VALIDATION OF THE PHOBOS ORBIT AND CONTROL POINT NETWORK WITH HRSC AND SRC ON MARS EXPRESS
K. Willner, J. Oberst, M. Waehlisch, K.-D. Matz, T. Roatsch, J. Bendig
German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany
Discrepancies between orbit models and observed Phobos positions in Mars Express images [Oberst et al., 2006]1 have renewed the interest in studies of the orbit evolution of this small Martian satellite. Here, we have analyzed observations of Phobos’ shadow on the Martian surface to obtain more constraints on the current orbit of Phobos and to validate existing orbit models.
Unlike direct observations of Phobos during spacecraft flybys, observations of the shadow position with respect to adjacent surface features at a given time do not suffer from uncertainties in the spacecraft orbit and pointing data. We evaluated shadow observations of HRSC (High Resolution Stereo Camera) on Mars Express, as well as those by MOC (Mars Orbiting Camera) on Mars Global Surveyor applying an ellipse fit method to determine the exact position of the shadow on the ground. We estimate that the accuracies for Phobos’ shadow position are ±0.81 km and ±0.4 km in east-west direction and north-south direction, respectively, for the HRSC images. Accuracies of ±5.9 km and ±1.6 km in east-west direction and north-south direction, respectively, were achieved for the measurements in low resolution MOC images (ranging from 1.5 km to 7 km/pixel). Results of the analysis showed remaining discrepancies of ±4.4 km to ±10.4 km with Phobos being ahead or behind the predicted position, depending on the orbit model that is used for comparison. Equally large across-track offsets were observed. Good agreement with observed Phobos positions was achieved using the [Lainey et al., 2005]2 orbit model.
In the meantime, Mars Express has completed several more flybys of Phobos, with a total number of flybys reaching 48. Analyses of the new flyby images obtained by HRSC and SRC (Super Resolution Channel of the HRSC camera) are currently under way. Unlike in previous Phobos images, background star observations through SRC are now available in all new flyby sequences to correct for camera pointing errors. These new image data will further constrain the Phobos orbit.
Furthermore, we are about to begin a detailed reanalysis of the existing Phobos control point network established by [Duxbury and Callahan, 1989]3 using SRC and HRSC observations to derive an improved global DTM of Phobos. High resolution images of the Mars Global Surveyor MOC and the Viking cameras, will be used to map the irregular shape of Phobos. Preliminary results will be reported at the conference.
 J. Oberst, K. D. Matz, T. Roatsch, B. Giese, H. Hoffmann, T. Duxbury, and G. Neukum. Astrometric observations of Phobos and Deimos with the SRC on Mars Express. A&A, 447:1145-1151, March 2006.
 V. Lainey, V. Dehant V., J. Oberst , M. Pätzold. New ephemerides of the Martian moons. EOS Trans AGU, G51A-0802, 2005
 T. C. Duxbury and J. D. Callahan. PHOBOS and Deimos control networks. Icarus, 77:275-286, February 1989