Apache Point Observatory Lunar Laser-ranging Operation - History and Motivation

History and Motivation

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, 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.

The McDonald and OCA stations are collecting data that is about as good as possible, given the number of photons they collect back from the reflectors. Although minor improvements are certainly possible, getting significantly better data requires a larger telescope and a better site. This is the basic goal of the APOLLO collaboration.

The APOLLO laser has been operational since October 2005, and routinely accomplishes sub-millimeter level range accuracy between the Earth and the Moon.