Validation of the Phobos Orbit and Control Point Network with HRSC and SRC on Mars Express

Abstract

Early astrometric measurements by the Super Resolution Channel (SRC) on Mars Express (MEX) , Mars Orbiter Laser Altimeter (MOLA) ranging data , and solar eclipse observations , showed large discrepancies between predictions of the Phobos position and observations of its position. As a consequence, new orbit models were developed and released by the Jet Propulsion Laboratory (JPL) and the Royal Observatory of Belgium (ROB). Since 2006 many more Phobos flybys were carried out by Mars Express, and more images were obtained by the SRC. We made new astrometric measurements in the new image data to validate the released orbit models. We devised new analysis methods, making use of a known control point network and background star observations to control the pointing of the camera. In addition we analyzed Phobos' shadow observations on the Martian surface obtained by the High Resolution Stereo Camera (HRSC) on MEX as well as by the Mars Observer Camera (MOC) on Mars Global Surveyor (MGS). Shadow observations are thought to be independent from spacecraft position and camera pointing. While good agreement of current orbit models with our flyby observations was achieved, the analysis of the shadow observations on the other hand showed large offsets between models and predictions.