MIRROR, MIRROR, ON THE MOON MARCH 9, 2011 | DAN LEWIS
On July 20, 1969, Apollo 11 and its crew landed on the moon. Famously, commander Neil Armstrong planted an American flag on the moon’s surface, as pictured above. The crew left the flag there. But it wasn’t the only thing they left.
Buzz Aldrin, the second man to walk on the moon, carried with him the “EASEP” — the “Early Apollo Scientific Experiment Package” — as seen below. In constituted two distinct experiments; on the left is the “Passive Seismic Experiment Package” (“PSEP”) and on the right is the “Lunar Laser Ranging Reflector Array” (“LRRR”). Both are still on there.
The PSEP was a short-term success — but quickly became moon junk. It measured shock waves on or below the moon’s surface; disturbances created by “moonquakes,” impacts by meteors or other objects, etc. The PSEP placed during the Apollo 11 mission ceased functioning a few weeks later, and although subsequent PSEPs were placed by other Apollo missions, the last one was turned off, remotely, in 1981.
More interesting is the LRRR — because it’s still in use.
The LRRR is a roughly two foot wide panel of mirrors — 100 mirrors, at that — which helps researchers measure the distance from the Earth to the moon. Researchers, on Earth, shoot a laser at the array, which disappates to become roughly four miles wide. (Hitting the array is a tough shot — Wikipedia likens it to “using a rifle to hit a moving dime 3 kilometers (two miles) away.”) The mirrors on the array are made of “corner-cubes,” so if the beam does indeed strike it, the array bounces the light straight back along the same path — as one scientist puts it, it’s “like hitting a ball into the corner of a squash court.” And the return volley isn’t visible to the naked eye. It’s typically only one photon, visible by telescope.)
(Here’s a video of the laser and telescope in action.)
That photon carries a lot of information — or, rather, the time it takes that photon to go round-trip can tell us a lot about the Moon and physics generally. First, the moon is moving away from the Earth, at a rate of 38mm a year. Second, the moon’s core is probably liquid. And third, we’ve been able to confirm that, over the now 40-plus years in which the LRRR experiment has been running, that the Newton’s gravitational constant has, in fact, remained constant.