Probing Fundamental Gravity via Lunar Laser Ranging

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APOLLO (Apache Point Observatory Lunar Laser-ranging Operation) is a next-generation laser-ranging apparatus capable of obtaining lunar ranges with millimeter accuracy. Relying on the still-operational retroreflector arrays placed on the lunar surface by Apollo astronauts, APOLLOconstitutes the ground-link of a dynamically clean orbital laboratory. APOLLO will bring new life to lunar ranging by applying advanced laser and detector technology on a large telescope, by incorporating precision gravimetry, millimeter-accuracy GPS positioning, and a meteorologicalarray on the site, and by forming a tight interface between the observation and data analysis efforts.The order-of-magnitude improvement in range precision expected from APOLLO will advance by a similar factor the scientific power of lunar ranging to determine a number of qualitative aspects of the fundamental properties of gravity. Among the tests that lunar ranging currently leads are tests of the equivalence principle, time variation of the gravitational constant, geodetic precession, gravitomagnetic effects, and the possible presence of long-range forces. Through its comprehensive investigation of the nature of gravity, lunar ranging addresses the question: is general relativity correct? The technique, ambitious goals, and connection to our remarkable history of space exploration make APOLLO especially appealing to the public, presenting excellent outreach potential. The APOLLO team is committed to sharing the excitement of this project with its immediate community and beyond.