NASA flight controllers now plan to move the space station's $190 million robot arm transporter back to its central work site on the lab's solar array truss just before a spacewalk Dec. 12 by Expedition 6 commander Kenneth Bowersox and flight engineer Nikolai Budarin.

The mobile transporter, or MT, was left at work site 7 on the end of the newly installed P1 truss segment Saturday evening during a spacewalk by Endeavour astronauts Michael Lopez-Alegria and John Herrington.

The plan that day was to move the MT from work site 4 on the central S0 truss to work site 7 and then to move the station's robot arm from the Destiny lab module to the transporter to support spacewalk activities. But the transporter stalled when it ran into a stowed UHF antenna shortly after moving onto P1. Herrington later deployed the antenna, freeing the transporter to continue toward work site 7.

But because the MT's automatic control sequence was interrupted when it ran into the UHF antenna, flight controllers had to manually enter a long series commands to resume its motion. By the time the transporter finally locked itself down at work site 7, plugging into power and data sockets, it was too late to either attach the arm as originally planned or to move the transporter back to work site 4 at the end of the spacewalk. Fortunately for NASA, Herrington and Lopez-Alegria were able to complete all of their tasks without using the arm.

Engineers prefer to store the transporter at work site 4 between uses because a long umbilical cable that plays out behind it as it creeps along the truss was not designed to stand up to possible impacts from space debris. That's not a problem when the transporter is back at work site 4 because the trailing umbilical system, or TUS, cable is rolled up on its spool. But out at work site 7, it is stretched out and exposed to the space environment. As a result, it faces a slightly higher risk of getting damaged.

But station flight director Mark Kirasich said today is "not a big deal" to leave the MT at work site 7 until Dec. 12. Flight controllers hope to have it back at work site 4 by the time Bowersox and Budarin leave the station's Quest airlock module. The move is being timed to coincide with the spacewalk in case any additional problems develop.

Engineers are still looking into how mission planners missed the UHF antenna interference issue in the first place. A preliminary review indicates computer models of the truss showed the interference existed before Endeavour took off. But for some reason, the issue never came to light.

In retrospect, Kirasich said, the transporter problem was a blessing in disguise.

"An objective in and of itself was to roll the MT down the track and make sure that we could move the MT, that we could get it to its destination work site, that we could plug it in and to learn from that experience," he said.

"From that perspective, what actually happened, there's a bit of good luck in it. On this particular flight, the impact was almost zero. ... But it was a very valuable lesson learned because now these will be things we'll take into consideration on future flights ... to make sure there is no interference.

"And that in and of itself probably paid for the task because there are flights in the future where we are moving elements of the space station on the MT and if you run into this sort of a problem, it can really, really cause complications," Kirasich said. "So it was a very good lesson learned."

The spacewalk by Bowersox and Budarin will begin around 10:30 a.m. on Dec. 12. Bowersox's call sign will be EV-1 while Budarin's will be EV-2. The goals of the excursion are to:

1. Reroute an electrical cable to enable flight controllers to deploy a folded set of radiator panels on P1. The truss segment features three sets of folded ammonia radiator panels, each one of which will stretch 75 feet when fully deployed.

2. Release 10 remaining launch locks holding the radiator panels in place. Flight controllers then will send commands to unfold the central array.

3. Reconfigure the electrical cable to route power back to a set of backup heaters.

4. Install a lighting fixture.

In the meantime, Bowersox, Budarin and science officer Donald Pettit face a relatively light schedule as they get used to living and working in space.

"We're going to give them a couple of light duty days and they'll get to recoup and set up shop the way they want to run it. But it isn't too long, about 11 days from today, things are going to get busy for them again."

As for Endeavour's mission, Kirasich said "the past seven docked days just went really, really well."

"We went into the flight with a list of 30 or so objectives and we were able to click off every single one," he said. "In addition to our primary mission objectives of rotating the Expedition 5 and Expedition 6 crews, we successfully installed and activated the port 1 truss segment and we transferred just over 2,000 pounds of supplies and new gear from the shuttle to the space station in order to keep the Expedition 6 crew safe, happy and busy for the next couple of months."

Endeavour is scheduled to land Wednesday afternoon at the Kennedy Space Center. But forecasters are predicting a chance of showers and thunderstorms Wednesday and Thursday that could cause problems. Conditions are expected to improve by Friday.

"We have enough (electrical power), food, everything we need to go four extension days beyond the nominal end of mission," said shuttle flight director Paul Dye. "Hopefully, we won't have to go that long."

But "the objective of the program is to get the orbiter back into KSC," he added.

"We will probably try for KSC only for a couple of days because we really would like to get it there and save the time that it takes if we land at Edwards getting it back."