UPDATE 2-Spacewalkers replace Hubble steering system

*NASA settles for spare set of gyroscopes

*Spacewalking astronauts replace two batteries (Recasts, spacewalk ends, other details)

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 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 (320 km) away. Three gyroscopes are kept as spares.