From: Takeo Kosugi
      Deputy Project Manager for Yohkoh, ISAS

To: NASA Project Manager for Yohkoh
    PPARC Representative for Yohkoh
    Yohkoh Key Members

Date: 13 May 2002

Subject: Concerning the Possibility for Resuming Scientific Observations 
         with the Solar-Physics X-Ray Satellite, "Yohkoh"


Dear Colleagues,

Yohkoh, a solar X-ray satellite of ISAS, launched on 30 August 1991, has 
observed the solar corona and solar flares for more than ten years. A total 
of four scientific instruments are on board the satellite, 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. Data were downlinked not only 
at Kagoshima Space Center, the ISAS station, but also at the DSN and GN 
Wallops stations of NASA as well. Thanks to these contributions from NASA, 
the total volume of data obtained by Yohkoh was more than double that which 
would have been possible using only the Japanese site alone, and these data 
have astonishingly renovated our understanding of the solar corona. 

The satellite, however, has lost its attitude control, and the on-board 
batteries for the system power supply have been discharged since 15 December 
2001; this situation resulted from complications associated with an annular 
eclipse observed over the central Pacific Ocean. Measures to charge the 
batteries and to regain the normal control have continued since the accident. 
Battery charging has been impeded by a diode switch in the charging control 
circuit and, unfortunately, the operation of the diode switch is controlled by 
the output of the battery system. The minimum voltage required to charge 
the batteries was estimated, and satellite power consumption was minimized. 
We awaited the best conditions to initiate battery charging, as the satellite 
was spinning and the battery voltages varied according to the spacecraft 
environment.

Despite these efforts, the stored electric power on board gradually decreased 
during this five-month period, and the voltage of the battery system never 
exceeded the required threshold. Moreover, we now believe that the spacecraft 
has undergone almost all available combination of orbital and attitudal 
circumstances over the past five months. Therefore, ISAS has concluded that 
resumption of the scientific operation of Yohkoh is not likely, and that the 
spacecraft should be continuously monitored only for engineering purpose 
until its re-entry.

Herewith, I, as deputy project manager of Yohkoh, thank you again for all
of your contributions and efforts so far, both before and after the launch
of the satellite. Despite the termination of scientific operation, Yohkoh
will still continue to produce scientific results, based on the data it
accumulated over more than ten years of operation. This on-going research
will continue to involve scientists throughout the world. It is ISAS's
understanding that all the parties so far engaged in the Yohkoh program
will establish the arrangement and support system for the Yohkoh data
archive shortly.

Sincerly Yours.

Takeo Kosugi, Deputy Project Manager for Yohkoh
Institute of Space & Astronautical Science (ISAS), Japan