The Radio Science technique enables to estimate the mass and other gravitational parameters of a solar system body from spacecraft observations very precisely. It uses the radio link between ground station and spacecraft. The frequency shift of the radio signal is proportional to the relative velocity change between spacecraft and ground station. If a spacecraft performs a close flyby at a solar system body, the velocity of the spacecraft is changed by the gravitational attraction of the body. If all other contributions on the radio signal are known, the remaining frequency change is solely due to the gravitational attraction. A least square fit can be performed on the frequency residuals to derive from it gravitational parameters. Within this thesis models were developed and merged into a software package with which it is possible to determine the orbit of a spacecraft precisely and to predict accurately the frequency to be observed at a ground station. Models for extracting the frequency shift caused by the propagation of the radio signal through the ionosphere and troposphere of the Earth were incorporated. The accuracy of the predicted frequency, i.e. the difference between measurement and predict, is in the same order as the total Doppler velocity error in X-band from the thermal noise of the ground station and the transponder phase noise. Filtering techniques were established improving the signal to noise ratio at least by a factor of three. A numerical stable least square fitting procedure was introduced to fit the frequency change due to the gravitational attraction of a body onto the measured frequency residuals. Measurements from the close flyby of the Rosetta spacecraft at the asteroid Steins were analyzed with the developed method. Due to the large flyby distance no mass estimate was possible. A feasibility study was carried out for the upcoming flyby of Rosetta in July 2010 at the asteroid Lutetia. It is possible to estimate from this flyby the mass of Lutetia with an error of 1 %. Moreover, the developed method was applied to measurements of the Radio Science Experiment onboard Mars Express MaRS from two close flybys at the Mars moon Phobos in March 2006 and July 2008. The mass of Phobos was estimated from these flybys. The solution provides the most accurate value currently available for the mass of Phobos from close flybys. Information about the interior were derived from the precise mass estimate. Phobos has a high porosity which is discussed with respect to its origin. It seems to be unlikely that Phobos is a captured asteroid as suggested from first spectral measurements. It seems to be more likely that Phobos is the remnant of the collision between a body originating from the asteroid belt and a body remaining from the formation process of Mars. Mars Express will perform another flyby in March 2010 with a closest distance of 62 km. A feasibility study was performed from which it was derived that the C20 term of the gravity field of Phobos can be estimated with an error of 1 % with the developed method
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