Accurate Mars Express orbit determination to improve the Martian moon masses and ephemerides

Details Accurate Mars Express orbit determination to improve the Martian moon masses and ephemerides Accurate Mars Express orbit determination to improve the Martian moon masses and ephemerides

Jul 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sf2a.conf..124r&link_type=abstract

SF2A-2007: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics held in Grenoble, France, July

Astronomy and Astrophysics

Astrophysics

Scientific paper

The determination of the ephemerides of the Martian moon Phobos and Deimos benefits from images taken by the cameras onboard spacecrafts orbiting Mars. Ones of the latest images have been obtained by the Super Resolution Camera (SRC) onboard Mars Express (MEX). From these images the position of the moons in the plane-of-sky are determined and then used to derive the ephemeris of each body on the basis of the fit of a complete dynamical model of their orbiting motion around Mars. However, the plane-of-sky positions are derived from the relative position of the spacecraft when images are taken, thus one needs to know the spacecraft position as accurately as possible. In this paper, we present accurate MEX orbit determination obtained by fitting two years of tracking data of MEX performed by the Mars Express Radio Science (MaRS) experiment onboard MEX. We show the gain in precision which is obtained for Phobos and Deimos' ephemerides at epoch of SRC camera observations. In addition, the accurate MEX orbit that we have performed allows re-estimating the mass of Phobos and Deimos. The new solutions have comparable or better formal uncertainties than previous estimates performed with the Mars Global surveyor (MGS) and Mars Odyssey (ODY) spacecrafts. With the MEX extended mission (up to 2009), it would be possible to refine these estimates by fitting also the second order coefficients of the gravity field of Phobos. Such a fit could greatly benefit from close-encounters (at a distance less than 300 km) between MEX and Phobos, given tracking data are performed at such events. This study also demonstrates the advantage of combining observations from spacecrafts and from the Earth to improve the precision of natural satellites ephemerides and thus to better understand their origin and evolution through the fine analysis of their orbital motion.

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