Mapping lunar mascons on the hidden side of the Moon: Gravitational field measurement through a micro-satellite mission

The gravitational anomalies of the Moon have never been completely mapped; in particular, those on the hidden side are known only through the interpolation of data obtained for the visible side. Many space agencies around the world are developing new plans for the exploration of the Moon, hence a more precise knowledge of lunar harmonics could be useful for the planning of future missions. The aim of this study is to demonstrate the feasibility of a micro-satellite mission for the accurate mapping of high order lunar harmonics, generated by density anomalies, on the hidden side of the Moon. The mission, conceived within the School of Aerospace Engineering, at the University of Rome “La Sapienza”, employs two small probes (whose masses will be under 100 kg), placed in two orbits around the Moon, with a fixed ratio between their orbital periods. The main probe will essentially be a relay satellite placed in a higher and eccentric orbit, always facing the Earth; a smaller probe will be in a low circular, near-polar, orbit. A reference signal would be sent from a net of Earth stations, and relayed by the relay satellite to the smaller probe, that reflects the signal back to the relay satellite, and then to Earth. A subsequent analysis of the Doppler effect on the downlink signal, with respect to the uplink, will allow to build a map of Moon gravitational field. The interesting aspect of this method of measurement is that there is no need to carry any kind of instrumentation for Doppler effect detection, including a stable frequency reference, on board. This allows the reduction of overall costs and technical complexity of the mission. The encouraging results of this study show the possibility of the realization of the mission in the short term, and with a limited budget, given the collaboration between universities, the aerospace industry, and any space agency.