Certainly all the molecular necessities of life are to be found on comets. Humans, like most other lifeforms on Earth, are made largely of water, and except possibly for a few of the moons in the outer solar system, there are no known worlds richer in water than the comets. Comets also contain large quantities of organic molecules, useful for agriculture and biological engineering, and rock and metal probably sufficient for practical purposes. The great quantities of water also mean that oxygen for breathing should be easily extractable, and rockets working as the Centaur booster does, off liquid hydrogen and oxygen, could be readily refueled on a cometary surface. In each of these respects comets are much more obliging bases and habitats than, say, the rocky and metallic asteroids.
But life on Earth, almost all of it, runs off the energy of sunlight. The plants harvest the sunlight and the animals harvest the plants. The inner solar system is flooded with light but, except for the Earth and Mars, is depleted in water. The outer solar system, by contrast, is rich in (frozen) water, but poor in sunlight. Equatorial noon on a cloudless world in the Saturn system is no brighter than twilight on Earth. The water is where the light isn’t, and vice versa—a point emphasized many years ago by the American science writer Isaac Asimov.
With modern technology, we can now imagine doing something to redress this oversight in solar system design. Comets (and icy boulders from the rings of Saturn) can be driven or towed to the inner solar system, where the ice would be mined directly from the surface or, for nearby extinct comets, by drilling through the lag deposit to the icy core below. The water would be dissociated to make rocket fuel and oxidant, and to supply oxygen to human outposts in space and on the other terrestrial planets. There is so much water available from comets that it is even possible to imagine water provided to selected regions of parched worlds, permitting life to be transplanted to previously desolate environments. The organic matter in a dead comet or a carbonaceous asteroid, if finely pulverized, might also be used—as a growth medium for living things, and to moderate the hellish climate of Venus, by the same mechanism invoked (Chapter 16) in studies of the extinction of the dinosaurs and of Nuclear Winter. Since extinct comets with icy cores may be already at our doorstep, they may prove to be a critical factor in human utilization of space during the next century or two.
The biological essentials that are not supplied by the comet, at least not directly, are heat, warmth, energy, power. These are ordinarily supplied to comets only when they come close to the Sun. We can readily envision vast arrays of solar panels deployed on and around a comet fairly near the Sun—conceivably, even as far away as the orbit of Saturn. Farther out, we can contemplate large nuclear fusion reactors powering cometary bases. If fusion reactors—powered by water itself—become commercially feasible in the middle of the next century, as some experts predict, they would represent an ideal power source for cometary bases because of the abundance of ordinary water ice, as well as frozen heavy water, HDO and D2O (where here D stands for deuterium, the form of heavy hydrogen that has a neutron as well as a proton in its nucleus).
A more romantic and environmentally sounder idea has been proposed by the British-born physicist Freeman Dyson, who suggests that through genetic engineering we will one day be able to design a special tree of unprecedented size to grow on comets far from the Sun. It would be planted in the organic snows, and grow enormously, so that its leaves could gather in enough of the sparse sunlight. Various requirements must be fulfilled, including heat insulation, no loss of gases to the adjacent vacuum, and the like. Because of the low gravity, growth of the trees is not restricted by their weight, and Dyson envisions forests larger than the comets from which they sprout (see plate XVI). The oxygen produced in photosynthesis “will be transported down to the roots and released into the region where men will live and take their ease among the tree trunks.” “Will” sounds a little optimistic, but there seems to be nothing impossible about the proposal. But very far from the Sun—within the Oort Cloud, for example—even such heroic measures will be unavailing; the Sun is simply too dim there, and something like a fusion reactor will be needed to power the biological cycles and keep the place tolerably warm.
Forested Comets
I propose to you then an optimistic view of the Galaxy as an abode of life. Countless millions of comets are out there, amply supplied with water, carbon, and nitrogen, the basic constituents of living cells. We see when they fall close to the sun that they contain all the common elements necessary to our existence. They lack only two essential requirements for human settlement, namely warmth and air. And now biological engineering will come to our rescue. We shall learn to grow trees on comets … From a comet of 10-mile diameter, trees can grow out for hundreds of miles, collecting the energy of sunlight from an area thousands of times as large as the area of the comet itself. Seen from far away, the comet will look like a small potato sprouting an immense growth of stems and foliage. When man comes to live on the comets, he will find himself returning to the arboreal existence of his ancestors. We shall bring to the comets not only trees but a great variety of other flora and fauna to create for ourselves an environment as beautiful as ever existed on Earth. Perhaps we shall teach our plants to make seeds which will sail out across the ocean of space to propagate life upon comets still unvisited by man.
—FREEMAN DYSON, “THE WORLD, THE FLESH, AND THE DEVIL,”
THIRD J. D. BERNAL LECTURE, DELIVERED AT BIRKBECK COLLEGE, LONDON.
REPRINTED IN COMMUNICATION WITH EXTRATERRESTRIAL INTELLIGENCE (CETI)
C. SAGAN, ED., MIT PRESS, CAMBRIDGE, MASSACHUSETTS, 1973
It is a law of biology as well as of social relations that isolation fosters diversity. Picture a time in the far future with millions of inhabited comets, each harboring no more than a few hundred individuals. Within the Oort Cloud, it would take a day or more for a radio message traveling at the speed of light to reach one colonized comet from another. That might be sufficient to maintain some cultural homogeneity among these many worlds, but the absence of frequent visits would permit a slow divergence of cultural and behavioral norms, and an enormous diversification of social, political, economic, religious, and other views—a development that might be of major benefit for the human species. It is hard to see, though, what advantage might thereby accrue to individual nation states, the only current entities wealthy enough to foot the bill; and the nation states have a history of preferring their own short-term advantage to the well-being of the species. Thus, the time when the majority of the human species will have been scattered among the comets is, for this and other reasons, far off. But in the long term, if space technology continues to develop, we will go to where the surface area, the water, and the organic matter are—to the comets.
What I conceive to be one of the main designs of the Creator in the formation of such a vast number of splendid bodies is, that they may serve as habitations for myriads of intelligent beings … If this position be admitted, then we ought to contemplate the approach of a comet, not as an object of terror or a harbinger of evil, but as a splendid world, of a different construction from ours, conveying millions of happy beings to survey a new region of the Divine empire …
—THOMAS DICK, THE SIDEREAL HEAVENS AND OTHER SUBJECTS CONNECTED
WITH ASTRONOMY, AS ILLUSTRATIVE OF THE CHARACTER OF THE DEITY AND
OF AN INFINITY OF WORLDS, PHILADELPHIA, 1850
Thomas Wright’s vision of innumerable suns surrounded by cometary orbital rosettes. From his An Original Theory of the Universe (1750). Courtesy Michael A. Hoskin.
If, in the remote future, we have populated not only all the nearby small worlds, but also some of the comets out to the Oort Cloud, we will have gone, through a succession of slow steps, halfway to the nearest star. A natural progression exists from there to the rest of the Galaxy. The colonization of the Galaxy would happen all by itself if the Oort Cloud becomes populated. Individual comets are so loosely bound that casual gravitational perturbations by pa
ssing stars release enormous numbers of them (Chapters 11 and 16) from bondage to the Sun; they then slowly pirouette through interstellar space on their own. In the far future, inhabited comets in the Oort Cloud would be shaken free from the shackles of the Sun’s gravity, to begin seeding at least the nearer parts of the Galaxy with humans.
At present, it seems that as many as forty or fifty Earth masses of cometary material have been ejected from the Oort Cloud since its formation. Much larger masses must have been ejected from the planetary part of the solar system when the vicinity of Jupiter and Saturn was populated by enormous numbers of small icy worlds (Chapter 12). Estimates of these masses range from a hundred to as much as a thousand Earth masses. All this material, originally derived from our neck of the galactic woods, is now tumbling through space between the stars, randomly perturbed by passing cosmic objects over billions of years of time, and diffusing until they are distributed through a major sector of the Milky Way.* But up to now, all these comets have been, so far as we know, uninhabited.
No comet has ever been observed on a trajectory originating outside the gravitational influence of the Sun. And yet, sooner or later, such comets should be seen. In our own system, we conclude that many comets have been ejected into interstellar space after close passages by the Sun or the major planets. Especially with the discovery of debris rings around nearby stars (Chapter 12), it is reasonable to think that many—perhaps most—of the stars in the sky are similarly enveloped in clouds of comets, which are also being ejected into interstellar space.*
Newton seems to have been the first to imagine comets around other stars:
This most beautiful system of the sun, planets, and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being. And if the fixed stars are the centres of other like systems, these, being formed by the like wise counsel, must be all subject to the dominion of the One.
And Laplace envisioned comets “which, moving in hyperbolic orbits, can wander from system to system.” But space is very empty and the stars are far apart. If every star in the Milky Way Galaxy had an Oort Cloud like ours, and a cometary ejection rate like ours, the average time between arrivals here of true interstellar comets would be hundreds of years. Astronomers are eagerly waiting.
As time goes on, the Milky Way acquires more and more interstellar comets. If every star in the Milky Way ejects a thousand Earth masses of comets into interstellar space every 4.5 billion years, as ours has, then there may be the equivalent of the mass of a hundred million suns floating as comets, undetected in the space between the stars. As great a mass as this represents, it is much less than one-tenth of a percent of the mass of the Milky Way itself.
There seems to be a community of comets that fills the Galaxy. Comets are likely to form in the accretion disk around every proto-star. If the case of the solar system is typical, every star will eject something like a trillion comets into interstellar space—mainly in the nebular stage, with a trickle of cometary ejections continuing for the life of the star. If there are a few hundred billion stars, each of which has ejected something like several trillion comets, the number of interstellar comets in the Galaxy is some 1024 (or 1,000,000,000,000,000,000,000,000), more than the number of stars in the universe. (The number of comets still bound to stars would be larger.) A multitude of interstellar comets should by now be distributed entirely at random, in and between the galactic spiral arms. The average distance between them, even far from stars, would be some tens of Astronomical Units, the same sort of distance that separates the comets in the outer Oort Cloud.
A comet orbits a binary system, composed of red and blue component stars. The length, color, and multiplicity of the tails vary with position in the orbit.
A comet orbits a widely separated binary star in a figure-eight orbit. Again, the tails are shown for various orbital positions. Diagrams by Jon Lomberg/BPS.
The Galaxy can then be pictured as a vast flattened disk of comets, in which are embedded the more massive but less abundant interstellar clouds, stars, and their planetary companions. In the vicinity of the stars, the concentration of comets is greater. Accordingly, nowhere in the Galaxy would there be a place more distant from a comet than the Earth is today from the planets we have already visited. Advanced civilizations may, for all we know, be able to cross the Galaxy in one fell swoop on some kind of interstellar express; but the comets provide an opportunity for backward civilizations like ours to construct a Toonerville Trolley that chugs off for a few years, stops at a comet, explores, refurbishes, and then squeaks off to the next comet. The principal problem is to find and catalogue the nearby interstellar comets.
The comets in the Oort Cloud of the Sun and the cometary clouds of other stars may even merge and intermingle. There may even be comets on figure-eight orbits, bound alternately to two different stars, if the stars themselves are close enough together. The comets then, are moving steppingstones to anywhere—like the ice floes on which Liza was able to cross the raging river in Uncle Tom’s Cabin.
Even if there is no colonization of the comets, we will one day set out to explore the trans-Plutonian spaces, and it will make sense to refuel on the comets. They would still be steppingstones to the stars. Perhaps eventually comets will themselves be converted into spaceships bound for other star systems, taking thousands of generations or more before they approach a new star and the sleeping cometary forest awakens to long-forgotten sunlight. The prospect recalls the image sketched by the German astronomer J. H. Lambert, who wrote in the eighteenth century:
Thus we can conceive comets which, being attached to no particular system, are in common to all, and which, roaming from one world to another, make the tour of the universe … I love to figure to myself those traveling globes, peopled with astronomers, who are stationed there for the express purpose of contemplating nature on a large, as we contemplate it on a small scale … We may suppose that their year is measured by the length of their route from one sun to another. Winter falls in the middle of their journey; each passage of a perihelion is the return of summer; each introduction to a new world is the revival of spring; and the period of quitting it is the beginning of their autumn.
But there is nothing peculiarly human about this story, not even the urge to colonize. We can imagine the comet clouds making every star a dandelion; periodically or episodically, some of the seeds are shaken loose, carrying the local life forms out into the Galaxy. Occasionally, a seed travels of its own volition. Sooner or later, it would seem, unless all technological civilizations have an unerring drive for self-destruction, the expanding inhabited cometary clouds will encounter one another somewhere in interstellar space. In our explorations of the Oort Cloud, will we find a stray object on which some alien technology is evident?
There are a great many cometary distractions between here and the nearest star. Some of them may be substantial worlds, hundreds or even thousands of kilometers across. If there are a hundred trillion comets filling the spaces between adjacent stars, any advanced civilization will expand outward very slowly. This may be why we have not found evidence of alien visitors in our solar system: The Galaxy is too interesting, and they have not discovered us yet.
“We must not let our people,” said he, “lose all interest in Earthly matters. They are still the children of the Earth. They are someday to return to the Earth, but even if they never return, it will be useful to attach them to our dear old mother world by at least memory and grateful recollection.”
—JULES VERNE, OFF ON A COMET, PARIS, 1878
This is the time of the historic maiden voyages by the human species to the comets. There will be more elaborate missions to a wider variety of comets, eventually extending to great distances from the Sun. One day—probably in the next century—these spacecraft will carry human crews. We will be living on the comets, and with the aid of rocket engines and Newton’s laws, piloting the comets. If such a day comes, we will have fully justified the faith implied by the !Kung
people—that perhaps unique surviving culture that considers cometary auspices benign. In the !Kung language, comets are “The Stars of the Great Captains.”
*A sustained excursion in this direction was once championed by a psychiatrist named Immanuel Velikovsky. Of the countless refutations, see Scientists Confront Velikovsky, edited by Donald Goldsmith (Cornell University Press, Ithaca, 1977).
*Occasionally, one may briefly enter the inner regions of some other planetary system and streak across alien skies, perhaps provoking some amalgam of fear and wonder in creatures very different from us.
*Martin Harwit and E. E. Salpeter, astrophysicists at Cornell University, have proposed that celestial gamma ray bursts, observed by satellite observatories, are due to the impact of comets with neutron stars. If this is true, comets must surround many stars, because these still unexplained bursts come from all over the sky.
CHAPTER 20
A Mote of Dust
I heard much of the comet of that year, 1577, and was taken by my mother to a high place to behold it.
—JOHANNES KEPLER, REMEMBERING AN EVENT
THAT TOOK PLACE WHEN HE WAS SIX YEARS OLD
Contemplated as one grand whole, astronomy is the most beautiful monument of the human mind; the noblest record of its intelligence. Seduced by the illusions of the senses, and of self-love, man considered himself, for a long time, as the centre of the motion of the celestial bodies, and his pride was justly punished by the vain terrors they inspired. The labour of many ages has at length withdrawn the veil which covered the system. Man appears, upon a small planet, almost imperceptible in the vast extent of the solar system, itself only an insensible point in the immensity of space. The sublime results to which this discovery has led may console him for the limited place assigned him in the universe.