The MoonLIGHT Pointing Actuator (MPAc) project

Laser Ranging is a technique used to perform accurate precision distance measurements between a laser ground station and an optical target, a Cube Corner Retroreflector (CCR). Since 1969 it is possible to realize Lunar Laser Ranging (LLR) measurements thanks to Apollo and Luna missions that placed some arrays of CCRs on the lunar surface. LLR outputs include accurate tests of General Relativity, information of the composition of the Moon, its ephemerides and its internal structure or geocentric positions and motions of ground stations: research uniquely enabled by the Moon.

Despite laser ground stations have significantly improved during the years, the current limitation of the lunar optical target is due to lunar librations. In order to achieve more precise LLR measurements, MoonLIGHT project is designed by SCF_Lab joined UMD. The aim of the project is designing a next-generation of retroreflectors, prototyping, manufacturing and qualify them for the Moon’s environment. Moving from a multi small CCRs array to a single large 100 mm CCR, called MoonLIGHT, unaffected by the lunar librations.

The field of view of each CCR is limited: the retroreflector needs to be pointed precisely to the ground station. The Apollo CCR arrays were manually arranged by the astronauts. In 2018 INFN proposed to ESA the MoonLIGHT Pointing Actuators (MPAc) project, able to perform unmanned pointing operation of MoonLIGHT. In 2019 ESA chose MPAc among 135 eligible scientific project proposals. In 2021 ESA agreed with NASA to launch MPAc to the Reiner Gamma region of the Moon, with a Commercial Lunar Payload Services (CLPS), which is part of the Artemis program. The lander on which MPAc will be integrated is designed by Intuitive Machines (IM). The launch expected date is in April 2024.

MPAc must be able to perform two continuous perpendicular rotations to accurately point the frontal face of the CCR towards the Earth. The device is continuously evolving to ensure the success of the mission, that will take place in Ultra High Vacuum space conditions, in a wide operating temperature range. Terrestrial prototypes, with all the characteristics of the final structure, have been developed for the study of mechanical and electronics components. Qualification tests for space are being planned as the components for the Proto Flight Model (PFM) arrived to the LNF. Payload delivery is scheduled for August 2023.

MPAc will contribute to attain lunar orbit range accuracy below few mm. This will improve, in turn, the precision of the Parametrized Post-Newtonian (PPN) parameters and put more stringent constraints on departures from GR predictions with observations.