Astronauts wrap up 4th spacewalk to repair Hubble

Two spacewalking astronauts overcame a stuck bolt, a power tool with a dead battery and other frustrations while working aboard the shuttle Atlantis to revive a long inactive science instrument inside the Hubble Space Telescope.

The astronauts embark Monday on the last of the daily spacewalks that formed the cornerstone of NASA's ambitious strategy to extend observations with Hubble at least another five years.

Sunday's outing by Mike Massimino and Mike Good to recover an instrument that identifies super massive black holes was considered to be the most intricate of the five spacewalks.

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.

Mission Control sympathized. "This is sort of like what happens to me when I go out on the driveway and try to work on my car," said Preston Burch, NASA Hubble program manager.

"I can't tell you how many times you try something simple and the next thing you know you are dealing with bolts that have heads rounded off and what not. I certainly sensed his frustration."

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

The spectrograph, which astronomers use to gather information about the chemical composition, temperature, 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.

"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, 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 inner most 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.

In addition to the repairs, the Atlantis astronauts have installed a pair of new science instruments, a data management computer as well as gyroscopes and batteries to sustain the pointing and power systems.

During Monday's spacewalk, the astronauts will equip Hubble with three more batteries, a pointing sensor and external shielding.

The shuttle crew rendezvoused 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.