Most efficient way to break up planets for material harvesting

Rocky planets are terrible. Convenient Earth like planets are rare, but most are too cold or too hot. They are covered with sand. I don't like sand. It's coarse, and rough, and irritating, and it gets everywhere.

Instead, our burgeoning K2 civilization doesn't bother with inhospitable planets anymore, when it can build trillions of nice climate controlled O'Neil cylinders with the perfect gravity levels and conditions for life. To build these, swarms of self-replicating Von Neumann probes are dispatched ahead of the colonization fleet and are tasked with breaking down the local planets into raw materials for use as construction materials.

Asteroids are a nice source of materials with their readily accessible mass, but asteroids only account a tiny fraction of a fraction of all the mass in the system; the rest is tied up in the strong gravity wells of planets.

So what is the most energy efficient method for our probes to break up the planets and get their materials out of the gravity well? A space elevator is nice, but a very complicated solution that relies on material manufacturing technology that does not yet exist. What about destabilizing the orbit of a moon and smashing it into the planet to "liberate" some of its mass? Could you "leverage" mass by using an asteroid to destabilize a small moon, which you use to alter the orbit of a planet so that in a thousand years it's orbit collides with another planet? Bonus points for creativity here and math based answers are always appreciated!

• "How long would it take for our fleet of probes to accomplish this?": This what? (And I don't get it why you believe that a civilization which can build autonomous von Neumann probes would have any difficulty in building space elevators. And why would you believe that getting things out of a gravity well using a space elevator is more energy efficient that any other way. And why would you believe that there is some sort of shortcut which could magically get stuff out of a gravity well without paying the full energy price.) – AlexP Jul 24 '20 at 13:54
• Materials for space elevators exist, we just can't make them outside a lab in any useful quantities. Also, if you're talking about casually taking apart planets, I don't think that some materials science that is at most 100 years out is gonna stop you – Dragongeek Jul 24 '20 at 13:54
• Sure you may be able to develop the tech in the future. But a space elevator is kind of a slow and boring solution that would take millions of years to ferry up each load of rock. Compare that to a little smashy smashy and then pick up the pieces. Much more fun. – SurpriseDog Jul 24 '20 at 14:13
• @AlexP using a space elevator is indeed more energy-efficient than any other known way. This is because you don't have to waste any energy accelerating the mass up to escape velocity, since the energy for that comes from the planet's rotation. If the cable is long enough you can even gain energy by removing mass from the planet. Of course, this does slightly slow the planet's rotation, and if your plan is to take the planet apart this way the rotation will eventually slow down to the point that the space elevator doesn't work any more. – Nathaniel Jul 25 '20 at 8:00

Freeman Dyson (the man who conceptualized the Dyson Sphere) had thought about this, and created the Dyson Planetary Spin Motor to break planets apart (this has nothing to do with the other Dyson who makes vacuum cleaners).

Dyson Planetary Spin Motor

Essentially, you turn the planet into a rotor by wrapping the planet with a metal grid and energizing it. For a planet like Earth, the rotation speed could be doubled in about 2500 years, and the planet accelerated to the breaking point in @ 40,000 years. Obviousy, you could gain efficiencies through various means, including using a superconducting grid instead of a metal one, and using orbital power stations to beam far more energy into the grid.

This would likely work on any rocky planet, and does not need conveinient other planets to collide with - even a single planet orbiting alone can be dissasembled this way.

If you do have more planets, another writer named Paul Birch had some interesting ideas. Using a device called a "light sail windmill" in near solar orbit to capture the energy of the Sun, he proposed to use streams of high speed particles to change the orbits of planets, or even their spin rates (the main idea was to spin up Venus so it would have a 24 hr day and put in a co orbital position to Earth). The details can be found in his papers here:

https://orionsarm.com/fm_store/MoveAPlanet.pdf

https://www.orionsarm.com/fm_store/SpinAPlanet.pdf

Since he is speaking of using far more energy than Dyson (about 0.2% of the Sun's radient energy for 30 years), the time frames are much shorter - a 30 year project to move Venus would be within the working lifespan of an engineer today. Moving planets rapidly around the solar system provides many opportunities to crash them into each other, use gravitational "slingshotting" to adjust the orbits of multiple planets, or, since the stream of pellets coming from the light sail windmill are carrying so much energy, use them directly like a sort of cosmic chainsaw to carve up the planet (the pellet stream cannot be reused if you do this).

So here are two theoretical methods to mine entire planets for material.

• What is the Dyson motor “pushing against” to increase rotation rate? Momentum in a closed system (i.e. a solar system) has to be preserved. – Michael Jul 25 '20 at 8:12
• Which takes more energy, spinning the planet up or moving it into a collision vector? – SurpriseDog Jul 25 '20 at 15:08
• @Michael A steady stream of asteroids that you slingshot around your nearest star. Basically it’s stealing angular momentum from the star to dump into the planet, and since the star is much, much more massive the planet will break apart long before the star cares. – Joe Bloggs Jul 25 '20 at 15:59
• This is used to great effect in one of the Long Earth books, with a swarm of Von Neumann probes that look like beetles tearing apart an alternate version of Earth. The descriptions of the latter days of the planet are quite trippy IIRC. – Joe Bloggs Jul 25 '20 at 16:01
• @michael: Or you can keep the rocks in orbit and use the fact that the sun will slow them down (tidal effects) to push against. It’s just slower that way I think. – Joe Bloggs Jul 25 '20 at 16:03

If you're trying to break up a planet for mining, you need to overcome its gravitational binding energy. It doesn't really matter how you do it -- blowing it up, lifting it piece by piece, vaporizing with a giant mirror -- you need to pay this energy cost to get rid of the gravity well. For an Earth-sized planet, this is roughly $$2*10^{32} J$$, or about seven days of solar output. It's a good thing you've got a Type II civilization -- this is an expensive proposition even by your standards.

As a slower alternative, you could send the planet on a close flyby of a gas giant, within the Roche limit, to break it up through tidal forces. This is cheaper: for Earth, it would only require about $$3*10^{31} J$$, one day of solar output, to change the orbit, with the breakup energy coming from the gas giant's kinetic energy.

Use Von Neumann swarms to build power plants, refineries, and magnetically accelerated launch platforms and systematically deconstruct the planet. Material payloads would be intercepted by orbital platforms and processed or rerouted accordingly.

The biggest hurdles to accomplish this are material availability and energy consumption.

Powering an O'Neil cylinder across interstellar distances probably requires fusion reactors, so we'll assume your tech level has mature fusion reactors. If not, then a combination of fission and solar panels will have to suffice to power our mining operation (and probably slow it down a few orders of magnitude).

Step 1: Locate metal rich asteroids with low gravity and deconstruct it to create a seed swarm of Von Neumann probes and processing space stations.

Step 2: Survey the planet for potential mineral deposits and disperse the probes accordingly.

Step 3: Harvest materials and construct power plants, processing facilities, and factories for more probes. Expand across the planet.

Step 4: Construct magnetic acceleration platforms (coil guns / rail guns), bundle valuable resources into launch vehicles, and hurl them into space.

Step 5: Intercept the bundles at apogee with your orbiting stations and redirect them to your orbiting ship yards.

Step 6: Watch your autonomous swarms continue to devour the planet and churn out more spaceships already in orbit.

• Love it because it includes railguns. I was about to propose "blind" railguns which just would land, pack up any material they find and shoot it into orbit in handy 10000 ton containers. All other facilities can stay out of the gravity well. You get your railguns back if you just wait long enough... the last rest of the planet just has no real gravity well anymore. While we're at it, we could mine the Sun, too. It is annoyingly bright and too hot and dangerous for the health. – Anderas Jul 24 '20 at 16:17
• Why mine the sun when you can use the material gathered from the planet to create a Dyson Swarm and harvest the power to help speed up your strip mining operation while providing some nice shade. – abestrange Jul 24 '20 at 16:22
• You know, if the container railguns would be pointed all in the same direction (exactly west, to be precise) you could spin up what remains of the planet, until the final rest breaks apart. Additional energy would be needed, though, but we're talking fusion driven railguns of a K2 civilization here, so that's ok. 👨‍🔬 – Anderas Jul 25 '20 at 6:19

Solar concentrator.

Build a big dish at the Lagrange point and heat it up using the power of its own star. The beauty of this method is that by controlling the heat, you can first vaporize off any elements you are not interested. Then carefully turn up the heat and vaporize off the elements you are interested in and capture them. This combines collection with purification (or more correctly, fractional distillation), and saves you schlepping impure materials to some other site for purification.

I was trying to work this out and the best solution I could think of on my own is the oldest technology ever: banging rocks together. As you suggested:

What about destabilizing the orbit of a moon and smashing it into the planet to "liberate" some of its mass?

But let's increase the scope of things here.

Boffins believe that the Moon formed when a planet the size of Mars collided with Earth some billions of years. The debris from both planets went into the Earth's orbit and after some unspecified amount of time coalesced into the Moon.

So what you really need is two rocky planets of about Earth size. Throw them against each other. The planets will mostly disassemble and you will get a lot of raw material floating in orbit of the star.

To illustrate how it would go: here is a video from the University of Bristol simulating two super Earths (i.e.: two rocky planets with each having the mass of 10 Earths) colliding head on: https://www.youtube.com/watch?v=0GvfoD-UW5A. The resulting debris would have the gravity of both planets, but you could easily scoop the material in flybys.

How long would it take for our fleet of probes to accomplish this?

Depends on the technology used to move the planets. You could either use gravity slings (could take millions of years) or you could move a gas giant by force and have it drag the rocky one around (could take millenia).

• I wonder if you could do this more efficiently by leveraging mass. Use an asteroid to destabilize a small moon, which you use to alter the orbit of a planet so that in a thousand years it's orbit collides with another one. – SurpriseDog Jul 24 '20 at 14:17
• @SurpriseDog sure thing. That's good engineering thinking and IMO not only the most efficient way to go around it, but also the most fun :) – The Square-Cube Law Jul 24 '20 at 14:21
• A major problem with huge asteroid/ planetoid collisions is that you've just kicked an insane amount of debris into orbit and beyond. Safely navigating around the former planet is going to be much more difficult for geological time scales. – abestrange Jul 24 '20 at 15:10
• I think if you have sufficient force to throw two planets together, you probably have sufficient force to scoop whatever you want out of them. – Mary Jul 24 '20 at 22:46
• Safe navigation isn't an issue. These are self replicating probes, they will replicate by the billion and dismantle the floating rocks bit by bit. If a few get smashed in the process, it's no big loss. – SurpriseDog Jul 25 '20 at 15:16

You could put all of your mineral processing equipment in one place and bring those pesky rocky planets to the processing center. On Earth, that's the strong preference. Ships, cars, and other movable objects are brought to centralized places to be broken down.

Your probes could move the planets with a series of solar sails as described in the journal Astrophysics and Space Science. As each planet arrives, it would smash into whatever planets are already there, breaking it down into smaller pieces. Your probes could then chew on these smaller pieces and convert them into materials. Whatever you want to do with those materials could be located in the same solar system, which would simplify your probes.

Space elevators are OK for planets that spin fairly fast & aren't too large. Earth is marginal for a space elevator of even the strongest possible material.

An ORBITAL RING would be far better. https://www.youtube.com/watch?v=LMbI6sk-62E&t=4s

• This is a good answer that avoids the need for exotic materials of a space elevator. You should add more detail to it. – SurpriseDog Jul 25 '20 at 15:13

Not sure if this helps any, but here it is-

in one of the star wars books i read once(I can't remember which one) we learn who built the death star. they had no idea it was built for destroying planets, but just thought it was for mining. the death star would be used on dying planets to allow the interior metals and resources to be easily accessed rather than digging all the way through the planet. of course, such a station would be nearly impossible, but i think it might help with your mining. just be careful not to point it at people or mess up orbits or anything.