Gravity field determination of a comet nucleus: Rosetta at P/Wirtanen

One of the prime objectives of the Rosetta Radio Science Investigations (RS I) experiment is the determination of the mass, the bulk density and the lo w degree and order gravity of the nucleus of comet P/Wirtanen, the target o bject of the international Rosetta mission. The RSI experiment will use the spacecraft's radio carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) in order to measure slight changes of the orbit velocity via the clas sical Doppler effect induced by the gravity attraction of the comet nucleus . Based on an estimate of the background Doppler noise, it is expected that a mass determination (assuming a representative radius of 700 m and a bulk density of 500 kg/m(3)) at an accuracy of 0.1% can be achieved if the spac ecraft's orbit is iteratively reduced below 7 km altitude. The gravity fiel d of degree and order two can be detected for reasonable tracking times bel ow 5 km altitude. The major competing forces acting on the spacecraft are t he radiation pressure and the gas mass flux from cometary activity. While t he radiation pressure may be predicted, it is recommended to begin a gravit y mapping campaign well before the onset of outgassing activity (>3.25 AU h eliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order g ravity field and will mask the contributions from the gravity field.