This is a fairly simple question with a likely rather complicated answer.

Basically, I worldbuilding a planet that has become a complete wasteland of ash, rock and flowing rivers of magma. The civilization that used to live there has since fled to floating space stations that circle the planet.

My question is: Assuming this civilization uses special mechanical drones to extract magma from the planet and fly it back up to the station, what sort of technology should exist for this to be even remotely plausible? Or, if other methods for energy production far better than this exist, what sort of technology should this civilization have not researched to make magma their best choice?

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    $\begingroup$ What's your magma made of, how high are your space stations, and how strong is gravity on this planet? With earth-like conditions, it's unlikely that the thermal energy contained in 1 kg of magma will be greater than the energy required to move said 1 kg of magma into orbit. It's probably better to extract the thermal energy on the surface, and send it up to the station some other way. $\endgroup$ – Nuclear Hoagie Aug 20 '18 at 17:32
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    $\begingroup$ Why use magma instead of solar energy? $\endgroup$ – RonJohn Aug 20 '18 at 17:55
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    $\begingroup$ Perhaps you are simply Very Very far away from the nearest star? $\endgroup$ – Ummdustry Aug 20 '18 at 18:52
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    $\begingroup$ Am I the first one to notice this? I think you mean magma -> lava :) $\endgroup$ – conman Aug 21 '18 at 4:09
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    $\begingroup$ As you have added the science-based tag I will mention this, magma is the state of molten rock before it has erupted when it is still beneath the earths surface. If the substance is flowing as rivers on the surface it's called lava, lava has a slightly different chemical composition than magma as gasses that are trapped by the pressures beneath the earth have a chance to vent out during the process of eruption. $\endgroup$ – Sarriesfan Aug 21 '18 at 6:14

14 Answers 14


Given that the science-based and reality-check tags are on this one, I'll start with the obvious refutation:

As liquid rock, magma is very heavy. There is practically no situation that would make transporting it out of a gravity well justifiable, because the energy expended by doing so would be greater than the energy that could be harvested from it.

The one potential exception I can think of would be if the mantle of the planet (pre-Cataclysm) was fantastically radioactive. Volcanoes, before the planet was completely ruined, were as dangerous as a reactor meltdown. I suspect that you might run into problems with natural criticality if the core of a planet is made up mostly of fissionables (or even denser materials), but it would provide a justification for scooping up the stuff to fly to orbit; fissionables would provide a power source that would likely be more valuable than the energy expended to lift them to orbit, provided you have really efficient reactors.

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    $\begingroup$ Mix in some super-dense handwavium neutron poison, which doesn't get expelled by the volcanoes because it's denser then the rest of the magma, and it could work. $\endgroup$ – Fake Name Aug 20 '18 at 22:48
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    $\begingroup$ Only answer here that even considered the thermodynamic. Which should always be the first thing you consider when dealing with energy/work/heat. $\endgroup$ – Aron Aug 21 '18 at 10:35
  • $\begingroup$ @jdunlop If the magma is so radioactive, would it pollute the planet long ago, when it already started to become such a wasteland? I would rather mine the fissionables when the magma is still liquid rather than heating it again, and then move the mined material up to the space stations. $\endgroup$ – Christmas Snow Aug 21 '18 at 13:26
  • $\begingroup$ @Christmas Snow - Separating molten rock into useful elements is even more difficult than handling it in the first place - assuming it had the normal concentration of fissionables in it (by our planet's standards), things go back to being uneconomical if you have to work to mine the stuff, rather than just scooping it up. There's a reason why we don't melt feldspar in the refining process. $\endgroup$ – jdunlop Aug 21 '18 at 15:15
  • $\begingroup$ Wouldnt it make more sense to separate out a chunk and wait for it to cool than transporting it hot? $\endgroup$ – Marie Aug 21 '18 at 19:38

You might be better off generating the power on the planet and beaming the energy to the station via microwaves.

The reverse of this article. You're creating the energy on the planet and sending it to space. https://phys.org/news/2015-03-japan-space-scientists-wireless-energy.html

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    $\begingroup$ Yes but then how are you going to have the super cool battle scene in the magma generating room. $\endgroup$ – BobTheAverage Aug 21 '18 at 0:06
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    $\begingroup$ But you could have a super cool battle scene around the exposed high-energy microwave beam that shines into the collector inexplicably located in the center of the station $\endgroup$ – divibisan Aug 21 '18 at 0:20
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    $\begingroup$ Generating power involves a heat engine. A magma planet would not support an effective one (lack of cooling). $\endgroup$ – Aron Aug 21 '18 at 3:21

There is a specific type of space station for which this would make total sense, namely a "space fountain" though this may differ from your original vision.

In this situation you have a large tower protruding into space (though not necessarily) that is kept aloft by a constant flow of lava through a pipe, the majority of the energy spent forcing the lava up this pipe is re-gained on its way back down. At the top of this tower you have your space station which has a large radiator array and a sterling (or other heat) engine. This sterling engine takes the "cold" of space from the radiator and the "hot" of the lava and uses it to produce electricity. This gets around the problem of lava's high mass as you need a lot of mass to make a space fountain work anyway and much of the energy you spend to get it out a gravity well is re-claimed.

This would be usefull in a couple ways.

  • It makes for a semi-space elevator able to serve as a launching platform for spacecraft.
  • It can send some of it's energy back to the planets surface to provide power to any installations there (espacily if this volcanic planet is coating in clouds of soot or greenhouse gases which seems likely).

Technology you need for this: a (low friction) pipe that can carry lava, reasonably efficent machinical generators and a few bits and pieces we've had for decades now.

The space station at the top of the space fountain can also contain other facilities, obviously.

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    $\begingroup$ a pipe that can carry lava, mechanical generators, and tremendously steady nerves as the ground station flings molten rock at you ;-) $\endgroup$ – Cort Ammon Aug 20 '18 at 18:59
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    $\begingroup$ space fountains require friction-less (or near frictionless) transport of your material in a closed loop. I don't think that is going to happen with lava in a pipe... $\endgroup$ – conman Aug 20 '18 at 20:53
  • $\begingroup$ @conman Not necessarily thats only for efficient space fountains, given that this space fountain is producing its own energy constantly that's not so much of a concern. There are perfectly viable with higher friction transport. You can also always simply scale things up. a bigger pipe will have less friction to worry about compared to the volume of lava it transports. Though i agree, i should have added "low friction" will now edit $\endgroup$ – Ummdustry Aug 21 '18 at 14:41
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    $\begingroup$ Science based and reality check were selected for this item. Space isn't that "cold". A vacuum has nothing that will receive the heated energy output from a Sterling. $\endgroup$ – Keeta - reinstate Monica Aug 21 '18 at 16:42
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    $\begingroup$ This is why I used cold in quotation marks yes you are in truth using the 'cold' of the space Borne radiators. You are in truth radiating heat into space $\endgroup$ – Ummdustry Aug 21 '18 at 17:48

Geothermal energy

Put a fluid in contact with some hot part of earth. Get it back when hot. Use this differential to make energy, be it steam-based or heatpump-like. This a now common way to get energy from earth undergrounds. Iceland is getting a large part of its energy from this.

In your case, you could use the lava on the ground to generate cheap energy, and store it in something easier to transport, like hydrogen.


On the ground, get some lava, put it close to water and use the steam to create electricity.

Then use this electricity to create some hydrogen. Put it in cans and send it to space.

Advantage The energy you get by kilo is higher and more usable than by sending hot rocks in space. Also it doesn't get cold, so you can use it later.

Problem Hydrogen needs oxygen to produce energy, oxygen is scarce in space.

Notes: Your problem is in fact similar to the problem we are facing on earth.

  1. Places where cheap energy is available (sunny deserts for photovoltaic, or windy seas for wind power) are not where the energy is needed. So we need to transport it.
  2. Moments when energy is needed, is not always when it is available (sun is down when people turn the lights and TVs on). So we need to store it.
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    $\begingroup$ Couldn't the energy be sent directly to space via laser? $\endgroup$ – Starpilot Aug 20 '18 at 21:24
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    $\begingroup$ I have the same comment for you that I did for L.Dutch. You are effectively proposing a heat pump. This requires a cold reservoir in which you can dump heat, and there isn't an obvious candidate in the question. Or, to put it simply: where exactly do you propose to find (liquid) water on a planet covered in lava? Even if you found some, heated it to steam with lava, and used it to generate energy, how are you going to cool it down to turn it back into water to use it to generate more energy? $\endgroup$ – conman Aug 21 '18 at 2:11
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    $\begingroup$ @conman You mean heat engine. Heat pump is the opposite of a heat engine (except in the hypothetical Carnot Engine). $\endgroup$ – Aron Aug 21 '18 at 3:24
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    $\begingroup$ @Aron You are 100% correct, thanks. Can't edit my comment but I'm sure my meaning will be clear (especially with your note). $\endgroup$ – conman Aug 21 '18 at 4:06
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    $\begingroup$ Re: Heat cycle - Heat up on the surface, cool it down (radiatively) in space, there's your cycle. It could be a future space lava that has a very high Specific Heat Capacity. For the surface, don't use cold water, use less hot lava. A heat gradient between areas of a convection zone could do the job. Although you'd already have a 'hot side' and a 'cold side' just by being in space and pointing things at the local star, but y'know, Future Space Lava. $\endgroup$ – Glen Davies Aug 21 '18 at 11:18

Given that the comments suggest you're willing to consider solutions that leave the magma on the planet, I have what I think to be a rather elegant solution: Turn your space stations into counterweights for a space elevator, and build a thermocouple into the tether.

Technology requirements: You need to be able to build a very strong, very light elevator cable at least 40,000 km long. Carbon nanotubes are usually the material of choice for settings that make use of Space Elevators.

You also need to be able to build an anchor on the planet's surface that's not bothered by liquid magma flowing over it. This problem is also easily solved with advanced carbon composites

Finally, you need a nice room temperature superconductor for your thermocouple. This can run right up the inside of your elevator cable and connect one end to the liquid magma and the other to the cold upper reaches of your tether. I need to do some further research here because I THINK you'd actually want your 'cold' end of the thermocouple to actually be in the high atmosphere, rather than in actual space, since in the high atmosphere you can rely on high-velocity winds to generate LOTS of convective heat exchange where in space you can only use radiative, which (again, i THINK) is more limiting in this application.

In either case, this solution gives you a very effective magma-based power source that has no moving parts whatsoever, making it extremely reliable.

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    $\begingroup$ This is my favorite answer thus far, but with one nitpick: I think you need something better than a "room temperature" super conductor. I think now you're looking for a "lava-temperature" super conductor :) $\endgroup$ – conman Aug 21 '18 at 4:08
  • $\begingroup$ you're missing 3 zeroes at your space elevator length. it should reach into geosync orbit. and why do you need a superconductor? $\endgroup$ – ths Aug 21 '18 at 8:51
  • $\begingroup$ Unless your tether ground and tether end are in a fixed position to the planets surface (meaning geostationary orbit) there will be substantial drag from the atmosphere (and there will atmosphere as lava is liquid and will gas out even if there are no volatile gasses in the lava itself). If the planet has a magnetic field you may run into some problems as it interacts with the current flowing through the conductor. $\endgroup$ – GretchenV Aug 21 '18 at 9:43
  • $\begingroup$ The length of the elevator depends on the planet mass. So for low mass planet, the gravity well is low and you can have a "small" elevator, with much less tensile force. I didn't do the math, but IIRC for Mars (which is only 0.3g) a space elevator would be feasible with steel. You then have both your thermal conductor (the steel cable is anchored on the ground and pick up the temperature and conducts it up) and, somewhere on the cable, you have the station that doing the TEG with the other side being space cooled. It's conveying the electricity on the last part of the cable. $\endgroup$ – xryl669 Aug 21 '18 at 12:09
  • $\begingroup$ @ths Whoops! Mistook a comma for a decimal point. Will correct. The Superconductor isn't ABSOLUTELY required, but your thermocouple will generate more energy the more conductive your connection between the "Hot' and "Cold" ends is. $\endgroup$ – Morris The Cat Aug 21 '18 at 13:14

Two possible usages come to my mind:

  1. Use the magma as high temperature thermostat in a Rankine cycle. In layman terms, use the magma to heat up pressurized water and use the (super)heated steam in a turbine. Use the space as low temp heat sink, by building radiators above the atmosphere.

Rankine cycle with superheated steam

  1. Use the magma as IR emitter and use some sort of Seebeck effect based device to generate electricity. Again, use the space as low temp heat sink, by building radiators above the atmosphere.

#1 dates way earlier than space travel, so it should not be a problem to master it for a space faring civilization.

#2 has been widely used for space equipment when no other viable power generator is available.

In both cases I would not bother transporting the magma out of the gravity well of the planet. You will need only to move the fluid away from the surface to cool it by radiating into space. Though it still requires energy, you are moving less mass than transporting whole volumes of magma. And transporting the energy, which is mass-less, is less of a struggle, as you can use microwave or laser beams.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Aug 22 '18 at 13:20

If the planet is already past the point of no return for sustaining human life, is it possible to make the 'problem' even worse, and therefore in to a solution?

Use High-temperature electrolysis to produce hydrogen in abundance across the entire planet. This of course requires heat (which sounds like you have in abundance) and likely water (which if the planet was previously inhabited, also probably exists, maybe in massive underground reservoirs).

A possible scenario may be that with all of the volcanic/geo-thermal activity, a lot of various heavy gasses were pumped out in to the atmosphere making it incredibly dense. This, along with appropriately strong gravity, would allow the hydrogen to naturally float to the top of the atmosphere and oxygen to be another layer below, where the space stations encircling the planet could harvest it for energy. The nice part about this is that no storage mechanism is necessary and could make for interesting plot ideas where there are more dense 'patches' of hydrogen, allowing for resource competition which is constantly changing.

While not an incredibly efficient system, improvements could be made such as more direct supply using actual plumbing or something more exotic (not sure on the sci-fi-iness of your world based on the tags). Plot advancement: These could also be designs currently being worked on to be fielded in the future.

As an added benefit, you've solved how to provide water to your space station inhabitants as well since this is a by product of the hydrogen fuel cell process.

Another possible plot idea would be that the thin layer of N2 separating the Oxygen from the Hydrogen gas layer could be tampered with or naturally wear thin and be a threat of a cataclysmic atmospheric 'event'.

Some science behind gas layers.


Magnetic fluctuation?

I admit I don't really know enough about this to provide a feasibility study on this method. But as the tag is science-based, here goes.

With the planets magma now flowing on the surface as lava, they magnetic poles and fields are in a constant state of flux. As these magnetic fields cross conductive metal, electricity is generated. So instead of directly taking the heat from the planet, use the magnetic fields of the molten flows to generate electricity in space.

  • $\begingroup$ The Space Tether Missions are a real proof of concept of this idea. $\endgroup$ – Steve Aug 22 '18 at 7:27
  • $\begingroup$ Any facility capable of doing this would also double as a Thruster system for the space station if used properly. $\endgroup$ – Ummdustry Aug 22 '18 at 19:26


Lasers is the answer for everything sci-fi. Truth be told, you could simple use modified solar panels to harvest radiated heat from the planet directly, but that's not the best way to do it. This is where lasers come in. Actually there's more than one way to use lasers to harness the energy of the magma, so I'll give you both and let you decide.

First choice is actually shooting the laser into space, with the laser being powered by some kind of thermal energy converter. You could use a steam engine or thermocouples, your choice. Then, you point the laser at whatever's in space that you can use for energy. You can point the laser at solar panels, which is fine, but require battery cells and stuff and isn't my first choice.

Personally I would fire this laser at a modified Salt Tower that's in space. It can store heat energy to be used as electrical energy later, both more efficiently than storage cells, and with less maintenance. Plus, if the laser needs repairs, you have a reserve of energy to use until the laser is operational again. Also, it doesn't have to use salt, so research if there's anything that may work better for you.

Okay, so those are the ways to use a laser shooting into space, but what if a dense atmosphere blocks the laser, or you want a power source that will last for a long time even away from the planet? Well lucky for you, there's a way to get a lot of energy off the planet in a condensed and usable form.

Turns out, lasers can be used to create nuclear fusion. So far we only really use hydrogen to create helium, but with just slight advances in technology, we could easily create materials such as Uranium, which is useful as a nuclear fuel. Of course, if our fusion is powered by a source of energy that consumes our own resources it won't work, but as far as getting a planet's energy into space, this is a good way to handle it.

  • $\begingroup$ Why did someone down vote my poor lasers? Who hurt you? $\endgroup$ – Clay Deitas Aug 20 '18 at 22:51
  • $\begingroup$ I was wondering that too... $\endgroup$ – Morris The Cat Aug 20 '18 at 23:27
  • $\begingroup$ I'm pretty sure it wasn't this guy. $\endgroup$ – Mazura Aug 21 '18 at 0:08
  • $\begingroup$ @Mazura I was just shouting into the void, but the void shouted back $\endgroup$ – Clay Deitas Aug 21 '18 at 0:14
  • $\begingroup$ don't worry @Clay Deitas , I upvoted your lasers. I love lasers. I think everybody should do everything with lasers! youtube.com/watch?v=Ee3TDO9fEqY $\endgroup$ – Nahshon paz Aug 21 '18 at 13:10

I know it's not quite what you asked - but others have covered the plausibility of 'lifting' heavy materials to orbit as heat stores.

The one possiblity I could imagine is eruption-driven.

E.g. a planet with a sufficiently low gravity (and sufficiently forceful volcanoes) that the magma reaches 'space' - either low orbit or even potentially escape velocity.

Then you'd have a double whammy - your 'shipment' of hot (ish) magma would be arriving on the platform for free (or at least, lower cost than a boost-to-orbit), and you'd also be reducing 'mass-loss' from the planet - because any planet that's flinging it's own mass out at more than escape velocity is going to be shrinking.

For real world examples, look to Io:


The higher vent temperatures and pressures associated with these plumes generate eruption speeds of up to 1 kilometre per second (0.62 mi/s), allowing them to reach heights of between 300 and 500 kilometres (190 and 310 mi).[57] Pele-type plumes form red (from short-chain sulfur) and black (from silicate pyroclastics) surface deposits, including large 1,000 kilometres (620 mi)-wide red rings, as seen at Pele

Note though - 1km/sec is less than Ios escape velocity, so the results of the mass-flinging do end up back on the surface eventually, unless Jupiter 'interferes'.


Since you've already got some of the 'heavy lifting' from ground to near-orbit done, you'd have a slightly easier ride of harnessing the energy.

  • $\begingroup$ One issue with this is being in the right place, at the right time, to catch something going by. Unless the eruptions could be reliably predicted (both in "launch time" and "launch vector"), it would be impossible. Even with this prediction, altering the orbit of something (a ship or station) to actually be in the proper place would take so much energy, and simply not be worth it. The difficulty is up there with lifting the mass off the planet in the first place. $\endgroup$ – Steve Aug 22 '18 at 7:30

The power generator itself could be something fairly low-technology - a steam engine. This is probably the most efficient way of extracting work from a heat differential.

Now the problems:

  1. Lifting magma into space is ridiculously expensive. If you have a space elevator, you can reuse some of that energy by dropping the spent rock down as a counterweight, but it's still a drain.

  2. The magma cools as soon as it's out of the ground. Ideally you want the generator as close to the source as possible.

  3. Cooling - to use a heat differential, you need to dump heat into something cold. You either need a constant supply of water, or you need to recapture and condense the steam by letting it cool off using air. Both are hard to come by in space. You can dump heat by radiating it off as infrared, but it's slower, and it strains an already very critical system - lose a heat pump and your living quarters suddenly get very toasty. (Waste heat is already a problem in space anyway; this just makes it worse.) ((Admittedly, if the planet itself has no liquid water or air, it's going to be tricky to generate power there too - but probably no more than in space.))

In summary, it would likely be a better idea to put your turbines planet-side, then use microwave lasers to beam it at collectors on the space station. There's some waste, but it's a lot more efficient and safe.

If the surface is too active to build on, you might need to put the power plants on big blimps that can move to safety as needed. (But that was already going to be an issue if you have only the magma-harvesting infrastructure on the ground.)

  1. As has already been stated, the first problem you have is that of getting your magma into space. Generally molten rock doesn't have enough energy in it to be able to lift its own mass very far, let alone into space. Developments in gravity manipulation technology would be needed to explain how the machines are able to lift the magma.
  2. Lifted off the surface of the planet is not the same as being in orbit. Things in actual orbit are moving very quickly, so the drones would also have to accelerate the magma to orbital speeds as well as lifting it. And if you already have a system for lifting and accelerating that much magma into space without much energy expended, your space station probably doesn't need a lot of energy to begin with, which could be a plus.
  3. From a thermodynamic perspective any power generating system that relies on heat is actually proportional to the temperature difference between the source (magma) and the heat sink. Due to this, a key technology that they would need is the ability to radiate that heat into space very quickly. The invention of an innovative radiator technology might very well be the key to making that sort of system viable.
  4. When you cool magma, it becomes rock, what do you do with the spent fuel (rocks)? If you just throw them out the window, over time there will be a literal asteroid belt of spent magma forming a ring around the planet.

Several answers have suggested geothermal energy to generate electrical energy, which might then be sent to space via laser or microwave transmitter, but one objection in the comments is that there is no "cold sink" for a Carnot cycle heat engine to work.

This problem may be overcome by the use of special materials which are designed to radiate at specific temperatures which the local atmosphere is transparent to. This has been demonstrated in principle and several companies are now working to commercialize this technology. Essentially the radiator on the ground is radiatively "coupled" to space, which has a temperature of 2.7K. Assuming the radiator is reasonably efficient, the problem of a "cold sink" is pretty much solved.

  • $\begingroup$ Is "radiate at specific temperatures" the same as "radiate at specific wavelengths"? $\endgroup$ – user42528 Aug 21 '18 at 8:25
  • $\begingroup$ @Thucydides You can use a gas compressor to make your own cold sink if need be, but probably the atmosphere will be colder than the magma and work good enough. $\endgroup$ – Clay Deitas Aug 21 '18 at 15:03

Lets assume a plannet nearly the size of earth (r = 6000 km) but lacking an iron core. The density of the plannet is on average similar to silicon dioxide (2650 kg / m^3). This puts the total mass of the plannet (MP) at 2.4 * 10^24 kilograms.

The atmosphere of the plannet has been blown off so you are able to orbit at an amzingly low 150 km (The ISS is at about 400 km)

With no atmosphere the scooper satellites move in eliptical orbits taking them back and forth between the surface and the station without any additional energy. When they pick up their lava payload they must use some energy to raise their payload. When they arrive at the station their energy is replenished from the lava powered heat reactor on the station.

The energy (E) to raise one kilogram of material from the ground to a height of 150 km is MP * 1kg * G * (1/r + 1/(r + 150km)) = 651 kilojoules

Lets say that the lava temperature is at 1500 kelvin and that the cold side of the reactor on your station runs at 300 kelvin. Lets also assume that the reactor runs at 90% of the theoretical maximum (carnot efficiency). The reactor efficiency (N) is then 1500 K / (1500K + 300K) * 90% = 75%.

If the reactor is 75% efficient then you must produce at least E / N = 651 kJ / 75% = 868 kJ /kg of material to come out ahead.

The only requirement left is that the specific heat of the lava material is greater than 868 kJ / kg / 1200 kelvin = 0.72 kJ / kg.

Silicon dioxide itself would barely meet the requirement. But with some lithium impurities to bring up the average you would have extra energy.

  • $\begingroup$ What you describe sounds like a Momentum Exchange Tether. $\endgroup$ – Steve Aug 22 '18 at 7:35
  • $\begingroup$ See on that page "Rotovator": If the tether is long enough and the rotation rate high enough, it is possible for the lower endpoint to completely cancel the orbital speed of the tether such that the lower endpoint is stationary with respect to the planetary surface that the tether is orbiting. $\endgroup$ – Steve Aug 22 '18 at 7:35

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