Voyage to Jupiter

These scientific instruments and their objectives were selected many years before the first Jupiter encounter in March 1977. Because Voyager was an exploratory mission, every effort was made to fly versatile instruments that could yield valuable results no matter what the nature of the Jovian system. In addition, the Voyager spacecraft control system permitted the instruments to receive commands from Earth to adjust their sensitivities and observing sequences in response to new information. By the spring of 1977, all the instruments were completed, ready to be installed in the Voyager spacecraft for testing and launch.

CHAPTER 5
THE VOYAGE TO JUPITER—GETTING THERE

Launch

On August 20, 1977, exactly two years after the launch of the Viking spacecraft to Mars, the first of the Voyagers—actually Voyager 2—was boosted into space at 10:29 a.m. EDT, less than five minutes after the launch window opened on the first day of the thirty-day launch period. Sixteen days later, at 8:56 a.m. EDT on Labor Day, September 5, 1977, Voyager 1 was hurled into space on a shorter, faster trajectory than its twin, zipping past the orbit of the Moon only ten hours after launch. Ultimately, Voyager 1 earned its title by overtaking Voyager 2 as both spacecraft journeyed through the asteroid belt, to arrive at Jupiter four months ahead of Voyager 2.

The Voyagers lifted off from Launch Complex 41, Air Force Eastern Test Range, Kennedy Space Center, Cape Canaveral, Florida, atop the giant Titan III-E/Centaur rocket. It was the last time such a launch vehicle was scheduled to be used, as, according to plan, the Space Shuttle would take over in the 1980s. Thus the launching of the two Voyagers signified both an end and a beginning: a once-in-a-lifetime opportunity to explore, in only 8½ years’ time, perhaps fifteen major bodies of the outer solar system.

But long before even the first Voyager was to make its closest approach to Jupiter—in fact, even before Voyager 2 was off the launch pad—there were problems to overcome.

In early August 1977, about three weeks before launch, failures in the attitude and articulation control subsystem (AACS) and the flight data subsystem (FDS), two of the spacecraft’s three main computer subsystems, prevented the VGR77-2 spacecraft, originally scheduled for launch on August 20, from becoming Voyager 2. Instead, the “spare” spacecraft VGR77-3 was substituted, becoming Voyager 2 upon launch August 20, and VGR77-2, after proper repairs, became Voyager 1. Minor problems continued right up to launch. The low energy charged particle instrument failed and had to be replaced, and as late as T minus five minutes there was a halt in the countdown to check on a stuck valve. Unlike a jinxed dress rehearsal, which is said to “assure” an opening-night success, Voyager 2’s prelaunch problems were a portent of difficulties to come.

The Voyager 2 launch was witnessed by thousands as the spacecraft ascended gracefully into the blue Florida sky, accompanied by the deep-throated rumblings of its rocket, echoing for miles across the beaches and scrub forests of Cape Canaveral. The Titan-Centaur performance was nearly flawless, and Voyager 2 quickly achieved an accurate trajectory toward Jupiter. However, even while the engines were still firing, the spacecraft began to experience a baffling series of problems that would absorb the attention of hundreds of persons from Pasadena to Washington, D.C., for the next several weeks until they were brought under control.

During the first minutes of flight, there seemed to be two difficulties with the AACS. The first was a problem with one of the three stabilizing gyroscopes, but fortunately, the gyroscope began operating normally without intervention from the ground. The other problem appeared to be with one of the AACS computers; the spacecraft switched to a backup computer during the Titan burn, and initial data transmissions were incomplete. Early analysis seemed to indicate that an event during the launch itself, rather than a faulty spacecraft computer system, was the cause of the data loss. At first, on August 23, officials suspected that perhaps the spacecraft had been bumped by the rocket motor one hour after liftoff and again about seventeen hours later, when telemetry signals indicated that the spacecraft had been jolted. However, by the next day, flight engineers determined that electronic gyrations in the AACS seemed to have caused the difficulty.

Within an hour after launch, Voyager 2’s science scan platform boom was to have been fully extended and locked. Instructions to deploy were given, and the boom moved outward; however, there was no signal to indicate that the boom was actually locked in place. Efforts to command the boom to move into the locked position were thwarted by the spacecraft. The first maneuver designed to try to lock the boom was aborted by the computer command subsystem (CCS) when the AACS erroneously indicated that it was in trouble. Three days later another maneuver was scheduled to reprogram the faulty computer in the AACS, to align the Sun sensors, and to try to lock the science boom. To provide a direct check of the boom position, the scan platform was turned so that the TV cameras could see the spacecraft. Careful measurement of these pictures verified that the boom was within ½ degree of full deployment, but still there was no indication that it was locked into place. Ultimately, it was decided that the sensor to signal actuation of the lock was at fault, and that the boom itself was almost certainly fully extended and operational.

Because of the postlaunch problems of Voyager 2, the launch of Voyager 1 was delayed twice—from September 1 to September 3 and then to September 5—in order to inspect Voyager 1’s science boom and to try to prevent a repetition of Voyager 2’s problems. An extra spring was attached to the science boom to assure its full extension. Finally, as if to make up for the troubles of the first launch, Voyager 1’s launch was both “flawless and accurate.” All launch and postlaunch events went smoothly. The launch window opened at 8:56 a.m. EDT, and Voyager took off promptly at 8:56:01. The booms and antennas deployed and locked in the first hours after launch; all instruments scheduled to be on were on and working well.

The First Year Is the Roughest

During the autumn of 1977 Voyager 2, and to a lesser extent Voyager 1, continued to plague controllers with erratic actions. Thrusters fired at inappropriate times, data modes shifted, instrument filter and analyzer wheels became stuck, and the various computer control systems occasionally overrode ground commands. Apparently, the spacecraft hardware was working properly, but the computers on board displayed certain traits that seemed almost humanly perverse—and perhaps a little psychotic. In general, these reactions were the result of programming too much sensitivity into the spacecraft systems, resulting in panic over-reaction by the onboard computers to minor fluctuations in the environment. Ultimately, part of the programming had to be rewritten on Earth and then transmitted to the Voyagers, to calm them down so that they would ignore minor perturbations, yet still be ready to perform automatic sequences required to protect the spacecraft from major threats. Meanwhile, however, more serious problems were developing.

On February 23, 1978, during a series of movements or slews, Voyager 1’s scan platform slowed and stopped before completing the maneuver. This failure caused a great deal of concern, since the scan platform houses the optical instruments that are crucial to the observation of the Jovian system—the ultraviolet spectrometer, the IRIS, the photopolarimeter, and the two TV cameras. At JPL, tests were run on a proof-test model—an exact copy of the Voyager spacecraft—to try to find out why Voyager 1’s scan platform had become stuck. On March 17, Voyager 1’s scan platform was tested—JPL engineers instructed the platform to move slowly for a short distance, and Voyager responded as ordered. Further tests were conducted on March 23. This time the scan platform was ordered to execute a sequence of four slews, moving away from the part of the sky where the original failure had occurred and ending with the position that it would be most useful to leave the platform in—just in case the platform should become stuck again. On April 4 the scan platform was commanded to perform a sequence of 38 slews, and fifty more slews were performed on April 5. All were successful. Yet engineers were still hesitant to force the platform to move through the region where it had originally stuck, and extensive discussions were held to determine if the Jupiter observations could be carried out without risking a return to the danger area. It was argued, however, that full mobility of the scan platform really was required, and on May 31 commands were sent to maneuver the scan platform through the danger region. It moved normally: The scan platform was operating properly again. After additional slewing tests were run in mid-June, the scan platform was pronounced fit for operation. Engineers suspected that the material caught in the platform gears must have been crushed or moved out of the way by the continued slewing, allowing the platform to move once more.

An even more serious crisis soon endangered the Voyager 2 spacecraft. In late November 1977, the S-band radio receiver began losing amplifier power in its high-gain mode, so the solid-state amplifier was switched to its low-power position. No further problems were noted until April 5, 1978, when Voyager 2’s primary radio receiver suddenly failed, and shocked engineers discovered that the backup receiver was also faulty. The trouble was detected after Voyager’s computer command subsystem directed the spacecraft to switch from the primary radio receiver to the backup receiver. This command was issued as part of a special protection sequence: If the primary radio receiver receives no commands from Earth for seven days, the backup receiver is switched on instead; if the secondary receiver in turn receives no instructions over a twelve-hour period, the system reverts to the main receiver. When, on April 5, Voyager 2’s radio reception was switched from the primary to the secondary receiver, flight engineers found that they were unable to communicate with the spacecraft—the secondary receiver’s tracking loop capacitor was malfunctioning. That meant that the secondary receiver could not follow a changing signal frequency sent out from Earth. The frequencies of signals transmitted from Earth are affected by the Doppler effect—just as the siren on a fire engine seems first to rise in pitch as the truck approaches, then falls as the truck speeds away, so the frequency of signals transmitted from Earth fluctuates with the Earth’s rotation as the Deep Space Network’s radio antennas move toward or away from the spacecraft. The engineers had to wait until the primary radio receiver was switched back on before they could communicate with the spacecraft. Once the primary receiver was on, Voyager 2 began receiving instructions from Earth, but approximately thirty minutes later, there was an apparent power surge in the receiver. The fuses blew. There was no recourse. The main receiver had failed; its loss was permanent. It remained for the engineers to devise a way to communicate with the slightly deaf spacecraft.

Voyager 1

Voyager 2

Jupiter-Saturn-Uranus

Sun

Earth 8/20/77

Earth 9/1/77

Mars 8/20/77

Jupiter 8/20/77

Jupiter 3/5/79

Jupiter 7/9/79

Saturn 8/20/77

Saturn 11/13/80

Saturn 8/27/81

Uranus 8/20/77

Uranus 1/30/86

Because the switching of the radio receivers was still controlled by the special protection sequence discussed earlier, flight engineers would have to wait for seven days—until April 13—before they could attempt communication with the spacecraft again. During that week special procedures were established and rehearsed so that commands could be sent to Voyager in the short time that the backup receiver would be on. On Thursday, April 13, 1978, the seven days were up and the spacecraft should have shifted from the dead main receiver to the sick backup system. There was just a twelve-hour “window” in which to restore communication. At about 3:30 a.m. PST the Madrid tracking station of the Deep Space Network sent its first order to the spacecraft, approximately 474 million kilometers away. Almost an hour later, word arrived from Voyager that the command had been accepted. (One-way light time for a signal to travel the distance from Earth to Voyager at that time was almost 27 minutes.) Elated flight controllers went ahead and transmitted nine hours of commands to the spacecraft.

Voyager 2 was successfully commanded again on April 18 and April 26. The April 26 commands included a course change maneuver that was executed properly on May 3. On June 23, Voyager 2 was programmed for a backup automatic mission at Saturn in the event that the secondary radio receiver should also fail. These backup mission instructions would operate all the science experiments, but only a minimum amount of data would be returned, since the scan platform would only be programmed to move through three positions rather than thousands as it would in normal operation. Instructions for a backup minimum automatic encounter at Jupiter were transmitted to Voyager 2 in two segments, the second of these on October 12, 1978.

With the backup instructions recorded on board the spacecraft, Voyager personnel felt their fears partially allayed. If Voyager 2’s secondary radio receiver failed, the spacecraft would still obtain some science data at Jupiter and Saturn. But that would mean that there would be no mission beyond Saturn; our first opportunity to explore Uranus, its satellites, its newly discovered ring system, and possibly even to get a look at Neptune, would not come in this century.

Another major concern affecting both Voyager spacecraft was the proper management of hydrazine fuel reserves. Hydrazine is used by the thrusters on the Voyagers for stabilization of the spacecraft and for trajectory correction maneuvers (TCM). Each Voyager was loaded with 105 kilograms of hydrazine budgeted for use on the long flight to Jupiter, Saturn, and beyond. Because of the excellent performance of the launch rockets, both Voyagers required less hydrazine than anticipated for their final boost into proper trajectory toward Jupiter, and at first it looked as though both spacecraft would have plenty of propellant to spare.

Charles E. Kolhase, Manager of Mission Analysis and Engineering for the Voyager Project, later explained the situation: “Voyager 1 should have been launched September 1. Had it been launched on September 1—and I’m glad it wasn’t—the maneuver to correct the trajectory for a Titan flyby would have required a change in velocity of 100-110 meters per second—an enormous maneuver—and we would have had a propellant margin for going on to Saturn of perhaps 4.5 kilograms. But, by launching on the fifth of September we increased our margin to 23 kilograms. Fortunately, for every launch date that went by, that velocity change maneuver was shrinking at a rate of 10 meters per second per day. Now, a 1 meter per second change uses about a pound of hydrazine [about 0.5 kilogram]. So when we launched on the fifth of September, now we suddenly had 40 pounds of hydrazine excess over what we would have had if we had launched on the first of September. As a result, Voyager 1 is in great shape as far as hydrazine is concerned.”

Problems with hydrazine management developed, however. Voyager 1’s first trajectory correction maneuver achieved only 80 percent of the required speed change. Exhaust plumes from the thrusters apparently struck part of the spacecraft, causing a 20 percent loss in velocity. That being the case, Voyager might require more fuel than had been expected to complete the mission. The extra fuel requirements did not threaten Voyager 1 itself, since it held ample fuel to reach Saturn; the concern was for Voyager 2, where the effective loss of fuel might be enough to jeopardize the Uranus mission.

Because of the plume impingement problem on Voyager 1, Voyager 2’s first trajectory correction maneuver was adjusted to allow for the possibility of a 20 percent loss in thrust. The Voyager 2 maneuver was successful, but controllers felt that additional action was required to conserve fuel. One way to save was by reducing requirements on control of the spacecraft orientation. Less control fuel would be needed if the already miniscule pressure exerted on the spacecraft by the solar wind could be reduced. Flight engineers at JPL calculated that the pressure would be reduced if the spacecraft were tipped upside down; however, to accomplish this, the spacecraft would have to be steered by a new set of guide stars. By reprogramming the attitude control system it was found possible to substitute the northern star, Deneb, in the constellation of Cygnus, for the original reference star, Canopus, in the southern constellation of Carina. With this change, as well as readjustment of Voyager 2’s trajectory near Jupiter, inflight consumption of hydrazine was reduced significantly.

In late August 1978 both Voyagers were reprogrammed to ensure better science results at Jupiter encounter; for example, the reprogramming would prevent imaging (TV) photographs from blurring when the tape recorder was operating. By early November, flight crews had begun training exercises to rehearse for the Voyager 1 flyby of Jupiter on March 5, 1979. A near encounter test was performed on December 12-14, 1978: a complete runthrough of Voyager 1’s 39-hour near encounter period, which would take place March 3-5, 1979. Participants included the flight team, the Deep Space Network tracking stations, the scientists, and the spacecraft itself. Results: Voyager and the Voyager team were all ready for the encounter.

Meanwhile, the spacecraft were busy returning scientific data to Earth. Technically, the Voyagers were in the cruise phase of the mission—a period that, for Voyager 2, would last until April 24, 1979, and for Voyager 1, until January 4, 1979, when each spacecraft would enter its respective observatory phase.

Cruise Phase Science

In the first few days after launch, the spacecrafts’ instruments were turned on and calibrated; various tests for each instrument would continue to be performed throughout the cruise phase. This period presented a great opportunity for the Voyagers to study the interplanetary magnetic fields, solar flares, and the solar wind. In addition, ultraviolet and infrared radiation studies of the sky were performed. In mid-September 1977 the television cameras on Voyager 1 recorded a number of photographs of the Earth and Moon. A photograph taken September 18 captured both crescent Moon and crescent Earth. It was the first time the two celestial bodies had ever been photographed together.

In November both Voyagers crossed the orbit of Mars, entering the asteroid belt a month later. On December 15, at a distance of about 170 million kilometers from Earth, Voyager 1 finally speeded past its slower twin. The journey through the asteroid belt was long but uneventful: Voyager 1 emerged safely in September 1978, and Voyager 2 in October. Unlike Pioneers 10 and 11, the Voyagers carried no instruments to look at debris in the asteroid belt.

By April both spacecraft were already halfway to Jupiter and, about two months later, Voyager 1, still approximately 265 million kilometers from Jupiter, began returning photographs of the planet that showed considerable detail, although less than could be obtained with telescopes on Earth. Both the imaging (TV) and the planetary radio astronomy instruments began observing Jupiter, and, by October 2, 1978, officials announced that “the polarization characteristics of Jupiter’s radio emissions have been defined. In the high frequencies, there is consistent right-hand circular polarization, while in the low frequencies, there is a consistent left-hand circular polarization. This was an unexpected result.” In addition to scientific studies of the interplanetary medium and a first look at Jupiter, the plasma wave instrument, which studies waves of charged particles over a range of frequencies that includes audio frequencies, was able to record the sound of the spacecraft thrusters firing, as hydrazine fuel is decomposed and ejected into space. The sound was described as being “somewhat like a 5-gallon can being hit with a leather-wrapped mallet.”

On December 10, 1978, 83 million kilometers from Jupiter, Voyager 1 took photographs that surpassed the best photographs ever taken from ground-based telescopes, and scientists were anxiously awaiting the start of continuous coverage of the rapidly changing cloud forms. These pictures, together with data from several ground-based observatories, were carefully scrutinized by a team of scientists at JPL to select the final targets in the atmosphere of Jupiter to be studied at high resolution during the flyby. The observatory phase of Voyager 1’s journey to Jupiter was about to begin.

The Observatory Phase

The observatory phase, originally scheduled to start on December 15, 1978, eighty days before encounter, was postponed until January 4, 1979, to provide the flight team a holiday-season break. For the next two months, Voyager would carry out a long-term scientific study—a “time history”—of Jupiter, its satellites, and its magnetosphere. On January 6, Voyager 1 began photographing Jupiter every two hours—each time taking a series of four photographs through different color filters as part of a long-duration study of large-scale atmospheric processes, so scientists could study the changing cloud patterns on Jupiter. Even the first of these pictures showed that the atmosphere was dynamic “with more convective structure than had previously been thought.” Particularly striking were the changes in the planet, especially near the Great Red Spot, that had taken place since the Pioneer flybys in 1973 and 1974. Jupiter was wearing a new face for Voyager.

In mid-January, photos of Jupiter were already being praised for “showing exceptional details of the planet’s multicolored bands of clouds.” Still 47 million kilometers from the giant, Voyager 1 had by now taken more than 500 photographs of Jupiter. Movements of cloud patterns were becoming more obvious; feathery structures seemed painted across some of the bands that encircle the planet; swirling features were huddled near the Great Red Spot. The satellites were also beginning to look more like worlds, with a few bright spots visible on Ganymede and dark red poles and a bright equatorial region clearly seen on Io when it passed once each orbit across the turbulent face of Jupiter.

From January 30 to February 3, Voyager sent back one photograph of Jupiter every 96 seconds over a 100-hour period. Using a total of three different color filters, the spacecraft thus produced one color picture every 4¾ minutes, in order to make a “movie” covering ten Jovian “days.” To receive these pictures, sent back over the high-rate X-band transmitter, the Deep Space Network’s 64-meter antennas provided round-the-clock coverage. Voyager 1 was ready for its far encounter phase.

Far Encounter Phase

By early February Jupiter loomed too large in the narrow-angle camera to be photographed in one piece; 2 × 2 three-color (violet, orange, green) sets of pictures were taken for the next two weeks; by February 21, Jupiter had grown too large even for that, and 3 × 3 sets were scheduled to begin. When these sets of pictures are pasted together to form a single picture, the result is called a mosaic.

While the imaging experiments were in the limelight, the other scientific instruments had also begun to concentrate on the Jovian system. The ultraviolet spectrometer had been scanning the region eight times a day; the infrared spectrometer (IRIS) spent 1½ hours a day analyzing infrared emissions from various longitudes in Jupiter’s atmosphere; the planetary radio astronomy and plasma wave instruments looked for radio bursts from Jupiter and for plasma disturbances in the region; the photopolarimeter had begun searching for the edge of Io’s sodium torus; and a watch was begun for the bow shock—the outer boundary of the Jovian magnetosphere.

On February 10, Voyager 1 crossed the orbit of Sinope, Jupiter’s outermost satellite. Yet the spacecraft even then had a long way to go—still 23 million kilometers from Jupiter, but closing in on the planet at nearly a million kilometers a day. A week later, targeted photographs of Callisto began to provide coverage of the satellite all around its orbit; similar photos of Ganymede began on February 25.

Meanwhile excitement was building. As early as February 8 and 9, delight with the mission and anticipation of the results of the encounter in March were already evident. Garry E. Hunt, from the Laboratory for Planetary Atmospheres, University College, London, a member of the Imaging Team, discussing the appearance of Jupiter’s atmosphere as seen by Voyager during the previous month, said “It seems to be far more photogenic now than it did during the Pioneer encounters; I’m more than delighted by it—it’s an incredible state of affairs. There are infinitely more details than ever imagined.”

The pictures from Voyager are “clearly spectacular,” said Lonne Lane, Assistant Project Scientist for Jupiter. “We’re getting even better results than we had anticipated. We have seen new phenomena in both optical and radio emissions. We have definitely seen things that are different—in at least in one case, unanticipated—and are begging for answers we haven’t got.” There was already, still almost a month from encounter, a strong feeling of accomplishment among the scientists and engineers; they had done a difficult task and it has been successful.

By the last week of February 1979, the attention of thousands of individuals was focused on the activities at JPL. Scientists had arrived from universities and laboratories around the country and from abroad, many bringing graduate students or faculty colleagues to assist them. Engineers and technicians from JPL contractors joined NASA officials as the Pasadena motels filled up. Special badges were issued and reserved parking areas set aside for the Voyager influx. Twenty-four hours a day, lights burned in the flight control rooms, the science offices, the computer areas, and the photo processing labs. In order to protect those with critical tasks to perform from the friendly distraction of the new arrivals, special computer-controlled locks were placed on doors, and security officers began to patrol the halls. By the end of the month the press had begun to arrive. Amid accelerating excitement, the Voyager 1 encounter was about to begin.

CHAPTER 6
THE FIRST ENCOUNTER

The Giant Is Full of Surprises

The Voyager 1 encounter took place at 4:42 a.m. PST, March 5, 1979. About six hours before, while the spacecraft continued to hurtle on toward Jupiter, overflow crowds had poured out of Beckman Auditorium on the campus of the California Institute of Technology. There had been a symposium entitled Jupiter and the Mind of Man. More than one face that evening had turned to look toward the Pasadena night, for there, glittering against the fabric of the sky, was the “star” of the show—a planet so huge that the Earth would be but a blemish on its Great Red Spot. One might have tried to imagine Voyager 1, large by spacecraft standards, rapidly closing in on Jupiter—a pesky, investigative “mosquito” buzzing around the Jovian system.

Meanwhile, at JPL, pictures taken by Voyager 1 were flashing back every 48 seconds, pictures that were already revealing more and more of Jupiter’s atmosphere and would soon disclose four new worlds—the Galilean satellites. The encounter was almost at hand.

4:42 a.m.—Voyager 1 had made its closest approach to Jupiter 37 minutes earlier. Only now were the signals that had been sent out from the spacecraft at 4:05 a.m. reaching the Earth.

But the moment of closest encounter did not have the same impact a landing would have had. Although the champagne would flow later for those who had worked so long and hard on this successful mission, there was none now. In the press room there were just four coffeepots working overtime to help keep the press alert. In the science areas, focus had shifted to the satellite encounters, which would stretch over the next 24 hours; meanwhile, a few persons tried to catch a little sleep at their desks before the first closeups of Io came in. The instant of encounter came ... and went ... with no screams, no New Year’s noisemakers. All the excitement of the mission lay both behind ... and ahead.

Thursday, February 22.

The encounter activity was kicked off in Washington, D.C., by a press conference at NASA Headquarters. After introductory statements by NASA and JPL officials, some of the scientific results from the observatory and far encounter phases were presented.

Both the plasma wave and the planetary radio astronomy instruments had detected very-low-frequency radio emission that generally increased in intensity whenever either the north or south magnetic pole of Jupiter tipped toward the spacecraft. The plasma wave experiment had also detected radiation that seemed to come from a region either near or beyond the orbit of Io—perhaps even as far as the outer magnetosphere. In addition, Frederick L. Scarf, Principal Investigator for the plasma wave instrument, released a recording of the ion sound waves created by 5 to 10 kilovolt energy protons traveling upstream from Jupiter.

James Warwick, Principal Investigator of the radio astronomy investigation, enthusiastically reported the detection of a striking new low-frequency radiation from Jupiter, at wavelengths of tens of kilometers. Such radio waves cannot penetrate the ionosphere of the Earth, and thus had never been detected before. Because of their huge scale, these bursts probably did not originate at Jupiter itself, but in magnetospheric regions above the planet—perhaps in association with the high-density plasma torus associated with Io. Warwick commented that “only from the magnificent perspective of the Voyager as it approaches Jupiter have we been able to get this complete picture.”

Voyager 1 trajectory

Launch date = 9/5/77

Jupiter arrival date = 3/5/79

Satellite closest approach

Sun occultation

Earth occultation

Io

Ganymede

Periapsis

Callisto

Amalthea

Europa

Another unexpected result was announced by Lyle Broadfoot, Principal Investigator for the ultraviolet spectrometer investigation. The scientists had expected to find very weak ultraviolet emissions on the sunlit side of Jupiter, caused by sunlight being scattered from hydrogen and helium in Jupiter’s upper atmosphere. “Instead we are seeing a spectacular auroral display. There are two features of the emission—the auroral emission which comes from the planet and a second type of emission which appears to come from a radiating torus or shell around the planet at the orbit of Io. The spectral content of these two radiating sources is distinctly different. What we find is that the auroral emission from Jupiter’s atmosphere is so strong that it completely dominates the emission spectrum even on the sunlit side of the atmosphere.”

“Not since Mariner 4 carried its TV camera to Mars fifteen years ago have we been less prepared—have we been less certain of what we are about to see over the next two weeks,” said Bradford Smith, Imaging Team Leader. He mentioned the time-lapse “rotation movie,” in which the colorful planet spun through ten full Jupiter days; tiny images of the satellites passed across Jupiter’s face as though being whipped along by the rotation of the giant. A week or so earlier, when this film had been shown for the first time to the full Imaging Team, it provided an occasion for good-humored rivalry between planet people and satellite people, with jokes about the satellites getting in the way of the important studies of Jupiter. For the next few days, the imaging focus remained on Jupiter; it shifted to Io, Ganymede, and Callisto, as each was passed in turn after closest approach to the planet.

Tuesday, February 27.

(Range to Jupiter, 7.1 million kilometers). At a distance of 660 million kilometers from Earth, within 90 Jovian radii (R_J) of Jupiter, Voyager 1 was prepared to begin the encounter with the planet’s magnetosphere. On the previous day the spacecraft had crossed the point, at 100 R_J, at which Pioneers 10 and 11 had found the bow shock, the first indication of the magnetospheric boundary. The start of Voyager’s plunge into the Jovian magnetosphere was overdue, and scientists anxiously watched the data from the particles and fields instruments, looking for the first indication of disordered magnetic fields and altered particle densities that would mark the bow shock. Apparently, higher solar wind pressure, associated with increased solar activity since 1974, had compressed the magnetosphere, but no one could predict how strong this compression might be.

For the first time since its discovery, Lyle Broadfoot and his UVS colleagues suggested a probable identification for the unexpected ultraviolet emission near the orbit of Io. The most likely candidate was sulfur atoms with two electrons removed (S III), at an inferred temperature perhaps as high as 200 000 K. An additional indication of sulfur came from Mike Krimigis, who reported that the low-energy charged particles instrument had detected bursts of sulfur ions streaming away from Jupiter that had apparently escaped from the inner magnetosphere. No explanations were offered, however, for the presence of large amounts of this element.

At JPL, a press room had been opened in Von Karman Auditorium to accommodate the hundred or so reporters expected to arrive. To keep all the interested people informed of Voyager progress, frequent television reports were beamed over closed-circuit TV throughout JPL. From an in-lab television studio called the Blue Room, JPL scientist Al Hibbs, who had played a similar role during the Viking Mission to Mars, provided hourly reports and interviewed members of the Voyager teams. As the pressure for constant commentary and instant analysis increased, Garry Hunt of the Imaging Team was also called on to host activities in the Blue Room, where his British accent added an additional touch of class to the operation.

As the encounter progressed, the JPL television reports reached a wider audience. In the Los Angeles area, KCET Public Television began a nightly “Jupiter Watch” program, with Dr. Hibbs as host. During the encounter days, service was extended to interested public television stations throughout the nation. In this way, tens of thousands of persons were able to experience the thrill of discovery, seeing the closeup pictures of Jupiter and its satellites at the same moment as the scientists at JPL saw them, and listening to the excited and frequently awestruck commentary as the first tentative interpretations were attempted. Unfortunately, the commercial television networks did not make use of this opportunity, and the greatest coverage available to most of the country was a 90-second commentary on the nightly network news.

Wednesday, February 28.

(Range to Jupiter, 5.9 million kilometers). At 6:33 a.m., at a range of 86 R_J, Voyager 1 finally reached Jupiter’s bow shock. But by 12:28 p.m. the solar wind had pushed the magnetosphere back toward Jupiter, and Voyager was once more outside, back in the solar wind. Not until March 2, at a distance of less than 45 R_J, would the spacecraft enter the magnetosphere for the final time.

At 11 a.m. the first daily briefing to the press was given. “After nearly two months of atmospheric imaging and perhaps a week or two of satellite viewing, [we’re] happily bewildered,” said Brad Smith. The Jovian atmosphere is “where our greatest state of confusion seems to exist right at the moment, although over the next several days we may find that some of our smirking geology friends will find themselves in a similar state. I think, for the most part, we have to say that the existing atmospheric circulation models have all been shot to hell by Voyager. Although these models can still explain some of the coarse zonal flow patterns, they fail entirely in explaining the detailed behavior that Voyager is now revealing.” It was thought, from Pioneer results, that Jupiter’s atmosphere showed primarily horizontal or zonal flow near the equatorial region, but that the zonal flow pattern broke down at high latitudes. But Voyager found that “zonal flow exists poleward as far as we can see.”

Smith also showed a time-lapse movie of Jupiter assembled from images obtained during the month of January. Once each rotation, approximately every ten hours, a color picture had been taken. Viewed consecutively, these frames displayed the complex cloud motions on a single hemisphere of Jupiter, as they would be seen from a fixed point above the equator of the planet. The film revealed that clouds move around the Great Red Spot in a period of about six days, at speeds of perhaps 100 meters per second. The Great Red Spot, as well as many of the smaller spots that dot the planet, appeared to be rotating anticyclonically. Anticyclonic motion is characteristic of high-pressure regions, unlike terrestrial storms. Smith noted that “Jupiter is far more complex in its atmospheric motions than we had ever imagined. We are seeing a much more complicated flow of cyclonic and anticyclonic vorticity, circulation. We see currents which flow along and seem to interact with an obstacle and turn around and flow back.” There is a Jovian jet stream that is “moving along at well over 100 meters per second. Several of these curious little dark features that appear to be small brown spots near Jupiter’s north temperate region have been seen to overtake one another and gobble each other up. And then they occasionally spit out a piece here and there as they move along.”

Thursday, March 1.

(Range to Jupiter, 4.8 million kilometers). At 5 a.m., at a distance of 71 R_J, Voyager crossed the bow shock for the third time, catching up with the contracting magnetosphere of the planet. About noon, at 66 R_J, the spacecraft finally reached the boundary of the magnetosphere, called the magnetopause. Herbert Bridge, the plasma instrument Principal Investigator, noted that the solar wind pressure as monitored by Voyager 2, still between the Sun and Jupiter, had been for several days from two to five times greater than its level during the Pioneer 10 and 11 encounters. Presumably, this high pressure was the cause of the compressed state of the magnetosphere. However, in the previous few hours the solar pressure had dropped, so Bridge anticipated that the Jovian magnetosphere might soon inflate and expand outward.