Two spacewalking astronauts overcame a stuck bolt, a fickle power tool and other aggravations Sunday to revive a long-inactive science instrument inside the Hubble Space Telescope.

The outing from space shuttle Atlantis by astronauts Mike Massimino and Mike Good to surgically repair a spectrograph that identifies super massive black holes was considered by NASA to be the most intricate spacewalk of the mission.

It turned equally frustrating for the two men as they were forced to contend with obstacles that required extra doses of ingenuity, patience and elbow grease and stretched their activities to just over eight hours.

"Oh, for Pete's sake," Massimino complained when the battery-operated ratchet he was using lost power. Later, the veteran astronaut cursed as he wrestled to discard the cover plate he'd pulled from the Space Telescope Imaging Spectrograph (STIS) into a storage bag.

Massimino and Good focused all of their efforts on the STIS, which was installed in the telescope by shuttle astronauts in 1997. The spectrograph, which astronomers use to gather information about the chemical composition, temperature, pressure and velocities 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 bolts, one of them badly stripped.

After several failed attempts to turn the bolt with wrenches, Massimino offered to snap the hand rail off using some old-fashioned muscle.

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

"Awesome," responded Mission Control.

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, but Hubble engineers planned to spend Sunday night conducting a more exhaustive test of the instrument.

Mission Control postponed plans to patch a damaged region of the telescope's exterior until Monday, when astronauts embark on the last of five gruelling spacewalks that form the cornerstone of an ambitious strategy to extend mind-boggling space observations with 19-year-old Hubble by at least another five years.

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 sapped 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, US space agency 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 and the discs of dust and gas around stars where planets are forming.

Hubble will remain anchored in the Atlantis payload bay until Tuesday. So far, refurbishments include the installations of two new science instruments, a data management computer as well as gyroscopes and batteries to sustain the pointing and power systems.

On Monday astronauts will replace three more batteries, a pointing sensor and external shielding.

The shuttle astronauts made their rendezvous with Hubble on Wednesday and gingerly hoisted it into the cargo bay to start the overhaul.

The 11-day Atlantis mission -- considered extra risky because Hubble circles Earth at a higher orbit than the International Space Station, making the telescope and the shuttle more susceptible to collisions with space junk -- is due to conclude Friday with a landing at the Kennedy Space Center in Florida.

Hubble is a cooperative project between NASA and the European Space Agency.

earlier related report
Tough 4th spacewalk to repair Hubble countinues
Two US astronauts struggled on a fourth spacewalk on Sunday to revive a long-inactive instrument aboard the 19-year-old Hubble telescope once used to study super massive black holes.

Mike Massimino and Mike Good emerged from the airlock of the shuttle Atlantis to begin work on the Space Telescope Imaging Spectrograph (STIS) at 1345 GMT, 30 minutes later than scheduled, to allow Good to adjust the fit of the right boot on his space suit.

The pair has struggled -- with Massimino cursing at one point -- to revive the inactive instrument they hope will help extend the life of the aging Hubble Space Telescope by at least five years.

They were forced to overcome a stripped bolt, a power tool with a weak battery and other obstacles during a marathon spacewalk that was headed beyond the eight hour mark.

"Oh, for Pete's sake," Massimino complained when a battery operated ratchet ground to a stop. Later, the veteran astronaut cursed twice as he wrestled to discard a cover plate into a storage bag.

Massimino and Good focused their efforts on the STIS, an instrument installed on the telescope by a shuttle crew in 1997.

The spectrograph, which is used by astronomers to study black holes and other celestial targets to gather information about chemical composition, temperature, pressures and velocity, was sidelined by a power failure in 2004.

In order to replace a failed power converter, Massimino and Good had to open the instrument with an arsenal of custom hand tools. The exacting task required Massimino to remove a protective cover held down by 111 small screws.

But access to the cover and its many tiny fasteners was obstructed by a hand rail that had to be removed first. The rail was secured by four bolts, One of them had been stripped so badly it defied several attempts by the spacewalkers to work it loose. Finally, Massimino was forced to pry the bulky hand rail until it tore away.

"Okay, here we go," said Massimino as he applied some old-fashioned muscle to break the fixture.

"Awesome," responded Mission Control.

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 also encountered a disabling electrical short in early 2007.

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

During an overnight testing session that concluded Sunday, US space agency NASA determined 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 the expansion rate of the universe.

The High Resolution channel, which could not be recovered, was used to study the innermost regions of galaxies and the discs of dust and gas around stars where planets are forming.

Hubble will remain anchored in the payload bay of Atlantis until Tuesday. So far, refurbishments include the installations of two new science instruments, a data management computer as well as gyroscopes and batteries to sustain the pointing and power systems.

Sunday's spacewalk to rehabilitate the instrument that hunts for black holes was considered the most intricate of the five daily outings.

A fifth and final spacewalk concludes on Monday, when astronauts will replace three more batteries, a pointing sensor and external shielding.

The shuttle astronauts made their rendezvous with Hubble on Wednesday and gingerly hoisted it into the cargo bay to start the overhaul.

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

Hubble is a cooperative project between NASA and the European Space Agency.