HOUSTON – U.S. and Japanese astronauts Sunita Williams and Akihiko Hoshide struggled unsuccessfully to replace a failing Main Bus Switching Unit (MBSU) outside the International Space Station during an Aug. 30 spacewalk, the first NASA-chartered excursion in nearly 14 months.

Though secured by just two bolts, the spacewalkers found it difficult to unscrew the fasteners holding the old device in place on the station’s long solar power truss. Once they did, they discovered small amounts of galling inside the bolt receptacles that prevented efforts to install and re-secure a new MBSU.

The device is one of four critical MBSUs that route power to the station’s internal and external electronic equipment. ISS managers expect to schedule a second spacewalk soon to finish the installation, NASA spokesman Josh Byerly said from Mission Control.

Astronauts attempted unbolting the partially installed MBSU replacement, spraying nitrogen into the bolt holes to blow away metal debris and trying everything over again with a range of power tool settings and torque multipliers. Once they managed to unbolt the new MBSU, the spacewalkers were instructed to temporarily  lash the electronics box to a handrail and head for the airlock.

“Tie it down,” instructed Fischer. “We’ll try this another day.” The spacewalk lasted 8 hr. 17 min.

The lengthy spacewalk got under way at 8:16 a.m. EDT, with high expectations. Williams and Hoshide intended to replace the balky MBSU, which arrived installed on the central segment of the station’s solar power truss in April 2002; extend a pair of U.S. power cables along the outside of the orbiting science laboratory, replace a failed video camera on the station’s Canadian robot arm; and perhaps install a protective shield on the docking port once used by NASA’s retired space shuttle fleet.

The MBSU began to fail in October 2011.