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.
