The Earth-Moon system is constantly bombarded by meteoroids of different size and impact speed. Observation of the impacts on the Moon can enable thorough characterization of the Lunar meteoroid flux, which is similar to that of the Earth. While Earth-based Lunar observations are restricted by weather, geometric and illumination conditions, a Lunar-based observation campaign can improve the detection rate and, when observing the Lunar far side, complement in both space and time the observations taken from Earth. The Lunar Meteoroid Impact Observer (LUMIO), one of the two winning concepts of the ESA SysNova Lunar CubeSats for Exploration challenge, is a mission designed to observe, quantify, and characterize the micro-meteoroid impacts on the Lunar far side. It is based on a 12U CubeSat that carries the LUMIO-Cam, a custom-designed optical instrument capable of detecting light flashes in the visible spectrum. The spacecraft is placed on a halo orbit about the Earth–Moon L2 point, where permanent full-disk observation of the Lunar far side can be performed with excellent quality, given the absence of Earth background noise. After passing Phase 0 and an independent feasibility study in the ESA Concurrent Design Facility, the mission has successfully completed its Phase A in March 2021. Although the Phase 0 design of the LUMIO spacecraft was assessed as feasible by the ESA CDF study, a number of critical issues were identified, which have been tackled by the Phase A design. The paper presents the outcome of this Phase A design effort for the LUMIO spacecraft. Particularly relevant changes or updates in the spacecraft design include: a consolidated design of the LUMIO-Cam, with longer baffle for straylight protection; a set of ADCS sensors and actuators with increased redundancy; a combination of Direct-to-Earth communication and inter-satellite link with a mothership in Lunar orbit; use of Earth ranging to complement and validate the current innovative autonomous navigation strategy based on optical observations of the Moon by means of the LUMIO-Cam; re-assessment of the COTS components selection for the power and propulsion systems. @en

The LUnar Meteoroid Impacts Observer (LUMIO) is a CubeSat mission to a halo orbit at Earth–Moon L2 that shall observe, quantify, and characterize meteoroid impacts on the Lunar farside, by detecting their flashes. In this way, LUMIO is expected to significantly contribute to Lunar Situational Awareness and to the current knowledge on the evolution of meteoroids in the cislunar space. This will allow, ultimately, to achieve a better understanding of the origins of the Solar System, the composition of its planets and the possible hazards caused by impacts between the Earth and Near Earth Objects. LUMIO was one of the proposals submitted to the SysNova LUnar CubeSats for Exploration call by the European Space Agency. The mission was awarded ex-aequo winner of the challenge, and its scientific relevance and technical feasibility were confirmed by an independent study conducted by the ESA Concurrent Design Facility. The LUMIO Phase A study is currently ongoing and is scheduled for completion by the end of 2020. This paper, after providing a short overview of the scientific relevance of the mission, presents in detail the status of the current LUMIO Phase A study, including an overview of all spacecraft sub-systems and the evolution of their design from Phase 0 to Phase A. @en