* Lunar Ranging Experiment: The primary purpose was to measure the distance between the Earth and the Moon with extreme precision.

* How it works: Retroreflectors are special mirrors that reflect light directly back to its source, no matter the angle of incidence. When lasers are fired from Earth at these reflectors, the reflected light is measured, allowing scientists to calculate the exact distance.

* Scientific value: This data helps us understand:

* The Moon's orbit and its relationship to Earth

* The rate at which the Moon is drifting away from Earth

* The gravitational forces at play within the Earth-Moon system

* The internal structure of the Moon

There are several sets of retroreflectors on the Moon, placed by:

* Apollo 11, 14, and 15 missions: These were the first sets, deployed by astronauts.

* Soviet Lunokhod 2 rover: It carried a set of retroreflectors, demonstrating the ability to place them robotically.

The retroreflectors are still used today, providing valuable data for scientists and furthering our understanding of the Moon and its relationship to Earth.