Figure 3.7-1. The Apollo 11 retroreflector. Detail from photo AS11-40-5952.
These passive devices, requiring no onboard power for their operation, were placed on the lunar surface by the crews of Apollo 11, 14 and 15. Even today, scientists can fire a high-power laser beam from Earth to the locations of these retroreflectors on the Moon and detect the light that they reflect back. The time it takes for the light to complete the round trip allows researchers to measure the Earth-Moon distance to within a few centimeters (inches) and to conduct many astronomy- and gravity-related studies.
One of the retroreflectors built for the Apollo program is on display at the National Air and Space Museum in Washington, DC.
Figure 3.7-2. High-power laser beams fired at precise locations on the Moon from the Goddard Geophysical and Astronomical Observatory strike the Apollo retroreflectors. Credit: NASA, 2007.
Strictly speaking, however, these devices cannot be used as indisputable evidence of crewed Moon landings, because the Russians managed to place their own retroreflectors on the Moon by using uncrewed probes (Luna 17 and Luna 21, in 1970 and in 1973). But they do prove that the United States, in 1969 and in 1971, were actually able to somehow place these instruments exactly where they claim to have landed astronauts on the Moon.