Canadarm2's Broken Wrist
It was a bad day at the aerospace office. Around 9 a.m. on March 5, NASA called Richard Rembala, a lead engineer for CANADARM2. There was a problem. Less than an hour earlier, astronauts aboard the International Space Station had put the huge, Canadian-built robotic arm, an essential tool in the station's construction, through a dry run for an upcoming mission. And the brakes in the wrist seized. When word reached Rembala at MacDonald Dettwiler Space and Advanced Robotics Ltd. in Brampton, Ont. - the arm's contractor - he and a team of elite engineers got busy. The cause of the malfunctioning joint, they knew, could lie anywhere along the lanky arm's 17-m length. They worked fast. "Within about an hour and a half," recalls Rembala, "we said yeah, the problem's definitely right in the wrist-roll joint."
That was the easy part. Now they have to fix it. That task, barring last-minute delays, begins next week, when the shuttle Endeavour is to blast off from the Kennedy Space Center in Florida a month behind schedule with the 220-kg spare part needed for the joint-replacement surgery. Also stowed in the shuttle's payload bay for the 13-day mission: the mobile base system (MBS), an aluminum framework bigger than two minivans parked side by side. An anchor for the arm, it will serve as a work platform and storage unit.
The crew will fasten the MBS, also built by MD Robotics, to a U.S.-made rail car, already in place, that will be able to roll more than 100 m along the outside of the station when it is all completed in 2006. Once the faulty joint is replaced, Canadarm2 is to cartwheel off the spot where it has been moored since Canadian astronaut Chris HADFIELD delivered it in April, 2001, and onto the MBS. The whole package, resembling a carbon-fibre praying mantis on wheels, will then be ready to move to various work sites in need of Canadarm2's lifting, holding and attaching skills.
The wrist problem, thought to be caused by a short-circuit, is Canadarm2's second major failure in its 14 months on the construction site. Engineers corrected a glitch in a computer chip in the arm's shoulder with new software last June. With the new setback, NASA had to postpone Endeavour's launch to allow the astronauts time to train for the joint replacement. That extended the mission of the station's current three-man crew, who will return on the Endeavour after more than five months' absence from earth. While unfortunate, problems are to be expected, says Chris Woodland, director of space station programs at MD Robotics. "I wouldn't tell anyone this isn't going to happen again."
Bear in mind what Canadarm2 has been through, says Woodland. For more than a year, the complex device has endured temperature swings of 200° C and back every 90 minutes as it darts in and out of sunlight in earth orbit. And, Woodland notes, Canadarm2 has already seen more service hours in space than any of the four versions of the original Canadarm used on shuttle missions for 20 years.
Still, thank goodness for backups. Throughout the latest orbital drama, the crew aboard the space station have used Canadarm2's secondary control systems to keep the wrist moving. New software for the primary system is telling the arm's computers to ignore the wrist failure. Even if the backup goes on the fritz, astronauts could still run the arm by using re-choreographed manoeuvres to compensate for going without one of the arm's seven joints. In Brampton, the engineers say they're ready. "It was seven days a week," says Woodland, "for as many hours as the guys could handle." Now, the payoff seems well within the robotic arm's reach.
Maclean's May 27, 2002