Isaac Asimov Comets
Isaac Asimov Comets
Isaac Asimov Comets
Isaac Asimov Isaac Asimov is a master storyteller, one of the worlds greatest writers of science fiction. He is also a noted expert on the history of scientific development, with a gift for explaining the wonders of science to nonexperts, both young and old. These stories are science-facts, but just as readable as science fiction.Before we found out about comets, the superstitious thought they were signs of bad times ahead. The ancient Greeks called comets aster kometes meaning hairy stars. Even to the modern day astronomer, these nomads of the solar system remain a puzzle. Isaac Asimov makes a difficult subject understandable and enjoyable to read.
But what happened if every once in a while, something unusual happened in the sky? What if something happened that was so out of the ordinary that it could not be predicted? Might that not mean something had gone wrong with the machinery? Might that not mean something unusual was going to happen on the Earth, too? As it happened, every once in a while, a new kind of shining object appeared in the night sky; one that was like nothing else you could see in the sky. It was not just a point of light like the stars or the planets. It was not a bright circle of light like the Sun or the full Moon. It was something that was larger than a star but it did not have sharp outlines. It was like a patch of shining fog, and out of one side of it there came a long, curved streamer of foggy light that grew fainter and fainter as it stretched outwards. It was as though it was a fuzzy star, with shining hair streaming out of it. The ancient Greeks called it aster kometes which, in their language, means hairy star. We keep the second part of the Greek name and call such an object in the sky a comet. At the center of the comet, there is sometimes a bright star-like dot called a nucleus. The hazy light around the nucleus is the coma. The long trail of hazy light from one side is the tail. The ancient astronomers could not predict when a comet would come, or where it would appear. They could not tell what path it would take across the sky, or when and where it would disappear. Even after astronomers had worked out the movement of all the other objects in the sky and had explained eclipses: they were still puzzled about comets. A comet just came, traveled across the sky, and disappeared. The more people came to expect everything in the sky to follow a regular path, the more frightened they became of comets. Maybe, when a comet came crashing into the smooth workings of the sky, it meant that some disaster would take place on Earth. Some important person might die; there might be a war or a plague. Whenever a comet did appear, there was sure to be something catastrophic happening on Earth (after all, there always is). People would say that the comet had served as a warning of this event. Then the next time a comet appeared, they would be all the more frightened. For instance, there was a comet in the sky in 44 BC, which was the same year that the great Roman dictator, Julius Caesar, was assassinated. There was also a comet in the sky in 1066 AD which was the year that William of Normandy invaded and conquered England. That was a disaster to the English, although the comet was lucky for William. People who dont really understand what is going on in the sky are frightened by comets even today. They think a comet will bring disaster or that it means the end of the world is coming.
Actually, comets are no more than objects in the sky. They have no effect at all on the Earth, one way or the other. But as long as people do not know what comets are, or where they come from, or where they go, or what makes them appear in the sky, they will worry about them. Fortunately, astronomers have learned, little by little, the answers to these questions about comets, and educated people are no longer afraid of them.
This was done by the ancient Greek astronomers. As long ago as 130 BC, one of them, Hipparchus, had calculated that the Moon was about 386,000 kilometers from the Earth. In 1577, Tycho decided to try to measure the parallax of the comet in the sky, as Hipparchus had measured the parallax of the Moon. If the comet was part of the air, it should be much closer than the Moon, and it should have a bigger parallax. Tycho arranged to have an astronomer in Germany, a few hundred kilometers away, note the position of the comet against the stars at a certain hour of a particular night. Another astronomer observed it at exactly the same time from Bohemia, while Tycho himself noted the position at the same time from his own observatory in Denmark. Tycho studied the results and found that the comet appeared to be in just about the same place compared to the stars no matter where the observer was standing. There was hardly any shift. The parallax was much smaller than that of the Moon. This meant that the comet had to be further away than the Moon. In fact, Tycho decided the comet had to be at least four times as far away as the Moon-it was, therefore, about 15 million kilometers away. This figure was not accurate. Actually, the comet was much further away than that. But Tychos result was still important. It showed that the comet could not be a burning patch of air, and that Aristotles theory was wrong. The comet was a heavenly body, just as the planets were. If comets were heavenly bodies, why did they look and act so differently from other heavenly bodies? Tycho could not say, that in his time, astronomers were beginning to take a new look at the Universe. Until then, astronomers had thought that the various planets moved in circles around the Earth. In 1543, however, a Polish astronomer, Nicholas Copernicus, showed that it made more sense to look at things in another way. He said that only the Moon moved in a circle around the Earth. The Earth itself moved in a circle around the Sun. All tire other planets also moved in circles around the Sun. If astronomers accepted this, it became much easier to figure out the motions of the planets. (The path followed by a body moving around another body is called an orbit, from a Latin word meaning circle.) The sun and all the objects that moved around it make up the solar system (so called because in Latin the sun is called sol). A German astronomer, Johannes Kepler, who had been one of Tychos assistants, disagreed with part of Copernicuss theory. After studying the motions of the planers in the sky. Kepler said, in 1609, that the planets moved around the Sun in orbits that were not circles. Each planet moved around the Sun in an ellipse. An ellipse looks like a flattened circle. It can be so slightly flattened that you cannot tell it from a circle. It can be more flattened so that you can see at a glance that it is not a circle. Or it can be so flattened that it looks long and thin, something like a cigar. The orbit of the Earth around the Sun is an ellipse that is only very slightly flattened. It is almost circular. The Moons orbit around the Earth is more flattened, and Mercurys orbit around the Sun is still more flattened. Even Mercurys orbit, which is more flattened than that of any other planet known in Keplers time, is not very flattened. Its orbit still looks like that of a circle. The Sun is not at the very center of the elliptical orbits of the planets around it. When the Earth moves around the Sun, it is only 147,250,000 kilometers from the Sun at one end of its orbit, but 152,078,850 kilometers from the Sun at the other end. The further distance is less than 4 per cent greater than the nearer distance.
Mercurys orbit around the Sun is more elliptical, so there is a bigger difference. When Mercury is at the end of the ellipse nearer the Sun, it is only 45,000,000 kilometers away. At the other end, it is only 70,800,000 kilometers from the Sun. The further distance is about 50 percent greater than the nearer distance. Kepler was able to work out elliptical orbits for all the planets, but what about the comets? If they were heavenly bodies, did that mean they had orbits, too? Kepler carefully studied the reports he had about the changing positions of comets in the sky. Finally, he decided that comets must move in straight lines. He thought they came from far out in space, passed near the Sun, then traveled onwards far out in space in the other direction. They could only be seen when they were close to the Sun and reflected its light. Before they came close enough to the Sun, they could not be seen. After they moved far enough from the Sun, they again could not be seen. According to Keplers view, comets were not part of the solar system. Each comet just passed through the solar system once and was never seen again. An Italian astronomer, Giovanni Alfonso Borelli, carefully studied the positions of a comet that appeared in the sky in 1664. He found he had to disagree with Kepler. The only way to make sense out of the path the comet took across the sky, Borelli said, was to suppose that it changed direction as it passed the Sun. It came closer and closer to the Sun, along a line that was nearly straight. Then it moved around the Sun, and left it along a line that was again nearly straight but had changed direction. The way Borelli explained this was to point out that ellipses could be very flattened indeed. They could be so flattened that they would resemble a very long thin cigar. In fact, if you imagined an ellipse that was more and more flattened, and longer and longer, you could eventually imagine one that was so flattened it just went on and on forever. Such an ellipse would be closed only at one end. In the other direction, it would never be closed, but would just go on and on. A one-ended ellipse that goes on and on forever is called a parabola. Borelli decided that a comets orbit was a parabola, with the Sun very near the closed end. The comet came in at one side of the parabola, went whizzing around the Sun, and then moved outwards along the other side of the parabola. Borellis view was like that of Kepler, except that the orbit he conceived was not a straight line. Like Kepler, Borelli thought the comet was originally so far away it could not be seen. As it came closer and closer to the Sun, it grew bright enough to be seen, and then as it went further and further from the Sun, it once more became too dim to be seen. In Borellis view, as in Keplers, the comets were not members of the solar system. Each comet just passed through the solar system once and never returned.
The results were quite amazing. Saturn, the planet furthest from the Sun (as far as was known in Halleys time) was never further from the Sun than 1,500,000,000 kilometers. The comet of 1682, however, moved out as far as 5,150,000,000 kilometers from the Sun before it reached the other end of its elliptical orbit and began moving inwards again. The comet moved over three times as far away from the Sun as Saturn ever moved. On the other hand, when the comet passed along the Orbit of Halleys comet Pluto Neptune Uranus Jupiter end of the ellipse that was near the Sun, it came as close as 87,000,000 kilometers to the Sun. This was only about half of Earths distance from the Sun.
After Halley had calculated the orbit, he announced that the comet of 1682 would return some time in 1758 and would follow a particular path across the sky. Halley did not live long enough to see the comets return. He was 86 years old when he died in 1742, but that was much too soon to see the return. There were, however, others who were watching for it. A French astronomer, Alexis Claude Clairault considered the orbit as outlined by Halley. He realized that the gravitational pull of the large planets, Jupiter and Saturn, would delay the comet a little bit. It would not pass around the Sun till some time in 1759. In 1758, astronomers eagerly watched that part of the sky in which Halley had said the comet should appear. They did not have to depend only on their eyes as Tycho and earlier astronomers had done. The telescope had been invented in 1609. On 25 December 1758, Christmas Day, a German farmer named Johann Georg Palitzch, who was an amateur astronomer, spotted the comet. The comet of 1682 had appeared in the sky where Halley said it would and proceeded to move along the path Halley had predicted for it. It moved around the Sun quite close to the time Clairault had predicted. There was no question that it was the comet of 1682 and that it had returned. That meant that some of the mystery of comets was cleared up. They followed the same rules as the other bodies of the solar system except that their orbits were more elliptical. Naturally, the comet of 1682 that returned and passed around the Sun in 1759 came to be called Halleys comet. Halleys comet is the most famous comet there is. It happens to be the one, which was in the sky in 1066 AD when William of Normandy was preparing to invade England. It was also in the sky in 11 BC about the time when Jesus may have been born. Some people think it may have been the Star of Bethlehem. Halleys comet has returned twice since Palitzch saw it. It came back in 1835 and was glowing in the sky when Mark Twain (the author of The Adventures of Tom Sawyer and Huckleberry Finn) was born. Then it came back in 1910 and Mark Twain died when it was glowing in the sky. It will come back again in 1986.
It is fortunate that Halleys comet has an orbit that is short enough for it to come back every 75 years or so. What if its orbit was longer? It might then take many hundreds, or even thousands, of years to return. Some comets appeared in the 1800sin 1812, in 1861, in 1882that were very large and bright. They seem to have orbits that are so long that they may take many thousands of years to return. The last time they were near the Sun, human beings were just primitive cave dwellers who did not think much about comets. When they come back again, who knows what the world and its people will be like! The orbits of such comets cannot be calculated from the little part of the orbit we see. Since they have made no previous returns when there were astronomers watching the sky, we cannot compare orbits, as Halley did for the orbits of the comet of 1682. Halleys comet has a shorter orbit than any other bright comet in existence and it is the only bright one whose orbit is known and whose returns can be surely predicted. Just the same, astronomers now know that comets are members of the solar system, and have orbits that are elliptical and can be calculated if only we can see enough of them.
4. Faint comets
Halleys prediction of the return of the comet of 1682 and its actual return on schedule in 1759 made astronomers pay more attention to comets. They did not have to wait for bright comets, which might not appear for long periods of time. With their telescopes they could locate many comets too faint to see with the naked eye. Many faint comets were found. In fact, every year a few were discovered. In 1770, a Swedish astronomer named Anders Jean Lexell discovered a comet. He followed its path as it traveled and found that its orbit was easy to calculate. It moved in an ellipse that was much shorter than that of Halleys comet. In fact, its orbit would bring it to the neighborhood of the Sun every 55 years. In that case, why was it that it had never been seen before? No comet had ever been seen passing along the sky on the route Lexells comet had taken. Lexell traced the path of the comet back into space to see where it had been before he first saw it. He found that it must have passed near Jupiter. It passed so close to Jupiter that it must have moved among the planets four large satellites. Lexell decided that the comet had had a long elliptical orbit to begin with, which was why it had not been seen before. When it passed close to Jupiter, however, the gravitational pull of that large planet made the comet curve out of its path. It took up a new orbit in an ellipse that was much shorter than the old one. However, the new short orbit did not stay put, either. Lexells comet never appeared again. There was more calculation and it seemed that, on its path away from the Sun after its 1770 approach, the comet once again passed close to Jupiter. Once again it curved out of its orbit. This time it took up an orbit that was spread out so widely that it did not close at the other end. It spread out even more widely than a parabola would. The curve of the new orbit was a hyperbola. Jupiter had hurled Lexells comet out of the solar system altogether. Every once in a while a comet is lost in this fashion. What happened to Lexells comet also showed how carefully one had to calculate a comets orbit. An orbit could always be changed by the various planets. Most important of all was the fact that Lexells comet passed very close to Jupiter and its satellites and yet produced no changes in their orbits. Lexells comet must have had so little mass that it had almost no gravitational pull.
Until then, it had been thought that comets might be large, very dangerous bodies that could destroy the Earth by colliding with it. Now astronomers knew they were actually tiny bodies. They might be surrounded by comas that took up a great deal of room, and they might have tails that were many millions of kilometers long. However, the amount of actual matter, or mass, in the coma and the tail was very small indeed. And it was the mass that counted. The comets were the first really small bodies discovered in the solar system. In one particular way, astronomers of the 1700s were disappointed in comets. After Halley had worked out the orbit of Halleys comet, there had been the feeling that many orbits might be worked outbut for a hundred years after Halleys time, no other comet had its orbit worked out. For a while, Lexell had thought he had one, but that orbit changed. Then in 1818, a French astronomer, Jean Louis Pens, found a comet he thought was new. A German astronomer, Johann Franz Encke, studied its path and found that several earlier comets in 1786, 1795, and 1805had followed the same path. With this information, Encke calculated the orbit of the comet and found its ellipse was so short that it returned to the neighborhood of the Sun every 33 years. The ellipse was so short that it didnt even extend outwards as far as Jupiters orbit. The comet came to be known as Enckes comet. It was the first comet after Halleys comet to have its orbit worked out and to actually follow that orbit and return when it was supposed to. Enckes comet is a short-period comet. Though many comets have had their orbits worked out since Enckes time, no other comet has been found to have an orbit so short, or to come back to the neighborhood of the Sun so often. Altogether, Enckes comet has been observed by astronomers on nearly 50 returns to the Sun.
Enckes comet can only be seen through a telescope since it is very dim. Even in a telescope one can only see a small coma around it, and there is never any tail. In fact, all the short-period comets those that return to the neighborhood of the Sun every few years are faint. The coma that is formed each time is blown away into the tail and never comes back. Each time the comet returns; there is less material in it to form a coma and tail. This means that a comet must get less bright at each return and slowly fade away. Comets that have small orbits and return frequently to the Sun have faded away long ago. Only comets with long orbits that return to the neighborhood of the Sun once in a long while are still bright when they return.
Nothing like that happened to Enckes comet. Perhaps that was because Enckes comet had a rocky core and Bielas comet had not. But was there really nothing at all left of Bielas comet? That question brings us to something else: every once in a while there is a shooting star or meteor in the sky. It is not a real star, of course. It is just a little bit of matter that has been moving through space and has collided with the Earth. As the matter moves through the atmosphere at great speed, it heats up because of friction with the air. It gets hot enough to glow and then you see it like a shining line of light that appears and then quickly comes to an end. Sometimes, the piece of matter heating up in air is so large that not all of it melts and burns away. What remains hits the ground as a lump of stone or iron that is called a meteorite. Such meteorites are very rare. Most meteors are very small objects in the air, objects far too small to end up as meteorites. They may be no larger than the head of a pin. Even such tiny objects can glow and appear as streaks of light. These tiny meteors are very common. Sometimes, in Fact, the Earth seems to pass through a whole cloud of such tiny pinhead bodies. In that case, there is a meteor shower. A particularly remarkable meteor shower was seen from the United States in 1833. The sky was alive with lines of light, seeming to fall as thickly as snowflakes. Some people who were watching thought that all the stars were falling from the sky and that it was the end of the world. But when the meteor shower ended, all the stars were still in the sky. These small bodies burn up completely in the air. No matter how thick a meteor shower is, nothing ever reaches the ground. The meteor shower of 1833 made astronomers wonder about such clouds of particles that the Earth passed through every once in a while. Could it be that they moved around the Sun in a regular orbit? The Italian astronomer, Giovanni Virginio Schiaparelli, considered the matter. He gathered all the information he could find about when meteor showers appeared and from what part of the sky they seemed to come. His calculations in the 1860s showed that clouds of meteors moved about the Sun in orbits that were long ellipses. Their orbits resembled those of comets. Could it be that there was some connection between those meteor clouds and comets? Schiaparelli thought so. There was one meteor shower that usually appeared in August and seemed to come out of a spot in the constellation Perseus. These meteors were therefore called Perseids. Schiaparelli showed that the Perseids moved in the same orbit as Turtles comet (This comet had been discovered in 1853 by the American astronomer Charles Wesley Turtle. It returned to the vicinity of the Sun every 14 years.)
It began to look as though a comet might he made up of material that would evaporate and become a gas in the heat of the Sun. Scattered through this gaseous material were fine bits of rock. When the gaseous material evaporated, fine bits of rock were left which glittered in the Sun. It was these that could be seen in the coma and the tail. Perhaps it was these fine bits of rock from heated comets that produced the meteors in the Earths atmosphere. Every time a comet passed near the Sun, meteor bits left it and then moved about the Sun on their own. Gradually, they spread out through the entire orbit of the comet. There were usually more bits near the comet than far away from it. Eventually, if the comet had no rocky core, all of it would turn into a cloud of particles. Could it be that this was what happened to Bielas comet? After Schiaparelli had shown that the Perseids were produced by Tuttles comet, astronomers applied this knowledge to Bielas comet. They knew what the orbit of Bielas comet was and they expected there would be meteors all along it. There would be a particularly thick cluster of them in the spot where the comet itself would have been. They waited for the time when the Earth would be near the spot. One astronomer, E. Weiss, predicted that there would be a meteor shower on 28 November 1872. He was wrong by just one day; it appeared on 27 November. This particular meteor shower was called the Bielids. It appeared several more times and then faded out. The particle cloud spread through the orbit but was not thick enough anywhere to make a good shower.
Every once in a while, one of these far-distant comets Action of solar wind on a comets tail might be slowed up as it made its huge circle about the Sun. This loss of speed might be caused by a collision between two of them, or because of the pull of a distant star.
The slowed comet would then drop in towards the Sun in a new orbit that would be a very long ellipse. It would pass near the planets, and if it came near enough to the Sun, it would shine brightly enough for us to see it. Then it would pass around the Sun and head back to the far distances. In its new orbit it might come close to the Sun only once every million years or so. When a comet approaches the Sun in this way, the icy materials it is composed of would evaporate. The rocky particles would be freed and would form the coma. In 1958, the American scientist Eugene Norman Parker showed that tiny particles, even smaller than atoms, were always hurtling out of the Sun at great speeds in all directions. These particles form the solar wind. It is this solar wind that strikes the coma of the comet and drives it away from the Sun to form a tail. Comets which are entering the neighborhood of the Sun for the first time can produce a huge coma that may take up more space than the Sun itself, and a long tail that may stretch for hundreds of millions of kilometers. Every once in a while, such a comet is curved out of its orbit by a planet, usually by Jupiter, the largest of the planets. It can then move along a shorter elliptical orbit. It loses so much of its material as it passes the Sun many times that it quickly grows faint. Even Halleys comet is growing fainter. A comet that came from the far outer reaches of space, perhaps for the first time, and was very bright, was seen in 1882.
Scientists have been waiting ever since 1882 for a new comet from the outer reaches to arrive so that they could study it with the new instruments and new knowledge they have developed in the last century. Finally, in 1973, a Czech astronomer, Lubes Kohoutek, sighted a comet quite a long distance from the Earth. That meant it had to be a large one or it would not reflect enough light to be seen. From its orbit, Comet Kohoutek or Kohouteks comet seemed to be arriving from the far-distant belt of small bodies. It made its closest approach to the Sun on 28 December 1973. It didnt become as bright as astronomers hoped it might; but it has been studied by humans in space on the Skylab satellite, and it will be studied further as it moves outwards from the Sun again. Perhaps some day in the future, when once again a new comet arrives from far-distant space, astronomers will be on a rocket ship ready to land on it. Then, instead of being afraid of a comet as a sign of bad luck, astronauts will be touching it and bringing back to Earth pieces of it for close study. END