4 - A New Look At The Planets: Distance In Our Solar System

The study of the planets reached a peak in the nineteenth century and then, towards its end, seemed to die down. Other subjects began to interest astronomers much more. There was nothing left, it would appear, for twentieth century astronomers to do about planets.

If, indeed, the planets seemed worked out by 1900, that is not surprising. After all, astronomers had been dealing with them for over 2,000 years, and what more could be left?

To be sure, the ancients had got off on the wrong foot. The Greeks had worked out careful and interesting theories concerning the motions of the planets as early as 350 B.C. They thought, however, that all the planets revolved about the Earth.

In 1543, the Polish astronomer Nicolaus Copernicus published a book which argued that the planets revolved about the sun. He also insisted that the Earth was one of the planets, too, and that it also revolved about the sun. (The moon, however, revolved about the Earth in the new system as well as the old.)

In 1609, the German astronomer Johannes Kepler worked out the fact that the planets revolved about the sun in ellipses, which resembled slightly flattened circles. Then, in 1683, the English scientist Isaac Newton showed how the sun and its planets (the solar system) were held together by gravitational force. All the motions of the planets could be worked out quite accurately by means of a clear formula which Newton presented.

Meanwhile in 1609, the Italian astronomer Galileo Galilei had devised a small telescope which he pointed at the heavens. At once, he saw numerous things no one had ever seen before. He discovered that there were spots on the sun, for instance, and that there were mountains on the moon. He also found that Jupiter had four moons that moved about it just as our moon goes about the Earth.

For a hundred and fifty years after Newton, astronomers worked hard to make new discoveries about the solar system with ever-improving telescopes. They showed more and more of its workings to be explained by Newton's simple law of gravitation.

New bodies were discovered. Saturn was found to have moons like Jupiter. It was found to have rings, thin circles of light, about its equator. Even a new planet was discovered in 1781 by the German-English astronomer William Herschel. It was found to circle the sun at a distance far beyond Saturn and it was named Uranus.

The climax of all this came in the middle of the nineteenth century. The motions of Uranus about the sun had been followed for over half a century and they did not quite follow Newton's law. This was very puzzling and upsetting to the astronomers of the time.

One or two of them wondered if there might not be a planet beyond Uranus; one that had not yet been discovered. Perhaps this unknown planet was exerting a gravitational pull on Uranus, a pull that wasn't being taken into account.

In 1845, two young astronomers actually tried to calculate where such a planet ought to be located if it were to produce just enough effect to make Uranus move as it did. One was an Englishman, John Couch Adams, and the other a Frenchman, Urbain Jean Joseph Leverrier. Neither knew the other was working on the problem, but both ended with just about the same answer.

When telescopes were turned on the spot where they said the planet ought to be, it was found! It was a new giant planet far beyond Uranus and it was given the name Neptune.

It was the greatest triumph in the history of astronomy and the great climax of the study of the solar system. There seemed nothing left to do among the planets that could possibly equal the drama of 1845.

This seemed all the more true as by the middle of the nineteenth century, the solar system was beginning to seem a small and insignificant thing anyway. Astronomers' attention was beginning to switch more and more to the distant stars.

In the 1830s, they had developed methods for determining the distance of the nearer stars. By 1860, methods were devised to analyze starlight. Astronomers could tell how hot a star was, whether it was moving towards us or away from us, even the materials of which it was made.

With all these exciting discoveries being made about the stars, fewer and fewer astronomers were left to concern themselves with the little worlds of our own sun's family.

The solar system wasn't entirely deserted, of course. Some new discoveries were made that were pretty exciting.

In 1877, for instance, Mars and Earth happened to be in those parts of their orbits that brought them only thirty-five million miles apart. That is as close together as they ever get. With Mars that close, the American astronomer Asaph Hall discovered that it possessed two tiny moons.

At the same time, an Italian astronomer, Giovanni Virginio Schiaparelli, found straight dark markings on Mars, which he called "canali" an Italian word for "channels." The word was mistranslated into English as "canals."

This made a great difference. Channels are merely narrow bodies of water, but canals are man-made. If Mars had "canals" that would mean there was intelligent life on it. Naturally, this excited people and there was considerable discussion about it in the newspapers.

Astronomers, however, did not get overly excited about the matter. Most of them couldn't see the markings that Schiaparelli had seen, and they suspected that even if they did, there was bound to be some explanation for it other than the presence of intelligent life.

As the twentieth century opened, however, one man brought the question of the canals to the fore. He was an American astronomer named Percival Lowell, who came of an old Boston family and had considerable money.

Using his private fortune, Lowell built an elaborate astronomical observatory in Arizona where the clean desert air and the absence of clouds made it easy to observe the heavens. This observatory was opened in 1894 and for fifteen years Lowell concentrated on watching Mars.

He was sure that Mars was covered with a fine network of straight lines. He made elaborate maps of these lines and was convinced they were indeed the work of intelligent beings. Many people who weren't astronomers were convinced by him. Most astronomers, however, remained skeptical. They insisted that whatever Lowell saw must be optical illusions.

Lowell created a stir in another direction as well. He was not satisfied that Neptune was indeed the farthest planet from the sun. Even after Neptune's gravitational pull was taken into account, Uranus still didn't travel in quite the path one would consider correct from gravitational theory.

Lowell insisted there was yet another planet beyond Neptune and that it, too, pulled on Uranus-though more weakly because it was farther away. He searched and searched for this "Planet X" but when he died in 1916, he had not yet found it.

The trouble is that the more distant a planet, the smaller and dimmer it appears and the harder it is to distinguish it from the stars that can be seen at the same time. Planet X was probably so far away that a telescope good enough to see it would also make out crowds of faint stars. The problem would then become that of telling one dot of light which was a planet from a million other dots of light which were stars.

Even if calculations were to tell an astronomer about where such a planet might be, it would still have to be picked out from among the many stars in the same neighbourhood.

After Lowell died, the observatory he had built kept on going and occasionally astronomers who worked there would do a bit of looking for Planet X. In 1929, the search went back into high gear when a twenty-three-year-old youngster, Clyde William Tombaugh, joined the staff.

Tombaugh's family were poor farmers who could not afford to send their son to college. Tombaugh, however, was fascinated by astronomy. He read all he could on the subject and when he was twelve years old he even built a small telescope for himself out of material he managed to get his hands on. By the time he was twenty, he had built a neat nine-inch telescope that worked very well indeed.

With his homemade telescope he studied Mars and managed to observe a few canals now and then. He grew interested, wrote to Lowell Observatory in the hope of getting a job, and got one.

Tombaugh set to work searching for Planet X. It might be just a point of light (if it were there) like any star, but it was different from a star in one important way. Planet X moved about the sun and that meant that it shifted its position in space.

Planet X was very far from the sun, of course, so that it moved slowly. That slow motion was even slower in appearance to astronomers on Earth because the planet was so far away. Even so, the motion could be spotted easily after two or three days in comparison with the surrounding stars, which didn't move at all!

Tombaugh's technique, then, was to take a photograph of a particular tiny portion of the sky. Then, two or three days later, that same tiny portion of the sky would be photographed again. If the only thing on the photographs were stars then nothing at all would have changed position in the slightest. All Tombaugh would have to do would be to check whether any of the tiny star-images on one plate was in a different position when compared to the other.

That was easier said than done. Each photograph contained, on the average, 160,000 stars, and it was just impractical to go over all of them. It would take too much time, and, unless Tombaugh had a tremendous stroke of luck, the chance of finding Planet X would be very small.

But Tombaugh did the following. The two photographic plates were placed side by side under a kind of viewer through which Tombaugh could look and see only one. He could adjust a tiny mirror that would enable him to see first the photograph on the left, then the one on the right.

He could adjust the two photographs so that both would be in exactly the same position. Then, if he flipped the lever that adjusted the mirror, he would view the photographs left, right, left, right, left, right, over and over. If both were properly adjusted, the photographs would be so alike that he wouldn't be able to tell them apart.

But if Planet X were somewhere on the plate, it would change position, for it would have moved during the several days between the taking of the first photograph and the second. As Tombaugh flipped the lever, the image of Planet X would shift back and forth, back and forth. All he had to do then was to adjust the photographs, flip the lever, and watch for any point that blinked. He could ignore all the thousands upon thousands of other points.

Even that wasn't very easy. He had to study the plates one tiny bit at a time. Sometimes he had to flip the photographs back and forth for six or seven hours before he could study all parts of them and be convinced that no point blinked. Then, too, sometimes there was a moving point but it was an asteroid, one of the tiny planets that moved about the sun between the orbits of Mars and Jupiter.

Such asteroids were much closer to the sun and to the Earth than Planet X was. This meant that they moved more quickly and that there was a much larger shift against the stars. If Tombaugh found a spot that moved too far, that was as bad as one that didn't move at all. It couldn't be Planet X.

In late January 1930, Tombaugh photographed the stars in a section of the constellation Gemini. For nearly a month he kept examining those photographs and on February 18, he caught a shift that was so small it had to be a very distant planet. For weeks he kept taking photographs of that spot and watching the way that the little dot moved until there was no doubt. The path of the object agreed with what would be expected of a planet beyond Neptune.

The discovery was announced on March 13, 1930, the day when Percival Lowell would have celebrated his seventy-fifth birthday if he had lived. The planet was named Pluto, partly because Pluto was the god of the dark underground in the Greek myths and the new planet was so far from the sun that it received less light than any other planet. Partly, also, the name was chosen because the first two letters were the initials of Percival Lowell.

 

At just about the time the discovery of the new planet Pluto was announced, astronomers were getting ready for a huge international project involving the solar system.

Ever since Kepler had worked out the elliptical orbits of the planets in 1609, astronomers had been able to draw an exact model of the solar system. What was lacking, though, was any notion of the actual size of this model. If only they could get the exact distance of any planet of the solar system, they could work out the distances of all the rest from the model. The closest planets were Mars and Venus. Mars sometimes was as close as thirty-five million miles from the Earth and Venus was even closer, sometimes only twenty-five million miles away.

If either Mars or Venus were viewed at the same time from two widely separated observatories on Earth, the planet would be seen against two slightly different backgrounds. That is, it would be seen from two different angles against the stars.

From the distance between the two observatories and from the size of the shift in the position of the planet, the distance of the planet could be calculated. Then the distance of all the other planets could be calculated, too. In particular, the distance of the sun from the Earth could be calculated.

There were problems, though. When Venus was closest to the Earth, it was more or less between the sun and the Earth and it couldn't be seen. Sometimes Venus passes exactly between the sun and the Earth and then it can be seen as a dark spot against the sun's brightness. If the moment at which Venus moves in front of before the sun is measured from two widely separated observatories, then the distance of the planet can be calculated.

Unfortunately, these "transits" don't happen often. Not a single transit will take place in the twentieth century, for instance. Another problem is that Venus has a thick atmosphere, which blurs the exact moment at which it begins to move before the sun.

Mars makes a better target, therefore, even though it is farther away and never passes in front of the sun. Using Mars careful astronomers were able to determine the size of the solar system pretty well. The distance of the sun was placed at somewhere between ninety-three and ninety-five million miles from the Earth.

Just the same, Mars has a thin atmosphere and it shows up as a small globe in the telescope so that its exact position is a little fuzzy. What is needed is a planet even closer than Mars or Venus and one that is so small it has no atmosphere and looks like a mere dot of light in the telescope.

Unfortunately, there is no such planet. Or is there?

Tiny planets do exist. There are the asteroids that circle in orbits between Mars and Jupiter. The largest is less than 500 miles in diameter (as compared with 8,000 miles for the Earth) and it was discovered on January 1, 1801. Most of those discovered afterwards were less than 100 miles across and there may be many thousands that are only a couple of miles across and are too dim to see.

In 1896, a German astronomer, G. Witt, discovered a new asteroid, which happened to be number 433. Another new asteroid wasn't much, but when Witt came to calculate its orbit, in 1898, he received a shock. Unlike all the other asteroid orbits known, this new one slipped inwards, so that much of the time the new asteroid was closer to the sun than Mars.

Ordinarily asteroids receive female names, but Witt named this one Eros, and ever since then asteroids with unusual orbits get male names.

The orbit of Eros is such that at long intervals it can approach the Earth much more closely than either Mars or Venus. In 1931, it was scheduled to pass within sixteen million miles of Earth, almost its minimum distance.

Astronomers thought it would be wonderful if Eros could be observed from different places. It was just a dot of light and would shift its position far more than either Mars or Venus. There would be no trouble making an accurate measurement of that shift.

The greatest international astronomical project ever attempted up to that time was set up. Fourteen observatories in nine different countries took part. Seven months were spent on the project and nearly three thousand photographs were taken. The position of Eros was carefully checked on each one of them.

It took ten years for the proper calculations to be made under the supervision of the English astronomer Harold Spencer-Jones. Finally, the results were announced. The solar system had been cut to size more accurately than ever before. The average distance of the sun from the Earth was found to be 93,005,000 miles.

 

The twentieth century saw a number of discoveries of new small members of the solar system. When the century opened, only five satellites of Jupiter were known, but between 1904 and 1951 seven more were discovered. All were small and all were distant from Jupiter. Astronomers feel they are asteroids that had managed to move too close to Jupiter and had got caught in its gravitational field.

The planet Uranus had four known satellites and Neptune one in 1900, but in 1948, a fifth satellite of Uranus, smaller than the rest and closer to the planet than any of the others, was discovered by the Dutch-American astronomer Gerard Peter Kuiper. It was named Miranda. The next year, 1949, Kuiper also discovered a second satellite of Neptune. It was small and circled Neptune at a great distance. He named it Nereid.

Nine newly discovered satellites had thus joined the lists of known members of the solar system between 1900 and 1966, bringing the total number to 31.

Saturn was the only one of the outer planets to have received no addition to its satellite family. It had nine known satellites and the ninth, Phoebe, had been discovered in 1898 by the American astronomer William Henry Pickering. Now, nearly seventy years had passed and nothing new had been added. To be sure, in 1905, Pickering had reported a tenth, which he named Thernis, but that seems to have been a mistake. No one has ever seen it since.

But Saturn has something other planets do not have. It has a set of thin, flat rings that circle the planet at its equator. They are composed of innumerable small fragments which may be no more than pebble-size and which may be largely ice.

Saturn's poles are tipped towards and away from the sun (just as Earth's poles are) and that means the rings are tipped, too. We see them either a little from above or a little from below, depending on where Earth and Saturn are in their orbits. Whether we see them from above or below the brightness of the rings makes it hard to see anything else that may be very near Saturn.

As Saturn's rings shift from a top view to a bottom view, however, there comes a short period, once every fourteen and a half years, in which we see the rings edge-on. The rings are so thin that they become invisible when seen from the edge and the area close to Saturn can then be studied.

In December 1966, the rings were edge-on and a French astronomer, Audoin Dollfuss, photographed the regions near the planet. He studied the photographs and was pleased to find a tenth satellite. It was closer to Saturn than any of the others and lay just outside the rings. Edge-on time was about the only moment when it could be seen easily. Because it was the first satellite, counting out from Saturn, and the last satellite to be discovered, Dollfuss named it Janus after the Roman god of first and last things.

New asteroids were also discovered in the twentieth century and some of them were even more remarkable than Eros. In 1906, the German astronomer Max Wolf discovered the 588th asteroid. It was odd, indeed, for its orbit was almost exactly that of Jupiter. He therefore gave it the masculine name of Achilles. A whole group of asteroids has been found in Jupiter's orbit since then, some moving about 480 million miles behind Jupiter and some about 480 million miles ahead of it. (Gravitational theory explains that such a situation is a stable one.) They were all given names of characters from Homer's poem about Troy and are called the "Trojan asteroids."

In 1920, the German astronomer Walter Baade discovered what is, even today, the farthest of all known asteroids. Its orbit carries it far beyond Jupiter and takes it nearly as far from the sun as Saturn. He named it Hidalgo.

Then in 1948, Baade (who by now had become an American citizen) discovered the satellite that approaches most closely to the sun. This is Icarus (named after a character in the Greek myths who flew through the air on feathered wings held together by wax but who flew so close to the sun that the wax melted so that he dropped to his death). Icarus approaches within seventeen million miles of the sun. This is considerably closer than the approach of Mercury, the innermost large planet.

The orbit of Icarus is such that it can approach within four million miles of Earth. This is a much closer approach than even that of Eros, so that Icarus is one of the group of asteroids now called "Earth-grazers." About half a dozen of these are now known, most having been discovered in the 1930s.

In 1937, the German astronomer Karl Reinmuth detected an asteroid which he named Hermes. Its orbit, when calculated, showed that it could approach as closely as 200,000 miles. It would then be even closer than the moon.

 

Yet none of all these discoveries of the first thirty years of the twentieth century seemed to make the solar system very exciting.

They lacked drama. The discovery of Pluto was the result of years of hard work, instead of the product of one great stroke. The work on Eros just resulted in a slight adjustment of the calculated distance of the sun. The discovery of a few small satellites and asteroids didn't seem like much.

The great excitement was going on far beyond the solar system. It was found that all the hundred billion stars of the Milky Way (of which the sun is one) make up a huge collection called the galaxy. Far outside that collection are many millions of other galaxies.

In the 1920s, moreover, it was discovered that the distant galaxies were moving away from us. The farther away they were, the faster they were moving. The whole universe was expanding.

It was a brand-new vision of endless space that broke on the eyes of the astronomers as the twentieth century progressed. There seemed little to compare with that in the solar system.

There were some interesting puzzles in the solar system, to be sure. There was still the question of canals on Mars. Were those marks really canals? Was there intelligent life on Mars? What lay under the mysterious blanket of clouds that hid the surface of Venus? What was on the other side of the moon, the side men never saw.

These were fascinating problems because they involved bodies that were so close to us, but there was no way astronomers could answer them. It seemed there would never be any way.

Yet although astronomers didn't realize it at the time, the 1920s and 1930s saw two enormous breakthroughs which were to revolutionize completely the study of the solar system in ways undreamed of in the nineteenth century.

One of these breakthroughs took place in 1926, when a professor at Clark University in Worcester, Massachusetts, fired a rocket into the air. This event, and what followed, will be considered in the next chapter. The other event, which took place in early 1932, will be described now.

 

One of the problems that faces astronomers is the fact that the Earth has an atmosphere. Naturally, people need the atmosphere to breathe; even astronomers do. But it is a problem when it comes to observing the heavens.

The atmosphere absorbs some of the light from the stars and planets. It curves the light that reaches it from objects near the horizon and makes those objects appear higher in the sky than they really are. There are temperature differences that cause light beams to waver, so that it is hard to get sharp pictures. There is often haze and smoke in the air and sometimes clouds that blank out everything.

Then, too, as human population grows, cities grow too and become more and more lit up at night. This light is scattered by the air and it becomes harder than ever to watch the sky. Astronomers can scarcely find sites high enough on the mountains and far enough from cities to make it possible to observe the skies in peace.

But need astronomers be confined to studying the sky by ordinary light?

Ordinary light is only a small section of a huge band of radiation, and it seems quite likely that stars and planets send out other radiations in this band. Unfortunately, the other sections of the band of radiation can't be detected by eye so that special instruments are needed to receive them. Furthermore, Earth's atmosphere, which lets ordinary visible light through, stops most other sections of the radiation band cold.

All these different types of radiation, including visible light, act as though they are made up of tiny waves. The difference between one type of radiation and another is in the size of these waves. When the waves are very long, we have what we call "radio waves." These were discovered in 1888 by a German physicist, Heinrich Rudolf Hertz.

Whereas the waves of ordinary light are so short that there are about 50,000 to the inch, the individual radio wave can be many miles long. Even the shortest radio waves (called "microwaves") can be several inches long.

Once radio waves were discovered, physicists began to try to use them to carry signals over long distances. The Italian engineer, Guglielmo Marconi, managed to send signals by radio waves from England to Newfoundland in 1901, and that can be considered the birth of our modern radio. Marconi's achievement was puzzling in a way. Radio waves travel in straight lines while the surface of the round Earth curves. How can radio waves manage to go round the curve? It turned out that the radio waves used by Marconi bounce off layers of ions in the upper atmosphere and zigzag up and down as they cross the Atlantic.

This does not happen if the radio waves are too short. The microwaves, for instance, go shooting through the layers of ions in the upper air (the "ionosphere") without trouble. Signals carried by microwaves would not travel along the Earth's surface for more than a few miles.

As a result, engineers who worked with radio (and there were many of them during the 1910s and 1920s) worked with long radio waves. Short radio waves were ignored because they seemed useless. No one paid attention to the fact that if they could go through the atmosphere as easily as ordinary light did, they might be useful to astronomers.

 

The man who first got a hint of that fact was Karl Jansky, a young American radio engineer, working for the Bell Telephone Laboratories. The people at Bell Telephone were interested in telephone conversations carried on over long distances with the help of radio waves. These were often interfered with by static and it was Jansky's job to try to pin down the causes of the static. Once the causes were known, the cures might be found.

Jansky, working in New Jersey, devised a large radio antenna which could be rotated to receive signals from any direction. When there was static, there were sure to be stray radio waves acting to produce it. Jansky's antenna could be rotated until the static was loudest and it would then be pointing to the source. If the source were known, then perhaps something could be done about it.

Jansky expected that a lot of the trouble arose from thunderstorms and the stray radio waves set up by the lightning. Sure enough, he did get a kind of crackling static from lightning, even when it was far off on the horizon, too far to see.

But then, in January 1932, he became aware of a faint hiss in his receivers, a sound quite unlike the lightning crackle. He might have thought it was just "noise" created by imperfections in his apparatus, but the hiss became louder and softer as he turned his antenna.

He found that the hiss was loudest in the direction of the sun. He wondered if he might be receiving radio waves from the sun.

If the sun had happened to have a great many sunspots at the time, the radio waves would indeed have been coming from the sun, for it was eventually discovered that the spots give rise to intense radio waves. In 1932, however, the sun was at a quiet period with few spots. It was producing very little in the way of radio waves.

Therefore as Jansky turned his antenna every day, he found that the spot from which the hiss was coming was not from the sun at all. In fact, it moved farther from the sun every day.

The sun moves slowly against the background of the stars (because the Earth, from which we watch the sun, is revolving about it so that we see the sun from a different angle every day) but the source of the hiss did not move. It remained at the same point in the constellation of Sagittarius.

Jansky realized he was getting radio waves not from the sun but from a different and possibly much more distant source. We now know he was getting it from the centre of our galaxy.

Jansky reported his findings, but they did not make much of a splash. The kind of radio waves that Jansky had detected coming from outer space were just those short microwaves with which nobody did any work. There were no instruments available that could really handle it. Astronomers preferred to work in fields where they had the instruments. .

They didn't seem to realize that they were ignoring something that was perhaps the greatest astronomical discovery of the twentieth century.

One youngster, in his twenties, was inspired by the report, however. He was Grote Reber. He built a device in the back yard of his home in Wheaton, Illinois. It was a curved reflector, thirty-one feet across, with which he received radio waves and reflected them into a detecting device at the centre. He put his "radio telescope" to work in 1937 and became the world's first radio astronomer.

All through the years of World War II, Reber kept carefully noting the quantity of radio waves coming from different portions of the sky. He was able, in this way, to produce the first radio map of the sky. He was also able to detect a few places from which radio waves seemed to be coming in particularly great quantities. These were the first "radio sources."

 

What eventually saved the situation was that during the 1930s interest grew in another angle of radio.

You can tell a great deal about an object if you bounce radiation off it and study the reflection. If you reflect light waves from a chair, the nature of the reflection will tell you the chair's shape, size, position, distance, colour, and so on.

Bats use sound waves for the purpose. Their squeaks are reflected by insects, twigs, and other objects and by listening to the echo, they can catch the insects or avoid the twigs. There are other examples of the same process.

Now suppose you wanted to detect an enemy aeroplane at night without letting the enemy pilot know he was detected. You could use a bright beam of ordinary light but the enemy would see it. Besides light is easily stopped by clouds, fog, mist, or smoke.

It would be much better to use some other form of radiation that he couldn't see and that would pass through clouds and other such obstructions. The longer the waves of the radiation, the better they would pass through clouds and the rest. If the waves were too long, however, there would be too much of a tendency for them to move around an object instead of being reflected by it.

It turned out that microwaves were just right. Their waves were long enough to go through clouds and short enough to be reflected by planes.

In Great Britain especially, methods were developed for sending out a tight beam of microwaves and receiving the echo.. Then, from the echo, you could tell the position and distance (or "range") of the reflecting object, which could be an enemy plane. The device was called "radio detection and ranging" and this was abbreviated as ra. d. a. r., which became the word "radar." Radar wave has therefore become another name for microwave.

Great Britain developed radar just in time to have it take part in the Battle of Britain in 1940. The British could detect the German planes coming in over the Channel by night as well as by day and were always waiting for them in the proper place. Without radar, Britain might have lost the war.

The important thing to astronomers was this: In developing radar, engineers had to learn to handle microwaves. Once they developed instruments to do that, those same instruments could be used to detect microwaves from outer space.

What's more, Great Britain became aware of microwaves from outer space in the course of the war.

In February 1942, Great Britain found severe interference with its radar network. The first thought was that the Germans had discovered the network and were jamming it in preparation for large new air strikes. A team under the British engineer Stanley Hey began to investigate the matter.

Hey discovered the source of the jamming in a few days. The sun was not quiet, as it had been when Jansky made his key discovery. It was loaded with sunspots and it was broadcasting radio waves. For the first time, radio waves from outer space were pinned down to a definite source-the sun. Immediately after the war, astronomers, using all the equipment and techniques worked out through radar developments, turned to the study of radio astronomy in a big way.

The "radio sky" was mapped in greater and greater detail, and certain radio sources were identified. It was found that stars that had once exploded were such strong sources of radio waves that they could be detected through all the vast distances that separated those stars from us.

Indeed, it was discovered that whole galaxies could be sources of radio waves of even greater intensities. Distant galaxies could be detected with greater ease by radio telescopes than by ordinary ones.

Radio astronomy in the 1960s uncovered mysterious objects which were named "quasars" by astronomers. There is no certainty as to exactly what they are, but some think that they are small but enormously bright objects farther away than anything else we know. The quasars may tell us a great deal about the youth of the universe billions of years ago, and about its edges billions of trillions of miles away.

In fact, in 1964, certain types of radio waves were studied which seemed to come from all directions and which some astronomers think is the radiation that was released when our universe was first formed.

Interestingly enough, the great discoveries of radio astronomy were not confined to far away places only. News was brought to mankind concerning its nearest neighbours in space, the planets of the solar system. Some of the news was so exciting and unexpected that the study of the planets, which seemed to have been played out, suddenly burst out into fascinating new directions.

 

For instance, if beams of microwaves can be reflected from enemy aircraft, and if the echoes can give us information, why can't such beams be reflected from objects of astronomical interest.

Hey, who discovered the radio wave radiation of the sun during the war, also noted certain echoes that seemed to be originating in the upper atmosphere. From the time it took the echoes to return, he could calculate the height, and he began to wonder if he weren't detecting meteors.

After the war, he studied these echoes in detail. Finally, in 1946, he was able to show that meteors leave so thick a trail of ions that some microwaves are reflected. One could therefore study meteor trails by radar.

This was useful, for only the larger meteors (about the size of pinheads or more) could be seen by their gleaming light, as friction with the air heated them white-hot, and even then they could only be seen at night. Using radar, however, small meteors could be detected day or night, if they were in sufficiently large clusters.

Certain large clusters of meteors move around the sun in what had once been the orbits of comets that had finally fallen apart. Once a year, the Earth will pass through a particular cloud and there will then be a shower of flashing trails left by many meteors moving quickly through the atmosphere.

Once in a longish while the Earth may move through the thickest part of such a cloud and then the trails may appear to be as thick as snowflakes. This happened over the eastern United States in November 1833.

There are about a dozen meteor clouds that have been observed in this way. Now that radar observations are made, at least three more have been found that always strike from the general direction of the sun. They always approach on the daylight side of the Earth, in other words, and can never be seen by eye.

But do we have to confine ourselves to Earth's atmosphere? Could not a beam of microwaves travel outside the air altogether? If it were aimed in the direction of the moon, it could reach the moon in one and one-quarter seconds, strike its surface, bounce off, and shoot back. The echo would reach Earth again after another one and one-quarter seconds. There would be two and a half seconds altogether between the time of sending and the time of return.

Naturally, the radar beam would spread out with distance. Some of it would be absorbed by the moon. Some of it would bounce off in directions away from the Earth. Then the returning echo would spread out again over the distance between moon and Earth. Only a very faint echo would be received.

To detect such a faint echo, either a very intense beam must be sent out in the first place, or very sensitive devices must be developed for detecting echoes or both.

Difficult as it was, the feat was accomplished almost as soon as the end of World War II freed radar equipment for the task. In early 1946, a Hungarian, Zoltan Lajos Bay, (who has since emigrated to the United States) reported receiving echoes. A very short time afterwards, the United States Army, with more powerful equipment, managed to do the job in an even more clear-cut way.

Reaching the moon by microwave was comparatively easy, because it is so close as compared with other astronomical bodies. The sun is much farther away but it is a giant in size so that it offers a large target. In 1959 astronomers aimed - a beam of microwaves at it and a group at Stanford University in California managed to get an echo back. The sun's own microwave radiations confused the echo, of course, but it could be made out.

The important target, however, was Venus. Venus was closer than the sun and echoes could be received from it much more sharply. Still, Venus was a much smaller body than the sun, a little smaller than the Earth, even. It made a tiny target in the heavens, and it would be a triumph, indeed, if a beam of microwaves could be made to strike Venus and return to Earth. The returning echo would be exceedingly feeble and to detect it would require the most delicate instruments and the most careful work.

If it could be done, however, a great deal could be gained. Scientists knew quite accurately how quickly a beam of microwaves traveled through space. It traveled at the speed of light which is a fraction over 186,282 miles per second. If one could measure the exact length of time it took for the microwaves to travel from Earth to Venus and back, one could calculate just how far Venus was at that moment.

Then all the other distances of the various bodies of the solar system could be calculated from that. In just a few days, the distance of the sun could be determined more accurately than through the entire ten-year project that involved the asteroid Eros.

Everyone was trying for the Venus echo and in 1961 three different American groups, one British group, and one Russian group all succeeded. Each calculated the distance of Venus and then of the sun. The best figures, obtained by a group from M.I.T., seem to show that the average distance of the sun from the Earth is about 92,955,600 miles. That is 50,000 miles closer than the results given by the Eros project.

After Venus was successfully touched, other planets were reached. In 1962, a Russian team made microwave contact with Mercury, a smaller and more distant target than Venus. In 1963, astronomers at the California Institute of Technology made contact with Mars. There have also been reports of contact with Jupiter, a planet more distant by far than any of the earlier targets, but this is still uncertain.

 

Microwave echoes can tell us far more than the distance of an object. It can tell us a great deal about the kind of surface that is reflecting the beam.

Suppose the microwaves were bouncing off a perfectly smooth sphere. Those waves that hit the exact centre of the side of the sphere facing us would bounce back perfectly.

The echo would come back right on the line along which the original wave had approached. The echo would return to the instrument that had sent out the wave and it would be detected.

Microwaves that hit the sphere a little way from the centre of the side facing us would bounce off to one side. (You can see why this would be so if you imagined yourself throwing a ball at a curved wall. If the ball hit the wall where it curved away from you, it would bounce to one side.) The farther from the centre that the radar touched, the farther to the side it would bounce.

But, of course, the moon is not a perfectly smooth sphere. It is uneven. It has mountains and craters, hills and rocks. A microwave striking the centre of the moon might hit the side of a hill or even the side of a rock and be reflected away from us, instead of coming straight back.

Then, too, if a microwave struck a point on the. moon quite a bit away from the centre, it might hit an uneven portion slanted in such a way that the wave would be reflected right back to us. So you see we would be getting some echoes from all over the moon.

But the moon's surface curves away from us and near the rim of the part we can see, the surface is over a thousand miles farther from us than is the surface in the very centre. This means that the microwave echo isn't absolutely clean and sharp. The part reflected from the centre of the moon comes back first and then small echoes come back from uneven surfaces a little farther along the curve of the moon, and then from uneven surfaces a little farther still, and so on.

The echo is a little fuzzier than the original wave. The fuzziness becomes greater or less as microwaves with different wavelengths are used, for the smaller the wavelength, the more the wave is affected by small unevennesses. From all this astronomers can get an idea of how rough the moon's surface is.

To work out the roughness of the moon's surface by "feeling" it with microwaves is exciting, but again Venus is much more important.

Venus is our nearest neighbour in space, next to the moon, but we know almost nothing about it. Its thick atmosphere is filled with clouds that never thin out. All we can see is the cloud layer so that Venus, in the telescope, looks like a shiny, white ball with no markings.

Microwaves can penetrate those clouds, though, and bounce off the rocky soil no one has ever seen. From the fuzziness of the echo, something can be worked out about the unevenness of that surface.

Late in 1965, for instance, it was decided that there were at least two huge mountain ranges on Venus. One of them runs from north to south for about 2,000 miles and is several hundred miles wide. The other is even larger and runs east and west. The two ranges are named for the first two letters of the Greek alphabet. They are the "Alpha Mountains" and the "Beta Mountains."

It is still uncertain as to how high these mountains are, but astronomers are using additional microwave measurements to work out a crude map of Venus-the map of a surface we have never seen.

Microwave measurements have also been used to test the roughness of Mars and by 1967 it was decided that Mars was about as rough as the Earth. This was a surprise, for studies by ordinary telescopes had made it seem that Mars was rather smooth.

It now seems that some Martian mountain peaks are as much as eight miles above the lowland depths. This is actually higher than Earth's mountain peaks, but then Mars has no ocean. If we measured the height of our mountains above our ocean bottom instead of above the top of the ocean water, some of our ranges would be over ten miles high.

Even that isn't all the information microwave echoes can give us.

Suppose that a microwave beam is reflected by a body that is turning,on its axis, and suppose the body is turning from left to right as we look at it.

The part of the body at the left is turning along the curve of its surface, towards the middle, which is closer to us than any other part is. The part of the body at the left is coming towards us, in other words. The part of the body at the right is naturally turning away from us.

If the microwave beam hits the left side of the body, which is coming towards us, then the waves are squeezed together. Those parts of the echo that reach us from there have shorter waves than the original beam had. In the same way, the radar beam that hits the right side bounces back from a part that is moving away and its waves are pulled apart. That part of the echo has longer waves than the original.

From the way in which the lengths of the radar waves have stretched out and pushed together as compared with the original, astronomers can tell how fast the body is turning.

This can be tried on the moon. We know how fast it is turning. Microwave echoes give the right answer. Astronomers were therefore confident they could try it on other bodies. What about Mercury, for instance? They thought they knew how fast Mercury rotated on its axis-once in eighty-eight days, exactly as long as it took to go around the sun once.

This is no coincidence. When a small body turns about a nearby large body, the gravitational force of the large body pulls some of the small body towards itself and makes a bulge in its direction. As the small body turns, this bulge is forced to remain pointing to the large body. It slips about the small body and as it does so, it sets up friction that slows down the rotation; just as the friction of a brake slows down a bicycle.

Finally, the small body slows its rotation till it is turning just once on its axis each time it moves around the big body. When this happens, the small body always turns the same side to the big body, so that the bulge is always in one place. There is no more friction.

The moon turns on its axis in just the time it takes to move once around the Earth so it always shows us the same side. It has a bulge in the centre of that side that faces us; a bulge about two miles high.

In order to tell how fast a planet turns on its axis (without the use of microwaves) astronomers would watch for certain markings on its surface and measure the time it took for those markings to disappear round the other side and come back. Accurate measurements can be made on even distant planets in this way.

The rotation of Mercury was hard to measure in this fashion, though. It is so close to the sun that it is difficult to make out its surface features in the glare.

In 1890, Schiaparelli (the astronomer who had first detected the "canals" on Mars) did follow certain features on Mercury. He found that when Mercury was in a certain position with respect to the sun, he could often make out the same markings in the same position. This would be what was to be expected if Mercury always turned with the same face towards the sun and this would happen if it turned on its axis in the same time that it turned about the sun-eighty eight days.

Astronomers were quite satisfied with that, for it made sense. The huge sun had slowed the rotation of nearby Mercury, as Earth had slowed the rotation of the moon. And, indeed, the first microwave contact made with Mercury seemed to show that that was so.

However, more and better contacts followed and in 1965, astronomers found themselves faced with surprising data. Careful work on microwave echoes from an observatory in

Puerto Rico showed that Mercury did not turn on its axis in eighty-eight days, but in a rather shorter time. Other laboratories pointed their microwaves at Mercury at once and the result was found to be correct. Mercury turns on its axis once in fifty-nine days.

But if that is the case, how could Schiaparelli have thought that the revolution was an eighty-eight-day one? Did he make a mistake in observing the markings?

Perhaps not. A period of fifty-nine days is just two-thirds of the eighty-eight-day swing about the sun. This means that every time Mercury moves about the sun two times, it turns on its axis three times.

Imagine that a certain spot on Mercury's surface faces the sun at a particular time. When Mercury has gone around the sun twice, it has turned on its axis three times. and the same spot is again facing the sun.

When Schiaparelli observed markings, he would have seen the same one in the same place every other time Mercury turned about the sun. He didn't see them in between but perhaps he paid little attention to that because Mercury was so close to the sun, one couldn't always be sure what one saw anyway. So he made the easy supposition that the markings were probably there every time, whether he saw them or not, and that Mercury rotated in eighty-eight days.

But again it was Venus that supplied the still greater surprise. That had happened a year before Mercury's rotation had been given a new look.

In the case of Mercury, astronomers at least thought they knew what the time of rotation was, even though they were wrong. In the case of Venus, no one knew. There were never any markings that could be followed.

That was so frustrating. All the other planets had definite rotation times that could be measured (even though Mercury's was measured wrong). Even distant Pluto, over 150 times as far as Venus, was not mysterious in this respect. Pluto is so distant it can only be seen as a dot of light even in a good telescope and no markings can be made out. However, it seems to grow slightly brighter and dimmer in a regular way. Astronomers have decided that this is the result of some part of it being brighter than the rest for some reason; and it is the bright part showing and vanishing as the planet rotates that makes the flicker. Judging by this, Pluto seems to rotate once every 6.4 days.

Yet Venus had no known period of rotation at all. Most astronomers thought that probably Venus's rotation was slowed by the sun and that it showed only one face to the sun. That would mean it would turn on its axis only once each time it turned about the sun-once in 225 days.

But what would radar say?

Radar had its say in 1964, and the answer was a startling one. Venus rotated not once in 225 days, but once in 243 days, so that it did not show only one face to the sun. But what really astonished astronomers was that Venus turned in the wrong direction!

To see what we mean by the wrong direction, imagine that you are viewing the solar system from a point high above the Earth's North Pole. All the planets would be seen to move around the sun in the same direction - counter-clockwise; that is, the direction opposite to that in which the hands of a clock move about its face. All the large satellites turn counter-clockwise about their planets, too, provided they move about the planet's equator. (Neptune's large satellite does not move about its equator and it is exceptional.)

The sun and the planets also rotate about their own axes in counter clockwise fashion. (Uranus is a partial exception. Its axis tips over so far that it seems to be rolling on its side. Astronomers don't know why.)

All these counter clockwise motions are thought to have arisen at the very beginning of the history of the solar system. The solar system began its life as a huge cloud of gas and dust turning slowly in a counterclockwise direction. That counterclockwise turning remains to this day in all the motions of the various parts of the solar system.

Yet Venus turns about its axis very slowly in the wrong direction. It turns clockwise. This is not because its axis is tipped, as in the case of Uranus. The axis of Venus is almost perfectly upright.... Astronomers can't explain this wrong-way motion.

There is an even greater mystery involved, for the period of rotation seems to be tied to Earth. Every once in a while, Earth and Venus reach positions in their orbits which place them as close together as they ever get. Venus manages to turn just four times in that period.

This means that every time Venus comes as close as possible to the Earth, it shows the same face to the Earth. We can't see this, because we can't see through the clouds, but it seems to be so.

But why is it so? Can Earth's gravitational pull have slowed the rotation of Venus and made it show the same face to us at every close approach? How could that be since Earth's gravitational pull is so much less than the sun's. Why would Venus respond to Earth instead of to the sun?

Astronomers don't know.... At least, not yet.

 

So far I have talked about microwaves being sent out from Earth to various bodies in the solar system. How about microwaves sent out from the various bodies to the Earth?

The sun sends out microwaves, of course. That has been known since 1942. But then every body in the solar system ought to be producing them too.

Every body contains a certain amount of heat and that means it produces a certain amount of radiation. The greater the temperature of the body, the greater the energy of the radiation it produces and, on the average, the shorter the waves making up that radiation.

If a body has a temperature of about 1000° F. or more, it sends out radiation that is so energetic and short wave that some of it appears in the visible light region. The body is "red hot," for it glows a deep red. As the temperature gets still higher, the light grows brighter and shorter in waves. The sun's surface is at 10,000° F. and it radiates brightly all the colours. It even radiates ultraviolet light, which is invisible, but which has more energy and shorter waves than ordinary light.

An object that has a temperature of less than 1000° F. doesn't radiate visible light, but it does radiate all the wavelengths longer than visible light. It radiates infrared light, for instance, which has less energy and longer waves than visible light. We can't see infrared but we can absorb it and feel it as heat. We can feel the heat of a hot iron from a small distance even though it isn't hot enough to glow.

These too-cool-to-glow bodies all radiate microwaves as well and even longer radio waves. Such waves are so long and have so little energy that even the coldest bodies can radiate them. They have so little energy that we can't feel them in any way, but we have instruments that can detect them.

Every body in the solar system radiates a certain quantity of long-wave radiation. The exact quantity and the exact length of the waves depend on the temperature of the body.

By studying the microwaves sent out by the moon or by a planet, we can therefore determine the temperature of the body. The first determination of this sort came in 1946 when two American astronomers, Robert Henry Dicke and R. Beringer, picked up radio waves sent out by the moon.

Promptly, this produced a puzzle. By studying the moon's infrared radiation, it had seemed that the temperature varied a great deal because there was no atmosphere on the moon to hold and spread the heat. At the height of the moon's day, the temperature reached 250° F. in some places, and this is well above the boiling point of water. At the close of the moon's long night, the temperature had dropped to 280° below 0° F. (which we can write as -280° F.).

The microwaves sent out by the moon, however, seemed to show much smaller variations in temperature. Astronomers decided that the infrared radiation comes from the very surface of the moon, while the radio waves come from some distance below the surface.

As the sun glares down on the moon, the surface heats up. The heat can't penetrate far beneath the moon's surface, however, and the lower layers remain cool. Then, in the moon's night time, the surface layer loses heat but the deeper layers don't.

It may be that about a yard below the surface of the moon, the temperature remains about -40° F. day and night. Naturally, astronomers went on to try to detect microwave radiation from other planets to see what that would tell them about the temperature of the planets. They could compare that with what they knew the temperature of the planet ought to be considering its distance from the sun.

They expected no surprises, but they got a big one from the very planet that has been turning everything upside down in the 1960s-Venus.

Earlier measurements of infrared radiation from Venus had showed the temperature to be -40° F. This may seem too cold for a planet that is closer to the sun than Earth is. Infrared radiation, however, reaches us from above the cloud layer of Venus. Naturally, that part of the atmosphere of Venus would be cold. It is cold on Earth, too; that is why high mountains have snow on them all year round even when they are located on Earth's equator.

Microwaves are another thing altogether. They can penetrate the cloud layer on Venus easily. Therefore if the solid surface of the planet gives off microwaves, those would go through the cloud layer and reach us. (Infrared radiation wouldn't.) The microwaves would give us the temperature of the solid surface of the planet.

In May of 1956, microwave emission from Venus was finally detected by C. H. Mayer at the Naval Research Laboratories in Washington. Surprisingly, the flood of microwaves was much greater than had been expected. They showed that the surface of Venus must be at a temperature of 600° F. and later measurements backed that up.

Astronomers expected Venus to be a warm world and, because of its thick clouds, sometimes visualized it as covered with a warm ocean. But now it seemed there was no ocean at all, for the planet was far hotter than the boiling point of water.

Any water on Venus would have to be in the form of steam and that might be why the cloud layer on the planet is so thick and permanent. (On the other hand, some astronomers believe that Venus has no water at all and that the clouds are something else.)

But why should Venus be so hot? One explanation involves its atmosphere.

When visible light strikes a planet it passes through the atmosphere and strikes the surface of the planet. The atmosphere doesn't interfere much with such visible light. Even clouds only stop part of the light.

The light that is absorbed by the planet's surface heats it up a little. The surface then gives off radiation of its own that is less energetic than visible light (after all, the planet's surface isn't as hot as the sun). Much of the light radiated by the planet's surface is infrared radiation.

This infrared ought to pass through the atmosphere and vanish into space and the planet, then, with light coming in and infrared going out, would be at a certain temperature.

But there are some gases which are transparent to visible light but not to infrared radiation. One of these is carbon dioxide. Earth's atmosphere has only three-hundredths of 1 percent carbon dioxide but even that small quantity is enough

to make it difficult for infrared to get through the atmosphere. The infrared leaks out so slowly that a considerable quantity accumulates and heats up the air and surface of the planet. The temperature of the Earth is higher than it would otherwise be, thanks to the small quantity of carbon dioxide in the atmosphere. (Water vapour also has this effect.)

The same thing happens in a greenhouse. The glass of the greenhouse lets sunlight in but doesn't let infrared radiation out. For that reason, the temperature inside the greenhouse stays warm on sunny days even in cold weather. The action of carbon dioxide and water vapour is therefore referred to as the "greenhouse effect."

The atmosphere of Venus is far richer in carbon dioxide than our own atmosphere. Not only does Venus get more heat from the sun than we do because it is closer to the sun, but the heat is trapped to a much greater extent. This is the most popular explanation for the unusually high temperature of Venus.

 

It is possible, to be sure, that some microwaves sent out by a planet may not be produced just by its heat. There may be other causes.

This came up as a strong possibility in 1955. In that year, two astronomers, Kenneth Linn Franklin and Bernard F. Burke, at the Carnegie Institution in Washington, were measuring radio waves from the heavens. They received strange interference at one point and wondered what it might be. It could just be static; perhaps some faulty electrical device was sparking somewhere in the vicinity.

However, they kept getting the interference night after night and it seemed to be coming from some particular place in the heavens; some place that was moving from night to night in a particular way. They studied the sky to see if something were in that place that might be moving in just that way, and they found the planet Jupiter in that place and moving in that way.

There was no mistake. Jupiter was sending out strong bursts of microwaves. Going back through the records, they found that strong bursts had been reported from the direction of Jupiter in 1950 and 1951, but no one had followed it up.

When a planet sends out radiation, it sends it out over a broad band of different wavelengths. In receiving the microwaves from Jupiter, then, one could study first one part of the band and then another.

Astronomers could, for instance, study those microwaves that were one or two inches long. When this was done, it was found that the quantity of microwaves received was about what one would expect of a body at a temperature of, say -200°F.

This was the temperature of Jupiter judging from infrared radiation, and about the temperature one would expect for a planet as far from the sun as Jupiter was.

So far, so good, but what about the microwaves with longer wavelengths. There the quantity rose unexpectedly. An object with a temperature of -200°F. couldn't possibly radiate as much long-wave microwaves as Jupiter did, if temperature were the only cause of the radiation.

Jupiter's radiation of four-inch microwaves was what would be expected of a body at a temperature of 700°F. or so. Its radiation of twelve-inch microwaves would have required a temperature of nearly 10,000°F., the temperature of the sun's surface. The radiation of twenty-seven-inch microwaves would have required 90,000°F., hotter than the surface of the hottest stars we can see.

This is quite impossible. Jupiter can't be that hot. It must be sending out long microwaves for other reasons.

One possible cause is related to the fact that Jupiter behaves like a strong magnet. Our own Earth behaves like a magnet, which is why the compass needle always points north, but Jupiter is apparently a much stronger one.

Electrons and other particles streaming out of the sun are trapped in Jupiter's magnetic field and are made to move in rapid spirals high above Jupiter's surface. Such spiraling particles would send out floods of microwaves.

In some wavelengths, though, the microwaves come off in unsteady bursts. Are they produced by gigantic thunderstorms in Jupiter's vast atmosphere, which is much thicker, deeper, and larger than ours? Are there lightning bolts a billion times as strong as those we witness on our own planet, each sending out a crackle of microwaves?

Then, too, as Jupiter rotates about its axis, the quantity of microwaves rises and falls regularly. There seem to be certain places on the planet that are particularly rich sources. What these might be nobody yet knows.

These bursts of microwaves also seem to be stronger than usual whenever Jupiter's innermost large satellite, lo, is in particular positions in its orbit around Jupiter. Why that should be no one knows.

Someday we will find answers and when we do, then through microwaves we will find out more about Jupiter than would have seemed possible just a couple of decades ago.

 

But all that followed from Jansky's discovery of radio waves from the sky does not exhaust the new studies of the solar system.

Even more dramatic is the other breakthrough I mentioned - the flight of the rocket in 1926. This I will now turn to in the book's last chapter.

From: Twentieth Century Discovery by Isaac Azimov