The Hubble Space Telescope fell into a safe mode after one of its remaining three fully operational gyros malfunctioned on 5 October 2018 preventing observations using the venerable astronomical spacecraft. The gyro concerned had been reportedly degrading for a year. The Hubble Space Telescope had its six gyros replaced in 2009 by Space Shuttle astronauts. However, in March and April two of these failed. This latest failure has officially left three working gyros, however, another is seriously “unwell” leaving only two fully operational ones.
The fourth faulty gyro of an “enhanced design” has been powered down after showing anomalous behaviour preventing a return to service for Hubble. NASA reports that a work around method of spacecraft attitude control may be possible using two or even just a single working gyro. This latter method has the advantage of “resting” one of the gyros which would act as a reserve for the other.
Update on 16 October 2018: NASA has revealed that its Chandra X-Ray observatory fell into a safe mode on 10 October 2018 after a suspected fault to one of its gyros (to be confirmed).