Gravity field determination of a Comet Nucleus: Rosetta at P/Wirtanen

¹ Institut für Geophysik und Meteorologie, Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln, Germany e-mail: wennmach@geo.Uni-Koeln.DE
² Institut für Raumfahrttechnik, Universität der Bundeswehr München, 85557 Neubiberg, Germany e-mail: Bernd.Haeusler@UniBW-Muenchen.DE; werner.eidel@UniBW-Muenchen.DE
³ Institute for Theoretical Astrophysics, University of Oslo, Oslo, Norway e-mail: Kaare.Aksnes@uio.no; Oystein.Olsen@uio.no
⁴ Jet Propulsion Laboratory, Caltech, 4800 Oak Grove drive, Pasadena, CA 91107, USA e-mail: John.D.Anderson@jpl.nasa.gov; Sami.W.Asmar@jpl.nasa.gov
⁵ Observatoire Midi-Pyrénées, Toulouse, France e-mail: jean-pierre.barriot@cnes.fr
⁶ European Southern Observatory, Santiago de Chile, Chile e-mail: hboehnha@eso.org
⁷ Max-Planck-Institut für Aeronomie, 37189 Katlenburg-Lindau, Germany e-mail: thomas@linmpi.mpg.de

Corresponding author: M. Pätzold, paetzold@geo.Uni-Koeln.DE

Received: 18 May 2001
Accepted: 7 June 2001

Abstract

One of the prime objectives of the Rosetta Radio Science Investigations (RSI) experiment is the determination of the mass, the bulk density and the low degree and order gravity of the nucleus of comet P/Wirtanen, the target object 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 classical 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/m³) at an accuracy of 0.1% can be achieved if the spacecraft's orbit is iteratively reduced below 7 km altitude. The gravity field of degree and order two can be detected for reasonable tracking times below 5 km altitude. The major competing forces acting on the spacecraft are the radiation pressure and the gas mass flux from cometary activity. While the radiation pressure may be predicted, it is recommended to begin a gravity mapping campaign well before the onset of outgassing activity (>3.25 AU heliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order gravity field and will mask the contributions from the gravity field.