## What if we could capture & gently land an asteroid on Earth?

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Beavertails
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### What if we could capture & gently land an asteroid on Earth?

OK, so..

Let's SciFi ourselves into believing that some sufficiently advanced alien had the motive and technology to ensnare, move, and lightly place a "typical" rocky asteroid onto the surface of the planet before fleeing undetected.

What would some of the effects be?

Let's assume a 5km asteroid radius, a 50km radius, and even a 500km radius. We can also assume a land or water placement if it matters. Say the Sahara Desert and/or somewhere in the Pacific.

Would any of those sizes be stable? Would the Earth remain stable? What about weather patterns, earthquakes (I assume the 500km would have some... settlement issues and a support problem in that the crust wouldn't support it), or even orbital problems (would the orbit slow, etc)?

I've tried to look online, but everything I've found has more to do with a large impact and associated destruction.

Any ideas?
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SDK
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### Re: What if we could capture & gently land an asteroid on Ea

Just to make conversation easier, and assuming 2g/cm3 density...

5 km radius = 1x1015 kg
50 km radius = 1x1018 kg
500 km radius = 1x1021 kg
Earth's mass = 6x1024 kg

Even in the 500 km radius case, Earth outweighs this asteroid 6000 to 1.

A "gentle landing" implies that this asteroid is being placed on the surface, therefore matching the rotation of the Earth already. The rotation of the Earth won't change because of that, but it will speed up slightly as the large asteroid is absorbed into the main body. Earth's orbit should be totally unchanged (again, because the asteroid was already moving at the same speed), but the moon will feel a slightly larger pull and might have to shift it's position a bit as a result. Overall, no big deal for the Earth overall, as you might expect.

Everything on the surface will be much more greatly affected, however. 500 km radius results in a volume of 5x108 km3. That's roughly equivalent to the volume of the Pacific Ocean (6.6x108 km3). I don't care how gently you drop that thing, entire continents are going to be flooded. The Earth's crust makes up less than 1% of Earth's volume, so about 1x1010 km3, making this beast of an asteroid just 20 times smaller than the entirety of the crust. I don't know plate techtonics, but that much pressure in a localised area is going to result in huge amounts of vulcanism if you drop this in the Sahara. I guess that's going to happen in the Pacific Ocean case anyway, so yeah, we're in for a wild ride! Mass extinctions for sure, but certainly not the end of life.
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LaserGuy
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### Re: What if we could capture & gently land an asteroid on Ea

The 5 km asteroid wouldn't be... too bad. It's volume is of order 600 km^3, which makes it about the size of a smallish mountain. If dropped on land, it would crush the crust below it down quite substantially and probably cause some extremely nasty local earthquakes as a result, but on a global scale it wouldn't be that major of a disturbance. Dropping it into the water would be bad for coastal areas. According to this paper, the largest tsunamis observed displace of order 100 km^3, so this would produce a pretty terrible one. Note that the asteroid that wiped out the dinosaurs is believed to have had a radius of ~5 km, but obviously much, much more kinetic energy.

The 50 km asteroid is quite a bit more severe. At a volume of 600000 km^3, it would have almost 10 times the mass of the world's largest mountain. Unprecedentedly massive earthquakes and tsunamis are probably pretty much inevitable here, regardless of how gently you're able to drop it: It's going to sink well into the mantle, which means it's going to have to displace an awful lot of material. Even at this size, you're still probably looking at a major extinction event due to the seismic activity. Note that on initial touchdown, the top of the asteroid would be at the edge of the stratosphere.

The 500 km asteroid would be very, very bad.

sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

You specified radius rather than diameter, so I'll refer to these asteroids as 10 km, 100 km, and 1000 km respectively.

The 10 km asteroid would be exciting for sure. The pressures involved will cause the crust to deform like a liquid, so we have a situation akin to an iceberg floating in water:

5 km.png (2.59 KiB) Viewed 5693 times

I've shown the Burj Khalifa as well as Puncak Jaya for scale (the latter because it is the highest island peak in the world and thus has the most immediately noticeable topographic prominence).

Of course, it's going to displace about 350 cubic km, and that material has to go somewhere. It will heap up into a ring of mountains around the asteroid about 700 meters high:

5 km heaping.png (1.03 KiB) Viewed 5693 times

The protruding spherical dome now has a height of around 3.35 km and is 9.3 km wide, but it won't stay that way. The weight of the upper portion pushes down on the lower portion with a pressure of about 37 MPa, which is slightly higher than the static compressive strength of rock in this density range. Fracturing and settling will occur, followed by the denser fractured portion sinking along with the denser core, causing the lighter material to rise into a secondary ring. The end result will be a very odd crater structure reminiscent of a volcanic dome:

5 km cratering.png (1.84 KiB) Viewed 5693 times

One interesting note is the amount of energy required to gently lower this asteroid from outer space to the Earth's surface. Even if we ignore the energy required to capture the asteroid and drag it into LEO and assume that the aliens doing this have a mechanism which negates gravity drag, it's still about 1e21 J, roughly 14% of the Earth's entire remaining petroleum reserves.

If it is placed into water, you won't really get any tsunamis (since it's being lowered in gently; the tsunamis of a meteor strike are the result of the kinetic energy). If it lands in the ocean, it will cause global sea level rise of 1.55 mm. Of course, if placed in a landlocked lake or sea, it will cause massive overflowing simply because there is no landlocked body of water deep enough to accommodate it.

The 100 km asteroid is definitely a bit worse. The energy required for this is a thousand times greater; 1e24 J is roughly the amount of solar energy the entire Earth receives every two months.

The asteroid is going to sink straight through the crust and into the mantle, coming to a "float" with a vertical prominence of 43 km:

50 km.png (1.97 KiB) Viewed 5693 times

At this scale, the Burj is less than a pixel high and Puncak Jaya is barely a ripple.

It will displace 316,000 cubic kilometers, which is a good deal more than before. This displacement will primarily take place in the crust, causing a dramatic heaping effect:

50 km heap.png (2.18 KiB) Viewed 5693 times

Basically, we have a gigantic mountain 43 kilometers high and 253 kilometers in diameter. It will sink vertically into the mantle but it will spread out over the mantle even faster, eventually flattening into a pancake about 360 kilometers in diameter:

pancake.png (1.58 KiB) Viewed 5693 times

At this point, we have completely destroyed/resurfaced a region roughly the size of Germany. The global effects are tremendous. If the touchdown took place in the ocean, then we will see an initial global sea level rise of about three feet, meaning that most coastlines worldwide will move inland by about 50 km. This results in a loss of 58 million square kilometers of land...roughly a third of all continental surface area, if my numbers are right.

And a 1,000 km asteroid? The only asteroid that even comes close is the dwarf planet Ceres, at a diameter of 950 km. The effects of gently lowering Ceres to Earth's surface will be left as an exercise for the reader...

earthandceres.png (117.48 KiB) Viewed 5693 times

...but it ain't pretty.

Beavertails
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### Re: What if we could capture & gently land an asteroid on Ea

The only thing that explanation was missing was great image hover text.

Love it. Thank you! (and thanks other folks too!)
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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

As it descends, the gravitational gradient would shift; at one point, the atmosphere and oceans would actually start to flow up into midair and partially cover the bottom pole of Ceres some time before Ceres actually made contact with the Earth's crust.

Of course the tidal effects at this point would have already ripped the former crust to shreds, but that's beside the point.

SDK
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### Re: What if we could capture & gently land an asteroid on Ea

At 0.27 m/s2, that can't be right. No matter how close Ceres got to Earth, Earth's gravity will always dominate, even at Ceres' surface. The upper atmosphere might move to cover a portion of Ceres' surface closer to the top pole, but it's not going to be sucking anything away from Earth.
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Flumble
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### Re: What if we could capture & gently land an asteroid on Ea

The way it's worded may insinuate that Ceres' gravity dominates at some point (and this might be intended sevenperforce), which is wrong of course, but he's right in that the pole will be covered in water before it hits the crust.
And the water will bulge a lot towards Ceres. The last image is a bit exaggerated, but think of weighted metaballs*: the shape may be concave at the right distance and threshold.**

*it's 2 AM here, metaballs are the best I can think of right now.
**it's 2 AM here, I have no clue what the conditions are to get concave metaballs. Actually, the only way that has something to do with the time is that I was convinced, for a moment, that every pair of metaballs must have a concave distance range.
***don't forget that water has low viscosity, so it'll creep up the surface anyway, though not for kilometres.

sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

No, SDK is right, I made a mistake. I was using the equation for acceleration rather than the equation for gravitational potential. The potential surface has its saddle point within the surface of Ceres regardless of the location of Ceres, so there's never any instance where water will be flowing up onto Ceres from the oceans. Though the tidal effects will still get messy.

Whizbang
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### Re: What if we could capture & gently land an asteroid on Ea

What if one of the asteroids was a mole of moles?

SDK
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### Re: What if we could capture & gently land an asteroid on Ea

Then the tidal effect will get even messier.

Real answer: a mole of moles would be far worse. Now we're talking one-hundredth the weight of Earth instead of one-six-thousandth (taking Randall's assumption of 4.5 x 1022 kg). The weight alone is going to completely shred the Earth's crust. You've just added the equivalent of thirty times the amount of water in all the oceans, leaving what's left of the continents flooded with blood. The composition of the Earth's atmosphere is going to change drastically as the moles decompose, likely having an even greater effect than the massive release of magma from beneath the surface. This will lead to the end of all higher forms of life, but at least the bacteria will love it since you've just multiplied the Earth's total biomass by about a trillion.
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Copper Bezel
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### Re: What if we could capture & gently land an asteroid on Ea

Beavertails wrote:The only thing that explanation was missing was great image hover text.

Love it. Thank you! (and thanks other folks too!)

No kidding. That was a totally legit What-If. I have to admit that the initial scale diagram for each of the three cases was the punchiest thing to me, but then, that's why it ends on the last one, so it works out. = ] I was most surprised by the way the 10 km asteroid "caved in" on itself - I know solid substances act like fluids at this scale, and models of asteroid impacts I've seen tend to make all the familiar water droplet shapes and things, but that was a shape I didn't expect.

So I guess Ceres would mostly pancake out above the mantle, too, and displace enough continental plate to cause obscene levels of volcanic activity globally?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

Copper Bezel wrote:I was most surprised by the way the 10 km asteroid "caved in" on itself - I know solid substances act like fluids at this scale, and models of asteroid impacts I've seen tend to make all the familiar water droplet shapes and things, but that was a shape I didn't expect.

To be fair, that was my conjecture about what would happen; I'm not an expert on what happens when gigantic asteroids are slowly lowered onto the surfaces of terrestrial planets. Then again, no one is an expert on that, because it has never happened.

But basically what we are dealing with is a situation similar to a salt dome. When you have a large layer of evaporites that is subsequently covered by sedimentary layers, the denser sedimentary layers will push down on the salt, causing it to flow upward in a giant bubble and form a dome. It's basically the same situation with the fractured surface and dense core of an asteroid; it will push down on the less dense rock around it and cause it to flow upward.

So I guess Ceres would mostly pancake out above the mantle, too, and displace enough continental plate to cause obscene levels of volcanic activity globally?

Well, in this particular case, it's kind of hard to tell what would happen without specifying the rate at which Ceres is lowered down. Ceres itself would (obviously) start to come apart from Earth's gravity, so we must assume that the lowering process also involves keeping it in one piece.

The effects are so massive that we also kind of need to specify the exact point at which it touches down. Ceres is roughly the size of Texas; the aftermath of its pseudo-impact is going to be completely dependent on where it hits. If it lands in an oceanic basin, for example, it will likely just carry the thin oceanic plate into the mantle with it, producing only minor perpendicular displacement of the surrounding crust. In contrast, if it lands on continental plate, it's going to fracture the plate along the nearest fault line and shove the pieces more than 300 miles away from where they started. For example, if it touches down in the Mediterranean, it will push France into Ireland, Italy into Romania, Greece into Turkey, and Egypt into Yemen.

Ordinarily, tidal effects cause a pair of bulges, but because we've specified that Ceres is being lowered against Earth's gravity, we can presume that Earth does not move toward Ceres. As Ceres descends, sea level will begin to rise on the near side and fall on the far side. If I wasn't lazy I'd solve for the gravitational potential to figure out the maximum rise/fall involved.

Tub
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### Re: What if we could capture & gently land an asteroid on Ea

Something else to keep in mind: dropping stuff onto Earth makes Earth bigger. With something as large as Ceres, Earth's radius would increase around 1km.

When the crust suddenly becomes too small for earth's surface, "obscene levels of volcanic activity globally" may still be an understatement.
If the crust stays intact (except for that one huge hole somewhere), we're in for a hot summer. Ceres will displace lava, which will seep out of that hole and cover the former surface under 1km of molten death.
Luckily the crust would probably break, resulting in a total area of around 160 000 km^2 (roughly the size of florida, or four times the size of the netherlands) where the earth's surface is suddenly raw magma; probably distributed across the globe along fault lines. Even if the initial shock from all that bulging and lifting and tearing doesn't get us, the long-term-effects aren't pleasant.

sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

It should be noted that unlike the 10-km and 100-km asteroids, Ceres cannot be modeled as having constant density. Its oblateness suggests that it is differentiated between a water-ice mantle with a density of roughly 950 kg/m3 and a rocky core with a density of roughly 3200 kg/m3.

For simplicity's sake, let's suppose an equator-to-equator meeting, and let's place it at the quad-point between the Pacific, Philippine, Indo-Australian, and Eurasian plates. Ground zero will be the small island of Waigeo just northwest of Papua.

ceres touchdown.png (6.06 KiB) Viewed 5064 times

It's not gonna be pretty.

The density of the crust is negligible here, so Ceres will come to a "float" when it displaces its mass in mantle. The upper mantle has a density of 3400 kg/m3, so this will correspond to a displaced volume of about 67 million cubic miles, 62% the volume of Ceres. It will float with the tip of Ceres about 390 km above sea level:

ceres float.png (2.92 KiB) Viewed 5064 times

West Papua will be obliterated along with Waigeo. The plate underlying the Philippine Sea will collapse, sliding down into the asthenosphere, while the thicker Australia Plate will buckle, forming massive but temporary mountains off the former coast of Queensland. The Eurasian Plate will buckle along the eastern borders of Indonesia but collapse at its intersection with the Australian Plate. Currently, the Pacific Plate is flowing into the Australian Plate, forming mountains (particularly along the center of Papua New Guinea), but this will stop and they will split apart, forming the largest fissure in the crust.

Before:

After:

Unfortunately, it gets worse.

Ice flows. The dense core of Ceres will sink down, into Earth's mantle, forcing the icy mantle of Ceres up while the top part collapses down and outward:

ceres sink.png (2.96 KiB) Viewed 5064 times

Once in Earth's mantle, the core of Ceres will be rapidly crushed to the density of the upper mantle. The interior of the Earth will only increase by about 1.98e8 km3, less than 0.02% of Earth's volume. Actually, it will be even less because the very minor increase in mass will produce a corresponding increase in density, but those effects are negligible. The surface area of the Earth will grow by 0.32%, about 600,000 square miles, but this expansion will be completely contained in the area local to the impact zone.

EDITED FOR ACCURACY:

All that ice, however, will melt rapidly, especially because it is dropping quite a distance and thus releases extraordinary potential energy. The potential energy can be roughly estimated by taking the center of mass of a hemispherical shell to be located at R/2. The total mass of ice is about 2e20 kg, 62% of which is below the surface and 38% of which is above the surface.

The potential energy of the top portion is thus equivalent to a 7.6e19 kg point mass at an altitude of 195 km (7.6e19 kg * 195 km * 9.8 m/s2)and the potential energy of the bottom portion is equivalent to the mass of the displaced crust minus 1.24e20 kg at an "altitude" of 295 km (8.23e20 kg * 295 km * 9.8 m/s2), for a total energy of 2.53e27 Joules. Thus, we can get an idea of what the aftermath looks like by supposing that an icy body with a mass of 2e20 kg were to hit the Earth at a speed of 5 km/s.

There are plenty of asteroid impact simulators out there, but I really like Earth Impact Effects Calculator because it's quite customizeable.

The sinking of the core and the collapse of the icy mantle produce a seismic event that would be a 12.5 on the Richter scale. Although there is no significant change to the Earth's orbit or rotation, the surface effects are tremendous. Total seismic destruction will extend 950 miles from the impact epicenter, covering a region roughly the area of the entire European Union. The ice itself will collapse in a tsunami that will ultimately drown nearly sixty million square kilometers, over 11% of Earth's total surface area.

But that's nothing compared to the air blast. The collapse of the upper ice shell will accelerate the atmosphere to nearly 50,000 mph in a devastating overpressure wave that sweeps around the entire planet, scraping it down to bedrock. Everything is obliterated.

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### Re: What if we could capture & gently land an asteroid on Ea

Simulating the collapse of a sphere of water that's already resting on Earth's surface as though it was an impactor coming from outside seems only a bit more reasonable than simulating a year's worth of eating as though it was a ton of TNT. Sure, it's the same amount of energy, but it's not going to be released the same way over the same length of time. (The speed of the air seems especially exaggerated. If anything it seems like the collapse would cause an underpressure wave due to the air rushing to fill the volume where the water previously extended up through the atmosphere. And it isn't going to somehow end up going much faster than any other part of the system.)
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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

Considering that this is basically the same as What-If #12 but with 32 million million billion times more water, I don't necessarily think I'm overestimating the destruction involved.

Because the air burst is the most devastating aspect and that's also the thing you seemed particularly skeptical of, I'll look more closely at that.

The biggest source of energy here is the crust pushing up on the water. But even if we ignore that, we still have to deal with 1.5e23 J of potential energy. I'm fairly certain the ice is going to fracture immediately and will flow as readily as water would under these kinds of forces.

The diameter of the upper spherical shell is about 940 km and its mass is 7.6e19 kg, so its weight will be 7.5e20 Newtons. Interaction with the atmosphere is a bit tricky to model due to the atmosphere's changing density, so to simplify and add conservatism I'll act as though the atmosphere has constant density. Since one square meter of atmosphere has a mass of roughly 10,330 kg and the density of air at STP is 1.225 kg/m3, the atmosphere would be roughly 8.4 km high. Modeling with constant density adds conservatism because greater density means it is harder to accelerate.

So then our constant-density atmosphere "hugs" the base of the sphere in a cylinder 8.4 km high and 940 km in diameter, a total area of 24,900 square km. The initial overpressure, then, is 3e9 Pascals, nearly enough pressure to synthesize diamond.

What about speed? Well, force isn't going to drop off much over the first meter, so let's use that. That first meter of air has a mass of 3.1e13 kg. Using W = F*x, we find a velocity of 6,956 m/s or a little over 15,000 miles per hour. Significantly less than what the asteroid impact calculator suggested for the entire energy dump, but definitely in the ballpark.

It's also worth noting that there will be no opportunity for underpressure until the ice tsunami has spread out enough to dip down below those 8.4 km. By that time, it is already 3,500 kilometers in diameter.

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### Re: What if we could capture & gently land an asteroid on Ea

I know there are other forces involved, but what I took issue with was just the calculation about the collapsing hemisphere of water, so forces from the crust are indeed not relevant. It's also not the same as what-if 12, because part of that was from falling 2km and hitting the ground as a mostly flat surface.

If I take your word for every mathematical step of your argument, you get a result that implies more than an order of magnitude less kinetic energy in the moving air than your asteroid-impact estimate.

I'm also not sure why I should take your word. The air isn't going to move at all without water pushing it out of the way, and water is 800x denser and harder to get moving.
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Beavertails
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### Re: What if we could capture & gently land an asteroid on Ea

Again, this is awesome. Thank you!

This is exactly what I hoped for when I asked the question.

I'm not sure why I thought that there might be a possibility of a remaining bulge that was Olympus-Monsian (or much, much larger) in nature. I'd imagine finding a material of such a mass / density for this to occur would be almost impossible.

I was also thinking if that were the case, then it might have interesting effects on orbital rates, climate phenomena, etc.

But no, general death and destruction. I'm ok with that too.

Well, not OK with it, but I'm sure you know what I mean.
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### Re: What if we could capture & gently land an asteroid on Ea

sevenperforce wrote:The initial overpressure, then, is 3e9 Pascals, nearly enough pressure to synthesize diamond.

According to the Bernoulli principle, a 1000kg/m^3 incompressible fluid flowing horizontally from a region of 3e9Pa to basically 0 will do so at sqrt(2*(3e9/1000)) = 2450m/s or 5500mph.

Now granted, that's what would happen if you attached a tube to a small hole in an otherwise closed container around the hemisphere of water, and if the water itself didn't compress at all (which isn't a good approximation at extremely high pressures, but higher density means even lower speed), so I don't know how well it translates to removing the entire container all at once, but it still seems like a better estimate than one which ignores the water completely.
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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

gmalivuk wrote:If I take your word for every mathematical step of your argument, you get a result that implies more than an order of magnitude less kinetic energy in the moving air than your asteroid-impact estimate.

The order-of-magnitude difference came from ignoring the contribution of the mantle shoving the larger portion of water upward.

I'm also not sure why I should take your word. The air isn't going to move at all without water pushing it out of the way, and water is 800x denser and harder to get moving.

Those calculations merely describe the peak overpressure and initial shockwave speed, not a sustained wind. I'm sure the sustained wind would be more in the range you describe...but still more than powerful enough to scour the planet clean.

Beavertails wrote:Again, this is awesome. Thank you!

This is exactly what I hoped for when I asked the question.

I'm not sure why I thought that there might be a possibility of a remaining bulge that was Olympus-Monsian (or much, much larger) in nature. I'd imagine finding a material of such a mass / density for this to occur would be almost impossible.

If you spread it out thinly enough and lowered it to the surface that way, it wouldn't have enough pressure to sink through the crust immediately.

But then the question has less to do with asteroids and more to do with "how big can you build a mountain before the crust under it collapses on a human timescale?"

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### Re: What if we could capture & gently land an asteroid on Ea

I still don't know why you pickeed 2e20kg at 5km/s instead of any of the other combinations that would give you the same total energy. Do you have some reason why that's more realistic, or was it just for numerical convenience?

Edit: Actually, it appears that the impact calculator itself mostly just treats energy as the important thing (though it does use size and density to determine whether the impactor will hit the ground or break up in the atmosphere). That makes me lose confidence in the calculator itself, because surely momentum has some effect on things.
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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

gmalivuk wrote:I still don't know why you pickeed 2e20kg at 5km/s instead of any of the other combinations that would give you the same total energy. Do you have some reason why that's more realistic, or was it just for numerical convenience?

Oh, I thought that was obvious. The total mass of Ceres's mantle is 2e20 kg. That's 0.76e20 kg above the surface and 1.24e20 kg below the surface in our scenario.

So I figured that just for the sake of a rough effects estimate, using the actual mass of material involved and the necessary speed to account for the total energy was a decent approach.

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### Re: What if we could capture & gently land an asteroid on Ea

As I said in my edit, though, it appears that the calculator itself just uses total energy, as though momentum doesn't matter.
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sevenperforce
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### Re: What if we could capture & gently land an asteroid on Ea

gmalivuk wrote:As I said in my edit, though, it appears that the calculator itself just uses total energy, as though momentum doesn't matter.

Oh, I missed that. The forum alerts you to new posts since you started replying, but not to edits.

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### Re: What if we could capture & gently land an asteroid on Ea

Yeah, I think I was writing the edit while you were writing your post.
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