Lunar SOURCE: Lunar SOUnding Radar Cubesat Experiment

Abstract

One important question is hanging over the feasibility of human settlement on the Moon: “Is there water ice on the Moon?”. And if so, “How much water is there?” and “At what depth?”. The first trace of water came from NASA Apollo 14 in the form of water vapour ions from a spectrometer near the landing site. Then Russian Luna 24 brought back subsurface samples and claimed evidence of 0.1% water content. The following other missions: American Clementine (which shown evidence later on denied by Arecibo planetary radar), NASA Deep Impact (with spectroscopy data), JAXA SELenological and ENgineering Explorer (with X and Gamma rays spectrometers), NASA Lunar Reconnaissance Orbiter (with six different instruments, including a neutron detector, associated with the impact mission Lunar CRater Observation and Sensing Satellite), and the first results of ISRO Chandrayaan-1 mission did not find unambiguous evidences to answer to these important questions. But in 2018 the first question received an answer. Indeed, Dr. Shuai Li published a paper titled “Direct evidence of surface exposed water ice in the lunar polar regions”, using the data from Chandrayaan-1, and in particular from Moon Mineralogical Mapper (M3), an imaging spectrometer developed by Brown University and JPL. However, the last two questions remain open and further investigations are needed. To this end, this paper proposes a microsat mission (e.g. a 16U Cubesat) carrying a sounding radar to analyze the lunar subsurface and quantify the mass of water ice on the Moon. The Lunar SOUnding Radar Cubesat Experiment (Lunar SOURCE) will use a 7.325 meters dipole radar antenna that transmits a 15-meter wavelength HF signal. Such antenna will have a beam-width of 78°, a maximum gain of 2.13 dBi and can be useful, if the orbit is lowered down to 50 km altitude from the moon surface. Such orbit, together with a 8 W average RF power, gives a nominal Signal to Noise Ratio (SNR) of 45 dB at the nadir point, which enables the analysis of the subsurface interfaces down to a useful depth for water ice detection. However, due to the low altitude expected for this mission, the irregularities of the mass distribution of the Moon have a significant impact on the orbit stability and thus mission lifetime. This paper presents some possible orbits for mapping the lunar surface considering the achievable space coverage, the mission lifetime and a mapping strategy. Preliminary results show that good coverage is achievable in fewer than 60 days (the expected lifetime of the mission) and that such coverage will be especially good for the polar regions, thanks to the 90° orbit inclination. The identification of the mission and system requirements develops the foundations for the feasibility study of the mission, enabling preliminary selection and sizing of the main subsystems of the Cubesat. The link budget is computed to ensure that Lunar SOURCE would be able to send back to Earth the scientific data with the High Gain telecommunication Antenna. The power system is designed considering the orbit, the mapping strategy, and HBDA.