Bottom line: The Hubble Space Telescope has been in safe mode since last Friday evening, following the failure of one of the gyros that helps stabilize it. NASA is analyzing the problem and hopes to resume operations soon.
Hubble is now down to two working gyroscopes and needs at least three for optimal operations but it can continue to provide observations with just one functioning gyroscope.
We know that the unit has a total of six gyroscopes, because it was designed with multiple redundancy of all systems. And so, that appears to be an issue that we don't understand fully at the moment.
Hubble has six gyroscopes it uses to point itself and keep itself steady for observations. The safe mode is meant to keep the telescope "precisely pointed" for a long duration, the space agency explains, stating that experts are working on fixing the problem.
Named after astronomer Edwin P. Hubble, the foremost American astronomer of the 20th century, the sophisticated optical observatory was placed into orbit about 600 kilometers (370 miles) above Earth by the crew of the Space Shuttle Discovery on April 25, 1990.
In connection with the damage to the telescope interrupted his work and was put into safe mode. An anomaly review board, which will include experts from the Hubble team and industry, familiar with the design and performance of this type of gyroscope, is being formed to investigate this issue and develop a recovery plan. "NASA is working to resume science operations", NASA said in an official statement.
The Register first noticed a questionmark over Hubble's health during the weekend, when Michigan State University astronomer Jay Strader tweeted rumours that it was in safe mode "following a gyro failure". Hubble can do good science with two gyroscopes, or even one, astrophysicist Grant Tremblay, of the Harvard-Smithsonian Center for Astrophysics, said via Twitter Sunday. As recently as previous year, Webb was scheduled for launch in 2018, but a series of engineering setbacks and missteps has delayed liftoff until 2021 at the earliest. Thus, at this time, Hubble has two functional gyroscopes.
"It's not a hard decision, @astrogrant: the plan has always been to drop to 1-gyro mode when two remain". Hubble usually uses three gyros at a time for maximum efficiency, but can continue to make scientific observations with just one.