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H. Krüger, R. Srama, S. Kempf, A, Graps (MPIK, Germany), M. Horányi (LASP, Boulder, Colorado), E. Grün (MPIK, Germany)
Streams of tiny dust grains emerging from the volcanic plumes on Jupiter's moon Io are dispersed throughout Jupiter's magnetosphere with a fraction of them swiftly ejected even into interplanetary space. Up till now, speeds and sizes of these grains could be derived only indirectly from theoretical modelling. The fly-by of the Cassini spacecraft at Jupiter -- and the Galileo spacecraft orbiting the planet -- offered the unique opportunity to simultaneously observe the escaping streams of grains and to measure their speeds in a time-of-flight experiment between the two spacecraft. Our time-of-flight measurements result in a characteristic dust particle velocity on the order of 400 ± 40 \rm km\,s-1. This speed -- based on our computer simulations -- indicates a characteristic grain radius of approximately 8 ± 2 nm. Our measurements imply a grain size distribution rising steeply towards smaller grains. The dust particle speeds are comparable with solar wind speeds and thus the Jovian dust stream particles are among the fastest objects in the solar system.