Mirror, Mirror on the Moon

When Neil Armstrong and Buzz Aldrin walked on the moon more than 40 years ago, I was just a youngster and don't remember it at all.  Most assuredly my mom explained to me the historical event that was happening, but it just didn't stick in my memory banks.  I just learned something about that trip that I had never known before.

Just about an hour before the end of that famous Apollo 11 mission on July 21, 1969, these two astronauts put down a 2-foot wide panel of 100 mirrors. 

 It is called the Apollo 11 lunar laser ranging retroreflector array.

 "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 Carroll Alley, the projects principal investigator during the Apollo years.  Back on Earth, telescopes intercept the returning pulse--'usually just a single photon,' he marvels."

If you arrange 3 mirrors in a shape like the corner of a rectangular box, with the reflectors on the inside, then any light which hits the reflectors, at essentially any angle, will bounce off each mirror and end up heading back exactly the direction from which it came. This makes such a mirror arrangement very useful, because you always get a nice strong reflection.

Here is another image that shows two different paths of light.  Even though the source is from two different places, the beam is returned to the source of the laser.

Scientists still use these mirrors on the moon.  They have learned some interesting things.  First, that the moon is spiraling away from the Earth at a rate of 3.8 cm per year.  The NASA web site says the ocean tides are responsible.  As the moon orbits Earth, it creates a bulge of water that travels round the planet behind it. This bulge - which we experience as tides - exerts a gravitational pull on the moon, slowing it down as it circles Earth at a distance of 240,000 miles. As a consequence of being held back by this pull, the orbit of the moon becomes altered and it moves slowly away from Earth

Second, it is now believed that the moon has a liquid core.  They have also learned that the universal force of gravity is very stable.

There are a total of five of these mirror arrays.  Apollo 11, 14, and 15 missions each placed one, as well as two Soviet Lunokhod landers.  They don't work as well as they used to work.  Dust has accumulated on them and sometimes light doesn't come back, and when it does it is much fainter.  The mirrors are losing their ability to reflect back laser light precisely, which hinders accurate measurements.

Now you have heard something interesting.