Then how did this get there?
31 years after landing on the moon, an Apollo 11 science instrument still shines
By Gregory R. Clark
For the past 30 years, a small reflective plate about 18 inches square has been sitting on the lunar surface quietly bouncing laser beams back to Earth. It continues to defy the predictions of some early Apollo planners who guessed the specialized mirror would quickly be buried in powdery moondust.
When Neil Armstrong placed and balanced the Laser Ranging Retro-Reflector on the white ground just a few giant moonsteps from the site where the Eagle touched down, nobody could have known the device would be as useful at the turn of the 21st century as it was then. But thanks to its simple, non-mechanical design and an extraordinarily placid lunar world, scientists still use the reflector to follow the most miniscule wobbles in the moon's spin and orbit.
The Laser Ranging Retro-Reflector, or LRRR, was one of the two primary science packages deployed by the Apollo 11 astronauts [See accompanying article, MoonScience, for more on the others]. It was a flat configuration of special reflectors that possess the special property of always reflecting light back to its point of origin.
It was used to reflect laser beams back to Earth so scientists could make very precise measurements of Earth-Moon distances. Although the moon is on average about 238,000 miles (385,000 kilometers) away from Earth, scientists were able to measure the Earth-moon distance to an accuracy of about 1 1/4 inches (3 centimeters).
Before Apollo 11, only very rough measurements could be made.
The Apollo 11 retro-reflector and similar units left later by the Apollo 14 and Apollo 15 missions, and one Soviet lunar probe, have produced many important measurements. These instruments are the only Apollo experiments still working, and, according to Carroll Alley, the University of Maryland physicist who was principal investigator for the reflector experiment, "There seems to be no measurable degradation in their performance."
The reflector array allowed scientists to precisely measure the moon's orbit and spin rate, and the speed at which the moon is receding from Earth (about 1.5 inches or 3.8 centimeters per year). They also know more about Earth's spin and as a result of reflector data.
The measurements allow scientists to learn all kinds of things about the moon, from the thickness of its crust to the makeup of its iron core, to the motion of its axial wobble.
"We now have a lot of material about the moon, but the main goal was to study gravitational theory," Alley said.
At the time of the Apollo missions, there was some debate about what system of physics governed the motion of bodies in the solar system.
"The ability to monitor precise point-to-point distances allows one to determine if the Earth-Sun-Moon system is governed by Newtonian physics, or Einsteinian relativity or by some other system, such as the one advanced by Dr. Robert Henry Dickey," he said.
A Princeton University physicist, Dickey was a profound theorist who some believe narrowly missed winning a Nobel Prize. He advanced an alternative to Einstein's theory of general relativity that predicted certain characteristics about the moon's orbit around the Earth.
Dickey's theory had gained a certain amount of influence, but it required measurements to validate or defy it. Such measurements were made using the Apollo 11 retro-reflector.
The measurements that ultimately disproved Dickey's system may have been the most significant of the Apollo 11 science results, Alley said. These helped scientists understand what governs the motion of the solar system, and better understand the laws of gravity.
And all that from a shiny-topped tile sitting in the dust of the moon.