likely that the HST system will be reduced to two operating gyros in the latter half of 2006. The HST engineering team is currently working on approaches to sustaining useful astronomical operations with only two gyros, and the team expects to have that capability by the time it becomes necessary. Two-gyro testing is scheduled to begin in March 2005. There are hopes that even a one-gyro operation mode might be feasible for limited telescope operations, but there are no detailed plans for this mode. The spacecraft can be held in a safe configuration with no operating gyros, but science operations would not be possible.

As is also discussed in detail in Chapter 4, battery failures are another likely cause of loss of science operations. HST now has six batteries, of which five are necessary for full operations. If battery levels fall too low, the temperature of structural elements in the Optical Telescope Assembly will fall below permissible levels, causing permanent damage.⁶ Recovery from this state is not possible.

A recent development is the failure on HST of the Space Telescope Imager and Spectrograph, a powerful ultraviolet/visible imager and spectrograph whose Side B electronics failed in August 2004 (Side A had failed earlier). The cause of the failure appears to be understood, and investigations are underway to understand the feasibility, if any, of a repair.

Details of the current status of the observatory are provided in Chapter 4.