This summer Americans have Apollo fever. July 20, as most everyone knows, is the 50th anniversary of the Apollo moon landing, when astronauts Neil Armstrong and Buzz Aldrin became the first humans to set foot on the moon. Many people, including some of our most eminent historians, have called it the greatest achievement in the history of mankind.

But when we think about Apollo and attempt to place it here on earth, we typically think about Apollo Mission Control Center in Houston, Texas. Who can forget Neil Armstrong’s famous words: “Houston, Tranquility Base here. The Eagle has landed.”

So what did this experiment accomplish? A great deal, it turns out.

Or maybe we think of the fiery liftoff of the Saturn V rocket from Kennedy Space Center on Merritt Island, Florida. And in the Northeast, many are also aware of the Massachusetts Institute of Technology’s huge contribution to the software written for the Apollo mission. A wonderful new book about the Apollo mission by Charles Fishman titled One Giant Leap: The Impossible Mission That Flew Us to the Moon, discusses at length the challenges faced by MIT computer programmers in the very early days of software development.

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But few people probably realize that Southern California can lay claim to a lot of Apollo 11 history. Where, you might ask? First of all, there is Downey.

North American Aviation factory in Downey, California

Both the New York Times and KCRW have interesting stories about how the Apollo 11 command module and the accompanying service module were actually built on a 160-acre swath of land in Downey called the North American Aviation factory. The site is now home to a strip mall with a Wal-Mart, TJ Maxx and 24-hour Fitness. Turns out, too, that the camera systems that flew on Apollo were also developed in Southern California, as was the second stage of the gigantic Saturn 5 rocket. The backstory of 84-year-old Shelby Jacobs, who helped develop the camera system that captured iconic scenes of the separation of the first and second stages of the Saturn 5 rocket, is heart-breaking. Jacobs, who is black, couldn’t live in Downey, as most black NASA employees lived in Watts.

But there’s more to California’s role in Apollo. In La Canada Flintridge, home of the Jet Propulsion Laboratory, one of the most important experiments of the whole mission was developed.

The Apollo 11 Laser Ranging Retroreflector Experiment was one of several major scientific endeavors that were part of the epic space mission. Developed and monitored at JPL, it consisted of a reflector that was positioned on the moon by Armstrong and Aldrin and aimed back at Earth. Lasers here on Earth fired pulses of concentrated light that were then reflected and detected by special receivers on the ground. The laser reflector consisted of 100 fused silica half-cubes, called corner cubes, mounted in a 46-centimeter (18-inch) square aluminum panel. Later, on Apollo 14 and 15 missions, two additional reflectors were set up on the surface of the moon.

“It’s very exciting,” says Turyshev. “This is the longest continuing experiment in the history of space science.” Slava G. Turyshev, NASA’s Jet Propulsion Laboratory

The reflectors are too small to be seen from Earth, and the task of actually hitting them was a major technical challenge. Even though a laser is highly concentrated light, by the time the light reaches the moon, the beam is roughly 2.5 kilometers (1.5 miles) wide, and detecting the light that reflected back was very difficult. Scientists back then likened the effort to using a rifle to hit a moving dime two miles away. Like the moon landing itself, it was audacious.

“It was a very clever experiment that changed the way we think about the moon,” Slava G. Turyshev, an astrophysicist at JPL involved with instrument design for the new generation of lunar laser operations, told California Science Weekly.

Laser Ranging Retroreflector – NASA

After the laser beam bounced off a reflector (it takes about 1.3 seconds for light to make the round trip), it came back to earth and was detected by ranging observatories in Texas, Hawaii and France, that use extremely sensitive amplification equipment. Even under the best atmospheric viewing conditions, just one photon hits the observatory every few seconds. But that one photon tells us a lot.

So what did this experiment accomplish? A great deal, it turns out.

Before the experiment, we had a decent idea how far the Moon is from Earth within a few feet. But based on data gathered from the Lunar Laser Ranging Experiment, we were able to determine that distance with amazing precision, down to under 5mm. The level of accuracy, said Dr. Jean Dickey, who was the Jet Propulsion Laboratory team investigator at the time (and who died in 2018), “represents one of the most precise distance measurements ever made. The degree of accuracy is equivalent to determining the distance between Los Angeles and New York to one-fiftieth of an inch.”

Lunar laser reflector on the moon. Credit: NASA

The experiment also verified Einstein’s theory of relativity, which states that all bodies fall with the same acceleration regardless of their mass. It determined that there are small scale variations in the length of an Earth day, changing by about one-thousandth of a second over the course of a year. The changes are caused by the atmosphere, tides, and the Earth’s core. The experiment also discovered that the Moon probably has a liquid and a solid core comprising some 20% of the Moon’s radius.

We all know that the moon causes tides on earth, but it turns out the tides also have a direct influence on the moon’s orbit, and measurements from the Lunar Laser Ranging Experiment showed that the moon is receding from Earth at a rate of about 3.8 centimeters (1.5 inches) per year. “That’s a big number, says Turyshev. “Initially, it puzzled people, but now it’s well understood and its an effect we’d expect.”

The reason that the moon is receding from earth has to do with what’s known as tidal energy dissipation. It takes gravitational energy to hold the moon in place, and over time, some of that energy dissipates, causing the moon to slowly slip away. That’s no cause for alarm in our lifetimes, but over millions of years, it means there will be an impact on the Earth’s tides and more.

Each of the two laser arrays that were left on the moon during later Apollo missions improved on the one that came before it. In fact, the array left behind by Apollo 15 is three times the size of the array left by Apollo 11. The Russians also placed two laser arrays on the moon called Lunokhod 1 and Lunokhod 2 in 1973.

The arrays from those Apollo missions and the Russian landers continue to provide valuable data to scientists here on earth, including tests of general relativity and other theories of gravity. New measurements are taken every year by several countries, all of whom regularly take ranging measurements for different experiments. JPL’s Turyshev estimates that more than 25,000 final range measurements have been taken since 1969.

“It’s very exciting,” says Turyshev. “This is the longest continuing experiment in the history of space science.”

And just in time for this year’s Apollo 11 anniversary, more arrays are being developed at JPL and elsewhere that will fly on upcoming lander missions to the moon as part of NASA’s ambitious Artemis program. The old reflectors are still working, says Turyshev, but they are losing their reflectance, and advances in laser technology and materials science will allow future reflectors to be a thousand times more powerful than the ones they replace.

NASA says it hopes to put a man on the moon by 2024 and have a sustained presence there by 2028, but several lander missions are planned sooner than that, and NASA expects each one may carry a laser ranging device designed to bring a flood of new science back to earth.

“We’re in a renaissance [of ranging technology],” says Turyshev.