The device had been expected to fail at some point this year, but the surprise came when a backup did not kick in properly after Friday’s failure, said Nasa on Monday.
As a result, Hubble remains in so-called safe mode and all science observations are on hold.
The 28-year-old telescope has had trouble with its gyroscopes before. Spacewalking shuttle astronauts replaced all six in 2009 during the final servicing mission. Three of the devices are now considered unusable.
“I think Hubble’s in good hands right now, I really do,” said Kenneth Sembach, director of the Space Telescope Science Institute, which operates Hubble. “The fact that we’re having some gyro problems, that’s kind of a long tradition with the observatory.”
Gyroscopes are needed to keep Hubble pointing in the right direction during its observations 340 miles (540km) above Earth. Precise pointing is crucial: Astronomers use the telescope to peer deep into the cosmos, revealing faraway solar systems as well as galaxies and black holes. Just last week, astronomers said they might have discovered the first moon outside our solar system, with Hubble’s help.
Since its launch in 1990, Hubble has made more than 1.3m observations.
Two of Hubble’s gyroscopes are working fine, Sembach said. The last one was in reserve; it was turned off some years ago after exhibiting some “funny behaviour” even though it was getting the job done, he said. That was the one flight controllers turned on on Saturday as a backup. As of Monday, it still wasn’t working as expected.
Sembach said everyone wanted to be careful in attempting a repair. “Obviously, we don’t want to make things worse,” he said.
Hubble normally uses three gyroscopes to function, but could get by with one or two, something it has done before. But that leaves little room for additional breakdowns. Besides redundancy, three functional gyroscopes also provide more flexibility in pointing, Sembach said.
“We’ll be fine,” he said from the Baltimore institute. “I’m sure Hubble has many years of good science ahead of it.”
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