Balloon satellite

A balloon satellite, sometimes referred to as a "satelloon", is a satellite inflated with gas after it has been put into orbit.

Echo 1 and Echo 2 balloon satellites

The first flying body of this type was Echo 1, which was launched into a 1,600-kilometer (990 mi) high orbit on August 12, 1960, by the United States. It originally had a spherical shape measuring 30 meters (98 ft), with a thin metal-coated plastic shell made of Mylar. It served for testing as a "passive" communication and geodetic satellite.

One of the first radio contacts using the satellite was successful at a distance of nearly 4,000 kilometers (2,500 mi) (between the east coast of the US and California). By the time Echo 1 burned up in 1968, the measurements of its orbit by several dozen earth stations had improved our knowledge of the precise shape of the planet by nearly a factor of ten.

Its successor was the similarly built Echo 2 (1964 to about 1970). This satellite circled the Earth about 400 kilometers (250 mi) lower, not at an angle of 47° like that of Echo 1, but in a polar orbit with an average angle of 81°. This enabled radio contact and measurements to be made at higher latitudes. Taking part in the Echo orbit checks to analyze disturbances in its orbit and in the Earth's gravitational field were thirty to fifty professional earth stations, as well as around two hundred amateur astronomers across the planet in "Moonwatch" stations; these contributed around half of all sightings.