Nor was space itself truly neutral. That interplanetary realm, which seemed like a void, actually consisted of hard and soft fields, each full of irregularities and neither fully mapped. Not all the hard geography of planetary objects was known. There were gravitational tugs from bodies unrecognized from Earth, not least undiscovered moons; the planets themselves had masses not previously measured with the accuracy required for precision navigation, and were surrounded by debris-laden rings, some visible, some only hypothesized; there were hits from dust that did little direct damage but that could nudge trajectories microscopically. The soft geography of magnetic currents and solar winds, perhaps even cosmic radiation, exerted its own subtle pressure and was occasionally aroused into tidal surges and storms. Even the minuscule movements of the spacecrafts’ magnetic tapes deflected attitudes and velocity. The Voyagers required ceaseless corrections by their internal guidance systems and earthbound steersmen and pilots. 55
Voyager’s targets—planetary encounters—had relatively unforgiving windows. If the spacecraft passed too far away, its instruments might not record what observers wanted; if too close, the immensity of the giant planets would blot out readings in a blur. The spacecraft had an exact route to travel, as intricate as threading the coral-infested Torres Strait, and they could make the pass only once. There would be no second chance. There was no opportunity to pause, to put to shore or lay at anchor while plans were reconsidered or advice sought or local pilots acquired. The reliance on gravity assistance, too, demanded precise steering as to not only speed but also direction, or the Voyagers might be flung wildly into space. The Voyagers had a “delivery error” of one hundred kilometers. The Grand Tour was premised on an ability, as the prevailing metaphor put it, to tee up a golf ball in New York and sink a hole-in-one in California.
An ancient distinction exists between navigation and pilotage. Pilots guided ships through local waters they knew from long apprenticeship, perhaps assisted by logbooks or rudders that spoke to the details of maneuvering through the hazards of particular harbors. The geographic oddity that rendered Europe a peninsula broken fractally into smaller peninsulas that in turn dissolved into isles meant that pilots steered along coasts and over small seas. They crossed the Mediterranean not all at once but through a series of lesser crossings, strait by strait, sea by sea. Not until mariners undertook long traverses over blue-water oceans did navigation, or the means to determine location when out of sight of land, become essential, and pilots and navigators begin to merge. And then they again searched sea by sea and strait by strait.
The means for navigating were primitive. A compass; an astrolabe, a Davis quadrant (or backstaff ), or sextant; a wooden log dragged on a knotted rope—by such means one could estimate location relative to the Earth’s magnetic pole, or its cosmological setting, and perhaps something of its speed. But all were faulty, little better than calculated magic. Experience counted more than instruments. The distribution of planetary magnetism—specifically the declination of the magnetic pole from true north—was unknown. Sightings of the Sun and polestar could determine with relative ease one’s latitude, which argued for sailing along fixed latitudes wherever winds and currents made such passages possible.56
At the onset of the Great Voyages the Portuguese assembled scholars to formalize, and if possible improve upon, known practices. To the rule of the North Star, they added the rule of the Sun, created a table for the rule for “raising the pole,” and made analogous calculations for the Southern Cross, all of which were gathered into a manual of navigation and nautical almanac, the Regimento do astrolabio e do quadrante, taught in formal schools for navigation established at Lisbon and, later, for Spain, at Seville. The exercise did for latitude what later state sponsorship did for longitude. It was thus no accident that the rediscoveries of the New World came from voyages that traveled from east to west, with the Norse sailing from isle to isle along a rude line of latitude (and, barring storms, never more than a couple of days out of sight of land) and with Columbus following favorable trade winds that blew roughly east and west.57
Far trickier was to determine longitude, for which there was no practical solution until the nineteenth century. The state of learning was not merely wanting but often dangerously inept. The fact is, the Great Voyages achieved their goals without adequate navigational techniques, and the Second Age accomplished most of its task before chronometers were both accurate and abundant enough to make a calculation of longitude generally sufficient. Instead explorers relied on an artful, if not quasi-superstitious, appeal to eclectic methods and a personal alloy of experience, hunch, whim, prayer, and luck, or the unfortunately named “dead reckoning.” Samuel de Champlain, one of the few explorers equally successful on both land and sea, shrugged his shoulders. He had never been able to learn from any mariners with whom he talked how they did it, “except that it be done by fanciful rules, all different, some better than others.” He trusted those who had actually voyaged more than those “others who often pretend to know more than they do.”58
The common practice was to recycle pilots, as da Gama did to double the Cape, or to seize them, as he then did at Malinda, and as Albuquerque did at Java before sailing to the Spice Isles.
By the time Voyager launched, pilotage referred to the minutely choreographed acts associated with planetary encounter; and navigation, to black-space sailing. For the latter, the Voyagers commanded a navigation team (NAV) of twelve. For the former, the spacecraft relied on an elaborate procedure for identifying the hundreds of tasks that an encounter required and then worked out a second-by-second sequence, coded it for the onboard computers, and uploaded the package.
The duties of the NAV team were three. The first was trajectory, or mission design. It tested options for routes by balancing size of payload and launch capacity with where program scientists wanted to go and what they wished to do when they arrived. There were tradeoffs, an infinity of tradeoffs. Unlike Cabot or Cabral, Voyager could not put to port to refit or replan or decide the season was too advanced to proceed or elect to revisit a site of special interest. Each planetary encounter had to be exquisitely choreographed down to seconds, which meant that the mission had to determine core trajectories well before launch, since prospective launch dates varied according to the desired routes past the planets.59
As chief navigator, Charles Kohlhase had overseen some 10,000 prospective trajectories for the Grand Tour, a change in any one of which could ripple through all the rest. Eventually mission planners winnowed that unruly swarm into a handful. The first charge was of course to survey Jupiter and Saturn, but behind that was the vision of the Grand Tour, such that the prospects for Voyager 2 depended on Voyager 1. Besides the anticipated hazards of asteroids and the Jovian radiation field, the mission had to make a critical assessment of Titan and avoid the Saturnian rings. Even after launch, various possibilities abounded through midcourse corrections, but only within a single-minded, multitasking furious passage at 39,000 kilometers per hour that resembled a descent through a cataract. Still, trajectory corrections were both possible and necessary.
These—the “orbit determinations” that specified where Voyager was and the “nudges” that refined or redirected its path—were the largest of the NAV group’s assignments. Orbit determination relied on both Earth-based and spacecraft-based methods. Earth-based navigation tracked the spacecraft through its telemetry, the messages it sent back to the Deep Space Network’s dishes. Because a regular Doppler shift occurred, it was possible to calculate distance, and this could be done over and over across months, if desired, to know exactly where the spacecraft was and how fast it was moving. In a sense, the triangulations took the place of historic methods for determining latitude, and the Doppler shift assumed the role of onboard chronometers for determining the equivalent of longitude.
But among the unknowns that made precision trajectories so daunting was the range of uncertainty about the target planets themselves. If navigators were to steer
the spacecraft within a one-hundred-kilometer window, they required more precise measurements of diameter and mass—better than those obtainable from Earth. A mistake of a thousand kilometers in the diameter of a giant planet was entirely within the range of instrumental error, yet could prove ruinous for Voyager. For such measurements, navigators needed an optical navigation apparatus housed on the spacecraft itself. Images of the planet against the background of fixed stars refined its dimensions, much as small variations in acceleration (by which the spacecraft felt the pull of the planet) honed its mass.
In this way the guidance team juggled with two numbers. One forecast distance to target, and the other, time of encounter. Both were inevitably flawed, but the magnitude of error could be trivial. Distance was the more critical, since the positioning of flyby decided what the instruments and images would record. As the spacecraft approached closer, both numbers sharpened, and argued (or not) for a final tweaking of Voyager’s trajectory. Such corrections were programmed into the formalized sequencing of encounter.
The third navigational duty was to decide how to make those course corrections. The sooner the adjustments, the lesser the variance as encounter approached. Some deviations resulted from the sum of minor perturbations. Others came about as the exact specifications of time and place for encounter made for a more precise if frenetic scenario of maneuvers. But either way, the exercise was harrowing, for it demanded that the Voyagers temporarily abandon their typical mode of navigation.
In normal flight, the Voyagers, like the Mariners from which they descended, stabilized themselves around three axes. To hold the craft’s position, controllers had to triangulate from two fixed points in space. One was simple: the Sun. For near-voyaging craft, the second point could be Earth itself. For far-voyaging craft, however, Earth could be confused with its Moon, and both lay too close to the Sun, so another mark was needed. The star of choice was Canopus, in the southern constellation Carina, the second brightest light in the sky. Between those two sensors, one on the Sun and one on Canopus, the Voyagers constantly recalibrated their location. But when they underwent a burn to accelerate and reposition, they had to surrender that cosmodetic baseline. They needed another means to stabilize.60
The procedure began by repositioning the spacecraft so its engine would propel it in the proper direction, then turning off the celestial guidance system and yielding stability to a set of three gyroscopes, one for each axis. At such times, along with a temporary loss of its navigational sensors, Voyager would no longer point its high-gain antenna to Earth; it had to surrender the thermal balance that its formal stability had allowed, and then, after the burn, rely on a small omnidirectional antenna to reacquire Earth’s location while its star sensors recaptured the Sun and Canopus. Until then the spacecraft was on its own, and dependent on gyros, devices long and well understood but still machines and therefore subject to their own electrical and mechanical gremlins. The maneuver was fraught with hazards, and given the difficulties of telemetry and tracking, it might be weeks before the exact outcome could be assessed.
The Voyagers’ long cruises were the ideal times to correct trajectories, for the coasting phase lacked the frenzy of encounter, and without knowing the correct velocity, both position and speed, well in advance, the encounters would fail. On his second voyage Columbus missed the westerly trades and nearly foundered. Da Gama mistimed the monsoon winds to Africa and narrowly escaped disaster.
STAR STEERAGE
Across the ages navigation had relied on mixed technologies, the search for a new celestial referent, the power of judgment, and simple trust to luck. What Voyager did was accelerate the level of technical knowledge and transfer more of the burden to the spacecraft machinery: the mission was itself a kind of midcourse correction in the trajectory of exploration history. Pilot, helmsman, rudder and log, sextant and compass metamorphosed into a complex machine over which human controllers exercised ever-shrinking capacity for tactile guidance even as the demands for precision maneuvering swelled. As with everything about Voyager, its mission fused the hoary with a high-tech modernity.
The explorer still looked to the stars for guidance. At the onset of the Great Voyages, this meant the Sun and Polaris, the polestar. Yet the latter’s value lay in its constancy about the North Pole, and as the Portuguese probed southward, it fell lower toward the horizon; and beyond the equator, it disappeared. Still, one could coast along Africa, though only at the cost of dreary daily tackings that made the Indies seem more remote rather than less so. If they wished to find those distant lands of their fevered imaginations, exploring marinheiros would have to sail from the mundane shorelines and into the Sea of Darkness, which was now all the murkier because the travelers had left behind the lights of both familiar constellations and the polestar itself.
Yet an astounding sight greeted them. Looming up was a striking constellation that resembled a kite or, to the eyes of the Portuguese, a cross. Instead of terrifying them with its novelties and its perhaps unknowable heavens, the southern sky seemed to beckon, as though they were crusaders. The Southern Cross summoned them to new worlds, and new possibilities, of navigation. As swaths of strange stars appeared, so would novel methods emerge to pilot mariners across those untracked seas and even an ocean an order of magnitude broader than Europe itself. Eventually Galileo’s telescope unveiled the inner moons of Jupiter and made a calculation of longitude feasible; surveys of geomagnetism plotted maps that traced the deviations that made a recalibration of the compass possible. Exploration and experiment would continue, each one provoking the other to new exertions. The Voyagers now raced toward the Jovian—the Galilean—moons that had caused a revolution in cosmography and navigation and were poised to announce another.
As the Portuguese sailed south, they spied also an extraordinarily bright star that rose in prominence as they pressed on, more luminous than anything in the night sky save Sirius. With providential calculation, it seemed to circle about the southern pole. They named it Canopus.
DAY 383 - 444
10. Encounter: Asteroid Belt
In late 1977 Voyager 2 entered the asteroid belt; Voyager 1 soon f ollowed. In mid-December, as planned, some 124 million kilo-I meters from Earth, Voyager 1, on a faster, tighter trajectory, passed its twin.
The swirl of asteroids was the first of the known hazards awaiting beyond the Earth-like planets. They were, collectively, a kind of failed planet, a vast composite of planetary particles, some, such as Ceres, as large as Texas, most smaller and more worrisome. Mission planners envisioned the cluster as a diffuse shoreline of sandlike debris that could strike with the speed of bullets. Moreover, the belt could stand, in surrogate, for the rings, both known and yet to be discovered, that encircled the giant planets. It had to be threaded or rounded for the Voyagers to find the open seas beyond. Within the barrier, the biggest blocks were mapped, and could be avoided; the others were a matter of blind reckoning and, for the frozen sand-storm of meteorites, a question of luck. The only way to know was to do it.
Fortunately, the Voyagers did not have to sail blind. A plucky pair of spacecraft, Pioneers 10 and 11, had already blazed a route to Jupiter and Saturn. Thanks to them, the passage to the Indies of the outer planets was open.
O, PIONEERS
Their names were appropriate. Pioneer was the hardy frontier scout, the first to Jupiter and Saturn, the tough traveler who discovered the pitfalls and fords, the pathfinder who made the Grand Tour possible. Voyager was the scientific sojourner, the one who rediscovered and elaborated the Jovian and Saturnian systems with rigor, and then plunged into the far unknown before overtaking the trailblazers. Pioneer was the indomitable Jedediah Smith, hunkered against the wind, crossing over South Pass; Voyager, the Pacific Railroad Surveys’ grand reconnaissance of the West.61
The Pioneer spacecraft were designed to be simple, durable, cheap, and, if necessary, expendable. The largest difference between their design and that of the Mariner series was that they maintained stabi
lity by spinning, rather than using a three-axis arrangement of thrusters. The earliest prototypes struggled through multiple launch failures until 1960, when Pioneer 5 roamed the interplanetary domains around Venus. Pioneers 6-9 were arrayed throughout the inner solar system as a “space weather network.” But the famous missions were Pioneers 10 and 11 to Jupiter and Saturn. In what they found and how they did it they not only foreshadowed the Voyagers but often led.
They were, as NASA put it, “precursor missions.” They were the first spacecraft to the two giant planets, they discovered the first of the new moons that the far-traveling robots would find, they helped sharpen the communications apparatus for deep-space communication, they confirmed the capacities of gravity-assist trajectories, they demonstrated how a lead spacecraft could inform and redirect the purposes of a second, and they blazed a trail through the asteroid belt, Jupiter’s lethal radiation, and Saturn’s rings. They were the first spacecraft to travel beyond the orbit of Pluto, and the first to carry engraved messages to any other intelligence that might lie beyond the solar wind. Dispatched the year after Voyager, Pioneer Venus 1 and 2—the last of their breed, turning inward to spiral around Venus—concluded the golden age of American planetary exploration.62
Pioneer 10 launched in March 1972, and Pioneer 11 a year later, in April 1973. It was Pioneer 10 that first encountered Jupiter, warned of its lethal radiation, and sent back real-time images of the planet as it approached periapsis. Its instruments sketched a new cartography of the solar system, a geography of magnetism and radiation. But the reports that captivated the public were the pictures that took shape, scan line by scan line, from Pioneer’s Pulsed Image Converter System (PICS). Its spin-stabilization design meant Pioneer did not have a camera, but it could broadcast data from the imaging photopolarimeter’s narrow-angle telescope in strips as it rotated. These were assembled into color images by computer, which added green to the reds and blues that Pioneer sent back, and then projected the result on television.
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