Apache Point Observatory Lunar Laser-ranging Operation

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 experiments, which use retroreflectors on the Moon to track changes in lunar orbital distance and motion.From our perspective on Earth, this would appear as a displacement, or polarization, of the lunarorbit away from the Sun with an amplitude of 13 meters. If the violation went the other way, with theself energy possessing inertial mass but not gravitational mass, the lunar orbit would appear to bepolarized toward the Sun by the same amplitude. The calculation of the amplitude is complicated, but a crude estimate may be derived by multiplying the Earth’s orbital radius of 1.5×1011 m by the 4.6×10−10 contribution to the Earth’s mass from the self-energy to yield 75 meters. 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 experiments, which use 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, scientists were able to measure and predict the orbit of the Moon to a precision of a few centimeters by the early 2000s. This precision provides the best known test of many aspects of our theories of gravity. APOLLO improves this precision even further, measuring the distance between the Moon and Earth to within a few millimeters. Using this information, scientists will be able to further test various aspects of gravity, such as; determining if the Earth and the Moon react the same to gravity despite their different compositions, investigating the predictions of Einstein with respect to the energy content of the Earth and the Moon and how they react to gravity, and evaluating if General Relativity correctly predicts the motion of the Moon. 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. 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. High precision Lunar Laser Ranging (LLR) started soon after the Apollo 11 astronauts left the first retroreflector on the Moon. Additional reflectors were left by the Apollo 14 and Apollo 15 astronauts, and two French-built reflector arrays were placed on the Moon by the Soviet Luna 17 (Lunokhod 1) and Luna 21 (Lunokhod 2) lunar rover missions. Over the years since, many groups and experiments have used this technique to study the behavior of the Earth-Moon system, investigating gravitational and other effects. For the first few years of the Lunar Laser Ranging Experiment, the distance between the observatory and the reflectors could be measured to about 25 cm accuracy. Improved techniques and equipment lead to accuracies of 12–16 cm until about 1984. Then McDonald Observatory built a special purpose system (MLRS) just for ranging, and achieved roughly 3 cm accuracies mid-to-late 1980s. In the early 1990s a French LLR system at the Observatoire de la Côte d’Azur (OCA) started operation, with similar precision.