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 
NASA's Echo 1 satellite was built by Gilmore Schjeldahl's G.T. Schjeldahl Company in Northfield, Minnesota. The balloon satellite functioned as a reflector, not a transmitter, so that after it was placed in a low Earth orbit, a signal would be sent to it, reflected or bounced off its surface, and then returned to Earth.
During ground inflation tests, 40,000 pounds of air was needed to fill the 150 pound balloon, while in orbit, several pounds of gas were all that was required to fill the sphere. 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."
Following the failure of the Delta rocket carrying Echo 1 on May 13, 1960, Echo 1A (commonly referred to as just Echo 1) was put successfully into a 944 to 1,048 mi orbit by another Thor-Delta  and a microwave transmission from the Jet Propulsion Laboratory in Pasadena, California was received at Bell Laboratories in Homdel, New Jersey on August 12, 1960.
The 30.5-meter (100 ft) diameter balloon was made of 0.5 mils (12.7 µm) thick metalized biaxially oriented PET film ("Mylar") material, and it was 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 easily visible to the unaided eye over most of the Earth. The spacecraft was nicknamed a 'satelloon' by those involved in the project, as a portmanteau of satellite-balloon.
It also had 107.9 MHz beacon transmitters for telemetry purposes, powered by five nickel-cadmium batteries that were charged by 70 solar cells mounted on the balloon. During the latter portion of its life, the spacecraft was used to evaluate the technical feasibility of satellite triangulation. It had a total mass of 180 kg.
Echo 1A reentered Earth's atmosphere and burned up on May 24, 1968.
Echo 2 
Echo 2 was a 41.1-meter (135 ft) diameter metalized PET film balloon, which was the last balloon satellite launched by Project Echo. It used an improved inflation system to improve the balloon's smoothness and sphericity. It was launched January 25, 1964 on a Thor Agena rocket.
Instrumentation included a beacon telemetry system that provided a tracking signal, monitored spacecraft skin temperature between -120 deg C and +16 deg C, 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.
In addition to the passive communications experiments, it was used to investigate the dynamics of large spacecraft and for global geometric geodesy.
Echo 2, being larger than Echo 1A and also orbiting in a near polar orbit, was conspicuously visible to the unaided eye over all of the Earth. Echo 2 reentered Earth's atmosphere and burned up on June 7, 1969. NASA abandoned passive communications systems in favor of active satellites following Echo 2.
Military use 
The Echo satellite program also provided the astronomical reference points required to accurately locate Moscow. This improved accuracy was sought by the US military for the purpose of targeting intercontinental ballistic missiles.
Echo I commemoration 
The U.S. Post office issued a postage stamp commemorating Project Echo on December 15, 1960.
See also 
- List of communications satellite firsts
- Project SCORE – world's first communications satellite, launched in 1958.
- Courier 1B – world’s first active repeater satellite, launched in 1960.
- Telstar – 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
- U.S. space exploration history on U.S. stamps
- "Echo 1, 1A, 2 Quicklook". Mission and Spacecraft Library. NASA. Retrieved February 6, 2010.
- Static Inflation Test of 135 Ft Satellite In Weeksville, NC NASA Langley Research Center (NASA-LaRC), 1961-06-28.
- Astronautix.com, Echo
- "Echo 1". NASA. Retrieved 13 July 2010.
- "Echo 2". NASA.
- Coulter, Dauna (Jumy 31, 2008). "A Brief History of Solar Sails". NASA. NASA. Retrieved February 4, 2010.
- Gray, Mike (1992). Angle of Attack: Harrison Storms and the Race to the Moon. W. W. Norton & Co. pp. 5–6. ISBN 0-393-01892-X.
Further reading 
- Elder, Donald C. (1995). Out from Behind the Eight-Ball: A History of Project Echo. AAS History Series 16. Univelt for the American Astronomical Society. ISBN 0-87703-388-9.
- Nick D'Alto "The Inflatable Satellite", Invention and Technology Summer 2007, Volume 23, Number 1 pp. 38–43.
External Links 
- A film clip "Big Bounce, The" is available for free download at the Internet Archive [more]
- A film clip "Space Triumph. Discoverer Capsule Recovered From Orbit , 1960/08/15 (1960)" is available for free download at the Internet Archive [more]