Astronauts tackle tricky repairs in third spacewalk

* Saturday spacewalk poses trickiest repair task so far

* Hubble to get new light-splitting spectrograph

* Astronauts have struggled with fiddly work in space

By Irene Klotz

HOUSTON, May 16 (Reuters) - Two spacewalking astronauts returned to the Hubble Space Telescope on Saturday for one of their toughest repair tasks so far to fix a broken camera and to install a spectrograph that reveals cosmic chemistry.

Astronauts John Grunsfeld and Andrew Feustel left shuttle Atlantis' airlock shortly before 10 a.m. EDT (1400 GMT) for the third of five spacewalks to upgrade the world-famous space observatory for another five to 10 years of work.

NASA expected Saturday's spacewalk to be the most challenging of the tasks planned during Atlantis' ongoing mission, the fifth and final servicing call to Hubble before the shuttle fleet is retired next year.

Hubble's observations have reshaped scientists' understanding of how galaxies form and change over time, of planet origins and of the mysterious "dark energy" force that is inflating the universe at a faster and faster rate.

To keep Hubble on the cutting edge of scientific discovery, it is being outfitted with two new instruments. One is a panchromatic, wide-field camera which was installed by Grunsfeld and Feustel during an initial spacewalk on Thursday.

The other, to be installed on Saturday, is an instrument known as the Cosmic Origins Spectrograph, which can break down light into its component wavelengths and reveal what chemicals and conditions the photons have passed through.

"The Cosmic Origins Spectrograph will be able to look through the universe and help us understand what the universe is made of and, and how it was formed," Grunsfeld, an astrophysicist, said during a preflight interview.

FIDDLY REPAIRS IN SPACE

The repair tasks are made all the more difficult by the fact that the gloved and suited astronauts are working in the free-floating environment of space.

Engineers created special tools to help the spacewalkers handle tiny screws and sharp-edged computer circuit boards, but Hubble managers can still never be sure of the outcome.

Grunsfeld, taking the lead on Saturday's repairs, won't even be able to see what he's doing as he tries to take out 32 screws and four circuit boards on Hubble's Advanced Camera for Surveys, which shut down in 2007 after a power failure.

He will have to rely on hours of painstaking practice to reach around a side of the camera to do the work.