(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.

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