Astronauts set for last spacewalk to repair Hubble

US astronauts are to embark on their final spacewalk under NASA's ambitious bid to extend the life of the Hubble Space Telescope for another five years at least.

After they venture outside the space shuttle Atlantis for the fifth and last time, the astronauts will equip the telescope with three more batteries, a pointing sensor and external shielding.

Astronauts Mike Massimino and Mike Good on Sunday overcame a string of frustrations while working aboard the shuttle to revive a long inactive science instrument inside Hubble.

That outing to recover the instrument that identifies super massive black holes was considered to be the most intricate of their spacewalk jobs.

It turned equally frustrating for the two men when they were forced to overcome a stripped bolt, a power tool with a dead battery and other obstacles that stretched their activities to more than eight hours.

"Oh, for Pete's sake," Massimino complained when the battery in the power ratchet he was holding died.

Later, the veteran astronaut cursed as he wrestled to discard the cover plate he'd pulled from the Space Telescope Imaging Spectrograph into a storage bag.

Massimino and Good focused all of their efforts on the imaging spectrograph, an instrument installed in the telescope by shuttle astronauts in 1997.

The spectrograph, which astronomers use to gather information about the chemical composition, temperature and pressure motions of celestial targets, was sidelined by a power failure in 2004.

In order to replace a failed power converter, Massimino and Good had to replace an internal circuit card. The extraction required Massimino to remove a protective cover secured by 111 small screws using an arsenal of custom-made hand tools.

But access to the cover and the many tiny fasteners was obstructed by a hand rail that had to be removed first. The rail was secured by four thin bolts, one of them badly stripped.

After several failed attempts to turn the bolt with wrenches, Massimino offered to snap the hand rail off by hand.

"Okay, here we go," said Massimino as he broke it away.

The two men then made quick work of removing a protective cover, replacing the bad circuit card and installing a new cover held down by a pair of latches rather than screws.

The spectrograph quickly passed an electrical test, the first step in a more thorough evaluation.

"Today was like a dream come true for the science community," said NASA astrophysicist Jennifer Wiseman, who predicted the revived spectrograph will experience heavy use.

"It has a very unique capability. That is why this is such a tremendous victory for us."

Mission Control postponed plans to patch a damaged region of the telecope's exterior until Monday.

On Saturday, spacewalkers John Grunsfeld and Drew Feustel breezed through a similar but less demanding repair of Hubble's Advanced Camera for Surveys, a heavily used seven-year-old instrument that encountered a disabling electrical short in early 2007.

The short disabled three internal imagers, though experts were able to recover one of them within a month of the power disruption.

When an overnight testing session ended on Sunday, NASA announced that Grunsfeld and Feustel had recovered one of the two long disabled internal imagers.

The revived Wide Field channel accounts for about 90 percent of the survey camera's observations, many of them focused on studies of galaxies and distant star systems used to calculate how rapidly the universe is expanding.

The High Resolution channel, which could not be recovered, was used to study the innermost regions of galaxies.

Hubble will remain anchored in the payload bay of Atlantis until Tuesday.

The shuttle crew made their rendez-vous with Hubble on Wednesday and hoisted it into the cargo bay to start the overhaul.

The shuttle's 11-day mission is scheduled to conclude Friday with a landing at the Kennedy Space Center in Florida.