Mass and density determinations of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraf t will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 an d 24 July 2008, respectively. The objective of the Rosetta Radio Science in vestigations (RSI) experiment at these flybys is a determination of the ast eroid's mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closet approach. The spacecraft's flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the m ass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20%. Unfortunately, a detectable trajector y perturbation seems to be hopeless for Otawara because of its small size a nd the large nominal flyby distance.