Orbiting Solar Observatory (OSO), series of eight orbiting observatories (see observatory, orbiting) launched between 1962 and 1971 by the National Aeronautics and Space Administration (NASA) to study the sun in the ultraviolet and X-ray wavelengths filtered out by the earth's atmosphere. The third to be launched, OSO-C, failed to achieve orbit. The other seven studied solar ultraviolet radiation and cosmic rays, measured radiation levels in the Van Allen radiation belts, detected neutrons, and investigated X-ray sources in the Milky Way and beyond. Through continuous monitoring of the sun over a 15-year period, the OSO program enhanced the understanding of the solar atmosphere and of the 11-year sunspot cycle. Among the significant achievements of the program were the first visible light and extreme ultraviolet photographs of the corona, the first full-disk photograph of the solar corona, and the first X-ray observations of a solar flare in the initial stage of eruption. See also ultraviolet astronomy.