LAGEOS is a Disco Ball Astronomers Used to Study Earth
In 1976, NASA launched a 900-lb. (408-kg.) metal ball covered with 426 reflectors into space. It was a satellite, dubbed LAGEOS (or Laser Geodynamics Satellite), that was designed to do one thing: reflect light. By shooting lasers at LAGEOS and measuring how long they took to bounce off its reflectors and make it back to Earth, NASA scientists could make extremely precise measurements of how far above the ground LAGEOS was, and therefore determine how the laser source on Earth was moving in relation to the planet's center of mass. This simple measurement told scientists a great deal, most importantly whether or not, and how fast, the tectonic plates were moving -- an important geological detail that science was missing at the time. Higher-tech satellites have since overtaken LAGEOS's role, but it's still up there, slowing its orbit by a tiny bit each day due to drag forces. Scientists don't expect it to fall out of the sky for another 8.4 million years.