4 Summary

The procedure of mass and bulk density determination of asteroids during spacecraft flybys was demonstrated and applied to the simulation of the Rosetta flyby at asteroid 140 Siwa on 24th July 2008. It is expected that the RSI radio science experiment on board Rosetta is able to determine the mass and bulk density of Siwa to an accuracy of 1% and 20%, respectively, for the nominal flyby distance of 3500 km and flyby velocity of 17 km s^-1

Attempts to determine the mass of the other Rosetta asteroid, 4979 Otawara, are unfortunately hopeless. Due to its small size (about 4 km) and the large flyby distance (1595 km), the expected Doppler frequency is far below the noise level.

Acknowledgements

We would like to thank E. Gill and O. Montenbruck for their contributions to the experiment over the first four years and Marcello Fulchignoni for first-hand information on the asteroids Siwa and Otawara. The RSI experiment is funded by the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Bonn, Germany, and by the National Aeronautics and Space Administration (NASA), Washington, USA.