Orbital evolution of the BepiColombo Mercury Planetary Orbiter (MPO) in the gravity field of Mercury

Abstract

We assessed the impact of the uncertainties in the gravity field harmonic coefficients of Mercury on the orbital evolution of the Mercury Planetary Orbiter (MPO), part of the European-Japanese BepiColombo mission. We used in our simulations the most recent estimation of the gravity field model of Mercury determined from radio tracking data of the NASA spacecraft MESSENGER. Thereby we propagated the orbital evolution of MPO by means of plausible gravity fields compatible with the covariance matrix measured gravity field coefficients. 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 3. With our simulations we could also demonstrate the importance of the correlation between the errors of the gravity field coefficients for the uncertainty in the trajectory evolution. 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 of 1000 days. However, the decrease rate varies significantly among the generated gravity fields. Moreover, the dispersion of the periherm altitude gets larger with increasing scale factor (optimistic to conservative scenario) and over the mission 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.