Gravity field and rotation state of Mercury from the BepiColombo Radio Science Experiments

The ESA mission BepiColombo will include a Mercury Planetary Orbiter equipp ed with a full complement of instruments to perform Radio Science Experimen ts. Very precise range and range-rate tracking from Earth, on-board acceler ometry, altimetry and accurate angular measurements with optical instrument s will provide large data sets. From these it will be possible to study (1) the global gravity field of Mercury and its temporal variations due to tid es, (2) the medium to short scale (down do 300 similar or equal to 400 km) gravity anomalies, (3) the rotation state of the planet, in particular the obliquity and the libration with respect to the 3/2 spin orbit resonance an d (4) the orbit of the center of mass of the planet. With the global gravity field and the rotation state it is possible to tigh tly constrain the internal structure of the planet, in particular to determ ine whether the solid surface of the planet is decoupled from the inner cor e by some liquid layer, as postulated by dynamo theories of Mercury's magne tic field. With the gravity anomalies and altimetry it is possible to study the geophysics of the planet's crust, mantle and impact basins. With the o rbit of the planet closest to the Sun it is possible to constrain relativis tic theories of gravitation. The possibility of achieving these scientific goals has been tested with a full cycle numerical simulation of the Radio Science Experiments. It includ es the generation of simulated tracking and accelerometer data, and the det ermination, by least squares fit, of a long list of variables including the initial conditions for each observed arc, calibration parameters, gravity field harmonic coefficients, and corrections to the orbit of Mercury. An er ror budget has been deduced both from the formal covariance matrices and fr om the actual difference between the nominal values used in the data simula tion and the solution. Thus the most complete error budget contains the eff ect of systematic measurement errors and is by far more reliable than a for mal one. For the rotation experiment an error budget has been computed on t he basis of dedicated studies on each separate error source. The results of the full cycle simulation are positive, that is the experime nts are feasible at the required level of accuracy. However, the extraction of the full accuracy results from the data will be by no means trivial, an d there are a number of open problems, both in the data processing (e.g., t he selection of the orbital arc length) and in the mission scheduling (e.g. , the selection of the target areas for the rotation experiment). (C) 2001 Elsevier Science Ltd. All rights reserved.