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]
