Lunar Laser Ranging Contributions to Relativity and Geodesy

AbstractLunar laser ranging (LLR) is used to conduct high-precision measurements of ranges betweenan observatory on Earth and a laser retro-reflector on the lunar surface. Over the years, LLR hasbenefited from a number of improvements both in observing technology and data modeling, which ledto the current accuracy of post-fit residuals of ∼ 2 cm. Today LLR is a primary technique to studythe dynamics of the Earth-Moon system and is especially important for gravitational physics, geodesyand studies of the lunar interior. When the gravitational physics is concerned, LLR is used to performhigh-accuracy tests of the equivalence principle, to search for a time-variation in the gravitationalconstant, and to test predictions of various alternative theories of gravity. The gravitational physicsparameters cause both secular and periodic effects on the lunar orbit that are detectable with thepresent day LLR; in addition, the accuracy of their determination benefits from the 35 years of theLLR data span. On the geodesy front, LLR contributes to the determination of Earth orientationparameters, 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 boththe terrestrial and selenocentric reference frames. The realization of a dynamically defined inertialreference frame, in contrast to the kinematically realized frame of VLBI, offers new possibilities formutual cross-checking and confirmation. Finally, LLR also investigates the processes related to theMoon’s interior dynamics. Here, we review the LLR technique focusing on its impact on relativityand give an outlook to further applications, e.g. in geodesy. We present results of our dedicatedstudies to investigate the sensitivity of LLR data with respect to the relativistic quantities; we alsopresent the computed corresponding spectra indicating the typical periods related to the relativisticeffects. We discuss the current observational situation and the level of LLR modeling implementedto 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.Keywords. Lunar Laser Ranging, Relativity, Earth-Moon dynamics