In "Dark Energy: The Biggest Mystery in the Universe" from the April issue of Smithsonian, writer Richard Panek describes an experiment that measures the distance between the Earth and the Moon:
Twenty times a second, a laser high in the Sacramento Mountains of New Mexico aims a pulse of light at the Moon, 239,000 miles away. The beam’s target is one of three suitcase-size reflectors that Apollo astronauts planted on the lunar surface four decades ago. Photons from the beam bounce off the mirror and return to New Mexico. Total round-trip travel time: 2.5 seconds, more or less.
There are actually five retroreflectors on the Moon: three placed by Apollo astronauts and two that sit atop Soviet rovers. But only one of the Soviet reflectors could be used by astronomers; the other, on board Lunokhod 1, the first robotic rover to traverse the Moon, was lost in 1971. Over the decades astronomers occasionally looked for the rover but without success. Even with the best telescopes it's difficult to spot something from here on Earth that's less than eight feet in length; the atmosphere interferes. And Hubble and other space telescopes aren't able to image the Moon in detail either.
But last month the Lunar Reconnaissance Orbiter imaged the area around Lunokhod 1's landing site, and a tiny glint could be seen, miles from where scientists had been looking all those years. Was it Lunokhod 1?
Astronomers at the Apache Point Observatory in New Mexico aimed their laser at the glint last week (when the Moon was finally in the right position) and received a return signal. Jackpot! A second observation let them triangulate the reflector's latitude and longitude. It turns out that the wait may have been worth it: Lunokhod 1 is better positioned for the laser experiments than Lunokhod 2, which does not work well when the Sun shines on it. Lunokhod 1's return signal was nearly three times as strong as that of Lunokhod 2.