About this Article

John MacKenty (mackenty[at]stsci.edu)

Published April 29, 2026

The primary electronics (Side B) of the Hubble Space Telescope’s (HST) Science Instrument Command and Data Handling (SI C&DH) computer system developed a fault in June 2021 that prevented it from reliably sending commands to the science instruments, although data transfer remained possible. HST was switched to its backup electronics (Side A) that July. However, this left the observatory without redundancy and at risk of a catastrophic failure. The HST team at the Space Telescope Science Institute (STScI) and NASA’s Goddard Space Flight Center (GSFC) has spent the past five years developing a software solution, called B-Side Operations (BSO), that restores full redundancy to this critical system. Final system-level testing is now underway, with installation of the flight software expected to proceed this summer. With the restoration of the redundancy in the electronics of the SI C&DH, the probability of this system being operational in 2030 is 85%.

HST’s onboard computing is divided into two systems. Responsibility for flight operations (e.g., attitude control, communications, executing command sequences, power management, safe modes) resides in the main flight computer, which was upgraded during Servicing Mission 3A in 1999 to the current 486 system. This highly redundant unit (3 CPUs and 6 memory banks) continues to function well and provides sufficient computing capacity to cope with the long-term aging of the gyroscopes and fine guidance sensors. The SI C&DH is a separate system responsible for the operations and safekeeping of the science instruments with two independent sets of electronics (Side A and Side B), providing redundancy against hardware failures.

The current SI C&DH unit was installed during Servicing Mission 4 (SM4) in 2009, replacing the original unit after it failed in 2008. That failure necessitated switching from the prior primary side (A) to the redundant side (B), a major operation impacting many components of HST but consistent with the existing flight and ground software systems. It also left HST only a single failure away from ceasing operations and led to the decision to delay SM4 from 2008 to 2009. The spare SI C&DH unit that was flown had been built in the 1980s and used continuously for ground testing of software changes and new HST instruments; it was refurbished and tested before launch, restoring this critical redundancy.

Upcoming Installation of Improved Flight Software: B-Side Operations

Given the difficulty and limited benefits of transitioning HST back to Side A after SM4, the observatory continued to operate on Side B with Side A as the backup. After just over 12 years, Side B developed the fault described above. The electrical issue is fairly well understood and isolated to an aging component rather than a broken connection or discrete part failure.

Testing with the ground-based engineering systems, which include the original SI C&DH unit returned by the astronauts during SM4, confirmed that pathways exist to restore full operation of the science instruments by sending commands from the 486 flight computer rather than the SI C&DH. The difficulty is that implementing this solution required extensive software modifications to both the 486 flight software and the ground systems at GSFC and STScI that create the weekly detailed observing schedules. Many of these systems had been developed in the 1980s and 1990s, were written in assembly or Fortran, and had remained essentially static for decades. The changes touch on the critical safety systems that autonomously handle software or hardware faults on the spacecraft and its science instruments, requiring rigorous design review and testing.

The HST team has now brought this project nearly to completion, with installation of the flight software expected to proceed in summer 2026. This work was accomplished in parallel with other major operational efforts, including the transition to single-gyro science mode, the commissioning of new Cosmic Origins Spectrograph (COS) FUV lifetime positions, and the development of mitigations for aging Fine Guidance Sensor hardware. BSO mode is expected to provide the same level of scientific performance as the current flight software, with at most one to two minutes of additional overhead for some commands.

In the best-case scenario, HST continues to operate on Side A indefinitely and BSO is never needed. However, should Side A develop the same problem that affected Side B, science operations can continue on Side A using the BSO software, which stands ready to go. Should a different type of failure on Side A require a switch to Side B, HST will then operate with the BSO software on Side B. The BSO development restores HST’s operations to a fully redundant and resilient status, giving confidence in continued scientific productivity into the 2030s.