Friday, August 24, 2018

[icjaclra] Mining Earth's core

How difficult would it be to mine the core of the earth for iron, nickel, and precious metals?  The core of the earth is much closer than asteroids, so seems like it could be easier to mine than asteroids, the latter which seems to be a seriously considered endeavor.  Plus, the earth stays still relative to us, unlike asteroids which move around.

We imagine a hole a mile wide to have space for lots of equipment, especially cooling.  Perhaps dug over a time frame of 1000 years, comparable with the longest human engineering projects (e.g., cathedrals), so 6 km per year or 17 meters per day.  Dig 10 times faster to complete in a century, perhaps to compete with asteroid mining.  We don't have to get to the exact center of the earth to get to the nice metals.

Although digging from the poles would save some distance due to the oblateness of the Earth, it seems better to put the "base", the top of the hole, somewhere more hospitable, as it will be surrounded by a lot of activity.  We will need a place to put a tremendous amount of removed material.  When we get to the core, that material could be dropped down as counterweight to bring the mined metal up, replacing the space the material removed from the core so the Earth doesn't collapse in on itself (very much) as we hollow out our planet.

What are the most difficult challenges for this project?

Perhaps cooling.  One intriguing idea is to do everything underwater, letting ocean water fill the hole and letting convection and boiling do its thing naturally.  Do some calculations of what diameter hole is needed for natural cooling to be able to vent enough heat, and how quickly we can dig.  This seems like the kind of thing that has to succeed at some large-enough diameter: the surface area of the sides of the shaft grows linearly with its diameter; the volume of water inside it grows as the square of the diameter, so at some diameter, there's enough coolant.  (This is the square-cube law but one dimension less, so the linear-square law.)  The general idea is, dig until it gets too hot for the digging equipment.  Let the ocean cool the freshly dug area, and, through the heat conduction of rock, cool beyond the freshly dug area (freezing the liquid mantle).  Dig again through the cooled volume.  Despite cooling thus being taken care of, it'll remain a very difficult environment to do work, with convecting water or boiling vapors constantly rushing by, causing erosion/wear on equipment and the walls of the hole.

As the hole gets deeper, the walls of the hole need to get stronger to withstand pressure of the weight of the earth.  This problem seems very difficult.  Maybe some active support structure, constantly circulating matter that exerts outward centrifugal force, inspired by space fountains and launch loops.  Can the temperature gradient be harnessed provide power for the active support?

How strong are the currents of liquid mantle?  Will they destroy the hole and its walls?  How quickly will they destroy the hole?  Hopefully only on a geologic time scale.

Perhaps the most difficult challenge is a political and economic structure stable and long-lived enough to survive such a long project.  The big payoff comes only at the end.

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