Two astronauts floated outside the International Space Station on Wednesday for the second time in six days to try to fix a power unit that was not properly installed during an earlier spacewalk last week.

NASA astronaut Sunita Williams and Japan's Akihiko Hoshide switched on the batteries on their spacesuits at 7:06 a.m. ET, signaling the official start of today's spacewalk.

The spacewalkers were expected to spend six and a half hours working in the vacuum of space to troubleshoot a pair of stuck bolts on a new replacement power unit that was to be attached to the space station's exterior. Wednesday's extra spacewalk was added after the troublesome bolts prevented the component from being properly installed on Aug. 30.

During Wednesday's outing, Williams and Hoshide were trying again to install the replacement power box called a main bus switching unit, or MBSU on the space station's backbone-like truss.

Last week, Williams and Hoshide removed a faulty MBSU and tried to install a new spare, but they were unable to drive in one of the bolts that fastens the unit to the station's truss. After repeated attempts failed, the astronauts used a tether to temporarily tie the MBSU down, and were forced to wrap up their marathon spacewalk. [Photos: Spacewalking Astronauts Fix Up Space Station]

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.

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 able to use only five of its eight available power channels.

With three power channels unavailable, flight controllers reallocated available resources aboard the outpost for critical systems and to keep the crew safe. Despite the outages, space station operations have suffered only minimal impact, and the crew was not be affected during preparations for Wednesday's spacewalk, agency officials said.

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

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Astronauts try, try again to fix space station