Planet disassembly

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Planet disassembly

Postby Robert'); DROP TABLE *; » Mon Aug 29, 2011 6:17 pm UTC

(This is in "real" science because, AFAIK, it's theoretically possible to actually do)

A common concept that seems to pop up in far-future hard SF is the idea of taking apart an entire planet into arbitrarily-sized chunks and doing stuff with those, like turning them into computronium or whatever. I'm just wondering, is this "practical?" What sort of energy do you need to put in, at bare minimum, to actually do that? I know the idea of gravitational binding energy is already studied, but I'm not sure that's useful if you only wanted to, e.g. turn Mercury into a Dyson swarm, since it assumes that you're moving everything "infinitely" far away.

Can anyone help? :D
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Re: Planet disassembly

Postby gmalivuk » Mon Aug 29, 2011 11:12 pm UTC

Robert'); DROP TABLE *; wrote:I know the idea of gravitational binding energy is already studied, but I'm not sure that's useful if you only wanted to, e.g. turn Mercury into a Dyson swarm, since it assumes that you're moving everything "infinitely" far away.
As far as Mercury's gravity is concerned, though, most of the space a swarm would take up around the Sun *is* effectively infinitely far away.
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Re: Planet disassembly

Postby HopDavid » Mon Aug 29, 2011 11:23 pm UTC

Robert'); DROP TABLE *; wrote:(This is in "real" science because, AFAIK, it's theoretically possible to actually do)

A common concept that seems to pop up in far-future hard SF is the idea of taking apart an entire planet into arbitrarily-sized chunks and doing stuff with those, like turning them into computronium or whatever. I'm just wondering, is this "practical?" What sort of energy do you need to put in, at bare minimum, to actually do that? I know the idea of gravitational binding energy is already studied, but I'm not sure that's useful if you only wanted to, e.g. turn Mercury into a Dyson swarm, since it assumes that you're moving everything "infinitely" far away.

Can anyone help? :D


Charles Stross is one of the science fiction writers who writes stories like that.

And he doesn't even believe colonizing the solar system is plausible, much less creating Dyson swarms. See his High Frontier Redux.

So far as a Dyson swarm goes I would start with the small bodies. The mass of the Main Belt may only be a small fraction of a planetary mass. But in terms of available surface area and resources, the asteroids beat the planets hands down. For a planet only the top 10 kilometers or so of the surface are accessible. As you go deeper, sheer stress and temperature prohibits further penetration. But with a typical asteroid, the entire volume is reachable. Travel from one asteroid to another is much less difficult due to their shallower gravity wells. With regard to gravity wells, this XKCD illustration is good. Most asteroids have gravity wells comparable to Phobos and Deimos in this illustration.
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Re: Planet disassembly

Postby Tass » Tue Aug 30, 2011 8:56 am UTC

HopDavid wrote:For a planet only the top 10 kilometers or so of the surface are accessible. As you go deeper, sheer stress and temperature prohibits further penetration.


Of course if you actually remove the top ten kilometers then the lower layers will start to cool and become accessible. Trouble is that most of the crust and mantle are relatively useless SiO2 and you have to expend a damned lot of energy to move all that mass out of the big gravity well before you get to the good metals in the core. A civilization would have to be very strapped for new resources to undertake that project. Kuiper belt and even Oort cloud objects are more accesible than this.

So yeah, If we ever get off this rock the progression will likely be (excluding surface bases and limited mining on other terrestrial bodies like the moon and Mars): Near earth asteroids, main belt asteroids, small moons, Kuiper belt, emmigration to other systems, then finally maybe disassembly of larger bodies if there is nothing else to get at within hundreds of light years.

Of course assuming we ever get started. It is sadly not as likely to happen, at least not soon, as I first believed when reading The High Frontier.
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Re: Planet disassembly

Postby Sagekilla » Sun Sep 04, 2011 3:07 am UTC

You may way to many gems this guy has here: http://qntm.org/geocide

He has a couple of interest ideas, and besides dissembling the earth is more or less like destroying it.

I'll warn you, it's quite a bit of reading. But it's rather entertaining ;)
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Re: Planet disassembly

Postby math-helper » Thu Sep 08, 2011 2:54 pm UTC

Yes, the idea of separating any planet is a fiction only. The mother nature have done it with a strong force and effort. I think the atomic bombs (the great countries on the planet, are holding these bombs) can undo the mother's work.
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Re: Planet disassembly

Postby Izawwlgood » Thu Sep 08, 2011 3:23 pm UTC

Out of curiosity, will temperature still be an issue within seismically dead planets? What about significantly smaller planets, like Mercury?
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Re: Planet disassembly

Postby gmalivuk » Thu Sep 08, 2011 5:19 pm UTC

math-helper wrote:Yes, the idea of separating any planet is a fiction only. The mother nature have done it with a strong force and effort. I think the atomic bombs (the great countries on the planet, are holding these bombs) can undo the mother's work.
No, the world's entire nuclear arsenal is nowhere near enough energy to separate a planet the size of Earth, or even the Moon.

If we figure the arsenal has on the order of 100,000 MT total yield, that is still only about 1 trillionth of Earth's gravitational binding energy, and about a billionth of the Moon's.
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Re: Planet disassembly

Postby Tass » Thu Sep 08, 2011 7:58 pm UTC

gmalivuk wrote:
math-helper wrote:Yes, the idea of separating any planet is a fiction only. The mother nature have done it with a strong force and effort. I think the atomic bombs (the great countries on the planet, are holding these bombs) can undo the mother's work.
No, the world's entire nuclear arsenal is nowhere near enough energy to separate a planet the size of Earth, or even the Moon.

If we figure the arsenal has on the order of 100,000 MT total yield, that is still only about 1 trillionth of Earth's gravitational binding energy, and about a billionth of the Moon's.


To put it further on scale it is actually way less than a single dinosaur killer asteroid impact.
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Re: Planet disassembly

Postby Technical Ben » Fri Sep 09, 2011 3:12 pm UTC

Another thing to remember, is atomic testing is done underground and in the sea. With little to no effect noticed by those living far away.

Can the two energies be compared though? Is the energy required to explode/vaporise the earth, and the energy required to disassemble it the same?
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Re: Planet disassembly

Postby Robert'); DROP TABLE *; » Fri Sep 09, 2011 4:58 pm UTC

I would have thought the energy required to vaporise the Earth would be orders of magnitude smaller than the energy needed to disassemble it.

Also, I ran some numbers through Wolfram, and it suggested that if you had a solar panel the size of Mercury to gather energy, (~330PW) it'd take 10,000 years to take apart the Moon. Can anyone say how accurate that is? I'd have thought that would be far too large, but I don't have much of a grip on the quantities involved, other than the raw numbers.
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Re: Planet disassembly

Postby gmalivuk » Fri Sep 09, 2011 6:30 pm UTC

Technical Ben wrote:Can the two energies be compared though? Is the energy required to explode/vaporise the earth, and the energy required to disassemble it the same?
The only difference between exploding and disassembling is the fact that explosions happen quickly. But the gravitational binding energy is what has to be overcome in either case, and that's the number I quoted before.

Vaporizing it in such a way that it's merely turned to gas but may end up recollapsing is probably far less energy. Vaporizing it in a way that the gas disperses means you've also overcome the gravitational binding energy, so of course the total is greater than just the one.
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Re: Planet disassembly

Postby HopDavid » Fri Sep 09, 2011 8:12 pm UTC

gmalivuk wrote:
Technical Ben wrote:Can the two energies be compared though? Is the energy required to explode/vaporise the earth, and the energy required to disassemble it the same?
The only difference between exploding and disassembling is the fact that explosions happen quickly. But the gravitational binding energy is what has to be overcome in either case, and that's the number I quoted before.


With a slow vertical ascent, gravity loss is incurred.

Image

So, if disassembly is done via ordinary rockets incurring gravity loss, I believe it would take much more energy.
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Re: Planet disassembly

Postby Robert'); DROP TABLE *; » Fri Sep 09, 2011 8:14 pm UTC

But we don't want the rockets going sideways? Unless I've misunderstood the diagram.
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Re: Planet disassembly

Postby Soralin » Fri Sep 09, 2011 8:29 pm UTC

Robert'); DROP TABLE *; wrote:Also, I ran some numbers through Wolfram, and it suggested that if you had a solar panel the size of Mercury to gather energy, (~330PW) it'd take 10,000 years to take apart the Moon. Can anyone say how accurate that is? I'd have thought that would be far too large, but I don't have much of a grip on the quantities involved, other than the raw numbers.


Well let's see, from http://en.wikipedia.org/wiki/Gravitatio ... ing_energy The gravitational binding energy of a spherically symmetrical object is 3GM2/5r So:

Gravitational constant = G = 6.67384*10-11 m3kg-1s-2
Mass of the Moon = M = 7.3477 × 1022 kg
Radius of the Moon = r = 1737100 m

Gravitational binding energy = U = 3 * 6.67384*10-11 m3kg-1s-2 * (7.3477*1022 kg)2 / 5 * 1737100 m
U = 2.00215*10-10 m3kg-1s-2 * 5.39887*1045kg2 / 8685500 m
U = 1.2445*1029 m2kg s-2
U = 1.2445*1029 J

A Joule is equivilent to 1 watt*second, and the human energy usage of Earth is about 15 TW, so with all the energy we currently use put to the task, it would take 262,911,117 years.

So, let's get a bigger power source

Is your solar panel the size of Mercury, at the distance from the sun that mercury is, or is it around where the Earth/moon is?

At Earth, there's on average 1.366 kW/m2
Mercury varies in distance a bit, but let's say 0.387 AU on average, which would put it at about 9.12 kW/m2

The radius of Mercury is about 2,440,000m, so a solar panel the same size as a cross section through the middle of it (about what you'd be able to get if you covered the surface in panels), would be about 1.87*1013 m2.

At the distance of the Moon, it would produce about 2.554 * 1016 W, which means that you could disassemble/blow apart the moon in about 4.87*1012s, or about 154,324 years.
At the distance of Mercury, it would produce about 1.705 * 1017 W, which cuts the time down to about 7.3*1011s, or about 23,132 years.

If you had something the surface area of Mercury, at the distance of the sun that Mercury is, that would probably get it down to 10k years, but if you just paneled the whole surface of Mercury, you'd only get it's cross-section worth of light at any given time. And note that all of this is assuming 100% efficiency.

So yeah, planets are huge, and getting stuff out of gravity wells takes a ton of energy, especially when it's the entire planet itself.
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Re: Planet disassembly

Postby Robert'); DROP TABLE *; » Fri Sep 09, 2011 8:36 pm UTC

Soralin wrote:Is your solar panel the size of Mercury, at the distance from the sun that mercury is, or is it around where the Earth/moon is?

I was panelling all of Mercury, for \frac{1}{2} \times area \times power. I realize that I directly asked Wolfram for Mercury's surface area, rather than calculating a flat disk of the same radius, so I'll have overestimated slightly.
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Re: Planet disassembly

Postby Soralin » Fri Sep 09, 2011 8:43 pm UTC

Robert'); DROP TABLE *; wrote:But we don't want the rockets going sideways? Unless I've misunderstood the diagram.

If you want to get something into orbit you do, if you want to to just send it out at escape velocity so it doesn't come back, you could just shoot it straight up. Really, this is only much of a problem if your rocket is slow, relative to the gravity of the planet you're taking off from. I mean, if you produced a rocket that could produce 1g of acceleration, and set it off on Earth, it would just hover there, burning fuel. You could give it a push, and it could float off into space, eventually, but it would use a ton of energy in doing so. http://www.projectrho.com/rocket/mission.php Has some good ways of calculating how much that gravitational drag will take on a given planet, with a given rocket.

Using the rockets we do now, taking off of Earth, it takes 9-10 km/s of delta-v, to get into low earth orbit, whereas without the drag it would be 7.8km/s. So you could add 15-25% or so because of that. Although taking off with rockets rockets of the same acceleration off of the moon would be a lesser percentage, faster rockets relative to the surface gravity would decrease it. And if you just tossed the rocks off of the surface with a rail gun or something, it wouldn't be a factor at all, since that would basically be the same thing as you'd get with blowing up the planet, with the rock flying away with sufficient speed that it didn't come back down again. One rock at a time, or all the rocks at once doesn't make a difference.
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Re: Planet disassembly

Postby HopDavid » Fri Sep 09, 2011 8:47 pm UTC

Robert'); DROP TABLE *; wrote:But we don't want the rockets going sideways? Unless I've misunderstood the diagram.


Horizontal burn is minimum gravity loss.

However achieving an 8 km/sec horizontal velocity vector in earth's troposphere is very bad. A vertical ascent is required before the rocket does the main horizontal burn. Even on an airless world, rocket acceleration must have a vertical component or spacecraft will contact planet surface and lithobraking occurs.

On the moon (or any airless world) a horizontal rail gun could achieve escape velocity with virtually no gravity loss. But that's less of an option on worlds with an atmosphere.
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Re: Planet disassembly

Postby HopDavid » Fri Sep 09, 2011 8:52 pm UTC

Soralin wrote:Using the rockets we do now, taking off of Earth, it takes 9-10 km/s of delta-v, to get into low earth orbit, whereas without the drag it would be 7.8km/s. So you could add 15-25% or so because of that.


15-25% increase in velocity is 32-56% increase in kinetic energy.
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Re: Planet disassembly

Postby Soralin » Fri Sep 09, 2011 9:09 pm UTC

HopDavid wrote:
Soralin wrote:Using the rockets we do now, taking off of Earth, it takes 9-10 km/s of delta-v, to get into low earth orbit, whereas without the drag it would be 7.8km/s. So you could add 15-25% or so because of that.


15-25% increase in velocity is 32-56% increase in kinetic energy.

And probably even more, when you take into account the extra fuel/propellent you have to carry, to carry the extra fuel/propellent that you have to carry for the extra delta-v, etc. (And much more, if you want to get your rocket back down again to bring up another load) That's why it's just simpler to launch them up from the ground at escape velocity without a rocket, since you can just ignore all that. :) (And since this was for the moon, you don't have to worry about atmosphere)
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Re: Planet disassembly

Postby mfb » Sat Sep 10, 2011 10:05 am UTC

HopDavid wrote:On the moon (or any airless world) a horizontal rail gun could achieve escape velocity with virtually no gravity loss. But that's less of an option on worlds with an atmosphere.

If you want to disassemble a planet with an atmosphere, it is a good idea to remove that first, as railguns look like the best way to do it piece by piece.
However, something more violent may work better. And a collision of an earth-sized object with Jupiter can be really interesting to watch.

1,4 billion km^3 of water on earth can be fused to Helium to get ~65GJ/g (but only 1/9 of the water is hydrogen) => ~1*10^34 J. That is ~50 times the required energy for the earth - so even with some inefficiencies, fusing all the hydrogen could be enough. It is much more difficult than D-T-fusion, but it doesn't look impossible.

With ~10% efficiency of the process and by heating up the surface to ~530°C, the earth will radiate away ~10^19W of waste heat, so the process is limited by that and will take at least 2.2*10^32 J / (10^18W) = 7 million years.
With 50% efficiency, this can be reduced to 700.000 years.

Using solar power can increase the efficiency, but direct photovoltaik is limited to ~1.7*10^17 W (170 PW).

The surface will shrink during disassembly, of course, but that is a small effect - as soon as it is down to 50%, you are almost done (M^2/R with 1/sqrt(8)M, 1/sqrt(2) R => only 1/(4*sqrt(2)) of the binding energy left).

Edit: Fixed, thanks
Last edited by mfb on Wed Sep 14, 2011 9:44 am UTC, edited 1 time in total.
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Re: Planet disassembly

Postby NonSequitur » Wed Sep 14, 2011 1:10 am UTC

GOOMH, Phil Plait. (He got 2^32 J for complete vaporization)

http://blastr.com/2011/09/astronomer-explains-why-w.php
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Re: Planet disassembly

Postby Tass » Wed Sep 14, 2011 9:08 am UTC

mfb wrote:The surface will shrink during disassembly, of course, but that is a small effect - as soon as it is down to 50%, you are almost done (M^2/R with 1/8M, 1/2 R => only 1/32 of the binding energy left).


1/2R means 1/4A, but you are right, binding energy shrinks much faster than area.
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Re: Planet disassembly

Postby scootwhoman » Mon Mar 12, 2012 6:58 pm UTC

Of all of the planets, Mercury probably is the most likely to be extensively mined. It has the highest density of any body in the solar system, it is very close to the sun, which makes energy easy to obtain, and also allows for solar sail export of the mass that is refined. If we are looking for heavy metals, we probably won't find high concentrations in asteroids, because the solar system is like a centrifuge working in reverse. The heavier materials collect at the bottom of the well, with the lighter materials on top. The asteroid belt represents the border between stony planets and the gas giants because the materials in it were too heavy to coalesce into a gaseous body, but too light to form a solid body.

Pressure mining will probably allow much deeper excavation than what has been achieved on Earth, so shafts several miles deep might be possible.
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Re: Planet disassembly

Postby starslayer » Mon Mar 12, 2012 10:20 pm UTC

That'd be nice if it weren't totally wrong. Earth is actually the densest body in the Solar System, not Mercury, though Mercury would be denser if gravitational compression were not a factor. The metallicity of the Solar System was essentially uniform everywhere at the time of its formation; the relative abundances change due to temperature, not angular momentum.* If we're looking for metals, the best places to go get them are from the asteroids, since as previously mentioned, they have small gravity wells and are thus easy and cheap to mine (as cheap as anything is in space, anyway). They're also non-differentiated; the surfaces of all the terrestrial planets are relatively heavy metal poor because they all sank to the core when the planets were still completely molten. The asteroids weren't big enough to have this happen to them, and are largely small enough that it doesn't matter even if the larger ones are.

Basically, ten'll get you twenty that the surface of Mercury is still largely basically useless silicates, and since it lacks plate tectonics, the crust is probably relatively iron and siderophile poor compared to Earth's; those all sank to the core, which is still under several hundred kilometers of the aforementioned silicates.

*The terrestrial planets are all too small to hang on to hydrogen and helium atmospheres, both due to their lower masses and their higher temperatures. The snow line at 4-5 AU is where the equilibrium state of water in a vacuum changes from gas to solid. The gas gaints probably started out as cores of rock, heavy metals and ice that, once they grew to about 10-15 Earth masses, were able to accrete hydrogen and helium. They subsequently snowballed from there.
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Re: Planet disassembly

Postby Tass » Wed Mar 14, 2012 5:06 pm UTC

starslayer wrote:If we're looking for metals, the best places to go get them are from the asteroids, since as previously mentioned, they have small gravity wells and are thus easy and cheap to mine (as cheap as anything is in space, anyway). They're also non-differentiated; the surfaces of all the terrestrial planets are relatively heavy metal poor because they all sank to the core when the planets were still completely molten. The asteroids weren't big enough to have this happen to them, and are largely small enough that it doesn't matter even if the larger ones are.


Even better, there are asteroids that were differentiated, but subsequently got smashed, exposing deliciously pure iron and siderophile material.
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