ASCA


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ASCA

Fourth in the series of small (500 kg) Japanese space science satellites devoted to X-ray astronomy. It was launched into a 560-km, 31° inclination orbit in Feb. 1993, carrying a payload of four identical grazing incidence X-ray telescopes (conical foil). Two telescopes had gas scintillation proportional counters in the focal plane and two had X-ray-sensitive CCDs (provided by NASA). The mission's main strength was X-ray spectroscopy. Both types of detector gave much improved energy resolutions over ASCA's working range of 0.5–10 keV, compared with the widely used imaging proportional counter. Apart from a gradual deterioration in the energy resolution of the CCDs caused by in-orbit radiation damage, all instruments aboard ASCA continued to function up to the middle of 2000, by which time nearly 4000 separate observations had been carried out. Among many scientific advances were the detection of a population of partly obscured active galactic nuclei (see active galaxy), whose hard X-ray spectra suggested that they formed a major component of the X-ray background radiation; the first evidence for strong gravity in the form of a broad iron fluorescence line; and the measurement of metal-rich spectra in the ejecta of several supernova remnants. The mission ended in July 2000 when a geomagnetic storm caused by an unusually large solar flare irrevocably altered the orbital attitude of the satellite and the orientation of its solar panels, taking the panels away from the Sun and effectively destroying ASCA's power supply. ASCA re-entered the Earth's atmosphere in 2001. See also Astro-E2; Ginga.