X-ray satellitesArtificial Earth satellites devoted to cosmic X-ray observations, the first of which was NASA's Uhuru, launched in Dec. 1970. Up to this date, i.e. throughout the period 1962–70, X-ray astronomy was carried out exclusively with sounding rockets and balloon experiments. Uhuru produced the first all-sky survey for cosmic X-ray sources and increased the catalog of known sources to 161 by 1974. The second X-ray astronomy satellite was the British satellite Ariel V, which was launched in Oct. 1974 and occupied a similar 500-km circular equatorial orbit to Uhuru. It successfully extended the Uhuru sky map and made detailed spectral and temporal studies of individual sources.
Although X-ray experiments were also carried on other satellites principally devoted to the research fields (Copernicus, OSO-7, ANS, OSO-8), the next two dedicated X-ray astronomy satellites were SAS-3 and HEAO-1, launched respectively in 1975 and 1977. Both included modulation collimator experiments in their payloads and these provided the first accurate positions of many X-ray sources, leading to additional and more reliable optical identifications. In Nov. 1978 HEA0-2 (the Einstein Observatory) was launched; its large grazing incidence telescope produced major advances. After the Einstein Observatory ceased operation in Apr. 1981 only two small satellites, Ariel VI and Hakucho, remained before the launch in 1983 of the Japanese Tenma and the European EXOSAT spacecraft. Subsequent X-ray satellites, of increasing sophistication, were the Japanese Ginga (launched 1987), German ROSAT (1990), and Japanese Asca (1993), with still larger missions in preparation, including the Russian Spectrum–X (1995), NASA's XTE (1997) and AXAF (1998/99), and ESA's XMM (1999).