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.
Targeting the mirrors and catching their faint reflections is a challenge, but astronomers have been doing it for 35 years. A key observing site is the McDonald Observatory in Texas where a 0.7 meter telescope regularly pings reflectors in the Sea of Tranquility (Apollo 11), at Fra Mauro (Apollo 14) and Hadley Rille (Apollo 15), and, sometimes, in the Sea of Serenity. In this way, for decades, researchers have carefully traced the moon's orbit, and they've learned some remarkable things, among them: (1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible. (2) The moon probably has a liquid core. (3) The universal force of gravity is very stable. Newton's gravitational constant G has changed less than 1 part in 100-billion since the laser experiments began.