by Carl Sagan
In the earlier scientific literature there are reported observations of cometary nuclei generally said to be hundreds or thousands of kilometers in diameter. Almost certainly, these measurements—made near the closest approach of the comet to the Earth—were in fact of the brightest part of the coma; the nucleus itself, much smaller, must have been lying hidden inside. A few observers have claimed that they could see a background star winking off and on as the nucleus passed in front of it.
Sometimes, by chance, a comet travels through the line of sight from the Earth to the Sun. If the nucleus were huge—1,000 kilometers across, say—it might appear as a small moving black dot silhouetted against the solar disk. But in every case where the observations have been attempted—as, for example, when the Great Comet of 1882 or Halley’s Comet in 1910 transited the Sun—the nucleus was too small for any trace of it to be seen.
Occasionally, a comet comes close to the Earth without a thick coma, and a small, bright point within the coma can be seen. This may be the cometary nucleus, but it may be only a dense inner coma of dust.
When the comets come close to the Earth they veil themselves from view by inquisitive astronomers. In general, they only undress in the dark—beyond the orbit of Jupiter. If you can catch the comet when it is far from the Sun, before it has generated a coma, then you might catch a glimpse of the naked nucleus. But because comets are so small, when they are at great distances from the Sun we can only see them as points of light no matter how bright they are, and no surface detail is discernible. There is an intrinsic limit to the resolution of ground-based telescopes, and to our ability to distinguish fine detail. So how can we know even the size of a naked nucleus? Astronomers have found another way.
The amount of sunlight reflected back from the nucleus to the Earth is measured: then, if only we knew how dark the nucleus is, we could calculate how big it has to be to return the amount of sunlight we measure. Bigger bodies reflect more light. The more reflective the surface, the smaller the nucleus must be to return the measured amount of light; conversely, the darker it is, the larger it must be. For dark, dirty cometary nuclei, for which there is other evidence, the calculated sizes are several kilometers across, or less.
A more direct way to determine the size of a cometary nucleus is to send a burst of radio waves out from the Earth to intercept the comet and be reflected back. Even a dense planetary atmosphere, like that on Venus, is transparent to such radar probing; these radio waves will pass through the coma with no difficulty at all. The few comets that have come close enough to Earth since the advent of advanced radar astronomy show nuclei between two hundred meters and a few kilometers across, in good agreement with the best estimates made by other techniques. The most direct measurement of nucleus size is with a spacecraft camera that flies through the coma (this page).
A cometary nucleus even five kilometers across, almost the size now estimated for Halley’s Comet, doesn’t sound like much; but if it were gently placed at a typical locale on the Earth’s ocean bottoms, it would stick up smartly above the ocean surface, making—at least until it melts—an unusual variety of tropical island.
The number of comets that have been so measured represents less than 1 percent of the known comets. How likely is it that much smaller or much larger comets exist? A comet a few kilometers across is the size of a city. Are there then comets as small as a house, or ones as large as Luxembourg, say, or Brunei? Very small comets in the inner solar system might be impossible to detect; they also would have an extremely brief lifetime as their ices vaporized and the comet dissipated. As comets split and break up, there are surely fragments released that are the size of a house or smaller, but they do not long survive. Oddly, astronomers do not find more and more small comets by looking at fainter and fainter objects. Some think this is merely because smaller comets are harder to find, but others suspect there is a real lack of comets smaller than a football field (a hundred meters across). No one knows why.*
There is indirect evidence that occasional long-period comets are considerably larger, perhaps a hundred kilometers across or even more. The most celebrated example is the Great Comet of 1729. It was visible to the naked eye, although its perihelion was in the exterior of the asteroid belt, almost at the orbit of Jupiter. If it had instead come all the way to the vicinity of the Earth, it would have been bright enough to read a newspaper by at midnight.
A family portrait of some of the irregular small moons in the solar system. Because their gravities are so weak they are not compressed into a spherical shape. (a) Deimos, the outermost moon of Mars. (b) Three views from different vantage points of the Saturnian moon Hyperion. (c) Some of the small moons of Saturn. (d) Phobos, the innermost moon of Mars. The shapes of cometary nuclei are expected to resemble what is depicted here. Photographs obtained by the Viking and Voyager spacecraft during their historic reconnaissance missions. Courtesy National Aeronautics and Space Administration.
For it to have been so bright so far away, it must have been both very large and outgassing great quantities of exotic ices. In the vicinity of Jupiter, it is too cold for significant vaporization of ordinary water ice. As we have seen, an iceberg of frozen nitrogen or carbon monoxide or methane begins vigorous evaporation near the orbit of Pluto, or beyond. Comets made of such materials would have spent much of their substance before ever coming close enough to Earth to be seen. Comets made of ammonia or carbon dioxide would vigorously evaporate between the orbits of Jupiter and Saturn. But unless such comets are particularly massive, their outgassing and jetting should remain indetectable from Earth.
Comet Kohoutek was exceptionally bright when it was still far from the Sun, from which widely publicized conclusions were drawn about how brilliant it would be as it passed the Earth in December 1973. But the apparition was far from spectacular; Comet Kohoutek was visible with the naked eye from the surface of the Earth, although it was much more clearly seen by the Skylab astronauts in Earth orbit. Apparently Kohoutek was so bright so far from the Sun because of the vaporization of exotic ices, a process entirely completed when the comet came close to the Earth. This is not expected for Hale-Bopp.
From the periodic appearance of jets, it has been possible to calculate the rotation period of the cometary nucleus, even though it is sequestered inside the coma. The spin rates of dozens of comets have now been measured by this or other methods. A typical comet turns out to rotate once every 15 hours, not too different from the length of the day on Earth. The direction of the spin axis in the sky seems to be entirely random: cometary spin axes do not, for instance, mainly point toward the North Star. For some comets, observations ranging back in time over a century or more can be used to determine rotation rates. Comet Encke, for example, has not changed its rotation period much in 140 years—although its period of revolution is erratic due to the rocket effect in ice (Chapter 6).
A photographic negative of Comet Ikeya (1963 I). Since the comet was moving slightly against the background of more distant stars, the stars appear as short, dark lines in this time exposure. The transparency of the tail is indicated by the stars that can be seen through it. The horizontal scale is one million kilometers long. Photograph taken by E. H. Geyer, Boyden Observatory, South Africa, February 24, 1963, courtesy K. Jockers.
Because the typical comet is so small, its gravitational pull is tiny. If you were standing on the surface of a cometary nucleus, you would weigh about as much as a lima bean does on Earth. You could readily leap tens of kilometers into the sky, and easily throw a snowball to escape velocity, as we imagined in Chapter 1. The Earth and the other planets tend to be almost perfect spheres because, as Newton showed, gravity is a central force—it pulls everything equally toward the center of the world, itself held together by the force of gravity. The mountains sticking up above the spherical surface of the Earth represent less of a deviation from a perfect sphere than does the layer of paint or enamel on the surface of a typical globe that represents the Earth. If you were able to pile a sizable mountain on
top of Mount Everest, it would not just sit there, poking in solitary magnificence into the stratosphere. The additional weight you had added would crush the base of Everest, and the new composite mountain would collapse until it was no larger than Everest is today. The Earth’s gravity severely limits how much deviation from a perfect sphere our planet is permitted.
On a comet, on the other hand, the gravity is so small that odd, lumpy, potato-like figures would not be squeezed into a sphere. Such shapes are already known for the small moons of Mars, Jupiter, Saturn, Uranus, and Neptune and for small asteroids. (See this page.) On a typical comet you could build a tower reaching a million kilometers into space, and it would not be crushed by the comet’s gravity—although it would certainly be flung off into space by the comet’s rotation. Both the small moons and the cometary nuclei stay lumpy because their gravities are low. But they get lumpy partly for different reasons—the moons because of the history of collision with other bodies, and the comets because they accreted irregularly, or because they have vaporized surface ices irregularly.
What is the pressure at the center of a cometary nucleus? Pick one that has a radius of a kilometer. Everyone has the sense that the pressure from the weight of the overlying rocks one kilometer beneath the Earth’s surface is considerable. We know about coal mine shorings collapsing and miners being crushed. But if the gravity were thirty thousand times less, then the overlying rocks would weigh thirty thousand times less. Put another way, the pressure at the center of such a comet is the same as it is 1/30,000 the comparable depth on the Earth. But 1/30,000 of a kilometer is 3 centimeters, about an inch. So the pressure at the center of a comet is roughly the same as that under a down comforter, or even a thin blanket. Even fragile structures can survive at the nucleus of a comet.
Suppose some comets had interior structure—a rocky inner core, say, or even an underground lake. How would we know? When a comet passes very close to the Sun and is split apart, its interior is suddenly exposed to space. Is there then a different kind of ice observed evaporating? Do we see previously undiscovered molecules in the spectrum of comets after fragmentation? A different ratio of gas to small dust particles? The answer to all these questions seems to be no. The interiors of these comets, at least, seem to be made of the same stuff as their exteriors—although involatile cores are hard to exclude. It is true that only a few comets have been examined as they split, and it is possible to imagine a quite different population of comets with interiors very different from their exteriors that have not in the past century come close to the Sun and therefore never been subjected to this test. There might be a reasonably abundant population of larger comets that are far from uniform throughout. We simply haven’t seen any yet.
From space, the atmosphere of the Earth is only a thin blue band hugging the horizon, compressed by gravity. On a comet, by contrast, the pathetically low gravity permits the atmosphere to stream over distances much larger than the size of the nucleus itself, producing comas tens of thousands of kilometers across. Indeed, much of a cometary atmosphere is not bound to the nucleus at all; the velocities in the spectacular jet fountains approach a kilometer a second—far in excess of escape velocity. At the surface of a cometary nucleus, the atmosphere is as thin as in the black sky 75 kilometers above the Earth’s surface.
The gases around a cometary nucleus do not form a permanent atmosphere as on Jupiter, or the Earth. Instead, these gases are in transition between their production from evaporating ices and their escape to interplanetary space. The gravity is simply too low to hold even the heaviest gases to the comet. Circumstances on a comet are unlike the situation on Earth, where the vaporization of the winter snows or the outgassing from a volcano or a fumarole generally makes negligible changes in the overall composition or pressure of the comparatively massive terrestrial atmosphere. On the comet, by contrast, the responsiveness of the thin cometary atmosphere to changes in the gases arriving from the nucleus should produce dramatic changes with time in the coma and the tails. This is exactly what is observed.
The coma is often an asymmetrical hood that develops from the sunward hemisphere of the cometary nucleus. It sometimes has a sharp boundary, and successive shells are regularly ejected by successive outgassing events. William Huggins described the coma as “a luminous fog surrounding the nucleus.” In the inner solar system, there is also an extended halo of atomic hydrogen, and OH, from the breakdown of water vapor by sunlight, that envelops each comet and that glows strongly in the ultraviolet; generally, the halo is larger than the Sun. Until the advent of orbiting telescopes in the early 1970s, no one had seen the hydrogen corona. Even in visible light, the head of a comet—the nucleus and coma, apart from the tail—can be larger than the Sun; the Great Comet of 1811 is an example. As the comet approaches the Earth’s orbit the size of the coma shrinks, although the comet becomes more active.
The new and long-period comets tend to be brighter and bigger than their short-period counterparts, because they are fresh from the outer solar system, and loaded with volatile ices that have not yet experienced the Sun’s heat. After they vaporize much of the ice on successive perihelion passages, they become smaller and less active.
The gas streaming off the nucleus is a kind of wind blowing into the interplanetary vacuum, and entraining cometary dust. Some comets are very dusty, others comparatively clean. Even in an aging cometary nucleus, there are still localized jets of gas and sources of fine dust. While unpredictable, there is a tendency for both jetting and nongravitational forces to increase the closer the comet is to the Sun, consistent with the vaporization of ices and the rocket effect.
Dusty comets have been observed to pour tons of fine particles into interplanetary space every second, and for most comets several times more water is lost than solids. Besides tiny individual particles of dust swept off into space in the jets, there may also be fragile clumps. A cloud of particles at least centimeters across surrounding a cometary nucleus has been found by radar. The relative amount of dust and ice near the surface of a cometary nucleus probably varies from comet to comet.
Comet Schwassmann-Wachmann 1, orbiting the Sun always beyond Jupiter, has produced over a hundred known outbursts, an average of about two a year. In successive perihelion passages, many comets seem to lose a meter or so of ices each perihelion passage for a number of orbits, and then close down shop. It may be that the refractory, rocky stuff that has not gushed away to space with the ices is now dominant, preventing sunlight from penetrating to the cold interior, and also preventing buried ices, even if warmed, from making their way to the surface. After successive evaporations, the surface of some comets may resemble the lag deposit on a glacier, the involatile rocky material covering a deep layer of still frozen ice. But for most comets, layer after layer of ice is lost in successive perihelion passages. Since the material in the coma and tail is never recaptured by the comet, it gradually dwindles, successive layers are peeled off and lost to space, and its interior parts are exposed to view. One way or another, every comet we see is dying.
We think ourselves unhappy when a comet appears, but the misfortune is the comet’s.
—BERNARD DE FONTENELLE, THE PLURALITY OF WORLDS, PARIS, 1686
*It is possible, however, to contrive such an eclipse artificially, at spacecraft altitudes, by using an opaque disk to block out the Sun.
*Charged particles in Jupiter’s magnetotail—formed by the interaction of the solar wind with Jupiter’s magnetic field—reach to the orbit of Saturn; this planetary magnetotail is larger than any known comet tail. But it was discovered only when a spacecraft flew through it. You cannot see it.
*Although there is an intriguing possibility that this paucity of small comets, if real, may be an echo of events in the building of the solar system, 4.6 billion years ago (Chapter 12).
CHAPTER 8
Poison Gas and Organic Matter
Hast thou ne’er seen the comet’s flaming flight?
The illustrious stranger passin
g, terror sheds
On gazing nations from his fiery train
Of length enormous; takes his ample round
Through depths of ether; coasts unnumber’d worlds
Of more than solar glory; doubles wide
Heaven’s mighty cape; and then revisits earth,
From the long travel of a thousand years …
—EDWARD YOUNG, NIGHT THOUGHTS, 1741
[The Laputans’] apprehensions arise from several changes they dread in the celestial bodies. For instance … that, the Earth very narrowly escaped a brush from the tail of the last comet, which would have infallibly reduced it to ashes; and that the next, which they have calulated for one and thirty years hence, will probably destroy us.
—JONATHAN SWIFT, GULLIVER’S TRAVELS, 1726
William Huggins scared the world. It was entirely unintentional. No one could have foreseen it. He was minding his own business, which happened to be astronomy. But because of Huggins, in 1910 there were sustained national panics in Japan and Russia lasting for weeks; a hundred thousand people in their nightclothes filled the rooftops of Constantinople; Chicago apartment dwellers anxiously stuffed rags under their doors; and Pope Pius X condemned the hoarding of cylinders of oxygen in Rome. A dispatch from Lexington, Kentucky—typical of reports from all over the world—announced that “excited people are tonight holding all-night services, praying and singing to prepare themselves … and meet their doom.” Through fear of the imminent catastrophe, a smattering of people in many lands took their own lives. But all this is getting ahead of the story.
Huggins was one of the first astronomical spectroscopists—scientists who decompose light into its constituent colors, or frequencies, and are able to deduce the motion and composition of a distant object. Spectroscopy traces back to another towering accomplishment of Isaac Newton. By passing a ray of sunlight, admitted into a darkened room, through a prism of glass and a slit Newton showed that ordinary white light is really a mixture of light of many colors. Different colors of light were bent by different angles when passing through the prism, so they could be spread out, or dispersed, against a surface on the other side of the prism. At first the surface was something like a white piece of cardboard. Much later, it became a photographic emulsion. The machine built around the prism was called a spectrometer, and the rainbow pattern generated was called a spectrum (plural, spectra).