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Subject:
HST safing
From:
Rodger Doxsey
Date:
Sun, 28 Jan 2007 08:01:50 -0500
To:
st_all@stsci.edu


HST entered inertial hold safemode this morning. The safing action
took place while the spacecraft was not in communication with the
ground, so it has taken a little longer to get engineering data off
the recorders and to the engineers. The initial indications are that
the safing was a result of a pressure gauge in the Aft Shroud
measuring an abnormal pressure. This gauge was originally installed
to check for leaks of neon from the cooling system, but is sensitive
to anything else that might introduce gas into the aft shroud. At
about the same time, there was an excessive current measured in the
power system, and the ACS power appeared to be shut off, likely by a
blown fuse. At this time, there is enough insight into the telemetry
to believe that the cooling system has not lost any neon, and that
something in the ACS on side 2 has failed. It is possible that an
electronic failure has generated enough "smoke" to trip the pressure
sensor. Those with long memories will remember that the final GHRS
failure was "seen" by the FOS pressure sensor in this way.

The situation with the ACS is very unclear because the engineers
have not had access yet to its engineering data from the time of the
event. STScI schedulers generated a Health and Safety SMS, which has
been processed and uplinked to HST. This provides commanding that
helps keep the spacecraft in a stable situation, including normal
communications paths, while the analysis continues.

Given the uncertainty in the ACS situation, please do not forward
this note further. I will update you as more is learned.


Rodger Doxsey