Abstract Despite many estimations found in the past, Phobos mass is still largely unknown. Strong correlations in the reduction techniques and some obvious systematic errors in the former results do not allow strong certitude on the Phobos internal structure (porosity, internal stress, etc.). We plan to use the opportunity of the close encounters between Mars Express and Phobos to deliver the most realistic value of Phobos mass. To achieve this goal, new high accurate ephemeris of Phobos is under development. In addition the observational data will be treated by two softwares exclusively specialized in respectively Mars Express orbital motion determination and the fit of Phobos mass. Mars Express and Phobos mass: a challenge for celestial mechanics V.Lainey(1), T.Andert(2), P.Rosenblatt(1), M.Patzold(2), V.Dehant(1), J.P.Barriot(3) (1)Royal Observatory of Belgium, 3 ringlaan B-1180 Brussels, Belgium (2)Institut für Geophysik und Meteorologie, Zülpicherstr. 49 a, Köln, Germany (3)Observatoire Midi Pyrénéees/CNES/CNRS, 14 Av. E. Belin, F-31400, Toulouse, France Motivation: The first determinations of Phobos mass have been obtained using Viking-1 spacecraft flybys respectively by Tolson et al. and Christensen et al. in the year Since that day, many efforts have been made to improve these determinations. Several combinations of each flyby have been used, involving mainly Viking 1, and Phobos 2 missions. Although many studies have converged on an unexpected low value of Phobos mass, no real agreement has been found yet. A better determination of Phobos mass (and so its density) would be of great importance regarding the still puzzling origin of the Martian satellites. HRSC/Mars Express Close encounters: Viking1 (1977) Phobos2 (1990) Mars Express (2004) None of the three published values can fit each other with a difference of more than 40% between the two latter  Systematic errors! Determinations of the density: Authors GM (10 -3 km 3 s -2 ) density (g/cm 3 ) Williams et al / /-0.2 Kolyuka et al / Smith et al / /-0.10 Method: To fit Phobos mass from Mars Express flybys (radio tracking data), we will use two softwares. The first one called GINS (Géodésie par Intégrations Numériques Simultanées) is primarily developed by CNES. It permits to determine an accurate orbital motion of MEX through a least squares fit on the Doppler and ranging tracking acquired by the MEX radio-science experiment (MaRS). In such a process the adjustment of the initial state vector (position and velocity) and several parameters related to the force budget models and to various biaises on tracking measurements allows to achieve an accurate determination of MEX orbital dynamics and thus position and velocity around the flyby time span. This accurate orbit will be used before and after the flyby over a time span of 30 minutes. Hence two initial conditions set (one just before and one just after the flyby) will be obtained. The second software called NOE (Numerical Orbit Elaboration) and developed at ROB will be used for fitting Phobos mass during the flyby time span. References: Kolyuka Yu.F., Efimov A.E., Kudryavstev S.M., Margorin O.K., Tarasov V.P. and Tikhonov V.F., Sov. Astron. Lett. 16, , Neumann G.A., Bills B.G., Smith D.E., Zuber M.T., 35th Lunar and Planetary Science Conference, Smith D.E., Lemoine G., Zuber M.T. Geophys. Res., 22, , Williams B.G., Duxbury T.C. and Hildebrand C.E., Lunar Planet. Sci Conf., XIX, 1274, YuanD., Sjogren W.L., Konopliv A.S. and Kucinskas, A.B., Geophys. Res., 106, p , A preliminary comparison between GINS and NOE has given residuals on the s/c positions of the order of the centimeter at the most. This work is supported by the MAGE network (Mars Geophysical European network). Observations database of the Martian satellites (N.Emelianov) Phobos ephemeris: work in progress The first complete numerical model (planet and satellites triaxiality, …) + + F.Colas, pic du midi Introduction of a rigorous tides formulation A lot of new observations! Mass, density? As the mass and the two Martian moon Phobos and Deimos are very small, mutual perturbations are not strong enough to be determined by mutual perturbation of one moon to the other. Hence, close encounters between a spacecraft and a moon is the only way to get an accurate