Testing general relativity with the bepicolombo radio science experiment

The ESA mission BepiColombo will explore the planet Mercury with equipment allowing an extremely accurate tracking. While determining its orbit around Mercury, it will be possible to indirectly observe the motion of its center of mass, with an accuracy several orders of magnitude better than what is possible by radar ranging to the planet's surface. This is an opportunity to conduct a relativity experiment which will be a modern version of the traditional tests of general relativity, based upon Mercury's perihelion advance and the relativistic light propagation near the Sun. We define the mathematical methods to be used to extract from the data of the BepiColombo mission, as presently designed, the best constraints on the main post-Newtonian parameters, especially beta,gamma and the Nordtvedt parameter eta, but also the dynamic oblateness of the Sun J2[sun] and the preferred frame parameters alpha1,alpha2. We have performed a full cycle simulation of the BepiColombo radio science experiments, including this relativity experiment, with the purpose of assessing in a realistic (as opposed to formal) way the accuracy achievable on each parameter of interest. For gamma the best constraint can be obtained by means of a dedicated superior conjunction experiment, with a realistic accuracy ~=2*10–6. For beta the main problem is the very strong correlation with J2[sun]; if the Nordtvedt relationship eta= 4beta–gamma–3 is used, as it is legitimate in the metric theories of gravitation, a realistic accuracy of ~=2*10–6 for beta and ~=2*10–9 for J2[sun] can be achieved, while eta itself is constrained within ~=10–5. If the preferred frame parameters alpha1,alpha2 are included in the analysis, they can be constrained within ~=8*10–6 and ~=10–6, respectively, at the price of some degradation in beta, J2[sun] and eta. It is also possible to test the change with time of the gravitational constant G, but the results are severely limited because of the problems of absolute calibration of the ranging transponder, to the point that the improvement as compared with other techniques (such as lunar laser ranging) is not so important.