phobos close encounters by mars express and its internal structure

PHOBOS CLOSE ENCOUNTERS BY MARS
EXPRESS AND ITS INTERNAL STRUCTURE
V. Lainey (1,2), P. Rosenblatt (1), V. Dehant (1), J.P. Barriot (3), M. Pätzold (4), T.
Andert (4)
(1) Royal Observatory of Belgium, (2) Institut de Mécanique Céleste et de Calcul des
Ephémérides, (3) Observatoire Midi-Pyrénées, GRGS/CNES, (4) Institut für Geophysik und
Meteorologie, Universität zu Köln
Several close encounters closer than 400 km of Phobos by Mars Express spacecraft are
scheduled for 2005. This will give us the opportunity to study its internal structure and
particularly the assumption of a probable constant density. Using the perturbations induced on the motion of Mars Express (MEX) by the ellipsoidal figure of Phobos, one
can fit its first gravitational coefficients as J2 and C22 and compare with computed
harmonics based on the shaped and assumed constant density. We present a simulation of this method. A first software called GINS (primary developed by CNES) is
used to represent MEX motion just before the close encounter. A second software has
been developed to fit the Phobos gravitational potential, using the observed velocity
perturbations of MEX. It is noteworthy that it will be the first direct observational
derivation of Phobos’ internal structure.
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