Echo 2 (satellite)

For The 2007 music festival, see The Echo Project.

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.[1]

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 transceiver, 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 (18,000 kg) of air was needed to fill the 150-pound (68 kg) balloon, but 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."[2]

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 orbit by another Thor-Delta,[3][4] 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 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 kilograms (397 lb).

Echo 1A reentered Earth's atmosphere and burned up on May 24, 1968.

Echo 2

Echo 2 was a 41.1-meter-diameter (135 ft), 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 and +16 °C (−184 and 61 °F), 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.[5]

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.

Of note is that both the Echo 1A and Echo 2 experienced a solar sail effect due to their large size and low mass.[6]

Other uses

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.[7]

The large horn antenna at Holmdel constructed by Bell Labs for the Echo project was later used by Arno Penzias and Robert Woodrow Wilson for their Nobel Prize-winning discovery of the cosmic microwave background radiation.[8]

Echo I commemoration

The U.S. Post office issued a postage stamp commemorating Project Echo on December 15, 1960.

See also


Further reading

  • Nick D'Alto "The Inflatable Satellite", Invention and Technology Summer 2007, Volume 23, Number 1 pp. 38–43.

External Links

  • A film clip ]
  • A film clip ]
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