CAPE CANAVERAL, Fla. -- NASA's flagship Hubble Space Telescope was in stable condition Wednesday after spacewalking astronauts performed chancy transplant surgery on the electrical heart of the observatory.

What's Next: Tonight in Space

Two spacewalking astronauts will set out Thursday to outfit the Hubble Space Telescope with an advanced planetary camera that is 10 times more powerful than its predecessor. Look for mission specialists James Newman and Michael Massimino to venture outside shuttle Columbia about 2:30 a.m. EST (0730 GMT), or about hour later than originally planned. The start-time was adjusted to give the shuttle crew more time to sleep after a long workday Wednesday. Once outside, Newman and Massimino will remove Hubble's Faint Object Camera and stow it in Columbia for a return to Earth. Its replacement: The $76 million Advanced Camera For Surveys, which is the most powerful planetary camera ever launched into orbit. Click here for mission updates and live NASA TV coverage beginning at 3 a.m. EST (0800 GMT) Thursday.

With shuttle Columbia's cargo bay serving as an orbital operating room, John Grunsfeld and Rick Linnehan worked with the precision of physicians, removing and replacing the telescope's crucial but flawed power switching station.

"You did it, buddy. You did it!" Linnehan called out with an incredulous laugh after his partner finished up the high-stakes procedure. "Good job!"

"Outstanding work," fellow astronaut Mario Runco told the spacewalkers from NASA's Mission Control Center in Houston. ""Words can't describe how people feel down here with the accomplishments of today."

Purposely halted for the operation, the electrical heartbeat of Hubble was restored four hours and 24 minutes after surgery started. Ground controllers then started pumping power to its eyes and its other body parts before extreme temperatures could cause irreparable damage.

The prognosis for complete recovery: "Very high," said NASA Hubble project manager Preston Burch. "One-hundred percent."

Considered the riskiest operation in Hubble history, the procedure called for the telescope to be turned off for the first time since it was launched 12 years ago.

A less-extensive power down was required when Hubble underwent eye surgery in 1993 to correct a severe case of myopia, one caused by a debilitating manufacturing flaw in the observatory's primary mirror.

And a partial outage was ordered when an electronic brain transplant was performed in 1999.

But to prevent the astronauts from being shocked or electrocuted during Wednesday's operation, all internal Hubble power was turned off at 4:37 a.m. EST (0937 GMT).

The flow of electricity through a shuttle electrical umbilical also was cut, creating a situation likened to performing transplant surgery without a life-sustaining heart-lung machine.

The only thing left ticking: a "thermal clock" that counted up toward the time - about 8 to 10 hours later -- when extreme cold or hot temperatures would begin to freeze or overheat sensitive science instruments and spacecraft control systems.

Anxious NASA officials, consequently, spent the day pacing around a waiting room, watching the astronauts as their precarious work unfolded.

"This morning we saw what I would have to call an Academy Award-winning performance by the astronauts, as they worked against the clock to bring Hubble Space Telescope back to life," said Anne Kinney, director of NASA's astronomy and physics branch.

"This dramatic and a masterful performance caused us to laugh, and caused us to gasp occasionally, and finally caused us to sigh," she added. "It was a moving performance."

Even after the surgery, there was the chance that Hubble might remain in a scientific coma, or that its control systems could be rendered useless, potentially putting the observatory into a slow but devastating tumble.

Fearful project officials, however, said the operation was a risk that had to be taken.

"As with any beloved relative, you're worried about sending them in for bypass surgery or even a heart transplant," said Hubble chief scientist David Leckrone. "But you realize the risk of not doing it is severe."

In this case, the telescope's old Power Control Unit had been faltering for the past nine years.

A simple loose screw was impeding its ability to route life-giving electricity from Hubble's solar wings to science instruments, control systems and six batteries that keep the observatory alive on the dark side of Earth.

Without the surgery, Hubble would have been left without adequate power to run all of its scientific instruments, including an advanced planetary camera to be installed during a spacewalk Thursday, and a dormant infrared instrument the astronauts hope to revive Friday.

And if the trouble was left to worsen, Hubble's batteries could have overheated and then ruptured. "And that would be catastrophic," Leckrone said.

Consequently, Grunsfeld and Linnehan were called upon to do the surgery.

An accomplished astronomer, Grunsfeld, 43, is an expert in high-energy astrophysics and a self-proclaimed "Hubble hugger." He helped bring Hubble out of a five-week stupor in 1999 after a gyroscope failure forced a shutdown of all the observatory's astronomical instruments.

Linnehan, 44, is a professional veterinarian whose previous surgical experience involved operating on whales, walruses and other large marine mammals. Prior to Columbia's March 1 launch, he likened the Hubble heart transplant to operating on a "big beast."

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