What is lunar ranging and how does it work? JILA's Dr. James Faller explains.
“Houston, Tranquility Base here. The Eagle has landed,” Neil Armstrong said as the Lunar Module landed on the moon, July 20, 1969.
While the world cheered, Dr. James Faller was running around Lick Observatory in the mountains near San Jose, California, anxiously waiting to run his own experiment. Getting to the moon was already a monumental achievement for science. But for Faller and other scientists back on Earth, the mission was only just beginning as they waited for Buzz Aldrin and Neil Armstrong to deploy their experiments on the lunar surface.
In the 50 years since the moon landing, most of those experiments have stopped working. Except one: Faller’s Laser Ranging Retroreflector, or LRR. Faller had joined JILA, then the Joint Institute for Laboratory Astrophysics, as a fellow.
The LRR is really fairly simple, Faller explained. It’s an array of corner cubes. These fused silicone cubes have one flat surface; the opposite side are three flat surfaces intersecting at perpendicular angels, which gives it a unique property.
“I always say it’s the Mona Lisa of optics,” Faller said. No matter where you stand in the Louvre, the eyes of the Mona Lisa seem to follow you. The corner cube sends light directly back to their source.
Faller thought up the lunar ranging experiment when he was a graduate student at Princeton. Put an array of corner cubes on the moon. Fire a beam of light at it, and time how long it take the signal to return to Earth. Knowing the speed of light, you could then calculate the exact distance from Earth to the moon.
“Does this idea make any sense?” wrote young Faller on his thesis paper. His thesis advisor Dr. Robert Dicke wrote back, “Maybe we can talk about it sometime.”
With the Apollo mission in development, Faller finally had an opportunity to get his experiment to the moon.
“It was simple physics, simple electronics, almost simple optics, but performed over an enormous distance,” Faller said.
The experiment almost didn’t happen. Faller’s experiment didn’t get moved up the priority list until a year or so before the launch. Mission leads began to worry about the amount of time astronauts needed to spend on the surface setting up different possible experiments. The LRR fit the bill. A hundred or so small corner cubes fit in an array the size of a briefcase. All the astronauts had to do was make sure it was level and pointed toward Earth.
As televisions broadcast the landing, Faller waited for the astronauts to report their exact position so he could start lunar ranging. But as the first men on the moon, Armstrong and Aldrin weren’t exactly sure where they were either, at least not within 10 miles. So Faller and his students began hunting and pecking with their laser trying to hit their target on the moon.
“Looking for this thing, which about this big [about the size of a briefcase] within a 10 mile-square on the moon, which is pretty far away, was not a trivial exercise,” Faller said. His former advisor, Dicke, came to the observatory to watch Faller’s ranging experiment play out, and was there when the signal finally bounced back to Earth.
The accuracy of lunar ranging has significantly improved since the first measurements in 1969. Scientists are now able to measure the distance to the moon down to the millimeter – on average 247,692 miles. More importantly, scientists could now accurately measure how that distance changes over time. We learned that the moon is pulling away from Earth at a rate of about 1.5 inches (about 38 millimeters) per year. Judging how its precise distance changes over time also tells us that the moon likely has a liquid core.
The last working experiment on the moon from Apollo 11 is the Laser Ranging Retroreflector. But it doesn't work quite as well as it once did. Its creator, JILA Fellow Adjunct Dr. James Faller, reflects on how it could be restored to its former glory.
Those changes also tell us how gravity affects objects in orbit, and how gravity affects time. These observations confirmed part of Einstein’s theory of general relativity: massive objects distort space-time, which is felt as gravity.
That information is not just for theoretical astrophysics. It has practical applications to modern space exploration. GPS satellites orbiting Earth have to adjust for the slight change in time based on their distance from the center of Earth’s gravity. Without that knowledge, they would give you the wrong position, causing a position error that would increase by about 10 kilometers every day.
While other retroreflectors have since been placed on the moon, the LRR from Apollo 11 is still in use today. Every so often, Faller looks up at the moon from his home in Boulder and thinks about his experiment up there. Getting it there took more just science, he reflected.
“Bob Dicke used to say that he felt he gave us some luck. Science also contains the need for luck, Faller said. “The Apollo program at the point needed some luck. And both our work and the Apollo program were lucky.”