ON THE MOON: Buzz Aldrin, on the Apollo 11 mission, prepares to place the Lunar Laser Retroreflector on the moon. Designed by a Princeton University graduate student, the device is still working on the moon today, 50 years later, making it possible to measure the exact distance between the Earth and the moon. (Courtesy of Heraeus Corporation)

By Donald Gilpin

In the late 1950s, Jim Faller, a Princeton University graduate student studying gravitational physics, wrote a research paper, “A Proposed Lunar Package: A Corner Reflector on the Moon,” in which he conjectured that a lightweight reflector could be placed on the moon and laser beams could be sent from Earth and reflected back to provide a precise measurement of the distance between the Earth and the moon.

Before he turned his paper in to Professor Robert Henry Dicke, one of the country’s leading astrophysics experts, Faller wrote on the top: “Professor Dicke, would you see if this makes any sense?”

It did make sense. It gained particular relevance a decade later when Faller’s Lunar Laser Ranging Retroreflector experiment traveled to the moon with Apollo 11. And its importance endures unabated to this day. This July 20 will mark the 50th anniversary of the Apollo 11 landing, and Faller’s experiment is the only piece of original equipment still functioning at the Apollo 11 landing site.

Scientists can now measure the distance from Earth to moon with the precision of a single millimeter, and the experiment’s success led to the development of Global Positioning System (GPS) technology.

Faller, professor emeritus at the Joint Institute for Laboratory Astrophysics (JILA) of the National Bureau of Standards and the University of Colorado, Boulder, described how in 1968 “the Apollo program at that point needed luck. Both our work and the Apollo program were lucky.”

In 1963 Faller had joined JILA in Boulder and established a Lunar Laser Ranging Experiment team to expand and advance his idea. Faller’s team proposed their Lunar Laser Retroreflector (LLRR) project in 1964, and, Faller noted, “the prospects were still in doubt in the fall of 1968, but we still continued to work.”

Size, weight, speed, and simplicity became crucial factors for consideration. “The astronauts had limited time to spend on the lunar surface to position the array aiming back towards the Earth,” Faller said. The LLRR was accepted and would later to become what Irwin Shapiro, then director of the Harvard-Smithsonian Center for Astrophysics, called “NASA’s most cost-effective experiment.”

Jeffery Oddo, senior manager, communications, for Heraeus Corporation, which provided the materials used to make the reflector, noted that “If this experiment had not been successful, the GPS we have today would not have been developed so rapidly. We might still be relying on those paper road maps.”

Heraeus’ fused silica used in the LLRR is a special material from quartz glass manufacturing which can withstand the harsh conditions of space and maintain its properties over time.

“We take for granted that GPS just works,” said Heraeus Global Director of Optics Todd Jaeger. “But if we hadn’t been able to measure this distance and verify Einstein’s theory of relativity, we wouldn’t have been able to apply that knowledge to the atomic clocks that are built into the GPS systems.”

Jaeger claimed bragging rights for Heraeus, a German-based company with regional headquarters in Yardley, Pennsylvania. “The Lunar Laser Retroreflector is the only still functioning equipment at the Apollo site. This is a testament to the quality of the fused silica that Heraeus produces.”

The array of 100 fused silica reflector mirrors on the LLRR enables scientists to send laser beam pulses to the moon and calculate distance based on the time it takes for the reflected laser beam to return to Earth.

“It worked for 50 years and is still working,” said Faller. “The transparency isn’t quite as good because lunar particles, small pieces of dust, have landed on these corner cubes, so over 50 years it’s gone from say 100 percent transmission to 10 percent transmission, but lasers have become 10 times brighter so that makes up for that.”

Fifty years later, the reflector continues to perform crucial studies of gravity, gathering information that has helped to answer important questions about the Earth and the moon.

For skeptics, the Lunar Laser Retroreflector, with its 100 triple prisms made of high-tech fused silica, still on the moon, still functioning, is also the single best proof that Apollo 11 was actually there on the moon.