The Lunar Retroreflector was invented by Jim Faller.

On July 21, 1969, the ascent stage of Apollo 11’s lunar module, with Neil Armstrong and Buzz Aldrin aboard, lifted off from Tranquility base to begin its journey back to Earth. Ever since that day the site where astronauts first walked on the moon has remained undisturbed.

The first men on the moon left behind more than footprints. In addition to the famous American flag, the astronauts left the descent stage of the lunar module, a golden olive branch, an Apollo 1 mission patch in honor of the three astronauts who died in a command module test, discarded tools including a hammer, sample scoops, scales, lunar overshoes, empty food packets, a TV camera, airsickness bags, armrests, and more than 100 other items.

During the first moonwalk the astronauts also deployed a number of experimental instruments designed to gather information about the moon. Since 1977 these machines have been powered down, their radios silent, no longer collecting data and sending it back to Earth.

To this day there is one, and only one, piece of equipment from the Apollo 11 mission still functioning half a century later. It is essentially a mirror — an array of 100 “corner cube”-shaped reflectors pointed toward Earth that simply reflects laser light back to the Earth. The Lunar Laser Rangefinder Experiment was invented by a Princeton student and made by Heraeus Incorporated, a company that has an office in Yardley. It has provided precise measurements of the distance between the Earth and the Moon, allowing scientists to test Einstein’s theory of general relativity and refine their understanding of gravity. With modern lasers rangefinders can measure the distance to the moon down to the millimeter.

The origins of the experiment go back to the 1950s, when Jim Faller, a graduate gravitational physics student at Princeton, wrote a research paper called “A Proposed Lunar Package: A Corner Reflector on the Moon.” The paper described how a reflector would allow scientists on earth to shoot a laser beam at the moon at a reflector, measure exactly how long it took the beam to return to Earth, and get a precise measurement of distance.

After President Kennedy committed the nation to landing on the moon in a 1961 speech, Faller’s idea became a real possibility. The Apollo missions would have the chance not only to claim the glory of being the first to the moon, but to use their time there to set up useful scientific experiments. Numerous scientists proposed ideas for experiments to be carried out on the lunar surface. But as the first mission was being planned it became apparent that the two astronauts would only have a very short window of time — planners were unsure how long the spacesuits could keep the astronauts cool. As it happened, the first moonwalk lasted just two-and-a-quarter hours.

This proved to be an advantage for Faller’s idea, since it was quick and easy to deploy. All the astronauts had to do was take the cover off of the array and set it down facing Earth. They could level it by kicking dirt under the corners. “The only way they could have gotten it wrong was putting the reflector wrong side up,” says Todd Jaeger, global sales director for commercial optics for Heraeus.

It only took a few minutes for Armstrong and Aldrin to set up the array, and it has been there ever since, faithfully serving as a target for laser beams along with two other reflectors from subsequent Apollo missions and two small reflectors on unmanned Russian space probes.

Anyone on earth with a powerful pulse laser, a photon multiplier tube, and a telescope with a three- and-a-half-meter primary mirror can get a reading from the lunar retroreflector. (A recent episode of “The Big Bang Theory” showed scientists performing this experiment from a rooftop, but it’s pretty expensive for a homebrewed experiment — a telescope that size costs around $14 million, and renting time on one isn’t cheap, either.)

Heraeus was just one company out of 20,000 organizations that contributed work on the Apollo program. There were several others in the Princeton area. RCA Astro Electronics in East Windsor built the remotely operated color TV cameras used for the last three Apollo missions. Engineers working for inventor Abram Nathaniel Spanel, working out of Drumthwacket, Spanel’s Princeton estate, designed the spacesuits the astronauts wore.

Jim Faller

Heraeus, based in Germany, specialized in making a material called fused silica. Fused silica is made from molten quartz sand and, unlike traditional glass, contains no other ingredients. Though more expensive to manufacture than other forms of glass, it contains far fewer impurities and is used in applications that call for high performance. It is used today, as it was then, for high-end optics such as telescope lenses, sensors for military aircraft, and fiber-optics.

For the retroreflector, NASA selected Heraeus’s fused quartz even though it was manufactured outside the United States. “Not just any transmissive material would have worked,” Jaeger says. “A standard glass, something melted from sand and soda lime, you would get metallic impurities.” This material would have darkened over time as it was exposed to solar radiation on the moon’s surface, turning it yellow and eventually black instead of crystal clear. The “corner cube” shape was used instead of a flat mirror because it reflects light directly back at the source instead of scattering it in all directions like a flat mirror does.

But if the retroreflector is basically just a mirror, why was it so important to make it as pure as possible? After all, laser light can bounce off just about any surface. In fact, scientists had bounced laser light off the moon and gotten a signal back as early as 1962.

These early experiments were not ideal for determining the Earth-Moon distance because scientists could not tell exactly where their laser beam had hit. The laser return could have been bouncing off a mountain top or a crater bottom hundreds of feet apart; there was no way to tell.

Another reason for using the high-end material is the dispersal of the laser. Over short distances a laser looks like a pinpoint beam. But the highly collimated laser light spreads out in a cone shape over long distances. By the time a laser beam reaches the moon about 238,900 miles away, it is a circle around two kilometers wide. (Even with this wide beam, hitting the reflector is no easy task. Scientists have described it as like hitting a moving dime with a rifle two miles away.) When it bounces back to Earth the laser beam is now 15 kilometers across and far too weak to see with the naked eye.

When the pulse of light is shot at the moon, it is extremely powerful, with about a quadrillion photons. By the time it gets back to the sensor that is down to a single photon. If the original signal were made of grains of sand instead of photons, it could make a beach 1,000 kilometers long, 30 kilometers deep, and 30 kilometers wide. The returning photon would be a single grain of sand, Jaeger says.

Fortunately, as the reflector has become weaker, lasers have become more powerful and able to fire in shorter bursts. This has allowed scientists to measure the distance to the moon with ever greater precision. These ongoing experiments have revealed some interesting facts: For one thing, the moon is moving away from the earth at a rate of three centimeters per year.

Another is that it has provided proof that Einstein’s theory of general relativity is correct. Einstein had predicted that spacetime would be warped by gravity. Previous astronomical experiments had confirmed that this was the case by observing light bending around planets, but the laser retroreflector provided the most precise proof up to that point.

Although the effects of gravity on spacetime may seem small at first, they are important when it comes to spacecraft, particularly GPS navigation satellites, which work by measuring how long it takes for radio signals to travel from GPS devices to satellites in orbit. Because of general relativity, time is passing more slowly for the satellite relative to the earthbound clock 12,000 miles away. The difference is only 38 millionths of a second, but the system relies on incredibly precise timing. Without taking relativity into account, GPS devices would be wildly inaccurate.

As lasers improve more and more, the retroreflectors are actually being used to find flaws in the theory of general relativity. While it is indisputably precise enough for the purposes of GPS satellites, quantum physicists believe further experiments may reveal tiny effects that Einstein’s theory does not account for.

While it has served faithfully for 50 years, the retroreflector is showing signs of age. Over time it has become dulled. The most likely culprit is moon dust, which is kicked up every time a tiny meteor hits the moon’s powdery surface.

But there might be a fix on the horizon — Jaeger says future astronauts from NASA’s planned upcoming moon missions might dust off the reflector if they return to the Apollo 11 site.

Heraeus Incorporated, 770 Township Line Road, Suite 300, Yardley, PA 19067. 215-944-9981. www.heraeus.com

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