Hubble Space Telescope team plots recovery plan
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: October 23, 2008
Engineers believe they understand what caused isolated problems during restart of the Hubble Space Telescope's science instruments last week in the wake of an earlier electrical glitch. They have successfully restarted Hubble's B-side payload computer, officials said today, and hope to bring the first of the telescope's major science instruments back on line this weekend.
The Hubble trouble began Sept. 27 when the telescope's control unit and science data formatter, or CU/SDF-A, suffered a "hard" failure, preventing ground controllers from receiving data from the science instruments. The A and B channels of the redundant science instrument data handing system are located on the same electronics tray and NASA managers decided to replace the entire unit with a flight spare to restore lost redundancy.
But the flight spare must be tested and recertified, forcing NASA to delay the shuttle Atlantis' launch on Hubble Servicing Mission 4, or SM-4, from mid October to mid February at the earliest.
In the meantime, Hubble managers wanted to restart the telescope's science instruments by reconfiguring six components in the observatory's data management system and five in the science instrument command and control system. Several of the components in question had not been turned on since launch in 1990.
The initial stages of the complex restart procedure went smoothly last week and the B-side science instrument computer powered up normally. But hours later, during re-activation of the Advanced Camera for Surveys solar blind channel, seemingly improper voltage levels triggered an abort. Later, the B-side science computer and science data formatter suffered simultaneous resets for unknown reasons. At the time, engineers did not know if the unexpected events were related.
Art Whipple, manager of the Hubble Systems Management Office at the Goddard Space Flight Center, said today engineers believe they understand what happened - and why - and after a thorough review, managers gave the engineering team permission to attempt another restart.
"One week ago, Hubble experienced two anomalous events at the end of an otherwise nominal reconfiguration to bring on line several backup components to restore our ability to take data with the science instruments," Whipple said. "The first event was the suspension of the Advanced Camera for Surveys on the afternoon of Oct. 16. The second event was the safing of the science payload later that evening. We've spent the last week supporting a detailed review of all data related to both events as well as a thorough assessment of all other systems on the spacecraft. And we're now ready to resume recovery of the science payload and if successful, science operations.
The Advanced Camera for Surveys, installed in 2002, suffered an earlier failure that knocked out its visible light wide-field and high-resolution cameras. Only its solar blind channel, which is sensitive to ultraviolet light, is still in operation. The Atlantis astronauts hope to repair the ACS during the upcoming servicing mission.
Whipple said the ACS solar blind channel power-up abort "was caused by a software test that runs in the ACS microprocessor and it tripped before a parallel data collection application had time to collect sufficient valid data."
"The engineering data clearly shows there were no actual problems with the ACS power supply at the time of the suspension," Whipple said. "Relative timing of these two software processes has always been tight and recent changes to the software to support the planned SM-4 ACS repair, as well as resetting of the onboard master clock that was a part of last week's reconfiguration, just made the timing too tight. The team will correct this problem by changes to the timing of the activation of the ACS software and will use this revision for all future ACS observations. We expect to resume science observations with the ACS solar blind channel later next week.
"Safing of the science payload was caused by a transient electrical event that simultaneously caused the reset of the (B-side) command unit science data formatter and the NASA standard spacecraft computer in the science instrument command and data handling system. That event was most likely caused by a momentary short or open circuit subsequently cleared. Events of these kind are not uncommon in electrical components that have been powered off for long periods of time and it is possible we may see another event of this type in the future."
Whipple said as far as the engineering team can tell from telemetry, "there does not appear to be any permanent damage. There was no harm done by this event to any other systems on the spacecraft."
"Following detailed review of these events, as well as the risk and benefits of resuming recovery of the science payload,, approvals were given ... to bring the SIC&DH back up on the B side," Whipple said. "This was accomplished today, shortly after 1 p.m. Eastern time. The NSSC is back up and running. If the NSSC continues to operate normally, science operations will resume with Wide Field Planetary Camera 2 this weekend and ACS later (next) week."
As of this writing, NASA is working toward a possible launch of Atlantis around Feb. 17. But that assumes the spare science instrument computer hardware undergoing tests at Goddard checks out and no other major problems crop up. Other possible launch targets include March and May. Bill Gerstenmaier, chief of NASA space operations, and Ed Weiler, director of space science, plan to meet Nov. 5 to review testing and launch readiness. A firm launch target date is expected shortly thereafter.