NASA ISS EVA: Spacewalkers to try power repair again Wednesday

Astronauts Sunita Williams and Akihiko Hoshide will venture back outside the International Space Station Wednesday for another attempt to install a replacement power switching unit that could not be plugged into the lab's electrical grid during a spacewalk last Thursday.

Equipped with an assortment of impromptu tools, the astronauts will attempt to clean the bolts needed to lock the 220-pound box in place, as well as the threaded bolt holes.

During Thursday's spacewalk, metal shavings were seen inside the bolt receptacles when main bus switching unit No. 1 -- MBSU No. 1 -- was removed.

While Williams and Hoshide attempted to blow out any remaining fragments using compressed nitrogen, they were unable to tighten down the replacement MBSU enough to engage cooling fins and gangs of electrical connectors.

Without MBSU No. 1, the space station was only drawing power from six of its eight solar panels, forcing flight controllers to carefully manage the lab's electrical useage while spacewalk planners studied the bolt problem and what might be done to fix it.

While that work was going on, the station's electrical system suffered an unrelated problem Saturday afternoon.

A direct current switching unit, or DCSU, dropped off line because of a presumed short somewhere in the system, effectively cutting a third solar array out of the station's power grid.

While the DCSU trip did not greatly worsen the station's power status, it marked the first time in several years that the lab complex has been forced to operate on just five of its eight power channels.

The DCSU problem will be addressed later, possibly with another spacewalk to install a replacement.

But in the near term, getting MBSU No. 1 bolted down and tied back into the lab's power system is the crew's top priority.

The space station is equipped with eight 115-foot-long solar panels, four on each end of a football-field-size truss that runs at right angles to the lab's pressurized modules.

The arrays rotate like giant paddle wheels as the station's orbits Earth to maximize power generation.

Each pair of arrays extend from integrated electronics assemblies containing batteries, cooling equipment, charge-discharge units and a direct current switching unit that passes power downstream to the station and back into the IEA.