HEA Heritage Missions: Uhuru

The first satellite devoted to X-ray astronomy, Small Astronomy Satellite-A, renamed Uhuru (Swahili for "Freedom"), was launched into a near-Earth equatorial orbit from the San Marco platform off the coast of Kenya on 12 December 1970, the seventh anniversary of Kenyan Independence Day.

UHURU operated until March 1973.

uhuru icon The science payload, at 64 kg, weighed no more than a typical rocket experiment at the time. It carried two sets of conventional proportional counters with simple honeycomb collimators which were used to undertake the first X-ray survey of the sky (details for the mission can be found in Giacconi et al. 1971, ApJ 165, L27). The spacecraft was spin stabilized at 12 minutes per revolution. One detector had a field of view of 1 degree x 10 degrees, so it viewed each source for 2 s during each scan. A second detector had a field of view of 10 x 10 degrees and spent 20 seconds on sources during each scan. Uhuru scanned many times (typically about 60 in a given day after which the scan region was usually moved) over the same region thereby greatly increasing its sensitivity to weak sources. The net result of this was that Uhuru was able to detect X-ray sources 10 times fainter than the faintest detectable on earlier rocket flights to a limiting sensitivity of about 0.001 of the intensity of Crab Nebula. Although not uniform in sensitivity, 95% of the sky was scanned during the 2.5 year lifetime of the mission. A tape recorder failed early on, and the transmitter failed and then miraculously recovered.
4U-catalog-sky-map icon

Despite these "challenges", the 4U (Fourth Uhuru; Forman et al. 1978, ApJS, 38, 357) Catalogue contains 339 sources. Strong sources were located with a few square arcminute accuracy, whereas weak sources could only be placed within a region covering several square degrees.
Spectacular light curves of the brightest sources proved that some were binary systems with the X-rays produced by accretion onto a compact object (neutron star or black hole), and spin and orbital periods were measured (see Giacconi et al. 1971, ApJL, 167, L67; Schreier et al. 1972, ApJL, 172, L79; Tananbaum et al. 1972, ApJ, 174, L143). Other scientific highlights from UHURU included the discovery of numerous eclipsing X-ray binaries (e.g., Jones et al. 1973, ApJL, 181, L43) and galaxy clusters as a class of X-ray sources (Gursky et al. 1972, ApJL, 173, L99). Kellogg and Murray (1974, ApJL, 193, 57) showed that X-ray emission from galaxy clusters was extended and likely arose from diffuse hot gas.

  Photo of Uhuru

UHURU Photo of GiacconiUhuruSatellite1970-flightspare

PI R Giacconi with the UHURU flight spare


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