It's possible to compute the energy and momentum that an asteroid impact would have to transfer to the Moon, assuming that two solid balls (classic Newtonian billiard balls) hit each other (either a direct impact or a glancing impact). There are two issues at play here, only one of which is real. The time for the impact would be about 1/4 of the Moons current orbital period, which is to say about a week. This is manifestly impossible without shattering the Moon, but in that case, the Moon would indeed hit the Earth. If it orbited closer than about 3000km to the surface of the Earth for long (the Roche limit) these forces would eventually pull it to pieces, and Earth would probably have a pretty set of rings for a short time before internal collisions between the bits caused them to rain down on Earth and kill everyone.įinally suppose the impact(s) was(were) so big that they actually put the Moon into an elliptical orbit whose innermost point was so close to Earth that the Earth and Moon touched. At this distance it starts to matter that the near side of the Moon is closer to Earth than the far side, so that Earth's gravity pulls on it more strongly.
FINETUNE ASTEROID HITTING EARTH SERIES
Now, suppose that the impact was really big, or perhaps there were a long series of impacts (starting to look like enemy action.) so that the innermost point of the ellipse was eventually driven down to within a few thousand miles of the Earth, somehow miraculously not smashing the Moon to fragments in the process. Over a long enough period the gravity of the Sun also comes into play and things may shift a bit, but that is a relatively small effect. Suppose it gets knocked into an orbit that is 220000 miles from the Earth at its closest and 240000 miles at its furthest, that is where it will stay. What is important though, is that this elliptical track is stable at least for a while.
Depending on the direction of the impact, it would either get a bit nearer to the Earth than it is now, once per orbit, or a bit further away (it also might swing North and South a bit).
The effect would be to shift the Moon from its present almost circular orbit around the Earth, into an elliptical one. So, suppose something large enough and fast enough to change its velocity noticeably, but not large enough or fast enough to shatter it, did hit the Moon. Part of the reason why is that the "circling the drain" effect you describe doesn't really happen for solid objects much less dense than black holes. As several people have said, this is incredibly unlikely.
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