Echo, NASA's first communications satellite, was a passive spacecraft based on a balloon design created by an engineer at NASA's Langley Research Center. Made of Mylar, the satellite measured 100 feet (30 meters) in diameter. Once in orbit, residual air inside the balloon expanded, and the balloon began its task of reflecting radio transmissions from one ground station back to another. Echo 1 satellites, like this one, generated a lot of interest because they could be seen with the naked eye from the ground as they passed overhead.