Space tests of the strong equivalence principle: BepiColombo and the Sun-Earth Lagrangian points opportunity

The validity of General Relativity (GR), after 100 years, is supported by solid experimental evidence. However, there is a lot of interest in pushing the limits of precision by other experiments. Here, we focus our attention on the equivalence principle (EP), in particular, the strong form. The results of ground experiments and Lunar Laser Ranging (LLR) have provided the best upper limit on the Nordtvedt parameter η that models
deviations from the strong EP. Its uncertainty is currently σ[η] = 4.4 × 10−4. In the first part of this paper, we will describe the experiment, to measure η, that will be done by the future mission BepiColombo (BC). The expected precision on η is ≈ 10−5. In the second part, we will consider the ranging between the Earth and a spacecraft (SC) orbiting near the Sun–Earth Lagrangian points to get an independent measurement of
η. In this case, we forecast a constraint similar to that achieved by LLR.