If the Moon were impacted by a suitably sized meteor, how long would it take to impact the Earth?

  • An answer to the question of How well would the Moon protect the Earth from a Meteor? mentions as a possibility that the Moon could get knocked into the Earth.

    What is the smallest change to the orbit of the Moon from being impacted by a large meteor that would cause it to eventually impact the Earth (i.e. "circling the drain")? What timeline would that look like (minutes, hours, days, years, etc)?

    Even if some unlikely event changed the Moon's orbit sufficiently for it to strike the Earth, once it got within the Roche limit it would be torn apart by tidal forces. How much of that would form a ring and how much would eventually impact the Earth as a lot of various sized meteorites is another matter, but the Moon itself would never impact the Earth.

    @BillDOe I wouldn't be so sure about that; the Roche limit for the Earth-Moon system is extremely close. Even if you assumed only the core is rigid enough to survive the tidal forces, that would still be a 320km diameter ball of iron and nickel.

    @BillDOe Even if it's torn apart, a lot of it will end up on Earth.

    The longest time is just the free-fall time from the Moon's current orbital distance to the Earth. Depending on the nature of the impact, it could be shorter than that.

  • As several people have said, this is incredibly unlikely. 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. Orbits are not "precarious" in that way.

    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. The effect would be to shift the Moon from its present almost circular orbit around the Earth, into an elliptical one. 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). What is important though, is that this elliptical track is stable at least for a while. 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. It will not "spiral in".

    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.

    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. 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. 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.

    Finally 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. This is manifestly impossible without shattering the Moon, but in that case, the Moon would indeed hit the Earth. The time for the impact would be about 1/4 of the Moons current orbital period, which is to say about a week.

    And another scenario would be to kick the moon out of orbit entirely, another highly unlikely scenario that'd almost certainly require enough energy to shatter the moon instead.

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Content dated before 7/24/2021 11:53 AM

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