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The bane of PC builders and Hubble Space Telescope controllers • The Register

Good news, Hubble fans – NASA believes it may have figured out what rocked the in-orbit observatory: a questionable Power Control Unit (PCU).

The agency only says the PCU is a “possible” cause of Hubble’s technical failure at this point, but the theory is strong enough that engineers have been given the green light to initiate a switchover procedure to components from emergency power supply in the spaceship.

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

The PCU hides within the Scientific Instrument Data Processing and Control Unit (SI C&DH) and is responsible for maintaining a constant voltage supply to the payload computer hardware. It contains a regulator which is supposed to ensure a constant flow of electricity of five volts to this computer and its memory.

If this voltage exceeds or exceeds acceptable levels, a secondary protection circuit shuts down the payload computer. The team believes that it is possible that the regulator is sending voltage outside the allowable levels or that the protection circuit has somehow stuck in its “inhibit” state.

Degradation over time appears to be the most likely culprit, although the team can only make a better guess based on the telemetry available. The PCU could not be reset by commands from the ground either.

The plan is therefore to switch to the backup side of the SI C&DH, which includes the backup PCU. This means changing more equipment than would be ideal, but if all went well, science operations could resume in just a few days.

A similar change occurred in 2008, however, as we noted in our previous coverage, a Space Shuttle maintenance mission was scheduled to visit the telescope the following year. If things don’t go as planned, there is a real danger that the telescope will simply continue to orbit serenely stuck in safe mode while engineers think about a plan B.

The switching process is expected to start today, July 15, and if all goes well, science operations are expected to resume by next week.

Since tomorrow is Friday, we’ll then throw a beer to the engineers working on the restoration of the telescope. And by crossing our fingers that the flow of science will resume before long. ®

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