SLEETING down to circle the Sun, twenty million meteors strike Earth each twenty-four hours, a hail of nickel-steel armor-piercing projectiles. On Earth to-day, man waits and hopes to send out a ship, a frail bubble of air wrapped in metal, a thing to reach other worlds beyond that sleet storm of death. Twelve miles a second -- twelve times as fast as the shells of the Big Bertha -- the meteors move when they strike Earth at their slowest. Twenty-five miles a second is an average speed; many have been observed thundering through the upper atmosphere at fifty miles a second.
What chance of survival would the fragile, metal bubble have if it went out beyond that near-invisible protective film of air that surrounds the Earth?
It would have one chance in two thousand of being hit if it went all the way from Earth to Mars through that "sleet"! Twenty million a day strike disk surface of fifty million square miles. It sweeps through eighty million million cubic miles of space in that twenty-four-hour period. Those millions of meteors are distributed so thinly through that vast volume that there must be less than one in every four million cubic miles of space.
One in four million cubic miles of space -- at Earth's orbit. Since they are circling the Sun, most of them in exceedingly eccentric orbits, there is a natural concentration of them as the Sun is approached. At Mars' distance there would be even fewer. Further, no space ship man ever built is going to equal the size of the Earth; there'd be no sense to it. But a ship fifty feet in diameter presents a front-surface area of about one ten-thousandth of a square mile instead of fifty million square miles. If such a ship makes the fifty million-mile trip to Mars, it sweeps only five thousand cubic miles of space, it has less than one chance in two thousand, probably, of sweeping the particular volume that contains a meteor.
But -- suppose by ill luck it does sweep into that one deadly volume. What then? The meteors, those twenty million a day that strike Earth, are not the kind you see in museums. It has been estimated that ten-thousand first-magnitude meteors could be held in one hand! Pinheads are huge by comparison.
But pinheads don't ordinarily move at speeds of dozens of miles a second. What damage would one of those minute things do if it did strike a ship? Although the results on striking Earth and in striking a ship are not strictly comparable, they may give indications. One upsetting factor is present: Earth's gravitational field. That accelerates any body falling to Earth from free space to a minimum velocity of seven miles a second, the least speed theoretically possible.
Actually, meteors appear to be true members of the solar system, revolving in true orbits, highly eccentric and distributed in any plane, any direction, at any angle. They act, so to speak, like individual comets, each on its own wild path. Certain great meteor showers are, of course, the remains of broken comets, fragments torn apart by close passage of some major planet. But since they rotate about the Sun as comets do, their velocities are naturally high; meteors do not fall to Earth; Earth gets in the way of a meteor with a rendezvous at the Sun. Therefore, meteors traveling only twelve miles a second are few and far between indeed. The average velocity of meteors appears to be about twenty-six point nine miles a second.
SINCE twelve miles a second is the minimum speed, let us work with this most conservative value. When a meteor enters our atmosphere, it has an energy represented by its motion, equal to 1/2MV2, M being its mass and V its velocity. In coming to rest, this energy is changed into some other form; practically, to heat. Since heat is a motion of molecules, the velocity of the body may be directly spoken of as "temperatures," for the molecules are all moving at twelve miles a second; it happens they are all moving in the same direction, so that the temperature isn't obvious, but it is a legitimate expression. The meteor, then, has a temperature of about 50,000° C. That is far more than sufficient to volatilize any substance in the universe; tungsten boils vigorously at a tenth of that temperature. How, then, can any meteor possibly survive to become a meteorite?
The atmospheric resistance a meteor encounters is directly proportional to the square of its velocity times the density of the air. At 12 miles a second and at sea level, this resistance would be 8 tons per square centimeter, a pressure readily capable of crushing the nickel-steel alloy of the metal meteors. At high altitudes this resistance is, of course, more reasonable. As the meteor descends it does work compressing the air directly in front of it; to only a limited extent does a meteor stir up air currents. A body moving through air at normal speeds displaces the air in front of it, and the air behind flows in as it passes. But a meteor, lashing through at that immense speed drills a hole through the air as though it were a solid body; air cannot move away from in front of it because the shock of its coming is so swift it cannot be transmitted before it. The air hasn't time to move out of the way, but can only pile up on the forward surface. Similarly, behind it is a space where the meteor has driven through, tearing the air out of place, and passing on long before surrounding air has had time to fill in the emptied space.
The meteor is doing enormous work, compressing, piling up air on its forward surface. More and more is jammed violently against it. Almost instantly the meteor is cushioned by a thick layer of terrifically compressed air. The work is done compressing air, not in rubbing against it. The air is heated, not the meteor. The result is that only the forward surface of the meteor is slightly heated, enough to fuse it superficially, perhaps, but nearly all the energy is released in the compressed air.
The air is heated to a fearful temperature. Only a comparatively small amount of air (about two thousand six hundred grams per square centimeter of front surface) is involved, and this is heated at a temperature of thousands of degrees. It radiates, consequently, because of the compression-heating effect. Nearly all that radiation is far in the invisible ultra-violet; what we see are the trickling dregs that have fallen far down the spectrum to the visible band. Ten thousand first-magnitude meteors in one hand -- yet each, during its brief flight, releases energy at a rate equaled only by something like a ten-million-dollar power house. The work done by a meteor moving at twelve miles a second through sea-level-density air would be at the rate of five billion, six hundred and sixty million watts per square centimeter of front surface. About thirty-five billion watts per square inch. Even seventy-five miles above the surface of Earth it would encounter a resistance that dissipated three hundred and twenty thousand watts per square inch of front surface.
That furious dissipation of energy will, obviously, stop any small meteor long before it reaches Earth. But the resistance varies according to the front-surface area. Now the greater the meteor is, the more mass it has behind each square inch of front surface; the more massive it is, the more chance it has of driving its way through the frightful resistance. A meteor weighing one thousand tones, for instance, would penetrate Earth's atmosphere almost unchecked, leaving a vast volume of ruptured air, a vacuum, in its wake. If a small, but sufficiently large meteor penetrates the atmosphere undestroyed, it is stopped, perhaps at an elevation of only a mile or so, to continue its fall as an ordinary, dropped stone. A little larger, and it might just strike the surface before the last of its velocity is braked away. A meteorite weighing two hundred pounds or more, and falling on soil, penetrates to several yards. But our one-thousand-ton meteorite would scarcely be checked by the air, and might strike at a velocity of a full twelve miles a second.
IT MAKES not the slightest difference whether that meteorite strikes soft sandstone, or plows into hard, igneous granite rock. The crystaline strength of the granite is absolutely unimportant. The meteorite has to move that resisting medium out of its way; that is the fundamental. The rock, the matter, must be accelerated, suddenly, to its own velocity of twelve miles a second, a terrific, instantaneous acceleration. The resistance of ordinary surface soil or rock, due to inertia alone, is the important thing, and that will amount to about two hundred thousand tons per square inch. About two thousand times the crushing resistance of good steel. The nickel steel of the meteorite, and the hard granite would, alike, flow like true gases; the granite would act precisely as the air did, with the exception that it now constitutes an immensely (two thousand times) denser gas. The energy released in a tenth of a second would volatilize both meteorite and surrounding material -- would, in effect -- explode it terrifically into flaming gases at thousands of degrees.
Heat or no heat, under that pressure both meteorite and stone would act as gases. Suppose it had encountered, instead, a mass of solid armor plate. Again, both would explode into flaming gas, the greater strength of the steel would merely make the resistance one part in two thousand greater, an utterly unimportant factor. But -- the steel would, nevertheless, offer a far greater resistance to penetration, because there is more mass per cubic inch that must be accelerated; steel is denser than granite.
Lead, or liquid mercury metal, however, would have a greater resistance to penetration than that hard steel! Osmium, density 3 times that of steel, twice that of lead, would be the most resistant of all. But it is inertia, not strength, that counts.
Now what of our space ship, the metal bubble in emptiness? Meteors are tiny things, pinheads moving at fearful speed. The penetrating power of a rifle bullet is quadrupled if the speed is doubled. A high-power rifle throws a bullet at close to a mile a second, and will penetrate some sixty inches of hard wood. At two miles a second -- it would penetrate about five inches and explode into gas. Would a meteor pierce the hull of a space ship? A wall, say, built of thin steel, covered with lead. Or, would it explode into gas at the surface of the lead, leaving the ship practically uninjured, or merely dented? Or would the sudden eruption of gas be so violent that the gas alone would force a huge breach in the wall, though the meteor originally was no more than a pinhead?
At any rate, it would seem that the pinhead meteors would not be apt to destroy a ship. Greater ones might. But meteors weighing one pound are wonderfully rare, those weighing one pound are wonderfully rare, those weighing ten pounds are far scarcer yet. A meteor weighing a ton --
Such a thing, furthermore, could be detected. Radio-wave reflection and electrostatic devices would warn of the approach of such a monster in the emptiness of space. Magnetic devices alone could not be relied on, for there are two types of meteors: the stony and the metallic.
THE METAL meteorites are composed of about ninety per cent metallic iron in alloy with various percentages of nickel (which may run as high as twenty-five per cent) and smaller quantities of cobalt, copper, phosphorus, sulphur and other elements. Those elements are joined in curious minerals never found on Earth; in fact, many of the meteorites in museums to-day have been recognized as such because of the non-Terrestrial minerals occurring in them.
Most of the meteorites exhibited in museums are of this type, though the metal meteorites are, actually, rarer than the stony type. Metal meteorites are hard, tough, relatively permanent, and much more readily recognized. The stony meteorites are easily confused, by the layman, with ordinary rock. The unaccustomed action of water and frost rapidly disintegrate them.
The metallic meteorites present one characteristic that has long puzzled metallurgists. Polished, and etched, the individual metal crystals are readily visible as large light-and-dark colored patches, shaped rather like a one-inch section of the broad end of a toothpick. The large network of crystals of metal are filled with silicate minerals peculiar to meteorites. Quite recently, a metallurgist has succeeded in crystallizing an iron alloy to form the same type of crystals observed in the meteorites, by slow, careful cooling from the molten stage. The question of how meteorites formed, however, remains very largely a question, for the conditions necessary for this type of crystallization are hard to understand.
The stony meteorites, too, indicate a slow cooling from a liquid stage. Both stony and metallic, on heating in a vacuum, yield large volumes of gases, including carbon and hydrogen compounds, but little or no oxygen. Helium is found in small quantities. Helium is the product of radioactivity, but the stony meteorites are less than one fourth as radioactive as Terrestrial granites, while the all-metal meteorites are almost wholly free of any radioactivity. This may be interpreted in a number of ways, primarily as being indicative of their origin, but in a way as yet undetermined, or as an indication of age; that they are so incredibly ancient that the long-lived uranium atoms themselves have at last broken down.
Under the latter interpretation, it would indicate an age about six billion years greater than that of Earth. This throws considerable doubt on the time clock interpretation, since dynamical considerations of the entire solar system indicate, vaguely, that something important happened two billion not eight billion years ago.
But in whatever way they may have originated, whether they will or will not constitute a menace to space ships to come, they represent to-day a thing entirely unique; they are the only material things that reach Earth from the regions of the stars: light -- and meteors. Those two things alone come to Earth from outside, to give any hint of things beyond our planet.