I've Been Thunderstruck

Last week I wrote about an interesting video I had watched called "Horizon : What On Earth Is Wrong With Gravity?" I learned quite a few interesting things while watching that video, but nothing shocked me as much to find out that there are mirrors on the moon!

When Apollo 11 landed on the moon almost 40 years ago, it was not only the first time we landed on the moon, but it was the start of an experiment that continues to this day. There’s an awesome article about the whole thing from "Science @ NASA," but the crux of the mission is stated as such:

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.

The mirrors were left of the surface of the moon so we, down on Earth, could shoot lasers at them and help understand the distance between the Earth and the moon in a very precise way.

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. There’s a set of mirrors there onboard the parked Soviet Lunokhud 2 moon rover–maybe the coolest-looking robot ever built.

Scientists have learned quite a bit about the Earth-moon relationship in the years that they’ve been hitting it with a laser. Three important points have emerged as a result of the experiements:

1. The moon is spiraling away from Earth at a rate of 3.8 cm per year.
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.

This is some pretty amazing work, and even more amazing that it’s been going on night after night for almost 40 years! In that time Einstein’s theories have been confirmed, as well as Newton’s Gravitational constant. But, it’s also cast some doubt on other theories, and continued exploration will ultimately flesh out the truth. This is a very interesting article, and I encourage you to read more!