How High the Moon

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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 it 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'sdespite 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. 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. Murphy’s primary method will be something called "laser ranging."Light from a laser fired from the Earth could hit a reflector on the surface of the moon 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. 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. 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 protons 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.