The astronauts also aim to revive Hubble's $110 million infrared camera and spectrometer, an instrument that was installed in 1997 but died two years later when its cryogenic cooling system shut down prematurely.
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The job at hand: Rigging up a new experimental cooling system so that the Near Infrared Camera and Multi-Object Spectrometer once again can peer through dark masses of interstellar gas and dust to witness the birth of planets, stars and galaxies.
These anxiously awaited scientific upgrades, however, are low priority compared to the electrical overhaul of Hubble, work intended to give the observatory -- which is expected to operate through 2010 -- a new lease on life.
A fresh set of solar wings will yield a power increase of 20 to 25 percent, enabling astronomers for the first time to operate all four Hubble science instruments simultaneously -- a capability that could lead to a new era of rapid-fire discovery.
Also due for replacement: the telescope's flawed power distribution system, original hardware now subject to devastating failure after a dozen years of orbital wear and tear.
"We need to change it out. It's become a health risk to the telescope," Austin said.
Considered the heart of Hubble's electrical system, the switching station routes power from the telescope's solar arrays to its science instruments, control systems and batteries.
However, replacing the so-called Power Control Unit -- or PCU -- will put the observatory on the precipice of disaster.
For the first time since its launch, Hubble will be totally powered down -- a move essential to ensuring that spacewalking mechanics aren't shocked, or electrocuted, on the job.
The astronauts, meanwhile, will have just eight hours to restore power to the observatory, and no one is entirely certain the telescope in fact will turn back on.
"That scares me a lot," said Weiler. "It kind a violates a longstanding policy in the space business that if something is working well, don't turn it off and just hope it comes back on."
If it doesn't, a powerless Hubble would be rendered blind, its instruments inoperable. Control systems would freeze up or overheat, faltering at first before failing outright. Lifeless, the telescope would tumble though orbit, with the tug of gravity gradually pulling it toward Earth.
An emergency Hubble retrieval mission, in that case, likely would be staged to make certain the dead craft wouldn't make an uncontrolled atmospheric reentry, potentially raining chunks of charred debris from Peoria to Paris.
Little wonder then, that the pucker factor is expected to reach an all-time when Hubble's electrical heartbeat is purposely brought to a halt.
Anne Kinney, director of the astronomy and physics division at NASA headquarters in Washington, expects to be "nervous as hell. Nervous as hell."
"We're going to be sitting on the edges of our seats," added NASA Hubble program manager Preston Burch.
"So the main, most important thing for this mission is to get that PCU installed, and hopefully everything works right," Linnehan said. "We have no reason to think it won't, but when the light switches come back on, and Hubble hums up again, that'll be a big sigh of relief."
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