In 1969, one object left the moon—and still works today

The Mirror That Never Sleeps

On July 21, 1969, Neil Armstrong and Buzz Aldrin planted a suitcase-sized panel of 100 quartz corner-cube prisms on the Sea of Tranquility, then left it there forever. The Lunar Laser Ranging Retroreflector Array was designed by a Princeton physicist named Carroll Alley, built to do one job: bounce laser beams fired from Earth straight back to their source. Corner cubes work by internal reflection—light enters, bounces off three perpendicular mirrors, and returns precisely along its incoming path no matter the angle. The Apollo 11 array measured 46 by 46 centimeters and weighed 22 kilograms. Two more retroreflectors arrived with Apollo 14 and 15 in 1971, and two Soviet Lunokhod rovers carried French-built arrays in 1970 and 1973. All five still work, and observatories fire pulses at them roughly 2,500 times a year. A single pulse contains around 100 quadrillion photons, but only one or two photons make the round trip—enough for an atomic clock to measure the 2.5-second journey and calculate distance to millimeter precision.

How a Corner Cube Became the Moon's Only Moving Experiment

The retroreflector is passive—it has no power source, no electronics, no moving parts—yet it is the only Apollo experiment still returning new data. Lunar seismometers went silent in 1977 when NASA shut off their transmitters to save money. The solar wind collectors, core samples, and flag stayed put. The retroreflector just sits there, waiting for light. Observatories in Texas, Arizona, New Mexico, France, and Russia fire lasers at the Moon during the two-week periods when the arrays are visible. The University of Texas McDonald Observatory fired the first successful pulse on August 1, 1969, eleven days after Armstrong and Aldrin left the Moon. The returning photon confirmed the retroreflector's position to within three meters. By the 1980s, improvements in laser technology and atomic clocks had sharpened the accuracy to a few centimeters. By the 2000s, it was a few millimeters. The current record is 1 millimeter across 385,000 kilometers, a precision of one part in 400 billion. No other measurement made by humans at this distance approaches it.

The Moon Is Leaving, and We're Watching

The retroreflector revealed something planetary scientists had suspected but never confirmed: the Moon is drifting away from Earth at 3.8 centimeters per year, about the speed fingernails grow. Tidal forces are the culprit. The Moon's gravity pulls Earth's oceans into bulges, creating high tides that rotate as Earth spins. The friction of water dragging across the ocean floor slows Earth's rotation by about 2 milliseconds per century, and conservation of angular momentum forces the Moon outward to compensate. Four billion years ago, the Moon orbited at a distance of 25,000 kilometers and appeared fifteen times larger in the sky; a day on Earth lasted five hours. In another billion years, a day will last 27 hours and the Moon will be too distant to fully cover the Sun—total solar eclipses will become impossible. Lunar laser ranging also confirmed Einstein's theory of general relativity by testing the equivalence principle to one part in 10 trillion, ruled out alternative theories of gravity, and measured the Moon's liquid core by detecting how much it wobbles.

Why It Still Matters

Every Apollo retroreflector still works because quartz and aluminum don't corrode in vacuum, and corner cubes have no alignment requirement—they reflect light back regardless of how they are tilted. The Apollo 11 array is still the most frequently targeted because it sits on a flat plain with no boulders or craters to scatter the beam. NASA considered adding retroreflectors to future Artemis missions, but the Apollo arrays remain sufficient for now. In 2021, the European Space Agency proposed placing a new array with larger mirrors at the lunar south pole to improve accuracy further. Scientists now use the retroreflector data to track continental drift on Earth, measure variations in Earth's rotation, and test whether the gravitational constant changes over time. The answer so far is no—gravity is the same today as it was in 1969, to within one part in 100 billion. The retroreflector will keep confirming that for as long as humans fire lasers at the Moon, making it the longest-running experiment in the history of space exploration.