Approaching Jupiter
At the beginning of July, the dry summer heat had returned to Pasadena, and so had the press. The scientists had come days or weeks earlier to look at data being transmitted from the second Voyager as it approached Jupiter and its satellites. The mood at JPL seemed quieter than it had been in March for Voyager 1, although the press room would once again be deluged with observers on the day of encounter. This would be our second good, close look at the Jovian system, but it was to be no summer rerun. Voyager 2 would have a different view of each world, and, in addition, both Io and Jupiter had undergone changes, as though to ensure that no one would become bored and fall asleep in front of a TV monitor. In a sense, this encounter was to be another first look at Jupiter and its satellites, with a view of each object that was quite different from what had been seen before.
Changes in Jupiter’s cloud formations became noticeable long before July. After a gap of six weeks following the first flyby, Voyager 2’s observatory phase began on April 24, 1979, seventy-six days before its July 9 encounter with Jupiter. During this time, the spacecraft’s ultraviolet and fields and particles instruments studied the Jovian system and its interaction with the solar wind. Between April 24 and May 27, Voyager 2’s imaging system concentrated on the motions in Jupiter’s atmosphere, creating another approach time-lapse “movie.” From May 27 to 29 photographs were taken in a more rapid sequence, showing the planet during five 10-hour rotations. From these studies it was apparent that Garry Hunt’s prediction had been right—the weather had changed by July. A month before the encounter JPL’s Voyager Bulletin—Mission Status Report announced that “Jupiter is sporting quite a different face than it did just four months ago. The bright ‘tongue’ extending upward from the Red Spot is interacting with a thin, bright cloud above it that has traveled twice around Jupiter in four months.” The turbulent region west of the Great Red Spot had begun to break up and separate from the Red Spot. The white ovals south of the Red Spot had drifted to the east (about 0.35 degrees a day), while the Red Spot itself had drifted west (about 0.26 degrees a day). The white zone seen just south of the Red Spot by Voyager 1 had become very narrow—like a thin white line just barely outlining the bottom of the spot. The Red Spot had also changed: It had become a more uniform orange-red, perhaps reverting to the color seen by Pioneers 10 and 11. The brown spots that had been seen in the north temperate region at the same longitude as the Red Spot were now on the other side of the planet. A dark brown spot not present during the Voyager 1 flyby had developed along the northern edge of the brown equatorial region on the Red Spot side of the planet. Some of the white markings that seemed to have protruded into the equatorial region at the time of the first flyby were missing in the Voyager 2 photographs.
As Voyager 2 entered the far encounter period on May 29, all instruments on the spacecraft (except for the photopolarimeter) seemed to be in good shape for encounter. As was the case with Voyager 1, the polarization wheel on Voyager 2’s photopolarimeter was stuck, so the instrument was able to obtain only color photometry measurements.
In early June, as Voyager 2 carried out its observatory phase, additional changes in Jupiter’s face began to be apparent. These two images, taken from a distance of 24 million kilometers, have a resolution of about 500 kilometers.
The Great Red Spot and the white oval south of it are seen to be followed on the west by regions of chaotic and turbulent clouds. This is not the same white oval that was near the Red Spot in March; the differential rotation of the planet carried a different oval close to the Red Spot during the intervening three months. [P-21713C]
Io is visible to the right of the planet, and the shadow of Ganymede falls on the colored clouds of Jupiter’s equatorial belt. [P-21714C]
The Voyager 2 trajectory was complementary to that of Voyager 1. This time, the satellites were encountered before Jupiter, revealing their other hemispheres. As shown in this drawing, the spacecraft flew by first Callisto, then Ganymede, then Europa. The ten-hour Io volcano watch took place immediately after closest approach to Jupiter. [260-533A]
Voyager 2 trajectory View normal to Jupiter equator Sun occultation Earth occultation Launch date = 8/20/77 Jupiter arrival date = 7/9/79 Periapsis Satellite closest approach Amalthea Europa Io Ganymede Callisto
These two faces of Jupiter were photographed by Voyager 2 on May 9 at a distance of 46 million kilometers from the planet. Voyager scientists began to detect significant changes in the cloud patterns since the Voyager 1 encounter two months earlier. [260-507]
The weather is changing over one of the northern hemisphere brown ovals in this picture taken July 6. The brown ovals are regions in which breaks in the upper layer of ammonia clouds reveal darker clouds below. A high, white cloud is seen moving over the darker cloud, providing an indication of the structure of the cloud layers. Thin white clouds are also seen within the dark cloud. At right, blue areas, free of high clouds, are seen. [P-21753C]
Although Voyager 2’s radio receiver still could not track a Doppler-shifted radio signal from Earth (the problem is that it “hears a monotone,” explained Deputy Project Manager Esker K. Davis), the Deep Space Network engineers had learned to work with the spacecraft, determining what frequency the spacecraft would listen to at any particular time. They had discovered that some of the “housekeeping” telemetry signals from the receiver were sensitive to the match between the incoming frequency and the receiver frequency. By monitoring these signals, they could detect a frequency drift in time to correct the transmission, thus keeping the system in tune in spite of slow changes in the receiver. The system was slow and demanding but effective; all the necessary command sequences were successfully loaded into the computer, and communications during the encounter were entirely successful.
The timing offset experienced by Voyager 1 as a result of Jupiter’s intense radiation environment was not expected to be a problem on Voyager 2 for two reasons: Even at closest approach, Voyager 2 would still be more than twice as far from Jupiter as Voyager 1 had been, and the Voyager 2 computer was programmed to resynchronize the spacecraft’s timing systems automatically every hour. In this way, even if the radiation environment proved to be much higher than anticipated, the image smear that might occur from a timing offset would be prevented.
Complex activity in the southern hemisphere of Jupiter continued during the Voyager 2 encounter, although changes had occurred in the region of the Great Red Spot. A white oval, different from the one observed in a similar position at the time of the Voyager 1 encounter, was situated south of the Red Spot. The region of white clouds extended from east of the Red Spot and around its northern boundary, preventing small cloud vortices from circling the feature. The disturbed region west of the Red Spot had also changed since the equivalent Voyager 1 image. The picture was taken on July 3 from a distance of 6 million kilometers. [P-21742C]
As a result of the discoveries made by Voyager 1, the project scientists decided to modify some of Voyager 2’s preplanned sequences. As early as April 1, the painstaking job of constructing new computer commands began. A ten-hour Io Volcano Watch was added to the spacecraft’s program, taking advantage of the fact that shortly after closest approach to Jupiter, the spacecraft would remain within about 1 million kilometers of Io for a long period, keeping nearly the same face in view. Provisions were also made to take extensive ultraviolet measurements of the emission from the glowing torus surrounding Jupiter near the orbit of Io. Further studies would be made of the dark side of Jupiter to search for lightning and auroral activity, and there was also the hope that the plasma wave instrument would be able to detect lightning whistlers (radio signals created by lightning bolts) as the Voyager 1 instrument had done. A high priority was given to observations of the newly discovered ring, which had not been in the original Voyager 2 sequence at all. The spacecraft would cross the ring plane twice, photographing the ring during both the inbound and the outbound passages.
As was originally planned, Voyager 2 would make its closest approaches to Callisto, Ganymede, Europa, and Amalthea before encounter with Jupiter. Because of the difference in the trajectories of the two spacecraft, Voyager 2 would see the faces of Callisto and Ganymede not seen by Voyager 1. The most important difference, however, was that the second Voyager would fly much closer to Europa than Voyager 1 did, giving scientists their first good look at the mysterious streaks scratched on the surface of that bright golden world. Voyager 2 would also have a closer flyby of Ganymede, giving us a second chance to examine its strange “snowmobile tracks.” The major loss, of course, was Io, which would be seen from Voyager 2 only at distances of a million kilometers or more.
Io appeared in front of Jupiter as seen by Voyager on June 25, at a range of 12 million kilometers. At a resolution of about 200 kilometers, the bright and dark spots on the satellite are just beginning to be resolved, but it was not possible to determine if any eruptions were still in progress. [P-21719C]