<h2>AAS  96-150</h2><h2>PERTURBATION FORCES ACTING ON THE ROSETTA SPACECRAFT IN A CLOSE ORBIT AROUND COMET P/WIRTANEN</h2><h4>E. Gill and O. Montenbruck, GSOC/DLR; M. Patzold, University of Cologne</h4><h2> Abstract </h2>The European Rosetta spacecraft is scheduled to approach comet P/Wirtanen in early 2012, following a series of one Mars and two Earth swing-by maneuvers.  Starting at a heliocentric distance of about 4 AU, a global characterization of the comet nucleus will be performed. In preparation of the cometary gravity field determination an analysis of the orbital perturbations acting on the Rosetta spacecraft has been performed. To this extent, the gravity field coefficients of an irregularly shaped sample body have been derived and used to compute representative accelerations as a function of spacecraft distance. In the absence of non-gravitational perturbations, gravity potential coefficients up to 4th degree may be determined.Among the non-gravitational perturbations, cometary outgasing is expected to result in pronounced perturbations, even at heliocentric distances of 3 AU and up. This result is important in view of the decision, to delay critical proximity operations and the lander ejection up to the time where the Earth-s/c distance reduces to 3.25 AU, due to lacking redundancy within the European deep space network.<br><br>