Echo

On 12 August 1960, NASA, Bell Telephone Laboratories and the Jet Propulsion Laboratory launched a passive reflector, Echo 1 ¹ on a ThorDM-19 Delta rocket into a ~47.2° inclined LEO orbit between ~966 km and ~2,157 km. Echo 1 (officially Echo 1A) was a 30-m plastic balloon (with a mass of 61.2 kg) with an aluminum coating. The balloon had 82 panels that were folded before launch and inflated in orbit.

Echo was used principally for FM voice, facsimile and data transmission. The first transmission was a recorded message from President Eisenhower sent from the NASA station at Goldstone, California to Bell Laboratories at Holmdel, New Jersey, followed by the first two-way telephone conversation via space. Echo was also used on 19 August 1960 to test picture transmission. Although a passive reflector, Echo contained two tracking beacons transmitting with a power of 10 mW at a frequency of 107.9 MHz.

Echo was simple and reliable but was severely limited by being a passive reflector of microwave frequencies between ~0.96 GHz and 2.39 GHz—despite the sphere having a reflectivity of 98%, Earth stations required extremely high-power transmitters of approximately 10 kW, with large 18-m antennas. Even with these high powers, usable receive powers were only possible because the reflector operated at a low height. At that height, however, the orbital periods were very small (118.3 minutes) so that the satellite was only accessible to ground stations for a brief time. Ground stations used FM receivers with phase-lock loops, and low-noise maser amplifiers.

Echo wrinkled and shrank with age, becoming unusable by late 1962. Echo 1 remained in orbit from 12 August 1960 until it re-entered the atmosphere on 24 May 1968—about 7.75 years of orbital life. There was no active de-orbit system; the satellite gradually decayed due to atmospheric drag, solar radiation pressure effects, and perturbations in its orbit until it burned up upon re-entry.

A follow-on satellite, Echo 2, launched on 25 January 1964 into near-polar orbit of ~81.5° inclination between ~1,030 km and ~1,315 km, was larger and constructed with a thicker skin to improve durability. In addition to the microwave frequencies used for Echo 1, Echo 2 conducted VHF experiments at ~162 MHz. Like Echo 1, Echo 2 decayed naturally until atmospheric drag and orbital perturbations caused it to re-enter and burn up in Earth’s atmosphere on 7 June 1969—after about 5.5 years in orbit.