The proof of the LRRR mirrors...
Of all the scientific experiments in the ALSEP package deployed on the lunar surface during the Apollo program, the Lunar Laser Ranging Retro Reflector (LRRR) is an extraordinary exception: it is the only one still working today, passive but relentlessly precise and it is overwhelming, unassailable physical proof of the crewed landings of the 1960s and '70s. These are not simple flat mirrors, which would require perfect perpendicular alignment to reflect a beam back to Earth, but complex panels made of fused silica prisms cut as "cube corners" (corner-cube retroreflectors). This specific and ingenious optical geometry ensures that every incoming photon undergoes three total internal reflections and is sent back exactly along the same vector it came from, regardless of the original angle of incidence . To withstand the brutal lunar temperature swings without warping and losing their optical perfection, the prisms were set in aerospace aluminum frames carefully designed for heat dissipation. There are currently five of these devices on the Moon: three placed by hand with extreme precision by the crews of Apollo 11 in the Sea of Tranquility, Apollo 14 at Fra Mauro and Apollo 15 (the largest and best-performing panel) at Hadley Rille, plus two French-made retroreflectors mounted on the Soviet robotic rovers Lunokhod 1 and 2. Interrogating these devices from Earth is no trivial task that can be done with a commercial laser: it requires a professional scientific infrastructure. Specialized astronomical observatories, such as the Apache Point Observatory in New Mexico or the Observatoire de la Côte d'Azur in France, fire laser pulses lasting a few picoseconds with peak powers in the gigawatt range through large-diameter telescopes. Because of optical divergence and turbulence in Earth's atmosphere, the laser beam spreads until it reaches a diameter of a couple of kilometers when it hits the lunar surface. Only an infinitesimal fraction of this light strikes the small LRRR panels and the return beam spreads out further on the way back. The physics of this operation is so extreme that, out of an original pulse of hundreds of millions of billions of photons, the observatory on Earth is lucky if it manages to catch even a single one coming back. It is precisely this technical detail that demolishes the objections of those who claim the lasers are simply bouncing off the lunar dust: regolith is highly reflective but diffusely so, meaning that it scatters light in all directions.
If an observatory aims its laser even just a few hundred meters away from the exact coordinates of the Apollo sites, the return signal instantly drops to zero, swallowed by background noise. Only by hitting the exact coordinates where the astronauts placed those geometrically perfect arrays do you get the return photon peak, which makes it possible to calculate the light's travel time precisely enough to measure the Earth-Moon distance to within less than a millimeter! Thanks to this continuous laser bombardment, carried on without interruption for over half a century, science has made landmark discoveries: we have learned that the Moon is moving away from us by 3.8 centimeters a year because of tidal friction, we have confirmed with staggering precision Einstein's Strong Equivalence Principle of General Relativity and, by analyzing the tiny physical wobbles of our satellite, we have inferred the existence of a lunar core that is still partly fluid.
The retroreflectors are therefore the final nail in the coffin of every denialist theory: anyone with a suitably sized telescope, a pulsed laser and a single-photon detector can aim their instruments at the landing sites and receive an unequivocal physical answer! Those optical devices are there, placed with extreme precision by human hands in very specific locations and they keep reflecting laser light from Earth like silent beacons in the void, bearing cold, mathematical and unfalsifiable witness to the truth of the greatest exploration in human history.
