Mission design techniques for satellites operating in low-lunar orbits

Several missions to the Moon are planned for the coming years. The international interest in the cislunar region is evident, as it is seen by many as the new frontier of space exploration and exploitation. However, designing a mission to reach an orbit close to the Moon is complex from many perspectives. Due to the non-linear dynamics characterising cislunar space, transfer design deals with chaotic trajectory behaviour, while the greater distance from Earth makes operations and the choice of satellite components complex. Moreover, miniaturised spacecraft are lately the favoured design choice for many missions, leading to a further increase in complexity due to limitations in the total AV budget. In this paper, a set of techniques for transfer design and orbit selection for satellites operating in Low-Lunar Orbits (LLOs) is proposed. LLOs are interesting for many scientific applications, but from a mission analysis perspective, they have been less studied. The main objective of this work is to show how the minimisation of station-keeping and transfer costs can be addressed together, respecting, at the same time, the typical mission requirements of a CubeSat mission. To minimise maintenance costs, a frozen orbit is proposed as the operational one. The frozen condition is designed with respect to the harmonics of the lunar gravitational field or the third body, depending on the altitude at which the LLO of interest is to be placed. On the other hand, an overview of the transfer design techniques by which such a LLO can be reached, minimising the AV budget, is presented, with a focus on the design of impulsive transfers. Strategies including low-energy transfers, weak stability boundary and endgame are considered, evaluated and compared in terms of total AV required, time of flight and sensitivity to the choice of the launch date. The techniques described in this paper are applied to the design of LunPAN, a study funded by the European Space Agency within a new initiative on Small Missions for Exploration, and its inception, Destination the Moon. Its objective is to reach an LLO to measure the particle flux related to solar flares, albedo and galactic cosmic rays with the Pix.PAN and NeuPix instruments.