[P-PS09] 火星と火星衛星

[PPS09-P05] Simulation of Improving Phobos Gravity Field Accuracy through Stepwise Reduction of Quasi-Satellite Orbit Altitudes

*山本 圭香¹、松本 晃治¹、池田 人²、Laurent-Varin Julien³、Marty Jean-Charles³、荒木 博志¹ (1.国立天文台、2.宇宙航空研究開発機構、3.フランス国立宇宙センター)

キーワード：MMX、フォボス、重力場決定、QSO

After arriving in the Martian system, the Martian Moons eXploration (MMX) spacecraft will finally be placed into Quasi-Satellite Orbit (QSO) to observe Phobos. To ensure the safety of the spacecraft, the mission will begin operations from a high-altitude QSO, gradually transitioning to lower altitude QSOs while updating various operational parameters as needed. In this study, based on the MMX mission plan, we simulate the stepwise gravity field determination process during the sequential descent of the orbital altitude. This approach enables obtaining the best estimated gravity field at each orbital altitude using the data acquired up to that time. Additionally, by reducing the errors in the low-degree gravity field, which have a relatively large contribution to gravity acceleration, the determination of higher-degree gravity field components at lower altitudes is expected to become easier. In the simulation, we first assume the true shape and internal density structure of Phobos and compute the corresponding gravity field. Next, based on this true gravity field, we generate spacecraft orbits in four different altitude QSOs and simulate Doppler, laser ranging, and landmark observations along these orbits. Then, starting with an initial gravity field and spacecraft orbit that differ from the true values, we estimate the true gravity field and spacecraft orbit using the observational data. For the estimation, we use the GINS software developed by the Centre National d’Études Spatiales (CNES). The initial estimation is conducted using data from the high-altitude QSO, and the estimated gravity field from this step is then used as an initial gravity field for estimating the orbit and gravity field at a lower altitude. This process is repeated until the orbit and gravity field are determined at the lowest altitude. We present the accuracy of estimating internal density structure of Phobos at different orbital altitudes, and compare the estimation efficiency and accuracy of the results with and without the use of this stepwise gravity field determination approach.