Mirror, Mirror on the Moon

By Sara Loy

Posted December 27, 2019

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Transcript

D: It’s been fifty years since humans first walked on the moon. Do you think we’ll go back anytime soon, Host 2?

Y: I suppose it depends on your definition of “soon.” Are you asking because you want to be a lunar tourist?

D: Well . . . yes! I love reading about the moon landings—it would be so cool if I could do that, too!

Y: I don’t know when space tourism will happen, but at least I can tell you that, in a way, we visit the moon all the time. It’s thanks to the Lunar Laser Ranging experiment. Observatories around the world shoot a laser at the moon. When the laser returns—often in the form of just a single photon!—scientists can make all sorts of discoveries, like the exact distance between the Earth and the moon.

D: You’re talking about the moon mirrors, aren’t you? Buzz Aldrin put down a two-foot-wide display of mirrors that faced Earth. Later moon missions added displays, too. And it’s these mirrors that reflect that laser, right? Neat! I never realized what they were used for.

Y: And they’re used for a lot! We’ve learned that the moon is moving away from the Earth at 3.8 centimeters per year. We also now know that the moon’s core is liquid, that Earth’s continents are still moving away from each other, that the force of gravity is extremely stable, and that many of Einstein’s predictions are correct.

D: Just think how much more we could learn with future lunar visits! Gosh I’d really love to be one of those visitors. Maybe someday…

When Buzz Aldrin went to the moon, he put down a two-foot-wide display of mirrors that faced Earth. (NASA, Wikimedia Commons)

It's been fifty years since humans first walked on the moon. Who knows if we'll ever go back again. Although it's not clear when space tourism will happen, it is true that, in a way, we visit the moon all the time.

This is thanks to the Lunar Laser Ranging experiment. Observatories around the world shoot a laser at the moon. When the laser returns, often in the form of just a single photon, scientists can make all sorts of discoveries, like the exact distance between the Earth and the moon.

This is all possible with moon mirrors. Buzz Aldrin put down a two-foot-wide display of mirrors that faced Earth. Later moon missions added displays, too. And it's these mirrors that reflect that laser.

From all of this we've learned that the moon is moving away from the Earth at 3.8 centimeters per year. We also know that the moon's core is liquid, that Earth's continents are still moving away from each other, that the force of gravity is extremely stable, and that many of Einstein's predictions are correct.