is not a good description - there are no absolute positions in space, and "at rest" doesn't make sense without specifying a reference frame first.
     
  4. Aug 6, 2017 #3
    Yes. I don't want to orbit the sun. I just want to stay in the same place where I stationed the ship and watch the moon and the earth slowly drift away from me, make their trip around the sun, and comeback to the same place i am.
     
  5. Aug 6, 2017 #4

    jim mcnamara

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    Your assumptions need changing.

    Well, you need to consider that the solar system as a whole has a large velocity relative to the Milky Way as well. The simple answer is: we cannot afford to spend the entire Gross Domestic Product of the Earth's economy for 5 years on a project like this. mfb explained it: too much cost for fuel is a big concern.
     
  6. Aug 6, 2017 #5

    mfb

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    No, relativity doesn't say that. If you don't use thrust, you won't feel any acceleration, but you will crash into the Sun after ~2 months. You can call that "the Sun crashes into me" - that is the freedom relativity gives you. It still means you'll die.
     
  7. Aug 6, 2017 #6

    Drakkith

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    You have to expend fuel because you are no longer orbiting the Sun and have to perform work against gravity to keep from falling towards it. If you choose to describe things as if the Sun and Earth are orbiting you, then you still need to expend fuel to accelerate away from them as they "fall towards you".
     
  8. Aug 6, 2017 #7
    Yeah. That makes sense. Thanks!
     
  9. Aug 6, 2017 #8
    It seems unlikely, but could matter get "stuck" and accumulate at the center of mass between two binary and tidally locked objects of similar mass?

    https://en.m.wikipedia.org/wiki/Binary_star#Center_of_mass_animations
     
  10. Aug 6, 2017 #9

    Drakkith

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    I haven't done the math, so I don't know for sure, but I suspect that any small amount of residual drift will place the matter closer to one binary member and the attraction of gravity will then attract it closer, making such an arrangement unstable. Also, note that matter occupies a non-zero volume, while the center of mass is a point.
     
  11. Aug 6, 2017 #10

    mfb

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    There is an unstable equilibrium - in general it is not at the center of mass, as that scales with distance, while forces scale with inverse distance squared.
    As an example, 1 kg at x=-1 and 9 kg at x=3 will create such an unstable equilibrium at x=0, but the center of mass is at x=2.6.

    It is unstable, so nothing will stay there for long. It is not even a stationary point as the objects orbit around the center of mass, not around the unstable equilibrium point.

    There is the Lagrange point closer to the smaller mass where you co-orbit so the rotation is fine, but that is unstable as well.
     
  12. Aug 6, 2017 #11
    Thanks guys. After further reading, I think what I was getting at is Lagrangian points, though this is something totally different than what the OP was talking about.

    Edit: Just realized mfb mentioned this in his response.
     
    Last edited: Aug 6, 2017
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