On May 4, 1976, a spacecraft resembling a disco ball entered orbit almost 3,700 miles (6,000 kilometers) above Earth. This shiny, twofootwide (60centimeter) sphere called the Laser Geodynamics Satellite, or LAGEOS, is covered with 426 retroreflectors-small mirrored prisms designed to bounce laser light directly back to where it came from. Beneath its aluminum exterior sits []