CAPE CANAVERAL, Fla. -- Trouble with the International Space Station's railroad car forced mission managers to re-plan Saturday's final spacewalk, but NASA officials said the astronauts were able to get everything done anyway.
"Today has gone quite well. We have accomplished all of our objectives," said Bob Castle, a NASA mission operations manager.
Trouble started after 10:20 a.m. EST (1520 GMT) Saturday when flight controllers ordered the Mobile Transporter (MT) -- a $190 million motorized platform designed to hold the station's robot arm -- to move a distance of about 55 feet (16.7 meters) down the truss assembly.
The move was planned so that the station's robot arm could "walk off" the Destiny science module and attach itself to the transporter. From there the arm would be used by spacewalkers as a stable work platform to help them access difficult to reach areas atop the P1 truss.
But about 10 feet (three meters) shy of its goal -- a place known as Work Site 7, where the platform could plug into a source of power -- the transporter stopped.
Unsure if there was a systems problem or an obstruction on the rail somewhere, flight controllers attempted to reconfigure the transporter and use a back up drive unit to move it. When that failed, managers knew that something on or near the rail was preventing the move.
As all of this was going on, Endeavour astronauts Michael Lopez-Alegria and John Herrington were preparing for their third and final planned spacewalk of this STS-113 mission.Given a "go" to proceed outside, Herrington was directed to float up to the P1 truss and look for anything that might be blocking the transporter. Almost immediately upon arriving in the area he made a discovery.
"Houston, I've found the problem," Herrington reported to Mission Control, telling them that a piece of hardware was bumping into a UHF radio antenna that was still stowed in the P1 truss. "I can deploy the UHF antenna and you'll have free clearance."
"John, we copy. Give us a few minutes to think about it," said astronaut Barbara Morgan from the Mission Control room in Houston.
Officials ran the idea around the room and, finding no objection, Herrington was told to press on with the task and deploy the antenna -- a task that was originally scheduled for a spacewalk on Dec. 12 by Expedition Six crewmembers Ken Bowersox and Nikolai Budarin.
A short while later the platform was commanded to move and was successfully positioned over Work Site 7, where managers decided it would remain for now.
Castle said engineers would investigate why it wasn't discovered before launch that the UHF antenna would get in the way of the MT.
While Herrington helped with the platform, Lopez-Alegria moved through his checklist of activities without incident. And then when Herrington was done with the platform, both spacewalkers were able to get back on their plan -- more or less.
Most of what they had to do on Saturday was to fasten as many as 33 metallic clamps on the end of various-sized quick-disconnect valves and other plumbing lines. The devices will make it easier for future spacewalkers to operate the valves, which were discovered to have a design flaw that made them prone to get stuck or difficult to open.
Attaching the clamps is difficult, even in normal gravity using your bare hands. In weightlessness and wearing bulky gloves it's a lot tougher -- a fact that was often acknowledged with humor throughout the day.
"How are you doing out there?" Lockhart asked Lopez-Alegria at one point.
"Fine. Loving life," Lopez-Alegria replied with a trace of sarcasm in his voice.
There were some other jobs done as well, all designed to complete the outfitting of the $390 million P1 truss segment that was carried into orbit by Endeavour and attached to the station on Tuesday.
With this work behind them the major goals of shuttle Endeavour's mission are all but complete. The only major task left undone: return the Expedition Five crew safely to Earth.
To get there, the crew will get some final packing done on Sunday -- and also enjoy a half-day off -- and then shuttle Endeavour is to undock from the station on Monday and land Wednesday afternoon at the Kennedy Space Center.
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