by Isaac Asimov
The experiment also showed that nature does not necessarily abhor a vacuum under all circumstances. The space left in the closed end of the tube after the mercury fell was a vacuum, containing nothing but a very small quantity of mercury vapor. This Torricellian vacuum was the first artificially produced vacuum.
The vacuum was pressed into the service of science almost at once. In 1650, the German scholar Athanasius Kircher demonstrated that sound could not be transmitted through a vacuum, thus upholding an Aristotelian theory (for once). In the next decade, Robert Boyle showed that very light objects will fall as rapidly as heavy ones in a vacuum, thus upholding Galileo’s theories of motion against the views of Aristotle.
If air has a finite weight, it must have some finite height. The weight of the atmosphere turned out to be 14.7 pounds per square inch; on this basis, the atmosphere was just about 5 miles high—if it was evenly dense all the way up. But, in 1662, Boyle showed that it could not be, because pressure increased air’s density. He stood up a tube shaped like the letter J and poured some mercury into the mouth of the tube, on the tall side of the J. The mercury trapped a little air in the closed end on the short side. As he poured in more mercury, the air pocket shrank. At the same time, its pressure increased, Boyle discovered, for it shrank less as the mercury grew weightier. By actual measurement, Boyle showed that reducing the volume of gas to one-half doubled its pressure; in other words, the volume varied in inverse ratio to the pressure (figure 5.2). This historic discovery, known as Boyle’s law, was the first step in the long series of discoveries about matter that eventually led to the atomic theory.
Figure 5.2. Diagram of Boyle’s experiment. When the left arm of the tube is stoppered and more mercury is poured into the right arm, the trapped air is compressed. Boyle showed that the volume of the trapped air varies inversely with the pressure, thus demonstrating Boyle’s law.
Since air contracts under pressure, it must be densest at sea level and steadily become thinner as the weight of the overlying air declines toward the top of the atmosphere. This notion was first demonstrated in 1648 by the French mathematician Blaise Pascal, who sent his brother-in-law Florin Perier nearly a mile up a mountainside and had him carry a barometer to note how the mercury level dropped as altitude increased.
Theoretical calculations showed that, if the temperature were the same all the way up, the air pressure would decrease tenfold with every 12 miles of rise in altitude. In other words, at 12 miles the column of mercury the air could support would have dropped from 30 inches to 3 inches; at 24 miles it would be .3 of an inch; at 36 miles, .03 of an inch; and so on. At 108 miles, the air pressure would amount to only 0.000000003 of an inch of mercury. This may not sound like much, but over the whole earth, the weight of the air above 108 miles would still total 6 million tons.
Actually all these figures are only approximations, because the air temperature changes with height. Nevertheless, they do clarify the picture, and we can see that the atmosphere has no definite boundary; it simply fades off gradually into the near emptiness of space. Meteor trails have been detected as high as 100 miles where the air pressure is only 1 millionth what it is on the earth’s surface, and the air density only I billionth. Yet that is enough to heat these tiny bits of matter to incandescence through air resistance. And the aurora borealis (northern lights), formed of glowing wisps of gas bombarded by particles from outer space, has been located as high as 500 to 600 miles above sea level.
AIR TRAVEL
From earliest times, there seems to have been a haunting desire on the part of human beings to travel through the air. The wind can, and does, carry light objects—leaves, feathers, seeds—through the air. More impressive are the gliding animals, such as flying squirrels, flying phalangers, even flying fish, and—to a far greater extent—the true fliers, such as insects, bats, and birds.
The yearning of human beings to follow suit leaves its mark in myth and legend. Gods and demons can routinely travel through air (angels and fairies are always pictured with wings); and there is Icarus, after whom an asteroid was named (see chapter 3); and the flying horse, Pegasus; and even flying carpets in Oriental legend.
The first artificial device that could at least glide at considerable heights for a considerable time was the kite, in which paper, or some similar material is stretched over a flimsy wooden framework, equipped with a tail for stability and a long cord by which it can be held. A kite is supposed to have been invented by the Greek philosopher, Archytas in the fourth century B.C.
Kites were used for thousands of years, chiefly for amusement, though practical uses were also possible. A kite can hold a lantern aloft as a signal over a wide area. It can carry a light cord across a river or a ravine; then the cord can be used to pull heavier cords across until a bridge is built.
The first attempt to use kites for scientific purposes came in 1749, when a Scottish astronomer, Alexander Wilson, attached thermometers to kites, hoping to measure temperatures at a height. Much more significant was the kite flying of Benjamin Franklin in 1752, to which I shall return in chapter 9.
Kites (or kindred gliding artifacts) did not become large enough and strong enough to carry human beings for another century and a half, but the problem was solved in another fashion in Franklin’s lifetime.
In 1782, two French brothers, Joseph Michel and Jacques Etienne Montgolfier, lit a fire under a large bag with an opening underneath and thus filled the bag with hot air. The bag rose slowly; the Montgolfiers had successfully launched the first balloon. Within a few months, balloons were being made with hydrogen, a gas only 1/14 as dense as air, so that each pound of hydrogen could carry aloft a payload of 13 pounds. Now gondolas went up carrying animals and, soon, men.
Within a year of the launching of the first balloon, an American named John Jeffries made a balloon flight over London with a barometer and other instruments, plus an arrangement to collect air at various heights. By 1804, the French scientist Joseph Louis Gay-Lussac had ascended nearly 4V2 miles and brought down samples of the rarefied air. Such adventures were made a little safer by the French balloonist Jean Pierre Blanchard, who, in 1785, at the very onset of the balloon age, invented the parachute.
This was nearly the limit for humans in an open gondola; three men rose to 6 miles in 1875, but only one, Gaston Tissandier, survived the lack of oxygen. He was able to describe the symptoms of air deficiency, and that was the birth of aviation medicine. Unmanned balloons carrying instruments were designed and put into action in 1892, and these could be sent higher and bring back information on temperature and pressure from hitherto unexplored regions.
In the first few miles of altitude rise, the temperature dropped, as was expected. At 7 miles or so, it was −55° C. But then came a surprise. Above this level, the temperature did not decrease; in fact, it even rose slightly.
The French meteorologist Leon Phillippe Teisserenc de Bort suggested, in 1902, that the atmosphere might have two layers: a turbulent lower layer containing clouds, winds, storms, and all the familiar weather changes (in 1908, he called this layer the troposphere, from the Greek for “sphere of change”); and a quiet upper layer containing sublayers of lighter gases, helium, and hydrogen (he named this the stratosphere, meaning “sphere of layers”).
Teisserenc de Bort called the level at which the temperature ceased to decline the tropopause (“end of change”), or the boundary between the troposphere and the stratosphere. The tropopause has since been found to vary from an altitude of about 10 miles above sea level at the Equator to only 5 miles above ut the poles.
During the Second World War, high-flying United States bombers discovered a dramatic phenomenon just below the tropopause—the jet stream, consisting of very strong, steady, west-to-east winds blowing at speeds up to 500 miles per hour. Actually there are two jet streams; one in the Northern Hemisphere at the general latitude of the United States, the Mediterranean, and north China; and one in the Southern at the latitude of New Zealand and Argentina. Th
e streams meander, often debouching into eddies far north or south of their usual course. Airplanes now take advantage of the opportunity to ride on these swift winds. But far more important is the discovery that the jet streams have a powerful influence on the movement of air masses at lower levels. This knowledge at once helped to advance the art of weather forecasting.
As human beings cannot survive in the thin, cold atmosphere of great heights, it was necessary to develop a sealed cabin, within which the pressures and temperatures of earth’s surface air can be maintained. Thus, in 1931, the Piccard brothers (Auguste and Jean Felix), the first of whom later invented the bathyscaphe, rose to 11 miles in a balloon carrying a sealed gondola. Then new balloons of plastic material, lighter and less porous than silk, made it possible to go higher and remain up longer. In 1938, a balloon named Explorer II went to 13 miles; and by the 1980s, manned balloons have gone as high as 23½ miles and unmanned balloons to more than 32 miles.
These higher flights showed that the zone of nearly constant temperature does not extend indefinitely upward. The stratosphere comes to an end at a height of about 20 miles, and above it the temperature starts to rise!
This upper atmosphere, above the stratosphere, containing only 2 percent of the earth’s total air mass, was penetrated in the 1940s, for further progress, by a new type of vehicle altogether—the rocket (see chapter 3).
The most direct way to read instruments that have recorded conditions high in the air is to bring them down and look at them. Instruments carried aloft by kites can easily be brought down, but balloons are less easily managed in this respect, and rockets may not come down at all. Of course, an instrument packet can be ejected from a rocket and may come down independently, but even it is hard to rely on. In fact, rockets alone would have accomplished little in the exploration of the atmosphere had it not been for a companion invention—telemetering. Telemetering was first applied to atmospheric research, in a balloon, in 1925 by a Russian scientist named Pyotr A. Molchanoff.
Essentially, this technique of “measuring at a distance” entails translating the conditions to be measured (for example, temperature) into electrical impulses that are transmitted back to earth by radio. The observations take the form of changes in intensity or spacing of the pulses. For instance, a temperature change affects the electrical resistance of a wire and so change. the nature of the pulse; a change in air pressure similarly is translated into a certain kind of pulse by the fact that air cools the wire, the extent of the cooling depending on the pressure; radiation sets off pulses in a detector; and so on. Nowadays, telemetering has become so elaborate that the rockets seem to do everything but talk, and their intricate messages have to be interpreted by rapid computers.
Rockets and telemetering, then, showed that above the stratosphere, the temperature rises to a maximum of some −10° C at a height of 30 miles and then drops again to a low of −90° C at a height of 50 miles. This region of rise and fall in temperature is called the mesosphere, a word coined in 1950 by the British geophysicist Sydney Chapman.
Beyond the mesosphere, what is left of the thin air amounts to only a few thousandths of 1 percent of the total mass of the atmosphere. But this scattering of air atoms steadily increases in temperature to an estimated 1,0000 C at 300 miles and probably to still higher levels above that height. It is therefore called the thermosphere (“sphere of heat”)—an odd echo of Aristotle’s original sphere of fire. Of course, temperature here does not signify heat in the usual sense: it is merely a measure of the speed of the particles.
Above 300 miles we come to the exosphere (a term first used by Lyman Spitzer in 1949), which may extend as high as 1,000 miles and gradually merges into interplanetary space.
Increasing knowledge of the atmosphere may enable us to do something about the weather some day and not merely talk about it. Already, a small start has been made. In the early 1940s, the American chemists Vincent Joseph Schaefer and Irving Langmuir noted that very low temperatures could produce nuclei about which raindrops would form. In 1946, an airplane dropped powdered carbon dioxide into a clo~d bank in order to form first nuclei and then raindrops (cloud seeding). Half an hour later, it was raining. Bernard Vonnegut later improved the technique when he discovered that powdered silver iodide generated on the ground and directed upward worked even better. Rainmakers, of a new scientific variety, are now used to end droughts—or to attempt to end them, for clouds must first be present before they can be seeded. In 1961, Soviet astronomers were partially successful in using cloud seeding to clear a patch of sky through which an eclipse might be glimpsed.
Other attempts at weather modification have included the seeding of hurricanes in an attempt to abort them or at least to moderate their force (seeding of clouds in order to abort crop-damaging hailstorms; dissipating fogs, and so on). Results in all cases have been hopeful at best, but never a clear-cut success. Furthermore, any attempt at deliberate modification of weather is bound to help some but hurt others (a farmer might want rain, while an amusement park owner does not), and lawsuits are an obvious side effect of weather modification programs. What the future holds in this direction is, therefore, uncertain.
Nor are rockets for exploration only (although those are the only uses mentioned in chapter 3). They can, and already have, been turned to the everyday service of humanity. In fact, even some forms of exploration can be of immediate practical use. If a satellite is rocketed into orbit, it need not look only away from our planet; it can turn its instruments upon Earth itself. In this way, satellites have made it possible, for the first time, to see our planet (or at least a good part of it at any one time) as a unit and to study the air circulation as a whole.
On 1 April 1960, the United States launched the first weather-eye satellite, Tiros I (Tiros standing for “Television Infrared Observation Satellite”). Then Tiros II was launched in November and, for ten weeks, sent down over 20,000 pictures of vast stretches of the earth’s surface and its cloud cover, including pictures of a cyclone in New Zealand and a patch of clouds in Oklahoma that was apparently spawning tornadoes. Tiros III, launched in July 1961, photographed eighteen tropical storms, and, in September, showed hurricane Esther developing in the Caribbean two days before it was located by more orthodox methods. The more sensitive Nimbus I satellite, launched on 28 August 1964, could send back cloud photographs taken at night. Eventually hundreds of automatic picture transmission stations were in operation in scores of nations, so that weather forecasting without satellite data has now become unthinkable. Every newspaper can run a cloud-pattern photograph of the United States daily, and weather forecasting, while still not mathematically certain, is not the crude guessing game it was only a quarter-century ago.
Most fascinating and useful is the manner in which meteorologists can now locate and track hurricanes. These severe storms have become far more damaging than in the past, since beach fronts have become much more built up and populous since the Second World War, and were there not a clear knowledge of the position and movements of these storms, there is no question but that loss of life and property would be many times what it is now. (In respect to the usefulness and value of the space program, satellite-tracking of hurricanes alone pays back far more than the program costs.)
Other earthbound uses of satellites have been developed. As early as 1945, the British science-fiction writer Arthur C. Clarke had pointed out that satellites could be used as relays by which radio messages could span continents and oceans, and that as few as three strategically placed satellites could afford world coverage. What then seemed a wild dream began to come true fifteen years later. On 12 August 1960, the United States launched Echo I, a thin polyester balloon coated with aluminum, which was inflated in space to a diameter of 100 feet in order to serve as a passive reflector of radio waves. A leader in this successful project was John Robinson Pierce of Bell Telephone Laboratories, who had himself written science-fiction stories under a pseudonym.
On 10 July 1962, Telstar I was
launched by the United States. It did more than reflect, it received the waves, amplified them, and sent them onward. By use of Telstar, television programs spanned the oceans for the first time (though that did not in itself improve their quality, of course). On 26 July 1963, Syncom II, a satellite that orbited at a distance of 22,300 miles above the earth’s surface, was put in orbit. Its orbital period was just 24 hours, so that it hovered indefinitely over the Atlantic Ocean, turning in synchronization with the earth. Syncom III, placed over the Indian Ocean in similar synchronous fashion, relayed the Olympic Games from Japan to the United States in October 1964.
A still more sophisticated communications satellite, Early Bird, was launched 6 April 1965; it made available 240 voice circuits and one television channel. (In that year, the Soviet Union began to send up communications satellites as well.) By the 1970s, television, radio, and radiotelephony had become essentially global, thanks to satellite relays. Technologically, Earth has become “one world,” and those political forces that work against that inescapable fact are increasingly archaic, anachronistic, and deadly dangerous.
The fact that satellites can be used to map Earth’s surface and study its clouds is obvious. Not quite so obvious but iust as true is the fact that satellites can study snow cover, glacier movements, and geological details on a large scale. From geological details, likely regions where oil may exist can be marked off. Crops on the large scale can be studied, as forests can; and regions of abnormality and disease can be pinpointed. Forest fires can be spotted, and irrigation needs located. The ocean can be studied, as can water currents and fish movements. Such earth resources satellites are the immediate answer to those critics who question the money spent on space in the face of great problems “right here at home.” It is often from space that such problems can best be studied and methods of solution demonstrated.
