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Calculating the mass of an impactor sufficient to fragment the Moon

So, Roland Emmerich has a new movie coming out...

Wait come back!

Look, yes it's as stupid as its other ones, but I lack the mathematical ability to determine exactly how stupid.

It's based on a novel called Moonfall which is about an interstellar comet hitting the Moon and causing it to fragment. The fragments then hit the Earth, cue standard Emmerich plotline.

So. Kinda stupid. But exactly how stupid? The gravitational binding energy of the Moon is 1.2 x 10^29 joules, which is arguably not the final number but still pretty close, given that we're only asking for, well, bits to come off.

The fastest any object in our galaxy has ever been recorded travelling is 8 percent the speed of light, which was around SgrA. In our neck of the woods, what's the highest speed we could expect?

Finally, exactly how big would this impactor have to be to get the, ahem, ball rolling, as it were?

"Occam" is the name of the alien race that will enslave us all eventually. And they've got razors for hands. I don't know if that's true but it seems like the simplest answer."

Stephen Colbert.