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The setting is basically earth. Our planet is unfortunately on a collision course with a large asteroid. However, humans have discovered and decoded a message from an ancient, advanced alien race (our parents). These aliens have left behind technology and knowledge that could save the planet, but this technology is encased in the earth's core (so that we may only access it when we are "ready"). If the whole world worked together for the next 5-10 years, would it be possible to detonate our way down to the outer core using our nuclear arsenal? I know that lateral pressure is a problem and if you dig a cylindrical hole it will collapse, but could we dig a cone-shaped hole?

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    $\begingroup$ Are you wedded to the idea of having this in the core of the earth? What about having it embedded somewhere in the Isua Greenstone Belt instead or another old rock formation. I don't know how long they have been undisturbed, but they have found 3900-3810 million year old zircons in there. $\endgroup$ – Martine Votvik Jul 4 '16 at 16:56
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    $\begingroup$ Predicting, more than ten million years ago, which surface rocks would still be on the surface, rather than having been subducted and melted, or had a volcanic hotspot erupt through them, seems to be very hard. If you're aliens who want to leave a message for the species you think will evolve on a planet, the plan in 2001: A Space Odyssey is pretty good: leave caches on geologically dead moons, and trust to luck that nothing big enough to destroy them will hit in just the right spot. $\endgroup$ – John Dallman Jul 4 '16 at 17:23
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    $\begingroup$ This is another example of a scale error. $\endgroup$ – JDługosz Jul 5 '16 at 11:05
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    $\begingroup$ Instead of having to physically reach the core and access it, perhaps humans need to figure out how to accurately image the device and "read" etchings on the surface in order to learn how to send a signal to activate the device so it comes up on its own. If humans have tech that allows us to physically reach the core of the Earth then dealing with an asteroid wouldn't present a problem. $\endgroup$ – Jason K Jul 5 '16 at 14:34
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    $\begingroup$ The already mentioned issues aside, I don't think "digging" with nuclear explosions is that good of an idea. Think what happens when detonating a firecracker in your closed fist vs on your palm... $\endgroup$ – Daniel Jour Jul 5 '16 at 22:19
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Instead of telling you it's impossible, I'll make a list of the problems you need to solve:

  1. Pressure:
    Pressure at Earth's center is $3.65 \times 10^{11} \ \mbox{Pa}$. Whatever enclosure you build is subject to that. If you made a solid block of diamond (one of the least compressible materials, with a bulk modulus of $4.43 \times 10^{11} \ \mbox{Pa}$), you'll find that it shrinks to $82.3\ \%$ of its size. If you make it out of "steel" (say, $\sim1.50 \times 10^{11}\ \mbox{Pa}$), it becomes $33.9\ \%$ its size.

    That's bad news, especially since your vehicle needs to be hollow. Most humans are not happy being compacted to $34\ \%$ their volume. You can't solve this by using unobtainium because whatever atoms unobtainium is made of need to actually exist. Bond dissociation energies are the physical limit of strength.

  2. Density and Viscosity:
    Earth's inner core is $12.8\ \mbox{g/cm}^3$. Something like lead has $11.34\ \mbox{g/cm}^3$. Your ship is going to float, and will have to actively propel itself downward. When it reaches the inner core, it will need to move through something solid. To fix this, you need propulsion and drilling. But, both are subject to the same crushing pressures mentioned above.

  3. Temperature:
    Temperature at the Earth's core is at least $5\,000\ ^\circ\mbox{C}$. Because of thermodynamics, the Earth's core will try to make your vehicle the same temperature. Most humans not so much "happy" at $5\,000\ ^\circ\mbox{C}$ as they are "charred-lumps-of-their-constituent-elements".

It is worth noting that if humans have difficulty solving these challenges, your aliens will have difficulty solving them too. If your aliens can solve them, this raises some serious unintended consequences.


Here's a possible solution. It is in the exterior realms of possibility and undoubtedly has Problems, but perhaps another worldbuilding question could help fix them:

Make a large, very long steel rod, and hollow out many small interior regions. Suspend from the foremost region your vehicle in a vacuum. Similarly, put nuclear warheads in the rear regions. The outer hull can compress, leaving inner components unharmed. After sinking through the mantle normally, the rear regions of the device successively detonate, pushing the device deeper (this is an inverted Orion nuclear pulse drive, with all attendant problems). Count on sacrificial outer hull to absorb heat and pressure for long enough to get to the center.

Speculative/imaginary/magic tech that would make easier solutions (use with caution):

  • Force fields
  • Neutronium
  • Teleportation of matter
  • Teleportation of energy (heat especially)
  • Reactionless drive
  • Arbitrary adjustment of magnetism of nearby materials
  • Universe editation
  • Asking your bloody aliens to come up with a nicer plan and stop being ruddy showoffs already.
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    $\begingroup$ I certainly wouldn't enjoy being compressed to 82.3% of my volume, let alone 34%. $\endgroup$ – Marc.2377 Jul 6 '16 at 3:14
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    $\begingroup$ @Marc.2377 You've never laid under a steamroller before? You're missing out on the good stuff in life. Also Teleportation of energy sounds pretty cool! As long it's sent to deep space and not towards the surface, that could get really messy really fast for all the earthlings! $\endgroup$ – Timmy Jul 6 '16 at 12:36
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    $\begingroup$ @imallett Your list of the problems that need to be overcome is impressive. The creativity of your proposed solutions is also impressive. Most of your speculative/imaginary/magic solutions including the inverted Orion earthship would be better solutions for getting rid of the asteroid. Then you could think about accessing the alien tech core cache at your leisure. The OP allowed 10-15 years in devising an earth-core solution. We can assume a year or two more to deploy it. Plenty of time. I had fun thinking of how to use your hypertech to fix the asteroid & getting to the core. $\endgroup$ – a4android Jul 7 '16 at 4:18
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    $\begingroup$ Excuse me, I watched a highly accurate documentary titled The Core that disagrees with everything in this post. $\endgroup$ – Nick T Jul 13 '16 at 8:32
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    $\begingroup$ @NickT Ah, but having anticipated your objection, I already cited this paragon of scientific accuracy! $\endgroup$ – imallett Jul 13 '16 at 9:54
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No.

On this scale, the Earth is not solid and rigid. It's more like extremely hot jello, with a thin and weak crust, a layer of hot floppy jello, the "mantle", a liquid outer core (actually molten iron) that's about 1,400 miles thick, and an inner core of iron about 750 miles in radius.

Films and TV programmes that show journeys to the centre of the Earth are exceptionally scientifically inaccurate, even by Hollywood standards.

A "cone-shaped hole" isn't possible, the Earth will just flow to fill it in once you get down a hundred miles or so. No, there isn't anything strong enough to brace the hole with. The only way to retrieve something from the Earth's core is to dismantle the planet, which will do more damage than any asteroid hit.

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    $\begingroup$ Could the sides be "frozen" in place by pumping in something very cold, like liquid nitrogen? I'm sure the answer is "no", but would it let you get somewhat deeper first? $\endgroup$ – Bobson Jul 4 '16 at 18:53
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    $\begingroup$ You might be able to get a little deeper that way if you were digging mechanically, rather than using hydrogen bombs as the OP suggested. Let's be generous and guess that lets you get down ten miles, rather than five. Only 3,990 miles left to go! And even if the Earth was cold all the way through, rock and metal still flow like liquid under the pressure a few tens of miles down. Matter simply isn't rigid on this scale. $\endgroup$ – John Dallman Jul 4 '16 at 19:30
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    $\begingroup$ @Bobson What would you do with all the heat? Ignoring the sheer quantity of liquid nitrogen you'd need, that heat has to go somewhere. When you make liquid nitrogen, you're simply moving heat from the nitrogen into the atmosphere (like a fridge does). Now imagine moving all the heat from the Earth's interior into the atmosphere... $\endgroup$ – Will Vousden Jul 4 '16 at 20:05
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    $\begingroup$ In addition to the physical impossibility of constructing the hole itself, such a project would be energetically infeasible; the required energy (assuming the Earth is uniformly dense and gravity varies linearly from surface to core) is about $\int_{0}^{R}dr'\int_{r'}^{R}dr\ \rho g(r/R)=\rho g R^2/3$, which is around 16 megatons of TNT per square foot; and this is assuming 100% efficiency in energy conversion from explosive yield to lifting. $\endgroup$ – 2012rcampion Jul 5 '16 at 3:57
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    $\begingroup$ "Films and TV programmes that show journeys to the centre of the Earth are exceptionally scientifically inaccurate, even by Hollywood standards." I watched The Core with a geology major. There was a lot of swearing. $\endgroup$ – ceejayoz Jul 6 '16 at 19:14
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I would agree with the NO answer already given.

For comparison: The deepest humans have ever dug is only a little over 12 kilometers.

https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole

And these are drilling shafts much less than a meter in diameter.

Also consider that blowing a hole in the earth with all of our nuclear weapons to reach the core would most likely make the earth just as lifeless as the possible asteroid impact.

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David J Stevenson has proposed a method to reach the Earth's core. It requires a nuclear device of only a few megatons to crack open the crust. The planetary mission vehicle descends using a large mass of about one million tons of molten iron to sink down to the core. This journey should take roughly one week.

The real technical problems your inner-earthonauts need to solve are how to survive the temperatures and pressures imposed on their vehicle during the descent for, at least, one week. We can safely assume there will be a human habitable base where the ancient alien technology is stored. So once they get there it is plain sailing.

But Stevenson has solved the technical problems and has the numbers to prove it too, of reaching the Earth's core. So this problem is already been solved. His probe isn't manned. Getting humans down there safely still remains to be solved. Possibly an extremely strong and highly refrigerated capsule needs to be built. Hopefully someone else on Worldbuilding SE has the answer.

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    $\begingroup$ "I have quite low confidence that it will work. However, my motivation in writing this paper is to get people to think about ideas like this.", he said. He mentions it under 'Crazy Stuff' on his website $\endgroup$ – AakashM Jul 5 '16 at 8:51
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    $\begingroup$ @JohnDallman In this scenario I expect getting back out will have something to do with the ancient alien technology. Under any other circumstances it would be madness. $\endgroup$ – a4android Jul 6 '16 at 6:32
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    $\begingroup$ Stevenson talks about sending something the size of a grapefruit to the core by essentially dropping it down a self-propagating crack that's a bit wider than a grapefruit. Humans are much wider than grapefruits so you're going to need a much larger crack, which is going to need much more explosion to get it started and much more iron to keep it going. $\endgroup$ – David Richerby Jul 6 '16 at 16:23
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    $\begingroup$ @a4android You've missed my point. Sure, you have billions of litres of iron. But you can't fit a human-carrying probe in that enormous volume because the volume itself is only 30cm wide. This is stated twice on the second page of the Nat Geo article you linked in your answer. $\endgroup$ – David Richerby Jul 7 '16 at 9:01
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    $\begingroup$ @DavidRicherby Ah hah! I had wondered if something like that was the case and might be your point. I had previously read other accounts of the Stevenson proposal, so I only skim read the Nat Geo article and missed that point. I stand corrected. $\endgroup$ – a4android Jul 8 '16 at 1:53
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I'll approach this problem from a different angle than I've seen in the current answers.

The radius of the Earth is about 3959 miles.

A cone shaped hole (assuming 1/10 ratio of base to height) will have a hole of almost 400 miles across at the surface.

Even if the composition of the earth was "only" dirt and rock, you would have to move 163,000,000 cubic miles of material during the excavation. Using nukes can break that material up for you, but you are still going to have to move that material out of the hole.

The material excavated by this project could be put into 271,666 piles, each larger than Mount Everest.

Changing the ratio of the cone to 1/100 would result in a much smaller number, but you would still be talking about moving many multiples of Mount Everest. Please note that pit mines generally use much more gradual slopes (actually wider than they are deep), which would result in a continent wide hole at the surface.

As another comparison, the amount of material moved is about half the volume of the entirety of the world's Oceans.

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TL;DR: Yes, but not the way you thought: Aliens left their message in the form a of punch card that can be read with a neutrino beam.

Neutrinos are elementary particles that only interact weekly with other particles of matter and can therefore travel through the earth. Because it is so advanced, the alien civilisation could prepare a material stopping neutrinos and embedded a “punch card” made of this material at the centre of the earth. It is already possible to product “neutrino beams” and to detect them, so we can imagine reading the “punch card” by emitting an intense neutrino beam towards the center from one side of the earth and reading it from the other side. Because we generate a very intense beam, it is easily distinguished from “universe's noise”.

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    $\begingroup$ Just make sure you don't send the neutrinos too fast, like OPERA did. Else you might violate causality, and perhaps the aliens will retroactively decide not to post the message... $\endgroup$ – leftaroundabout Jul 6 '16 at 14:04
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    $\begingroup$ Fun typo: "Neutrinos [...] interact weekly" - usually on fridays. $\endgroup$ – Grüse Jul 6 '16 at 14:07
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    $\begingroup$ @Grüse: actually neutrinos interact more like every 6.8 weeks, I checked that back. (But my initial calculation gave 0.4 seconds as the result, so...) $\endgroup$ – leftaroundabout Jul 6 '16 at 20:45
  • $\begingroup$ @MichaelGrunewald An excellent idea. Reaching the core using a neutrino beam. It's not that the aliens left a message, the important thing is their cache of advanced technology needed to fix the asteroid. Sending a neutrino beam could be the key to releasing the tech cache for human use. Now the technical challenge is how to create the neutrinos on a scale to give a sign this was how the core was being accessed. The punch card concept is unnecessary. A beam alone would work -- assuming that's what our progenitors want. $\endgroup$ – a4android Jul 7 '16 at 4:03
  • $\begingroup$ @a4android Aliens left behind “technology and knowledge” so this could be technological artifacts as well as technological information, the latter could be carried by a punch card. $\endgroup$ – Michael Le Barbier Grünewald Jul 7 '16 at 8:51
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Considering that about 1,000 above-ground nuclear tests were conducted between 1946 and 1964 by the superpowers, huge numbers of underground tests followed, and nations not subscribing to treaty conducted many more, the fact that the earth is still here and not noticeably different should dispel any notion about the power of even fusion devices for excavation purposes of such magnitude. One large volcanic explosion subsumes the power of many fusion devices (see article on Krakatoa in Wikipedia, e.g.). Add to this that once (or if) you get through the crust you hit magma. Underneath this impenetrable barrier, what you might find is purely theoretical.

Perhaps more disheartening is the fact that nuclear weapons shot at an asteroid would have little to no chance of affecting it. The reaction is quite momentary, and in the vacuum of space it does nothing other than get very bright and very hot for an instant. There's no surrounding matter to create a blast effect. There's little chance of intercepting something at aggregate velocity of perhaps 60,000mph with any chance of timing the reaction properly. Perhaps if, as in movies, you could bore a (very) deep hole in the thing and detonate the device there, the thermal shock would either fracture it or at least eject enough matter to alter its course a bit. But the odds of landing on a 40,000mph object with almost no gravity and then conducting a drilling operation difficult even on earth are, to put it mildly, not encouraging.

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  • $\begingroup$ Also, cracking open a big rock coming towards earth only creates a lot of little rocks of the same total mass still headed towards earth at now slightly different velocities. $\endgroup$ – user2943160 Jul 5 '16 at 17:02
  • $\begingroup$ The hope would be that a lot of the trajectories would be altered laterally enough to miss or skim the earth. But certainly many would not be. And the larger fragments may not be so small. $\endgroup$ – leoman Jul 5 '16 at 17:11
  • $\begingroup$ Amusing game based on this idea. $\endgroup$ – Ross Presser Jul 5 '16 at 17:21
  • $\begingroup$ @ user2943160 - lots of small rocks will burn up in the atmosphere, one big rock won't. $\endgroup$ – Scott Jul 6 '16 at 1:07
  • $\begingroup$ @Scott - The resulting smog might be worse than an impact. $\endgroup$ – Noah Spurrier Jul 6 '16 at 4:16
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So, supposing we have alien directions that fix the location of the artifact (it's not moving relative to a location on the crust), perhaps the solution would be to drill with the intent of causing an eruption, using the pressure of the core to push out through a weakened mantle, saving us from drilling all the distance to the artifact (perhaps), thereby ejecting the nearly indestructible artifact, and recovering the item from the ejecta.

Let's hope the artifact isn't buried under New York City....

Obviously, the environmental consequences would be catastrophic, if we can't engineer for them. I'm not proposing solutions to such engineering problems here - I don't have any. This is just a post to suggest a new line of reasoning if someone would like to follow up on it.

Maybe the aliens put it under a mid oceanic rift?

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If you knew, where it was, maybe this would be better.

  1. Disassemble all those nuclear weapons and build power plants.
  2. Make a giant, focus magnet so that the magnetic forces were aimed at the alien device.

Using all the electricity these power plants could muster, maybe they could pull it up. Throw is some graphene for good measure. At least with my idea you wouldn't have to deal with crazy amounts of heat and pressure.

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