NASA became aware of the issue last Friday when a gyro, already displaying "end of life" behavior, went out of action.
Hubble has carried a total of six gyroscopes - three standard and three enhanced - since astronauts installed them during a spacewalk in 2009, typically using three at once. This is one reason why NASA sought to install three enhanced gyros on the telescope in 2009. With the shuttle now retired, NASA has no present capability to send humans to fix the telescope. That gyro, which had been powered down, is not "performing at the level required for operations", according to spacecraft telemetry after it was commanded to turn on.
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.
Only two of those enhanced gyros are now running.
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Gyroscopes are needed to keep Hubble pointed in the right direction during observations. But when the third one was powered up, it wasn't operating as it should be, so NASA Goddard engineers placed the telescope in safe mode while they try to figure out the problem.
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"The plan has always been to drop to 1-gyro mode when two remain", Rachel Osten, deputy head of the Hubble mission said on Twitter. Staff at NASA's Goddard Space Flight Center and the Space Telescope Science Institute are now performing analyses and tests to determine what options are available to recover the gyro to operational performance. "Which the Astro community wants desperately", Osten tweeted.
Science operations with Hubble have been suspended while NASA investigates the anomaly.
As a result, Hubble is in so-called safe mode with non-essential systems turned off, putting all science observations on hold.
Even if that particular gyroscope stays out of order, Hubble can get back to work - while it works best with three gyroscopes, the telescope can run on just one without losing too much scientific power.