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What Would Happen If the Moon Crashed To Earth?
angkor writes: "What would happen if the Moon crashed into the Earth? We'd die. But there seem to be a lot of variables involved in answering this. I wonder if /.ers have any other ideas..."
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...indicate that, assuming the Moon's orbital momentum were bled off at some reasonable (!) rate, that it would spend sufficient time within its Roche limit (5.5*2.42/3.34 = ~4 earth radii) to be torn to bits and redistributed into a massive, chunky ring. Once that transformation was accomplished, the Earth's surface would rapidly restabilize; since the ring would have a somewhat evenly distributed mass, its major tidal effect would be to increase Earth's equatorial bulge.
Of course, the large number of collisions in NEO among kilometer-and-greater-diameter objects would result in many thousands of catastrophic KT-magnitude impacts, meaning that any current life on earth would be kaput without a primary lunar impact ever occurring.
Mr. Bailey answered this question well, but seemed to believe that this kind of event was impossible. I'd suggest running an N-body simulation in which a small, massive object such as a black hole passes near the Earth-Moon system, in such a way as to impart a differential acceleration to the two bodies. It's easy to find (by trial and error, if need be) the appropriate criteria for a fly-by that would drop the Moon right in our laps, with little or no lateral motion. I'd guess that this was probably the kind of impact that the questioner envisioned.
Remind anyone else of the climactic scene of When Worlds Collide?