Four symmetrically placed jets in the illuminated (upper) hemisphere of a non-rotating cometary nucleus (top). If the nucleus were rapidly spinning (counterclockwise in this schematic representation), the arms would trail the rotation and something like a swastika would be produced (bottom). Diagram by Jon Lomberg/BPS.
There is something anthropomorphic about the swastika. We read it as arms and legs in motion. It is one of the few symbols, not ordinarily found in nature, that is both simple and compelling. If something like it slowly materialized in your night sky amidst shrouds and fountains of dust—something self-propelled, animate, almost purposeful—you would surely find the experience noteworthy. You would speculate on its meaning, its religious significance, its portent. People would copy the symbol down so others would know about it, so that this marvel would not be forgotten. Whether you view it as an auspicious sign or as a harbinger of disaster, no one need explain to you that the thing is important.
The swastika form is not very different from the pinwheel structures seen in many comets, and brought out in short exposure photography with a large telescope. Comet Bennett 1970 II is a recent example. For Comet Bennett, at least, the color of the pinwheel was yellow, implying that the structure is in the dust. You look at these forms and perhaps you will grant at least that if enough spinning, jetting comets pass by the Earth, sooner or later there will be one that presents something like a swastika to view. But the argument alone is insufficient to convince you, or us, of a cometary origin for the symbol. For so speculative a subject, at least one piece of more direct evidence is needed.
Under these circumstances it is arresting to find, in the culture with the longest tradition of careful observations of comets, a straightforward, apparently unambiguous description of a swastika as just another comet. Such is the case of the twenty-ninth and final comet to appear in the ancient silk atlas of cometary forms that was unearthed in a Han Dynasty tomb at Mawangdui, China (Chapter 2). It dates from the third or fourth century B.C., but is an anthology of observations that must be much older. The twenty-ninth comet is called “Di-Xing,” “the long-tailed pheasant star.” The caption is the lengthiest of all twenty-nine, because the swastika comet alone is subject to differing interpretations. This comet is associated with change. “Appearing in spring means good harvest, in summer means drought, in autumn means flood, in winter means small battles.” Of course, these auspices are fanciful, but the origin of the swastika in a cometary apparition now seems to us a real possibility. We wonder if the connection is drawn in other little-noticed artifacts of ancient cultures.
Long ago, the swastika achieved worldwide distribution, and was, unlike the usual cometary attributes, almost everywhere considered benign. Thousands of years later, the Nazi brand of racism and plunder arises and seeks a symbol to represent the allegedly superior, allegedly racially homogeneous peoples of Northern Europe. They already call themselves Aryans, after the light-skinned Persian invaders of dark-skinned India in the middle of the second millennium B.C. Under the Nazis, the swastika is emblazoned on uniforms, weapons, stationery, aircraft, and assorted regalia. Children all over the world practice drawing the symbol. Under the banner of the swastika, the Nazis murder tens of millions and usher in an era in which humans are able to destroy their global civilization and perhaps the human species as well. A pinwheel of evaporating ice appears in solitary magnificence in the skies of Earth, and 3,500 or 4,000 years later its form— remembered through all the intervening generations of humans—is still employed to symbolize both good and evil. When we peer out into space we see the many varieties of our own nature reflected back.
The last seven comets shown in the Mawangdui silk from third or fourth century B.C. China. Nothing in the caption indicates that the first form, in the shape of a swastika, was considered fundamentally different from the other comets shown. Wen Wu. “Ma Wang Tui po shu ‘T’ien wen ch’i hsiang tsa chan’ nei jung chien shu” and “Ma Wang Tui Han ts’ao po shu chung to hui hsing t’u.” Volume 2, pp. 1–9 (Beijing, Wen wu ch’u pan she, 1978).
Both the form of this luminous ejection, and the direction in which it issued from the nucleus, underwent singular and capricious alterations, the different phases succeeding each other with such rapidity that on no two successive nights were the appearances alike. At one time the emitted jet was single, and confined within narrow limits of divergence from the nucleus. At others it presented a fan-shaped, or swallow-tailed form, analogous to that of a gas flame issuing from a flattened orifice; while at others again two, three, or even more jets were darted forth in different directions.
—JOHN HERSCHEL, “ON THE 1835 APPARITION OF HALLEY’S COMET,”
OUTLINES OF ASTRONOMY, LONDON, 1858
PART II
Origins and Fates of the Comets
CHAPTER 11
At the Heart of a Trillion Worlds
How many other bodies besides these comets move in secret, never rising before the eyes of men! For god has not made all things for man.
—SENECA, NATURAL QUESTIONS, BOOK 7, “COMETS”
We have reason to suspect that there are a great many more comets, which being at remote distances from the Sun, and being obscure and without a tail, may for that reason escape our observation.
—EDMOND HALLEY, TRANSACTIONS OF THE ROYAL
SOCIETY OF LONDON, VOLUME 24, PAGE 882, 1706
The ancients imagined the planets to be attached to invisible machinery—transparent crystal spheres, elegantly coupled and geared. We now know that the ancients were wrong. The planets orbit the Sun guided only by the invisible hand of Newtonian gravitation. Some worlds are rock, some gas, some ice, and nowhere, from Mercury to Pluto, is there anything like a crystal sphere. But imagine ourselves leaving the solar system at some impossible speed, until even the orbits of the outermost planets are too small for us to see, until even the Sun is only a point of light no brighter than the brightest stars seen from Earth. Then we do encounter something like a crystal sphere, but a shattered one—a cloud of a trillion shards and fragments of ice, little worlds each the size of a city, feebly illuminated in the great dark between the stars.
We live at the heart of a trillion worlds, all of them invisible. It sounds like the teaching of some New Age sect. And we are not talking of metaphorical worlds; rather a trillion places, every one of them as distinct a world as ours is, every one gravitationally bound to the Sun, every one with a surface and an interior and on occasion even an atmosphere.
If there is a ceiling above you, step outside. Cast your eye upward. Concentrate on the smallest piece of sky you can make out. Imagine it extending in a widening wedge far out into space, to the stars. In that little patch of sky are a hundred thousand worlds or more, worlds unseen, unnamed, but in some sense known. These distant cousins of the Earth are the cometary nuclei—cold, silent, inactive, slowly tumbling in the interstellar blackness. But when they are induced to fall into our part of the solar system, they creak and rumble, begin to evaporate and jet and eventually produce the tails so admired by the inhabitants of Earth. How we know of this invisible multitude of icy worlds is one more scientific detective story that begins with Halley.
It seems however not agreeable to the uniformity of the universe, that after a short view of the Sun [the comets] should be continually flying farther off, in that wide void beyond the planetary bounds, to creep along that dark cold region for millions of years … but that they should rather revolve about the Sun, in certain, though long periods.
—THOMAS BARKER, OF THE DISCOVERIES CONCERNING COMETS, LONDON, 1757
After Edmond Halley made the first inventory of cometary orbits, it was clear that many comets return infrequently, once every few centuries or even longer (see Chapter 4). At any given moment, he knew, there must be unseen long-period comets that have not lately visited the Sun. And if by chance we see comets with periods of years to centuries, perhaps there are others with periods measured in millennia or more. As one of the epigraphs to th
is chapter shows, Halley was prepared to believe in a large population of undiscovered comets with very long periods and high eccentricities. But he did not envision truly immense numbers of comets. When Thomas Wright drew a rosette of orbits surrounding the Sun, he was not tempted to include more than the few known comets, although he did conclude “the Comets … I judge to be by far the most numerous Part of the Creation.”
The key to the discovery of the comet cloud is the orbits of the comets we see. We must bear in mind that this constitutes only a small sample of all cometary orbits. For all we know, our sample may not even be representative of the larger population. But it is our only starting point.
An elliptical cometary orbit has a certain size. Its near point to the Sun is called its perihelion and its far point, its aphelion, terms we have been using throughout this book. The line from perihelion to aphelion, running through the Sun, is the major axis of the ellipse, and half the major axis is called the semi-major axis. The semi-major axis of the Earth’s orbit is, of course, one Astronomical Unit (A.U.). Comets with small semi-major axes never leave the planetary part of the solar system, and comprise the kingdom of short-period comets. Such comets are tightly bound to the Sun’s gravity; it would take a very major influence to perturb their motion significantly. But comets with large semi-major axes spend most of their time far beyond the region of the planets, and less often than once a human lifetime they make a brief foray into the inner solar system. Such long-period comets are much more loosely bound to the Sun, and are more easily perturbed. The convention is to call a comet with a period less than two hundred years a short-period comet, and one with a period longer than two hundred years a long-period comet. There is nothing magic about two hundred years; it is chosen only because this is very roughly (now a little less than) the period of modern astronomical study of the comets. So a comet like Encke’s or (by this definition) Halley’s are short-period comets, while one like Comet Kohoutek, which passed by the Earth in 1973 and will not return for another ten million years, is a long-period comet.
Laplace once imagined the Sun stationary in space, but surrounded by a vast population of randomly moving interstellar comets. Some would, by chance, find themselves moving very slowly with respect to the Sun; they would be attracted by its gravity, and would fall into the inner solar system. He showed (Chapter 5) that the net result in the vicinity of the Earth would be many comets on highly eccentric orbits, but bound to the Sun, and more rarely a comet on a hyperbolic orbit, dipping once into the inner solar system, never to return again. And this is just what we seem to see. Because the calculation agrees with the actual observations of comets, Laplace took it to be a confirmation of the existence of a great cloud of interstellar comets in which the Sun and planets are embedded.
But later investigators pointed out that the Sun has a proper motion of its own, at present moving at a goodly clip toward a point in the constellation Hercules.* When the fall of randomly moving interstellar comets is calculated for a Sun in motion, a large number of hyperbolic comets is predicted, contrary to observation. By the late nineteenth century, Laplace’s concept of a cloud of interstellar comets was firmly rejected. The resolution of this difficulty—to imagine the exterior comets loosely bound to the Sun, so that the Sun is not moving with respect to them—seems not to have been thought of until the second third of the twentieth century.
Laplace had also calculated that the short-period comets were being destroyed—by gravitational ejection from the solar system, by running into a planet now and then, or (we can now add) merely by dissipating into interplanetary space after a sufficient number of perihelion passages. If there were a vast cloud of interstellar comets, then the population of short-period comets could be resupplied by the cascade from interstellar comet to long-period comet to short-period comet, the planetary billiards we have already discussed. But if the Sun isn’t sweeping up new comets from interstellar space, how is the population of old comets in the inner solar system replenished?
There are only two possibilities: Either comets are being made today somewhere in the solar system, or there is a vast repository of hidden comets that supplies a steady trickle of samples. All suggestions about how comets might be manufactured lately, in anything like sufficient numbers, have failed. That leaves only the possibility that the comets are sequestered. If they were stored nearby, we would have some sign of them. It follows that they must be stored at a great distance from the Earth (and the Sun). But where? And how many?
If we are able to see a few long-period comets, on highly eccentric orbits, plummeting into the inner solar system, might there not be much vaster numbers in slow, circular orbits, disdainful of the inner solar system, out there beyond Pluto? That might account for the nearly random orbital inclinations of the long-period comets; we can imagine them as decoupled from whatever influence confines the planets and the short-period comets to the ecliptic plane. The cloud of comets would move as Newton imagined the planets should had God not intervened at the Beginning. Such comets would be much too far from the Sun to develop comas or tails. They would be invisible from Earth.
[This] article indicates how three facts concerning the long-period comets, which hitherto were not well-understood, namely the random distribution of orbital planes and of perihelia, and the preponderance of nearly-parabolic orbits, may be considered as necessary consequences of the [stellar] perturbations acting on the comets.
—J. H. OORT, “THE STRUCTURE OF THE CLOUD OF COMETS SURROUNDING THE SOLAR SYSTEM, AND A HYPOTHESIS CONCERNING ITS ORIGIN,” BULLETIN OF THE ASTRONOMICAL INSTITUTE OF THE NETHERLANDS, VOLUME 11, PAGE 91, 1950.
Jan Oort was for many decades the dean of the distinguished modern school of Dutch astronomers. Among his many contributions to his science are the first correct estimate of the distance of the Sun from the center of the Milky Way, the first use of radio astronomy to map the spiral structure of the Milky Way, and the discovery of episodic and titanic explosions at the hub of the Milky Way—which may indicate the presence of a massive black hole there. It was also Oort who, shortly after the end of World War II, proposed the existence of a distant cloud of comets, loosely bound to the Sun. Although some aspects of the theory were anticipated by the Estonian-Irish astronomer Ernst Öpik, the full beauty of the idea was first glimpsed and developed by Oort.
Just as Halley had examined the orbital characteristics of the handful of comets available to him, Oort studied nineteen long-period comets with well-determined orbits. He found a smattering of long-period comets with semi-major axes of a few thousand Astronomical Units (A.U.), and even a few tens of thousands of A.U. These are already very far from the Sun, hundreds of times farther away than Pluto is. But the bulk of the comets seemed to be clustered in the vicinity of 20,000 A.U. or more. Nineteen comets is not a large sample, but it is enough. Since Oort’s pioneering study in 1950, the statistics have improved, but the conclusion remains the same: Most long-period comets come to us from a region roughly 50,000 A.U. from the Sun.
Oort proposed that a vast cloud of unseen comets surrounds the Sun at these immense distances, and that all the comets we see are the deserters and refugees from that distant assemblage. Most of these comets are on fairly circular orbits, with modest eccentricity. They never enter the planetary part of the solar system, and we never see them. But occasionally a cometary nucleus leaves its fellows and plummets into the inner solar system, where it may come close enough to the Sun for us to designate it as a long-period comet; or else it might make a close pass by one or more of the major planets, and have its orbit progressively altered, so that eventually we describe it as a short-period comet.
But what induces this occasional comet, weakly held by the Sun’s gravity, to enter the inner solar system? Oort calculated that the Sun, in its motion about the center of the Milky Way Galaxy, would sometimes come close enough to other stars to make a kind of gravitational flurry in the comet cloud—spilling numbers of them in all directions, including to the vic
inity of the Sun. A typical comet in the Oort Cloud is circuiting the Sun at the leisurely pace of about a hundred meters per second, around 220 miles an hour. The change in speed administered by the passing star is only a few tens of centimeters a second, near the top speed your fingers can manage to walk across a tabletop. It represents a very small change in the overall speed of the comet, but it’s enough to send a few of them careening down among the planets. No single gravitational impulse from a passing star causes the comets to flutter about. Rather, the accumulation of a few dozen close stellar passages has produced a jittery population of faster-moving comets, and the latest stellar encounter provides the small increment needed to drive some of them down toward the Sun or out into the interstellar medium. It’s the straw that breaks the camel’s back.
Even if a star were to plow straight through the comet cloud the consequences would not be spectacular. Öpik likened it to a bullet traversing a swarm of gnats: comparatively few of the gnats are scattered or destroyed; the swarm continues almost undisturbed.* And comets deep within the Oort Cloud are not ejected by stellar perturbations at all; residing closer to the Sun, they are more tightly bound to it by gravity, and cannot readily be shaken loose by a passing star, unless it comes very near the Sun.