When we calculate the fate of that original population of small worlds, we discover that gravitational interactions with the newly finished jovian planets would have ejected multitudes of kilometer-sized worlds into the outermost gravitational frontier of the solar system, like an automatic pitching machine throwing baseballs into the bleachers once a minute for a hundred million years. This is how the Oort Cloud was generated. There is a population of primitive bodies that four and a half billion years ago were sequestered so far from the Sun that no vaporization, no collisions, nothing at all could transform them. They are the stuff from which the solar system was formed, and they are waiting for us in the Oort Cloud. Even a single comet newly arrived from the solar system frontiers is the answer to an astronomer’s dream.
It appears reasonable to suppose that during the phase of the formation of the major planets there were besides the planets many small condensations, which may have had comet-like structures … many of these stray bodies must ultimately have been absorbed by the planets (or proto-planets), but it would be unavoidable that during this period of conglomeration a number of the small condensations suffered large perturbations, bringing them into orbits of considerable eccentricity. The perturbations at subsequent perihelion passages would then start a process of diffusion … in the outward direction … The minute stellar perturbations required to bring the perihelion outside the region of the major planets do not … contribute appreciably to the escape of comets. Their main effect is to … “catch” them semi-permanently in the large cloud surrounding the solar system … Once the stellar perturbations had deviated the orbit so that they no longer pass through the inner part of the solar system, evaporation would practically cease, and the comets could easily retain their volatile components up to the present time.
—J. H. OORT, “EMPIRICAL DATA ON THE ORIGIN OF COMETS,”
CHAPTER 20, IN G. P. KUIPER AND B. M. MIDDLEHURST, EDS.
THE SOLAR SYSTEM, VOLUME 4, CHICAGO, 1965
In the 1960s, V. S. Safronov, a Russian specialist in the early history of the solar system, and in 1981, J. A. Fernandez, a young Uruguayan astronomer, and W. H. Ip, in Germany, showed that if primitive cometary bodies (those kilometer-sized objects) were formed in the vicinity of Jupiter and Saturn, gravitational perturbations by these massive planets would eject them out of the solar system altogether. But if these protocomets were born in the vicinity of the less massive planets, Uranus and Neptune, their gravitational influence would tend to eject the cometary bodies into the Oort Cloud, but not out of the solar system. So if these primitive icy and rocky worlds had condensed throughout the solar system, most would have been used up in making planets and in being ejected into interstellar space. But trillions at least would have been relocated to the Oort Cloud.
If the protocomets had been formed in the vicinity of Jupiter, exotic ices would not have survived; and, if formed still closer to the Sun, even ordinary water ice would not be retained. Thus, two independent considerations—making the primitive comets out of the right stuff, and ejecting them into the right orbits—point to an origin in the rough vicinity of Uranus and Neptune.
Comets, it seems, were formed ultimately out of interstellar grains within the solar nebula, just a little before the moons and planets formed, some 4.6 billion years ago. Many comets collided with each other, forming larger bodies, and sacrificing themselves so the planets would be made. Our planet also seems to have been formed from such objects, poor in ice, rich in rock. Many other comets were gravitationally ejected from the solar system altogether as, sooner or later, they made close passes by the jovian planets, and especially Jupiter. But the calculations show very clearly that a substantial population of the original comets must have been ejected to the far reaches of the solar system, where the random gravitational shuffling of passing stars and interstellar clouds would have forced them into more circular, randomly inclined orbits. Not all would have been ejected out to the very periphery of the solar system, and the calculations predict a substantial population of comets on near-circular orbits from hundreds to tens of thousands of Astronomical Units out—a population of comets fairly impervious to gravitational disturbances by passing stars. Comets may also have formed at these distances in the accretion disk of the solar nebula. It is therefore possible that a typical denizen of the inner Oort Cloud has never been seen by astronomers on Earth. It is entirely plausible that much bigger comets than those several kilometers across were ejected into the Oort Cloud. But there are far fewer of these, and much more rarely will we see one redirected into our small but well-lit volume of space.
If this currently popular picture is correct, a typical short-period comet is an aggregate of interstellar matter condensed during the origin of the solar system almost five billion years ago, ejected by the newly formed planets, Uranus or Neptune, to the solar system frontiers, its orbit there circularized by gravitational encounters with passing stars. A few billion years later, the cumulative gravitational influence of further stars and interstellar clouds drives the comet back into the planetary part of the solar system, where close planetary encounters—this time especially with Jupiter—reduce the large elliptical orbit into the more modest dimensions of a short-period comet. The homecoming has been long delayed, and the solar system has changed considerably in the interim.
Like everything else of which we have evidence, comets are born, live for a time, and then die—or at least disappear. After a short-period comet wends its long way from Uranus to the Oort Cloud to Jupiter, what happens next? Each time the comet passes through the inner solar system, it runs a gauntlet of risks. Eventually, the odds catch up. For some, every perihelion passage shrinks the comet by a meter, until in the end there is hardly anything left. Other comets collide with something in their path, transmogrify themselves into a different world, or set out for the interstellar void. These several fates have, it turns out, deep consequences for the planets today and, it seems likely, for ourselves. We trace these connections in the following chapters. A graphic summary of the evolution of the solar system and the birth and deaths of the comets is provided.
Because of these various ways for a comet to die, there would after a while be no short-period comets at all—if new comets were not being recruited into the inner solar system by Jupiter’s gravity. As on Earth, the places of those recently departed are taken by a new, exuberant, but relatively inexperienced generation.
The comets are way stations in the evolution of planets. They have seen much. As remnants of the forming solar system they can tell us much. Both the comets and the planets are formed of interstellar materials. The difference is that the planets have been enormously reworked, physically and chemically, since the beginning of the solar system, while the comets of the Oort Cloud remain comparatively unscathed by the ravages of time. This is the principal motivation for the dawning age of spacecraft exploration of comets. When we study the comets, we study our own beginnings.
*This direction of rotary motion—counterclockwise if you were looking down on the solar system from high above the North Pole—is called direct or prograde. Motion in the opposite direction is called retrograde.
*And for this reason some scientists have supposed that Pluto is an escaped satellite from Neptune, returning periodically to the scene of the crime.
CHAPTER 13
The Ghosts of Comets Past
Oft you shall see the stars, when wind is near,
Shoot headlong from the sky, and through the night
Leave in their wake long whitening seas of flame.
—VIRGIL, GEORGICS, BOOK 1
… Through the tranquil and pure evening skies, a sudden fire shoots from time to time, moving the eyes which were steady, and seems to be a star which changes place, save that from the region whence it was kindled nothing is lost, and it lasts a short while.
—DANTE ALIGHIERI, PARADISE, CANTO XV
You hear about them before you ever see them. “Did you see the falling stars last ni
ght?” the adults ask one another. You wonder how old you’ll have to be before they’ll let you stay up late and see for yourself. Falling stars. The words conjure up something tragic—a star, high and proud for years and then, for some secret transgression, humbled before our eyes. Cosmic justice.
When you’re older, ten say, and you finally get to see your first falling star, you’re pleased; it’s like fireworks. An “ooh” or an “ah” might even escape your lips. You try to remember back, and see whether any star that used to be up there is now missing. But it’s hard to do. There are so many faint stars. Even so, with stars falling every night, you wonder why there are any stars left at all.
The phrase “falling star” has a model of the universe built into it: Stars can be loosened from their attachments to the firmament and plummet to Earth. Stars, therefore, must be little things. You see the point of light streaking from the horizon toward the zenith, brightening and then fading. Where did it go? It seems more natural when it falls the other way, from zenith toward horizon. Then you might be tempted to look for a fallen star—by taking a brisk walk over to where the trail of light seemed to end. In the next county, maybe. At age ten, what do you imagine you might find? Something with five points, wrapped in silver paper, glistening in the snow? Perhaps.
“Falling star” and “shooting star” are of course not scientific terms. The proper word is meteor. A meteor is an object that as it falls through the Earth’s atmosphere produces a trail of light. It looks something like a magnesium flare. Unlike the comets with which they are sometimes confused, meteors, of course, do streak across the sky. They are very tiny. If one falls in solitary splendor, it is called a sporadic meteor. If it is a member of a group of meteors all falling on the same night from the same part of the sky, it is a part of a meteor shower. The brightest meteors are called fireballs, and the brightest fireballs can outshine the Moon or even the Sun. Its brilliant head is teardrop-shaped and accompanied by a streak of light and scattered sparks. After a daylight fireball falls, a trail of dark smoke is sometimes seen. By definition, a meteor does not strike the ground. If you follow the streak toward the horizon, rush over to the next county and indeed recover a rock newly fallen from the sky, it is not a meteor, but a meteorite. The suffix suggests that meteorites come from meteors, and thus are smaller than meteors—which, in general, is untrue. That big hole in the ground in Arizona is called Meteor Crater—but the object that dug the crater was much too big to be a meteor. It was a meteorite. Meteorites, though, are pieces of other worlds. It is a reasonable guess that meteors are also.
“The meteors whisper greenly overhead” is a lovely and evocative line by the American writer Loren Eiseley. In fact, meteors whisper only to themselves. They streak too silently through the upper air to be heard down here. Like comets, and children in Victorian dramas, they are seen but never heard. Meteorites—fragments chipped off the asteroids or the Moon or Mars or extinct comets—can be heard; they and the fireballs produce on occasion a sonic boom or a deep rumbling roar, the only sounds made by another celestial body that Earthbound ears unaided have ever heard.
Daimachus, in his Treatise on Religion … says that … for seventy-five days continually, there was seen in the heavens a vast fiery body, as if it had been a flaming cloud, not resting, but carried about with several intricate and broken movements, so that the flaming pieces, which were broken off by this commotion and running about, were carried in all directions, shining as falling stars do. But when it afterwards came down to the ground in this district, and the people of the place recovering from their fear and astonishment came together, there was no fire to be seen, neither any sign of it; there was only a stone lying, big indeed, but which bore no proportion, to speak of, to that fiery compass. It is manifest that Daimachus needs to have indulgent hearers.
—PLUTARCH (CA. 46 TO CA. 120 A.D.), LYSANDER
Prescientific cultures held meteors—like comets—to be portents of something or other, usually impending evil. More rarely, other explanations are proffered. In West Africa, there are traditions which hold meteors and meteorites to be a kind of solar excrement. Like the notion of a meteor as a star falling to Earth, this teaching of the Atak-pame people has more than a grain of truth to it, as we shall see. The Herero call them “buzzing stones,” which doubtless reflects some direct experience with a meteor fall. In other traditions, the meteors are the souls of the dead returning to Earth to be reborn; or a thunder ax; or the heralds of Mbomvei, the supreme being. The Jukun hold that a meteor is a gift of food carried from one star to another—an extraterrestrial take-out service. According to the Kamba, a meteor is a kind of royal ensign, signifying that the beings who live on the stars are this day visiting the Earth. In Islamic Africa, a shooting star used to be described as a dagger thrown by angels to thwart those genies who aspire to ascend to heaven. But in general—in Africa and throughout the world—meteors, like comets, were considered portents of pestilence, disaster, witchcraft, and death. Perhaps because meteors are so much more common than comets—if you’re patient, you can see them fall on any clear dark night—the evils attributed to meteors tend to be more humdrum than those for which the comets are held accountable.
From early times, the Chinese (of course) kept meticulous records of meteor showers, with careful attention to color. The earliest known description, “Stars fell like a shower,” is in the Ch’un Ch’iu, the Spring and Autumn Annals, describing an event that happened on March 23, 687 B.C. In ancient and medieval Europe, careful record-keeping on meteors was virtually unknown, but the occasional bright fireball was considered noteworthy. For example, in the year 1000—which had been widely advertised to be the date of the end of the world—a contemporary account relates:
The heavens opened, and a kind of flaming torch fell upon the Earth, leaving behind a long track of light like a path of a flash of lightning. Its brightness was so great that it frightened not only those who were in the fields, but even those who were in their houses. As this opening in the sky slowly closed, men saw with horror the figure of a dragon, whose feet were blue and whose head seemed to grow larger and larger.
Modern scientific interest in meteors was prodded by the following account by the German scientist Alexander von Humboldt in Camana, Venezuela, on the night of November 11, 1799:
From half after two in the morning, the most extraordinary luminous meteors were seen in the direction of the East. M. Bonpland, who had risen to enjoy the freshness of the air, perceived them first. Thousands of bolides and falling stars succeeded each other during the space of four hours … From the first appearance of the phenomenon, there was not in the firmament a space equal in extent to three diameters of the Moon, which was not filled every instant with bolides and falling stats … All these meteors left luminous traces from five to ten degrees in length … the phosphorescence of these traces, or luminous bands, lasted seven or eight seconds. Many of the falling stars had a very distinct nucleus, as large as the disk of Jupiter, from which darted sparks of vivid light.… The light of these meteors was white, and not reddish … The phenomenon ceased by degrees after four o’clock, and the bolides and falling stars became less frequent; but we still distinguished some to North-East by their whitish light, and the rapidity of their movement, a quarter of an hour after sunrise.
An eyewitness rendition of the great Leonid Meteor Shower of November 13, 1833. After Fletcher Watson, Between the Planets (Harvard University Press, 1956).
Humboldt found that many observers, including some in Europe, had seen the same marvel that night, and concluded that a meteor shower was something that happened over a large area of the Earth and high up in the atmosphere. But this straightforward conclusion posed numerous problems:
Whatever may be the origin of these luminous meteors, it is difficult to conceive an instantaneous inflammation taking place in a region where there is less air than in the vacuum of our air pumps … Does the periodical recurrence of this great [meteor shower] phenomenon depe
nd upon the state of the atmosphere? Or upon something which the atmosphere receives from without, while the Earth advances in the ecliptic? Of all this we are still as ignorant as mankind were in the days of Anaxagoras.
It is indeed a remarkable fact that meteor showers recur nearly on the same date of every year—on August 11, say, or December 14—although it might take a day or two to reach maximum intensity and a day or two to fall off. You can go out on a clear night, and count the numbers of bright meteors. With an intense shower you might have to count dozens every second. With a pedestrian sort of shower, you might have to wait a minute between bright meteors. You note where the meteors appear to be coming from. They are not randomly distributed over the sky, but instead are concentrated toward a particular place in a particular constellation. This focus, from which the meteors appear to be radiating, is called the radiant. As the constellation rises and sets, the radiant moves with it. So the meteor shower is characterized by the constellation from which it seems to emerge. The Leonid shower, around November 17, pours out of the constellation Leo; the Perseids, around August 11, out of the constellation Perseus, and so on. Now how, the nineteenth-century astronomers asked themselves, could meteor showers know what day of the year it was, and how did they manage this conjurer’s trick of pouring out of a tiny spot of sky, which rises and sets with the stars?
The night was fine; the moon was absent. The meteors were distinguished not only by their enormous multitude, but by their intrinsic magnificence. I shall never forget that night.… For the next two or three hours, we witnessed a spectacle which can never fade from my memory. The shooting stars gradually increased in number until sometimes several were seen at once. Sometimes they swept over our heads, sometimes to the right, sometimes to the left, but they all diverged from the East. As the night wore on, the constellation Leo ascended above the horizon, and then the remarkable character of the shower was disclosed. All the tracks of the meteors radiated from Leo. Sometimes a meteor appeared to come almost directly towards us, and then its path was so foreshortened that it had hardly any appreciable length, and looked like an ordinary fixed star swelling into brilliancy and then as rapidly vanishing. Occasionally luminous trains would linger on for many minutes after the meteor had flashed across, but the great majority of the trains in this shower were evanescent. It would be impossible to say how many thousands of meteors were seen, each one of which was bright enough to have elicited a note of admiration on any ordinary night.