In relation to one of my questions, Earth was destroyed after hit with a weapon that tore the planet apart by magnetic and gravitational force which does leave a large segment left floating. The rest of the mass is about in either a ring or impacted back into the remaining mass.

Now, with the distortion of and loss of pull in gravity, the pull on the moon might differ or the orbits might change. I don't quite know what the outcome would be for the moon. It's important for me to know because there's a whole mining colony stationed on the moon; I'm not expecting survivors since I expect earth-debris would rain down on the moon at one point or another.

Would the moon stay in orbit, of either earth's remains or around the sun? Or would the loss of earth cause the Moon to turn rogue and get ejected from the system/pulled into the orbit of another planet (like mars)? or would the moon's pull start to move the earth? I don't know which of these theories would be most likely to happen.

So, what would happen to the moon if earth broke apart?

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    $\begingroup$ "The distortion of and loss of pull in gravity": where does this come from? Simply breaking the Earth into pieces won't change its mass, nor will it move its center of mass. To have an impact on the orbit of the Moon you need to somehow propel the pieces beyond said orbit. (As long as the pieces remain inside the orbit of the moon, the total gravitational attraction will remain equal to what it was before.) Which means that the question cannot be answered unless you describe specifically what exactly happens to the Earth, that is, where do the pieces go. $\endgroup$ – AlexP Jun 19 '20 at 19:18
  • $\begingroup$ @AlexP That's more in relation to the previous question. the weapon donated inside the planet after crashing into the planet. An alien weapon so I think some things can have some leverage to allow this aspect to work. It was designed for the planet to break apart after it more or less got to the core where most of the magnetic field is generated. $\endgroup$ – SKKennell Jun 19 '20 at 19:23
  • $\begingroup$ Yeah, I got that. But the point is that in order to understand what will be the impact on the motion of the Moon, we need to know where the pieces of Earth go. In a first order approximation, simply breaking up Earth won't affect the motion of the Moon. (From the point of view of an external object, the gravitational pull of a system appears to originate in the center of mass of that system, and the center of mass of Earth cannot be moved by an explosion or anything else within Earth.) Yes, if we go beyond a first order approximation then things are vastly more complicated. $\endgroup$ – AlexP Jun 19 '20 at 19:27
  • $\begingroup$ @AlexP No one's yet given me an acceptable answer so I'm leaning towards most of the debris in ring-like orbit for the time being. I expect some to return to bombard the planet of course. Either one half or one third would untimely be left as the largest remaining piece. $\endgroup$ – SKKennell Jun 19 '20 at 19:34
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    $\begingroup$ Note it can take considerably longer than one hour for someone to answer a question on WB SE. It's not uncommon for new answers to be appearing days after a question is posted. Note that comments are not considered answers - they're typically used to clarify or point out issues with a question. $\endgroup$ – StephenG Jun 19 '20 at 20:42

The Moon does not orbit the Earth. She orbits the barycenter of the system she is part of together with Earth.

That barycenter is located within Earth (and changes location as the Earth rotates), but does not coincide with the center of Earth. It is located 4,670 km away from the center of Earth (for comparison, Earth's radius is about 6,380 km).

Earth was destroyed after hit with a weapon that tore the planet apart by magnetic and gravitational force which does leave a large segment left floating. The rest of the mass is about in either a ring or impacted back into the remaining mass.

If all the mass of the Earth stays within its original sphere of influence and Hill sphere, then the orbit of the Moon will remain largely the same. There might be disturbances as the barycenter of the system is displaced as the Earth is torn apart, but that's it.

In an alternate scenario in which the Earth disappeared, or if some of the Earth's mass disappeared/escaped and the remaining mass is not enough to hold the Moon in orbit, the Moon would escape the Earth-Moon system in a tangent (i.e.: in the direction it was moving when it escaped). Its new orbit would be directly around the Sun, and though it would not be exactly the same orbit as Earth's original orbit, it would be almost exactly the same. The new orbit's apoapsis and periapsis would be just a few thousand km closer or farther from the Sun (remember that 1 AU ~ 150 million KM), and it might be a fraction of a degree tilted in relation to the original Earth orbit.


It would depend on the details of how the Earth was shattered and where the debris went. But in outline the Moon would probably be bombarded by Earth debris imparting huge amounts of energy and possibly liquefying large parts of it. It is possible that the Moon itself would also be disrupted.

Which ever way it went the debris of the Earth and the wrecked Moon would continue on in a roughly Earth shaped orbit although the Moon and the debris might well spread out into an inner asteroid belt. There would be some variation in the Moons orbit depending on the position with respect to the Earth and the Sun at the point of impact, but this would not deviate greatly from Earths orbit.

If the Moon survived it is reasonable to assume it would be repeatedly hit by debris in the following years and might even eventually form the basis of a new planet sweeping up the debris, but this could take a very long time.


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