Radio Science Observations of the Mars Express December 2013 Phobos Flyby and Implications for the Satellite’s Gravity Field

Abstract:

On 29^th December 2013, the European spacecraft Mars Express performed a very close flyby at the Martian moon Phobos dedicated to the radio science experiment MaRS. The flyby distance was 58 km, the closest ever. Almost 32 hours of continuous tracking data were collected by ESTRACK (35 m) and DSN (70 m) ground station antennas. 31 VLBI antennas worldwide also recorded the radio signal. The tracking data were interrupted by occultations of approximately 1 hour duration in each orbit revolution, when the spacecraft in Mars orbit disappeared behind the planet as seen from the ground station. Images were taken with the Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC) onboard Mars Express before and after the flyby in order to improve the ephemeris of Phobos.

The gravity field of Phobos was estimated from a close MEX flyby in 2010 at a distance of 77 km. The derived second degree and order gravity coefficients, however, showed large errors and could not resolve the interior structure of Phobos. Hence, the close flyby in 2013 was the opportunity to estimate the gravity field of Phobos at a higher precision because of the closer flyby distance, improved Phobos ephemeris obtained from the HRSC/SRC camera, and longer observation times with the ground station antennas.

We aim at measurements of the gravity coefficients C₂₀ and C₂₂, which are linked with the main moments of inertia of the body. By comparison with the Phobos shape model and assuming a homogeneous mass distribution these can help in interpretations of the internal structure of Phobos. The main contribution to the error budget of the gravity field is caused by the uncertainty of the Phobos ephemeris, which potentially can be improved by HRSC/SRC observations.