Say a large, stray Kuiper Belt object manages to enter the inner solar system and hits the moon, causing the moon to fracture. Say the impact wasn't devastating and left most of the moon intact while creating fairly small moon fragments. What would be some conceivable short and long-term consequences for earth after such an event (eg. altered tidal patterns for a number of years)?

From what I gather reading the 2 other threads here that relate to this topic, a smaller moon may mean less gravity/drag on earth, resulting in shorter days on our planet. Some of the small moon fragments may also form a ring around earth. If this happens, is it conceivable that some of these fragments would be visible to the naked eye at night, illuminated by the sun just like the moon?

Someone also mentioned that over time, the fragments still bound by moon's gravity could re-coalesce. Is that to say in time the moon could regain some of its original mass?

Sorry for the handful of questions here. What I'm most interested in is whether (and how) life/the environment on earth could be altered slightly following an impact on the moon. Any insights on the possible scenarios would be GREATLY appreciated!

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    $\begingroup$ Welcome to the site Leigh, nice first question. $\endgroup$
    – James
    Mar 9, 2016 at 14:54
  • $\begingroup$ For a scy-fi setting where moon chunks devastated Earth, check the anime series Aldnoah Zero. $\endgroup$ Mar 9, 2016 at 15:12
  • $\begingroup$ I think you should take a look at Seveneves $\endgroup$
    – F-Gamma
    Mar 9, 2016 at 16:59

3 Answers 3


Shrinking the moon now wouldn't speed up the earth's rotation - if you removed the moon millions of years ago the days would be shorter now, but that's something different. The days are actually getting longer now (obviously not very fast!) because of the moon dragging on the earth, and reducing the size of the moon would only make the slowdown happen slower. So forget about that route.

You'd have to hit the moon pretty hard to form a ring, and it wouldn't last long (probably no more than a month) before it was swept up again by the moon. Also, if you did hit it that hard, you'd have a lot of fragments raining down on earth, which would cause a lot more problems than the 'minor' effects you're looking for!

If you hit the moon hard enough at the right angle, you might be able to disrupt its orbit, leading to a greater 'wobble'. There would be some times that the moon would be closer to earth than it is now, and other times that it would be farther away. Speeding up or slowing down the moon could change its average distance from the earth as well. This might have noticable effects on the tides and weather.

If you want to go the 'orbit disrupted' route, I would suggest having the object not hitting the moon (which would probably cause the moon-fragment apocalypse mentioned above) but passing nearby and disrupting its orbit with its gravity instead. If you must have the moon getting hit by something, maybe you could have a pair of gravitationally-bound objects passing through the area instead - the smaller one hits the moon, and the bigger one passes close enough to disrupt its orbit.

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    $\begingroup$ +rep for very nice answer. However you probably forgot to mention the gravitational effects of that passing body upon Earth and its orbit :) $\endgroup$ Mar 9, 2016 at 12:03
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    $\begingroup$ @YoustayIgo If the object is the right mass and the right distance away, it could plausibly alter the orbit of the Moon without having a noticable effect on the orbit of the Earth. The Moon's gravity doesn't have a huge effect on the Earth's orbit (it causes some wobbling but not much), so another Moon-sized object passing at about the same distance shouldn't affect Earth much either. $\endgroup$ Mar 9, 2016 at 12:13
  • $\begingroup$ That's because the moon already has orbital velocity around the sun, so the wobble is because the common center of gravity of the earth+moon is orbiting the sun with the planet and moon orbiting that point. $\endgroup$
    – Tim B
    Mar 9, 2016 at 12:21
  • $\begingroup$ maybe the moon could be broken apart by a body of a specific size passing on the far side of the moon such that the moon fell within the Roche limit but the Earth was far enough away to avoid any major disruptions... $\endgroup$
    – Michael
    Mar 9, 2016 at 19:08
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    $\begingroup$ @LeighP. If the moon's orbit had a high eccentricity (which is a better word in this scenario than 'wobble', I suppose) you might have a cycle where on one side of the month the Moon would come very close, and might appear many times its normal size. During a 'big moon', the tides would be much more 'extreme' than during the 'small moon' on the other side of the month - the high tide would be higher and the low tide would be lower, possible much different, depending on how close the moon got. I'm not sure what this would do to the weather, but I expect it would do something. $\endgroup$ Mar 9, 2016 at 21:01

I strongly suspect that Earth will be peppered with moon chunks large and small. This would be catastrophic for anything on the surface.[citation needed]

Also the moon only affects the Earth's rotation in a small way, fractions of a second per year, so day length will not be noticeably different.

Finally, the moon would re-coalesce, but it would take millions of years. The fragments would reflect light and would be visible at night and possibly even during the day.


First of all, I would suggest that you read this wikipedia article and this article. And watch this youtube video

Basically these are articles about the collision of a protoplanet with Earth far back during the early stages after its formation (sometime between 4.5 and 4.45 billion years ago).

An object as large as to fracture the moon would have extremely disastrous effects on Earth. Considering that it was a ~10 km long asteroid which brought about an abrupt end to the reign of dinosaurs and their relatives 65 million years ago, the effects of having a somewhat smaller (1-3 km) moon piece falling on Earth would be cataclysmic. Talk about giant tsunamis, extreme volcanism, earthquakes ... and nearly 70% of human population would be wiped off the face of Earth (and that is a very generous estimate for survival of our species!).

For some reason, which I am not aware of yes, we have not discovered any Earth-sized or smaller planet with rings. So I cannot say if rings would/could be formed as a result of the debris falling into space and circling the Earth. But hopefully, after all the doomsday apocalypse, Moon would perhaps come a bit closer to Earth and the next intelligent species would see a bigger, larger moon without fearing that it would leave Earth's gravitational pull forever.


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