John Grunsfeld and Drew Feustel floated from the airlock of the shuttle Atlantis at 8:52 am (1252 GMT) to begin a spacewalk expected to span six to seven hours with a burst of enthusiasm.

"It's a great Hubble day," said Grunsfeld, a 50-year-old astronomer who is making his third visit to the iconic science instrument, the most by any astronaut.

"Pretty cool," said Feustel, a 43-year-old geologist and rookie astronaut, as he followed Grunsfeld from the shuttle's airlock. "Fantastic."

The seven Atlantis astronauts rendezvoused with Hubble on Wednesday, capturing the 13.2 meter (43 and half feet) telescope with the shuttle's robot arm and mounting it upright in the cargo bay.

In all, five daily spacewalks are planned to extend observations by the telescope for at least five years.

The ambitious refurbishment will upgrade the power and pointing system with new batteries and gyroscopes and a more robust science computer.

The new Wide Field Camera-3 installed on Thursday, replaces the Wide Field and Planetary Camera-2, a 16-year-old workhorse imager.

The new more versatile camera is capable of looking deep into the far reaches of the cosmos for signs of the earliest star systems as well as studying the closest planets.

The Cosmic Origins Spectrograph will join the new camera on Saturday. The spectrograph has been developed to study the grand scale structure of the universe as well as map the appearance and distribution of carbon and the other chemical elements necessary for life.

In addition, the astronauts will attempt repairs to the internal circuitry of two science instruments that experienced earlier electrical problems.

But Thursday's outing was not problem free.

Two bolts that secured the Wide Field and Planetary Camera-2 proved difficult to remove. The astronauts reached for an assortment of ratchet tools before finding one with enough force to free them. The struggle put them about 30 minutes behind schedule.

As Grunsfeld reached into a tool bag for one of the ratchets, a rivet floated out. Grunsfeld reacted quickly by reaching out and grabbing the fastener before it could float away.

The spacewalkers also exercised caution to avoid a dusting of a white material spotted on a piece of of the shuttle's equipment. NASA feared the material might contaminate the telescope's optics.

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

Thursday's spacewalk includes a second high priority task, the installation of a new science computer to replace a processor that experienced a partial electrical failure in late September.

The breakdown of the Science Instrument Command and Data Handling System prompted NASA to postpone plans to launch the Hubble mission in October so engineers could prepare a replacement.

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

The third task on the first spacewalk was the installation of a new docking system.

Although NASA has no plans for a future shuttle visit to Hubble, the new docking device 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.

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