(OGO), series of six orbiting observatories (see observatory, orbiting) launched between 1964 and 1969 by the National Aeronautics and Space Administration (NASA) to study the earth's atmosphere, ionosphere, and magnetosphere and the solar wind. In order to provide global coverage, three of the satellites (OGO-2, OGO-4, and OGO-6) were launched into low polar orbits while the other three (OGO-1, OGO-3, and OGO-5) were placed in eccentric orbits ranging out as far as almost 92,830 mi (150,000 km). Each observatory carried 20–25 instruments, including cosmic ray detectors, counters, and telescopes, ionization chambers, Geiger counters, magnetometers, spectrometers, photometers, micrometeoroid detectors, and ion and electron traps. Among the significant achievements of the OGO program were the first observations of daytime auroras and of the protons responsible for the ring of electric current that surrounds the earth during magnetic storms, the collection of data showing the interaction of the solar wind with the earth's magnetosphere, and the verification of the existence of the plasmapause, the inner boundary of the region of trapped radiation in the magnetosphere.

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