Lead spacewalker Sunita Williams and Japan's Akihiko Hoshide floated outside the station for the second time in a week, hoping to resolve a problem that left a replacement power router tethered to its attachment plate.

The astronauts attempted to install the 220-pound (100-kg)device, known as a main bus switching unit, to the station's framework during a spacewalk last Thursday, but were stymied by a jammed bolt.

The unit is one of four needed to route power from eight solar array wings to transformers that distribute electricity to run the $100 billion orbital outpost.

Williams, who is making her sixth spacewalk, and Hoshide, on his second, left the station for another try shortly after 7 a.m. EDT (1100 GMT). Their gear included an assortment of brushes including one they fashioned out of a toothbrush.

NASA hopes that a thorough cleaning of the bolts and their housings will resolve the problem, though engineers came up with 15 pages of options and procedures.

If the new unit cannot be plugged into the station's power grid within four hours of the planned 6.5-hour spacewalk, Williams and Hoshide plan to bring it into the airlock with them for additional assessments inside the station, said NASA spokesman Josh Byerly from Mission Control in Houston.

Replacing the main bus switching unit was the primary goal of last week's spacewalk. The old unit was routing power but could not be commanded. Without the new unit installed, the station cannot get power from two of its eight solar panel wings.

An unrelated problem on Sunday took out power from a third wing.

NASA said the station is not in any danger, but until full power is restored all of its equipment, backup systems and experiments cannot be operated simultaneously.

The station, a project of 15 countries that flies about 250 miles above Earth, is staffed by rotating crews of six astronauts and cosmonauts and used for dozens of medical, materials science, physics and other experiments.

(Editing by Kevin Gray and Vicki Allen)

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