NASA teams have successfully
resurrected the iconic Hubble Space Telescope after it first experienced
problems with an onboard computer on June 13 and suspended all science
operations. The telescope is now fully operational and the switch to the backup
hardware — which included powering on the backup payload computer — was
performed successfully on July 16. With all its science instruments back
online, the collection of science data resumed on July 17.
Hubble is an icon, giving us
incredible insight into the cosmos over the past three decades,” said NASA
Administrator Bill Nelson. “I’m proud of the Hubble team, from current members
to Hubble alumni who stepped in to lend their support and expertise. Thanks to
their dedication and thoughtful work, Hubble will continue to build on its
31-year legacy, broadening our horizons with its view of the universe.
On Sunday, June 13, at 20:00
UTC, Hubble’s main
computer stopped receiving a “keep-alive” signal from the telescope’s payload
computer. Once Hubble’s main computer recognized the payload computer halt, it
immediately put the telescope into safe mode, where all science instruments
were shut down and only essential telescope systems, such as power and
communication systems, are operational.
Hubble teams in the control
center at the Goddard Space Flight Center in Maryland recognized that Hubble
was in safe mode and later attempted to restart the telescope the following
day. However, the halt occurred yet again, and Hubble went back into safe mode.
All instruments on the Hubble Space Telescope are now in operational status, and science data is once again being collected to further our understanding of the universe. https://t.co/1pskum8dXY
— Hubble (@NASAHubble) July 17, 2021
Thankfully though, the payload
computer to blame for this issue was built with redundancy. Thus, a second
computer is available for use in orbit in the event that a failure occurs.
The Hubble team was able to
pinpoint the underlying problem in the Power Control Unit (PCU) of the
telescope’s payload computer. It contains a power regulator which ensures a
steady voltage supply of five volts to the payload’s computer hardware.
The PCU and the payload
computer reside in the Science Instrument Command and Data Handling (SI
C&DH) unit, which controls, coordinates, and monitors Hubble’s science
instruments. To check if the PCU is working optimally, a secondary protection
circuit senses the voltage levels leaving the power regulator. If the voltage
falls below or exceeds the allowable levels, this secondary circuit alerts the
payload computers to cease operations and enter into “safe mode.”
Safe mode is a capability built
into most modern spacecraft that pauses all non-critical functions when a
technical issue occurs. This keeps the spacecraft in a stable configuration and
prevents further damage while controllers troubleshoot the issue.
The teams conducted a series of
multi-day tests including attempts to restart and reconfigure the computer and
the backup computer. Although these tests were initially unsuccessful, they
gave ample insight into the underlying problem, and the teams discerned 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, yet another indication of Hubble’s
growing age.
Without the ability to service the
telescope and update its instruments, Hubble teams have had to work around
Hubble’s various failures throughout the past 12 years. One of Hubble’s most
recent failures occurred in March of this year.
The team was not able to reset
the PCU using ground commands, so the decision was made to switch over to the
backup SI C&DH unit, which contains the backup PCU. The reviews and testing
procedures for the switch were completed and the go-ahead was given by NASA
management. The switch was a multi-day process that started on Thursday, July
15, and was completed successfully the next day.
After a successful switch, the
teams monitored the hardware, ensured everything was working as expected and
started the process of recovering the science instruments out of their safe
mode configurations. They ran various procedures and ensured that instruments
were at stable temperatures. After some initial calibrations, normal science
operations were resumed on Saturday, July 17.
Scheduled operations which were
missed over the month that the spacecraft was in safe mode will be rescheduled.
This switch was similar to the
one performed in 2008, which allowed Hubble to continue normal science
operations after a Command/Unit Science Data Formatter (CU/SDF) module, also a
part of the SI C&DH unit, failed. The final Hubble servicing mission, STS-125, which
included NASA astronauts
Megan McArthur, Scott Altman, Gregory C. Johnson, Michael T. Good, John M.
Grunsfeld, Mike Massimino, and Andrew Feustel, replaced the entire SI C&DH
unit, including the faulty CU/SDF module.
Hubble’s payload computer is
tasked with coordinating and operating the many science instruments onboard the
telescope. Currently, there are four operational instruments on Hubble,
including the Advanced Camera for Surveys (ACS), the Cosmic Origins
Spectrograph (COS), the Fine Guidance Sensor (FGS), and Wide Field Camera Three
(WFC3).
This payload computer is a
1980s NASA Standard Spacecraft Computer-1 (NSSC-1) system located in Hubble’s
Science Instrument Command and Data Handling Unit. The NSSC-1 system features
four memory modules containing 64K of Complementary Metal-Oxide Semiconductor
(CMOS) memory.
