Comets - Mysterious Visitors From Outer Space

Comets: Mysterious Visitors from Outer Space

In 1994 the Comet Shoemaker-Levy was torn apart as it entered the planet Jupiter’s gravitational

field, creating explosions that could be seen from Earth with a telescope. The following feature

article on comets for the 1995 Collier’s Year Book traces the history of scientific research into

comets, as well as the history of human responses to these trailing balls of light in the sky.

Comets: Mysterious Visitors from Outer Space

By Ronald A. Schorn

Comets have fascinated people for millennia, but until 1994 no one had ever had the chance to witness firsthand the destructive power of these enigmatic celestial wanderers. From July 16 to 22, fragments of Comet Shoemaker-Levy 9 smashed into Jupiter, creating a series of spectacular fireballs in the giant planet's atmosphere. The string of explosions was awesome, yet the discovery that comets can wreak havoc would not have come as a surprise to our distant ancestors.

People of widely different cultures have long regarded comets with fear and dread. The unheralded, unpredictable appearances of these ephemeral celestial visitors were believed to be omens of disaster for rulers, realms, and entire populations. Comets, it was thought, signaled or caused wars, revolutions, plagues, and other calamities. Halley's Comet made an appearance in 1066, which just happened to be the year that William the Conqueror crossed the English Channel from Normandy and won the British crown by defeating King Harold II. The episode is vividly illustrated in the medieval Bayeux Tapestry, which tells the story of the Norman conquest. In one scene awed courtiers gaze at the comet and then tell Harold of the evil omen—which in his case indeed proved fatal.

Modern scientists may not believe in omens, but they have come to realize that comets, which are only mountain-sized bodies, are capable of causing worldwide disasters as terrible as any that were imagined of old—and paradoxically might also be responsible for our very existence.

Exotic Visitors

When observed from Earth, a typical comet may show several different features. A starlike or fuzzy nucleus may be visible inside a surrounding coma, which is a bigger, hazy blob of light that might appear as large as the Moon in the sky, but is usually smaller. Nucleus and coma together—or coma alone—are often called the head. The tail, which may be narrow or wide, long or short, issues from the coma and usually is brightest nearest the head, gradually dimming to invisibility along its length. The tail generally points away from the Sun and in rare cases may span a good fraction of the sky. Comet tails come in a wide variety of shapes and must be extremely rarefied, for stars shine through them without a trace of dimming.

Not every comet shows all these features. Some never reveal a nucleus, but only a coma, while others display just a starlike nucleus. In the latter case they resemble asteroids, the miniplanets that orbit the Sun mainly between the paths of Mars and Jupiter. Many faint comets never develop a tail but come and go merely as ghostly comas. Comet Shoemaker-

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Levy 9 broke up into fragments in 1992 and thereafter seemed to observers to resemble a "string of pearls."

Comets in the past have been described in such terms as "brilliant," "awesome," "spectacular," or "splendid," so it might seem puzzling that most people alive today have never seen one. But even so-called great comets are usually not all that bright. The tail of a typical comet has roughly the same surface brightness as the Milky Way—or even a lower brightness—and the head is not much brighter. Even a first-quarter or last-quarter Moon will wash out much of the show. In addition, most people nowadays live in or near cities and towns where electric lights illuminate the night sky and smog dims the view.

But it was different in the past. A great comet is indeed a noble sight when seen in a truly clear, dark sky—though it is nowhere as bright or prominent as implied by such startling old descriptions as "blazing sword."

If a comet's orbit takes it very close to the Sun (a few million miles or so), the comet can become very bright—brighter than the Moon. Unfortunately, to be seen at that time the celestial visitor must appear near to the Sun in our sky, and you have to look very carefully quite close to the Sun (in daylight) to see it—and even then the comet looks faint in comparison to the Sun. Moreover, such a comet sets shortly after—or rises shortly before—the Sun, so it cannot be seen in a dark sky. Only on extremely rare occasions do circumstances conspire to produce a sight that rivets the attention of even the most casual viewer—a startlingly bright object just above the western horizon just after sunset or the eastern horizon at sunrise.

Comets have properties that set them apart from all other celestial bodies. They can appear anywhere in the heavens—in sharp contrast to the Sun, Moon, and planets, which are restricted to the rather narrow belt around the sky called the zodiac. And comets can move in any direction at weirdly variable speeds with respect to the background stars. As far back as 2,000 years ago astronomers could predict solar, lunar, and planetary movements and positions accurately, but comets left them baffled. Moreover, different comets have different shapes, which sometimes change from night to night. While the Moon's appearance varies in the same regular way month after month, comets change almost whimsically.

Perhaps most terrifying to the ancients was the fact that comets appeared unannounced and unpredicted with no regular interval between them and were visible for anywhere from a few days to many months. Since they did not conform to the strict regularity of the rest of the heavens, it was thought that they might be special emissaries of God or the gods. For millennia that seemed to be the most logical—and the safest—solution.

Reining Comets In

For some 2,000 years Western ideas on the nature of comets were essentially those of the ancient Greek scientist and philosopher Aristotle. (The word "comet" comes from the Greek aster kometes, meaning "long-haired star.") In Aristotle's view the starry heavens above were changeless; the planetary, solar, and lunar regions below could change, but they were subject to strict laws. Only in his lowest sphere (Earth's atmosphere in the broad sense) could there be unpredictable changes of any sort. He reasoned, therefore, that comets were fairly close to us, exhalations of some sort from the Earth that took fire as they rose up to near the level of the moving heavens. So matters stood for a long time, for there was no evidence contradicting Aristotle.

The Danish astronomer Tycho Brahe (1546-1601) dealt a death blow to Aristotle's ideas by proving from observations that the great comet of 1577 had to be farther from the Earth than



the Moon and thus was a heavenly body. Still, the nature and motions of comets were hotly argued, though astronomers gradually swung around to the view that they were celestial objects that passed by or around the Sun in some kind of orbit. Just what those orbits were, or just what comets were for that matter, was still unknown.

It was the English astronomer Isaac Newton (1642-1727) who provided the key to how comets moved. In 1687 he published Principia Mathematica, a book that laid out the law of gravity and the laws of motion for all material bodies—laws that were essential to explaining how the gravitational force of the Sun affected the orbits of celestial bodies. Subsequently, Newton's close friend Edmond Halley (1656-1742) "captured" comets and made them full-fledged members of the solar system.

Halley was one of the few contemporary scientists who understood Newton's writings well enough to make effective use of them. He was able to show that the comets of 1531, 1607, and 1682 had the same orbit—a very elongated ellipse with a period of about 76 years—and thus were actually the same comet. For the first time a comet became a well-regulated member of the Sun's family, and as a result that object has been known ever since as Halley's Comet. Subsequent calculations were able to verify earlier recorded appearances of Halley's Comet as far back as 239 B.C. It is now generally accepted that Halley's is the comet rendered (with notable accuracy) in the Adoration of the Magi by the great Italian painter Giotto di Bondone. The fresco has been dated to around 1304, three years after Halley's Comet streaked across the sky.

Like most known comets Halley's spends most of its time loitering near the most distant part of its orbit, making periodic dashes toward, around, and then away from the Sun in what is for comets a relatively short time. Halley predicted that his comet would return in 1758 or 1759. Before that happened, theoretical astronomers were able to refine the estimate so well that it differed from the actual telescopic sighting of the object by only about a month. A comet had been captured at last. Halley was very lucky, for the comet named after him is the only bright predictable one known in the entire solar system.

During the 18th and first half of the 19th centuries there were great advances in determining the orbits of comets, but little was discovered regarding their nature. New knowledge in that area finally came from directions undreamed of by earlier astronomers.

Surprises

The 19th century saw several unexpected developments in cometary astronomy. The curious history of Comet Encke is a typical example. First spotted in 1786, it proved to be the same one seen again in 1795, in 1805, and in 1819, when its orbit was calculated by German astronomer Johann Franz Encke. In fact, accounting for times when it was badly placed for observation from Earth, this object had a period of only a little over three years. This was an incredibly short time for a comet—more like an asteroid. But a comet it was, with tail and all. This object was the first of a class of bodies known as short-period comets, whose orbits are more nearly circular than those of objects such as Comet Halley.

However, Encke's Comet had an even greater surprise in store. After the gravitational effects of other planets had been allowed for, the period between its appearances was still shortening. One popular explanation was that there had to be some "resisting medium" slowing the body's motion, decreasing its period and causing it to spiral toward the Sun; but no planet's orbit showed evidence of a similar change. The German astronomer Friedrich Bessel (1784-1846) suggested that the "rocket effect" of erupting jets of gas from the sunward side of a cometary nucleus could do the trick. It turned out that he was right, but it took more than a century to prove it.



A different case was the curious demise of Comet Biela, which astronomers in 1845 and 1846 saw split into two pieces. The pair—more widely separated than before—were observed again in 1852, but since then there has been no trace of either. The year 1872 saw a spectacular meteor shower whose components all had the same orbit as that of the missing comet. Evidently Comet Biela had disintegrated into a swarm of small particles.

Some astronomers had already suspected a connection between at least some comets and meteors. A particularly great display by the annual Leonid meteor shower in 1833 provided the catalyst, and by the 1860s the Leonid shower had been shown to be due to debris from Comet Swift-Tuttle. Similarly, astronomers realized that the Perseid meteors that appear every August are rubble from Comet Tempel-Tuttle. These showers also taught scientists something very important about the physical makeup of comets, for of all the billions of such shower meteors seen to enter our atmosphere, not one is known to have reached the Earth's surface. Thus the stuff of comets must be weak and fragile indeed (although, traveling as fast as they do, they can still pack a wallop).

At about the same time, the newly invented spectroscope (a device that analyzes matter by spreading out into a rainbow the light that it emits, absorbs, or scatters) showed that the light of comets comes partly from sunlight reflecting from small particles and partly from glowing gas, with the ratio between the two varying from comet to comet—and sometimes from day to day for the same object. However, in most cases the only gas that could be definitely identified was molecular carbon (a molecule made of two carbon atoms), a situation that remained unchanged until the 20th century. Two exceptions were comets that passed very close to the Sun in 1882 and displayed evidence of sodium and even iron vapor. Unfortunately, with the passage of time these observations tended to become discounted and forgotten by astronomers.

In the 19th and early 20th centuries scientists generally believed that a comet's nucleus was a swarm of small, separate particles following the same orbit around the Sun. This "flying gravel bank" or "flying sand bank" view was supported by the connection between comets and meteor showers, but astronomers now know that, at least in some cases, it is wrong.

Modern Analysis

The 1910 appearance of Comet Halley caused a worldwide sensation. Astronomers carried out extensive observations, and the public was swept up in an excitement bordering on hysteria. The chance that the visitor would hit and destroy the Earth worried many (in actuality it never came close), as did the prediction that the Earth would pass through the comet's tail, whose poisonous gases would kill us all (the tail was so rarefied that the danger was nil). Unfortunately, after all the excitement had subsided, it turned out that scientists had learned relatively little that was new about comets.

The next decades saw scientific knowledge about comets grow slowly, steadily, but unspectacularly; public interest was at a low level due in part to a lack of bright examples in the decades after Halley's appearance. Improved equipment and better physics helped astronomers identify a number of simple molecules—ionized carbon monoxide, for example—in the spectra of comets. (When an atom or molecule is ionized, it gains or loses one or more electrons, thereby acquiring an electric charge.) Since these substances are short-lived, it was conjectured that they may be produced when ultraviolet light from the Sun breaks up more complex "parent" molecules driven off from the comet's nucleus. The "daughter" molecules are "excited" by sunlight and then give off light as they return to a "deexcited" state. But the search for the parents was a long one.

A real advance came in midcentury when Fred L. Whipple, a professor of astronomy at



Harvard University, published his "icy conglomerate," or "dirty snowball," model of a comet's nucleus. In this view the nucleus is a single solid body (though only weakly welded together) a few miles across, composed of assorted ices shot through with grains of a metallic and stony nature that are much like the stuff meteorites are made of. When far from the Sun, a comet is only an inert lump of frozen material, but as it approaches our star, sunlight heats the nucleus. The surface ices evaporate, releasing gas and dust that easily escape the snowball's feeble gravity. The dust is pushed away from the Sun by the pressure of sunlight, while the gas molecules, broken apart and ionized by solar ultraviolet light, are swept up by the "solar wind," a stream of ionized particles flowing outward at high speed from the Sun. As a result of these two different processes acting on two different kinds of materials, a comet can have two separate tails. Generally, the "dust" tail is shorter and more curved, while the "ion" tail is longer and straighter.

Whipple suggested that common substances such as methane, ammonia, and especially water were the parent molecules that formed the ices. In addition, he proposed that as a comet aged it would develop a dark covering of dust that managed to avoid being swept away with the gas (a remnant pile of snow in the springtime shows what such a crust may look like). Finally, he revived Bessel's old "rocket" idea, suggesting that sunlight produces jets of gas and dust by heating isolated, exposed areas of surface ice.

Whipple has been proved correct on every major point, and his ideas on the nature of a comet are the ones that today's scientists hold—with some modifications. For example, astronomer Mark V. Sykes of the University of Arizona has argued that in at least some comets the proportion of the nucleus made up of rock (as opposed to ice) may be greater than once thought, and that comets may be more like frozen mudballs than dirty snowballs. Also, recent observations have again raised the possibility that the nucleus, rather than being a single solid dirty snowball, may be composed of many smaller bodies held together loosely by gravity. This might explain how the units of Shoemaker-Levy broke apart so easily in 1992 under the influence of Jupiter's powerful gravitational pull.

But advances in theory were not the whole story. Until the middle of the 20th century, astronomers essentially were limited to observations made at wavelengths of light that the human eye could see. But after World War II that straitjacket was loosened, first of all by the development of infrared and radio astronomy. The advent of space vehicles was the next step, for from above the Earth's protecting yet obscuring atmosphere comets can be studied at all wavelengths. Finally, there came probes that could fly by comets and take close looks. Because of these advances scientists were finally able to detect the parent molecules and examine the nucleus in detail.

Halleymania Returns

As 1986 approached, scientists were determined that the mediocre scientific results from the 1910 appearance of Halley's Comet would not be repeated, and they made elaborate preparations for its return. Observations were planned and carried out from a wide variety of terrestrial observatories, satellites, and space probes.

The comet, as seen from the Earth, did not put on much of a show. Astronomers took great pains to warn the public not to expect spectacular views. Their caution was spurred by the fiasco of Comet Kohoutek in 1973 and 1974, when tremendous publicity and confident predictions of the "comet of the century" were followed by a dismal flop of a spectacle. Scientifically, however, things went well with Halley's Comet. The most prominent parent molecule proved to be ordinary water, along with carbon monoxide and carbon dioxide. The presence of methane and ammonia was inferred, and the presence of hydrocyanic acid was confirmed. Surprisingly, formaldehyde was detected in the form of polymers, or complex



chains forming very complex molecules. This finding might hold some very important clues regarding the origin of life on Earth.

Close-up images of the heretofore unseen nucleus of Comet Halley showed a solid, potato-shaped body mostly covered with a coal-black crust, with jets of gas and dust erupting from a few active areas of exposed ices. This comet, at least, has a solid nucleus.

The Origins of Comets

The jury is still out on when, where, and how comets are formed. Unlike the cases of Earth minerals, Moon rocks, and meteorites, scientists do not have any hard evidence for the age of a single comet. However, they assume that comets formed when the rest of the solar system did, some 4.5 billion years ago. That great age is a problem, since Comet Halley, for example—not to mention the short-period comets—would have been gone by now if it had always been in its present orbit, for passing near the Sun every 76 years, it would have evaporated early in the history of our solar system, which has been around for almost 5 billion years.

As to where comets formed, it is clear that their nursery must have been far from the Sun, for they are mostly made up of water ice and of other ices that evaporate at even lower temperatures. It would have been necessary, therefore, for them to have been created in very cold conditions. On the other hand, it is known that they did not form around some other star. Astronomers have computed orbits for many comets and have concluded that not one has come in from outside the solar system, though a few appear to have left forever.

While no one is sure where comets formed, it appears that today most of them are in the so-called Oort cloud, a shell of some 100,000 million comets at the far reaches of the solar system beyond the orbits of the planets; it is named after the Dutch astronomer Jan H. Oort (1900-1992), who proposed its existence in 1950. In the Oort cloud cometary nuclei can last almost indefinitely, except for those that are lost to the Sun because of the gravitational pulls of nearby stars and the Milky Way. However, sometimes gravitational forces can change the orbits of cometary nuclei in the Oort cloud so as to cause them to pass close to the Sun. We see only those that come very near to our star; those that do not come close pass unseen—small and extremely faint mountains of dormant ice with no comas or tails to attract attention.

The Comet-Asteroid Connection

At the beginning of the 20th century, astronomers were convinced that comets died by disintegrating into clouds of small particles, as did Comet Biela. It turns out that things are not that simple. If a comet is far from the Sun, for example, it is just a small body that shines by reflected sunlight, and to observers it cannot be distinguished from an asteroid. Are "asteroids" in the outer solar system—which astronomers have been discovering recently—just dormant comets? They may be, but no one has been able to find out.

Closer to Earth there are a number of "asteroids" that may be the nonvolatile remains (what is left after the ices evaporate) of short-period comets. It may take a space probe that can analyze samples of such an object to decide if that theory is correct.

Comets and Us

In the 18th and 19th centuries the prospect of collisions of comets and Earth was taken very seriously. Then early in the 20th century the possibility of such an event largely lost its terror. Space is large, planets and cometary nuclei are small, and the odds seemed against such a meeting. But in the past few decades that comforting idea has been reexamined. Geologists



have proved that the Earth has undergone sudden and terrible catastrophes in its long history, events that seem to have come from the outside. In particular, encounters with both comets and asteroids have been blamed for episodes of enormous extinctions of life on our planet, especially the demise of the dinosaurs (the extinctions at the end of the Permian era were even more drastic and came close to eliminating all life on Earth).

But there is another side to the story. The dinosaurs were dominant for tens of millions of years and had no competitors in sight. Had they not been eliminated, mammals—and, in particular, humans—might never have had the chance to come to the fore. So we might owe our very existence to a comet.

To go back even further, life might not have had a chance to get started on Earth if it were not for comets. Our planet formed relatively close to the Sun, where volatile substances such as water and organic matter would find it hard to condense. Comets may have brought such material to Earth from the outer reaches of the solar system, and thus all life on our planet may be due to these visitors from distant regions.

Jupiter Jolted

Though similar events must have occurred before, the impacts of Comet Shoemaker-Levy 9 on Jupiter in July 1994 provided the first opportunity for scientists to study such collisions as they happened. Moreover, astronomers had over a year of warning to prepare. As a result, observatories around the world—along with satellites and space probes—were able to point their instruments toward the giant planet at the right times. Sensibly, astronomers were cautious in their predictions of what might happen, pointing out that Jupiter's enormous, nonsolid bulk might simply swallow the fragments and that not much of anything might be visible from Earth. But what actually happened exceeded even the most optimistic forecasts.

All the impacts were at about the same latitude on Jupiter but at different longitudes. They all occurred just before Jovian sunrise, and all on the side of the giant planet away from Earth. The explosions were enormous by earthly standards—as much as the equivalent of millions of megatons of TNT. (The largest thermonuclear device ever tested was about 60 megatons.) Some of the fireballs caused by the collisions were as large as Earth, and as they expanded, telescopic images clearly showed them rising above Jupiter's limb (the edge of the planet as seen from Earth).

Jupiter's rapid spin soon brought the impact sites into view and, surprisingly, revealed dark spots large enough to be easily visible even through relatively small amateur telescopes. Infrared images showed the crash locations as "hot spots," where the energy of the collisions had heated Jupiter's atmosphere. The dark spots visible to the eye through a telescope were among the most prominent features on the planet. They proved to be surprisingly durable, for although strong Jovian winds quickly began to distort and tear them apart, they were expected to remain visible for a year or more.

Jupiter is wrapped in clouds, has an extensive atmosphere composed mostly of hydrogen gas, and has an interior made up primarily of the same element in liquid form. The planet has no solid "surface," so before the impacts there was much speculation about changes that might take place in the planet's upper atmosphere. That the resultant spots were dark was not completely unexpected, for astronomers have known for some time that the white areas visible on the planet represent clouds that are at higher altitudes than those of the darker regions. Presumably the collisions made by the fragments, which were all in a light zone, would dredge up some of the lower-lying darker material, along with gases such as water vapor that are known to exist below the cloud tops.



Spectroscopic observations revealed the presence of elements and compounds not otherwise found in Jupiter's upper atmosphere. Presumably these substances came from the comet itself and from material dredged up from lower layers of Jupiter's atmosphere. The presence of water vapor was not confirmed. This negative result suggested first that there was none in the comet, and second that the fragments did not penetrate deep enough to reach the layers in the planet's atmosphere where water vapor resides. Evidently the pieces of Comet Shoemaker-Levy 9 exploded relatively high up in Jupiter's atmosphere, which is consistent with the resultant fireballs' prominence.

The results of the impacts intensified debate about whether Shoemaker-Levy 9 really was a comet after all. All of its pieces had tails, just like normal comets, but these shone only by reflected sunlight, showing that they were made of dust particles. There was no trace of the glowing gas—glowing because of the effects of energetic ultraviolet radiation from the Sun—that is given off by most comets. Could this, therefore, have been some unusual sort of asteroid (asteroids are generally thought to be made of rock and metal)? If it was, why would it disintegrate and shed fine material so far from the Sun? The most fascinating possibility was that it was an entirely new type of object, one that seldom comes close to the Sun.

Whatever the nature of Comet Shoemaker-Levy 9, its spectacular demise had some very concrete results. The incident showed plainly that life on our planet could be wiped out by such impacts. We now have the technology to avert such a disaster (a rocket armed with a nuclear device would presumably be able to change the object's course), but scientists lack the means to find such a potential killer before it is too late. Fortunately, at the very moment that the fireballs were exploding on Jupiter, the Science Committee of the U.S. House of Representatives voted to require the National Aeronautics and Space Administration (NASA) to track any comets or asteroids that pose a real threat to Earth. Soon afterward the space agency named a study panel to examine the feasibility of an early-warning system that would survey the inner solar system and detect all such objects.

The wheel has come full circle, but it has done so with a vengeance, for now we know for certain that comets and their ilk can indeed be the agents of our doom. Not as omens, as was thought in the past, but in a very dramatic, direct, and drastic way.

About the author: Ronald A. Schorn, former chief of ground-based astronomy for NASA, is a planetary astronomer and historian for Intaglio, Inc., a private firm in College Station, TX, that specializes in researching and writing social, economic, technical, and scientific histories.

Source: 1995 Collier’s Year Book.




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Comets - Mysterious Visitors From Outer Space