This story was updated at 12:54 p.m. EDT.
HOUSTON ? Two spacewalking astronauts resorted to ?Plan C? Sunday to remove a stuck bolt on the Hubble Space Telescope that blocked their efforts to begin the second daunting repair of their mission: resurrecting a long-broken instrument that can sample the atmosphere of distant alien planets.
Atlantis astronauts Michael Massimino and Michael Good floated outside their shuttle at 9:45 a.m. EDT (1345 GMT) to resuscitate Hubble?s jack-of-all-trades spectrograph, which failed after a power failure in 2004.
NASA expected the repair to be one the mission?s most challenging tasks, primarily because the instrument was never built to be fixed in space. Its power supply sits behind a panel locked down by more than 117 tiny screws.
But a stuck bolt on a handrail blocking access the panels stymied the astronauts. The handrail had to be removed, but the bolt was stripped and two attempts failed to unscrew it. Mission Control finally told Massimino to rip the handrail from Hubble with brute force, shearing the bolt off. The move, Plan C, would take some serious strength, Mission Control said.
?I think you?ve got that in you,? astronaut Andrew Feustel told the spacewalker from inside Atlantis.
?I can try,? Massimino said.
It is the fourth of five crucial back-to-back spacewalks by Atlantis astronauts to upgrade and repair Hubble to extend its mission life through at least 2014. Atlantis? 11-day mission is NASA?s fifth and final flight to the iconic observatory before the agency retires its three-orbiter fleet next year.
A vital tool
The target of Sunday?s spacewalk - the Space Telescope Imaging Spectrograph, or STIS - was installed on Hubble in 1997. It can pick beams of light apart into their component wavelengths to find the chemical make-up of objects like planets, comets and galaxies. But unlike other spectrographs, STIS can build images, too, and was built for versatility.
?It has many bells and whistles,? Hubble?s senior project scientist Dave Leckrone told reporters here at NASA?s Johnson Space Center on Saturday.
In addition to being the first to discover the chemical composition of the atmosphere around an extrasolar planet, the spectrograph was also the first to detect the tell-tale signs of a supermassive black hole in the heart of galaxy, Leckrone said. So scientists understandably want it repaired if it?s possible, he added.
Tiny screws in space
Massimino and Good plan to spend nearly five hours replacing the circuit board-like power supply that sits behind a panel secured by 117 tiny screws, many of different sizes and some which have washers. Each of those screws and washers need to be removed (but not lost in space) in order to reach the power supply board. A handrail must be removed as well.
?The problem is that this thing was not supposed to be changed out in space,? Massimino has said. ?It?s hard enough to change the thing out on the ground.?
NASA engineers built a custom-made capture plate with color-coded holes for each screw size. The holes are large enough to fit over a screw and allow a screwdriver bit in, but secure enough to lock loose screws inside so they don?t drift away. The engineers also built a new array of tools so Massimino could tackle the intricate, hand-intensive repair while wearing a bulky spacesuit and thick gloves.
Hubble scientists and mission managers had confidence in the repair based on the apparent ease of similar repair work on the telescope?s Advanced Camera for Surveys, which a different pair of spacewalkers pulled off on Saturday.
Engineers on Earth declared that camera repair a partial fix - but still successful - late Saturday since it revived two of the camera?s three imaging channels, including a vital wide-field camera used by Hubble to build its trademark deep views into the cosmos.
?I think it?s going to go extremely smoothly,? Leckrone said of today?s spacewalk, adding that the astronauts also plan to install much-needed thermal insulation on Hubble as well.
Massimino, in particular, has practiced removing the screws on Earth exhaustively until it became second nature, Leckrone said.
His best time? A swift 40 minutes, mission managers said.
?That?s extraordinary,? Leckrone said.
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.
- New Video - Hubble's STIS: The Ultimate Repair Job, Repaired Camera
- Image Gallery - The Hubble Repair Missions: Part 1, Part 2
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