NASA has identified the possible cause of the payload computer problem that suspended Hubble Space Telescope science operations on June 13. The telescope itself and science instruments remain healthy and in a safe configuration.

The payload computer resides in the Science Instrument Command and Data Handling (SI C&DH) unit. It controls, coordinates, and monitors Hubble's science instruments. When the payload computer halted, Hubble's science instruments were automatically placed into a safe configuration.

A series of multi-day tests, which included attempts to restart and reconfigure the computer and the backup computer, were not successful, but the information gathered from those activities has led the Hubble team to determine that the possible cause of the problem is in the Power Control Unit (PCU).

The PCU also resides on the SI C&DH unit. 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.

Because no ground commands were able to reset the PCU, the Hubble team will be switching over to the backup side of the SI C&DH unit that contains the backup PCU. All testing of procedures for the switch and associated reviews have been completed, and NASA management has given approval to proceed. The switch will begin Thursday, July 15, and, if successful, it will take several days to completely return the observatory to normal science operations.

The team performed a similar switch in 2008, which allowed Hubble to continue normal science operations after a Command Unit/Science Data Formatter (CU/SDF) module, another part of the SI C&DH, failed. A servicing mission in 2009 then replaced the entire SI C&DH unit, including the faulty CU/SDF module, with the SI C&DH unit currently in use.

Related Links
Hubble at NASA
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NASA grapples with Hubble Telescope's most serious problem in years
Washington DC (UPI) Jul 12, 2021
NASA's Hubble Space Telescope is broken, and the agency is struggling to understand the workhorse observatory's most serious problem in more than a decade. Hubble stopped working suddenly June 13 while astronomers were using the 31-year-old telescope to examine pulsating stars 200 million miles away. NASA is trying to understand what went wrong on the orbiting telescope, without which hundreds of astronomy investigations had to be put on hold or canceled. At stake are efforts to understa ... read more