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.