Advanced Satellite for Cosmology and Astrophysics (ASCA) is in Safe Mode

Dear ASCA users,

As was announced in July, ASCA has transferred into a safe-hold mode
(SH-mode). In spite of our serious efforts for about two months after
the event, we are not able to recover ASCA to the normal observation
mode. Here, we wish to report briefly on the event and current status
of ASCA.

A historically big solar flare occured on July 14, 2000, with extremely
strong solar proton flux. Subsequently, the solar flare triggered a
big geomagnetic storm that continued for a few days after July 15. The
orbital altitude of ASCA has been gradually decreasing for the seven and
a half years of operation, and its perigee was 440 km at the time. Thus
we expected the re-entry of ASCA into the atmosphere to be the middle of
next year.

Due to the geomagnetic storm, the atmosphere sporadically expanded and
the atmospheric gas density at the ASCA altitude sudenly increased to
several times the normal value. This caused the external torque on the
satellite due to the air drag to increase. Thus, the ASCA attitude was
perturbed and the on-board attitude control system (ACS) transferred
ASCA to the SH-mode. Since the accumulated external torque was stronger
than the compensable internal torque stored in the on-board momentum
wheels, the ASCA attitude was not locked to the nominal aspect of
SH-mode, and further moved away the solar paddles from the nominal
direction normal to the sun. This reduced power generation by the
solar cells and finally exhausted the battery power completely.

After the event, we attempted all possible and considerable operations
to recover the aspect and to charge up the battery. However, there has
been no improvement so far. We suspect that the battery cells may have
suffered serious unrecoverable damage.

Currently we can operate ASCA only during real-time aquisition of ASCA
at KSC in daytime using the power that is generated by the solar cells
and connected to the equipment directly through the busline. We have
continued to monitor the rough ASCA aspect using the geomagnetic aspect
sensors. ASCA is now flying in a free spin mode with a period of about
3 min and a nutation angle of about 40 degree. After the long unrewarding
efforts to recover ASCA, we decided to change the ASCA operation from
early September; thereafter we operate ASCA just once a day during the
daytime aquisition at KSC for monitoring the status of ASCA.

Taking all of the situation explained above into account, we regret
that we must announce that the possibility that ASCA will return to
observation mode is very small, almost hopeless, even though we will
monitor the ASCA status untill its re-entry into the atmosphere. We
appreciate your understanding that we must cancel all the programmed
long observations after July 15, 2000. We will report more details at
the HEAD meeting in November.

Finally, we wish to thank all the astronomers who have participated to
the ASCA observations, and all the members of the ASCA operation team in
Japan and the members of the NASA ASCA/GOF facility in the United States
for their close collaboration in the ASCA operation from proposal handling,
observation planning, satellite tracking, and data aquisition, through
data processing and archiving. ASCA lived twice as long as the planned
mission life, and we believe that the successful collaboration between
Japanese and US scientists has produced tremendous fruitful scientific
results from ASCA.

2000, September, 20
H. Inoue, F. Nagase and ASCA operation team