Conclusions

The sensitivity of both CDA targets was determined from calibration data. On the basis of this the measured impact rate was compared with existing models for the interplanetary and interstellar dust flux. Since Cassini is not a spinning spacecraft, the measured event rate cannot be compared with an isotropic model. The only interplanetary dust flux model which was constrained by directional information of impacts is the model by Staubach [1], However, at 1AU the Staubach model underestimates the measured flux by more than one order of magnitude and the interplanetary flux cannot explain the impact rate.

Our attempt to classify the measured events into impacts on the large and small target, indicated that impacts elsewhere in the detector (e.g. the side walls) can lead to measureable signals on the targets. A preliminary criterion classified 6 out of 27 impacts not to be due to a primary impact on one of the targets. The classification as it is described here, is not able to uniquely identify whether the primary impact hit one of the targets. To do this calibration data with shots on the side-walls are required.

The impacts which did not hit one of the targets bring the measured flux closer to the flux predicted by the Staubach model. However, they cannot explain the large discrepancy between model and measurements. Therefore the comparison of the model with the measurements suggests that interstellar particle impacts were recorded. It is difficult to judge how the presented model calculations are influenced by dust impacts measured onboard Galileo and Ulysses. Therefore more direct comparison of measured dust fluxes at 1 AU, but at other ecliptic longitudes, could decide more quantitatively how many of the measured impacts are likely to be of interstellar origin. Ultimately, velocity estimations for individual events could prove that the orbit of the impacting particle was hyperbolic.

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