Orbiting Solar Observatory
The Orbiting Solar Observatory (abbreviated OSO) Program was the name of a series of nine American science satellites primarily intended to study the Sun, though they also included important non-solar experiments. Eight were launched successfully by NASA between 1962 and 1975 using Delta rockets. Their primary mission was to observe an 11-year sun spot cycle in UV and X-ray spectra. The initial seven, OSO 1, OSO 2, OSO 3, OSO 4, OSO 5, OSO 6 and OSO 7, were built by Ball Aerospace, then known as Ball Brothers Research Corporation (BBRC), in Boulder Colorado. OSO 8 was built by Hughes Space and Communications Company, in Culver City, California.
The basic design of the entire series featured a rotating section, the "Wheel," to provide gyroscopic stability. A second section, the "Sail," was driven electrically against the Wheel's rotation, and stabilized to point at the Sun. The Sail carried pointed solar instruments, and also the array of solar photovoltaic cells which powered the spacecraft. The critical bearing between the Wheel and the Sail was a major feature of the design, as it had to operate smoothly for months in the hard vacuum of space without normal lubrication. It also carried both the power from the Sail and the data from the pointed solar instruments to the Wheel, where most of the spacecraft functions were located. Additional science instruments could also be located in the Wheel, generally looking out on a rotating radius vector which scanned the sky, and also across the Sun, every few seconds.
As for a number of other NASA programs, the OSO spacecraft were labeled by letter designations before launch, and only given numbers when successfully placed in orbit. Thus OSO 1 was originally called OSO A, and the other satellites were OSO B, OSO C, OSO D, OSO E, OSO F, OSO G and OSO I which became OSO 8. The OSO J was planned but never launched.