Lunar Laser Ranging experiment

Lunar Laser Ranging experiment 
I think, the most famous thing Neil Armstrong left on the moon is not a boot-shaped depression in the gray moondust but rather the lunar laser ranging retroreflector array.  

The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the Moon during the Apollo program (11, 14, and 15), and the time for the reflected light to return is determined.

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. Back on Earth, telescopes intercept the returning pulse-"usually just a single photon".

The round-trip travel time pinpoints the moon's distance with staggering precision: better than a few centimeters out of 385,000 km, typically.

In this way, for decades, researchers have carefully traced the moon's orbit, and they've learned some remarkable things, among them:
(1) The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible.
(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.

References:
https://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/lroc-20100413-apollo15-LRRR.html#.VolHr1R97IU
http://science.nasa.gov/science-news/science-at-nasa/2004/21jul_llr/

Photo credits NASA

#nasa   #apollomissions   #LLRR   #moon   #space   #science