In the wake of an unsuccessful attempt to install a replacement power switching unit on the truss of the International Space Station (ISS), two crew members will venture outside for a second time in six days to complete the work.
The spacewalk is scheduled to begin at about 6:15 a.m. CDT (1115 GMT) on Wednesday (Sept. 5).
Expedition 32 flight engineers Sunita Williams of NASA and Akihiko Hoshide of the Japan Aerospace Exploration Agency (JAXA) will again don U.S. spacesuits and exit the Quest airlock to complete the installation of a spare Main Bus Switching Unit (MBSU).
The MBSU is a boxy, 236 pound (107 kilogram) component that relays power from the station's solar arrays to its systems. An 8- hour, 17-minute spacewalk on Aug. 30 — the third longest in history — ended with the spacewalkers unable to finish bolting the switching unit into its housing on the truss.
Engineering teams at the Johnson Space Center in Houston have worked since last Thursday (Aug. 30) 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 prior to bolting and possible damage to the threads of the receptacle posts on the 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 science research, mission managers decided to remove and replace the unit.
There are four MBSUs on the station. They are the primary electrical power routing devices on the orbiting laboratory. Each MBSU is 28 by 40 by 12 inches (71 by 102 by 30.5 centimeters).
In preparation for the additional spacewalk, Williams and Hoshide spent the weekend reviewing procedures and fabricating a series of tools to add to their arsenal for the cleaning and lubricating of the MBSU's bolts and the S-zero (S0) 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 that routes power 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 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 spacewalk.
This spacewalk will be the 165th in support of space station assembly and maintenance, the sixth in Williams' career and the second for Hoshide. Williams will wear a spacesuit bearing red stripes. Hoshide will be clad in a spacesuit with no markings.