Scientists Aims to Measure Distance to Moon in Millimeters

ByLee Dye

Jan. 17, 2002 -- 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.

By using a laser system mounted on a 3.5-meter telescope at Apache Point, N.M., and equipped with a sophisticated array of detectors that can capture and isolate every photon of light that bounces back from the moon, Murphy hopes to get the number down well below a millimeter.

The laser will blast a 1-billion-watt "bullet" of light at the moon 20 times every second. But the Earth's atmosphere will distort the beam once it leaves the telescope, so by the time it gets to the moon the beam will cover an area more than a mile wide. Murphy is hoping that at least one out of every 30 million photons hits a reflector and bounces back toward Earth. That would send about a billion photons back from each bullet.

But by the time the reflected laser beam reaches Earth, it will have spread out to nearly 10 miles in diameter, so probably only about one out of 30 million reflected photons will actually be captured by the detectors.

That data will be fed into a powerful computer, but all it will tell is the distance between the telescope and the reflector.

Factoring in Swelling Earth, Atmospheres

"The difficulty is in converting that into a useful number," Murphy says.

What the scientists want to know is the distance between the center of the Earth and the center of the moon, and there are just a whole bunch of things that make that a difficult conversion.

"The Earth swells about a foot every day, so you have to know where you are in that tidal cycle," Murphy says. "You have to know how much atmospheric delay is presented because light travels a little bit slower though air, so you better know how much atmosphere you're going through. When you have a high pressure system [in the atmosphere], the local crust is pushed down by the weight of the air."

We're talking feet in some of these cases, and the scientists are thinking millimeters.

But if they've done everything right, Murphy expects to get the measurement down to below a millimeter within the first minute of observation.

Then he can get down to the real science.

Challenging Einstein

The foundation of Einstein's theory of General Relativity is the equivalence principal, which postulates "all types of mass, regardless of composition, will accelerate or fall at the same rate in a gravitational field," Murphy says.

The measurements he hopes to collect should tell if the moon and the Earth fall toward the sun at the same rate, as Einstein predicted. If there's a difference in the rate between the two bodies, then something is really wrong with the equivalence principal.

Beyond that, the laser ranging system will work sort of like a telescope, detecting virtually every object in the solar system on the basis of its effect on the orbit of the moon.

"Even asteroids exert enough force on the moon that they should be detectable at the millimeter level," Murphy says.

And eventually, if Murphy can keep the program running for four or five years, he might be able to determine if gravity weakens as the universe expands, a proposition that has tantalized some scientists for many years.

So as a young man he stands on an interesting threshold. His experiment could turn up all sorts of surprises.

"That's the exciting thing," he says. "Any time you probe to a new level of precision like this, you never know what monsters are going to rear their heads. Sometimes, monsters are a terrible thing that you don't want to deal with, and sometimes they are very interesting."

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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