You Can't Get There from Here

There is a striking though clumsy pharase from the autobiography of the 19th Century writer Richard Jefferies that has stuck in my mind for many years: "The unattainable blue of the flower of the sky." Unattainable: that is a word we seldom use these days, now that men have reached the greatest heights and depths of Earth and are preparing to journey far beyond the sky. Yet only a century ago the North and South Poles were utterly unknown, much of Africa was still as mysterious as in the time of King Solomon, and no human being had descended a hundred feet into the sea or risen more than a mile into the air. We have gone so far in so short a time, and will obviously go so much further if our species survives its adolescence, that I should like to pose a question which would have seemed very odd to our ancestors. It is this: "Is there any place which will always remain inaccessible to us, whatever scientific advances the future may bring?"

One candidate springs to mind at once. Only 4000 miles from where I am sitting is a point far more difficult to reach than the other side of the Moon -- or, for that matter, than the other side of Pluto. It is also 4000 miles from you; as you have probably guessed, I refer to the center of the Earth.

With all apologies to Jules Verne, one cannot reach this interesting spot by descending into the crater of Mount Sneffels. In fact, it is impossible to descend more than a couple of miles through any system of craters, caves or tunnels -- natural nor artificial. The deepest mine goes down only 7000 feet.

Just as it does in the sea, the pressure below the Earth's surface increases with death, owing to the weight of the material above. The surface rocks of our planet are about three times as dense as water; therefore, as we go downward into the Earth the pressure rises three times as quickly as in the sea. When the bathyscaphe Trieste reached the Challenger Deep, seven miles below the surface of the Pacific, there was a pressure of over a thousand tons on every square foot of its surface, and the walls of the observation sphere had to be made of steel five inches thick. The same pressure would be reached only two miles down inside the Earth, and this is a mere scratch on the surface of the globe. At the Earth's center, the pressure is estimated to be over 3,000,000 tons per square foot, or 3000 times that which Trieste encountered.

Under such pressures, rocks and metals flow like liquids. In addition, the temperature rises steadily toward the interior, reaching perhaps 6000 degrees Fahrenheit at the center. It is obvious, therefore, that we cannot hope to find a ready-made road into the heart of our planet, and the old idea of a "Hollow Earth" (once put forward as a serious scientific theory) must be reluctantly dismissed -- together with a whole host of subterranean fantasies such as Edgar Rice Burroughs' At the Earth's Core.

The greatest depth to which the oil companies -- the most energetic of underground explorers -- have so far drilled is just over five miles. This is a quarter of the way through the solid crust of the Earth, which is about 20 miles thick beneath the continents; under the oceans, the crust is much thinner and plans are now being made to drill through it (the so-called Mohole Project) to obtain samples of the unknown material upon which it floats.

The conventional drilling technique involves turning a bit at the end of thousands of feet of pipe, rotated by an engine at the surface. As the drill goes deeper, more and more energy is lost in friction against the wall of the hole, and it takes hours to lift and lower the miles of piping every time a bit has to be changed. Newer methods do away with the rotating pipe and put the power source on the drill itself, driving it electrically or by hydraulic pressure. The Russians, who have pioneered in this field, have also developed what is effectively a rocket drill, which burns its way into the ground behind a 6000-degree oxykerosene jet. Using one or another of these techniques, it would now be possible to drill a 10-mile shaft at the cost of several million dollars. This would take us halfway through the crust of the Earth -- or a four-hundredth of the way to the center.

A six-inch drill hole is not what most people have in mind when they speak of underground exploration, so let us look at some more exciting possibilities. Russian mining engineers have already built man-carrying mechanical moles for tunneling at shallow depths; they are very similar to the device that Burroughs' hero employed to reach Pellucidar, the world inside the Earth. These machines solve the problem of soil disposal in exactly the same way as does the common or garden mole, which was the prototype on which their design was based; the earth loosened by the drilling head is compacted and tamped to form the tunnel wall.

Even in fairly soft soil, the mechanical mole is very slow-moving. Its speed is limited to a mile or so a day by the power available (electricity is supplied through a trailing cable) and by the wear and tear on the drilling mechanism. An "earth probe" that really hoped to get anywhere would have to have a fundamentally new type of excavating technique, and a very considerable supply of energy.

Nuclear reactions could provide the energy underground, as they already do undersea. As for the method of excavation, here again the Russians have suggested one answer. They are now using high-frequency electric currents to blast a way through rocks by sheer heat, and an underground arc could burn its way through the Earth just as fast as one could pour energy into it. Ultrasonic vibrations might also do the trick; they are now being employed on a small scale for cutting through materials too hard to be worked with ordinary tools.

A man-carrying, nuclear-powered "subterrene" is a nice concept for any claustrophobe to meditate upon. For most purposes, there would be little point in putting a man in it; he would have to rely entirely upon the machine's instruments, and his own senses could contribute nothing to the enterprise. All the scientific observations and collection of samples could be done automatically according to a prearranged program. Moreover, with no human crew to sustain, the vehicle could take its time. It might spend weeks or months wandering around the roots of the Himalayas or under the bed of the Atlantic before it headed for home with its cargo of knowledge.

The depth that such an earth probe could reach would be limited by the pressure its walls could sustain. This might be very high indeed, if it were designed as a solid body and the empty spaces inside it were filled with liquid to provide additional strength (another argument for having no crew).

In the laboratory, steady pressures of a quarter of a million tons per square foot have now been produced; this is equivalent to the pressure 400 miles inside the Earth. This does not mean that we can build vehicles theoretically capable of going 400 miles down, but a 10th of this figure does not seem beyond the bounds of possibility. Temperature is a less serious problem; apart from occasional hot spots like volcanoes, the temperatures in the crust do not exceed six or seven hundred degrees Fahrenheit. It appears, therefore, that we may eventually explore most of the Earth's crust, if we really wish to do so, with machines which can be visualized in terms of today's engineering techniques.

Difficult though the problems of physically exploring the outer layers of the Earth may be, they are quite trivial compared with those we have to face if we hope to travel into the mantle (the next 1800 miles) or the core (from 1800 miles down to the center). No existing technology could help us here; all the materials and forces now available are hopelessly inadequate to deal with the combined effects of 6000 degrees F and 3,000,000 tons to the square foot. We could not hold open a hollow space as large as a pinhead under such conditions for more than a fraction of a second; our toughest metals would not only flow like water, but would be converted into new and denser materials.

Any exploration of the Earth's deep interior cannot, therefore, be carried out by direct physical means, until and unless we gain control of forces many times more powerful than those that we posess today. But where we cannot travel, we may yet observe.

To see into the Earth with the precision and the definition with which we can explore the interior of our own bodies would be a marvelous achievement, of the greatest scientific and practical value. An X-ray photograph would have been unbelievable to an 1860 doctor; yet now we are building up what are virtually crude X-ray photos of the Earth, from the wave patterns produced by natural earthquakes or by explosions. (We can now make bangs big enough to shake our planet; it is not generally realized that the greatest explosion ever recorded -- that of the volcano Krakatoa in 1883 -- could be matched by a large fusion bomb.)

The pictures are still very crude and lacking in fine detail; in particular, they tell us virtually nothing about the dense central core, which is almost 4000 miles in diameter. We do not even know what it is composed of; the old theory that it is made of iron has been somewhat discredited lately, and it may well turn out to be some fairly conventional rock compressed by the enormous pressure into a form denser than lead.

What we want in order to explore this region are waves that will pass through the solid Earth as easily as X rays pass through a human body, or light waves through the atmosphere, bringing back to us the information they gather on their journey. But such an idea is obviously absurd; you have only to think of the 8000 miles of impenetrable rock and metal that screen you from the antipodes----

Well, think again. There are, if not waves, entities to which this massive Earth is as transparent as a soap bubble. One is gravitation. Though I have never met a physicist who would give me a straight answer to the question "Is gravity propagated in waves?" there is no doubt that it goes straight through the Earth as if it weren't there.

Something equally penetrating is that most peculiar and elusive of atomic particles, the neutrino. All other particles are stopped by a few inches, or at most a few feet, of materials such as lead. But the incredible neutrino, having no mass and no charge (to put you out of your misery, it does have spin), can shoot through a lead screen 50 light-years thick without being noticeably inconvenienced. Torrents of them are sweeping at the velocity of light through our so-called solid Earth at this very moment, and only one in a million million notices the trifling obstruction.

I am not suggesting that we could use either gravity or neutrino beams to give us close-ups of the Earth's core; both are probably too penetrating for the job, since you cannot scan an object with rays that go through it completely. But if such extraordinary entities exist in nature, there may be others that possess the properties we need, and which we can use to map the interior of our planet as the radiologists map the insides of our bodies.

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We may well discover, when such a survey is made, that there is nothing particularly interesting deep down inside the Earth -- merely homogeneous shells of rock or metal, growing denser and denser toward the center. Almost invariably, however, the Universe turns out to be more complicated and surprising than we could have supposed; consider the way in which "empty" space was found to be crowded with radio waves, cosmic dust, stray atoms, charged particles, and heaven knows what, just as soon as we started to explore it. If nature runs true to form, we shall discover something deep inside the Earth that we will not be content merely to survey from a distance. We'll want to get at it.

It may want to get at us, as I suggested some years ago in a short story called The Fires Within. This was based on the fact that forms of matter exist, under high pressure, so dense that by comparison ordinary rock would seem more tenuous than air. Indeed, this is a gross understatement; granite is about 2000 times as dense as air, but the "collapsed matter" in the heart of a dwarf star is 100,000 times, and in some cases 10,000,000 times, as dense as granite. Although even the pressures inside the Earth are far too small to crush atoms to this inconceivable density, I assumed, for purely fictional purposes, that creatures made of compressed matter might be swimming round inside the Earth as fish swim in the sea. I hope that no one takes the idea any more seriously than I did, but it may serve as a fable to prepare us for facts almost equally surprising, and much more subtle.

If our descendants -- or their machines -- ever succeed in sinking far down into the molten interior of the Earth, it may be through the use of techniques developed very far from home for quite different purposes. To consider these, let us take a detour far out into space -- to the giant planet Jupiter, which our first automatic probes will be circling and surveying in the 1970s.

I am a little tired of reading in books about space travel that Jupiter is a planet upon which men will "certainly" never land -- although I cannot pretend that I am very anxious to go there myself. Here is a world with 11 times the diameter of Earth, and more than a hundred times its area; if our entire planet were spread out across the face of Jupiter, it would appear about the size of India on the terrestrial globe. But we have never made any maps of Jupiter, for we have never seen its surface; like that of Venus, it is perpetually hidden by clouds -- or what, for want of a better word, we may call clouds.

They are drawn out in ever shifting parallel bands by the swift spin of the planet, and across half a billion miles of space we can watch the progress of mammoth storms or disturbances, many of them larger than Earth. The meteorology of Jupiter is a science whose very foundations are not yet laid; out there in the cold twilight so far from the Sun, a huge atmosphere of hydrogen and helium is being torn by unknown forces. Yet despite these convulsions, some features manage to survive for years at a time; the most famous of these is the Great Red Spot, an immense oval object some 25,000 miles long which has been observed, on and off, certainly for 120 years and perhaps for three centuries.

Because of Jupiter's size, and the scale of the events taking place there, it is natural to assume that its atmosphere is very much deeper than ours -- perhaps a thousand, rather than a hundred, miles in thickness. But this is not the case; because Jupiter's gravity is more than two and a half times Earth's, the planet's atmosphere is compressed into a layer which may be only 50 miles deep.

At the bottom of that layer, the pressure must mount to values which we know only in the depths of our oceans. To enter the atmosphere of Jupiter we would need not merely a spaceship, but a bathyscaphe. There may be no definite solid surface on which any vehicle could land; the hydrogen may become steadily more dense until it turns first to a liquid slush, then -- when the pressure reaches a thousand times that at the bottom of the Challenger Deep -- to a metallic solid.

Yet, someday men are going to visit this world; the exploration of Jupiter may be one of the greatest enterprises of the 21st Century. Jupiter will be the laboratory in which we shall learn to withstand, control and use really high pressures, and from this work may arise vast new industries in the years to come. (There is no lack of raw materials on a world that weighs 300 times as much as Earth.) When we have learned how to survive in the lower levels of the Jovian atmosphere, we shall be better prepared to burrow into our own planet.

On Jupiter our main problem will be pressure -- and perhaps the sheer violence of gales that may blow at hundreds of miles an hour. We shall not have to contend with high temperatures; the outer layers of the atmosphere are at about 250 degrees below zero Fahren-heit, but at "ground level" it may be slightly tropical, though that is now anyone's guess. If there are places in the Solar System that are unattainable because of temperature alone, we must look for them much closer to the Sun.

The planet Mercury is an obvious choice. This little world -- just over 3000 miles in diameter -- knows neither day nor night, since one face is turned perpetually toward the Sun and the other is in eternal darkness. At the center of the illuminated hemisphere, in that unimaginable endless noon where the Sun hangs forever vertically overhead, the temperature must rise to seven or eight hundred degrees Fahrenheit. And on the dark side, where the only heat received is the feeble glow of starlight, it is at least 400 degrees below zero.

These temperatures, extreme though they are by ordinary standards, are well inside the range of today's industrial and scientific techniques. The conquest of Mercury will not be an easy project, and not a few men and machines will perish in the attempt. But we shall have to get closer -- much closer -- to the Sun before we run into real trouble.

The temperature rises quite slowly at first as we move in toward the central fire of the Sun; here are some figures which show what would happen to a spaceship whose hull was at a comfortable 65 degrees F in the vicinity of Earth.

As the ship went past Venus, 67,000,000 miles from the Sun, the hull would reach 160 degrees F; at the orbit of Mercury, 36,000,000 miles from the Sun, it would touch 400 degrees F. We would have to approach the Sun to within 10,000,000 miles before the temperature passed 1000 degrees F.

Five million miles out from the center of the Sun, it would be approaching 2000 degrees F; 1,000,000 miles, 4500 degrees F. This last distance is only half a million miles above the surface of the Sun, which is at a temperature of about 9000 degrees F.

Materials are known which remain solid at temperatures above 6000 degrees F; graphite starts evaporating around 6800 degrees F, while hafnium carbide holds out to 7500 degrees F -- the record, to the best of my knowledge. Thus we could send a hafnium carbide nose cone to well within a million miles of the Sun -- a hundredth of the Earth's distance -- and hope to get it back in one piece. Instrument-carrying, expendable probes, well protected with layers of refractory material which slowly boiled away, could even reach the surface of the Sun before they disintegrated.

But how close to the Sun could a man-carrying ship approach in safety? The answer to this question depends upon the skill and ingenuity of the refrigeration experts: my guess is that 5,000,000 miles is an attainable distance even with a crew-carrying vehicle.

There is one useful trick we may employ to get quite close to the Sun in (almost) perfect safety. This is to use a convenient asteroid or comet as a sunshade, and the best choice known at the moment is the little flying mountain appropriately named Icarus.

This minor planet travels on an orbit that every 13 months brings it within a mere 17,000,000 miles of the Sun. Occasionally, it also passes quite close to Earth; it will be within 4,000,000 miles of us in 1968.

Icarus is an irregular chunk of rock one or two miles in diameter, and at perihelion, beneath a sun that appears 30 times as big in the sky as it does from Earth, the surface of this little world may reach temperatures not far short of 1000 degrees Fahrenheit. But it casts a cone of shadow into space; and in the cold shelter of that shadow, a ship could safely ride around the Sun.

In a short story called Summertime on Icarus I described how scientists might embark on such a (somewhat) hair-raising sleigh ride to get themselves and their instruments close to the Sun, which would be unable to touch them as long as they remained on the cool side of their mile-thick shield of rock. Though it would be possible to construct artificial heat shields, like today's reentry nose cones, it will be a long time before we can give ourselves the protection that Icarus would provide for nothing. Small though it is, this minor planet must weigh about 10 billion tons.

There may be other asteroids that go even closer to the Sun; if there are not, we may one day make them do so by a nudge at the right point in the orbit. And then, dug well in below the surface, scientists would be able to skim the atmosphere of the Sun, whipping across it and out again into space on a tight hairpin bend.

It is interesting to work out how long the ride would take. Being a rather small star, the Sun is "only" 3,000,000 miles in circumference. A satellite just outside its atmosphere would move at about a million miles an hour, so would circle it every three hours.

A comet or asteroid falling toward the Sun from the distance of Earth would be moving somewhat faster than this at its point of closest approach. It would flash across the surface of the Sun at a million and a quarter miles an hour, and so would make its swing round the Sun in little more than an hour, before heading off into space again. Even if a few megatons of rock boiled away in the process, the instruments and observers deep inside the asteroid would be safe -- unless, of course, there was a navigational error and they plunged too deeply into the solar atmosphere, to burn up through friction as so many artificial satellites of Earth have already done.

What a ride that would be! Imagine flashing high above the center of a giant sunspot, a gaping crater 100,000 miles across, spanned by bridges of fire over which our planet Earth could roll like a child's hoop along a sidewalk. The explosion of the most powerful hydrogen bomb would pass unnoticed in that inferno, where whole continents of incandescent gas leap skyward at hundreds of miles a second, sometimes escaping completely into space.

Ray Bradbury, in his short story The Golden Apples of the Sun, described the descent of a spaceship into the solar atmosphere to obtain a sample of the Sun (which we now know, incidentally, to be 90 percent hydrogen, 10 percent helium, plus a mere trace of all the other elements). When I first read this story, I dismissed it as charming fantasy; now I am not so sure. In one sense we have already reached out and touched the Sun, for we made radar contact with it in 1959 -- and how unbelievable that would have seemed a generation ago! Even a close physical approach no longer seems completely out of the question, thanks to the development of the new science of plasma physics, born within the last 10 years.

Plasma physics, sometimes known by the jawbreaking name of magnetohydrodynamics, is concerned with the handling of very hot gases in magnetic fields. Already it has enabled us to produce temperatures of tens of millions of degrees in the laboratory, and ultimately it may lead to the goal of limitless power from hydrogen fusion. I suggest that, when we have acquired some real mastery of this infant science, it will also give us magnetic or electric shields that can provide for more effective protection against both temperature and pressure than can be obtained from any walls of metal. The old science-fiction idea of the impenetrable shield of force may no longer be a dream; we may be forced to discover it, as the only real answer to the ICBM. When we possess it, we may have a key not only to the interior of the Earth, but even, perhaps, to the interior of the Sun.

• • •

This search for the unattainable has taken us, in imagination, to some strange and hostile places. The center of the Earth, the depths of the Jovian atmosphere, the surface of the Sun -- though these are certainly beyond the reach of today's technologies, I have given reasons for thinking that they need not be forever out of bounds, if we really desire to visit them.