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Thread: if a moon is spiralling towards the planet?

  1. #1
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    if a moon is spiralling towards the planet?

    Does it reach the Roche limit and then just break up and form a ring?
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    Quote Originally Posted by Mudskipper View Post
    Does it reach the Roche limit and then just break up and form a ring?
    That is what I would expect. The spiraling in is too slow to bring the moon into an impact while still more or less intact.

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    Quote Originally Posted by Hornblower View Post
    That is what I would expect. The spiraling in is too slow to bring the moon into an impact while still more or less intact.
    Surely that depends on the size and tensional strength of the satellite?

    Also: how small pieces does the satellite need to break into in order to stop further inspiralling?
    If a satellite breaks into infinite number of pieces that form a ring of perfect central symmetry, the ring raises no tides and therefore loses no energy.
    How about a finite number of pieces held together by cohesion?
    If two satellites are orbiting at equal period 180 degrees then the tidal bulges they create on planet add up, not subtract. What would be the angle at which tides cancel?

    Does the Lagrange stability analysis (unstable at 180 degrees, stable at 60 degrees) still hold if secondary and tertiary are inside Roche limit?

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    Quote Originally Posted by chornedsnorkack View Post
    Surely that depends on the size and tensional strength of the satellite?
    Yes, which is why there is a fluid Roche limit and a rigid Roche limit. Phobos is inside the fluid Roche limit but will stay intact for quite a while.

    Also: how small pieces does the satellite need to break into in order to stop further inspiralling?
    If a satellite breaks into infinite number of pieces that form a ring of perfect central symmetry, the ring raises no tides and therefore loses no energy.
    How about a finite number of pieces held together by cohesion?
    You might want to look at what happen with Shoemaker-Levy as it broke up into a number of large pieces, small as well, no doubt.
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    Quote Originally Posted by Mudskipper View Post
    Does it reach the Roche limit and then just break up and form a ring?
    What would make a moon spiral into a planet?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    What would make a moon spiral into a planet?
    if it orbited in the opposite direction of the planet's spin. It would be dragging the tidal bulge behind it.

    I read that this might have happened with Venus...
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    Quote Originally Posted by DaCaptain View Post
    What would make a moon spiral into a planet?
    If it’s orbital period is less than the planet’s rotation period.

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    Quote Originally Posted by swampyankee View Post
    If it’s orbital period is less than the planet’s rotation period.
    but isn't that is true of Earth's moon...?

    I thought it had to orbit against the planet's rotation.
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    Quote Originally Posted by Mudskipper View Post
    but isn't that is true of Earth's moon...?

    I thought it had to orbit against the planet's rotation.
    They both drag down the satellite. Viewing from the primary, the relative motion is the same direction.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by Mudskipper View Post
    but isn't that is true of Earth's moon...?

    I thought it had to orbit against the planet's rotation.
    Earth's rotational period is 24 hours; the Moon's orbital period (sidereal) is about 650 hours. One example in the Solar System is the case of Phobos, which has a period of less than a Martian day; it's likely to crash into the surface of Mars (or break up into pieces that will do so) in the next 10 million years or so* (https://arxiv.org/abs/0805.1454, https://www.universetoday.com/14258/...years-to-live/)





    * The number depends on the internal properties of Mars, including the radial mass distribution and the internal damping of the materials that make up the Martian interior.

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



  11. #11
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    Quote Originally Posted by George View Post
    Yes, which is why there is a fluid Roche limit and a rigid Roche limit. Phobos is inside the fluid Roche limit but will stay intact for quite a while.

    You might want to look at what happen with Shoemaker-Levy as it broke up into a number of large pieces, small as well, no doubt.
    Were small pieces seen collide as well?

    Also, Shoemaker-Levy was not on a stable circular orbit.

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    Quote Originally Posted by swampyankee View Post
    If itís orbital period is less than the planetís rotation period.
    Quote Originally Posted by Mudskipper View Post
    but isn't that is true of Earth's moon...?

    I thought it had to orbit against the planet's rotation.
    oh, sorry, I misunderstood; I was thinking of 'speed' when I read 'period'.
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