The PAGEOS balloon satellite

The PAGEOS balloon satellite, built for the C&GS Worldwide Geometric Satellite Program, was a 30-meter-diameter sphere made from 0.0125-millimeter-thick Mylar externally coated with vapor-deposited aluminum. The satellite was launched June 23, 1966, into an orbit approximately 4,000 kilometers high. It was inflated in orbit to serve as a giant reflector of sunlight that could be photographed from the Earth. PAGEOS was the second (following GEOS 1) satellite in the National Geodetic Satellites Program.