Today was perhaps one of the most difficult days for the crews on the International Space Station (ISS) and Space Shuttle Discovery. Tuesday was the third of five scheduled spacewalks and tasks included reattaching a huge solar panel plant that had been relocated, and inspecting the gears which rotate the solar panels to face the Sun. All went well despite the difficulty of the task but during the unfurling of the solar array, ISS commander Peggy Whitson noticed the solar blanket ripping and aborted the operation. In spite of the damage, the array is producing 97 percent of anticipated power.
Tuesday's spacewalk lasted just over seven hours. Discovery astronauts Scott Parazynski and Doug Wheelock reattached the 17.5 ton, P6 solar truss segment to the left end of the station's main power truss, completing a two day, 145 foot move to its permanent location.
The P6 segment was originally attached to a central truss in 2000 during early station construction. With the main power truss now built, and the P4 and S4 segments in place, it was time to relocate the P6 segment to its permanent location. A fourth set of solar arrays, S6 will be installed next year.
Following P6 being bolted into place, the plan was for mission control Houston to unfurl the solar arrays which extend 249 feet. Each side of the array, called 2B and 4B, unfurls 124 feet. 2B deployed with no problems but when 4B had unfurled about 90 feet, the edge of one of the 31 panels ripped.
Engineers estimate the rip is about 2.5 feet long. Despite this, station managers report the array is producing 97 percent of full power, and the array is tracking the Sun. That's good news because it means that the feed wires were not damaged.
"It's not a situation where anybody is particularly panicked," said ISS program manager Mike Suffredini. "It's getting all the power we need. That's all we need out of that array is power. It doesn't have to look good; it just needs to give us power. So it's not about style points at this point with this array."
While not an immediate problem, engineers want to figure out the cause of the rip and be sure that no further damage will occur. This needs to be done before the next shuttle mission in December. Atlantis and her crew are bringing up the European Space Agency's Columbus laboratory. Adding another module to the station will require additional power, making the P6 solar array vital.
Atlantis has a short launch window from December 6 through December 13. If the shuttle is unable to launch during that time, the next window will not open until January 2. This restricted launch window is due to the solar angle on the station making it impossible to generate enough power, and dissipate enough heat, to support a shuttle assembly mission at the station.
"In my mind, we have a path through the wilderness on these problems," Suffredini said. "What we want to do is get this fixed to a point where we can continue assembly the way we had planned. This is a significant challenge for us to deal with, but we will deal with it."
Tuesday's spacewalking astronauts also deployed a forty foot radiator which dissipates heat generated by the station. They also moved a power distribution box about the size of a refrigerator, relocating it to a stowage platform atop the station.
Astronaut Scott Parazynski also inspected the joint which rotates the P6 solar arrays to face the Sun. Engineers wanted to compare this joint, which is operating properly, with a joint on another array which has been having problems. The joint had been drawing more power than it should have been for several months and Dan Tani discovered why during an earlier spacewalk.
Tani found metal shavings in the starboard array's rotary joint. The shavings have been causing friction, making it harder for the joint to turn, thus drawing more power. Upon this discovery, mission managers parked the wing into a stationary position to avoid further damage. With this array no longer rotating to follow the Sun, power production is significantly reduced, making the P6 array even more crucial. Tani was able to collect some of the dark colored shavings and they will be returned on Discovery so that engineers may examine them.
Discovery's mission has also been extended an additional day to allow for a closer inspection of the flawed joint. The inspection will require about six hours work and will take place Thursday. If time permits, astronauts may also carry out additional inspections of the damaged P6 solar array. Discovery will return to Earth November 7 instead of November 6.
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Kathy Miles, Author, and Chuck Peters, Systems Administrator