How Glass On The Moon Made GPS Possible

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The laser reflector, which features 100 triple prisms of quartz glass mirrors / NASA-Johnson Space Center

Fifty years ago, Buzz Aldrin left a device on the moon that has supported Einstein’s theory of relativity and allowed GPS as we know it to be possible. 

The quartz glass retro-reflector array was designed by the Bendix Corporation and Heraeus, a German company with a facility in Buford that is one of the few producers of this nearly indestructible material.

It was one of three scientific experiments that the Apollo 11 astronauts brought with them during the first moon landing on July 20, 1969. 

The most basic goal of this experiment was to determine the distance between earth and the moon, according to Heraeus’ Global Sales Director of Commercial Optics Dr. Todd Jaeger. 

To do this, scientists fired an infrared laser beam at the reflectors on the moon, and measured how long it took the infrared light to reflect back to earth. They then plugged the travel time into a formula that could determine the distance travelled. 

Then they explored how the distance between the earth and the moon related to their differences in gravity, and the differences in the speed of time at both locations.

What they found supported Einstein’s theory of general relativity: the smaller the distance between an object and a heavy mass, such as earth, the slower time moves for that object. This specifically means that a clock on earth would move slower than an identical clock on the moon or a satellite.

Designers were able to account for that difference in time when they made GPS, which determines a person’s location based off the time it takes for a signal to travel between them and several nearby satellites.

If the difference in time’s speed between the satellites and earth was not accounted for, GPS directions would “virtually be useless,” said Jaeger. 

Jaeger says he is especially proud of this experiment because of its longevity. It is the only piece of equipment from the Apollo 11 mission that is still functioning.

“I actually worked at NASA on the Laser Risk Reduction Program and we had trouble keeping optics rotating around the planet for more than 50 days sometimes,” he said, “So one heck of an achievement there.”

He is also excited for the future, in which new lasers may allow the distance from the earth to the moon to be calculated to within fractions of millimeters. Currently, the distance between the earth and the moon is known to within a millimeter. 

The retro-reflector also serves as the “only discernible proof” that men have been to the moon, because lasers now reflect off glass that did not exist before the moon landing.