Chapter One

THE PLURALITY

OF WORLDS

What is the nature of the Universe? Where do we come from? Why are we here? Where are we going? Where do we, as living intelligences, fit into creation? What are those countless heavenly bodies really like? What, if anything, are they good for? Are they habitable worlds? Do they, or did they ever, actually harbor life -- even intelligent life?

These questions are now raised almost daily by a flood of news stories relating the discovery of planets orbiting about other stars, or reporting evidence of simple life forms in meteorites that come from Mars. Certainly, all these questions are timely. But, astonishingly, they are also among the most ancient questions that have intrigued mankind. They have occupied the best minds of philosophy, religion, and science since the dawn of history. Indeed, the evolving answers to these questions are themselves a documentation of how we have changed -- and where we are going. The philosophers of the Greek golden age; the Hindu sages who wrote the Shrimad Bhagavatam Mahapurana and the early Jyotish literature; Moses, the Old Testament prophets, and the early church fathers; and countless folk traditions around the world give us windows into the ancient human soul, allowing us to see -- at least to glimpse -- the world as it seemed to our forebears. Today, as we stare in astonishment at the Hubble Space Telescope deep-field image of layer upon layer of galaxies retreating back into the deepest depths of time, and as we read almost weekly of the discovery of strange worlds orbiting other suns, we share the deep feelings of awe and mystery, of reverence before an unfathomable and endlessly diverse Universe, that are no less real than those felt by the ancient seers. Whether scientist or artist or mystic, we are stunned by the beauty and complexity of what we see. But whether we look with the eyes of science or religion, we see order and lawfulness underlying superficial complexity. There is no trait more human, or more ancient, than the desire to seek understanding -- to bring order to our experience and express that order in universal laws.

QUESTIONS ABOUT the heavens were discussed by the earliest Greek philosophers, starting with Thales and Orpheus in the sixth century B.G. Their naked-eye Universe consisted of Sun and Moon and several small planets ("wanderers") that moved against the background of "fixed" stars. Both argued that the Moon was, in some general way, similar to Earth, and that Earth, Moon, planets, and the pinpoints of light in the sky were all alike in kind; Thales called them all "stars."

In the fifth century B.C., members of two very different philosophical schools became intrigued with these questions. The early atomists Democritus and Leucippus emphasized their belief that all creation must be made out of vast (in fact, probably infinite) numbers of a few basic kinds of elementary building blocks. These individual particles, or atoms, all obey the laws of nature, which are everywhere the same. The atomists were inclined to conclude that there must be other Earths, other life, even other people in space. But the Greek concept of space was very different from ours. For example, Anaximander, a student of Thales', saw the Solar System as fitting snugly inside a sphere decorated with countless tiny specks of light, the "vault of the heavens," centered on a cylindrical Earth. This kosmos , or "world," was in effect a capsule surrounding Earth. The fourth-century B.C. Greek atomist Epicurus imagined countless spherical (perfect) kosmoi packed like a barrel of bubbles, each independent and self-sufficient, having no interaction with any of the others. Thus the idea of an aperoi kosmoi , or "plurality of worlds," was inspired by philosophical principles, not observational evidence, and was in any case devoid of practical significance: other kosmoi must lie outside our Universe, and were not even in principle observable. We would say that they existed outside the scope of physics, that they were "meta-physical."

The Pythagorean mystery school, in which Philolaus was trained, argued from the fact that the Moon and Earth moved around a common center; this reciprocity in their motions bespoke a symmetry in their natures. The Moon must, according to the Pythagoreans, be another Earth. Thus two worlds, even two inhabited worlds, might reside in the same kosmos. Such an interpretation conflicted irresolvably with the idea that the heavens are heaven, whose contents are spiritual and perfect in nature, not material and imperfect. This idea of the supposed heavenliness of the heavens was furthered by Plato's doctrine of the ideality of spheres -- the heavens, when viewed as spiritual, ideal, or immaterial, could relate to Earth only in metaphor, not in physical reality.

The redoubtable Aristotle, in the third century B.C., argued in his De Gaelo (Of the Heavens ) that each of the four elements (earth, water, air, and fire) strove to attain its own natural place or plane in our perfect Platonic, and therefore spherical, kosmos. The natural place of earth (solid matter) is in the center. Therefore it is quite impossible to speak of other earthy bodies, as a sphere can have only one center. Aristotle revisited this issue years later in his Metaphysics , asserting that a postulated plurality of worlds would require a plurality of "First Movers," which he rejected as an obvious impossibility.

The Judeo-Christian scriptures have almost nothing explicit to say about other worlds. Even the planets clearly visible to the unaided eye in the night sky were ignored by the ancient Israelites. If they left no record of Mercury, Venus, Mars, Jupiter, and Saturn, why should we expect them to have discussed other, more remote, and quite unseen worlds? Paul's Epistle to the Hebrews, dating from the first century A.D., begins with the first and only biblical reference to a multitude of worlds: "God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom he made the worlds." But even here it is not clear what Paul means by worlds -- whether he is referring to the visible bodies of the Solar System, or speaking in broader terms.

The first century A.D. Roman natural philosopher Lucretius, in his De rerum natura (On the Nature of Things ), fully adopted the atomist position and argued for an infinity of worlds: "There is, as I have said, no end; this truth speaks for itself, and blazes forth from the very depths of creation. Neither can we, seeing space reaching in all directions, infinite and free, and seeds [atoms] infinite in number flying in eternal motion through the void, suppose that only this one Earth and sky of ours has been created, and that those teeming remote bodies of matter serve no other use but ours. Further, as this world was fashioned by Nature through the intrinsic motion, collision, and joining of seeds [atoms], thrown about at random without any design, at best accreting when thrown together by chance, so must all great things begin -- Earth, and sea and sky and races of living creatures. There must, I repeat, be such accretions of matter elsewhere, clasped, like our Earth, in the embrace of infinite space." Lucretius, who denies intelligence and purpose in creation, and attributes all change to the mindless collisions of a myriad of atoms "without any design," nonetheless explicitly assumes that every material object in the infinite, unbounded Universe exists for a purpose, to fulfill some function or serve some use. Lucretius's "argument from utility," after passing through numerous incarnations, survives in our own day in the form of proposals to make practical use of solar power and the material resources of space. This theme is explored in an earlier book of mine, Mining the Sky (Addison-Wesley, 1996).

The third-century Neoplatonist philosopher Plotinus raised a fascinating question with regard to these putative other worlds: "Could God make something better than that which he actually has made?" If He is omnipotent, He clearly could. If so, did He? If He did not do so, why didn't He? Note the implication that an all-powerful God would be capable of creating a world better than Earth. Christians, driven by the logic of divine omnipotence, it would seem, must accept the plausibility of Lucretius's opinion that habitable worlds are possible and even abundant; that life is widespread in space; and that there exists a plurality of Earths, with all that that implies. Thus predictions based on the theories of Plotinus and Lucretius, which respectively attributed Creation to divine power and to blind, godless chance, seem shockingly similar.

There were many points of Christian doctrine that seemed to bear on the issue of a plurality of worlds. For example, theologians generally agree that Christ's atonement was intended as a lifting of the burden of sin from the posterity of Adam. But the weight of original sin, which the people of Earth inherited from Adam, never fell upon the residents of other worlds. Therefore Christ's atonement could not apply to them. Those putative beings might therefore be lower life-forms incapable of sin (animals), or intelligent beings that had not yet fallen, or intelligent beings that had incurred sin but had never received an atonement. The last case points to irredeemably lost souls in Satan's grasp. But the idea that Christ may have sacrificed himself more than once on other worlds was, and is still, found offensive by most Christians. To some extent, the situation could be saved by denying the simultaneous existence of multiple populated worlds. Plotinus's contemporary Origen offered another possibility: that multiple peopled creations were acceptable so long as they were sequential, rather than parallel.

After the third century A.D., Western thought was put on hold for nine centuries. The Greek classics were largely lost -- in fact, apparently irretrievable -- after the burning of the Library of Alexandria in A.D. 391 by order of the Christian emperor Theodosius, whose pious hope was to purge the world of heathen writings. But many of the Greek classics survived in Arabic for centuries, unsuspected in the West. In the seventh century, during the long sleep of European civilization, Muhammad commented that there were "many worlds with lands and seas, and with human inhabitants," an idea whose antecedents seem more Greek than Arab.

In the wake of the first Crusades, much Arabic literature became available to Western scholars. In A.D. 1170 Gerard of Cremona translated Aristotle's De Caelo from Arabic into Latin, inspiring a searching examination of Greek wisdom by the Church. A thirteenth-century sage, Albertus Magnus, recognized the compelling interest of the issues raised by the Greek philosophers: "Since one of the most wondrous and noble questions in nature is whether there is one world or many, a question that the human mind desires to understand per se , it seems desirable for us to inquire about it." That spirit of inquiry was to mark the coming age.

In A.D. l266, Roger Bacon, in his Opus maius , like Michael Scot and the Bishop of Paris, William of Auvergne, rejected other worlds on ostensibly Platonic grounds, arguing from the supposed perfection of the sphere: "If there were another mundus [kosmos] it would be of spherical shape, like this one, and there cannot be any distance between them, because there would then be a vacant space without a body between them, which is false. Therefore they must touch; but they cannot touch each other except at one point (by the twelfth proposition of Euclid's Elements ), as has already been shown for circles. Hence everywhere except at that point of contact there must be void between them." Thus the principle that "nature abhors a void" became an argument against the plurality of worlds.

A year later, in 1267, Thomas Aquinas wrote in his Summa theologica that "the only ones who can assert that many worlds exist are they who do not acknowledge any ordaining wisdom, but rather believe in chance, as did Democritus, who said that this world, along with an infinite number of other worlds, was made from a casual confluence of atoms." Thus Aquinas made the concept of a plurality of worlds out to be a pagan philosophy, incompatible with Christianity. Ironically, this brought his conclusions nicely into line with those of the pagan philosopher Aristotle. Aquinas's argument would be echoed four centuries later by no less than Sir Isaac Newton.

Aristotle's writings were widely read, especially in the many universities that had been founded in the thirteenth century. In response to the spread of Aristotelian philosophy, Etienne Tempier, the Bishop of Paris, issued a Condemnation in 1277, attacking 219 common beliefs current in the universities. One of the propositions he denounced was the assertion that "the First Cause cannot make multiple worlds." In effect, Tempier affirmed the principle of the omnipotence of God. Because of Tempier's remarks, the trustworthiness of the Aristotelian basis of many of the arguments held by academia came into serious question, leading to a veritable landslide of anti-Aristotelian philosophizing.

The fourteenth century saw an acceleration and polarization of the debate over the plurality of worlds. The English philosopher William of Ockham, famed as the man who honed Occam's razor, was obliged as part of his requirements for the master of theology degree at Oxford to write a commentary on the Sentences (c. 1150) of Peter Lombard, Bishop of Paris. Ockham took on Lombard's Distinction XLIIII, which dealt with the recurrent question of Plotinus and others as to whether God could make the world better than that which he had made. Ockham boldly argued a slightly different question, "Could God make a world that is better than this one?" Ockham used this opening as an opportunity to argue for the plurality of worlds. His forceful challenge to this and other points led to Oxford's refusal to grant him his degree, and later to his excommunication.

In 1350 Jean Buridan, the rector of the University of Paris, argued that God had the power to create other worlds, and that the laws of motion may be different on other worlds. Although this sounds to the modern ear like heretical science, Buridan's meaning was rather less sensational: he argued that the "law" that dense materials (the element earth) returns naturally toward the center of Earth when displaced does not apply to other planets, whose dense matter would return toward their centers if displaced. In other words, he accepted a multitude of gravitating, material bodies as plausible. His theory presents a qualitative appreciation of gravitation, but is devoid of predictive power.

In the mid-fifteenth century, the German cardinal Nicholas of Cusa proposed that the Universe was infinite and unbounded (and therefore without a defined center), in which every location was more or less on an equal footing. In such a Universe, there was no Obvious objection to the presence of vast numbers of worlds, many like Earth and each an independent center of attraction. In Nicholas's view, each world could, like Earth, be inhabited: "Life, as it exists here on Earth in the form of men, animals, and plants, is to be found ... in a higher form in the solar and stellar regions." This seems to be a case of confusing "the heavens" with "Heaven."

The crystallization of the heliocentric theory of the Solar System by Copernicus in his De revolutionibus orbium coelestium (On the Revolutions of Heavenly Spheres ) (1543) explicitly and successfully treated all the planets alike, drawing no distinction between the nature and location of Earth and those of the other members of the Solar System. This theory was criticized strongly by both Roman Catholic and Protestant clerics. Martin Luther himself fumed, "People give ear to an upstart astronomer who tries to show that the Earth revolves, not the Sun and the Moon. This fool wishes to reverse the entire science of astronomy."

One of the most remarkable characters in this drama, the exuberant hermetic mystic Giordano Bruno, heartily endorsed the Copernican system in his La Cena de la Ceneri (The Ash Wednesday Dinner ) in 1584. Arguing from a principle of unity, and heavily influenced by Lucretius, Bruno claimed that all the glowing, fiery bodies of the heavens were stars, analogous with our Sun, each of which could rightly serve is a center for a system of planets. Bruno, on one of his many extended journeys to evade the Inquisition, brought the doctrine of the plurality of worlds to England and established it there, where it was to flourish for centuries to come.

Tycho Brahe, unconvinced by the Copernican system (he held out for the Sun as the center of motion of the other planets, which then circled Earth), remained resolved about the uniqueness of Earth. The absence of any detectable shifts in thc positions of the stars as Earth orbited the Sun suggested to Brahe that they were huge bodies at enormous distances. He envisioned a vast gulf between Saturn and the stars, seeing these stars as so huge and hot as to be uninhabitable. From his argument that almost all of creation was a sterile wasteland, Brahe concluded that the idea of other worlds was false.

In 1605, Johannes Kepler expressed his opinion that "the Moon is correctly called by Plutarch a body such as Earth, uneven and mountainous, with even more mountains in proportion to its size than Earth," and suggested the moon's habitability. Kepler followed Brahe's argument from utility to a point, but reached the conclusion that those vast and distant suns could not be useless, and therefore must be attended by populated planets. But, curiously, Kepler's most powerful influence on this debate arose from his demonstration that the motions of the planets could be most easily explained by having them all pursue elliptical orbits around the Sun. Why was this so important? Because it called attention to the critical role of observation as the inspiration for and ultimate test of theory. In the dawning age of science, precise quantitative tests of theories quickly showed the inadequacy of the medieval penchant for reasoning by qualitative analogy.

In 1616, Tomasso Campanella, in his Apologia pro Galileo , addressed two of the most serious and damning attacks on the Italian astronomer Galileo Galilei. First, Galileo's assertion that there was water on the Moon and planets had been attacked on the Platonic and Aristotelian grounds that heavenly bodies were perfect, unchanging, and incorruptible. Allowing seas and mountains on the Moon "vilifies immeasurably the homes of the angels, and lessens our hope regarding heaven." In response, Campanella cited Genesis and Psalms regarding the "waters above the Earth" to dismiss this attack as an example of doctrinal impurity, neglecting Scripture in favor of slavish devotion to the opinions of the heathen Aristotle.

Secondly, it had been alleged by Galileo's critics that "if the four elements which form our world exist in the stars, it follows from the doctrine of Galileo that, as Mohammed declared, there are many worlds with lands and seas, and with human inhabitants. However, Scripture speaks of only one world and of one created man [but see Hebrews 1:2 and I Corinthians 15:45 -- 49] so that this belief is opposed to Scripture." Campanella countered by arguing that having many small systems within a great Universe created by God was in no way a contradiction of scripture, only of Aristotle.

In 1632 Galileo, in his Dialogue on the Two Chief World Systems , argued that neither rain nor liquid water could exist on the Moon; nonetheless, life of a kind vastly different from that on Earth was still possible, adapted to local conditions. This appears to be the first clear statement of the possibility of truly alien life.

The great essayist and chronic insomniac Robert Burton wrote an admirably concise summary of this debate in his Anatomy of Melancholy (sixth edition, 1651): "We may likewise insert, with Campanella and Brunus, that which Pythagoras, Aristarchus Samius, Heraclitus, Epicurus, Melissus, Democritus, Leucippus, maintained in their ages, there be infinite Worlds, and infinite Earths or systems, in infinite aether, which Eusebius collects out of their tenents, because infinite stars and planets like unto this of ours, which some stick not still to maintain and publickly defend: I look for innumerable worlds wandering in eternity."

Elsewhere, Burton writes, "Kepler (I confess) will by no means admit of Brunus' infinite worlds, or that the fixed stars should be so many Suns, with their compassing Planets, yet the said Kepler, betwixt jest and earnest in his Perspectives, Lunar Geography , and his Dream , besides his Dissertation with the Sidereal Messenger , seems in part to agree with this, and partly to contradict. For the planets, he yields them to be inhabited, he doubts of the Stars: and so doth Tycho in his Astronomical Epistles , out of a consideration of their vastity and greatness, break into some such like speeches, that he will never belive those great and huge bodies were made to no other use than this that we perceive, to illuminate the earth, a point insensible, in respect of the whole. But who shall dwell in these vast bodies, Earths, Worlds, if they be inhabited? rational creatures? as Kepler demands, or have they souls to be saved? or do they inhabit a better part of the World than we do? Are we or they the Lords of the World?"

Many writers have offered richly contradictory answers to these questions. In a memorable example, Voltaire's Candide (1759) has the character Pangloss insist, with inane optimism, that this is indeed "the best of all possible worlds." We may view this as an irreverent, sarcastic, and belated answer to Plotinus.

Bruno received further English support for the idea of a plurality of worlds in 1638, when John Wilkins's Discovery of a New World in the Moone advocated the habitability of the Moon. The absence of references to other worlds in the Bible was, according to Wilkins, no more significant than the Good Book's failure to mention the known planets.

The French philosopher Rend Descartes's Principia philosophiae (1644) developed an elaborate pre-Newtonian theory of tourbillons , or "whirlpools," of material space-stuff. These whirlpools filled all space, butting up against each other with no intervening void. (This model ignores Bacon's objection from basic Euclidean geometry that any round structure cannot be stacked without leaving prohibited voids.) The static, eternal, unchanging kosmoi of Epicurus here become dynamic, evolving entities. Planets were in fact stationary in the invisible, incompressible medium in which they were embedded, transported by the whirlpool motion around their primaries. All stars, more or less, had systems, but these systems were by no means identical. The boundaries between whirlpools could, according to Descartes, be crossed by material objects such as comets. Logically, however, this theory is incapable of explaining the motion of comets, or of anything in an eccentric orbit, or of any bodies in intersecting orbits, since each of these violates the assumption of the embedding of planets in a whirlpool of space.

Descartes's entire basis for understanding the dynamics of stars and planets was overthrown by Newton in 1687. The idea of whirlpools of space, never more than an overgrown metaphor, was discarded in favor of the quantitative idea of gravitational force. The planets were maintained in motion not by a force pushing them, Newton posited, but by the absence of frictional dissipation of their energy of motion, which they had inherited at the time of their formation. The simple postulate of a gravitational force, dropping off as the square of the distance, was shown by Newton to give rise to Kepler's laws of motion. Newton, however, was very cautious in applying his theories to other worlds. His scientific speculations were always tempered by his theological beliefs.

The astronomer Thomas Wright, in his Original Theory or New Hypothesis of the Universe (1750), while sharing Newton's concern for theological correctness, set himself the task of placing the plurality of worlds on a firm Newtonian footing. He placed great emphasis on proving, both from observation and theory, that other stars are suns. Although he argued on observational grounds that the other planets of the Solar System were grossly similar to Earth, he based his arguments for planetary systems orbiting other stars squarely upon the principle of utility.

Immanuel Kant similarly argued, in his Allgemeine Naturgeschichte und Theorie des Himmels (General Natural History and Theory of the Heavens , 1755), that the material Universe is governed by natural law, and that that lawfulness constitutes direct evidence of the wisdom and power of God. Kant claimed that Newtonian forces could bring order to a chaotic, atomistic Universe and bring about the origin of stars and planetary systems. He argued by analogy with our Solar System that "these systems were formed and made in the same way as our own, out of the elementary atoms of matter that pervaded the void." He further argued that the Milky Way was a vast assemblage of stellar systems, and that other faint nebulae were in fact also such systems (we now call them galaxies). Kant links the development of intelligence and spirituality to the "quality" of the matter in which they develop. The densest regions he associates with minimal rationality, with successive levels of higher spiritual and rational attainment possible toward the more rarefied periphery. This theory was recently put to striking use in the science fiction novel A Fire Upon the Deep by Vernor Vinge.

By 1808 the French astronomer Jerome de Lalande could casually dismiss the Cartesian model and endorse plurality without whirlpools, basing his arguments, like those of Kant, on thc Newtonian theory of universal gravitation. The transplantation of Descartes's vision of infinite spaces and numberless worlds into the Newtonian Universe was by then an unqualified success.

The early nineteenth century was a time of progress on the observational front. The widely reported 1834-1838 South African expedition of John Herschel to survey the southern skies occasioned a great public furor when Richard Locke, a writer for the New York Sun , authored a series of stories purporting to reveal exciting results from the expedition. The stories built up in great detail, and with an occasional hint of plausibility, a wholly fictitious account of Herschel's observations of the Moon, which culminated in a revelation of vast forests, large animals, intelligent beings, and cities on the lunar orb. Even competing newspapers nodded in solemn approbation. Public opinion was prepared to accept life almost anywhere.

The first American edition of William Whewell's The Plurality of Worlds (1854) generally doubted the existence of life off Earth, but allowed that: "The surface of the Moon, or of Jupiter, or of Saturn, even if well peopled, may be peopled only with tribes as barbarous and ignorant as Tartars, Esquimaux, or Australians." Whewell approvingly cites the opinion of the German mathematician-astronomer Friedrich Bessel, who wrote in his Populare Vorlesungen Uber Wissenschaftliche Gegenstande (Popular Lectures on Scientific Subjects , 1848) that "those who imagine inhabitants in the Moon and Planets suppose them, in spite of all their protestations, as like to men as one egg to another." But Whewell, who wrote anonymously because he apparently anticipated opposition to his negative assessment of life on other worlds, concludes that he would not be surprised if humans were the only intelligent, moral race in the entire Galaxy. So positive was the prevailing American attitude on life in space, however, that a special introduction, written by the president of Amherst College, Edward Hitchcock, was added to the American edition of Whewell's text to commend the author's attempts to reconcile religion and science and to argue for a more liberal interpretation of the astronomical evidence. Curiously, Hitchcock, the nineteenth-century scientifically literate Protestant theologian and educator, asks the ancient question, "Of what possible use to man are those numberless worlds visible only through the most powerful telescopes?" His answer, echoing that of so many of his forebears, is that they are there for the benefit of other intelligent species: "When we see how vast is the variety of organic beings on this globe, and how manifold the conditions of their existence; how exactly adapted they are to the solid, the liquid, and the gaseous states of matter, can we doubt that rational and intelligent beings may be adapted to physical conditions in other worlds widely diverse from those on this globe? May not spirits be connected with bodies much heavier, or much lighter, than on Earth; nay, with mere tenuous ether; and those bodies, perhaps, be better adapted to the play of intellect than ours; and be unaffected by temperatures which, on Earth, would be fatal?" A daring conjecture indeed; that some intelligent beings may live free in "tenuous ether," without need of a planet on which to dwell!

The same general attitude was favored by the great French popularizer of astronomy, Camille Flammarion. Flammarion summarized the arguments for countless alien worlds in his book, La Pluralite des Mondes Habitees (The Plurality of Inhabited Worlds ), (1889), a book written in 1861 but unable to attract a publisher until Mars became a hot topic due to Schiaparelli's famous "discovery" of canals on the planet during the 1877, 1879, and 1881 close oppositions of Mars. (Oppositions occur when Mars and the Sun are opposite in the sky; i.e., when Earth is passing close to Mars between it and the Sun.) The American astronomer Percival Lowell built an observatory near Flagstaff, Arizona, dedicated to the study of Mars and jumped on the canal bandwagon with an enormously influential book, Mars , published in 1895. In response, Flammarion's book was promptly translated into German, English, Spanish, Italian, Portuguese, Russian, Danish, Swedish, Polish, Czech, Arabic, Turkish, Chinese, and braille. Flammarion personally presented an autographed copy of the book to Napoleon III. The ideas in it were not only widely familiar to experts; they were widely discussed by the public. Flammarion's central theme was "Il approprie a chaque astre une humanite." (A human race takes over at every star.) Victor Hugo, upon reading Pluralite , wrote to Flammarion, "Je pense comme vous." (I think as you do.)

Thus, by stages, the idea of a cozy, Earth-centered kosmos, in which Earth was the sole physical body, yielded to the conception of a full-blown stellar Universe of countless stars, each the center of a planetary system of its own, with life and intelligence widely distributed through them. Worlds unlike Earth were conceived as able to bear their own indigenous forms of life, well adapted by origin and habituation to a wide range of conditions, many of them perhaps un-Earthly in degree, or even in kind. The question whether we, or they, were the true lords of creation remained unanswered. Flammarion ushers in the familiar twentieth-century Universe within which space travel and contact with alien races become at least conceivable, and perhaps inevitable.

THE LONG human tradition of imaginative voyages, when fertilized by the dawning acceptance of the plurality of worlds, gave rise to both literary and scientific interest in these new possible arenas of activity. Through astronomy, the planets become not "immaterial or "celestial" lights in the sky, or mythological figures, or divine portents, but other places, subject not only to study and theory but even to fictionalization. In the light of the age of exploration, it was at least conceivable that one could visit other places and explore them; and it remained only for chemistry, physics, and engineering to make the conceivable practical. It is from such a wedding of vision and science that the space age, the era of human exploration of other worlds, has sprung.

Human imagination long preceded human technology in probing the mysteries of space. The first tale of travel to another world, by Lucian of Samosata in the first century A.D., invokes a waterspout to carry its hero into space. There he finds the Sun and Moon not only populated, but their denizens at war with each other over colonial rights on Jupiter. The whole piece is an airy fantasy, lacking even such discipline as might have been provided by heeding the rudimentary science of Aristotle.

The popular imagination was also stirred by fabulous tales of journeys to foreign and exotic lands. Marco Polo's thirteenth-century accounts of his exploits on the road to China and in the court of the Great Khan, although generally factual, were received as fantasy and mocked by many in Europe. John Mandeville's bizarre tales of his fourteenth-century travels in the land of Prester John seemed to confirm to many readers the idea that travel stories were nothing but rank fantasies, even though many credulous souls read them as factual accounts. Both those predisposed to accept these tales of fabulous travels and those inclined toward skepticism would find ample stimulus in the next round of imaginative innovation, the extension of these voyages to the Moon and beyond.

Marco Polo brought back from the East knowledge of gunpowder and of small rockets powered by special formulations of gunpowder. These small solid-propellant rockets were suited for light military use, or for launching fireworks for entertainment. Use of these rockets spread rapidly from east Asia through south Asia to Europe. But gunpowder rockets are teacherous and unsuited for scaling up to sizes more than a few feet long. The culmination of this technology was the Congreve rocket used by England during the War of 1812, by whose "red glare" the American national anthem was penned. These rockets were modest improvements over the Chinese and Persian war rockets of six centuries earlier.

The seventeenth century saw not only the introduction of the astronomical telescope and the wide publication of its discoveries, but also a liberation of the imagination. Soon after Galileo reported mountains on the Moon, revealing the lunar orb to be a physical, imperfect body rather than an immaterial, perfect sphere, Kepler's Somnium seu astronomia lunar (Dream ) told of an imaginary trip to the Moon. As the title suggests, no attempt was made in the narrative to provide a viable means of travel through space. The purpose of the piece was simply didactic; to instruct the reader in the lore of the Moon without seeming overly academic. But, in 1634, Kepler did foresee and affirm the possible physical exploration of space, even though the means of such travel were beyond the scope of his science: "When ships to sail the void between the stars have been invented, there will be men who come forward to sail those ships."

The year 1638 saw the publication of Bishop Francis Godwin's The Man in the Moone or a Discourse of a Voyage Thither by Domingo Gonsales . In this story, the shipwrecked Gonsales is transported by means of a flock of trained swans which, unexpectedly, migrate to the Moon. Equally fantastically, in Cyrano de Bergerac's Voyage dans la Lune (Trip to the Moon , 1657) the protagonists is carried aloft by the power of the Sun's attraction on vials of dew.

Margaret Cavendish, an amateur scientist of some note, wrote in The Description of a New World, Called the Blazing World (1668) of a fictional visit to a world, unaccountably invisible, which is in contact with Earth at the North Pole. The hapless visitors are blown there by a freak storm. Cavendish characterized this work as "meerly Fancy," perhaps an apt description of a world abounding in both reason and wealth.

Daniel Defoe's 1705 fantasy novel, The Consolidator: or Memoirs of Sundry Transactions from the World in the Moon , was a true "fantastic voyage," really no more than an elaborate political satire, purporting to be set on the Moon as a means of piquing interest and disavowing realism. It is plausible that Defoe was encouraged to experiment with such a theme by the success of Margaret Cavendish's novel a generation earlier. Jonathan Swift's Travels into Several Remote Nations of the World (1726), reprinted as Gulliver's Travels , was yet another of these fantastic voyages, clearly inspired in its turn by Defoe's Robinson Crusoe . Swift confined Gulliver's wanderings, like those of John Mandeville, to Earth; but again, like Defoe's Consolidator , the principal purpose was political satire and social commentary.

The first seed of a physically plausible means of spaceflight was planted obscurely in the ordinary book of the polymath Erasmus Darwin, grandfather of Charles Darwin. In these notes, dating from about 1779, Darwin's biographer Desmond King-Hele has found an evocative sketch of a simple liquid-fuel rocket engine, with hydrogen and oxygen tanks connected by plumbing and pumps to an elongated combustion chamber and expansion nozzle, a concept not to be seen again until the 1890s. The design also features the ability to use air as the oxidizer, making it technically a ramjet/rocket hybrid, an engine type that would eventually attract attention again in the 1990s. The design, like those in Leonardo da Vinci's notebooks, lay unrecognized for two centuries. Since Darwin was aware of the diminution of air pressure at high altitudes, the inclusion of an oxygen tank makes it clear that he was thinking of, at the least, very high-altitude flight. But Darwin provided no explanatory or descriptive text, and the idea was lost to posterity until King-Hele expounded on the diagram.

It was not until 1865 that Jules Verne rendered into prose the first physically-based space flight, in De la Terre a la Lune (From the Earth to the Moon ). Verne employed a titanic gun, based loosely on Civil War technology, to fire a three-man sealed capsule to the Moon from a Florida launch site. There are numerous physical flaws in the story: the acceleration achieved by such a launch vehicle would have utterly crushed the hapless passengers, and Verne erroneously has them experience weightlessness only at the point of equal gravitational force between the Moon and Earth.

The famous American writer and orator Edward Everett Hale wrote of the launching of an artificial Earth satellite in his story "The Brick Moon," published in the Atlantic Monthly in 1869. Hale invokes centrifugal force tapped from two gigantic counter-rotating flywheels (ultimately powered by a waterfall in Maine) to launch his masonry sputnik. It is unclear whether he had a well-defined (but mistaken) idea about the physics of acceleration, or whether he simply invoked scientific-sounding terminology to obscure the absence of an idea. (It was this same Hale who delivered the eloquent hours-long declamation at the dedication of the Gettysburg battlefield, only to be upstaged by Lincoln's polished little gem of a speech.)

In 1870 Jules Verne published Autour de la lune (Around the Moon ), the sequel to De la Terre a la lune (From the Earth to the Moon .) The interest in this book comes mainly from the fact that the author invokes a "midcourse correction" while flying near the Moon, an idea ridiculed in a later editorial, written to attack Robert Goddard, in the New York Times of January 13, 1920. The author of the editorial pontificated with glorious arrogance and total lack of understanding of physics, "That Professor Goddard, with his `chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react -- to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools." Although unacceptable to the opinion shapers at the Times , Verne's technique, embraced by Goddard, was actually used by the Apollo missions to the Moon. (Nowadays, high schools daily ladle out the knowledge that the exhaust gases of a rocket and the rocket's thrust chamber push against each other, satisfying Newton's laws while working admirably in a vacuum.) Verne's astronauts, peering at the Moon through a glass, also debate endlessly about the existence of forests, seas, and cities on the lunar surface.

Percy Greg's 1880 novel, Across the Zodiac , features an antigravity-powered ship complete with a hydroponic air-regeneration system, in which the hero visits a utopian civilization on Mars. The device of antigravity is also invoked in the 1894 novel, A Journey in Other Worlds , by Harvard graduate J. J. Astor to escape from the humdrum existence on a utopian Earth in the year 2000 by visiting Jupiter and Saturn. On Jupiter the novel's protagonist finds a sort of Jurassic dinosaur playground; on Saturn, a civilization of spirits that teach biblical principles. Later in his career, John Jacob Astor IV was to build the Waldorf-Astoria Hotel in New York before going down with the Titanic .

The German physicist Hermann Ganswindt, as early as 1891, was lecturing on the physics of reaction engines and advocating space travel using rocket propulsion. Unfortunately, it appears that he left no published record of his ideas. The Russian engineer A. P. Fedorov wrote a narrative titled A New Principle of Aeronautics (1896) on spaceflight.

Kurd Lasswitz's 1897 science fiction novel Auf Zwei Planeten (On Two Planets ) describes a Martian invasion of Earth. The aliens quickly sink the British Navy and establish a utopian world government. The connection between these two events was presumably evident to Lasswitz's German audience. Sensing a potential English market for such fiction, in 1898 the British novelist Herbert George Wells wrote The War of the Worlds , in which the Martians, shot in giant artillery shells from Martian cannons, land in England. The alien invaders, with "intellects vast and cool and unsympathetic," are militarily unstoppable, but fortunately succumb to terrestrial microbes. American audiences are more familiar with Orson Welles's radio version, which was set largely in New Jersey, and a printed adaptation that recasts the novel in the geographical context of the Boston area.

The year 1898 also saw the premiere of Edmond Rostand's classic play, Cyrano de Bergerac . In one scene the redoubtable Cyrano, trying to divert the attention of the Comte de Guiche, pretends to have just fallen to Earth from the Moon, and promises to describe six different methods of interplanetary travel.

At the very time that Rostand was writing, the young Russian genius Konstantin Edouardovich Tsiolkovskii was already hard at work on the theory of rocket propulsion. In 1903 Tsiolkovskii published his treatise, Exploration of Cosmic Space with Reaction Engines , in which the theory of rocket propulsion was developed for the first time. This visionary work was unfortunately available only in Russian. Among its limited readership, it seems to have impressed only one prominent scientist, the eminent chemist Dmitri Mendeleyev.

Contemporary fiction was still stuck in the previous millennium. Edwin Lester Arnold's 1905 "scientific romance," Lieut. Gullivar Jones: His Vacation , has his protagonist make a wish and travel to Mars on a flying carpet. In 1917 the famed author of the Tarzan books, Edgar Rice Burroughs, published A Princess of Mars , in which his protagonist, John Carter, with similar technical sophistication, is drawn "with the suddenness of thought" to Mars. This nonphysical approach to interplanetary travel seems to have reached the end of the road in 1922, when E. R. Eddison has the hero of his fantasy novel, The Worm Ouroboros , travel to Mercury in a dream (the character thinks it is a dream -- he can pass his hand through any material object he sees -- but a Mercurian insists on his own authority that it is not a dream).

As the fantasy of space travel faded away like the dream it was, the dawning technology of rocket propulsion advanced energetically. From 1905 through 1907 the Norwegian astronomer Sverre Birkeland carried out experiments with small hydrogen-oxygen rocket engines, of which there are no known surviving records. In 1912 the French inventor Robert Esnault-Pelterie held a conference on rocket propulsion in Paris. He concluded that rocket flight into space would not be possible until nuclear energy could be mastered.

Having failed to make much of an impression on the world with his technical publications, in 1913 Tsiolkovskii published a science fiction novel, In the Year 2000 (On the Rocket ), about a group of rich men who build a rocket powered by hydrocarbons and liquid oxygen. These pre-Bolshevik investor-inventors can best be described as venture capitalists. One can only wonder how history might have been changed had Tsiolkovskii somehow come to the attention of his contemporary John Jacob Astor IV. Also in 1912-1913, Robert Goddard, having received his doctorate at Clark University, spent a fruitful year at Princeton University's Palmer Physical Laboratories, researching rocketry and pursuing basic patents in his spare time. While there he had many interactions with the great astronomer Henry Norris Russell, who advised him on his rocket patents.

In 1915 the Transylvanian inventor Hermann Julius Oberth submitted his designs for war rockets to the Austro-Hungarian Ministry of War, which invited him to abandon "this fantasy." Only months later, Professor G. Tiekov of Pulkovo gave a public lecture on the importance of rocketry, in which he claimed that "rocket propulsion will remain necessary until gravitation can be annulled -- probably by electrical means." As of this time, Tsiolkovskii, Oberth, and Goddard were all completely unaware of each other's existence.

Goddard, in 1918, wrote an astonishing futuristic essay, "The Ultimate Migration," in which he predicted the eventual flight of mankind out of the Solar System in hollowed-out asteroids to escape the deaththroes of the Sun. But Goddard was anxious lest his practical efforts to build a high-altitude rocket be discredited by association with such grandiose schemes. So concerned was Goddard with professional credibility and public perception that he sealed up the unpublished manuscript in an envelope labeled "Special Formulas for Silvering Mirrors" and set it aside. It was finally opened over fifty years later, long after Goddard's death. The infamous 1920 editorial mentioned earlier clearly demonstrates that Goddard was correct in his perceptions.

The climate of opinion was no kinder to young Oberth. His doctoral dissertation on the physics of rocket propulsion and space travel was rejected by the University of Heidelberg in 1922, and remained unpublished until Oberth issued it at his own expense in 1929, under the title Wege zur Raumschiffahrt (Routes to Space Travel ). But in the meantime, Goddard had built, tested, and flown liquid-fuel rockets. The space age was dawning. Ironically Oberth, unaware of Goddard's essay "The Ultimate Migration," often criticized Goddard as an unimaginative plodder who could not see as far as space.

In the same year, British physicist J. D. Bernal wrote in his essay "The World, the Flesh, and the Devil," of solar sails, beamed microwave power, solar power satellites, and the use of space resources. He proposed, quite independently of Goddard, the idea of hollowing out ten-mile asteroids for use as interplanetary habitats and interstellar "slowboats."

The idea of rocket travel was suddenly everywhere, its spores spread by pulp magazines. Hugo Gernsback's Amazing Stories carried, beginning in August 1928, E. E. "Doc" Smith's "The Skylark of Space," in which the hero flits about space in an antigravity ship, visiting not only the Solar System but also other stars. In his mind-blowing 1931 novel Last and First Men , British writer Olaf Stapledon casually assumed the use of rockets to settle Venus and Neptune. Rockets and space travel had entered public awareness. It was only a matter of time before rocketry would give us firsthand access to the planets and satellites of the Solar System, letting us for the first time appreciate the enormous diversity of worlds.

In the decade from 1930 to 1939 the German Verein flit Raumschiffahrt (Union for Spaceflight), the British Interplanetary Society, the American Interplanetary Society (later the American Rocket Society), and the Russian GIRD were founded to investigate and advocate rocket propulsion and interplanetary travel. And Goddard's rockets were flying to show the way.

One of the proponents of the practical benefits of space was a young British engineer who suggested as early as 1939 in the Journal of the British Interplanetary Society that manned missions to the Mars system might derive great benefit from extracting water and other volatiles from the Martian moons Phobos and Deimos to refuel their rockets for descent to Mars or return to Earth. A few years later he proposed that satellites placed in high orbits above Earth's equator could serve as communications relays covering nearly the entire planet. That young man, Arthur C. Clarke, is today best known for his 2001: A Space Odyssey and numerous other science fiction stories and novels.

From the 1930s through the 1960s, practical engineers such as Wernher von Braun in Nazi Germany and Sergei Korolyev in the Soviet Union brought the liquid-fuel rocket to a high level of performance and reliability. When von Braun was being interrogated by American officers at the end of the Second World War, he was astonished by their questions probing into the "secrets" of Nazi rocketry. He answered in apparent sincerity that the theory had been worked out by the American Goddard, and German engineers were just following his lead. The American officers were equally astonished. They had never heard of Goddard. But, accelerated and encouraged by the performance of the German V2 rocket, the world hastened on into space with considerable confidence that the goal could at last be realized. From the launching of Sputnik 1 in 1957 to the first Apollo landing on the Moon in 1969, American and Russian scientists and engineers fairly leaped into the space age.

By 1974 Princeton University physicist Gerard K. O'Neill had proposed permanent large space colonies capitalized by exporting power to Earth. In 1977, at the instigation of physicist Peter Glaser of Arthur D. Little, Inc., and O'Neill, a NASA study of solar power satellites was carried out, and another NASA study in 1996 explored more advanced technical options that promise to make SPS systems more economical. Princeton's Space Studies Institute, founded by O'Neill, continues to advance both the study of economic development of space and public awareness of its potential.

THE EDIFICE of our insights into other worlds rests in part on two distinct foundations: the "imaginary voyages" of Verne, Wells, Stapledon, and their fellow writers of fiction, and the "visionary technologies" of Tsiolkovskii, Goddard, Oberth, and their fellow scientists and engineers who have carried us to other worlds. It should come as no surprise to hear that the practitioners of these two fields inspired and encouraged each other. Goddard acknowledged his inspiration from Wells, and Tsiolkovskii spoke of the formative influence of Verne on his work. Gernsback tried to commission Goddard to write articles for his magazines. Likewise, the leading writers of science fiction built firmly upon the best available scientific data and technology: the rocket moved quickly from Goddard's test stand to the pages of Stapledon and others, and Wells hastened to build on the latest Mars observations and conjectures of Percival Lowell.

The space age made possible by these visionaries has given us true, firsthand access to our Solar System. We are no longer dependent upon astronomical observations, made through Earth's cloudy, dusty, hazy, and turbulent atmosphere, of planets that are mere dots in the sky. Since 1960, spacecraft built on Earth have been heading out to visit the Moon, Mars, Venus, and even Mercury, Jupiter, Saturn, Uranus, and Neptune. The first tentative missions to comets and asteroids have already returned priceless data. Vicariously, through the eyes and hands and sensors of these robot craft, we have begun to visit alien worlds and immerse ourselves in their mysteries.

In a way, the recent discovery of planets orbiting stars transports us back to where we were in the 1920s in our study of the Solar System. In fact, the situation is even worse: although our instruments are vastly more sensitive and capable than those we had then, we are charged with the task of observing planets that arc roughly a million times as far away as our neighbors in the Solar System. In the usual sense of the word, we have not yet "seen" these planets at any wavelength, but simply detected the gravitational effects of their presence on the motions of their parent stars. Even the crudest images of these bodies do not yet exist. We can, from the observations that are available, often deduce their orbital periods, eccentricities, masses, and distance from their stars, and hence estimate their surface temperatures, diameters, escape velocities, and gravities. But as long as we cannot "see" them in any way, our conclusions must remain indirect and inferential.

Fortunately, we already know something about planets: our Solar System contains a group of bodies of very diverse size, distance, temperature, composition, and structure. We see enough diversity in our planetary neighbors to realize that these bodies are a sample -- though incomplete -- of all the possible kinds of planets that may exist. We are inspired to examine the countless types of alien worlds that would result from only modest changes in the origin and evolution of our own system. Our basis for understanding the worlds of other suns must be the planets we see nearby, the very ones that our spacecraft can fly by, orbit, land on, and explore. We therefore turn our attention to the Solar System before leaping into the interstellar void. We do so not because it holds all the keys to understanding the kosmos, but because it is convenient. Pragmatically, we start our search under the nearest streetlight -- the Sun.

Introduction	p. 1
The Plurality of Worlds	p. 7
Worlds As We Know Them	p. 29
Genesis and Evolution	p. 34
Recipes for a Rocky Planet	p. 51
More of the Same Is Not the Same	p. 60
Mercuries: Too Little to Compete	p. 68
Giant Impacts: Chips and Broken Planets	p. 76
A Suite of Earths	p. 81
A Family of Giants	p. 98
Brown Dwarfs and Their Close Kin	p. 118
Planets Around Planets	p. 129
Dippy and Merely Eccentric Motions	p. 142
'Tis the Star, the Star Above Us, Governs Our Conditions	p. 152
Starry Nights: Life in a Globular Cluster	p. 164
Interstellar Orphans	p. 172
A Planet's Lot Is Not a Happy One	p. 181
The Plurality of Habitable Worlds	p. 191
Where Do We Go from Here?	p. 202
Afterword	p. 215
Glossary	p. 219
Suggested Reading	p. 225
Acknowledgments	p. 227
Index	p. 229
Table of Contents provided by Syndetics. All Rights Reserved.

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Worlds Without End : The Exploration of Planets Known and Unknown

9780738201702

0738201707

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