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Hubble Telescope Science Instrument Experiences Anomaly During Restart
A cooling system associated with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) science instrument aboard the Hubble telescope experienced an anomaly during a restart.

As part of preparations for the new science instruments that will be installed during the SM4 mission, planned flight software updates to the computer that controls Hubble’s five science instruments were uploaded last week. Installation of the software requires putting all of the telescope's science instruments into safe mode configuration for a short period of time.

Just before 4 a.m. EDT on Sept. 11, the NICMOS anomaly was seen. This was approximately six hours after the system was reactivated. The cooling system put itself into safe mode after seeing too high a speed in the circulator pump operation. After studying data, flight controllers modified operating protection parameters and attempted a restart of the system on Sunday, Sept. 14. The circulator system again indicated a high speed violation so the system was returned to safe mode.

Analysis of the flight telemetry indicates an erratic increase in speed of the circulator rotor during turn on. The data are consistent with a perturbation of the rotor by ice particles (~10 microns to 1-mm in size). Such perturbations can produce an erroneous sensing of excessively high rotor speeds.

Source of the ice particles is suspected to be contamination that has been in the cooling loops used before launch to solidify the liquid nitrogen in the NICMOS cryostat. The NICMOS Cooling System was installed on servicing mission 3B following exhaustion of the solid nitrogen in the cryostat, thereby extending the lifetime of the NICMOS science instrument by several years.

Engineers believe the ice particle condition is understood and that with some small adjustments in start-up procedures, the cooling system can be successfully reactivated. The flight team tried another restart Monday evening (9/15). The anomaly was still seen after that restart, so the Hubble Project's plan now is to stand down from any additional attempts to restart . Engineers will study the anomaly while waiting until the cooling system has been allowed to warm somewhat, which may take several weeks.

The impact to planned NICMOS science operations involves approximately 70 exposures from three guest observer programs and additional exposures from two NICMOS internal calibration programs. Additionally, all NICMOS science has been removed from this week’s observation schedule. Sixty-one orbits of NICMOS science were scheduled for the week between September 15 and September 21.