Lunar Laser Ranging Contributions to Relativity and Geodesy

Lunar laser ranging (LLR) is used to conduct high-precision measurements of ranges between an observatory on Earth and a laser retro-reflector on the lunar surface. Over the years, LLR has benefited from a number of improvements both in observing technology and data modeling, which led to the current accuracy of post-fit residuals of ∼ 2 cm. Today LLR is a primary technique to study the dynamics of the Earth-Moon system and is especially important for gravitational physics, geodesy and studies of the lunar interior. When the gravitational physics is concerned, LLR is used to perform high-accuracy tests of the equivalence principle, to search for a time-variation in the gravitational constant, and to test predictions of various alternative theories of gravity. The gravitational physics parameters cause both secular and periodic effects on the lunar orbit that are detectable with the present day LLR; in addition, the accuracy of their determination benefits from the 35 years of the LLR data span. On the geodesy front, LLR contributes to the determination of Earth orientation parameters, such as nutation, precession (including relativistic precession), polar motion, and UT1, i.e. especially to the long-term variation of these effects. LLR contributes to the realization of both the terrestrial and selenocentric reference frames. The realization of a dynamically defined inertial reference frame, in contrast to the kinematically realized frame of VLBI, offers new possibilities for mutual cross-checking and confirmation. Finally, LLR also investigates the processes related to the Moon’s interior dynamics. Here, we review the LLR technique focusing on its impact on relativity and give an outlook to further applications, e.g. in geodesy. We present results of our dedicated studies to investigate the sensitivity of LLR data with respect to the relativistic quantities; we also present the computed corresponding spectra indicating the typical periods related to the relativistic effects. We discuss the current observational situation and the level of LLR modeling implemented to date. We emphasis the need for the modeling improvement for the near future LLR opportunities. We also address improvements needed to fully utilize the scientific potential of LLR.