NASA believes it has discovered the issue with Hubble, after the telescope has been in safe mode for over a month.

The Hubble Telescope’s payload computer started malfunctioning in June. Despite various attempts to get it working properly, NASA has not had any success. NASA has gained enough understanding from the failed attempts to now have a clearer picture of what’s happening.

NASA believes the problem is in the Power Control Unit (PCU). 

It ensures a steady voltage supply to the payload computer’s hardware. The PCU contains a power regulator that provides a constant five volts of electricity to the payload computer and its memory. A secondary protection circuit senses the voltage levels leaving the power regulator. If the voltage falls below or exceeds allowable levels, this secondary circuit tells the payload computer that it should cease operations. The team’s analysis suggests that either the voltage level from the regulator is outside of acceptable levels (thereby tripping the secondary protection circuit), or the secondary protection circuit has degraded over time and is stuck in this inhibit state.

Like much of the systems on Hubble, there is a backup PCU. NASA is now trying to switch to the backup, a process that was also done in 2008 and will take several days to complete.