Astronauts Mike Massimino and Mike Good floated from the airlock of the shuttle Atlantis on Friday for a second spacewalk as part of an ambitious overhaul of the Hubble Space Telescope.

Their six- to seven-hour spacewalk got under way at 8:49 am (1249 GMT).

Massimino, a 46-year-old mechanical engineer, took part in a 2002 overhaul mission to Hubble.

Good, a 46-year-old US Air Force colonel and flight test engineer, is making his first spaceflight. The Atlantis astronauts carried out the first of five spacewalks on Thursday.

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Astronauts prepare for second Hubble spacewalk
Astronauts aboard the shuttle Atlantis are set to perform the second of five spacewalks on Friday to carry out vital work on the Hubble telescope.

Spacewalkers John Grunsfeld and Drew Feustel upgraded the telescope for the first time in seven years on Thursday, equipping the 19-year-old stargazer with a powerful new camera and science computer during a seven-hour outing.

On Friday at 1216 GMT, it will be the turn of Mike Good and Mike Massimino, who are to replace sensor units for the Hubble's gyroscopes and swap the first of two batteries.

The second spacewalk is set to last six and a half hours. If the time allows, the astronauts also hope to prepare for the mission's third outing by installing a power input harness for Hubble's new advanced camera.

Good and Massimino, both 46 years old, will be hoping not to encounter any problems like the dodgy bolt that threatened to prevent Grunsfeld and Feustel on Thursday from removing the telescope's old camera.

Feustel, a 43-year-old geologist and a rookie astronaut, provided the muscle that eventually freed the bolt.

His companion Grunsfeld used the baseball parlance for tricky curveball pitches to describe the incident.

"We gave Hubble a hug, and true to form Hubble gave us a few curves," said the 50-year-old, who is making his third visit to the telescope -- more than any other astronaut.

The refurbishments should mean the iconic telescope -- named after the great US astronomer Edwin Hubble -- can send back its fantastic images to Earth for an extra five years.

The new camera will equip Hubble, which whizzes around the Earth at more than 17,500 miles per hour (28,000 kilometers per hour), to search for the most distant star systems, probing the mysteries of dark matter and dark energy.

Atlantis and its seven-member crew arrived on Wednesday and hoisted the 13.2-meter telescope aboard using the shuttle's robotic arm, which was operated by mission specialist Megan McArthur.

The spacewalkers struggled on Thursday to replace the telescope's Wide Field and Planetary Camera-2, a 16-year-old imager, with the new Wide Field Camera-3, a more versatile and capable instrument.

The older camera was stubborn about making way for its replacement.

Grunsfeld and Feustel reached for an assortment of ratchet tools to remove two bolts that secured the 16-year-old imager inside the telescope, throwing them about 30 minutes behind their timeline.

As Grunsfeld reached into a tool bag for one of the ratchets, a rivet floated out. The veteran spacewalker reacted quickly by grabbing the fastener before it could float away becoming yet another piece of space debris.

The spacewalkers also worked with caution to avoid disturbing a dusting of white material spotted near the shuttle's airlock on Wednesday. NASA feared the material might float free and contaminate the telescope's optics.

"I see a small amount of what looks like dust, but it's pretty minor," Feustel assured Mission Control.

The installation of a new science computer proved to be a much easier task.

The Science Instrument Command and Data Handling System experienced an electrical problem in late September. The setback prompted NASA to postpone plans to launch the Hubble mission in October so engineers could prepare a replacement.

The computer prepares each of the telescope's science instruments for astronomical observations and formats the findings for transmission to Earth.

Although NASA has no plans for a future shuttle visit to Hubble, a new docking device also installed on the first spacewalk would allow a robotic spacecraft with a propulsion module to latch onto the observatory.

When Hubble is no longer able to conduct observations, NASA plans to steer the space telescope into the Pacific Ocean rather than allowing it to plunge back to Earth uncontrolled, potentially endangering a populated area.

During a third spacewalk on Saturday, astronauts will install the Cosmic Origins Spectrograph, an instrument developed to study the grand scale structure of the universe, including the star-driven chemical evolution that produced carbon and the other elements necessary for life.