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Echo 1 sits fully inflated at a Navy hangar in Weeksville, North Carolina. The spacecraft measured 100 feet across when deployed, and was nicknamed a 'satelloon' by those involved in the project. Credit: NASA.

The Echo satellites were NASA's first communications satellite experiment. Each spacecraft was designed as a metallized balloon satellite acting as a passive reflector of microwave signals. Communication signals were bounced off of it from one point on Earth to another.^[1]

Following the failure of the Delta rocket carrying Echo 1 on May 13, 1960, Echo 1A (commonly referred to as just Echo 1) was successfully put in a 1519 x 1687 km orbit on August 12, 1960. The 30.5 meter (100 foot) diameter balloon was made of 0.127 mm (0.005 inch) thick metallized Mylar polyester film and was successfully used to redirect transcontinental and intercontinental telephone, radio, and television signals. The satellite also aided the calculation of atmospheric density and solar pressure due to its large area-to-mass ratio. As its shiny surface was also reflective in the range of visible light, Echo 1A was visible to the unaided eye over most of the Earth. Brighter than most stars, it was probably seen by more people than any other man-made object in space. Echo 1A reentered Earth's atmosphere and burned up on May 24, 1968.

Echo 1 was a passive communication satellite. It was not yet a two-way transmission satellite system, and rather functioned as a reflector. After it was placed in a low orbit of the Earth, a signal would be relayed to Echo, relfected or bounced off its surface, then returned to Earth. We know that the Echo 1 was visible to the eye because of its shiny surface, but it was also because it was placed in the low orbit of the Earth. It would appeared from below one side of the horizon and disapeared below the opposite horizon after running accros the sky. It happened not only to the Echo series; it happened to all placed-in-the-low-orbits-of-the-Earth satellites.

Echo 2, a 41.1 m diameter metallized PET film balloon with an improved inflation system to improve the balloon's smoothness and sphericity, was launched January 25, 1964 on a Thor Agena rocket. communications experiments, and also to investigate the dynamics of large spacecraft and for global geometric geodesy. NASA abandoned passive communications systems in favor of active satellites following Echo 2. Echo 2 reentered on June 7, 1969.

The Echo satellite program also provided the astronomical reference points required to accurately locate the Russian city of Moscow geographically. This improved accuracy was sought by the US Military for the purpose of targeting intercontinental ballistic missiles. ^[2]

• Project SCORE, the world's first communications satellite, launched in 1958.
• Courier 1B, launched in 1960.
• Telstar, the first active, direct relay communications satellite, launched in 1962.
• AO-51, AMSAT-OSCAR 51 (also known as Phase 2E, or ECHO), an amateur radio communications satellite launched in 2004.
• PAGEOS - a similar balloon satellite project