1 Introduction

The Rosetta spacecraft will be launched in January 2003 to intercept comet P/Wirtanen in 2011 and to escort the comet in an orbit about its nucleus from rendez-vous (about one year after the comet's aphelion) to its perihelion in July 2013. During its eight year long interplanetary cruise to its prime target, Rosetta will flyby at two main belt asteroids: 4979 Otawara on 11th July 2006 and 140 Siwa on 24th July 2008.

Apart from the cometary science objectives (Pätzold et al. 2000), the Radio Science Investigations (RSI) experiment onboard of Rosetta will also focus on the determination of the mass and density of the asteroids.

An asteroid's mass can be determined to a high degree of precision by analyzing spacecraft tracking data (Doppler frequency and range) received from a spacecraft flying by an asteroid. The gravitational attraction of the asteroid acting onto the spacecraft will influence the flyby trajectory by an amount that is proportional to the asteroid's mass for a given flyby distance and velocity.

So far, the masses of only seven asteroids have been determined (see Yeomans et al. 1997 for a summary): four large main belt asteroids (1 Ceres, 2 Pallas, 4 Vesta and 11 Parthenope) from orbit perturbations and another three asteroids (243 Ida, 253 Mathilde and 433 Eros) from spacecraft encounters. The flybys of NEAR at Mathilde (Yeomans et al. 1997) and Eros (Yeomans et al. 1999, 2000) have provided the only mass determinations from spacecraft tracking. The mass of 243 Ida could only be constrained following the discovery of its moon Dactyl (Belton et al. 1995) from images from the Galileo solid state imager. Radio tracking was not sensitive enough to detect the slight change in Galileo's flyby trajectory because of the large flyby distance.

Bulk densities of asteroids are important parameters to understand their internal structure, any alteration during their life span and their origin.

Once the surface of the asteroid is mapped by imaging during the flyby, a shape model and the volume can be determined. The bulk density can then be estimated from the mass and volume.