The effects of a major asteroid impact on the Earth have been well explored. But what about a major impact on the Moon?
How big an impact would be required to disrupt the Moon?
How would that affect the Earth? I know that the Moon stabilises the Earth, but would a ring of debris do the same? Would there be prolonged orbital bombardment or would it quickly settle?
How quickly would the Moon reform? Presumably millions of years? For that matter, how quickly would the ring form? Centuries? Millennia?
No impact big enough to disrupt the Moon is likely; it would take a Ceres-sized planetesimal to do it. Comets or Earth-crossing asteroids just aren’t big enough. The only effect on Earth might be if enough secondary debris subsequently impacted the Earth.
Only if the orbit of the moon is affected, or
if the impacting object significantly adds or subtracts from the moon’s mass (which would also likely change the orbit), or
the moon is actually broken apart (though significant changes in tides wouldn’t start until the mass of the moon starts to spread out or spiral into the earth).
I would have to agree with Lumpy. The size/mass and velocity of the object hitting the moon would have to be something beyond what we have even contemplated in scenarios of objects hitting earth to really destabilize the Earth/Moon system as we know it.
Now, if such an object did hit the moon, the event would be nothing short of fantastic (to watch) and profoundly influential to Earth and our future. All depending on when/where and from what angle it hit the Moon. The different outcomes are fine for speculation, but impossible to anticipate accurately.
Just for the sake of contemplation, wouldn’t it just be wild for the Moon’s orbit to fail and it slowly spiral (the whole moon) into Earth over say, a dozen years? Not that I wish it or anything but, that would be AWESOME!
Note, of course, that it would be even harder for an impacter to disrupt the Earth. That’s not what people are worried about in these scenarios: We’re just worried about something disrupting the Earth’s biosphere, which is a lot more fragile than the planet itself.
This is comparable to talking about gravity failing and people gradually drifting to the Moon over the course of a dozen years. Orbits are conic sections, not spirals, and therefore any orbit for an object which collides with Earth must do so in a half orbit or less, or not at all. So for the Moon to eventually hit the Earth, we’d be talking weeks, not years. Nor is it possible for a thing in space to not be in orbit: Everything’s in some orbit or another.
You’d need one of the larger asteroids, something with a mean diameter of a few hundred kilometers.
Well, something bigger than anything that is at all likely to hit the Moon now, anyway. We’ve contemplated scenarios of Mars-sized objects hitting the Earth, but only early in the history of the Solar System.
That is why I said just for contemplation purposes only. I like to think of the prolonged implications over years and imagine the social chaos that would ensue. Just for kicks - not trying to re-invent science here.
(Kind of like watching the Death Star collide with a planet - Yumm)
Actually, you could get a situation in which the Moon starts out in an orbit comparable to its current orbit (but inclined 90 degrees out of the ecliptic) and crashes into the Earth in 6.5 years due to perturbations from the Sun, at least according to a simulation my undergrad advisor did.
Sorry for the derail, but what happens then if a spaceship is traversing between planets and something is ejected from the craft? Does it drop, fall behind, stay in the same place, or what?
It’ll be in some orbit or other. Precisely what orbit depends on how fast and in what direction it’s ejected, but practically speaking, it probably won’t be ejected very fast, so it’ll be in very close to the same orbit as the ship.
I’m having trouble imagining how one might change the Moon’s orbit enough to get it to crash as shown in the simulation with just an asteroid. You’d probably need something bigger to do that.
Mr. Neville watched an anime show, Gurren Lagann, where in one story arc some bad guys were planning to do that.
Well, if it came down that slowly, it would just be lame. There needs to be a big impact at the end, or it’s just no fun.
Given the rotational momentum of the Earth plus whatever resultant velocity the Earth and Moon are accelerated to, the impact would be a lot more impressive than just sinking. We can safely say that neither body would remain intact; the seismic stresses would be enough to tear the Earth’s crust apart, and the Moon, which is basically a large and highly fractured cinder block, would crumble and crack into pieces. Think of the gyrations the basket of a washing machine makes when it has an unbalanced load, magnify to celestial scale, and you get an idea of what would happen.
An object making an unpowered interplanetary jaunt (Hohmann transfer) is on an elliptical orbit about the primary; in the case of, say, Earth and Mars, that would of course be the Sun, so the object remains in a Solar orbit. Any object ejected with only a small velocity will essentially follow the same trajectory as the vehicle from which it was ejected, although given the distances involved, even a small change in velocity is sufficient to cause it to miss an intercept, so it’ll likely fly right on by the destination. And the behavior, in terms of terrestrial ballistics, is odd; throwing it backwards will end up with it falling inward, throwing it in will have it speed up and pass by the radial position of the vehicle, et cetera.
Oh, and to address the o.p.: the Moon would reform, at least as an amorphous collection of rocks vacuum cemented together by dust and trace water, rapidly; on the scale of hundreds or thousands of years. Earth is too small to maintain anything more than a tenuous ring via known mechanisms, and the Moon is way too far out to be affected by Roche tides, even if you can considered it to be an essentially fluid body after impact. Depending on the energy of the impact, it may fling some material into an escape orbit and/or on an intercept toward Earth, spreading the material around some, but the larger masses will tend to aggregate together and disrupt any nascent ring structures, and eventually drag from coupling forces (as one overtaking mass makes a close pass on another it pulls the other along with it and is simultaneously dragged slower and higher) will draw most of the material back together. An impact big enough to both shatter the Moon and send a significant amount of material Earthward is unlikely to an extreme.
As far as the “Moon stabiliz[ing] the Earth,” there is no real consensus on how much effect it actually has, but it probably isn’t the biggest problem, at least for the foreseeable future; the lack of tides, however, would massively disrupt the life and feeding cycles of littoral ecosystems upon which much ocean life is directly or indirectly dependent. It’s hard to say exactly what the impact on marine life would be, but without the regular tidal action we can assume that littoral and sublittoral primary production (which depends on nutrients being drawn in and material being moved out to sea to support life in oceanic zones) will be a fraction of what it is currently, severely restricting deep ocean life, and thus, both fishing and oxygen production.
If the Moon is really giving you fits, instead of smashing it to bits, why not try covering it with carbon black, or putting a big sunshade ahead of it, or painting it pink and erecting a Somebody Else’s Field around it? That’ll cost you far less, and it won’t disrupt any vital biospheres.
