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Uhuru(oo-hoor -oo) (‘Freedom’ in Swahili) The first artificial Earth satellite for X-ray astronomy (SAS–1), launched into a 500 km circular equatorial orbit in December 1970 from the San Marco platform off the coast of Kenya. Two sets of proportional gas counters, each 840 cm2 in area and mounted on opposite sides of the slowly spinning spacecraft, scanned the sky for X-ray sources. Uhuru provided the first detailed X-ray sky map, leading to the 3U catalog of 161 sources, published in 1974. Its most important discoveries were of X-ray binaries (Cen X-3, Her X-1, etc.) and of extended regions of X-ray emission associated with clusters of galaxies. Uhuru operated successfully until July 1971, when its transmitter failed. The 3U catalog is based on the data to that time. Subsequently the star sensors, used to determine the precise attitude of the spacecraft and hence the sky coordinates of each X-ray source, also degraded; surveying recommenced in Dec. 1971, however, after partial transmitter recovery. These later observations, with the aid of a magnetometer attitude solution, continued until Apr. 1973, when the Uhuru battery failed. All the Uhuru observations were finally put together to form the 4U catalog, issued in 1978 and containing 339 X-ray sources.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006