1. Apollo Left Science Stations on the Moon, and Most Went Silent in 1977
The Apollo missions did not only bring rocks home. The crews also set up instruments on the Moon that kept working after they left. The sets of experiments left on the Moon by the Apollo 12 through 17 missions were called ALSEP, short for Apollo Lunar Surface Experiments Package. NASA's final report says data reception from the equipment on the lunar surface ended on September 30, 1977[1].
Apollo 11 was the first to try. Its seismometer, a device that feels ground shaking, was the first one ever placed on the Moon. It detected "moonquakes" and meteorite hits, about 100 to 200 of them during its short life[2]. So the question is a fair one. If the Apollo 11 crew also left a mirror, why is that still in use, when the seismometer worked for only weeks?
2. Machines That Need Power Are Fragile on a Moon With Two-Week Nights
The Apollo 11 seismometer ran on two panels of solar cells, so it could only do its measuring in daylight. NASA's description says the lunar night lasts about 340 hours, and temperatures can drop to minus 170°C (minus 274°F). To keep the instrument from being damaged, a small heater powered by a radioactive material held it above minus 54°C. NASA calls this the first major use of nuclear energy in one of its crewed missions[2].
Daytime was a problem too. Engineers tried to protect the electronics from the midday heat by cutting off the electrical power in short bursts. Even so, the instrument stopped responding to commands on August 25, 1969, probably from overheating in the hot midday Sun[2]. It returned data for just three weeks. More advanced seismometers at the Apollo 12, 14, 15, and 16 sites kept sending data to Earth until September 1977[2].
Anything with electronics has a long list of ways to fail: heat, cold, power, radio. And even a working station needs people and money back on Earth to receive and process its data. Now imagine an instrument with none of those weak points.
3. A Moon Mirror Has Nothing to Switch Off Because It Only Sends Light Back
The Apollo 11 astronauts placed a laser reflector in the Sea of Tranquillity. NASA's Goddard Space Flight Center describes it as 100 fused silica "corner cubes" (glass pieces cut like the inside corner of a box), each 3.8 centimeters across, mounted in a 46-centimeter-square aluminum panel[3]. A corner cube sends a beam of light directly back toward the point it came from[3].
Compare it with a hand mirror. Tilt a hand mirror, and the reflected light swings off in a different direction. A corner cube is not fussy about the angle like a flat mirror: light that comes in from Earth goes back toward Earth. That is why a plain, unpowered piece of glass can be a target that people on Earth can aim at. The reflector itself is passive. The laser and the detector that catches the returning light are at the observatories on Earth[3].
So the mirror outlived the powered stations for a simple reason. It never depended on electricity, a radio link, or anyone on Earth listening to it. It was always passive.
4. How Laser Ranging Measures the Moon's Distance From One Light Particle at a Time
Aiming at the mirror is hard. A scientist at NASA's Jet Propulsion Laboratory explained that the reflectors are too small to be seen from Earth. At the Moon, the laser beam has spread to roughly four miles (6 kilometers) wide, and aiming it was compared to hitting a moving dime two miles (3 kilometers) away[3].
The return trip is just as hard. Even in good conditions, only one photon, a single particle of light, reaches the telescope every few seconds[3]. Observatories use extremely sensitive filtering and amplification to catch it[3]. The trick is timing. The laser fires a very short pulse, the equipment records exactly when it left and when a returning photon arrives, and the round-trip time gives the distance. A simple way to picture it is an echo: shout at a far cliff, time the echo, and you know how far away the cliff is. The difference is that light travels about 300,000 kilometers every second, so the timing has to be extremely fine.
Because so few photons come back, one pulse is never enough. Observatories repeat the shots many times and combine the results. By 1994, NASA reported that the measurements could give the distance to about 3 centimeters, and the average distance between the centers of Earth and Moon, about 385,000 kilometers, to better than one part in 10 billion[3]. (That works out to a few centimeters, by the writer's own arithmetic.) Later, the APOLLO station in New Mexico reported a median nightly accuracy of 1.7 millimeters for its 2006 to 2020 data, bouncing laser light off five reflector arrays on the Moon[4].
5. What Decades of Moon Distance Measurements Revealed
Measuring the same distance for decades turns out to be powerful. NASA's 1994 summary says the measurements showed that the Moon is moving away from Earth at about 3.8 centimeters per year[3]. If that rate stayed the same for 50 years, the Moon would be about 1.9 meters farther away. That is the writer's simple multiplication, and there is no guarantee the rate stays constant.
The same data told scientists other things. The slight wobbles in the Moon's rotation gave strong evidence that the Moon has a liquid core. And because Earth's tides, atmosphere, and core affect the spin of our planet, the length of an Earth day was found to vary by about a thousandth of a second over a year[3]. A mirror left for a moon walk became a ruler for the Moon, and a very slow clock for Earth.
6. Old Seismic Records Gave a Second Clue About the Moon's Core
The powered instruments were not wasted. Their records stayed in the archives. NASA's technical reports server lists a presentation that re-analyzed Apollo's deep moonquake recordings using methods from Earth's seismology. Its abstract says the results indicate a solid inner core and a fluid outer core. The same abstract adds that significant questions remain[5]. I only read this abstract, not the full study, so the details here are limited.
Put the two together. Laser ranging suggested a liquid core by measuring how the Moon wobbles. Old seismic data, by listening to the Moon shake, hinted at a solid inner core inside a fluid outer one. Two very different methods, one from a still-working mirror and one from a long-silent machine, seem to point toward a similar sort of inside, though the abstract itself says questions remain. Neither is the last word, but this is why scientists keep both datasets.
7. A New Moon Mirror Landed in 2025 to Measure Even More Precisely
The University of Maryland reports that a new reflector, designed by physicists including Doug Currie, who also worked on the original Apollo reflectors, launched on January 15, 2025, aboard Firefly Aerospace's Blue Ghost Mission One lander. The lander reached the Moon on March 2, 2025, and the next day the instrument successfully communicated with the French Lunar Laser Ranging Observatory at Grasse[6].
It is different from the Apollo reflector: instead of an array of small mirrors, it is a single corner cube 10 centimeters across. The team hopes it will allow Earth-Moon distance measurements to within 1 millimeter, which should help study the Moon's liquid core and test general relativity, Einstein's theory of gravity[6]. The Apollo mirrors set the pattern: leave something simple and passive behind, and let observers on Earth do the hard work.
Try It Yourself: Find a Mirror That Sends Light Back, and Read the Original Reports
- Compare a hand mirror with a bike reflector. Ask an adult to help, and stay indoors or somewhere safe, away from roads and traffic. In a dim room, shine a flashlight at a bicycle reflector or reflective tape on a bag from a few steps away. Hold the flashlight right next to your cheek and look at the reflector. It glows back toward you. Now do the same with a hand mirror, tilting it a little. The light jumps somewhere else. Never point a flashlight or laser at anyone's eyes. Real Moon reflectors are made much more precisely than a bike reflector, but the idea is the same.
- Read the NASA summary. The page "Measuring the Moon's Distance" is short and mentions the dime comparison. See whether you can find the numbers used in this article.
- Look up the new reflector. Search for "NGLR" and "Blue Ghost" and see what the first measurements say.