*NASA settles for spare set of gyroscopes *Spacewalking astronauts replace two batteries (Adds official comment on gyroscopes) By Chris Baltimore HOUSTON, May 15 (Reuters) - Struggling with balky hardware, spacewalking astronauts on 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 two new pairs of gyroscopes designed to thwart the corrosion which felled earlier models, and a third refurbished pair that NASA officials said are nearly as reliable as the new ones. "The difference in the projected longevity of the observatory in the out years is very small" due to the refurbished gyroscopes, said Preston Burch, Hubble's program manager. "We don't see this is a significant detriment at all to the observatory." With three gyroscopes, Hubble can stay fixed on celestial targets as precisely as a laser beam hitting a dime 200 miles (320 km) away. Three gyroscopes are kept as spares. It was the second day of spacewalks beset by technical hurdles, after a balky bolt on 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 on Monday. (Additional reporting by Irene Klotz; Editing by Eric Walsh)
