Project Echo was the first passive communications satellite experiment. Each of the two American spacecraft, launched in 1960 and 1964, was a metalized balloon satellite acting as a passive reflector of microwave signals. Communication signals were bounced off them from one point on Earth to another.
During ground inflation tests, 40,000 pounds (18,000 kg) of air were needed to fill the balloon, but while in orbit, several pounds of gas were all that was required to fill the sphere. At launch, the balloon weighed 156.995 pounds (71.212 kg), including 33.34 pounds (15.12 kg) of sublimating powders of two types. According to NASA, “To keep the sphere inflated in spite of meteorite punctures and skin permeability, a make-up gas system using evaporating liquid or crystals of a subliming solid were incorporated inside the satellite.” One of the powders weighed 10 pounds (4.5 kg), with a very high vapor pressure; the other had a much lower vapor pressure.
Echo 2 was a 41.1-meter-diameter (135 ft) balloon satellite, the last launched by Project Echo. A revised inflation system was used for the balloon, to improve its smoothness and sphericity. Echo 2’s skin was rigidizable, unlike that of Echo 1A. Therefore, the balloon was capable of maintaining its shape without a constant internal pressure; a long-term supply of inflation gas was not needed, and it could easily survive strikes from micrometeoroids. The balloon was constructed from “a 0.35 mil (9 µm) thick mylar film sandwiched between two layers of 0.18 mil (4.5 µm) thick aluminum foil and bonded together.” It was inflated to a pressure that caused the metal layers of the laminate to slightly plastically deform, while the polymer was still in the elastic range. This resulted in a rigid and very smooth spherical shell.
Echo 2 was launched January 25, 1964, on a Thor Agena rocket. In addition to passive communications experiments, it was used to investigate the dynamics of large spacecraft and for global geometric geodesy. Since it was larger than Echo 1A and orbiting in a near-polar orbit, Echo 2 was conspicuously visible to the unaided eye over all of the Earth. It reentered Earth’s atmosphere and burned up on June 7, 1969.
Both Echo 1A and Echo 2 experienced a solar sail effect due to their large size and low mass. Later passive communications satellites, such as OV1-08 PasComSat, solved the problems associated with this by using a grid-sphere design instead of a covered surface. Later yet, NASA abandoned passive communications systems altogether, in favor of active satellites.