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A What is orientation of a needle shaped spacecraft?

  1. Sep 11, 2016 #1
    this is a small part of a problem on tidal forces and I wasn't sure what the question asks as it seems to me that more information is needed. Am I right or is there something im missing? the question goes as:

    "A spacecraft approaches a neutron star of radius 10 km and mass 1.5 times mass of sun. what is its likely orientation at its closes approach of 20 km from the center of the star?"

    I appreciate someone clarify what the question is asking.
    Thank you.
     
  2. jcsd
  3. Sep 11, 2016 #2
    The question seems quite obvoius: Will the spaceship be orientated horizontally or vertically ore someting between at its closes approach? However, I do not know if this question can be answered without the starting orientation and angular momentum.
     
  4. Sep 11, 2016 #3

    Dale

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    No, actually much less information is needed. None of the details of the mass or distance matter. The question could be simplified:

    "A spacecraft approaches close to a neutron star, what is its likely orientation at its closest approach?"

    Think about what makes one state more likely than another state.
     
  5. Sep 12, 2016 #4

    jbriggs444

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    There are two ways of answering this that give opposing answers. Is the rotation of the approaching spacecraft damped in any way? [Larry Niven had to admit that Beowulf Schaeffer does not survive].
    Please ignore the above. My recollection of the sequence of events was erroneous. There is an unambiguous answer.
     
    Last edited: Sep 12, 2016
  6. Sep 12, 2016 #5

    Bystander

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    Huh? PM me w' the explanation ... we don't need to give away answers.

    Edit: Well, shucks ... trying to figure out what I'd missed in the Fleet of Worlds n-volume whatever it is.
     
    Last edited: Sep 12, 2016
  7. Sep 12, 2016 #6

    jbriggs444

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    Tried to PM, failed: "You may not start a conversation with the following recipients: Bystander."
     
  8. Sep 17, 2016 #7
    After much thinking I have found that we need to use the equation for tidal forces acting on the spaceship as it approaches the star. and it appears that there is a big difference (a few hundred g's if i remember correctly) in gravitational acceleration between tip of the spaceship and its tail hence the ships orientations is most likely radial towards the center of the star.

    Thank you.
     
  9. Sep 17, 2016 #8

    Dale

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    That is correct. You can also come to the same realization by considering potential energy. The radial orientation has a lower PE.
     
  10. Sep 25, 2016 #9
    How does that lead to the maximum propability for the vertical position independent from the starting conditions? The the lower potential energy results in an increased kinetic energy and therefore in an increased angular velocity compared to the horizontal position. Shouldn't that result in an increased propability for the horizontal position in case of a rotating ship?
     
  11. Sep 25, 2016 #10

    Dale

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    It is just an observation that a system is most likely to be found in the lowest energy state. For a gravitational system that is the tidally locked state with the long axis pointed towards the primary.
     
  12. Sep 25, 2016 #11
    The gravitational potential energy of a planet in an elliptic orbit is a simple counter example. What makes you sure that the potential energy resulting from the tidal forces acting on the rocket is not another exception?

    Is the rocket tidally locked during the closest approach?
     
  13. Sep 25, 2016 #12

    Dale

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    It isn't a counter example. Even for a planet in an elliptical orbit the most likely configuration is tidally locked. It certainly doesn't mean that all planets in elliptical orbits are tidally locked, just that it is more likely than any other configuration.

    We don't know, but it is more likely than any other possibility.
     
  14. Sep 26, 2016 #13

    rcgldr

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    Assuming there's no internal friction related to rotation of the spacecraft, would it tend to oscillate or be out of sync with respect to its path, similar to the moons libration (the moon rotates about about the same rate, but it's orbit is slightly elliptical).
     
    Last edited: Sep 26, 2016
  15. Sep 26, 2016 #14
    That's not what I was talking about. It is more likely to find the planet in a position with a higher potential energy - no matter if it is tidally locked or not.

    I would like to see your calculation.
     
  16. Sep 26, 2016 #15

    jbriggs444

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    Consider a pendulum in free fall in an elevator shaft. We do not know its initial orientation. Assume that its initial angular velocity is small. We apply a small upward force on the axis about which the pendulum is free to swing. The pendulum begins swinging back and forth. As time passes we gradually increase the upward force that is applied. What effect does this have on the amplitude of the swings of the pendulum (measured in displacement from the "down" orientation)?

    What is the likelihood of finding the pendulum at 90 degrees from straight down after the upward force has been increased by a factor of 1000 from its starting value?
     
  17. Sep 26, 2016 #16

    Dale

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    OK, but that isn't relevant to the thread. The thread was asking about the orientation at the position of closest approach, which is most likely to be "vertical" because the tidally locked configuration is the most likely.


    Well, in the solar system there are 173 natural satellites. Of those, 34 are tidally locked, so that is just under 20% likelihood. So the orientation at closest approach for all of those is vertical. All other orientations have less than 1% likelihood. So at least in our solar system it is the most likely orientation empirically.
     
  18. Sep 27, 2016 #17
    Apparently, not likely at all. Just type x''(t) = -(g+k t) x(t) into wolfram alpha and it will give you the answer which turns out to be a pair of Airy functions which look like this, except there is a minus sign in the argument so time goes towards negative x on this graph:
    800px-Airy_Functions.svg.png
    The amplitude gradually decreases while the frequency increases indefinitely (or rather until the whole thing shatters to pieces)
     
  19. Sep 28, 2016 #18
    As the pendulum will get closer to its stable equilibrium (hanging straight down) the likehood is very low. This is indeed a good point. It suggests that strongly increasing tidal forces will result in an oscillation close around the vertical orientation of the ship during the closest approach. However, this argumentation requires that the period of the oscillation is short compared to the time of the flyby and therefore also depends on the starting conditions. Without a corresponding calculation (or at least rough estimation) I cannot say if this is the case - especially if the process starts near the instable equilibrium (horizontal orientation of the ship or standing upside-down in case of the pendulum analogy).



    It might be irrelevant for the OP but not for the thread. The example shows that the "observation that a system is most likely to be found in the lowest energy state" can't be generalized to the potential energy of any system. Thus you need to show that it applies to the OP if you want to use it in your argumentation.

    In contrast to jbriggs444 you did not yet provided a proper justification for this claim. As you do not refer to the given details regarding mass and distance (which are important for his argumentation) you need another explanation.

    The most likely orientation of bullets with hyperbolic trajectories would be different (just to give you a single counter example). Why need a spacecraft approaching a neutron star needs to behave like a natural sattellite in the solar system and not like the bullets?
     
  20. Sep 28, 2016 #19

    Dale

    Staff: Mentor

    I agree with you on this point now. I think that I have overstated it above.

    What we can say is something along the lines that the tidally locked state is the most likely state for gravitational system. There might need to be some further qualifications also, but I am not certain of that.

    I don't know why. I observe the fact that the tidally locked orientation is the most likely orientation. From that fact I can immediately answer the OP. The question isn't asking which orientation it actually has, just which is most likely.

    I don't think that is a counterexample. What makes you believe that the most likely orientation for hyperbolic trajectories would be different than for elliptical trajectories?
     
  21. Sep 28, 2016 #20
    I'm still missing a proper justification for this claim. A statistic based on a non-representative selection of objects is not sufficient. You need to show it for any kind of gravitational systems or at least for the system we are talking about.

    Because there is not anough time to change the initial orientation and angular momentum of the bullet during a single flyby in the gravitational field of a body in the solar system.
     
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