Astronauts try, try again to fix space station

After an unsuccessful attempt last week to install a new power unit to the exterior of the International Space Station, two astronauts are trying again during a spacewalk on Wednesday.

NASA astronaut Sunita Williams and Japanese spaceflier Akihiko Hoshide will battle the stuck bolts that prevented them from properly installing the power box on the space station's backbone-like truss during their first spacewalk. The outing, which is due to begin around 7:15 a.m. ET, will mark the second time the duo has worked outside the orbiting complex in six days.

Williams and Hoshide spent more than eight hours upgrading the exterior of the station last Thursday, but a stubborn bolt on the so-called main bus switching unit (MBSU) prevented them from securing the power box to the outpost and attaching its electrical connections.

The International Space Station has four MBSUs that harness power from the outpost's solar arrays and distribute it throughout the orbiting complex. Without the use of one unit, the station is unable to relay power from two of the eight solar arrays on the massive orbiting complex.

In an unrelated mishap, a component that ensures power is distributed at the proper amperage and voltage suffered a glitch late Saturday, NASA officials said. As a result, the station is only able to use five of its eight available power channels. [Photos: Spacewalking Astronauts Fix Up Space Station]

With three power channels unavailable, flight controllers re-allocated available resources aboard the outpost for critical systems and to keep the crew safe. Despite the outages, space station operations have only suffered a minimal impact, and the crew will not be affected as they prepare for Wednesday's spacewalk, agency officials said.

Mission managers opted to proceed with the extra spacewalk after working around the clock since Thursday to analyze the problem.

"The most probable cause is likely a combination of a slight misalignment in the positioning of the spare unit for its installation prior to bolting and possible damage to the threads of the receptacle posts on the S-zero truss to which the MBSU must be bolted in place," NASA officials said in an update.

Last week, Williams and Hoshide removed a faulty MBSU and tried to install the spare, but they were unable to drive in one of the bolts that fastens the unit to the space station's truss. After repeated attempts failed, the astronauts used a tether to temporarily tie the MBSU down, and were forced to wrap up the marathon spacewalk.

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The outing lasted 8 hours and 17 minutes, making it the third longest spacewalk in history and the longest ever performed by a space station crew.

Williams and Hoshide spent the weekend reviewing the choreography for the upcoming spacewalk, and preparing tools that will be used to clean and lubricate the MBSU bolts and their corresponding receptacles on the space station's truss.

If the power unit cannot be properly installed, mission managers may opt to have the astronauts bring the box inside for more analysis, NASA officials said.

The proceedings will be broadcast live on NASA Television beginning at 6 a.m. ET: http://www.nasa.gov/ntv

Follow Denise Chow on Twitter @denisechow or Space.com @Spacedotcom. We're also on Facebook and Google+.

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Source: http://www.msnbc.msn.com/id/48906172/ns/technology_and_science-space/

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