It's not widely known that Apollo mission astronauts left behind small arrays of specialized optical devices called retro-reflectors when they visited the moon in the 1970's. Retro-reflectors have a unique optical feature: they reflect light directly back towards its source, regardless of the angle of incidence. The lunar retroreflectors are of the "corner-cube" type, which have prisms with three mutually orthogonal reflecting surfaces. Corner cubes have a variety of other applications, including land surveying, radar reflectors on boats, and reflectors on bicycle pedals. The lunar retro-reflectors have made it possible to monitor the distance to the moon to remarkable precision (centimeters) ever since, using intense pulsed laser beams, earth-based telescopes, and sensitive light detectors. Data from these studies have tested general relativity and given new insights into the internal structure of the moon.

The goal of our project is to understand the numerous physics challenges involved in the lunar ranging experiments and to experiment with and understand the optical characteristics of corner cube reflectors. The main physics challenge is the inevitable spreading (diffraction) of the transmitted and reflected laser pulses. Because of this most of the transmitted light misses the lunar reflector and most of the light that is reflected misses the earth-based telescope. Diffraction and signal loss can be minimized by using a relatively large telescope, typically one meter in diameter. Even under the best conditions the return signal is just a few photons per second.