Validation of the Phobos orbit with HRSC and SRC on Mars Express

Abstract

There is renewed interest in studies of the orbit evolution of the small Martian satellite since the discovery of discrepancies between orbit models and observed Phobos positions in Mars Express (MEX) images [Oberst et al., 2006]. To obtain more constrains on the current orbit of Phobos we evaluated shadow observations in High Resolution Stereo Camera (HRSC) and Mars Orbiter Camera (MOC) image data. By registering images with the aid of adjacent surface features to existing maps, the shadow observations do not suffer from spacecraft position and camera pointing uncertainties. Using ellipse fits, we estimate the accuracies for Phobos’ shadow position measurement to be ±0.81 km and ±0.4 km in east-west direction and north-south direction, respectively, for the HRSC images as well as ±5.9 km and ±1.6 km for MOC images. Furthermore, Super Resolution Channel images (SRC) of 34 Phobos flybys were used to directly determine the position of Phobos center of figure with respect to the stellar background by identification of known control points [Duxbury and Callahan, 1989]. Camera pointing uncertainties are minimized with the aid of especially obtained background star observations during each flyby. The Phobos positions are found to agree with the new orbit model of [Lainey et al., 2005] to within 1.5 km.