By Lyman Spitzer Jr. *37 *38: Travel Through Space!

Mr. Spitzer is chairman of the Department of Astronomy at Princeton and occupies the Charles A. Young Professorship established by the Class of 1897. The present article is adapted from a Baker Lecture delivered in Toledo last month under the auspices of the Princeton Alumni Association of Northwestern Ohio.

For more than three thousand years man has gazed at the planets and stars, observed their motions, and speculated as to their nature. During most of this time, observations have been limited by the imperfection of the human eye. Since the Renaissance, however, the eye has been aided by. Telescopes of increasing size and power. At the present time, with photographic plates and photo-electric cells recording and analyzing starlight, the human eye is no longer of primary importance, and its imperfections are no longer an obstacle.

With the perfection of super-telescopes and ultra-sensitive electronic detectors, another barrier now confronts the astronomer. Now it is the atmosphere itself, the shielding life-bearing ocean of air, that hinders the modern search for truth in the heavens. The air has two effects on radiation from the stars. First, much radiation gets absorbed before it reaches the earth’s surface. Second, what rays do get through are bent and distorted, so that an accurate, detailed picture is difficult to obtain.

The absorption of radiation deprives the astronomer of much valuable information. For example, the sun sends out not only visible light, which does get through the atmosphere, but also a variety of other types of radiation – X-rays, ultraviolet light, infra-red heat rays, and radio waves. Very little of this “invisible light” reaches the earth’s surface.

The bending of the visible light that does get through is also a very serious obstacle in astronomical research. The familiar twinkling of a star is produced by this bending of starlight back and forth, as a result of winds. Suppose one takes a photograph of the planet Mars through a very large telescope. Because of the air motions, each portion of the planet will seem to dance about, and the photograph is blurred. Because of this same effect, if an Aqualung diver looks up through the water at objects in the sky, everything will seem blurred if there are waves at the surface. This blurring is a very serious limitation on the performance of a big telescope. For example, because of this effect the 200-inch telescope practically never has the full effective resolving power it would have in a vacuum.

We see that the atmosphere now constitutes a serious barrier to astronomy, because of the absorption and bending of radiation. The one simple way to surmount this obstacle is to put a telescope above the atmosphere. This proposal may seem visionary, but I am convinced that an observatory several hundred miles up is the next major goal of astronomical instrumentation.

Already there have been several attempts in this direction. Big rockets have risen some 50 to 100 miles above the earth’s surface carrying small telescopes to photograph the ultra-violet light which does not penetrate the atmosphere. Measures of the solar spectrum in ultra-violet light have yielded very interesting results. The program has been somewhat limited, since rockets are above the atmosphere for only a few minutes, and they do not carry much instrumental equipment. Besides, all the equipment gets smashed when the rocket falls down to earth again. High-altitude research with great rockets was at first a rather discouraging program; when the rockets were in the experimental stage, quite a few of them blew up before they got very high. Now the use of rockets is becoming a standard technique for making observations from great heights.

These high-altitude rockets constitute only a preliminary step. The logical solution to the astronomer’s problem is to send up an observatory that will stay permanently above the atmosphere. If a rocket some five hundred miles up reaches a speed of five miles per second, parallel to the earth’s surface, and then turns off its rocket blast, the laws of physics show that such a rocket will then go round and round the earth practically indefinitely. Such an object would be going fast enough so that the centrifugal force would just balance the centripetal force, and the object would revolve in a permanent orbit around the earth, exactly as the moon does. In this way it would be possible to give the earth a new satellite 500 miles up would go around the earth in an hour and a half, and could command a fine view of the earth’s surface.

There have been many studies of artificial satellites, how they could be launched, and how they could be used. Most of these studies are secret and cannot be discussed openly. Enough has been published, however, to show that existing rocket techniques are sufficient to carry an appreciable weight up 500 miles and give it the necessary speed of five miles per second in the right direction. This speed may seem high, but is only five times as fast as the German V-2 rockets, used by the Germans to bombard London during the last war.

Actually five miles a second, the speed needed for an artificial satellite, is near the limit of what can be reached with the familiar type of rocket. Such rockets obtain their energy from a mixture of oxygen and alcohol, or of some similar chemical, which burns fiercely, shooting a jet of hot gases out the back. Under extreme conditions, the jet can shoot out with a speed of a mile per second or so, when the rocket is motionless. By the time all the fuel is burned the rocket then has a top speed of several miles per second. To give a rocket a speed of five miles a second, 500 miles up, is not easy but it seems to be possible.

Such an enterprise is not cheap. If it were only the astronomers that were interested in getting out into space, we might have to wait a long time before an artificial satellite were launched. Fortunately or unfortunately, the military have a very live interest in bigger and bigger rockets with longer and longer range. An artificial satellite may well be the natural result of this research.

Let us suppose than artificial satellite has been sent up, complete with telescopes, detectors, and other scientific paraphernalia. For what purpose would this observatory in space be useful? Two examples may be given. The first example is a type of important astronomical observation which would be possible only from such a satellite observatory. The second example is a type of further exploration, which would not be possible without a satellite as a base.

As our first example we may take the study of nebulae. One type of nebula is essentially a cloud of gas drifting about between the stars. These clouds are so vast that it takes a light ray some ten to a hundred years to travel from one side to the other. Since light travels 186,000 miles a second, or six trillion miles every year, it is clear that these clouds are enormous. The different parts of a typical nebula are moving and twisting at very great speeds, a few miles a second, and possess a complicated and intricate structure. Some clouds appear bright, usually because of light reflected from a star near the cloud. If there is no bright star nearby, the nebula will appear dark, absorbing the light from more distant stars.

These gaseous nebulae are of much interest to astronomers, as they are believed to be the birthplace of new stars. Most stars are thought to be about five billion years old, but some stars are relatively young, only a few million years old. To an astronomer a star one million years old is practically a newly born baby! The gas in these nebulae is believed to condense relatively slowly, forming many new stars.

Our present earth-bound telescopes do not have nearly enough power to see in detail how stars are being formed in these gaseous nebulae. A telescope above the atmosphere would be an ideal tool for studying the structure of these clouds. For example, a 40-inch telescope on a satellite could photograph a nebula clearly with ten times the magnification practical with the existing 200-inch. If so great a magnification is attempted with the 200-inch, on the earth’s surface, the picture is blurred because of motions in the air above the telescope. In the almost perfect vacuum between the stars light travels in straight lines, and enormous magnifications are possible with a telescope of sufficient size.

SPIRAL GALAXY: Billions of stars form this nebula in the Big Dipper. Astronomers want to know how and why these vast island universes arise.

Princeton Alumni Weekly. March 18, 1955.

 There is another type of nebula also. This is a gigantic aggregation of many billion stars, each star about as bright as the sun. These stellar systems, which are usually shaped like discs or wheels, often show spiral arms, reaching from the center out to the periphery. How did these island universes arise? Why are they spiral in form? Why are most of them revolving, with a period of revolution about a hundred million years long? What is their ultimate fate? These questions are very difficult to answer now, restricted as we are to earthbound telescopes. In another few generations perhaps we shall see gigantic telescopes, even greater than any now on earth, scanning the sky from a satellite observatory. Who knows what secrets of the universe we may then discover!

Observation would be only one function of a satellite. Another function would be to serve as a base for further travel. Once man has established himself in a satellite all the planetary system is easily within reach. A new type of rocket can then be provided to travel anywhere in the solar system within a time of several months. Let us see how this is possible.

Every reader of science fiction knows that in the vacuum above the atmosphere only a rocket can provide continual acceleration. People are sometimes surprised that a rocket works in a vacuum. Actually, the rocket works by the simple principle of recoil. When a gun is fired, the bullet goes on way, the gun gets pushed in the opposite direction by recoil. No air is needed. The rocket, instead of shooting out bullets, shoots out a stream of gas. However, the basic limitation of a rocket is that its maximum speed, when its fuel is all gone, cannot exceed more than a few times the initial backwards speed of these gases, measured when the rocket is at rest. Since the gas speed is at most one or two miles a second, a rocket speed of five miles a second is about the most that can be achieved by recoil from hot gases. To get to Mars in a few months, a speed of 15 miles a second is needed, quite outside the range of this kind of rocket.

Fortunately, for interplanetary travel an entirely different kind of rocket, an electrical rocket, is also possible. IN this rocket individual atoms are accelerated by electricity, and shot out backwards at speeds of about a hundred miles a second. The recoil from these atomic bullets then pushes the rocket ship in the other direction. The electrical techniques required for accelerating individual atoms have been worked out by physicists for their atom-smashing machines. With a backwards speed of 100 miles a second for the atomic bullets, about a hundred times the speed attainable with simply a jet of hot gas, a rocket can easily reach a speed of several hundred miles a second.

Some source of power is needed to generate the electricity. Atomic power here comes to our aid. A small uranium reactor could provide the energy for about a century before the uranium were used up, so enormous is the reserve of power locked up in the uranium atom.

There is, of course, a fly in every ointment, and the proposed electrical rocket suffers from two very serious disadvantages. The first disadvantage is the deadly radiation emitted by uranium atoms when they are releasing their enormous power. A single atomic bomb can destroy all life in thousands of square miles by this lethal radiation. Unless some precautions were taken, the crew of the electrical rocket would never live to reach the Moon, let alone Mars.

The many feet of solid concrete needed to absorb this radiation would weigh too much for an interplanetary ship. Fortunately, another protection is possible, and that is simply distance. The crew can stay in a gondola, towed at a safe distance from the main atomic power plant. However, the distance required for complete safety turns out to be fifty miles! On the earth this is a big distance, but in the infinite reaches of space fifty miles is scarcely any distance at all, and so great a separation between the gondola and the uranium reactor does not seem entirely impractical, surprising though it may be at first sight.

Another disadvantage of such a rocket is that it would have a very low acceleration. In a ship weighing ten tons, for example, the amount of electrical power that can be generated is not very great, only a few thousand kilowatts, and the acceleration developed by such a ship would be almost absurdly low. Starting from rest, floating in space, it would take this ship a full hour to attain a speed of 25 miles per hour, as compared with the few seconds needed for an automobile to reach this peed. Unlike an auto, however, an electrical rocket could continue the same acceleration for many weeks. After a month the speed would be up to four miles a second.

Evidently such a rocket is worse than useless on the earth’s surface; it could never lift its own weight up with the recoil from its atomic bullets. Once such a rocket were hauled up to a satellite, perhaps taken up in pieces and assembled there, it would be the ideal vehicle for cruising about the solar system. It could not itself land on any planet, since if it did it could not take off again, but it could go to any planet in the planetary system and circle round it, perhaps waiting there while men landed on the planet’s surface and came back up with more conventional rockets. Somewhat like the Flying Dutchman of the legend, this ship would sail forever back and forth in the emptiness of interplanetary space, never touching solid ground.

Where would such a ship take us? There are nine planets circling around the sun, and many of these have natural satellites. Which would be the most interesting to visit?

The closest astronomical body to the Earth is of course, the Moon. As a goal for space travel, the Moon has the one advantage that it is only 250,000 miles away – a trip of 20 days even at the relatively sedate speed of 500 miles per hour. However, the Moon is devoid of either air or water. A more complete desert can scarcely be imagined, and as far as we know there is no life there.

Image

This painting by Howard Russell Butler ’76 shows Mars as it might appear from its own satellite. Astronomers do not agree, of course, whether there are canals on Mars. Lent by H.R. Butler Jr. ’20, the painting hangs in the Observatory’s Class of 1920 Room.

Howard Russell Butler ’76

 Of the nine planets, the innermost is Mercury, which is too close to the Sun for comfort. On the sunlit side of Mercury the rocks are at a temperature of 650 degrees Fahrenheit, hot enough to melt lead. Since Mercury has no atmosphere as far as we know, it must be as arid and lifeless as the Moon.

The next planet out is Venus. Since this planet is about the same size as the Earth, and has an extensive atmosphere, it would be very interesting to visit. Since Venus is always covered with dense clouds of some unknown substance we have no idea what the surface is like. Perhaps some day we can travel thirty million miles and find out.

The most alluring target would seem to be Mars, the next planet beyond the Earth, and some forty million miles away at closest approach. Like Venus, this planet has a respectable atmosphere. The surface can be seen through occasional clouds and we can tell that water is present from the white frost that covers the ground at each pole when it is winter there. There is so little oxygen on Mars that an explorer would have to wear an oxygen mask. The temperature is lower than on Earth, but rises as high as 50 degrees Fahrenheit on the Martian equator during the day. We suspect that plant life may exist on Mars, since the surface turns green in summer and brown in winter. Perhaps there is animal life too, possibly even intelligent life.

The possibility that intelligent beings may exist on Mars bring us to one of the great controversies in astronomy, the so-called canals of Mars. Some observers claim to have seen a network of sharp dark lines on the surface; others, equally reliable, cannot see any such thing. Photography of such sharp features on a distant planet is very difficult; and we simply do not know whether the canals are there or not. Possibly a trip to Mars will find the planet criss-crossed with irrigation ditches to distribute and conserve the scarce and precious water.

Beyond Mars are two giant planets, Jupiter and Saturn, several hundred million miles away. These gigantic but cold, lifeless and unfriendly planets have atmospheres thousands of miles thick, composed of hydrogen, methane, and ammonia. The surface temperature is some 200 degrees below zero Fahrenheit. While these planets are unattractive places, some of their satellites are of respectable size, and have visible surface and some atmosphere; they would provide interesting exploration. There are the planets beyond Saturn, but they are even colder and less attractive.

Evidently the climbing of Everest, the deep dive sin the ocean and the many Antarctic expeditions do not mean the end of our era of exploration. Above us, beyond our atmosphere there are new worlds to explore and to possess. The exploration will not be easy. There are many practical problems which have not been mentioned here. Perhaps the greatest problem is the human one: how would man behave in the lonely emptiness of interplanetary space, with the blazing hot sun in one direction and black, black night all around, with earth and home millions of miles away, with even the main power plant at the far end of a slender wire fifty miles long? I believe that the spirit of man can meet this challenge as it has met others.

How far in the future is the satellite? How far away is interplanetary travel? Any country able and willing to spend billions of dollars on the enterprise could, I believe, achieve a successful Martian expedition in ten years. The project does not seem urgent enough to warrant such an all-out effort, and travel in space will doubtless come more slowly. How soon a successful voyage to the Moon or Mars may be expected is anyone’s guess, but I would not be too surprised if such a trip were made within my own lifetime.

This was originally published in the March 18, 1955 issue of PAW.