Testing relativistic gravity with lunar laser retroreflectors and PEP

Since 1969, Lunar Laser Ranging (LLR) has been a vital technique for measuring the distance between Earth and the Moon by using laser pulses reflected from retroreflectors placed on the lunar surface. This method has offered crucial information about the Moon’s internal structure, lunar ephemerides, and important tests of General Relativity (GR), relying on retroreflectors from the Apollo and Lunokhod missions. Over time, improvements in LLR station capabilities have reduced measurement errors, but the impact of lunar librations caused by the Moon’s orbital eccentricity and inclination now dominates the error budget. To address this, next-generation libration- free retroreflectors are necessary, such as the MoonLIGHT project, developed by the INFN-LNF in collaboration with the University of Maryland and supported by the Italian Space Agency (ASI). MoonLIGHT features a $\mathbf{1 0 0 ~ m m}$ uncoated cube- corner retroreflector that is unaffected by librations. To further enhance lunar measurement precision, the INFN- LNF has proposed the MoonLIGHT Pointing Actuator (MPAc), a robotic actuator designed to align the MoonLIGHT retroreflector towards Earth after the spacecraft lands. ESA and NASA have agreed to launch MPAc as part of NASA’s IM-3 mission, expected to take place in late 2025 or early 2026. In addition, the INRRI instrument, developed by INFN, will land on China’s Chang’e–6 lander in June 2024. This instrument will provide high-precision control points on the lunar far side, assisting lunar surface mapping, orbit determination, and future lunar mission navigation. These advancements will significantly improve lunar orbit range accuracy to millimeter-level precision, facilitating more precise gravitational tests and offering new insights into al- ternative theories of gravity beyond General Relativity. The Planetary Ephemeris Program (PEP), developed by the Harvard-Smithsonian Center for Astrophysics, plays a key role in analyzing LLR data, offering refined constraints on GR (General Relativity and facilitating the exploration of alternative gravitational theories, paving the way for future breakthroughs in space-based gravitational measurements.