By the early eighteenth century the sheen had tarnished. Daniel Defoe maroons the vagabond Robinson Crusoe on a wretched desert isle in the Caribbean; Jonathan Swift casts his voyaging cipher, Lemuel Gulliver, onto one isle after another, ending with the fantastic vision of an island metropole, Laputa, suspended in the clouds, the creation of visionary “projectors.” The bright prospect of new Madeiras and Tenerifes ended with the horrors of Hispaniola and the wreckage of the Antilles; the Arcadian gardens imagined beyond the horizon collapsed into the scalped forests, goat-plagued hills, and gullied slopes that washed over the boles of felled dragon trees and the bones of extinct dodos. Instead of wistful new Gran Canarias, there were grim St. Helenas, and in place of another hypothetical Ile de France, there was Red Madagascar, destined to become a symbol of habitat degradation, ever burning and bleeding its soils into the sea. In the New World the Black Legend acquired an ecological edge. The shimmering mirage of Columbus met the bleak reality of Las Casas. Exploring utopias ended as environmental dystopias.88
Yet still they beckoned. And since better worlds had to be somewhere else, exploration repeatedly opened up prospects for uninhabited lands—new frontiers that might avoid the mistakes of the past. The modern founder of the genre, Thomas More, published Utopia in 1516. As a Platonic ideal, an updated Republic, Utopia did not have to be any place; but the rapid discovery of unknown lands made its literary location as a New World isle plausible, along with the use of a weather-beaten Portuguese, Raphael Nonsenso, as a protagonist and prototype for the Ancient Mariners and Old Men who would hound and hector the enterprise.89
The Second Age found it harder to place its utopias: there were few terrestrial locales yet unvisited. For a while Tahiti served as a tropical Teide, the first and most enduring of the Enlightenment’s new Edens. Jules Verne stationed Captain Nemo on Deception Island near Antarctica, and James Hilton sited Shangri-La in the remote mountains of Tibet, but most visionaries had to turn to the past or the future. They invented modern versions of lost golden ages and former Edens, or they projected, along evolutionary trajectories, perhaps outfitted with steam power, a more perfect future. Instead of places, they turned to peoples, remote tribes as relics of a primitive virtue or exotic folk such as the Polynesians of apparent plenty and ease. Then the Third Age in space suddenly made possible a revival of place-sited utopias.
For space seers there were three possibilities. One of course was Mars, seeming to stand to Earth as the New World did to the Old, awaiting only a more benevolent terraforming. How Mars might avoid the irony of past colonization and not sink into a planetary Haiti was unclear: it would simply happen. Proponents proclaimed, in a more secular declamation to those which Franciscan missionaries had made five hundred years earlier, that scientific knowledge was great enough to guide the technology to righteous ends.
A second possibility was more Platonic, to establish a space station among the Lagrange points, specifically at L-5, at which the gravitational attraction of Earth and Moon are equal, and which would thus suspend the colony in perfect weightlessness. Here was an invented, high-tech island free from the weight of the past—a solipsistic society, though one founded on ideals and first principles. If More’s ideal was a kind of monastery, Gerard O’Neill’s L-5 colony Dyson sphere is a kind of space suburb absolved of the past injustices and unmarred by the pull of Earthly vices. To advocates it occupies itself a kind of historical Lagrange point: it can neither evolve nor decay. To critics it might seem like a burnished version of Laputa. Neither utopia is likely to happen.90
Instead, exploration will probe the third alternative, the new isles of the solar system, the moons, now revealed as diverse, fascinating, and abundant. For visionaries these places come, as they had for the Great Voyages, with a window of opportunity between discovery and full-bodied exploration. The first ignites imagination, while the second tends to extinguish those utopian flames. The places’ real value lies with a more worldly if still idealistic pursuit. Here are new worlds that, if closed to prospective colonists, still hold interest for science. Here are the experiments in natural history, equivalent to those scrutinized by Wallace, Darwin, and Joseph Hooker, that might reveal fundamental truths about the origin of Earth, life, and perhaps human purpose. Here are the promises less of a New Jerusalem than of a New Galapagos. Here are the Spice Isles of an exploring science: a new Madeira among the moons of Jupiter, a new Antilles in orbit about Saturn, a new Ternate at Io.
In brief, while the moons of the outer planets could never evoke the sensuousness of the earthly tropics, nor spark an equivalent lushness of imagination, they were the best the solar system had, and a few possessed the organic molecules that might be the byproduct or forerunner for life. The Third Age’s answer to the tropical Edens that so enchanted intellectuals in earlier times, enticing them to imagine other worlds, would be satellites that beat to geologic rhythms different from plate tectonics or were rich with organic molecules like methane. The new Tahiti would be a place like Saturn’s Titan.
TO THE SIRENS OF TITAN
That is where the Voyagers now trekked.
By now, Pioneer 11, having doubled Jupiter and sprinted across the solar system, was a scant four months away from its rendezvous with Saturn on September 1, 1979. Once there, it found a new ring and even a new moon (indirectly, through magnetic disturbances), and most of all it blazed a route outside that great reef of rings that would preserve the option for Voyager 2 to continue the Grand Tour.
Meanwhile, the colossal Jovian gravitational field that had quickened the Voyagers’ velocity during their planetary fling now pulled them back; but they had still gained more than they lost. Even as they slowed, they sped along some twenty-five thousand kilometers per hour faster than before they began encounter, with Voyager 1 pulling still farther ahead. For Voyager 2 its long course correction put it at a velocity and trajectory that would save precious hydrazine for its maneuvering thrusters. The Voyagers reentered cruise phase. They would continue this way for twenty and twenty-five months, respectively, chatting with Earth, sampling the solar wind and cosmic rays, reorienting themselves by the Sun and stars.
On they sailed to Saturn, across a void over 648 million kilometers wide, like two gnats crossing the Pacific.
DAY 587-914
13. Cruise
The spacecraft now had the velocities they needed to complete their mission. Voyager 1 had entered its Jovian observation phase at 48,960 kph; it exited at 86,000 kph. Voyager 2 had begun encounter at 38,016 kph and ended at 75,600 kph. Liftoff had granted escape from Earth. Their swing around Jupiter granted them escape from the Sun. They had sufficient inertia to carry them from giant planet to giant planet.
The program, too, had a quickened purpose. The spacecraft had worked. They remained sentient. They had completed what the pilots of the First Age had termed a gran volta.
PORTALS AND PASSAGES
Past exploration had launched with muscle, wind, current, and tide. An exploring party might begin with nothing more than a long stride, a horse’s gait, a floating coracle, or an unfurled sail. Long voyages required cracking the codes of wind and current—understanding their constancies, their seasonal variations, their propensity for storms. Discovery unfolded as those motive forces permitted and as mariners learned to harness them to their own ends.
For the North Atlantic, this meant deciphering the southwesterly trade winds, the seasonal paths of storms, and the Gulf Stream. The process began with coasting down Africa and some ventures into the Atlantic that discovered Madeira and the Azores. It proved difficult (and dangerous) to sail up and down Africa, however, and as the magnitude of the continent became slowly apparent it was obvious that while long coasting could serve cautious probes, it could never support the high-volume far voyaging demanded for trade with the Indies.
The solution was what the Portuguese called a volta do mar largo. This required a journey westward until, after reaching the far isles, one could turn into the westerlies and
then ride them back to Madeira or the Azores. The volta involved a great arc by which one sailed out and then back. Over time, this pattern was enlarged until it could cross the North Atlantic altogether. In this emerging geography of wind and water, the Canary and Cape Verde islands were ideally situated to ride the southwestern trade winds to the New World, and Madeira and the Azores well sited to capture them on return. The short volta between near-seas became a long one over the Ocean Sea.
Crossing the equator unsettled that lore. The doldrums were a nasty mix of thunderstorms and calms. The inherited learning of the Ancients had nothing to contribute, the winds below were reversed, and even the North Star vanished. One could not travel to India coastal port by coastal port, as one could sail across the Mediterranean. Bartolomeu Dias could inch down Africa before being blown by storms southwesterly and returning around the Cape; a successor would have to find another strategy. Perhaps outfitted with some intuition that the world had to be symmetrical, that what happened in the north should, in reverse, happen in the south, probably aided with a dose of accident and happenstance, Vasco da Gama, once past the Cape Verdes and Sierra Leone, rode the trades out to sea and then turned south to capture the austral westerlies. Those powerful gales hurled him back to Africa, just above the Cape of Good Hope, which took another week to round. When Pedro Cabral did the same with the next Indies fleet, he veered still farther west and ran into Brazil; but he also rode the winds farther south and rounded—or “doubled”—the Cape. The great turn, pivoting with the winds, made the passage to India possible by flinging fleets around the gravitational mass of the Cape and into the Indian Ocean, where they would ride the monsoons to India.91
That was half the problem: tracing the patterns that characterized each discovered sea. But to truly sail the world ocean required discovering the links by which to get from one decoded sea to another. The brachiated peninsulas and islands that constituted Europe defined interior seas between which straits allowed passage. The Mediterranean was, for mariners, a collection of smaller seas—the Tyrrhenian, the Adriatic, the Aegean, the Ionian, the Ligurian—broader seas east and west, and innumerable gulfs. The composite had one eastern strait, the Bosporus, that joined it to the Black Sea, and one to the west, Gibraltar, that connected it to the Ocean Sea. A similar logic applied to the north, with its own complex quilt of seas, gulfs, and straits. That experience, like the creation of the volta, expanded across the earth.
The quest for the Indies that underwrote the Great Voyages was a search for passages across seas and a search for those straits that would permit passage from one sea to another. The barrier that was Africa required a southern passage; the barrier that was Eurasia, a northeast passage; the barrier that was the novo mundo, a strait somewhere—at its narrow middle at Panama, through its icy northwest, and ultimately around its southern extremities, the Strait at Magellan and the Drake Passage. The chronicle of discovery arranges itself around those pursuits like iron filings around the poles of a magnet.
Continental exploration obeyed a different logic, though it still began with the obligatory seaport that linked to the metropole. Beyond the coast, however, exploration depended on unlocking the geography of rivers and interior lakes.
Those lands that had rivers capable of ready access were discovered quickly; those that did not, lagged. Its enormous rivers, though they drained to the frozen Arctic, allowed Russians to cross Eurasia with breathtaking speed, by hopping from one tributary to another, like a squirrel jumping between branches. Timofeyevich Yermak crossed the Urals in 1581, roughly twenty-five years before Jamestown was founded in Virginia; by 1632, Cossacks founded Yakutsk, two years after émigré Puritans founded Boston; by 1649, Yerofey Khabarov had sailed down the Amur and established Khabarovsk, some fifteen years before the Dutch established New Amsterdam. The drainage of the Mississippi River opened up the interior of North America like a split log, and despite its snags, seasonal flows, and shallows, exploring parties habitually made the Missouri River the entry to the Rockies.
Lands that lacked access, particularly by galliot (or later, steamboat), stalled or redirected exploration. The St. Lawrence River proved little more than an extension of its gulf, stymied by rapids and Niagara Falls, so that travel portaged to the Ottawa River in order to cross to the Great Lakes. So it was also with the Rio de la Plata, which proved a similar dead end. The one major river in Australia, the Murray, was unusable as a point of entry to the interior, causing expeditions to stagger across stony desert and sandy spinifex by foot or camel. The most curious case was Africa, which was both the first and last of the continents penetrated by Europeans. The reasons for its lag were several, not least its horrific diseases; but the absence of usable streams north and south, the endless cataracts on its major rivers, and the confusing contortions and delta of the Niger all prevented serious exploration. The world’s largest river by volume, the Congo, and its longest, the Nile, were the last in each category to be navigated.
The hope for simple ways through those great landmasses surrendered to a search for a means across them. Commercial rivers and navigable lakes were replaced by cross sections of natural history; and the hope of finding Great Khans, Prester Johns, and the bullion of new Mexicos, by inventories of natural wealth and nature’s wonders. As steam power overcame the motive geography of wind and currents, so railroads replaced rivers as means of transit. By 1869 the last unknown river and mountain range in the continental United States were discovered, though not fully explored, curiously the same year as the completion of the transcontinental railroad. Within another decade cross-continental surveys had traversed Australia and Africa.
For the solar system, a still different logic governed travel, for this was a journeying geography defined by gravity. It had its equivalent to seas and straits, an imprinted dynamic of forces that one could sail with or against. The solar system had its doldrums and trade winds and, in Lagrange points, its Sargasso Seas that suspended dust and asteroids. Exploration could advance only if it found a way to harness those forces to its own ambitions, to ride over gravity’s waves. For that it required a new gran volta.
The exploration of space could begin only by overcoming Earth’s gravitational field, and it could proceed in a timely way only by navigating the gravitational fields of the outer planets. In the 1920s Walter Hohmann worked out for each planet the lowest energy speed to achieve that goal, which is to say, the trajectory that would require the least departure velocity from Earth, what became known as Hohmann transfer ellipses. For the outer planets the time required to travel would run thirty years to Neptune and fifty to Pluto. No spacecraft could survive that long, no scientist could sustain a career across so many decades without results, and no institutional program could expect to thrive over such a duration. Planetary exploration seemed doomed either to sail among the inner planets or to devise some additional propulsion beyond what an Earth-launched rocket could provide. 92
The initial thrust, then, was to concentrate on hardware—bigger rockets were an annual event. It might be possible to strap on a snazzy new propulsion system that, once beyond Earth’s gravitational field, could accelerate the spacecraft to velocities that made interplanetary flight feasible. But it was also possible that better trajectories might simplify and shorten the requirements. In the early years, when getting a payload into Earth orbit, much less to the Moon or to Venus, stretched their liftoff capabilities, engineers were eager to try anything. The best one might hope for in sustained travel was a flyby mission to Jupiter.
VOYAGER’S THREE-BODY PROBLEM
In June 1961 JPL hired a UCLA math and physics graduate student, Michael Minovitch, as a summer intern to work on some tricky calculations involving trajectories to and around planets. The work absorbed, then obsessed the twenty-six-year-old, who continued to labor over it for the next two years, not only during summers but while at school on borrowed big-computer time at both UCLA and JPL. What emerged from his analysis was the realization that as a spacecra
ft swung past a planet, its velocity could undergo a permanent change. Specifically, moving in the same direction as the planet could add the thrust that bigger rockets or exotic propulsion systems could not, and thus make planetary exploration accessible with existing technologies.93
Minovitch quickly recognized the issue as a variant of the classic three-body problem in that it involved two large objects and one small one in mutual gravitational attraction—in this case, the spacecraft, the target planet, and the Sun. A solution had bedeviled Newtonian physics since its origins; it was to celestial mechanics what Fermat’s Last Theorem was to number theory. No exact solution was possible, but with the advent of computers it was feasible to generate hundreds of approximations, each of them a possible trajectory for interplanetary travel. Enthralled, Minovitch, who termed his insight “gravity propulsion,” learned FORTRAN to better run endless calculations and tried to work out as many solutions as possible, defaulting to his training as a mathematician to fashion a universal set of curves. He soon appreciated the seminal status of Jupiter by virtue of both its immense mass and its sentinel location between the inner and outer planets. At summer’s end he wrote up his results in a JPL technical report.94
The initial reaction to his study was mixed. The mathematics was formidable, and the physics, in some respects counterintuitive, since it required a shift in reference frames from Earth to the Sun. Viewed from Earth, what a planet gave by gravity to an incoming spacecraft it then took away as the spacecraft departed; but viewed from the Sun, there was a gain or loss depending on the directions in which the spacecraft came and went. With a monster planet such as Jupiter, the momentum added could be significant for the spacecraft, though infinitesimal relative to the mass of Jupiter. For Voyager 2 the impulse acquired was 35,700 kilometers per hour. For Jupiter the impulse shed was a foot of orbital velocity per trillion years.95
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