The moonwalking crew of Apollo 11, which landed on the moon 50 years ago this month, put special retroreflectors on the lunar surface, as did the late

Why Is the Apollo Reflector Experiment Still Operating, 50 Years Later?

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2021-05-30 20:30:05

The moonwalking crew of Apollo 11, which landed on the moon 50 years ago this month, put special retroreflectors on the lunar surface, as did the later crews of Apollo 14 and 15, in 1971. (Another retroreflector, built by the French, sits on the Soviet Lunokhod 2 rover that landed without a crew in 1973.)

The NASA experiment, called the laser ranging retroreflector, is "a special type of mirror with the property of always reflecting an incoming light beam back in the direction it came from," explained the Lunar and Planetary Institute (LPI) in a statement. And the reflector is key for measuring the distance between the Earth and the moon, the institute added.

The way it works is an observatory — quite often the McDonald Observatory in Texas, although many others have participated — shoots a laser beam at the moon. "They remain tightly focused for large distances," the institute explained, although there is some dispersion. By the time the beam bounces off the mirror and returns to Earth, it has widened to about 12 miles (20 kilometers) in diameter. This beam dispersion makes it more difficult to view the reflection.

The reflector itself doesn't require any power, which is why it's still operating decades after other instruments on the moon fell silent. This instrument includes 100 "corner cubes" (or fused silica half-cubes), which are placed in an 18-inch-square (46 centimeters square) aluminum panel, according to NASA's Jet Propulsion Laboratory (JPL). It's due to the shape of these corner cubes that any laser shot at them reflects light directly back. Over the years, measurements of the moon's distance have improved thanks to better laser and computing equipment.

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