Lunar Laser Ranging (LLR) experiments accurately measure the round-trip travel time for a laser pulse from an observing station on Earth bouncing off a retroreflector array on the surface of the Moon. Such observations have been made at the McDonald Laser Ranging Station (MLRS) for almost 40 years. These data contribute to varied, multi-disciplinary and inter-disciplinary results in solid Earth sciences, geodesy and geodynamics, solar system ephemerides, terrestrial and celestial reference frames, lunar physics, general relativity and gravitational physics. LLR expands our understanding of the precession of the Earth?s spin axis, the lunar nutation, lunar and solar solid body tides, lunar tidal deceleration, lunar physical and free librations, and energy dissipation in the lunar interior. The present support allows the MLRS LLR experiment to run through at least late 2009. These LLR data will provide better results, due to the improvement of observing equipment and data analysis with time, expansion of the database, and learning to apply more, and cleverer, observation strategies.
The LLR project shares the MLRS with artificial satellite laser ranging studies, and cooperates with the International Laser Ranging Service. It is involved with communication, cooperation, and coordination of both observations and data analysis efforts around the country and across the world. The MLRS has many visitors and provides regular lectures and tours.
The McDonald Observatory Laser Ranging System (MLRS) is one of two lunar laser ranging stations in the United States and one of three observing stations in the world that produce routine Lunar Laser Ranging (LLR) data. LLR is a technique that accurately measures the round-trip travel time of a laser pulse, emitted from an observing station on the Earth and returns, after being reflected by a reflector array on the surface of the Moon. The analysis of this constantly changing distance, using different observatories on the Earth and different retroreflectors on the Moon, provides for a wide array of scientific results. However, getting reflected photons back from the Moon has always been a technical challenge. With hardware and software products created and expanded upon by this project, we continued to improve the quality and quantity of the LLR data set over the past more than 40 years. Many challenges were met and successfully conquered. The effort was worthwhile since LLR is one of the most modern and exotic of the astrometric techniques that are used in basic scientific studies. In terms of fundamental physics, the analysis of the orbit of the Moon provides input for four important tests of gravitational physics. It provides an extraordinary test of the Equivalence Principle, a fundamental tenet in Einstein's theory of General Relativity. The test of the Strong Equivalence Principle (SEP), i.e., the extent to which the gravitational fields of large bodies might alter their gravitational or inertial masses, has not been tested in any other way. With the MLRS’s LLR observations, the Parameterized Post-Newtonian (PPN) quantity, beta, which is identical to 1 in General Relativity, is now known to an accuracy of 0.0001, one of the most accurate determinations available. In gravitational theories the PPN quantity, beta, measures the non-linearity in the superposition of gravity. The analyses of LLR data provides one of the most accurate checks on the constancy of the gravitational constant G, to 1 part in 10^12 per yr. This result is important in light of more recent reports of variation of another of the fundamental physical constants (alpha) on cosmological time and distance scales. With respect to gravitational physics, the analyses of LLR data also provides one of the most accurate determinations of the geodetic precession, the relativistic precession of the Earth due to its annual motion around the sun. With respect to lunar science, the results from LLR data analyses focuses on the interior structure and properties of the Moon. These analyses provide strong evidence of a liquid lunar core with a radius about 20% of the lunar radius. Also measured are solid body tides on the moon and lunar tidal dissipation. There is evidence for oblateness of the core and, with additional effort, one may confirm lunar core oblateness and detect an inner core. Tidal dissipation causes the moon to move outward in its orbit about the Earth by about 4 cm/yr. With the parameters of the lunar orbit, this rate is accurately determined from the analysis of LLR data. We now know that the Earth and moon were much closer together when they were young and the evolution to their larger separation was accompanied by heating in both bodies and a decrease in the Earth's spin rate. Evolutionary studies, based upon these observables, help us understand the evolution and early history of both bodies. LLR and very long baseline interferometry (VLBI) are the only existing techniques that are capable of directly observing accurate and continuous UT1 information for Earth rotation. The LLR series is the longer of the two, starting in 1970 with data from McDonald Observatory. It is true that the density of LLR observations are far less than the VLBI-based data, but the pair combine to provide the defining parameters of Earth rotation. A wealth of fundamental scientific results were accomplished with the analysis of LLR data, over the past more than 40 years. Because of the passive nature of the lunar surface retroreflectors and the steady improvement in equipment, the MLRS and the LLR data type continues to provide new results in many disciplines. Similar to other astrometric techniques, LLR provides broad results and its gains are steady as the data base expands and its accuracy and precision improves. As a basic astronomical data type this longevity and continuity is assured by observations such as those of the MLRS. We recently celebrated the 40th anniversary of the first emplacement of a retroreflector package on the surface of the Moon. The LLR experiment remains as the last active Apollo experiment and, with the efforts of the MLRS, it is producing ever expanding and improving results. LLR, and the science it is able to accomplish, is a source of special pride to the entire scientific community.
