Which 1970-launched X-ray astronomy satellite observed Cygnus X-1? The satellite was Uhuru, NASA’s first spacecraft dedicated specifically to X-ray astronomy.
Uhuru launched on December 12, 1970, from the Italian San Marco platform near Mombasa, Kenya. Its name means “freedom” in Swahili and honored Kenya’s hospitality. The spacecraft was also known as SAS-A, SAS 1, or Explorer 42, reflecting its place in NASA’s Small Astronomy Satellite program.
Operating until 1973, Uhuru performed the first comprehensive survey of the sky in X-rays. Its extended observations of Cygnus X-1 detected rapid fluctuations in X-ray intensity, showing that the emitting region had to be extremely compact. This evidence complemented optical measurements of the visible companion and helped establish Cygnus X-1 as a leading black-hole candidate.
Uhuru did much more than observe one famous system. It catalogued hundreds of X-ray sources, including accreting binaries and sources beyond the Milky Way. Its final 4U catalogue contained 339 objects. The other choices are different spacecraft: IUE studied ultraviolet light, COBE investigated the cosmic background, and Hipparcos measured stellar positions and motions.