For a still unknown reason, the Science Instrument Command and Data Handling system caused Hubble's science computer to stop issuing its "keep alive" signal.
When the signal was absent for 20 seconds, Hubble shut itself down again, retreating into safe mode.
"At this time it's not known if these two events are related," Whipple said.
Despite the loss of science communication, engineers were able to obtain a memory dump from a Hubble computer that tracks systems status. Initial data suggest that the failure is related to a component in the so-called side B data formatter.
Side B, which was designed as the formatter's backup system, had not been turned on since Hubble's 1990 launch.
Still Optimistic
Whipple and Morse were quick to point out silver linings to Hubble's challenges.
For one, the spacecraft is still heeding commands from engineers at Goddard, and "all of its subsystems appear to be functioning normally," Whipple said.
"Being able to get a memory dump out … is itself a very good sign," he added.
All of the issues the team has experienced so far fall within the realm of scenarios that had been imagined as part of the recovery plan, he added.
"We expect that we will work through it," he said.
The team will begin testing next week on a replacement data formatter—which includes a primary and backup system—that has been in safe storage on Earth since Hubble's launch.
Once it checks out, the duplicate formatter can be flown to Hubble for installation during the telescope's final servicing mission, which was postponed following the September equipment failure. The team expects to know by early November when the shuttle might make its rescheduled flight.
Meanwhile, engineers may try to restart side B or design a hybrid configuration that combines working parts in the primary and backup formatters.
Morse pointed out that the current glitches in switching to side B mean that engineers will have a better sense of what to do should the primary system in the replacement formatter fail before the end of Hubble's official operations.
If all efforts to remotely recover science data transmission fail, the worst case is that Hubble will be mum—except for star-mapping functions, which don't rely on the data formatter—until the servicing mission.
The Hubble program loses an estimated U.S. $10 million a month when the telescope doesn't send science data.
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