December 20, 2008
Hubble Servicing Mission Back On Track
Servicing Mission 4 to the Hubble Space Telescope is currently on track for a mid-May 2009 launch based on a successful Pre-Environmental Readiness Review (PERR) held December 16 at NASA's Goddard Space Flight Center for the flight spare Science Instrument Command and Data Handling (SI/C&DH).
Extensive testing and inspection of the flight spare Control Unit/Science Data Formatter-A (CU/SDF-A) did not reveal the cause of a previously observed intermittent anomaly. The anomaly is observed very infrequently and can be mitigated simply by re-issuing commands to the unit.The CU/SDF-A workmanship was judged to be of high quality. Therefore, on December 16 the team began a rigorous three month environmental testing program (Electro Magnetic Interference/Electro Magnetic Compatibility, vibration, and thermal vacuum testing). Hubble engineers will continue to monitor the CU/SDF-A closely during this time to see if the anomaly re-occurs under a variety of test conditions.
Inspection of the flight spare Power Control Unit (PCU) revealed tin whiskers and tin nodules. The HST team anticipated this based on the in-flight anomaly when the SI/C&DH onboard Hubble was switched to the B-side in October and it shut down. This scenario was explained at that the time as probably being due to a tin whisker producing an electrical short that quickly cleared itself when the tin whisker evaporated. Tin whiskers are commonly seen as thin, needle-like growths on the surface of aging components coated with tin.
After this in-orbit event, Side B of the PCU was subsequently restarted with no problems and continues to perform nominally. Hubble engineers have removed the tin whiskers and nodules in the flight spare PCU and applied a special barrier to mitigate the effects of any potential future reoccurrence. The flight spare CU/SDF-B was also opened and inspected and determined to have no workmanship issues. The HST Program expects to complete certification of the flight spare on schedule for a May shuttle flight.
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