  uhuru was the first satellite launched specifically for the purpose of x-ray astronomy. it was also known as the x-ray explorer satellite, sas-a (for "small astronomy satellite" a, being first of the three-spacecraft sas series), sas 1, or explorer 42. the observatory was launched on december 12, 1970 into an initial orbit of about 560&nbsp;km apogee, 520&nbsp;km perigee, 3 degrees inclination, with a period of 96 minutes. the mission ended in march 1973. uhuru was a scanning mission, with a spin period of ~12 minutes. it performed the first comprehensive survey of the entire sky for x-ray sources, with a sensitivity of about 0.001 times the intensity of the crab nebula. ==objectives== the main objectives of the mission were:heasarc "the uhuru satellite" * to survey the sky for cosmic x-ray sources in the 2-20 kev range to a limiting sensitivity of 1.5 Ã 10â11 ergs/(cm2-sec), 5 Ã 10â4 the flux from the crab nebula * to determine discrete source locations with a precision of a few square minutes of arc for strong sources and a few tenths of a square degree at the sensitivity limit * to study the structure of extended sources or complex regions with a resolution of about 30 arc minutes * to determine gross spectral features and variability of x-ray sources * to perform, wherever possible, coordinated and/or simultaneous observations of x-ray objects with other observers. ==instrumentation== the payload consisted of two sets of proportional counters, each with ~0.084 m2 effective area. the counters were sensitive with more than 10% efficiency to x-ray 
Artificial_satellites_formerly_orbiting_Earth Explorers_program_(NASA) Space_observatories Spacecraft_launched_in_1970 Spacecraft_which_reentered_in_1979 X-ray_telescopes