Two US astronauts struggled to refurbish the Hubble Space Telescope's pointing and power systems Friday, on a marathon spacewalk to equip the 19-year-old observatory with new gyroscopes.

In a spacewalk which stretched to seven hours and 56 minutes, Mike Massimino and Mike Good successfully installed new gyroscopes and new batteries on the revolutionary stargazer.

Their persistence with the installation of a half-dozen new gyroscopes, the highest priority of an 11-day mission to extend operations of the telescope by five years, paid off.

Much of their space work was spent in the cramped instrument bay at the base of the telescope, which juts 13.2 meters (43.5 feet) out of the ship's cargo bay.

"At times, I felt like I was wrestling a bear," Massimino complained as he and Good improvised to achieve the most difficult task, the gyro installation.

Mission managers rewarded the shuttle astronauts for their perseverance with an extra hour of sleep before they tackle the third of the five daily spacewalks on Saturday.

During the first of the mission's five daily spacewalks on Thursday, two other astronauts also equipped Hubble with a powerful new camera and science computer, the mission's second and third highest priorities.

The seven Atlantis astronauts rendezvoused with Hubble on Wednesday and hoisted it aboard the shuttle for a final overhaul.

When the work is finished, the iconic telescope should achieve a new summit of discovery, able to scan the distant universe in search of the oldest star systems, contribute to an understanding of dark matter and dark energy as well as observe planet forming processes around other stars.

Astronaut mechanics last worked on the telescope in March 2002. As Atlantis launched, only three of the six gyroscopes responsible for Hubble's precision pointing were functioning.

An equal number of batteries aboard the solar-powered telescope were losing their ability to re-charge and provide electricity when the observatory was in darkness.

Massimino, a 46-year-old mechanical engineer who was making his second trip to the space telescope, paired comfortably with Good, a 46-year-old US Air Force flight test engineer.

The gyro work took them deep inside the cramped telescope. Dressed in restrictive spacesuits, they stretched out on their backs and reached up with power ratchets to install the new hardware.

The gyros have to be installed in pairs deep. But one of the three new sets would not align properly, forcing the spacewalkers to improvise, drawing out the time required for the critical task.

"It felt like it has been aligned a couple of times, but then it doesn't want to go," Good, a 46-year-old US Air Force flight test engineer on his first space voyage, informed Mission Control.

"I know it's frustrating, but keep up the great work," shuttle communicator Dan Burbank calmly told Massimino and Good from the control center. "Hopefully, we will get lucky in a minute."

Their fortunes changed for the better when the spacewalkers reached for a spare gyro set that had been removed from the telescope in 1999, then rebuilt on Earth and flown back aboard Atlantis. With some coaxing, the spare fit.

"Nice job," said Mission Control.

Though nearing the end of what had been planned as a six- and-a-half-hour spacewalk, Mission Control urged the two men to work overtime to install three of the six new batteries.

"I'm on the edge now," said Good. "But I'm willing to give it a try." Three more new batteries will be installed on Monday.

Astronauts John Grunsfeld and Drew Feustel will embark on the mission's third spacewalk early Saturday.

They plan to equip Hubble with a second new science instrument, the Cosmic Origins Spectrograph, and repair another, the Advanced Camera for Surveys.

The spectrograph was developed to study the grand scale structure of the universe to explain how galaxies formed and clustered. It will also chart how the chemical elements, especially those necessary for life, emerged and changed in abundance over time.

The advanced camera increased Hubble's field of view and observing efficiency 10-fold when it was installed in 2002.

However, an internal electrical short five years later shut down the imager's ability to observe distant galaxies. The spacewalkers hope to resurrect the camera by replacing an internal electronics box and attaching a new power supply.