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: http://www.stsci.edu/hst/HST_overview/TwoGyroMode/2GyroFAQs/WhenNeeded
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Expectations for when HST might enter two-gyro mode are somewhat uncertain because of the limited amount of lifetime data available for the types of mechanical gyros HST uses. A model has been developed that includes random gyro failures as well as a gyroscope wear-out function. The coefficients in this model are poorly known due to small number statistics. Some changes were made in the assembly procedures for the gyros which should have improved their lifetime, although no statistics are available for these units. The model based on HST flight experience predicts that two-gyro mode will be entered in spring 2006, part way through Cycle 14. A brief summary of the failure probabilities for Cycle 14 is given below. The probabilities in the table should be considered approximate estimates only, as past performance does not necessarily translate directly into future survival.
The present (October 2004) status of the HST gyros is as follows:
Gyro 1 - in use, 2.1 years of operation
Gyro 2 - in use, 3.6 years of operation
Gyro 3 - failed
Gyro 4 - in use, 5.1 years of operation
Gyro 5 - failed
Gyro 6 - in reserve, 1.4 years of operation
For more information about the Hubble gyros, see the HST Gyroscope Factsheet.