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Hubble Space Telescope
The Hubble Space Telescope hangs in orbit after a 2002 servicing mission. (NASA Photo)

Things are looking up for two of NASA’s Great Observatories, the Hubble Space Telescope and the Chandra X-Ray Observatory.

Both telescopes had to go out of service this month due to different types of problems experienced by their gyroscopic pointing systems.

On Oct. 5, one of the three active gyros being used by the 28-year-old Hubble Space Telescope failed, triggering a safe-mode condition.

Hubble has six gyros on board, but two of them had failed already and a third one was glitchy. Having four non-working gyros forced the Hubble team members to consider whether they’d have to go to a less efficient mode of operation, in which only one gyro would be used for pointing at celestial targets and the other one would be reserved as a backup.

The more attractive alternative was to get the glitchy gyro working again and return Hubble to its standard three-gyro pointing mode. The main problem was that the gyro’s rotation rate was significantly higher than the usual 19,200 revolutions per minute. To put it back into service, engineers had to slow down the spin.

First, they tried turning the gyro off for a second and then turning it back on. That didn’t work. Then they put the spacecraft through a series of back-and-forth turns while switching between pointing modes, in hopes that the jiggling would dislodge any blockage inside the gyro and bring the rotation rate down to normal.

An initial round of jiggling on Oct. 18 resulted in a significant reduction in the rate. The next day, the team commanded Hubble to go through additional maneuvers and mode shifts. That apparently cleared up the issue: The gyro started registering normal rates for high-precision as well as low-precision pointing modes.

Engineers put Hubble through yet another round of maneuvers, just to make sure the gyro’s rate stayed sufficiently stable while the telescope was in motion. The team monitored the performance over the weekend, and as of this week, Hubble seems to be back in shape.

The gyros “are working again, in case you’re wondering, and we do hope to be back on sky with Hubble very soon,” Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy, said today during a Washington Post forum on spaceflight. AURA is responsible for managing the Space Telescope Science Institute and other telescope centers.

The telescope will be put through a series of tests to evaluate the revived gyro’s performance during typical observing conditions — including moving to targets, locking on and performing precision pointing. Once the pointing system passes those tests, Hubble should be cleared to resume normal science operations.

The 19-year-old Chandra X-ray telescope, meanwhile, has fully recovered from a gyro glitch that triggered a safe-mode condition on Oct. 10. Chandra’s operations team tracked down the problem and placed the gyro that experienced the glitch in reserve. Then the team returned the spacecraft to normal pointing mode and applied a series of pre-tested software patches.

Today NASA reported that Chandra has resumed science operations. “We are back in science mode!” Thomas Zurbuchen, associate administrator for the NASA’s Science Mission Directorate, said in a tweet.

Sending a repair crew to the scene wouldn’t have been an option for either telescope. Chandra wasn’t designed for on-orbit servicing, and Hubble has been inaccessible since the retirement of NASA’s space shuttle fleet in 2011.

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