The Most Important Thing Armstrong Left on the Moon: "Ringed by footprints, sitting in the moondust, lies a 2-foot wide panel studded with 100 mirrors pointing at Earth: the 'lunar laser ranging retroreflector array.' Apollo 11 astronauts Buzz Aldrin and Neil Armstrong put it there on July 21, 1969, about an hour before the end of their final moonwalk. Thirty-five years later, it's the only Apollo science experiment still running.

"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."