Before 13 September 1959, no human-made object had ever touched an extraterrestrial body. On that date, Luna 2, the Second Soviet Cosmic Rocket, impacted the moon. Since then, numerous other spacecraft and various humans have visited our satellite.
All told, nearly 500,000 pounds of Earth stuff adorns the surface of the Moon.
Some of the alien debris is logistical (heavy ships and tools), some symbolic (plaques, statues, photographs), some goofy (golf balls), and some downright gross (bags of human urine and feces, anyone?). Though leaving most of the refuse was a practical necessity, we did place a few non-metaphoric entities in hopes of continuing the great scientific experiment in the sky.
Perhaps the most important items were mirrors.
Neil Armstrong and Buzz Aldrin left a retroreflector array on the Moon during the Apollo 11 mission. A retroreflector is a mirror that sends light back to its source with minimum scattering. The crews of Apollo 14 and 15 each left another array on the surface. The Soviets deployed two arrays during their rover missions, called Lunokhod 1 and Lunokhod 2.
Why leave a series of mirrors on the Moon?
So we can shoot it with lasers!
Scientists have long beamed light waves at the Moon. As early as 1940, the British theorized the satellite could be used for “passive communications.” Radio and microwaves, they figured, could leave Earth, hit the moon, and bounce back to a receiver somewhere else on Earth. In 1943, a German scientist successfully sent radio waves to the Moon and back, becoming one of the first celestial radar usages. Radar stands for radio detection and ranging. We call this type of interplay Earth-Moon-Earth Communication or Moon Bounce.
By 1962, scientists had moved from basic radio waves to lasers. The focused beams of a laser allowed astrophysicists to make accurate measurements about the moon. We might associate radar more with the speeds a police officer snags on our automobiles, but the “ranging” aspect is the real crux of the technique. By timing how long it takes for a beam to hit an object and then return, we can measure the distance – the range – between the two objects. Fancy machines make calculations to figure out how fast you were speeding.
After they applied the laser technique in 1962, a Princeton University graduate student named James Faller suggested placing special mirrors on the Moon to further improve the accuracy of the ranging. If we could drop corner reflectors on the surface, we could produce astonishingly precise measurements. Unlike regular mirrors, which need to be exactly perpendicular to a light source to reflect that source back to itself, corner mirrors work mathematical magic to remove the angle of incidence problems.
Faller’s fantastic notion had one issue: it was 1962 and humans had never been to the Moon.
Nearly a decade later, we rectified that shortcoming. Now, scientists had bullseyes for which to aim. Form and function are often two different beasts, however. Aiming a laser from Earth to a tiny array on the Moon is no easy feat. Despite the coherent beam of a laser, by the time the light reaches the moon the beam is four miles wide. That width sounds great for hitting a target, but it’s still extremely difficult. Further, even if scientists hit the arrays, the reflected information we receive is infinitesimal. According to NASA, under good atmospheric conditions, sensors on Earth might receive a single photon every few seconds. Without sensitive, finely tuned equipment, that sort of data could easily be missed.
Fortunately, we have wonderful theorists, engineers, and computers at the helm. Since the advent of the Lunar Laser Ranging Experiment, we have acquired a slew of important information. It takes 2.5 seconds for light to go from here to there and back to here, but, over 50 years, we’ve learned a lot via repetition.
We know the average distance to the Moon is 240,000 miles (average because the satellite is not at a fixed distance from us). That measurement is precise to within a millimeter! We have also discerned that the two bodies are separating. The distance between the pair grows about 1.5 inches a year, about as much as your fingernails.
The measurements have also provided evidence of secondary attributes. Scientists used information from the lasers to determine the core of the Moon is liquid. The experiment proved one of the world’s greatest scientists correct, as well. Einstein’s General Theory of Relativity predicted the Moon’s orbit. When we applied the laser information to Al’s prognostication, his mathematical work was impeccable.
These days, the arrays on the Moon seem to produce far less reflected light than they should. Some scientists speculate they might be covered with dust. If we want to keep tabs on gravity and the Moon’s distance from us, perhaps we should send someone there with a broom!
BONUS FACT: The Perseverance Rover. which landed on Mars in 2020, brought along a retroreflector array. You don’t have to go to Mars to see one, though. Your bicycle is probably outfitted with a few!
Further Reading and Exploration
Laser Beams Reflected Between Earth and Moon Boost Science – NASA
MEASURING THE MOON’S DISTANCE – NASA
Shooting Lasers at the Moon: Hal Walker and the Lunar Retroreflector – Smithsonian National Air and Space Museum
Why Is the Apollo Reflector Experiment Still Operating, 50 Years Later? – Space.com