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We describe the flight performance of the cryogenic system of the infraredastronomical satellite AKARI, which was successfully launched on 2006 February21 (UT). AKARI carries a 68.5 cm telescope together with two focal planeinstruments, Infrared Cameras (IRC) and Far Infrared Surveyor (FIS), all ofwhich are cooled down to cryogenic temperature to achieve superior sensitivity.The AKARI cryogenic system is a unique hybrid system, which consists of cryogen(liquid helium) and mechanical coolers (2-stage Stirling coolers). With thehelp of the mechanical coolers, 179 L (26.0 kg) of super-fluid liquid heliumcan keep the instruments cryogenically cooled for more than 500 days. Theon-orbit performance of the AKARI cryogenics is consistent with the design andpre-flight test, and the boil-off gas flow rate is as small as 0.32 mg/s. Weobserved the increase of the major axis of the AKARI orbit, which can beexplained by the thrust due to thermal pressure of vented helium gas.

Flight Performance of the AKARI Cryogenic System