Tuesday, July 21, 2009

The Apollo experiment that's still working

The experiment that Apollo 11 left on the Moon are still working, The Lunar Laser Ranging Experiment. "it studies the Earth-Moon system and returns data to scientific centers around the world, including NASA's Jet Propulsion Laboratory".

Three things the experiment has proved:
-The Moon is moving away from the Earth
-The Moon has a fluid core
-Einstein's Theory of Relativity is accurate

University of Maryland physics professor Carroll Alley was the project's principal investigator during the Apollo years, and he follows its progress today. "Using these mirrors," explains Alley, "we can 'ping' the moon with laser pulses and measure the Earth-moon distance very precisely. This is a wonderful way to learn about the moon's orbit and to test theories of gravity."

Here's how it works: A laser pulse shoots out of a telescope on Earth, crosses the Earth-moon divide, and hits the array. Because the mirrors are "corner-cube reflectors," they send the pulse straight back where it came from. "It's like hitting a ball into the corner of a squash court," explains Alley. Back on Earth, telescopes intercept the returning pulse--"usually just a single photon," he marvels.

The round-trip travel time pinpoints the moon's distance with staggering precision: better than a few centimeters out of 385,000 km, typically.

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