The MoonLIGHT-2 experiment, funded by INFN’s National Scientific Commission 2 (CSN2), has spent recent years developing next-generation laser retroreflectors for Earth-Moon distance measurements. Starting in 2026, it will expand its scientific activities to the study of the Earth-Moon system for gravity and fundamental physics tests—as well as terrestrial and lunar geophysics—using the Lunar Laser Ranging (LLR) technique. For over 56 years, LLR has monitored the distance to our satellite using laser pulses sent from a small number of stations within the International Laser Ranging Service (ILRS), one of which is located near Matera at the Italian Space Agency (ASI) Space Center. The theoretical heart of this challenge is the PEP (Planetary Ephemeris Program) software, developed beginning in the 1960s by the Harvard-Smithsonian Center for Astrophysics in Cambridge (MA).
While the first 50 years of LLR relied on mirror arrays from the Apollo and Lunokhod missions (which achieved centimeter-level precision), the challenge modern physics presents today in the study of gravitational interaction requires a higher resolution: millimeter or sub-millimeter. The main issue with historical arrays is their sensitivity to lunar librations—oscillations that “blur” the reflected laser signal, thereby limiting final precision. To meet this need, the SCF_Lab at INFN-Laboratori Nazionali di Frascati, in collaboration with the University of Maryland and ASI, developed MoonLIGHT (Moon Laser Instrumentation for General relativity High-accuracy Tests). It is characterized by an Innovative Design (instead of an extended array of small mirrors, MoonLIGHT uses a single large CCR (Cube Corner Retroreflector) with a frontal diameter of 10 cm) and a significant reduction in Libration effects (the 10 cm single-reflector configuration eliminates uncertainty due to lunar oscillations, enabling millimeter precision). To date, the Next Generation Lunar Retroreflector-1 (NGLR-1) system, known as fixed-pointing MoonLIGHT, has been operational on the Moon since March 2025. An even more advanced version, integrated into the MPAc (MoonLIGHT Pointing Actuator) robotic pointing system, will be deployed in 2026 to actively align itself toward Earth
With the millimeter precision guaranteed by the new 10 cm CCRs and PEP analysis, MoonLIGHT-2 aims to improve several gravitational measurements characteristic of LLR experiments, such as:
- Strong Equivalence Principle (SEP). Verifying whether the Earth and the Moon fall toward the Sun with the same acceleration, testing the validity of this cornerstone principle of General Relativity.
- Geodetic Effect. Measuring the precession of the lunar orbit due to the motion of the Earth-Moon system within the Sun’s gravitational field.
- Lorentz Invariance Violations. Detecting potential spacetime anisotropies that could indicate new physics beyond the Standard Model.
[1] Muccino, M., et al. MoonLIGHT and MPAc: The European Space Agency’s Next-Generation Lunar Laser Retroreflector for NASA’s CLPS/PRISM1A (CP-11) Mission. Remote Sensing, 2025, 17(5), 813, DOI: 10.3390/rs17050813. [2] Molli, S., et al. NovaMoon: A Strategic Lunar Reference Station for Positioning, Timing, and Largely Enhanced Science in the Earth-Moon System. arXiv:2602.08432.