Voyager - Exploration, Space, And The Third Great Age Of Discovery

by Stephen J. Pyne

  There remained a final course correction, the last tweaking of a trajectory that had spanned nearly five billion kilometers. But so accurate was the spacecraft’s path that no further adjustments were needed. Voyager 2 hurtled within twenty kilometers of its ideal aim point, testimony to a masterful triangulation among it, JPL, and the Deep Space Network.151

  Compared to past exploration, Uranus was unfathomably remote in space but close in time. It could substitute speed for distance.

  Lewis and Clark traveled 13,700 kilometers in 28 months; Voyager 2 would take a few months longer than that to travel almost eight billion kilometers. The Corps of Discovery was basically out of contact for two years, and the collective journals were not published (and then in abridged form) for another century. At Saturn a message from Voyager took 86 minutes to reach the DSN; at Uranus, it would take 164 minutes, and as long to return. The first fruits of discovery were offered at daily press conferences during encounters, and published in scientific journals within months. While the ever-lengthening lag in electronic communications required Voyager to be semiautonomous, compared with historic precedents their correspondence and publication were practically simultaneous.

  Such communications had always served to tell just where an explorer was and where he was headed. With space travel, however, the exchanges were the irreplaceable means needed to guide the spacecraft to its destination. This was a novelty. During the Great Voyages armadas had vanished over the horizon with hardly the prospect of a reply until they, or some fraction of them, returned. The last port of call was the final opportunity to exchange commands, information, personal mail, or warnings. While in the Canaries, Magellan received an alert that a Portuguese squadron was waiting to intercept him on his way to the New World, and so rerouted his voyage southward. One of his ships, the San Antonio, broke off and deserted near the Strait and returned with news (much false) to Spain. Otherwise, no one heard from the expedition again until, two years later, the Victoria limped into Lisbon. No camera watched as Columbus set foot on San Salvador, no radio message signaled da Gama’s arrival in Calicut, no transmission measured Dias’s occultation around the Cape of Good Hope. No one heard until someone returned, and if no one returned, no one knew what had happened or where. Lewis and Clark simply disappeared up the Missouri River, with no word about their whereabouts until they reappeared at St. Louis. When John Cabot disappeared in his search for a Northwest Passage, no one knew when or where.

  Increased commercial traffic improved communication during the eighteenth century, and the telegraph boosted opportunities during the nineteenth. Humboldt sent letters to his sister Caroline in Prussia, who published them in newspapers. Dispatches from Russian American Company outposts in Alaska could take two hard years to reach Moscow and return. From Greenland, the 1845 Franklin Expedition sent letters home with the returning supply ship and military escort, though they might have used the whaler that spotted them afterward in Melville Bay; but they could not have expected any replies (and they themselves were never heard from again). Thomas Huxley on the HMS Rattlesnake could anticipate sending and receiving letters at Australian ports via the Royal Navy or commercial shipping. Henry Stanley had to wait until he returned to Zanzibar before he could telegraph his “discovery” of Livingstone to the New York Herald; but his more breathtaking traverse across central Africa down the Congo had him out of contact for three years. Newspaper rumors about his death preceded John Wesley Powell’s successful run of the Colorado River through Grand Canyon. When Robert Scott’s party died on their trek to the South Pole, it took two weeks for a search party to find their remains, another two months to inform the crew of the returning Terra Nova, and still another two weeks by ship before they could tell the world at Lyttleton, New Zealand. Seventeen years later Robert Byrd flew over the pole in a Ford Trimotor and then reported the feat by wireless radio upon his return to base at Little America.

  Voyager had its missed, and mixed, messages, beginning years before, with JPL’s error to send it an expected signal, which caused it to default to backup procedures, and continuing to moments when its cantankerous receiver would lose contact and when the spacecraft disappeared for painful minutes behind a planet. But it had what few prior expeditions did: a system of redundancy. As it pushed across the Outback of the solar system, it also had encoded in its protocols the wrenching history of past exploration. It could accept new orders. It could adapt new procedures. It could learn.

  The immense distances meant that Voyager could not wait for commands from Earth. That it could communicate routinely with JPL was an astounding accomplishment; but the lengthening lines of communication meant that its managers had to allow Voyager more and more leeway. The velocity of encounters left no other option, for although communication—if compared to that available to Louis-Antoine de Bougainville in the Pacific or Nikolai Przhevalsky in Central Asia—was nearly instantaneous, “nearly” wasn’t good enough. Voyager had to communicate with itself.

  THE DARKEST WORLD

  Still, Voyager 2 could only report on what it was told to examine, and Uranus was a dark world, only dimly fathomed. With Jupiter and Saturn enough had been known to predict what the scientific priorities might be. But Voyager 2 now required detailed scenarios for an encounter with a planet only first discovered in 1781, two centuries before the spacecraft had left Saturn, and a body about which almost nothing was known. Its planetary plane was tilted: that anomaly was only the most apparent of Uranus’s obliquities.

  During its long, silent cruise phase, Voyager’s staff had sought to sharpen that blurry vision and to heighten the return of Voyager information as the spacecraft blasted through the planetary plane of an eccentrically oblique Uranus. The task was formidable. There would be no Pioneers 10 and 11 to forge trails, and no Voyager 1 to do a sweeping reconnaissance. The skewed rotational plane made trajectories stranger still, and the path to Neptune trickier. Near-encounter would have to compress almost everything into a scant six hours, roughly the time it took to exchange messages between Voyager and Earth. Long distances, limited computing power, a bottomless thirst for data, and a scarcely known target—encounter demanded an unusually intricate scenario, which is to say, advance scientific scouting and planning.152

  That began with a conference on February 4-6, 1984, which assembled more than one hundred scientists to assess the state of knowledge regarding Uranus and Neptune. The resulting proceedings revealed ignorance about even such basic parameters as rotation periods, magnetic fields, radiation, rings, and major satellites. Remoteness and a thick methane atmosphere made Uranus’s surface opaque. Nothing was known of its weather, and heating at the poles rather than the equator introduced unprecedented variables in comparison with the other gaseous planets. Five satellites were known; almost certainly there were others. Nine narrow rings had been identified; again, there were surely more. (The rings had been discovered by stellar occultation only in 1977, the year of Voyager’s launch.)153

  Necessarily, much of the expected data and anticipated measurements would draw on analogies to the two gaseous giants Voyager had already visited. But some features did not translate—that oblique rotational plane, for one, and worse, Uranus’s exceptional dimness, which rendered its rings and moons among the darkest objects observed in the solar system. Since sunlight decreased with the square of the distance, Uranus received 1 unit of solar radiation for every 360 at Earth. That left Uranus plenty dark, its moons nearly invisible, and its rings, reflecting some 2 percent of even incident sunlight, with half the brightness of “ground-up charcoal.”154

  Researchers were organized into three Uranus Science Working Groups, one each for atmospheres, rings, and satellites and magnetospheres. (To assist deliberations, Bradford Smith and Richard Terrile of the Voyager imaging team conducted special observations at the Las Campanas Observatory, in Chile, to gather extra data on the rings.) Between April and May the three working groups met repeatedly and independently to identify the major themes of in
quiry and then to work with the flight science office to reconcile what they wanted with what the instruments could do and how to sequence the desired observations, each of which was designated as a “link,” and which collectively made a chain of encounter events that could be graphed into a timeline. Each group also prioritized its scientific goals. In July 1984 they issued their final report to the Voyager Science Steering Group. Then the three groups had to reconcile their ambitions and produce a master (if still incomplete) timeline. Iteration followed iteration, with the work lasting from February 11, 1985, to July 19, 1985.155

  One intractable issue concerned Miranda, the principal moon of Uranus. Planetary astronomers desperately wanted to know its mass, which would affect their calculations of the planetary system’s orbits as well as the path of Voyager 2, and this required real-time tracking to detect the gravitational pull of the moon on the spacecraft. But planetary geologists wanted equally to image the surface, which required image motion compensation; that is, the spacecraft had to turn along with the camera to keep the narrow-angle lens pointed steadily on target. The first needed constant contact with Earth, while the second forced the antenna to turn away, breaking that connection; there was no way for the hardware to be in two places at the same time. But it was possible for the software to pretend otherwise. During the iteration exercise, with its successive refinements of timing, planners realized that the image requirements could be satisfied best just prior to closest approach, and the mass determination during and just after closest approach, which left a difference between the two tasks of roughly five minutes during which the antenna had to reposition itself. The hardware could run only one way, but the software could be run backward, as it were; the effect on image motion compensation was the same. And so it proved: both experiments yielded excellent returns.156

  The experience was no less a symbolic reversal of public expectations of how Voyager actually worked. The public face was rockets and robots. The reality was software and communications.

  Over and again, hardware failures—faulty platform gears, broken receivers, the need for extended imaging—were compensated for by software fixes and the ability to send them across the immensity of interplanetary space. Programs for image motion compensation tweaked spacecraft gyroscopes to allow long exposures even as the spacecraft hurtled onward. Special software compressed data, and even allowed for onboard processing, which reduced the time and burden of transmission; an onboard Reed-Solomon data encoder bolstered the accuracy of data transmission while shrinking its digital “overhead.” Other programs adjusted actuators for slewing, torque, and rolls.

  But the grandest software, constantly rewritten and uploaded, remained in the human brain. It was the vision of the Grand Tour, and in late 1985, made real in Voyager, it sailed boldly beyond any previous encounter and toward a world darker than any other in the solar system.

  DAY 2,493 -2,635

  16. Encounter: Uranus

  From March 26 through November 3, 1985, preparations for encounter commenced with a routine that included regular imaging of Uranus, a recalibration of instruments, and a trajectory course correction. For almost three and a half years, Voyager had run in maintenance mode; now it had to be resuscitated, and its crew retrained for the intricate tacking and close hauling executed at velocities ten times that of a bullet. Staffing gradually scaled up. There were training sessions and trial runs, a shakedown rehearsal. As it felt the gravitational tug of Uranus, the mass of the planet altered Voyager’s velocity, and the quickening events bulked up time until its history, too, seemed to acquire momentum. 157

  From October 7 to November 4, over four years after leaving Saturn and eight since it broke free from Earth, Voyager 2 put its instruments, mechanisms, and procedures through a series of checks. The power system, scan slewing, maneuvers for image motion compensation and radio occultation, likely frequency drift in the spacecraft radio reception, wide-angle photography of background stars—everything that the spacecraft would have to do in less than three months when any prospect for corrections would be lost in the blistering rush past the planet was tried now. The process culminated in a full-dress rehearsal of near-encounter. The exercise revealed assorted problems, primarily in new and inadequate staff, which JPL and DSN addressed by calling up reserves and scheduling more training. Meanwhile, engineers worked feverishly to ready final adjustments, engage the so-called target maneuver that would calibrate the image science subsystem and IRIS, and upload software patches and the all-important sequence commands.

  Near-encounter would last a mere 6 hours; an exchange of radio messages between Earth and Uranus would take 5.5 hours; as Voyager hurtled around Uranus in another volta, there would be no opportunity to do anything but let the program play out. Expectation and anxiety tumbled along like wood chips through a cataract.

  CLOSE ENCOUNTER

  On November 4, Voyager 2 entered its observatory phase.

  Since July the extended scrutiny of the planet offered by Voyager’s instruments had exceeded the best Earth-based sources. One after another the remote inventory of what JPL’s Voyager Uranus Travel Guide called “our human quest to understand the world beyond Earth” began its third grand iteration. The ultraviolet spectrometer sought out gaseous emissions and the Uranian aurora. The planetary radio astronomy instruments searched for evidence of a Uranian magnetosphere. Of particular interest was timed imaging of the atmosphere; these photos were assembled into thirty-eight-hour movies (hence, two complete rotations of the planet) although the oddities of tilted Uranus meant that the imaging recorded the south pole, which allowed far less variety than comparable movies of the Jovian and Saturnian equatorial belts. Steadily, strikingly, the blue murk that was Uranus began to dissolve into features and movements, and the hard matrix that was the Uranian system resolved itself out of a starry background. The planet became real.158

  Voyager’s arcing trajectory around Uranus offered the usual occasion for occultation, with measurements of rings and planetary atmosphere, but halfway through its observatory phase, the spacecraft, as viewed from Earth, would pass behind the Sun. This solar conjunction allowed for occultation of the solar atmosphere as well, and because of the Sun’s immense mass, for a test of the general theory of relativity, which predicted that the rays would bend (and therefore slow) under the influence of gravity, or more precisely, from the curved geometry of space around the Sun. This backside transit of Voyager yielded results consistent with prediction.159

  Mostly, observation meant preparations: more measurements of the interplanetary medium, continuing instrumental calibrations, further testing of scan platforms, torque margins, and gyroscopic drifting. The various clocks had to be synchronized. Increasingly large images had to be edited into movies. Eventually an acrobatic Voyager underwent four yaws and four rolls that helped reset its magnetometer and furnished detailed star maps for precision guidance. The observatory phase ended with a final (of five) trajectory correction maneuver. 160

  After ninety-five days, observation segued into far-encounter on January 10, 1986. For the next dozen days the pace of monitoring matched the acceleration of the spacecraft. The pull of Uranus speeded up everything.

  By now the bulk of Uranus could no longer fit into a narrow-angle lens; it took four such images to make a planetary mosaic; and soon the scope of the scene would overwhelm capabilities for whole-planet movies. Measuring the gravitational tug allowed for more precise calculations of planetary mass, which refined further trajectory corrections. Dormant instruments came back to life. The temperature of IRIS stabilized, readying it to transmit infrared data. The photopolarimeter revived and swept over the planet, satellites, and rings. The radio astronomy and plasma wave instruments added to particle and field measurements. New command packages, B721 and B723, were uploaded. A final “operational readiness test” put both the Voyager JPL team and the DSN through their paces. After years of mechanical hibernation, Voyager 2 was fully roused and ravenous, and on January 22 it
began its near-encounter. 161

  In something like six hours Voyager threaded its way through the Uranian system, conducted more than ninety priority science experiments, and sped at still greater velocity toward Neptune. It was not simply the number of maneuvers but their condensation that astonishes. At Jupiter and Saturn, the Voyagers had sailed along the plane of planetary rotation, joining the flow of the rings and moons, faster than those other satellites but moving with and through a shared orbital plane over several days. Now Voyager 2 passed across them, almost at right angles. Everything had to happen in sharp, crisp succession. The spacecraft had to roll, which meant it had to shift its reference stars from Alkaid (in Ursa Major) to Canopus and then Fomalhaut in Piscis Austrinus and Achernar in Eridanus. Voyager soaked up so much data that it could not unload it, even with compression algorithms, except by replaying tapes over several days.

  As it passed across the backside of Uranus, Voyager conducted almost continual occultation experiments with rings and atmosphere. The unblinking eyes of Voyager’s instruments, especially IRIS and the radio science sensors, recorded atmospheric temperature, pressure, and chemical composition. Full-ring mosaics were composed both coming and going. Passage through the rings recorded particle impacts. Other instruments sampled the north polar region, invisible as Voyager had approached but now exposed as it swung behind the planet. The spacecraft interrogated the mysteries of the Uranian magnetosphere—whether it existed at all, and if so, with what dynamic geometry, since the south pole pointed into the solar wind. After passing repeatedly into and out of bow shock, Voyager could sketch a map of that magnetic shorescape. It imaged and studied the brightness of the five major moons—Ariel, Umbriel, Titania, Oberon, and Miranda—and used their perturbing gravitational effects on the spacecraft to measure more precisely their separate masses; of Miranda, the innermost moon, it made particularly close observations, passing within a mere 29,000 kilometers. But most complicated were the full-color and monochromatic photos of the Uranian satellites, all of which demanded image motion compensation, which required the spacecraft to pitch, yaw, and roll with intricate timing to hold the bleakly dark scenes steady while the spacecraft sped past. All this occurred amid constant moderate-rate slewing to capture as many objects as possible. And so it did: Voyager unveiled new rings and discovered ten new moons. For the largest, Puck, it captured a hurried but still usable image.162