Scientists Aims to Measure Distance to Moon in Millimeters

Tom Murphy wants to know how far it is to the moon. Not just how many miles, or how many feet, or even how many inches.

He wants to know to within a millimeter, and that's less than a 25th of an inch. And he wants to know exactly how far it is every moment of every day. That's despite the fact that the distance changes minute by minute because the moon is in an elliptical 28-day orbit ranging from 220,000 to about 252,000 miles from Earth.

To achieve his goal, the University of Washington postdoctoral researcher in physics and astronomy will use the latest in laser technology, a large telescope in New Mexico, a team of other experts, a little funding from NASA, and a whole lot of luck.

The stakes are high. If it doesn't work, he could spend the rest of his life trying to live down "Murphy's folly." If it does, he might disprove part of Einstein's theory of General Relativity, or he might discover evidence of an unseen celestial body in our solar system, either of which would earn him a page in the history books.

Bouncing Lasers Off the Moon

But here's a bit of irony. Murphy doesn't really give a rip exactly how far it is to the moon.

"We're not interested in the distance itself," he says. "It's just a boring, arbitrary number."

But that number, he says, could be used to learn much about one of the most mysterious forces in the universe: gravity.

What Murphy is really doing is using the Earth and the moon as his laboratory, because the questions he is asking can't be answered in an ordinary lab. The tools just aren't big enough.

To succeed, he and his colleagues will have to pull off an experiment that requires a level of precision that is almost beyond imagination. It's a little like standing in Los Angeles and shooting a fly out of the sky over New York City.

Their primary method will be something called "laser ranging," and it wouldn't be possible without the foresight of people like Kenneth Nordtvedt. In the 1960s, Nordtvedt, a physicist at Montana State University, came up with an intriguing idea. He reasoned that a series of reflectors on the surface of the moon could be helpful in determining the exact distance between the moon and the Earth.

Light from a laser fired from the Earth could hit a reflector and bounce back. The time it took for the light to travel from Earth to the reflector and back would reveal the exact distance, or at least within a few inches, Nordtvedt argued. That would reveal much about the lunar orbit, and that data could in turn be used to test some of the tantalizing ideas in Einstein's theories.

His idea was convincing enough that NASA sent three arrays of 100 to 300 prisms, called retroreflectors, to the moon during the Apollo program. The prisms reflect light back to its point of origin, and each array is about the size of a suitcase. Two other arrays were delivered to the moon by Soviet unmanned missions.

The retroreflectors allowed scientists to pin down the distance to the moon to within about 10 inches by the early 1970s, and that number was reduced to less than an inch within a decade.

More ‘Bullets’ for More Precision

But that isn't good enough to carry out the kind of research that Murphy and his colleagues want to do.

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