The first Project Echo satelloon may have started out as a 100-meter sphere, but it didn’t stay that way. Echo IA launched on August 12, 1960, and it stayed in orbit and visible to the naked eye until May 24, 1968. It inflated successfully, but as a paper in Bell Labs Report, Sept. 1961 explains, by May of that year, its shape had already been somewhat deformed in orbit:
On several early passes the average “scatering cross section” was equal to that corresponding to a perfectly conducting 100-foot sphere. From this it is assumed that the balloon inflated as planned.
There apparently has been a long-term decrease, of a few db, in the average “scattering cross section.” As of last May, Echo I [technically IA, since Echo I burned up soon after launch in March 1960] was probably an approximately spherical object with a diameter of no less than 70 feet, and a somewhat wrinkled skin. There may have been a few flattened areas, as indicated by occasional deep fades in the radar signal, but voice communication was then still possible as shown by successful tests with NRL on May 25.
One factor may have been the solar sail effect, the slight pressure generated by photons from the sun bombarding the satelloon’s skin.
image: The Odyssey of Project Echo [history.nasa.gov]