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  • To the moon and back, many times a minute

Photonics Spectra
Jul 2009
Hank Hogan,

FORT DAVIS, Texas – The moon is spiraling away at 3.8 cm, or about an inch and a half, a year. But don’t worry. Given that it sits at a distance of about 385,000 km, or just under 240,000 miles, there will be no noticeable change in its appearance for a long, long time.

For almost 20 years from the late 1960s, astronomers measured the Earth-to-moon distance by beaming a laser from the 2.7-m McDonald Observatory Smith Telescope to retroreflectors placed on the moon. Courtesy of McDonald Observatory.

Detecting that slight lunar movement has been possible because of photonics technologies and the only Apollo experiment still running. For almost 40 years, scientists have periodically pinged the moon with a laser beam, measuring how long light takes to make the round trip.

The answer, for the curious, is about 2.5 s, varying because the moon’s elliptical orbit results in a constantly changing distance. From precise measurements of that number, researchers have confirmed not only a key tenet of Einstein’s theory of relativity, but have shown that the force of gravity is very stable and have discovered that the moon probably has a liquid core.

Earth-moon measurements are now preformed with a dedicated telescope, the McDonald Laser Ranging Station. Photo by Kathryn Gessas, McDonald Observatory.

The last is a consequence of an early and unexpected finding, said Peter J. Shelus, project manager and principal investigator for the McDonald Observatory and Center for Space Research Laser Ranging Program at the University of Texas at Austin, a job he has held for almost four decades.

“The rotation of the moon is a lot more complicated than originally thought,” he said, adding that, once it was better understood, scientists got a bonus from the lunar laser ranging experiment. “It’s a probe into the interior of the moon.”

The finding involving relativity arises because the experiments have confirmed that gravitational and inertial mass are equivalent, at least to current accuracy levels. That proves that Albert Einstein’s formulation of relativity is correct.

Measuring the distance from Earth to the moon is dependent upon retroreflectors left on the surface by Apollo astronauts and by two Russian robot vehicles. Researchers on Earth hit the reflectors with a laser pulse, a feat that has been likened to successfully targeting a moving dime with a rifle at a distance of 3 km.

The retroreflectors send the pulse back to Earth. Usually only one or, at most, a few photons make it back to be collected and detected to provide the total trip time. That and the speed of light yield the distance to the moon to an accuracy of a few centimeters or better, depending on the equipment.

Today, the McDonald measurements are done on a dedicated system, one that also does ranging to orbiting artificial satellites. This setup enables cost sharing and avoids the scheduling conflicts of a larger system. The drawback is a lower data rate because of smaller collecting optics.

Besides McDonald, two other sites collect lunar laser ranging data, one in New Mexico and the other in France. The latter has been off-line for years while undergoing an upgrade, and the former has scheduling constraints.

As for how long the experiments can go on, Shelus noted that the retroreflectors should be observable for years to come. The science itself, he added, gets better with each additional year of observations.

Given all that, there is no technical reason not to continue the lunar distance measurements for another two score years or more, he said. “They can go on as long as there is financial support – and the interest – to do so.”

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