This story was updated at 10:41 a.m. EDT.
HOUSTON — Atlantis astronauts ventured back out to the Hubble Space Telescope on Saturday for their most ambitious spacewalk yet in an attempt to resurrect the observatory's broken main camera.
It is the third spacewalk in as many days for Atlantis shuttle astronauts as they work to repair the 19-year-old Hubble for the final time. But this time, spacewalkers John Grunsfeld and Andrew Feustel will try and fix a camera that was never designed to be opened in space.
"These repairs...go well beyond anything we've ever tried in space before," said Grunsfeld, an astrophysicist-turned-astronaut making his third trip to Hubble.
The spacewalkers began their work, which includes adding a new $88 million spectrograph to Hubble, at 9:35 a.m. EDT (1335 GMT) as they flew 350 miles (563 km) above the central Indian Ocean.
"Welcome back out, Drew," Grunsfeld, a veteran spacewalker, told Feustel, who is on his second spacewalk.
Atlantis' 11-day mission is NASA's fifth and final service call on Hubble and includes five back-to-back spacewalks to extend the telescope's lifetime through 2014.
Restoring Hubble's vision
he camera at the heart of today's spacewalk is Hubble's Advanced Camera for Surveys. It was installed in 2002 but suffered a debilitating electronics failure in 2007 that left all but one data channel offline. At the time, it was Hubble's most-used camera and had taken some of the space telescope's most breathtaking images, including the Hubble Ultra Deep Field that caught nearly 10,000 galaxies in one snapshot.
It needs an unprecedented repair — one that requires astronauts to dig deep inside the camera and tinker with its inner workings — and there's no guarantee of success, Hubble scientists have said. There may not even be enough time in the 6 1/2-hour spacewalk to finish it, but the astronauts are determined to try.
"John Grunsfeld has trained within an inch of his life to do this," Dave Leckrone, Hubble's senior project scientist, told reporters here at Johnson Space Center late Friday. "There's no way you're going to be able to get him back in. He's going to finish this."
The last two spacewalks by Atlantis astronauts have hit unexpected glitches that led them to run long. Mission Control gave the astronauts extra time to sleep in today to make sure they were rested. Leckrone said the fact that this is Hubble's first service call since 2002 may have something to do with it.
"My theory is that after seven years of not having people around, Hubble has lost its accommodation for people," said Leckrone. "It's gone wild again, so we have to tame it. And that will happen, I'm sure."
A tricky task
But fixing Hubble's broken main camera is no easy task. The electrical short that killed its main instruments in 2007 occurred in a backup system that Hubble was using because an earlier glitch had already shut down the primary side.
Grunsfeld and Feustel have to open up Hubble, use custom-made tools to cut their way through bolts on two cover plates, remove 32 screws (taking care not to lose any) and then pry out four electronics boards so they can be replaced.
"They're going to go inside, get into the guts of it and they're going to take out some of the boards that aren't working and put in new ones," said a hopeful Keith Walyus, the Hubble mission's operations manager at NASA's Goddard Space Flight Center in Greenbelt, Md., late Friday. "After that they're going to button it up again and it should work fine."
The astronauts also have to hot-wire the repaired camera to a new power source they'll attach to Hubble's hull since they can't patch into to its original source, which is out of reach.
Hubble goes COSmic
Grunsfeld and Feustel have one other main task in today's spacewalk. They plan to replace COSTAR, a refrigerator-sized box that contains the corrective mirrors which canceled out the myopia caused by Hubble's flawed primary mirror in 1993. Since then, Hubble's images have been crystal clear.
Hubble's newer instruments automatically take its flaw into account, so COSTAR is no longer needed. It will be returned to Earth aboard Atlantis, open a slot in Hubble for the new Cosmic Origins Spectrograph, or COS.
"It's sort of the quiet instrument back in the background waiting to become a superstar," Leckrone said of the new spectrograph. "COS is the most sensitive spectrograph ever to fly in space."
The new instrument is expected to peer into the cosmos to study the structure of the universe and delve deeper into the mysteries of dark energy and dark matter.
SPACE.com is providing continuous coverage of NASA's last mission to the Hubble Space Telescope with senior editor Tariq Malik in Houston and reporter Clara Moskowitz in New York. Click here for mission updates, live spacewalk coverage and SPACE.com's live NASA TV video feed.