Paula Cleggett-Haleim
Headquarters, Washington, D.C.                   December 12, 1991
(Phone:  202/453-1547)

RELEASE:  91-204

HUBBLE TELESCOPE RESUMES OPERATIONS AFTER BRIEF ANOMALY

	The Hubble Space Telescope (HST) resumed science data collection at
7 a.m. EST this morning.  For the past 24 hours, the spacecraft has been
safely returning from a standby condition which began Dec. 9 at 7:47 a.m.

	Standby conditions, or safe modes, are a capability built into all NASA
spacecraft.  Safe modes are invaluable "safety nets" to protect against
spacecraft anomalies caused by on-orbit hardware problems or erroneous
commands sent from the ground.

	A subtle error in the ground software system that generates command
sequences for HST triggered this event.  The error caused an incorrect
command to be sent, which made the high gain antenna slew at a rate beyond
allowable limits.  The onboard flight computer sensed this "out-of-range" rate
and put the spacecraft in a standby and safe condition.

	According to Dr. Charles Pellerin, Director for NASA's Astrophysics
Division, "The HST flight system functioned exactly as designed and protected
itself from human errors on the ground."

	This particular type of anomaly is the first since launch of HST in
April of 1990.  The last time HST was in a safed mode of operation was in May
1991 due to a different type of computer anomaly.

	Corrections in the ground software are being worked and interim
guidelines are in place for manual inspection of command loads to ensure
against any repetition of this particular problem.

	The level of standby HST was in, called an "inertial hold mode," is the
most benign of several levels of safe configurations.  When initiated, it
basically stops executing further commands, maintains current spacecraft
attitude and alerts controllers to a potential problem.  This gives ground
personnel time to evaluate the situation, analyze the telemetry and start
appropriate actions.