August 31, 2012
Second Expedition 32 Spacewalk Ends With Unsuccessful Attempt To Secure Power Unit
Lawrence LeBlond for redOrbit.com - Your Universe Online
Two International Space Station (ISS) astronauts completed a scheduled spacewalk Thursday afternoon at 4:33 p.m. (EDT) after spending several hours trying to fix a flawed power unit. The spacewalk began at 8:16 a.m. and was originally scheduled for 6.5 hours, but problems installing a new power-switching box took them into overtime.
NASA Flight Engineer Sunita Williams and JAXA Flight Engineer Akihiko Hoshide were unable to replace the Main Bus Switching Unit (MBSU) for the ISS and were forced to leave the unit dangling from the orbiting lab rather than bolting it in place. NASA scrambled to reduce the power demands of the station to balance out the electrical load, while outlining a plan to get astronauts back out as early as next week to fix the issue.
After removing the failed unit, Williams and Hoshide had difficulties driving the bolts to secure the new device in the station´s s-zero truss. The duo used a long-duration tie-down to secure the new unit until a future mission could see them returning to complete the install.
The spacewalk, although the third longest walk for any NASA astronaut, was a disappointing feat. They spent hours struggling with the bolts, using all sorts of tools and tactics to try to get the device in place, only to end with failure. As time dwindled, Mission Control told them to tie off the device and head back to the safety of the lab.
“We'll figure this out another day,” Mission Control radioed to the team.
The spacewalk did have some success, however. Williams was able to successfully connect one of two power cables in preparation for a future arrival of a Russian lab module. A third task, replacing a camera on the Canadarm2 robotic arm, was not completed and will be rescheduled for a future walk.
The MBSU needed to be replaced after crew discovered earlier that it had faltered. The old unit was still supplying power to the station, but could not be commanded by crew.
“We uncovered this problem some months ago and had been living with it ever since,” station program manager Mike Suffredini told reporters during a press conference on Thursday. “We waited to conduct the (spacewalk) until we were ready and had every thing in place.”
However, the problems were evident from the get-go. As Hoshide removed the old unit, Williams reported finding metal shavings on one of the bolts and around the housing. The spacewalkers used nitrogen to spray the shavings away from the housing before attaching the new unit, but problems continued: a bolt jammed while trying to secure the MBSU in place, and after repeated attempts to fix the issue, the situation ended with the astronauts giving in to defeat.
“We're kind of at a loss of what else we can try,” astronaut Jack Fischer at NASA's Mission Control Center in Houston told the crew after more than an hour of trouble-shooting. “If you guys have any thoughts or ideas or brilliant schemes on what we can do, let us know.”
Hoshide suggested using a tool that provides more force on the bolts, but NASA engineers were concerned that more force could equal more damage.
While the loss of use in one power distributor means the engineers will have to work to balance out the electrical payload, they affirmed that three other power distributors were still in good working order and could still handle station operations while they figure out what to do about the fourth. Still, the loss of one power distributor will reduce electrical output by 25 percent.
“We have to do a balancing act,” said NASA flight director Ed Van Cise.
“The team may have to manage power loads a little bit, but this is familiar territory,” Suffredini told Marcia Dunn of the Associated Press. “We'll be able to deal with that while we decide what our next plan is.”
To try to keep the team from discontent after the failed spacewalk, Mission Control said: “You guys are rock stars, just so you know.”
At a news conference later Thursday, NASA officials said possible solutions may include lubricating the bolts and applying more torque.
This was Williams´ fifth spacewalk and Hoshide´s first. Hoshide is the third Japanese astronaut to conduct a spacewalk. The spacewalk was the 164th mission in support of station assembly and also marked the first US-based spacewalk since the retirement of the shuttle fleet last year.
This was also the second Expedition 32 spacewalk. Commander Gennady Padalka and Flight Engineer Yuri Malenchenko went for a stroll in space on August 20 to move the Strela-2 cargo boom from the Pirs docking compartment to the Zarya module. That mission lasted 5 hours and 51 minutes. That mission also included the installation of micrometeoroid debris shields on the exterior of the Zvezda service module and the deployment of a small science satellite.