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.
Echo 1
's Echo 1 satellite was built by Gilmore Schjeldahl's G.T. Schjeldahl Company in Northfield, Minnesota. The balloon satellite would function as a reflector, not a transceiver; after it was placed in a low Earth orbit, a signal could be sent to it, reflected by its surface, and returned to Earth. During ground inflation tests, 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, including 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 incorporated inside the satellite." One of the powders weighed, with a very high vapor pressure; the other had a much lower vapor pressure. The first attempt to orbit an Echo satellite miscarried when Echo 1 lifted off from Cape Canaveral's LC-17A on the morning of May 13, 1960. The Thor stage performed properly, but during the coasting phase, the attitude control jets on the unproven Delta stage failed to ignite, sending the payload into the Atlantic Ocean instead of into orbit. Echo 1A was successfully put into a orbit of by another Thor-Delta, and a microwave transmission from the Jet Propulsion Laboratory in Pasadena, California, was relayed by the satellite to Bell Laboratories in Holmdel, New Jersey, on August 12, 1960. The diameter balloon was made of biaxially oriented PET film, metalized at a thickness of , and had a total mass of. It was used to redirect transcontinental and intercontinental telephone, radio, and television signals. It also had 107.9 MHz telemetry beacons, powered by five nickel-cadmium batteries that were charged by 70 solar cells mounted on the balloon. The spacecraft aided the calculation of atmospheric density and solar pressure, due to its large area-to-mass ratio. During the latter portion of its life, it was used to evaluate the technical feasibility of satellite triangulation. As its shiny surface was also reflective in the range of visible light, Echo 1A was easily visible to the unaided eye over most of the Earth. The spacecraft was nicknamed a "satelloon" by those involved in the project. It was originally expected that Echo 1A would not survive long after its fourth dip into the atmosphere in July 1963, although estimates allowed the possibility that it would continue to orbit until 1964 or beyond. It ended up surviving much longer than expected, and finally reentered Earth's atmosphere and burned up on May 24, 1968.
Echo 2 was a 41.1-meter-diameter 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 thick mylar film sandwiched between two layers of 0.18 mil 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. Instrumentation included a beacon telemetry system that provided a tracking signal, monitored spacecraft skin temperature between, and measured the internal pressure of the spacecraft between 0.00005 mm of mercury and 0.5 mm of mercury, especially during the initial inflation stages. The system consisted of two beacon assemblies powered by solar cell panels, and had a minimum power output of 45 mW at 136.02 MHz and 136.17 MHz. 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.
