The crew patch for STS-111. Photo: NASA

The primary goal of Endeavour's mission, scheduled for launch Thursday, is to ferry a fresh three-person crew to the international outpost - the fifth set of full-time residents since permanent occupation began in November 2000 - and to deliver science equipment, fresh food, water and other supplies.

Endeavour also is carrying up a critical robot arm anchor platform that will be bolted to the top of a compact rail car installed during the last station assembly mission in April.

The rail car, in turn, is attached to the central element of a huge truss that eventually will stretch the length of a football field and carry the solar arrays and ammonia coolant lines and radiators needed to keep the station electrically alive and prevent overheating.

Only the central element of the truss currently is in place, bolted to the top of the U.S. laboratory module, Destiny.

The Canadarm2 space crane was designed to latch onto the anchor platform so it can move along the rails to attach additional truss elements. Eight more truss sections are scheduled for installation this year and next, and all require a healthy robot arm.

"The mobile base is going to allow us to transport, move the arm around. It gives us a mobile platform for the arm," said station astronaut Peggy Whitson, a member of the lab's next full-time crew. "That's going to be absolutely critical for the continued assembly of the station. Without our ability to have the mobile base sitting on top of the transporter, we would not be able to complete that assembly."

But NASA has a problem. Shortly after the space station remote manipulator system - SSRMS - robotic crane was installed last year, engineers discovered a subtle electronic glitch in the redundant circuitry used to control the movement of the arm's wrist-roll joint.

For redundancy, independent electronic control systems are built into each of the arm's seven joints. In one of the two systems that can be used to control the wrist-roll joint, a malfunction can occur that prevents the release of brakes locking the joint in one position.

That, in turn, can trigger an automatic shutdown, preventing controllers from using the arm even with the healthy avionics "string."

After intensive troubleshooting, Canadian software engineers developed a computer programming patch that forces the arm's main computer to ignore the problem in the wrist-roll joint and not to order a shutdown before switching to the healthy avionics string.

Before the most recent shuttle-station assembly mission, another patch was put in place that essentially tells the arm's computer to ignore the joint entirely when working through the faulty avionics string.

The wrist-roll joint works normally with the other control system, but NASA managers want full redundancy to protect against problems down the road. As it now stands, a failure in the healthy control system could shut the arm down, interrupting assembly operations.

"Certainly for normal operations, for any of the build outboard of where we are now, we need a fully functional arm," said shuttle flight director Paul Hill. "The arm we have now is fully functional on (one) string but if we fail this string, that takes us down to either one degraded string or, depending on the failure, two degraded strings."

And so, on March 20, station managers decided to add a third spacewalk to Endeavour's mission so French astronaut Philippe Perrin and Franklin Chang-Diaz could replace the faulty wrist-roll joint. Launch was delayed a month to give engineers time to develop a payload bay mounting fixture for the replacement joint and to give the spacewalkers additional time to train.

The astronauts to launch aboard Endeavour pose in front of pad 39A. Standing, left to right, are Expedition 5 crew members Sergei Treschev, commander Valeri Korzun and Peggy Whitson; Endeavour commander Ken Cockrell, pilot Paul Lockhart, mission specialists Philippe Perrin and Franklin Chang-Diaz. Photo: NASA

That work is now complete, and Endeavour's crew is ready for flight.

"We've got one of those missions that has almost got too much in it to get done," said shuttle commander Kenneth Cockrell. "We're looking forward to a very jam-packed timeline when we get on orbit. Aside from the normal excitement of launch, landing, docking and undocking, we've got a tremendous amount of work to do while we're docked to the international space station."

"We've got three EVAs to pull off, one of which we've only been training for the last month, which is the launch on need repair of the big arm on space station, and two other previously planned assembly EVAs," he said. "We have a crew exchange ... and then we have a very full (cargo canister) with hopes of filling it to 90 percent capacity for return. So we have a full plate in front of us."

Endeavour is scheduled for launch May 30 from pad 39A at the Kennedy Space Center. Cockrell, pilot Paul Lockhart, Perrin and Chang-Diaz, acting as the shuttle's flight engineer, will be seated on the upper deck of the cockpit. Cockrell is known by his nickname "Taco" while Lockhart, a fellow University of Texas graduate, is known as "Paco."

The space station's next crew - Expedition 5 commander Valeri Korzun, Whitson and Sergei Treschev - will make the climb into orbit seated on Endeavour's lower deck.

Only three of the shuttle's seven crew members are spaceflight veterans. Cockrell is making his fifth flight, Korzun his second and Costa Rica-born Chang-Diaz his seventh, tying a record set by astronaut Jerry Ross on the most recent shuttle mission in April.

"Franklin is well known, well respected for his consummate abilities and consummate knowledge of the space shuttle and systems," Cockrell said. "So he's the perfect guy to have as the flight engineer. He knows my job, he knows Paco's job and he backs us up, directs us and guides us in every way having to do with the shuttle flying.

"He's also a guy who takes to space very well by all accounts. He doesn't get sick and he's a natural at maneuvering around in zero G. And he puts together pretty good meals! ... So on just about every front having to do with living life on the space shuttle, Franklin's going to be a great guy to have on the crew."

For his part, Chang-Diaz, a nuclear physicist who is helping develop an innovative interplanetary propulsion system when he's not training for a shuttle mission, is typically modest about his achievements.

"Being an astronaut has been a dream of my life, ever since I was very little," he said. "I tried to make that dream come true, obviously successfully, and I'm hoping that perhaps this is in the minds of many other children as we open up space for human exploration, that many other children all over the world will have the opportunity to dream about it and someday realize those dreams."

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