mid-2011, the status was: All 5 reflectors (three Apollo and two Lunokhod) ranged routinely. As many as 12 photons in a single pulse (limited by detector – might have been more). Sustained rate of about 3 photons per pulse over several minutes. This is about 65 times more photons detected than previous efforts. As many as 50,000 return photons detected in a single lunation (during 5 hours total operation). As of mid-2011, the range precision (per session) was believed to be about per reflector, while the orbit of the Moon is being determined to roughly the 15 mm level. The gap between the measurements and the theory could be due to systematic errors in the ranging, insufficient modeling of various conventional effects that become important at this level, or limitations of our theory of gravity. Although it is possible that this discrepancy is due to new physics, the primary suspect is insufficient modeling, since this is known to be both complex and difficult. To enable APOLLO to improve beyond the part per trillion measurement accuracy level, in 2016 it added a cesium atomic clock and improved calibration system. With the new system in place, the possible accuracy can be increased to better than 2 mm. The new system has confirmed the accuracy of previous measurements. It revealed that the previous estimate of 10 ps of error (corresponding to 1.5 mm of distance uncertainty) attributed to APOLLO's GPS-synchronized oven-controlled crystal oscillator was too low; the true figure was closer to 20 ps (3 mm). However, careful record-keeping allowed the old data to be reanalyzed in light of the new understanding of the clock's variations and most of the accuracy recovered. By confirming the accuracy of previous measurements, and making new even more accurate measurements, the still unresolved discrepancy between theory and experiment is now placed more firmly on the theoretical models. The collaboration APOLLO is collaboration between: University of California, San Diego (Tom Murphy Principal investigator), University of Washington, Harvard, Jet Propulsion Laboratory, Lincoln Laboratory, Northwest Analysis, Apache Point Observatory, and Humboldt State. References External links What Neil & Buzz Left on the Moon NASA description of the basics of Lunar Laser Ranging Main web page for the Apache Point Lunar Laser Ranging Project Lunar science Tests of general relativity 2005 establishments in New Mexico