Mercury is the smallest and the less explored planet of the Solar System. It is also the closest planet to the Sun. This proximity makes the study of its dynamics a fundamental input for testing general relativity. The effect of general relativity is indeed for Mercury orbit the strongest among all planets in the Solar System (43 arcseconds per century of perihelia advance). Despites its proximity, until recently, Mercury was only visited three times by the Mariner 10 spacecraft in the seventies (2 times in 1974 and once in 1975).

The MESSENGER spacecraft launched in 2004 by NASA in the frame of the Discovery program of Solar System exploration was the first probe to orbit Mercury since March 18th 2011. The main goal of the mission is to map Mercury surface and to study its inner structure.

Researchers of research units of the institut UTINAM (Université de Franche-Comté, CNRS), of GéoAzur (UNS-CNRS-OCA-IRD), of IMCCE (Observatoire de Paris, CNRS) and of CNES have studied the spacecraft orbit using more than one-year of navigation data. This work allows to set up a new planetary ephemeris (INPOP13a) and to perform new tests for general relativity.

Several models are presently under consideration aiming to solve some major problems of quantic physics and cosmology such as the existence of dark matter or our understanding of the gravitation mechanism at large and quantic scales.

In studying the motions of natural and artificial objects in the Solar System, the researchers of this paper provide new limits confirming or denying the proposed models. The INPOP (Intégrateur Numérique Planétaire de l’Observatoire de Paris) team working together since 2003 for the construction of the INPOP planetary ephemerides, is the only team in the word to address these questions by the construction of spacecraft and planetary orbits with the use of raw navigation data of space missions.

MESSENGER data are regular and accurate at a level never reached before. The team has built up the most precise ephemerides of Mercury and obtained new constraints on PPN parameters (Parametrized Post-Newtonian) of light deflection γ and of gravity non-linearity β.

In a paper recently published in Astronomy and Astrophysics (Verma et al. 2014), the obtained accuracy gives an improvement of about a factor 4 for the parameter β in comparison to previous estimations obtained by the team in 2010 (Fienga et al. 2011). For the γ parameter, the 2003 Cassini experiment published in Nature (Bertotti et al. 2003) gave a possible maximum violation to general relativity of about 4.10^-5. The present study reduces the possible range of such violation by a factor 2.

Furthermore, this study shows that it was possible to produce a disentangled estimation of the parameters β and γ based on the analysis of spacecraft data and the construction of planetary ephemerides.

Collaborations are installed in order to test other theories at the scale of the Solar System in particular with the Institut Astrophysique de Paris and Paris Observatory.

This work was supported by a PhD grant from the CNES and the Région Franche-Comté, in the framework of the GRGS (Groupe de Recherche en Géodésie Spatiale) and of the CESDN (Consortium pour l’Exploitation Scientifique des Données de Navigation).

Last update on 21 December 2021