The Second Encounter With Jupiters Ring

The exciting discovery of a Jupiter ring, combined with a number of other important scientific discoveries, led to a strong desire to redo the previously designed Voyager 2 encounter sequences. The project had four months to understand the first Jupiter encounter results, figure out what observations should be added to answer questions raised by that encounter; and then make the painful decisions about what observations to delete to make room for the new ones. As an indication of how low the expectations were of finding a Jupiter ring, Voyager team members had planned no ring observations for the second Jupiter encounter [13]. The large quantity of high-priority science that could only be captured by Voyager 2, even if Voyager 1 collected all of its planned encounter observations, had led investigators to forgo future ring observations.

The Voyager 2 encounter with Jupiter began on April 24,1979, approximately 76 days before closest approach [14]. The dimness of the rings in the Voyager 1 image led scientists to speculate that the ring particles were very small, there was therefore a desire to image the rings from beyond Jupiter, where expected forward-scattering would make the rings brighter. Ring observations would have to wait until after the July 9, 1979, closest approach when the spacecraft was behind the planet.

On July 10, 1979, the Voyager 2 cameras were commanded to look back towards the Sun (which was hidden from the spacecraft by Jupiter) and search for aurorae and lightning. Nestled into these high-phase-angle observations were imaging frames designed to detect scattered light from the newly discovered ring. Data transmitted back to Earth showed that these high-phase-angle ring observations were a spectacular success. A series of narrow-angle frames revealed a faint ring structure that was all but absent from the Voyager 1 images. Figure 4.3 shows a narrow-angle frame that captures one edge of the Jupiter ring system.

Figure 4.3. This Voyager 2 image of one of Jupiter's ring ansae was shuttered when the spacecraft was 1.5 million km away. The ring has a sharp outer edge with a much more diffuse inner edge. (PIA00377)

From this and other narrow-angle frames, imaging scientists were able to determine that the Jupiter ring was composed of two very tenuous rings. The inner ring extended from the upper atmosphere to a distance of 53,000 km above the cloud tops [15]. The outer ring, which is the more pronounced portion of the ring system, extended further by another 5,000 km. The thickness of the outer ring was not determined but was probably less than 30 km and more likely less than 1 km [16]. There were also some indications that the ring particles extended out of the equatorial plane [17].

Subsequent analysis of Voyager 2 data revealed a third ring residing outside the two already discovered. This additional ring was extremely faint and extended to approximately 210,000 km [18]. Barely observable, this "gossamer" ring would require more data from a more sensitive instrument to ascertain its true nature.

The two Voyager Jupiter encounters were a dramatic success. They greatly increased the understanding of the planet's atmosphere, magnetosphere, satellites, and rings. However, every question that was answered seemed to give rise to many more questions. Of the new questions raised, many were about the rings themselves. These questions included, "What was the source of the ring material?''; "Why were there multiple rings?''; and "Why were some ring particles out of the equatorial plane?''. Another mission to Jupiter was needed to attempt to answer these questions, a mission that would spend years, not months, making detailed observations of all aspects of the system, including its rings.

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