Good news, Hubble fans – NASA reckons it may have worked out what has upset the orbiting observatory: an iffy Power Control Unit (PCU).

The agency is only saying the PCU is a “possible” cause of the Hubble’s technical breakdown at this stage, but the theory is strong enough that engineers have been given the green light to start a procedure to switch to backup power components within the spacecraft.

As a reminder, Hubble is unable to carry out any science right now: its sensors are inactive and in a safe mode because the payload computer that controls the instruments mysteriously halts. NASA is trying to find the cause of the crashes, and now suspicion has fallen on the power supply.

The PCU lurks within the Science Instrument Command and Data Handling (SI C&DH) unit, and is responsible for maintaining a steady voltage supply to the hardware of the payload computer. It contains a regulator that is supposed to ensure a constant five volts of electricity flows to that computer and its memory.

Should that voltage go above or below acceptable levels, a secondary protection circuit shuts off the payload computer. The team reckons it is possible that either the regulator is sending voltage outside of allowable levels or the protection circuit has somehow got stuck in its “inhibit” state.

Degradation over time looks like the most likely culprit, although the team can only make a best guess based the telemetry available. The PCU was also not able to be reset through commands from the ground.

The plan is therefore to switch to the backup side of the SI C&DH, which includes the backup PCU. This does mean switching over more hardware than would be ideal but, if all goes well, science operations could resume in a just a few days.

A similar switch happened in 2008 though, as we noted in our previous coverage, a Space Shuttle servicing mission was due to visit the telescope in the following year. If things don’t go to plan there is a real danger the telescope will simply carry on serenely orbiting stuck in safe mode while engineers ponder a plan B.

The switching process is set to kick off today, July 15 and, if all goes well, science operations should resume by next week.

Since tomorrow is Friday, we shall be raising a beer then to the engineers working to restore to the telescope. And keeping our fingers crossed that the flow of science resumes before long. ®

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