An Interruption to Scientific Observations by the Solar-Physics X-ray Satellite, "Yohkoh" 18 December 2001 Institute of Space & Astronautical Science (ISAS), Japan Yohkoh, a solar X-ray satellite of ISAS, was launched on 30 August 1991 and has been observing the solar corona and solar flares for more than ten years. Data obtained from a total of four scientific instruments, including an X-ray telescope (SXT; Soft X-ray Telescope) and a spectrometer (BCS; Bragg Crystal Spectrometer) which were jointly developed by U.S.-Japan and U.K.-U.S.-Japan international teams respectively, are on board the satellite. All the data have been made available for solar physics studies to researchers worldwide, and used for real-time monitoring of solar activity, both domestic and abroad, for the sake of space weather studies of the Earth environment. On 15 December 2001, at around 3:50 pm (time in JST, hereafter), the operators at the Kagoshima Space Center (KSC) noticed that the power supply to the scientific instruments had been shut down, and henceforth scientific observations were suspended. ISAS immediately started to take necessary measures and to investigate the cause of this trouble. The investigation so far has revealed that at around 8:00 am, the satellite power system detected a low battery voltage, which triggered a UVC (Under Voltage Control) automatically, and shut down the power system to the scientific instruments. The above situation was initiated in relation to the annular eclipse observed over the Pacific Ocean at around 7:00 am, the same day. Yohkoh experienced an unexpected satellite-body rotation, when the satellite attitude control switched from the normal Sun-acquisition mode to the stand-by mode, as the orbit crossed the annular-eclipse belt. As a result, the total power generation started to decrease and the battery was discharged, as the solar panels could not accept stable and sufficient solar light. During ten contacts at KSC so far since the beginning of the UVC, the operation has been successful in minimizing the power consumption of the satellite by shutting down the power lines of the other instruments and heaters. The orbital average of the satellite power balance is achieved, and no more decrease of the battery voltage is expected at the moment. However, the satellite is still spinning and has not been controlled yet as of 18 December 2001 and scientific observations have been suspended. The measures to retain the normal control of the satellite are continuing. However, many steps will be required for recharging the battery, re-establishing attitude control, and so forth. Therefore, it may take some time before scientific observations can resume.