- diamondoid microsaws for fine hull tube cutting with shearing drive material liftup
- callenges of combined heat and pressure
- preservation of drill cores -- usage of structural elements
Earthquake controlled tension release cables (& why its probably a bad idea). A very dense net of tension cables would be needed in critical zones replacing much of the lithospheres structure (that must be dumped -prefferably in a structurally preserved way - at a supersurface location). Controlled release of a sudden ride in tension could avoid earthquakes while simultaneously providing immense amounts of energy. But if this is not sufficiently undestood this unnatural slow relaxations might lead to very unpleasant consequences. [to investigate: how woud one handle shearing on a wavy fracture "plane"]
disposal of radioactive waste into outer earth core (the well to hell)
APM and nuclear technology If it really stays there it will really stay there for the immense amount of time it takes to decay. If not we have made ourselves a radioactive volcano. Also radioactive waste might become valuable when Transmutation will be started. Doing Transmutation and all highly radioactive technology remote controlled right at these depths might be a good idea. Heating up the earth massively from the inside sounds like a bad idea.
Note: The highly problematic waste of today may become the very valuable resource of the future. There are several examples of this pattern. Both with and without humans involved. So permanently irreversibly getting rid of it might not be so good after all. See: "Recycling#General"
Earth core probes
Warning! you are moving into more speculative areas. (level: hallucinogenic drugs)
Forget about all those ridiculous things you've seen in SciFi a real probe would (if at all possible) need to be very very different.
It must be very compact and densely built without macroscopic voids to withstand the extreme pressure. Carrying a human is impossible.
If one manages to build a device capable of cruising in one of the few (horrifying) open lava lakes one may be able to go deeper in a continuous fashion. Pushing the limits step by step.
While the mantle is assumed to be mostly solid there should be liquid channels that supply our active volcanoes and those lava lakes. The outer core of earth is presumably globally liquid posing little to no obstacles for a probes navigation.
Reaching these depths is alone temperature wise highly questionable. Even the best refractory materials (melting points up to 4488K) barely exceed the estimated 3000K at the upper edge of the melted outer core. Since melting points may drastically change at very high pressures the situation may be even worse.
The refractory material must not dissolve into the surrounding magma (if it does than it should be a rather slow rate). This is likely a linchpin of the whole idea.
- Diamond is completely unsuitable. It isn't sufficiently thermally stable and is excellently soluble in liquid iron.
- Looking at materials used for long lasting crucibles in steelworks, periclase (MgO) may be a start to investigate. See: Magnesium#Misc
Maybe (a very strong maybe) it's possible to filter material from the magma and use it to regenerate the probes refractory shell from the inside (and to collect some new nuclear fuel - more on that later).
With rising depth the rising pressure must not crush the structural framework materials of the probe (including the refractory shell) into different crystal structures with lower volume and higher density. Instead one wants to use gemstone like compounds that are crystal structure invariant from normal conditions (1atm 300K) all the way to the target conditions.
- Quartz (a polymorph of SiO2) is unsuitable it gets crushed into various other crystal structures.
- Stishovite (a polymorph of SiO2) is crystal structure invariant under pressure (but maybe not under temperature?)
- Moissanite (SiC) is crystal structure invariant under temperature (but maybe not under pressure?)
- (TODO: search suitable compounds)
- Side-note: water gets crushed at these depths too
Deliberate inclusion of crushable compounds combined with actuators could be used for buoyancy adjustment.
To have a probe that can actually perform complex tasks/operations (data gathering processing storage) it's inside needs to be cooled down a lot. Every heat pump of course has a hot side so at least parts of the outer shell of the probe needs to be heated even above the already extreme outer temperature such that it can act as a thermal radiator. These refractory heat radiator parts are most susceptible for dissolution. Driven convection may improve heat removal but also worsen dissolution (by either mechanical friction of raising of the diffusion gradient). The method of cooling is basically equivalent to active spacecraft cooling.
Creating a highly effective thermal isolation that can resist extreme pressures is likely rather difficult. (An interesting problem!). Nanoscale voids can only be used in a minimal manner. To have a probe that actually can be cooled faster than the heat flows back it needs to be pretty big and have a bulky shape not too far from a sphere. This way it has a low surface area relative to a big internal volume. (Hopefully not bigger than the magma channels.)
Active cooling of course needs a powerful energy source.
- Chemical would lack a resupply of oxidizer and the temperatures are too high anyway.
- Nuclear fusion needs large empty spaces that can't really resist the extreme pressure
- Nuclear fission seems to be to only option that seems somewhat plausible.
The reactor of course produces even more heat that too needs to be isolated from the more thermally delicate parts of the probe.
Motion: A magma diving probe would probably be rather slow too "drill" / "dig" / "screw" (flagella like propulsion?) its big volume through a hot viscous liquid. Fast sharp cube cutting as in underground working will pretty much certainly not work.
- how to find the way back out a lava tube?
- how to avoid getting stuck in to viscous places?
Sonar is pretty much the only thing that works (beside SciFi neutrino navigation) For dead reckoning the movement is way to slow.
Communication: You basically have to wait until the probe returns which may take a long time given its slow movement. Would direct communication without waiting for the probe to return be possible
- by setting of extreme sonic impulses? (Seems more practical with a daisy chain of many probes.)
- with neutrinos? While a truly giant probe could maybe detect neutrinos how the heck should it send neutrinos?
Will the probe be fast enough to return before its nuclear fuel runs out? If it can gather fuel while moving around there's no time limit.
To finally succumb to total bio-analogy insanity: How about in-situ self replication? There are certainly no competitors in this big homogenous "ecological niche". Perfect prerequisites for a chain reaction.