by Carl Sagan
The comet is barreling in toward the Sun, approaching the orbit of Jupiter—as, it may be, it has done a dozen times before when Jupiter was on the other side of the Sun and nothing untoward transpired. But this time, by chance, as it crosses the orbit of Jupiter, Jupiter happens to be nearby. It is the most massive of the planets, and the comet is, by comparison, a little thing, a puff of gas surrounding a speck of dust. Jupiter’s gravity attracts the comet—not enough to draw it into Jupiter (the comet, after all, is traveling at some tens of kilometers a second), but enough to deflect it toward Jupiter, and thus to change its orbit. The comet is still traveling in an ellipse about the Sun, but the gravitational encounter with Jupiter has dramatically changed its orbit.
A similar maneuver has been successfully negotiated by four spacecraft from Earth—Pioneers 10 and 11, and Voyagers 1 and 2—which in the 1970s were sent to Jupiter with such exquisitely crafted trajectories that the gravitational acceleration of Jupiter swung each of them like a slingshot off toward a precisely preplanned point in the sky. Voyager 2, for example, swung by Jupiter in such a way that it was propelled toward a close encounter with Saturn two years later. (The close encounter of Saturn was also designed to swing it by Uranus in 1986, and the Uranus encounter to swing it by Neptune in 1989.) Now imagine the Pioneer or Voyager trajectories run backward in time. The spacecraft approaches Jupiter from the outer solar system; it races around Jupiter and is carried on its new orbit toward the Earth.
This kind of gravitational billiards explains the Jupiter family of comets. The inner solar system is sprayed with many comets, a few of which by accident come close to Jupiter. Some are immediately ejected by the encounter out of the solar system, some run into Jupiter or one of its moons, but many others have their orbits converted so that they become short-period comets of low inclination, with aphelion and node near Jupiter’s orbit. Most short-period comets may have achieved their orbits by multiple gravitational encounters with Jupiter, or even by multiple encounters with more distant planets and, eventually, with Jupiter itself.
A comet arriving from the depths of space in a direct or prograde orbit—one that goes around the Sun in the same direction as the planets (clockwise as seen from above the North Pole)—is more likely to suffer a large perturbation by an encounter with Jupiter. A comet coming in with a retrograde orbit—going around the Sun in the opposite direction—is more likely to experience a small perturbation. But only large perturbations suffice to carry the comet into the inner solar system. This is why the motion of short-period comets is direct. The comets supplied from the depths of space are as likely to be on retrograde orbits as direct ones, but only the comets in direct orbits tend to be captured. Jupiter’s gravity makes distinctions.
It is estimated that there are about 2,000 long-period comets bigger than about a kilometer that cross Jupiter’s orbit every year. The number of comparably-sized short-period comets in the Jupiter family is about 1,400. When we talk of the orbital evolution of the comets we see, we are talking about thousands of worlds.
Laplace showed that new and long-period comets could be converted into short-period comets by a gravitational machinery working before our very eyes. His work also seemed to demonstrate that interstellar comets, visitors from beyond the solar system, could be converted into short-period comets about the Sun.
A few comets have been seen in their passage through the inner solar system on very slightly hyperbolic trajectories—untied to the solar system, bound for the stars. It is natural to think of them as interstellar nomads, perhaps comets from some other star system, long wandering through interstellar space, and by lucky chance just passing through on our watch. Indeed, Laplace believed that both the short- and the long-period comets as well as many “new” comets were captured by the Sun from a population of free interstellar comets. If so, then some short-period comets might have evolved through an orbital cascade, a succession of changing trajectories determined by successive planetary encounters. The implication, which was hardly lost on Laplace, was that comets are ultimately denizens of the interstellar cold and dark, a fact fundamental to the modern understanding of comets.
Nevertheless, despite their unconstrained trajectories, these hyperbolic comets do not come from interstellar space, at least none observed so far. Using the same sort of mathematical orrery that Laplace pioneered, we can track the orbits of the apparently hyperbolic comets back in time. Remarkably, every such hyperbolic comet is only slightly hyperbolic; if it were moving only a little more slowly it would be gravitationally bound to the Sun. When we track the orbits of such hyperbolic comets we find, in their recent past, an approach to one of the major planets close enough to perturb the cometary orbits. All of them seem to be comets that have been on long elliptical orbits about the Sun, and then slingshot out of the solar system by gravitational encounters with Jupiter or another of the giant planets. We see them on their exit trajectories. Not a single interstellar comet has ever been observed.
By the middle of the nineteenth century, the study of comets was an established part of professional astronomy. Their fundamental motions could be grasped. Some new and long-period comets might eventually evolve into short-period comets, and in the tumult of gravitational encounters, comets might collide with the planets or the Sun or be ejected from the solar system. There was something chaotic, slightly unsettling, about the motions of the comets. Some fell to pieces unexpectedly, and at least one delivered itself of small non-Newtonian feints and lurches, interrupting the stately circumsolar procession. But by and large comets were considered well understood. By now it was possible to illustrate public lectures with photographs of comets, and some of these stirred the multitudes. For a glimpse of what a professional astronomer, expert on comets and with a flair for lucid explication, was saying to the general public on the subject, the following is an excerpt from a talk delivered in 1882 by William Huggins at one of the public lectures (Friday-evening discourses), held then and now for the general public at The Royal Institution, London:
With the aid of a telescope, in the heads of most comets a minute bright point may be found. The apparently insignificant speck is truly the heart and kernel of the whole thing—potentially it is the comet. It is this small part alone which conforms rigorously to the laws of gravitation … If we could see a great comet during its distant wanderings when it has put off the gala trappings of perihelion, it would be a very sober object, and consist of little more than nucleus alone … Under the Sun’s influence, luminous jets issue from the matter of the nucleus on the side exposed to the Sun’s heat. These are almost immediately arrested in their motion Sunwards, and form a luminous cap; the matter of this cap then appears to stream out into the tail, as if by a violent wind setting against it. Now, one hypothesis supposes these appearances to correspond to the real state of things in the comet, and that there exists a repulsive force of some kind acting between the Sun and the gaseous matter, after it has been emitted by the nucleus … Great electrical disturbances are set up by the Sun’s action in connection with the vaporization of some of the matter of the nucleus, and … the tail is matter carried away, possibly in connection with electrical discharges, in consequence of the repulsive influence of the Sun …
A comet would, of course, suffer a large waste of material at each return to perihelion, as the nucleus would be unable to gather up again to itself the scattered matter of the tail: and this view is in accordance with the fact that no comet of short period has a tail of any considerable magnitude.
The Great Comet of 1882, in what may be the earliest successful photograph ever obtained of a comet. Photograph by David Gill, in South Africa.
Virtually every one of these remarks of Huggins is in reasonable accord with the modern understanding of comets. Some were far ahead of the knowledge of his time. The subject was considered respectable and mature. Nevertheless, the central fact about the comets—their composition, and the nature of the spectacular variations in the appearance of comets—
was at best only dimly glimpsed. In reading the literature of the time, we are stuck by how rarely it was even acknowledged that these were important matters awaiting future discovery.
*Among the erroneous possibilities offered were friction by an enormous quantity of otherwise undetected interplanetary dust; the resistance of a postulated “luminiferous aether,” through which light waves were once thought to propagate; and a small departure of gravity from the inverse square law.
*A mechanical model of the solar system with the periods and/or distance of the moons and planets rendered to scale. Laplace and Lexell built no machinery, however; their orrery was purely computational.
*Halley’s Comet, which has a period less than 200 years, is today classified as a short-period comet.
CHAPTER 6
Ice
The frightful ice that covers the whole face of the land.
—HANS EGEDE, A DESCRIPTION OF GREENLAND, 1745
One of the central questions about comets, and surely the key to many mysteries, is their composition. What is a comet made of? Are they all made of the same stuff? In the sixteenth and seventeenth centuries, it was still customary to think of comets as Aristotle had—as gases, vapors, “exhalations” from the Earth, and perhaps from the Sun and the planets as well. Newton, clear-sighted as usual, thought otherwise. He noticed that the Comet of 1680 came very close to the Sun; at perihelion, 0.006 of an Astronomical Unit, a little under a million kilometers. This should, he estimated, have heated the comet to the temperature of red-hot iron, from which he deduced that it could not be composed only of vapors and exhalations—because then its substance would have been rapidly dissipated during perihelion passage. Instead, he concluded that “the bodies of comets are solid, compact, fixed, and durable, like the bodies of the planets.” Since the tail of this comet was “much more splendid” just after perihelion than before, Newton concluded that the heat of the Sun produces the tail: “The tail is nothing else but a very fine vapor, which the head or nucleus of the comet emits by its heat.”
Fair enough. But what is the nucleus made of? Just what is this “very fine vapor” that constitutes the tail? This is the problem with which Kant and many others had tentatively grappled. When a comet comes as close to the Sun as the Great Comet of 1680 did, then virtually any common material should start to vaporize. But many comets begin to develop comas and tails when they are between the orbits of Mars and Jupiter. Heated only by sunlight, the temperatures of these comets, as they begin to pour vapor out into space, are something like a hundred degrees below zero on the Centigrade or Celsius scale (–100°C). Materials like iron that do not vaporize until they reach a high temperature are called involatile or refractory. Materials like ice that turn into gas after relatively modest heating are called volatile. The comets, therefore, must be composed of something quite volatile. But what?
The propensity of some comets to split suggests a nucleus not very strongly compacted. The forces that hold it together must be rather weak. As we have seen, every now and then comets depart from their scheduled arrival times in the inner solar system, or even exhibit a tiny darting motion, entirely at variance with the languid Newtonian sweep that the comets usually present to outsiders as they fall in towards the Sun. These erratic and unpredictable nongravitational motions of the comets recall Kepler’s image of comets as fishes darting through the cosmic ocean. Encke described his comet as deviating “wildly”* from the motion predicted by Newtonian gravitation, and attributed the anomalous and unpredictable movements to some resisting gas in interplanetary space, retarding its motion. But the accelerations are much too abrupt, and we now know there is not nearly enough material between the planets to have any detectable effect on the motion of comets. Some very different explanation is needed.
Until quite recently, the prevailing cometary fashions were dominated by the known association between comets and meteor streams—as when Comets Biela/Gambart disappeared, leaving the Andromedid meteor shower in their wake (Chapter 5). As late as 1945 the leading American college textbook on astronomy accepted without question the idea that comets are “loose swarms of separate particles moving on parallel orbits through interplanetary space.” Some scientists believed this swarm of small meteors, imagined to make up the nucleus of the comet, to be gravitationally bound; others thought there was not enough mass in the nucleus to hold it together, and that instead an enormous number of small particles were traveling very closely in the same orbit through space. There was a certain tendency for advocates of this flying gravel-bank model to favor pointillist renditions of the comet head; a representative example is shown in the drawing on this page.
The gravel- or sand-bank hypothesis deftly explained why an aging comet might one day be replaced by a cloud of fine particles. The spectra of meteors, as they burn up in the Earth’s atmosphere, show the presence of such materials as iron, magnesium, aluminum, and silicon, typical constituents of rocks on Earth. If meteors are made of rocky stuff, and comets in turn are made mainly of meteors, it followed that comets were rocks and stones. Then what about the coma and the tail? It was proposed that the sand particles were coated by some more volatile solid that evaporated as the swarm approached the Sun, or that gases were baked out of the stones as they were heated. But it was hard to see that much of this material—whatever it was—could be left after a single passage close to the Sun, if in the first place it was only a thin coating around a grain of sand, or the gas trapped near the surface of a rocky particle. And there were other difficulties—the jet fountains, for example, which have no ready explanation in a loosely bound swarm of gravel.
Comet Pons-Winnecke, as sketched by Baldet. Courtesy R. A. Lyttleton from his book The Comets and Their Origin (Cambridge University Press, 1953).
For Comet Encke, for example, the issue is in our time definitely resolved: when probed by radar from a large radio telescope on the surface of the Earth, it shows a single solid nucleus, not a swarm of particles. The size of the nucleus detected by radar—a kilometer or two in radius—is consistent with other estimates. The nuclei of several other comets have also been detected by radar, accordingly sounding the death knell for the orbiting gravel-bank hypothesis of the cometary nucleus.* But before 1950 even the idea of a compact cometary nucleus was disreputable, and the nature of the cometary volatiles only vaguely glimpsed.
A summary describes briefly the observational success of the [icy conglomerate] comet model, both quantitatively and qualitatively. The surprising aspect of the model is its usefulness in spite of its vagueness …
—FRED L. WHIPPLE, “PRESENT STATUS OF THE ICY CONGLOMERATE MODEL,” HARVARD/SMITHSONIAN CENTER FOR ASTROPHYSICS, PREPRINT 1966 (1984)
Fred Whipple describes himself as an Iowa farm boy turned astronomer. He served as chairman of the Department of Astronomy at Harvard University and, for many years, as director of the Smithsonian Institution’s Astrophysical Observatory in Cambridge, Massachusetts. He had for years been thinking about small objects in the solar system, including the physics of meteors entering the Earth’s atmosphere, and the nature of comets (of which he has discovered half a dozen). By the late 1940s, Whipple was convinced that large quantities of matter poured out of comets near perihelion—much more than could be accounted for by ice coatings on sand, or the driving out of small amounts of vapor that might be trapped inside the individual grains. It was also clear that interplanetary space was too good a vacuum for comets to be able to refurbish their supply of volatiles when their orbits took them far from the Sun (as Huggins and many others had pointed out). The problem was especially severe for comets like Encke, which become very warm as they pass close to the Sun, and which have made many perihelion passages.
As a convenient shorthand, Whipple called the refractory materials “dust” and the volatile materials “ice,” and decided that the problem would be solved if there were a great deal more ice than the sand-bank model of the cometary nucleus admitted. He describes the idea as “obvious,” altho
ugh, remarkably, it had never been stated this baldly before. Such luminaries as Newton, Kant, and Laplace had all toyed with something similar, but it was Whipple who first stated the idea lucidly and coherently. He then showed that a number of other mysteries—including the splitting of comets, their dissipation into meteor showers, and the worrisome nongravitational forces acting on cometary motion—could all be explained if we revised our thinking and imagined the cometary nucleus as a ball of dirty ice, with mineral grains and perhaps other materials scattered throughout.
If comets are indeed made of dirty ice, then to understand comets we must understand something about ice. To begin, let’s suppose that a comet is made of ordinary water ice. There are some 92 kinds of atoms occurring in nature, the most abundant of which are hydrogen, helium, oxygen, carbon, and nitrogen. These atoms combine with each other according to specific laws that are collectively called chemistry. Because there is more hydrogen than any other kind of atom in the universe, typical pieces of cold cosmic matter tend to be rich in hydrogen. Atoms such as oxygen, carbon, and nitrogen often are attached to as many hydrogen atoms as they can accommodate. Oxygen, for example, likes to combine with two hydrogen atoms, forming a molecule symbolized H2O. The H, of course, stands for hydrogen, the O for oxygen, and the molecule in question, justly famous, is called water. A nitrogen atom likes to combine with three hydrogens, forming NH3, also called ammonia; and carbon likes to combine with four hydrogen atoms, forming CH4, a.k.a. methane. These abbreviations are a kind of shorthand picture of how the atoms are combined, what—if you could see it—the molecule actually looks like. There are many other simple combinations: CO, carbon monoxide; CO2, carbon dioxide; HCN, hydrogen cyanide … and an enormous variety of more complicated* molecules, such as HCOOCH3, CH3CCCN and HC10CH.