NASA Looks to Next Week for ISS Power System Spacewalk

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Cosmonaut Yuri Malenchenko, foreground, greets Sunita Williams, right, and Akihiko Hoshide in the U.S. airlock after a frustrating spacewalk. Photo Credit: NASA TV

NASA's lengthy spacewalk drought may come to an end with an encore of Thursday's hard luck outing for NASA astronaut Sunita Williams and Japanese astronaut Akihiko Hoshihiko, who were unable to accomplish their primary task -- the installation of a new Main Bus Switching Unit -- during an excursion outside the International Space Station that stretched to an unexpected eight hours.

Several stubborn bolts that prevented them from quickly removing an aging MBSU and replacing it with a spare were to blame.

Williams and Hoshiko could make an unscheduled walk early next week -- if NASA's station program managers, flight control team and resident engineers can pinpoint a cause for the difficulties and devise a plan to overcome them. In the meantime, the six-person space station crew will have to make due with 75 percent of the electricity normally available.

That means some science experiments will likely have to be re-scheduled to adjust the power draw. Heaters that warm the station's outer walls will likely be adjusted manually rather than by thermostats.

"From a systems perspective we are stable and in good shape," Mike Suffredini, NASA's ISS program manager, assured a news briefing called to address the implications of the difficult spacewalk.

"I would tell you this is not a configuration we want to stay in for a long period of time, even if this configuration is robust to many failures," Suffredini added. "We will try to get out the door early next week if we can come up with a plan. The biggest driver is knowing what we can do."

Thursday’s spacewalk, which was scheduled for 6 1/2 hours, marked the first NASA organized excursion outside the orbiting science lab since July 2011. The previous walk was incorporated into the final shuttle mission, which marked the end of NASA's long-running assembly of the station's U.S. segment. At that point, NASA turned its focus to science experiments and engineering demonstrations rather than construction.

Repair tasks outside the station are allowed to accumulate until a spacewalk is warranted.

Williams, a veteran of previous spacewalks, and Hoshide, a rookie, departed the airlock early Thursday with an ambitious agenda: replace the faulty MBSU, one of four power distribution boxes fastened to the station's solar power truss to route power to electrical components inside and outside the lab; string two power cables for a future Russian science module; replace a failed video camera on the Canadian robot arm; and perhaps fasten a protective shield on the docking port once used by visiting shuttles.
 
Williams managed to fully string one of the cables along the station's U.S. segment and partially string the second.

The spacewalkers ran into difficulties, though, when they tried to remove two bolts securing MBSU-1. The switching unit arrived at the station in 2002 as part of the central solar power truss segment and stopped responding to remote commands in October 2011. Though Williams and Hoshide finally succeeded in freeing the old switching unit, they encountered similar problems when they attempted to secure a replacement MBSU with new bolts.

Further inspection revealed some galling in the bolt receptacle, which Williams attempted to remove with pliers and puffs of nitrogen gas. That helped but didn't solve the issue. The spacewalkers attempted to drive, then remove one of the new bolts with a power ratchet and a torque multiplier.

Their efforts left the new MBSU partially secured by a single bolt and strapped down to an external hand rail with a strap. They didn't attempt to temporarily connect the circuit box to the power grid.

In the coming days, NASA's experts will look to lubricants as a possible solution, inspect MBSU production records for clues of a hidden assembly issue and sort through the station's big tool inventory in search of a solution.

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