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Figure 7. Galileo trajectory during the passage of the Jovian gossamer rings in November 2002. The position of Amalthea during closest approach is shown as a filled circle.

will fly by several of Saturn's satellites during its orbital tour about the giant planet. It will provide a unique opportunity to study the dust environments of many of the small Saturnian satellites.

Although considered to be the archetype of an ethereal dusty planetary ring, the Jovian gossamer and main ring system has been relatively incompletely studied to date. The in-situ measurements of ejecta grains escaping from the circum-satellite dust clouds and images of Jupiter's main and gossamer rings have demonstrated that impact-ejecta derived from hypervelocity impacts onto satellites is the major - if not the only - constituent of these dusty planetary rings. The details of the complex dynamics of grains over a large size range and under the various forces acting on the grains are as yet only poorly understood.

In November 2002 - during its final orbit about Jupiter - Galileo will traverse the gossamer ring system and fly by Amalthea (Figure 7). Detailed in-situ studies of the dust grains in the gossamer rings will provide a better understanding of the forces dominating the grain dynamics in the rings (gravity, Lorentz force, plasma drag, Poynting-Robertson drag, radiation pressure). The relative importance of each force varies strongly with grain size and distance from the planet and leads to drastically different size distributions at different locations along the gossamer rings and in the main ring. Investigation of why the Poynting-Robertson drag dominates over the other forces will lead to a comprehensive picture of the grain dynamics in the gossamer ring, a necessary step in deriving a full picture of the dust dynamics throughout the Jovian magnetosphere. In-situ studies of the ring material can provide valuable information about the surface properties of the source moons. Comparative studies of ejecta from the large Galilean moons and the smaller ones embedded in the gossamer rings will provide information about the ejection process over a large range in speed not accessible in the laboratory. Especially the close fly-by at Amalthea (< 300 km) will allow to test on a small moon the models for the impact-ejection process which have been developed for the much larger Galilean satellites.

In-situ dust measurements provide information about the physical properties of the dust environment not accessible with imaging techniques. Since all dusty planetary rings in our Solar System are most likely dominated by impact-ejecta, studies of Jupiter's gossamer ring provide valuable information not only about the mechanism feeding this ring system but also about the processes that govern planetary rings in general. Studies of the Jovian ring with Galileo and of the Saturnian ring with Cassini will lead to a vastly improved understanding of the formation and evolution of dusty planetary rings.

Acknowledgements. We wish to thank the Ulysses and Galileo projects at ESA and NASA/JPL for effective and successful mission operations. We also wish to express our gratefulness to our co-investigators. Without their continuous efforts over the years the results presented here would not have been achievable. We are also grateful to John R. Spencer and John Stansberry who provided thermal near-infrared data of Io. This research has been supported by the German Bundesministerium für Bildung und Forschung through Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR, grant 50 QJ 9503 3). Support by Deutsche Forschungsgemeinschaft (DFG) and Max-Planck-Institut für Kernphysik (MPIK) are also gratefully acknowledged.

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