For the first time ever, Canadian space hardware will be repaired in orbit.
Space engineers say the repair will be tedious but no more complicated than changing the tire on a car.
Endeavour's scheduled launch, now planned for the end of this month, nevertheless was delayed nearly a month so that its crew could train for the job, which will require a spacewalk from the International Space Station.
At stake is the health of the most complex robotic device ever flown in space, Canadarm2, the Canadian-built robot arm without which the orbital outpost cannot be completed.
The $1.4-billion arm has been malfunctioning since March 5, about a year after its installation. One of its seven joints needs to be replaced.
"Any part that is mechanical or electronic is liable to fail. We were hopeful that there wouldn't be a failure. However, there was always that potential," Canadian Space Agency director of operations Benoît Marcotte said at a technical briefing Tuesday.
"We're seeing this [problem]probably earlier than we expected."
The glitch occurs only in Canadarm2's primary systems; the arm's computer says that it cannot release the brake in one of the wrist joints.
The backup systems still operate, but officials want Canadarm2 to be fully functional because it will be very heavily loaded in future missions, assembling station elements weighing more than 10 tonnes.
"It's the first time we will do a repair like this in orbit," Mr. Marcotte said. "Canadarm2 is modular. It was conceived so its elements could be dismantled in space."
The two spacewalkers who will mend it will be Franklin Chang-Diaz, a Costa-Rican-born American plasma physicist, and Colonel Philippe Perrin of the French Air Force.
Dr. Chang-Diaz is an astronaut with the U.S. National Aeronautics and Space Administration; Col. Perrin works for CNES, the French space agency.
"The third spacewalk will involve the repair of something that has failed. And that is really what humans are all about in space, to keep things going," Dr. Chang-Diaz said in an interview posted Tuesday on a NASA Web site.
"We'll have many other failures along the way that will require intervention by humans to keep things moving along."
The shuttle mission was initially supposed only to ferry a replacement crew to the station and to install another piece of Canadian hardware, the Mobile Base System.
The MBS is a work platform and storage facility that will move along the length of the station on electrical rails and on which Canadarm2 will ride.
The arm's troubles postponed the launch of the flight, labelled STS-111, so the two men could train for the additional spacewalk. That took place first in a virtual-reality lab, with computerized images projected on tiny screens in a helmet head mount, then diving in their space suits in a giant swimming pool that simulates weightlessness.
The 12-day flight is to be launched the afternoon of May 30. The repair will take place on the mission's third spacewalk, scheduled for June 7.
The repair will take four to five hours, in which two astronauts will essentially detach the arm's hand (not the Canada Hand, which will be installed on a future mission) so they can snap in a new wrist.
"It's a pretty simple operation," said Canadian Space Agency official Adam Mizera, the mission's operations engineer,
The two will have to take the arm apart, removing its latching end and stowing it in a workbench. Then, by removing six thumb-thick expendable bolts, they will disconnect the defective wrist and replace it.
The new joint, a 100-kilogram device the size and shape of a beer keg, needs to be ratcheted into place with a power tool to ensure the computer connections are tight.
"It will be a very methodical process, very slow," Dr. Chang-Diaz said on the NASA Web site.
Despite the faulty joint, the Canadian-made robot systems have repeatedly been lauded.
"They've done a terrific job at creating a very nice arm with a lot of capabilities and a very smart interface," Col. Perrin said on the Web site.
"I think it's unique to have one agency being so focused on one specific piece of the hardware. It makes them extremely talented and knowledgeable. I think it makes a lot of sense for the [Canadians]to keep working on . . . robotic arms. I hope they do more on the Mars mission."
Tu Thanh Ha