the moon is not held in space a particular distance from the earth by some weird "gravity point" that keeps it that far away. the earth's gravity keeps trying to pull the moon toward the earth's center, but the moon is traveling fast enough through space that the earth's gravity can't do any more than cause it to orbit. change the moon's mass seriously enough, you might change things so that the earth's gravity was finally enough to cause the moon to fall.
First of all, sorry for responding to a months-old post.
I think you might have been wrong to correct pv61. I think pv61 was pointing out the problem I have in science-fiction whenever a planet-scale body (like the Moon) explodes, unless the story gives a nod to the real scale of energy it would take for some or all of the chunks of the body to reach escape velocity and not just fall back toward the center of gravity. The problem isn't that a "weird 'gravity point'" would keep the chunks away from Earth, but how powerful the explosions would have to be to make the chunks escape the Moon. As others on this board have pointed out, nukes aren't that powerful.
And this may not really alter your point, but I find the way you put it misleading. Changing the Moon's mass, in and of itself, shouldn't affect its orbit. Remember how Galileo found that falling bodies of different mass fall at the same rate? And escape velocity is the same no matter how great the mass being launched. And the Moon would orbit pretty much the same way no matter its mass. Earth might wobble differently around a new center of gravity in the Earth-Moon system (particularly if the Moon increases its mass enough), but decreasing the Moon's mass isn't going to increase the effect of Earth's gravity so it falls. The effect of Earth's gravity is proportional to whatever mass is up there, so a body of whatever mass (until it crosses some line between Moon-mass and Earth-mass) could orbit the same way at the same speed (cross that line, and you end up with an Earth-Moon center of gravity outside the Earth, and then, while the Moon's orbit is much the same, Earth is orbiting the center of gravity as well).
Possibly you meant "change the moon's mass" to refer to the other mechanics of blowing the Moon apart (again, assuming explosions powerful enough). The explosions would give each chunk of the moon, big or small, a new arc to trace through Earth's gravitational field, so they would end up in altered orbits, or fall to Earth, or fly free.
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