Apollo Left Mirrors on the Moon. Scientists Still Bounce Lasers Off Them

Astronauts on Apollo 11, 14, and 15 each deployed a Lunar Laser Ranging Retroreflector array on the surface before lifting off. The devices are panels of corner-cube prisms, the same geometry used in bicycle reflectors, machined from fused silica to survive the Moon's temperature swings of roughly 250 degrees Celsius between lunar day and night. Apollo 15's array, placed near Hadley Rille in 1971, is the largest, carrying 300 individual prisms. The Soviet Lunokhod rovers independently placed two more reflectors in 1970 and 1973, so five targets currently sit on the lunar surface.

An observatory fires a short laser pulse, typically a few hundred picoseconds long, at one of the arrays. The corner-cube geometry sends the light straight back along the direction it came from regardless of the panel's exact angle, which is the property that makes retroreflectors useful across vast distances. The return signal, after a round trip of roughly 770,000 kilometres, arrives faint: observatories typically catch a few photons per pulse, sometimes fewer. By timing the round trip to within a few picoseconds, researchers calculate the Earth-Moon distance to within a millimetre. The Apache Point Observatory Lunar Laser-ranging Operation in New Mexico and the Observatoire de la Côte d'Azur in France have been the most active stations in recent decades, though several others contribute data.

Decades of ranging measurements have produced results that no other method could match. The Moon is moving away from Earth at approximately 3.8 centimetres per year, a figure derived almost entirely from this dataset. The same measurements have tested the equivalence principle, the idea that all objects fall at the same rate regardless of their composition, by watching whether the Earth and Moon fall toward the Sun identically. So far, Einstein holds. Ranging data also constrained the size of the Moon's fluid core: the way the Moon very slightly wobbles, a motion called free libration, is detectable in the millimetre-precision distance measurements and points to a liquid or partially molten interior. None of this was the original goal of the experiment, which was simply to demonstrate that the reflectors worked.

For years, the Apollo 11 and Apollo 14 reflectors returned signals far weaker than models predicted, while the Lunokhod 1 reflector went almost silent for decades after 1971 and was effectively lost until 2010, when the Lunar Reconnaissance Orbiter photographed its location precisely enough for ranging teams to find it again. The degraded performance from the Apollo panels puzzled researchers until studies suggested that lunar dust, electrostatically lofted and settling on the prisms, was scattering light and reducing reflectivity. The problem is worse during the lunar day when the surface charges up under solar radiation. The dust explanation remains the leading hypothesis, though it has not been tested directly.

The retroreflectors require no power, no maintenance, and no communication link. They are passive, and they will remain usable for as long as the prisms survive and observatories on Earth choose to point lasers at them. Future missions are already planning to leave next-generation reflectors with tighter prism tolerances and better thermal management. ISRO's Chandrayaan-3 lander carried a NASA-supplied retroreflector as a passive payload when it touched down near the lunar south pole in August 2023, the first new ranging target placed on the Moon in more than fifty years. The original Apollo arrays are not retired; they are still ranged regularly. The light that left Earth in the 1970s and came back a few seconds later is still coming back, every time an observatory chooses to ask.