LAUSR

Abstract We assessed the orbital evolution of the Mercury Planetary Orbiter (MPO), part of the European-Japanese BepiColombo mission, using a gravity field model determined from radio tracking data of the NASA spacecraft MESSENGER. Owing to MESSENGER’s eccentric orbit and the resulting uncertainties in the estimation of the gravity field coefficients, the orbital evolution is not precisely predictable. In this work we used the most recent estimation of Mercury’s gravity field and assessed the impact of the uncertainties in the gravity field harmonic coefficients on the orbit evolution of MPO. For that purpose we used the covariance matrix of the gravity field (which holds information on the correlation between the different coefficients), rather than pure standard errors. In order to assess the impact of the coefficients’ covariance we generated 1000 gravity fields compatible with the covariance matrix. Thereby we considered two scenarios: 1) the optimistic case, where the formal errors were considered plausible, i.e. scaled by 1; and 2) the conservative case where the formal errors were scaled with a factor of 10. With each of the generated gravity fields we have propagated the orbit of MPO over 1,000 days, which includes the nominal MPO mission phase of 1 year as well as an extended mission. The propagation of the spacecraft trajectory was performed with a numerical integrator taking into account the gravitational potential of Mercury and the gravity force of the Sun acting upon the spacecraft. Other perturbations, e.g. solar radiation pressure or gravitational forces by other planets were found to be negligible for this analysis. By evaluating the dispersion of the propagation results we could assess the influence of the gravity field uncertainties, including the covariance, on the evolution of critical orbital elements of MPO. As the altitude of the spacecraft above the surface is most important in Mercury’s harsh thermal environment, we mostly focused our analysis on the evolution of the periherm altitude. The results of the MPO orbit propagation show a non-linear decrease in the periherm altitude over the simulated timeframe. However, the decrease rate varies significantly among the generated gravity fields. Moreover, the dispersion of the periherm altitude propagations outcomes gets larger with increasing scale factor (optimistic to conservative scenario) and over time. While the values of the orbital elements of MPO are still in an acceptable range after the first year, the periherm altitude may fall below a critical value of 200 km after 2 years in orbit about Mercury. In fact, when considering the conservative scenario and without any preventive actions there is a possibility of about 24% that the spacecraft collides with the planet after 1,000 days in orbit.