Hubble will transition to single-gyro mode to gain a few more years of operational life

So about that commercial servicing mission, hmm?

NASA has confirmed that the time has come: the venerable Hubble Space Telescope (HST) is to run in one-gyro mode from now on.

The observatory has six gyroscopes, installed during the final Space Shuttle servicing mission in 2009, but over the years, three have failed. A fourth began showing signs of wear, resulting in the spacecraft dropping into safe mode in April. That event has now resulted in engineers deciding the time has come for the HST to transition into a mode whereby it can be pointed using only one gyro.

The mode requires other instruments onboard the spacecraft to be used for pointing the telescope as well as a single gyroscope. According to NASA, the technique was already demonstrated in 2008 "with no impact to science observation quality."

However, the agency noted that "there are some expected minor limitations." It will take longer for the observatory to slew and lock onto a science target, and it won't be able to track moving objects closer than Mars, "though these are rare targets for Hubble."

The optimism of NASA officials needs to be balanced with comments from the scientific community. The Space Telescope Science Institute concluded in a 2016 paper [PDF] that "Entry into RGM [Reduced Gyro Mode] will reduce the scientific productivity of HST by ~25 percent, preclude several existing science observing strategies, reduce synergies with other observatories, and decrease the likelihood of responding to time critical events."

Without another servicing mission scheduled, managers do not have much choice.

The failing gyros are not the only issues to have beset Hubble. In recent years, engineers have also had to deal with computer problems that sent the telescope offline for more than a month in 2021.

NASA hopes that switching to a single gyro mode will extend the observatory's operational life for many years to come, even if its efficiency might be lowered. It also extends the period for a potential servicing mission or reboost in the future.

The spacecraft was outfitted with a Soft Capture Mechanism during the final Space Shuttle servicing mission in 2009. Once the Hubble finally reaches the end of its operational life, an uncrewed vehicle will dock with it to perform a controlled reentry.

A 2005 study estimated that more than 2,000 kg of debris could survive reentry, meaning targeting will be essential. ®

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