Explorer is the name of a series of scientific satellites sponsored first by the U.S. Army and later by the National Aeronautics and Space Administration (NASA). Explorer 1, the first U.S. artificial satellite, was named for the high-altitude balloon flights of the 1930s. It was launched from Cape Canaveral on Jan. 31, 1958, by a Juno 1 rocket (see rockets and missiles). This craft and subsequent explorer-class vehicles were categorized as lighter scientific satellites, as compared to the observatory class of heavier scientific satellites, but this distinction was sometimes blurred. In addition, many explorer-class satellites were not actually named Explorers. Explorer-type craft have included upper atmosphere explorers, air density satellites, direct measurement explorers, the IMP (interplanetary monitoring platform), ionosphere explorers, radio astronomy explorers, small astronomy satellites (including Uhuru), geodetic explorer satellites, Injuns, Sun-Earth explorers, and ultraviolet explorers. Of the satellites named Explorer, the last in the series was Explorer 59, launched on Oct. 6, 1981, which studied solar-terrestrial interactions.In variety and purpose, the Explorers were similar to entries in the concurrent Soviet/Russian Cosmos series. Among the major contributions of the Explorers was the discovery by Explorer 1 of the Van Allen radiation belts surrounding the Earth, the transmission of Earth's first televised cloud-cover portrait by Explorer 6 in 1959, and the first detection of gamma rays from space by Explorer 11 in 1961.