Moon crashing into earth, different scenarios

[Traab]

So, I was watching this Which is a great cover of The Show Must Go On, and the video made me think about this scenario. What kind of effects would we see before impact if the moon, or even worse, another planet, were set to collide with us? I imagine that if it was travelling like a comet at absurdly high speeds it would be academic that the atmospheric compression would ignite the air around it in a life destroying level of power since 5 ms later it would impact the surface and destroy the planet anyways, but I wondered about something slower, like, the moon is nudged JUST out of its proper orbit and gets dragged into earth by gravitational pull. Im not sure how slow of a change is possible, but lets work with the theoretical slowest speed it could collide with us. Would that make any difference to what we would see happen? Would it take long enough to impact the surface that we would see any interesting effects of something that massive entering our atmosphere? Or are we still talking a matter of seconds before the full collision happens?

[NichG]

I don't think you can do it gently. In its current orbit, the moon is going about 1km/sec relative to the Earth. Unless you're actually going to do a powered landing, the best you're likely to manage is to push retrograde until the moon just barely clips the Earth's surface. However, just the change of potential energy from lunar orbit to Earth's surface is enough to get the moon to 1.4km/sec from rest.

I suppose you could try a gravitational aerobrake maneuver - get the moon going just above the Earth's atmosphere and have it rip off gas with every pass, very very gradually slowing itself down. I'm not sure how long that orbit could continue before a 'top of atmosphere' orbit becomes a 'top of crust' orbit, but it's pulling the long part of the semimajor axis in rather than pulling the near part in, so I'd guess it'd actually take quite awhile. You'd definitely not enjoy the tides though. Or the faster-than-soundspeed winds. Or whatever heating effects that kind of thing would have.

[Eldan]

The question is if you try anything like "gently" (which will not be gentle, there are always enormous energies involved), would the moon just break apart from tidal forces before hitting the Earth? In which case, you get a rain of meteors that sterilizes the planet.

[Traab]

Well i never thought it would be gentle, i was just curious about what the difference in speed of impact might achieve. I admit I wasnt thinking about its orbit around the earth and thus "horizontal" for lack of a better term, movement would change its impact. WOULD the tidal forces break apart the moon before it slams down onto earth?

[NichG]

That's a good point. It looks like the Roche limit (https://en.wikipedia.org/wiki/Roche_limit) for the Earth-Moon system is about 9500km, which is bigger than the radius of the Earth. So if you have the moon sit in orbit nearer than that, it'll start to get torn apart. I'm not sure what happens if the orbit is elliptical and the moon just dips into that distance for one day a month. If the moon has any cohesion beyond the gravitational, it might be able to pull that off without tearing apart during the period of exposure. So you'd get some kind of material transfer from loose surface dusts and the like, but anything that's fused rock might stay put.

[Ibrinar]

https://youtu.be/jGzsub7gh2g?t=65 this says we would be getting cool 7600 meter high waves!

[Traab]

On a related note, does the roche limit get effected by the speed of a traveling object? By which I mean, if a chunk of rock the size of the moon was heading right for us at top speed, would it be torn apart before it could collide? Or does it take time to tear off appreciable sized chunks of the moon sized meteor? Meaning the majority of it would still slam into earth as a single solid object but there would be some minor flaking/breaking from its surface as it comes in.

[tyckspoon]

On a related note, does the roche limit get effected by the speed of a traveling object? By which I mean, if a chunk of rock the size of the moon was heading right for us at top speed, would it be torn apart before it could collide? Or does it take time to tear off appreciable sized chunks of the moon sized meteor? Meaning the majority of it would still slam into earth as a single solid object but there would be some minor flaking/breaking from its surface as it comes in.

There would not, I suspect, be a meaningful difference to the planet being impacted in this case, as the options are 'get impacted by a solid mass' or 'get impacted by a cluster of somewhat smaller masses that still in sum have enough energy to sterilize the planet.' (I do not know the answer to the actual question, but I'm pretty sure either case is functionally the same scenario for the planet being impacted.)

[Lord Torath]

Once you are inside the Roche limit, you will start being pulled apart. I don't know how quickly you get pulled apart; I think it depends on the size of the object crossing the limit.

Does the Roche limit vary by the size of the object crossing it? If Earth's Roche limit is outside its surface (as stated up-thread), the limit would almost have to vary, considering that objects in freefall don't seem to spaghettify themselves before hitting the ground.

Probably time for me to go read up on the Roche limit. I think I'm spouting off from the top of Dunning–Kruger hill.

[Ibrinar]

Checking it appears to depend on the density but not the size. However Roche is only about gravity so a rigid object can survive being inside the Roche limit. And I think what depends on the size is how great the force differential is so for a large object the point where it gets rapped apart after passing the limit comes sooner. So our gravity is too small to hold us together inside earth roche limit but the tidal forces aren't big enough to rip us apart because we are only about 2 meter long.

[tyckspoon]

Does the Roche limit vary by the size of the object crossing it? If Earth's Roche limit is outside its surface (as stated up-thread), the limit would almost have to vary, considering that objects in freefall don't seem to spaghettify themselves before hitting the ground.

It's primarily related to the conflict between the object's own gravity (which, on a sufficiently massive object in space, will pull it into an approximately round shape) and the pull of a more massive object, which will try to distort that round object toward itself. Nothing on Earth is massive enough to have sufficient gravity that it will conflict with Earth's, and as mentioned nothing is big enough to have a notable gravity differential over its size, which the hypothetical moon-sized impactor would. Basically everything we have on or in orbit of Earth currently is too small for gravitational influence to matter much to it in this sense.

I think the hypothetical moon, if it were coming straight in (and not rotating. If you add rotation and the varying forces moving because of different parts experiencing different rotation speeds due to differing levels of gravitational influence things get.. complicated) would not experience significant breakup, at least not due to gravitational effects - Earth's gravity would be accelerating it in the direction it's already going, not pulling part of it in a different direction than the main body as it would if it was skimming across the Roche line/going in an orbital path.

[Fat Rooster]

On a related note, does the roche limit get effected by the speed of a traveling object? By which I mean, if a chunk of rock the size of the moon was heading right for us at top speed, would it be torn apart before it could collide? Or does it take time to tear off appreciable sized chunks of the moon sized meteor? Meaning the majority of it would still slam into earth as a single solid object but there would be some minor flaking/breaking from its surface as it comes in.

In many ways objects that size are better thought of as fluids than solids. It would probably be stretched out a little, but still one piece when it gets to earth.

The Roche limit is independent of velocity, but we need to be clear exactly what we mean. An object inside the Roche limit will begin to break up, but it is not an instant thing. Objects on elliptical orbits can drop inside the Roche limit temporarily. They begin to break up, getting stretched along a direction, but might get far enough away before breaking completely. Gravity will then squeeze it back into a more spherical shape when further away, before repeating.

You can think of it like gravity being reversed in one direction for a while. Obviously if gravity is reversed forever everything will escape, but if gravity is only reversed a little bit for a little while, then turned back on again, everything will fall back together again. Tidal accelerations are pretty tiny, so don't expect any spaghettification of anything travelling at escape velocity. It is not that it isn't happening, just that it takes too long to notice.

If planets changing shape like putty sounds like it should involve a lot of energy, you would be right. Look At Io for the best example. It is only in a moderately elliptical orbit far from the Roche limit, but is the most energetic landscape in the solar system. The energy comes from the eccentricity, so if the moon were in a highly eccentric orbit that hit the roche limit it would not stay in one for long. These tides would reduce the eccentricity, and it would probably melt, before breaking up completely. Io avoids this by being in a resonance with Ganymede and Europa, which keeps it's orbit elliptical. Io is basically syphoning the orbital energy of the other too moons into heat. /tangent.

As for how something like that could be possible; quite easily. If the moon were in a retrograde orbit, the tides would act to reduce it's orbital velocity rather than increase it. It's orbit would decay, until it broke up and formed a ring system. That is believed to be how many ring systems form. As for an object hitting the earth that has not spent a lot of time inside the Roche limit, that is harder. I guess you could get it happening if we were near the Roche limit of a larger body, such as a gas giant, but then the earth would probably already be melted from the tides around that.

[Rydiro]

What would happen if the moon just gently landed on earth?

[factotum]

What would happen if the moon just gently landed on earth?

I don't think there's any realistic way of soft landing it. Even if you did, you're still ending up with a rock 2000 miles across sitting on the surface of the Earth, which is not going to result in anything "gentle" happening...

[Traab]

I don't think there's any realistic way of soft landing it. Even if you did, you're still ending up with a rock 2000 miles across sitting on the surface of the Earth, which is not going to result in anything "gentle" happening...

This is a link to the xkcd comic of dropping a mountain. The most minor version, dropping it an inch could produce a tremor on par with a 3.5 earthquake, or possibly trigger an actual one depending on how the fault reacts to the impact. Now, im not entirely sure how many quadrillion times larger and therefore more destructive the MOON would be in this scenario, but my gut tells me... alot. Thats not even counting what sort of damage adding the mass of the moon to the earth would cause just by existing. Im pretty sure we either lost a continent or gained another. Maybe even joined some in a land bridge if it fell in the right place of the oceans? My sense of scale is kinda wonky at this level. If it landed in the water, it probably flooded most of the current world by displacing that much water. So we may be down to one continent and a few scattered islands. The moon, and the highest points of the other continents.

[Ibrinar]

For comparison the moon diameter is 3,5 million meter, the distance between delaware and portugal about 5,6. After it falls apart yeah it can easily bridge a major ocean. But yeah would also flood many areas. Edit looked up the number about 16 times the volume as all oceans.

[NichG]

This is a link to the xkcd comic of dropping a mountain. The most minor version, dropping it an inch could produce a tremor on par with a 3.5 earthquake, or possibly trigger an actual one depending on how the fault reacts to the impact. Now, im not entirely sure how many quadrillion times larger and therefore more destructive the MOON would be in this scenario, but my gut tells me... alot. Thats not even counting what sort of damage adding the mass of the moon to the earth would cause just by existing. Im pretty sure we either lost a continent or gained another. Maybe even joined some in a land bridge if it fell in the right place of the oceans? My sense of scale is kinda wonky at this level. If it landed in the water, it probably flooded most of the current world by displacing that much water. So we may be down to one continent and a few scattered islands. The moon, and the highest points of the other continents.

It's a bit more than a continent. Continents are surface features, like the skin of a grape. The moon is 1/80th of the mass of the whole grape. The entire crust of the Earth accounts for 0.6~1% of the Earth's total mass, so the Moon is 'from 1-2 entire Earth surfaces down to 5-30km deep' in total mass. You're not just adding one continent, you're adding all of them, twice.

[Traab]

It's a bit more than a continent. Continents are surface features, like the skin of a grape. The moon is 1/80th of the mass of the whole grape. The entire crust of the Earth accounts for 0.6~1% of the Earth's total mass, so the Moon is 'from 1-2 entire Earth surfaces down to 5-30km deep' in total mass. You're not just adding one continent, you're adding all of them, twice.

Fair point. Huh, Thats an interesting thought. If the moon did appear on our planet, a very significant portion of it would still be in space as its larger than our atmosphere. What kind of effect would THAT have? The more I think about it, the more insane it becomes. For one thing, it would be cutting the flow of air and water currents to shreds by acting as an impassable barrier, would having like 2/3 of it sticking into space also act as an absurd heat sink? The sheer weight of the moon, even resting on our surface would likely shatter the earths crust like dropping a boulder onto a water bed, bursting all that fluid out in all directions underneath the surface. Sweet baby jeebus, it doesnt even have to collide to wipe out all life, it just has to EXIST.

[Fat Rooster]

Fair point. Huh, Thats an interesting thought. If the moon did appear on our planet, a very significant portion of it would still be in space as its larger than our atmosphere. What kind of effect would THAT have? The more I think about it, the more insane it becomes. For one thing, it would be cutting the flow of air and water currents to shreds by acting as an impassable barrier, would having like 2/3 of it sticking into space also act as an absurd heat sink? The sheer weight of the moon, even resting on our surface would likely shatter the earths crust like dropping a boulder onto a water bed, bursting all that fluid out in all directions underneath the surface. Sweet baby jeebus, it doesnt even have to collide to wipe out all life, it just has to EXIST.

Objects that size are in hydrostatic equilibrium, so it would behave like a fluid. It would just kind of bloop together rapidly into a larger sphere. The effects on air and water are extremely minor compared to the fact that it would be causing currents in rock.

The energy involved is not comparable to orbital speed, but still crust melting levels.

[Eldan]

Yeah. The moon would crumble, as we already established upthread that it woudl be inside the roche limit. Quite likely, it would also partially melt, just from gravity potential energy. If you set down the moon on the Earth, it rips the atmosphere off, and the water, and then melts into a massive new surface layer.

Which is sort of the reverse of how it was created.

[Lord Torath]

And to clarify, the Roche limit depends on the mass of both objects, the distance between the two objects, and the size of the smaller object (and a few other things). So the Roche limit for the Moon and Earth is outside the surface of the Earth, but the Roche limit for, say the ISS and the Earth is well inside the surface of the Earth, which is why the ISS hasn't "spaghettified' itself. Yes, I've done some reading.

Next up: Compare the kinetic energy of the Moon at 2.4 km/s to the 'disintegration' energy of the Earth. (Yes, I know there's a proper term for it, but I can't remember it just now. ) Also: how fast would the Moon have to strike the Earth to completely scatter it to interplanetary space?

[factotum]

Next up: Compare the kinetic energy of the Moon at 2.4 km/s to the 'disintegration' energy of the Earth. (Yes, I know there's a proper term for it, but I can't remember it just now. ) Also: how fast would the Moon have to strike the Earth to completely scatter it to interplanetary space?

Gravitational binding energy. For Earth it's about 2.25 x 10^32 J. Given the mass of the Moon then the minimum speed it would need to have that much kinetic energy is 78km/s, but in reality it would have to be going faster than that in order to completely destroy the Earth because the transfer of energy during the impact isn't going to be perfect.

[Eldan]

Depends on your definition of "completely destroy the Earth". You could melt the entire planet to slag and it would cool down and reform at some point, in an entirely new configuration, but technically, that was below the gravitational binding energy, because it's still here.

[factotum]

I was taking "completely destroy" to mean what Lord Torath said, namely, "Completely scatter it to interplanetary space". Obviously the Moon impacting with far less than 78km/s would utterly destroy the Earth for the intents and purposes of the people living on it!

[Mastikator]

What would happen if the moon just gently landed on earth?

If you had a rocket that could emit enough thrust to do a powered landing of the moon on the Earth, and point that rocket at the Earth... well we ded just from the rocket. There's no way of getting the moon to the surface of the Earth without sterilizing the crust.

[Bohandas]

Next up: Compare the kinetic energy of the Moon at 2.4 km/s to the 'disintegration' energy of the Earth. (Yes, I know there's a proper term for it, but I can't remember it just now. )

Well the moon is currently theorized to have once previously, about 4.5 billion years ago, crashed into the earth going at a speed at a little under 4 km/s, and they;re both still here (although the moon lost much of its theorized original mass)

https://en.wikipedia.org/wiki/Giant-...is#Basic_model

[Strigon]

I don't think there's any realistic way of soft landing it. Even if you did, you're still ending up with a rock 2000 miles across sitting on the surface of the Earth, which is not going to result in anything "gentle" happening...

Even if you did "land" it, the moon is round currently^{[CITATION NEEDED]} and would not be stable on the earth. Landing the moon intact, even if physically possible, would still result in most of the moon crashing hundreds or thousands of miles to the surface.