HOUSTON — International Space Station program managers imposed power restrictions as they prepared a response to a failed attempt on Aug. 30 to replace a degraded Main Bus Switching Unit (MBSU) outside the six-person orbiting science laboratory during the first -orchestrated spacewalk in 14 months.
Mission planners juggled experiment schedules, with the exception of the high-priority work of the Alpha Magnetic Spectrometer, turned to manual rather than thermostat control of internal heaters, and took other measures to contend with a temporary 25% loss in capability to distribute solar power.
A second spacewalk by U.S. and Japanese astronauts Sunita Williams and Akihiko Hoshide next week is high on the list of response options — if’s Mission Control team and station engineers can develop a nondestructive strategy to unseat a single stuck bolt holding the replacement for MBSU-1 on the solar power truss.
Other factors driving the urgency of a second spacewalk include the scheduled Sept. 16 return to Earth of U.S. astronaut Joe Acaba, who served both as the internal choreographer for the well-rehearsed excursion and the operator of the Canadian robot arm that assisted the spacewalkers. Acaba is preparing to return to Earth with Soyuz 30 crewmates Gennady Padalka and Sergei Revin on Sept. 16.
“If we can, we would try to get outdoors early next week — if we can come up with a plan. That is the biggest driver,” Mike Suffredini, NASA’s ISS program manager, told a news briefing late Aug. 30.
“This is not a configuration we want to stay in for a long period of time,” Suffredini says. “But even in this configuration, we are robust to many failures. So, in that respect we are not rushing out the door. We are trying to take advantage of the experience of the crew on board. There is an advantage in going outdoors while Joe Acaba is still on orbit for us.”
The station is outfitted with four MBSUs that distribute solar power to internal and external electrical components, including life support and communications systems as well as dozens of science experiments. With just three functioning MBSUs, power from two of the station’s eight outstretched solar arrays cannot be fed into the power grid.
All of the devices were launched installed on the station’s central solar power truss segment in 2002. MBSU-1 began to degrade in October 2011 and no longer responds to external commands.
During the Aug. 30 spacewalk, Williams and Hoshide struggled to remove one of two bolts securing MBSU-1 to the truss. They prevailed, but found galling in the truss receptacle for the first bolt required to secure the replacement.
The spacewalkers used pliers and a nitrogen gas spray to unseat some of the debris. Nonetheless, the first bolt for the new MBSU jammed as they attempted to drive it with a power ratchet. As their planned 6.5-hr. spacewalk stretched to more than 8 hr., Williams and Hoshide were instructed to strap the box down without mating the electrical connections.
Aboard the station, the astronauts also worked to overcome a cooling system problem with Hoshide’s spacesuit.
The spacewalk was the first sponsored by NASA since July 2011. That outing coincided with NASA’s final shuttle flight and the end of the long-running assembly of the station’s U.S. segment.