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It has now been more than a month since the Hubble Space Telescope reverted to safe mode following an apparent memory failure. NASA attempted all the standard fixes such as turning it off and on, but the aging observatory has stubbornly refused to come back online. After investigating the issue, NASA now believes the failure stems from a power component on the Science Instrument Command and Data Handling module. The team is working to switching Hubble over to its backup computer system, a process that will take several days. 

The issues started on July 13th when Hubble’s payload computer stopped working. This is separate from the spacecraft’s main computer, so NASA has not lost communication. However, the payload computer controls and coordinates the science instruments on Hubble. Without it, Hubble has no reason to continue operations. When the payload computer failed, the main computer put Hubble in safe mode, and little has changed in the last month. 

Initially, NASA engineers believed the problem was tied to memory modules on the payload computer. The team tried to swap in one of the four spare memory chips, but that didn’t work. Now, NASA says the issue is most likely at a higher level. The payload computer is part of the Science Instrument Command and Data Handling (SI C&DH) unit, which astronauts replaced during Hubble’s final servicing mission in 2009. The information gathered over the last several weeks has convinced NASA that the issue lies with the Power Control Unit (PCU) on the SI C&DH. Unfortunately, NASA has not been able to reset the PSU via ground commands. 

A Hubble SI C&DH unit prior to its installation in the telescope.

There’s still hope, though. Like most expensive hardware destined for space, Hubble was designed with numerous redundancies. The SI C&DH is a two-sided computer, so NASA now plans to switch to the backup side of the system, which has its own components unconnected to the malfunctioning ones. 

The Hubble team has already completed testing and gotten NASA leadership’s approval to switch to the backup system. This process began on July 15th and should be complete in a few days. At that point, we expect Hubble to come back online. NASA has had to rely on backup hardware to keep the telescope operational in recent years, but it’s running out of backups. After 30 years, Hubble is nearing the end of its useful life. It’s already lasted much longer than anyone anticipated, and if we’re lucky, it will share the sky with the James Webb Space Telescope. Webb is scheduled to launch by the end of 2021. Unless it gets delayed again, which would not be surprising at this point.

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