HOUSTON Struggling with balky hardware, spacewalking astronauts Friday replaced gyroscopes that will allow the Hubble Space Telescope to steady its gaze on distant galaxies.
Replacing Hubble's six gyroscopes was the top priority for shuttle Atlantis' 11-day servicing mission, NASA's fifth and final visit to the observatory before the shuttles are retired next year.
NASA hopes the improvements will keep Hubble operational until at least 2014 so it can work in tandem with its replacement, the James Webb Space Telescope.
Hubble ended up with five new gyroscopes designed to thwart the corrosion which felled earlier models, and a sixth refurbished older device that is more failure-prone.
"All it means is that one of our six gyroscopes is an earlier design," said Ray Villard, with the Space Telescope Science Institute in Baltimore, which operates Hubble for NASA.
With three gyroscopes, Hubble can stay fixed on celestial targets as precisely as a laser beam hitting a dime 200 miles away. Three gyroscopes are kept as spares.
It was the second day of spacewalks beset by technical hurdles, after a balky bolt Thursday nearly prevented astronauts from installing a new wide-field camera that will allow the telescope to see closer to the origins of the universe.
The frustration level was evident during Friday's spacewalk by astronauts Michael Massimino and Michael Good, which spanned nearly eight hours and was the eighth-longest in history.
"I felt like it's been aligned a couple of times, but it just doesn't want to go," Massimino radioed to his crewmates aboard the shuttle and ground control, as he labored from within the telescope's body to install the new gyroscopes.
"I know it's kind of frustrating," replied astronaut Dan Burbank from Mission Control. "Hopefully we'll get luckier."
Eventually NASA engineers opted to install a refurbished model that had been removed from the telescope during a 1999 shuttle mission.
Massimino and Good also replaced three of Hubble's batteries. The remaining three are to be replaced during the mission's last spacewalk Monday.
(Additional reporting by Irene Klotz; Editing by Eric Walsh)