The determination of the post-newtonian parameter γ during the cruise phase of BepiColombo

The post-Newtonian parameter γ, unity in general relativity, controls the delay and the deflection of photons induced by the space-time curvature produced by any mass. A precise determination of this parameter can be attained by exploiting the Shapiro time delay and the corresponding Doppler shift affecting the propagation of radio signals between a ground antenna and a spacecraft near a superior solar conjunction. In its cruise phase to Mercury (from October 2018 to December 2025), the spacecraft BepiColombo will experience eleven superior solar conjunctions, thus offering an excellent opportunity to test general relativity. The experiment will benefit from very accurate range and Doppler measurements, enabled by a multi-frequency radio link in X and Ka bands (8 and 34 GHz) and a high rate (24 Mcps) pseudo-noise ranging code. The multi-frequency link configuration allows the suppression of the plasma noise due to the solar corona. In this paper, we report the results of a comprehensive analysis based on the latest spacecraft trajectory and numerical simulations. The ultimate accuracies will be determined by operational constraints, number of exploitable conjunctions, and dynamical noise acting on the spacecraft. We find that γ can be determined up to 6.2×10^−6 with just one conjunction in July 2022, thus improving by a factor of 4 the result obtained by the Cassini mission in 2002. This accuracy can be further increased by selecting a limited number of favourable superior solar conjunctions, down to 2.5×10−6 when all of them are combined.