The Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO, is a project at the Apache Point Observatory in New Mexico. It is an extension and advancement of previous Lunar Laser Ranging Experiment, which uses retroreflectors on the Moon to track changes in lunar orbital distance and motion.
Using telescopes on Earth, the reflectors on the Moon, and accurate timing of laser pulses, by the early 2000s scientists could measure and predict the orbit of the Moon to an accuracy of a few centimeters. This already impressive accuracy (the Moon is typically about 385,000 km away) provides the best known test of many aspects of our theories of gravity. APOLLO improves this even further, measuring the distance between the Moon to an accuracy of a few millimeters. Using this information, scientists will be able to further test various aspects of gravity: do the Earth and the Moon react the same to gravity despite their different compositions? Does the energy content of the Earth and the Moon react to gravity in the same way as Einstein predicts? In general, does Einstein's General Relativity correctly predict the motion of the Moon, or are new theories required?
The APOLLO collaboration built their apparatus on the 3.5 meter telescope at Apache Point in southern New Mexico. By using a large telescope at a site with good atmospheric "seeing", the APOLLO collaboration gets much stronger reflections than any existing facilities. (Strong is a relative term here—APOLLO records approximately one returned laser photon per pulse, as opposed to the roughly 0.01 photon-per-pulse average experienced by previous LLR facilities.) The stronger return signal from APOLLO translates to much more accurate measurements.
Read more about Apache Point Observatory Lunar Laser-ranging Operation: History and Motivation, Science Goals, Principles of Operation, Status, The Collaboration
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