International Space Station Expedition 32 Flight Engineers Sunita Williams of NASA and Akihiko Hoshide of the Japan Aerospace Exploration Agency will conduct a spacewalk Wednesday, Sept. 5 to attempt to complete installation of a spare power unit on the station's truss. The spacewalkers will venture outside the station for the second time in six days to complete the installation of a Main Bus Switching Unit (MBSU). NASA Television will air live coverage of the spacewalk beginning at 6 a.m. EDT. The spacewalk is scheduled to begin at approximately 7:15 a.m.
Following a thorough review of preparations, the International Space Station Mission Management Team, including all international partners, agreed to press ahead for the follow-up spacewalk. A final readiness review by station program officials will be conducted on Tuesday, Sept. 4.
On Aug. 30, Williams and Hoshide removed a faulty MBSU from its slot in the S-zero truss, but were unable to properly secure a spare unit to the station. Engineering teams at the Johnson Space Center in Houston have worked around the clock since last Thursday to troubleshoot the problem that prevented the spare MBSU from being installed. The most probable cause is likely a combination of a slight misalignment in the positioning of the spare unit for its installation prior to bolting and possible damage to the threads of the receptacle posts on the S-zero truss to which the MBSU must be bolted in place.
MBSU-1 had been presenting "intermittent bit flip errors," also known as circuit reconfiguration confirmation errors, since Oct. 19, 2011. An analysis indicated there was a component hardware failure internal to the MBSU, which was causing the problem. Although the unit continued to provide power without interruption before its removal on Aug. 30, and had not been a threat to the health or safety of the crew, station or continued research, mission managers decided to remove and replace the unit.
There are four MBSUs on the space station. They are the primary electrical power routing devices on the orbiting laboratory. Each MBSU is 28 by 40 by 12 inches and weighs 220 pounds.
In preparation for the additional spacewalk, Williams and Hoshide have spent the weekend reviewing procedures and fabricating a series of tools to add to their handyman arsenal for the cleaning and lubricating of the MBSU's bolts and the S-zero truss post receptacles. If the MBSU cannot be bolted in place during Wednesday's spacewalk, an option exists for the spacewalkers to bring the unit inside the station for further analysis and troubleshooting.
In a coincidental but unrelated occurrence, a direct current power switching unit component that enables power to be routed at the proper amperage and voltage tripped late Saturday, Sept. 1, causing one of the station array's power channels to default to a parallel channel. Although the power trip is not linked to the station operating on only three MBSUs at the moment, the station is currently operating on only five of its eight power channels. Even with three such channels unavailable, flight controllers have been able to reroute power to critical station systems and payloads with only a minimal impact on operations. The crew is not affected by the power configuration as they prepare for the next spacewalk.