July 20, 1969, a historic day for the Apollo space program, was also the start of a laser experiment that is still going strong more than 50 years later. The ongoing lunar laser ranging experiment, also known as the Apollo reflector experiment, measures the distance between the moon and Earth using laser ranging. Laser light pulses are transmitted and reflected back to Earth, and the round-trip duration is measured. The Apollo 14 and 15 missions also left reflector arrays, in addition to the one left behind by the Apollo 11 crew. The Apollo 11 and 14 arrays each have 100 quartz glass prisms — known as corner cubes — and the Apollo 15 array has 300. Because of their shape, these corner-cube reflectors, which are made from fused silica, always reflect an incoming light beam back in the direction from which it came. When a laser beam is shined at the moon, the beam is bounced off the reflector mirror and reflected back to Earth. The return signal is weak, but it can be detected over a period of time using sensitive filtering and amplification equipment. The arrays have been able to provide continuous service for over 50 years because they do not need power to operate. Like the Energizer Bunny, they “just keep going and going and going,” only no batteries are required. Scientists observe the signal over several hours and then average out their observations to calculate the moon’s distance down to less than 2 cm, which is pretty impressive, considering that the arrays are almost 239,000 miles from Earth. Lunar laser ranging can accurately determine the orbit, rotation, and orientation of Earth’s moon. The lunar orbit and the orientation of the moon are needed by spacecraft that orbit and land on it. For example, cameras on spacecraft in lunar orbit can “see” the reflecting arrays and use them to identify locations at an accuracy of less than a foot. Goddard’s Laser Ranging Facility in Greenbelt, Md., directing a laser (green beam) toward the Lunar Reconnaissance Orbiter spacecraft in orbit around the moon (white disk). The moon has been deliberately overexposed to show the laser. Courtesy of Tom Zagwodzki/Goddard Space Flight Center. Laser ranging has shown that the distance between the moon and Earth increases 3.8 cm a year. The gravitational force between the tidal bulges and the moon slow Earth’s rotation while also pulling the moon forward along the direction it moves in its orbit around Earth. The forward force causes the moon to spiral away from Earth by 3 mm each month. Earth’s gravity tugs on the moon, causing the positions of the reflecting arrays to vary as much as 15 cm up and down each month as the moon flexes. Measuring how much the arrays move has enabled scientists to better understand the elastic properties of the moon. Analysis of lunar laser data has shown that the moon has a fluid core and supports Einstein’s theory of gravity, which assumes that the gravitational attraction between two bodies does not depend on their composition. Analysis of data from the lunar laser ranging experiment finds no difference in how gravity attracts the moon and Earth due to their makeup. NASA has approved a new generation of reflectors to be placed on the moon’s surface within the next decade. The improved performance of new reflectors and their wider geographical distribution on the moon would allow improved tests of Einstein’s relativity theory, deeper study into the lunar interior, and a more rigorous investigation of the moon’s history. A remarkable legacy, for a few simple arrays, each about the size of the laptop.