The Hubble Space Telescope has resumed science operations in single-gyro mode after one of its three remaining gyros was declared suspect.

Science operations resumed on Friday, June 15, 2024, albeit with what the Hubble team described as "minor limitations." Engineers plan to monitor the suspect gyro in the hope that it stabilizes and can be used again.

For now, however, the Hubble is down to one gyroscope, with a second in reserve "for future use."

According to NASA, there is a 12 percent decrease in efficiency in operations during one-gyro mode due to the extra time that's needed to lock onto a science target. The mode swaps the failed gyros for a multi-step process using the observatory's magnetometers, sun sensors, star trackers, fine guidance sensors, and a single working gyro to point the telescope.

In addition to the extra time needed to get the telescope pointing in the correct direction, the mode also reduces the area of sky that the Hubble can point to, so transient events or targets of opportunity - an exploding star or an impact on Jupiter - might be missed. Any moving objects closer to Earth than the orbit of Mars might also be missed - Hubble needs three gyros to move fast enough to track them.

All told, NASA reckons that the total decrease in productivity will be "roughly 20 to 25 percent" from when three gyros were in use. A 2016 paper [PDF] from the Space Telescope Science Institute reckoned the figure was 25 percent.

Still, three-quarters productivity is far better than nothing at all, and the Hubble has been a highly productive observatory during the 34 years since its launch.

The Space Shuttle was used to service the observatory, and the final servicing mission - STS-125 - replaced all six gyros. In the 15 years since, four of the six have degraded, meaning engineers have had to wheel out plans developed more than 20 years ago for operating the telescope with less than its full complement of gyros.

The Space Shuttles have long been retired, and there is currently no commercial servicing mission on the books for the veteran observatory. Still, it is at least back in operation now, even if scientists might need to rethink some planned observations.

The US space agency said: "NASA anticipates Hubble will continue making discoveries throughout this decade and possibly into the next, working with other observatories, such as the agency’s James Webb Space Telescope for the benefit of humanity." ®

https://www.theregister.com//2024/06/17/hubble_space_telescope_is_back/