Posted on 02/10/2005 10:59:13 AM PST by vannrox
Adventure stories involving the exploration of the interior of Planet Earth have a long and distinguished history in science fiction. Jules Verne’s Journey to the Center of the Earth (1864) was perhaps the first such tale. Despite the title, the story involves explorers following the instructions of a 17th century runic message on a trip that descends into the crater of an Icelandic volcano and into a long tunnel connecting to a vast cave containing a conveniently phosphorescent ceiling, an ocean, islands, dinosaurs, and mastodons, all in the interior of the Earth some miles beneath the surface.
Now, however, there’s some new writing about the exploration of the Earth’s core, but this time it’s not fiction, but a serious scientific proposal. David Stevenson, a Professor of Planetary Science at CalTech, has proposed mounting an ambitious NASA-style mission to the Earth’s core. He describes his “modest proposal” (in the Swiftian sense) in a paper recently published in the journal Nature. Since Stevenson has not yet mastered the use of the twistor effect, however, he has to do things the hard way. He cannot be accused of thinking small. He proposes to use a multi-megaton nuclear weapon and one hour’s worth of the net iron production of the Earth’s iron smelter facilities (~108 kg). In this column I want to describe this proposal.. Stevenson is faced with the basic problem is how to get through all the rock between the surface and the core. Anyone who has ever dug a post-hole recognizes the problem. Something like Abner Perry’s mole machine that took Perry and David Innes to Pellucidar couldn’t really do the job. We now have well engineered digging machines for tunneling, and they can’t go down more than a few thousand meters. Deep well-drilling techniques are not much better. The deepest drill hole, dug in the Kola Peninsula in Russia, goes down only 12 km. So Stevenson has proposed a more radical approach: melt your way through the rock. It takes about a mega-joule of energy to melt each cubic meter or rock, assuming that the rock is already hot enough to be near its melting point. Therefore, melting a tunnel that is 3 square-meters across and 6,380 km long, all the way to the Earth’s center, would use an energy of about 2 ´ 1013 joules. That sounds like a lot of energy, but consider that a large nuclear power plant produces about 8 ´ 1013 joules per day, so we are in the right energy ballpark. The challenge is to find a vehicle that can withstand the heat and pressure of the Earth’s interior while making the trip. Stevenson would start the process by finding a suitable fissure in the Earth’s crust, setting off a multi-megaton underground nuclear explosion to widen the fissure to a sizable crack, and then dumping in an instrumented blob of liquid iron (melting point 1535 C) with a mass of about 108 kilograms, the amount of iron in a sphere about 30 meters in diameter. The blob would then assume an elongated shape that would fill a part of the crack and cause the crack to propagate downward under the pull of gravity, melting the path in front, with liquid magma flowing around the outside of the iron mass and sealing the path behind. As the iron blob moved downward, despite the high pressure from below, it would achieve a fairly high velocity. Assuming that the iron elongates to melt a path about 1 meter across, its downward speed would be about 30 meters per second. At that speed it could reach the Earth’s core in about two and a half days. Today’s microprocessors are made of silicon (melting point 1410 C) and can’t operate at high temperatures. The instrumentation package would either have to be locally insulated and cooled or would require a presently unknown microprocessor technology. Further, getting the measurement information from the probe to the Earth’s surface is very difficult. There could be no trailing wires, no light beams, no radio waves, so Stevenson proposes to use acoustic signals with a radiated power of about 10 watts for the duration of the mission. The frequency of the acoustic waves is limited at the high end by absorption in the rock, and at the low end by seismic noise and the rate of information transfer. Stevenson proposes a signal frequency around 100 Hz for sending signals to a surface detector similar to the LIGO gravity-wave detector, but coupled to rather than insulated from the Earth’s vibrations. This, he estimates, should allow the transfer of 10 megabytes or so of information during the duration of the mission. The power supply for the mission is also a challenging problem. Conventional batteries and fuel cells do not tolerate high temperatures any better than microprocessors. The thermo-electric nuclear isotope power generation used in some spacecraft would not work because the molten-iron environment is already hotter than the decaying radioactive isotope. Stevenson proposes to use a Stirling-cycle engine to tap into a part of the energy flow that occurs as the iron melts its way to the core, using the temperature difference between the molten iron and the cooler surrounding rock. To me, that sounds difficult, and I also foresee a problem with the generator that the Stirling engine drives, since most magnetic materials, on which standard generators depend, lost most of their magnetic properties in a high temperature environment. Writing the environmental impact should also be interesting. The proposal for crack-creation process wilt a nuclear explosion is probably in collision with various international treaties and is sure to raise the ire of anti-nuclear activists. And it would probably be necessary to set very low probabilities that the project would not produce a new active volcano at the launch site or generate massive earthquakes as it moved downward. I doubt that either of these scenarios is likely, but “proving’ that with our present understanding of geology is a formidable problem. That’s a lot of money, but as I see it the proposal would do more for society than the current administration's tax break for millionaires, and it costs a lot less. In any case, the first steps would not be implementation, but research into all the technical issues that the proposal raises. This research should begin. It may be a long time until we can probe the Earth’s core, but we should make a start.
Following Verne’s lead and doing considerably more violence to geology, paleontology, and physics, Edgar Rice Burroughs wrote seven novels beginning with At the Earth’s Core (1922) that were set in Pellucidar, a “land” occupying the inner surface of a vast spherical hole in the Earth’s hollow interior. Pellucidar had a sizable ocean and more land area than Earth’s outer surface, had its own internal sun and moon, and was populated by mastodons, dinosaurs, and an intelligent but rather nasty reptilian species called the Mahars. Burroughs’ various protagonists (including Tarzan) traveled to Pellucidar in a variety of ways, including a mechanical mole machine, arctic pirate expeditions, and a vacuum-filled magnesium dirigible.
Unfortunately, Burroughs got the physics of hollow planets completely wrong. The Mahars, dinosaurs, and explorers would not be pulled to the inner surface of Pellucidar by inside-out gravity. As Isaac Newton first proved, because of the inverse-square law the pull of gravity anywhere in the cavity in a thick massive hollow sphere is zero, because the gravity pulls from below and above any point exactly cancel. Potential inhabitants of Pellucidar would find themselves floating around in free fall.
Not immune to the pull of the Earth’s interior, I also once wrote an almost-published piece about the exploration of the Earth’s core. Near the end of the original manuscript of my first hard SF novel Twistor there is a long scene in which my protagonists David and Vickie, with some help from Boeing Aerospace, build a special “inner-space craft” vehicle that uses gravity and the twistor effect (a “rotational” interchange of normal matter and shadow matter) to do a 38 minute in-vacuum free-fall through the Earth’s interior gravitational field to the other side of the planet, sampling snippets of the Earth’s interior all along the trajectory to the center and back and exploring for the first time the “inner space” of our world.
Unfortunately my editor, in his wisdom, decided to halt the narrative at an earlier point and removed this scene from the published version of Twistor, so few people have actually read my inner space adventure.
Stevenson’s “vehicle” is a large blob of molten iron. Iron is very heavy, with a density of 7.87 grams per cubic centimeter, as compared to a density of about 2 g/cm3 for rock. Therefore, a large blob of sufficiently hot molten iron would tend to produce a “China Syndrome”, melting its way through the Earth’s crust, losing thermal energy but gaining gravitational energy as it went. Planetologists believe that the iron at the Earth’s core got there in just that way, melting its way through the crust of the primordial planet until it reached the core.
The problem with this scheme, of course, is that a blob of molten iron that is subjected to the very high pressures in the Earth’s interior does not provide a very good mode of travel. Thus, the “passenger” would have to be neutral buoyancy micro-miniaturized robotic instrumentation that would relay measurements from the core to the surface of the Earth. How to accomplish that is also a challenging problem.
There also may be other problems with the scheme. If the propagating crack containing the blob splits, it may also split the iron mass into two blobs, which may not individually be massive enough to continue propagating to the Earth’s core. Also, the envisioned communication link is one way. As NASA-watchers know, space probes work best when there is two-way communication, permitting course alterations and program alterations to deal with unforeseen problems. Further, the Earth’s gravity pull downward diminishes linearly as the probe moves downward, and I see nothing in Stevenson’s calculations that takes this into account. Presumably there is some critical depth at which the iron blob would stall because the pull of gravity is insufficient to move it further or provide more gravitational energy.
How much would the project cost? Stevenson’s proposal has no budget attached, and his a bit cagy about the cost. He points out that the cumulative cost of unmanned space exploration has been more that $10 billion, and that the exploration of the Earth’s interior should deserve “a comparable or lower amount”. My guess is that the price tag would be several billion dollars.
WTF ? a 30 meter BALL of melted iron is gonna weigh MUCH MORE.
Nope, the mass distribution and gravity is only perfectly cancelled at the exact center. Everywhere else inside the sphere you will get microgravity which will start you falling to the inner edge of the sphere.
Of course if that sphere is large enough, it's going to hurt when you hit the sphere.
Because as you approach any point on the wall, even though the material you are receding from exerts less and less gravitational force on you, there is more and more of it in that general direction?
Exactly.
RE: 108 kilograms, the amount of iron in a sphere about 30 meters in diameter.
????? Oooops! That's some pretty light iron there!
"Ok, zero g in the center of the earth, 1 g at the surface of the earth, and gets less and less the further you go out. What is the max g point?"
The maximum g point would be at the earth's surface, or 1 g. As one went beneath the surface, the pull would decrease until it reached zero g at the center. If the earth were hollow (which seems unlikely), it would reach zero g as soon as one reached the hollow cavity.
"Stevenson would start the process by finding a suitable fissure in the Earth’s crust, setting off a multi-megaton underground nuclear explosion to widen the fissure to a sizable crack, and then dumping in an instrumented blob of liquid iron (melting point 1535 C) with a mass of about 108 kilograms, the amount of iron in a sphere about 30 meters in diameter."
Something's wrong here -- a solid iron sphere about 30 meters in diameter would weigh more by a factor of about a million . . .
Cheap metals, potentially. The planet's interior to some degree acts as a giant smelter, and metals separate themselves, often with excellent purity.
If there were methods in place that allowed the efficient and inexpensive extraction of these metals, it could potentially make the United States an industrial economy again, exporting all the metals required to satiate world demand. :)
Additionally, if mankind became adept at underground engineering and construction, we lay the groundwork for innoculation from nuclear attack in the near term and a hostile environment long-term in the event of ice age, solar instability, or asteroid impact.
Wouldn't that cause an object located at any point inside the sphere to be drawn towards the center?
Carlos Castaneda fan? Did anyone ever find out if he actually knew someone named Don Juan?
If I were a bettin' man, I would take you up on that. It is true that since gravity follows the inverse square rule, gravity increases greatly as you approach the mass, but the aggregate of the gravity from the much larger mass in the other direction cancels it out when inside a hollow sphere. Picture yourself standing (if it were possible) on the inside surface of the hollow sphere. There is a very small mass pulling you down (under your feet), but the rest of the sphere pulling you up (everywhere else). Cancels out. Anybody else out there agree or disagree? Now, if the hollow sphere is John Kerry's cranium...that's a different story.
I'm scratching my head on this one.
I'm still thinking that an object will be drawn to the center, for the very reasons you point out. Once in the center, the mass distribution and gravitational force is equal on all sides, resulting in zero-G.
SF ping.
Physicist, what do you think? Zero gravity inside a massive hollow sphere?
Everything inside Kerry's hollow cranium tends to the left side...definitely not the center.
LOL!
Actually, that's incorrect. I proved it in an analytic geometry class in college. The gravity is exactly canceled out at every point inside the hollow sphere.
The reason for this is as follows: Starting at the center of the sphere, as you move to a non-central point (say, to your right), the gravitational attraction between you and the part of the sphere you're approaching increases...but you're putting more of the sphere behind you than in front of you, in a ratio and at a distance that exactly balances your proximity to the point you're approaching.
It sounds complicated, but it works. Honest.
I'm interested to see the proof, and I've got an open mind on this one at this point.
The cut-and-paste failed...the actual number was probably supposed to be 10^8, or 10 to the 8th power kilograms. Make it 100 million kilos of iron.
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